Progress In Oceanography

Published by Elsevier
Online ISSN: 0079-6611
Publications
Article
The upwelling systems of the eastern boundaries of the world's oceans are susceptible to harmful algal blooms (HABs) because they are highly productive, nutrient-rich environments, prone to high-biomass blooms. This review identifies those aspects of the physical environment important in the development of HABs in upwelling systems through description and comparison of bloom events in the Benguela, California and Iberia systems. HAB development is dictated by the influence of wind stress on the surface boundary layer through a combination of its influence on surface mixed-layer characteristics and shelf circulation patterns. The timing of HABs is controlled by windstress fluctuations and buoyancy inputs at the seasonal, event and interannual scales. Within this temporal framework, various mesoscale features that interrupt typical upwelling circulation patterns, determine the spatial distribution of HABs. The inner shelf in particular provides a mosaic of shifting habitats, some of which favour HABs. Changes in coastline configuration and orientation, and bottom topography are important in determining the distribution of HABs through their influence on water stratification and retention. A spectrum of coastline configurations, including headlands, capes, peninsulas, Rías, bays and estuaries, representing systems of increasing isolation from the open coast and consequent increasing retention times, are assessed in terms of their vulnerability to HABs.
 
Article
The ecology of bottom-living fishes was investigated from a series of collections off northwest Africa in the area 08°–27°N and 14°–30°W from sounding of 261–6059 m. The variety of sampling gear included epibenthic sledges, a semi-balloon otter trawl, benthic traps and longlines. A total of 92 operations collected more than 4600 specimens of at least 148 species. The catches of both types towed gear were shown to have a high degree of similarity, but sampled a different spectrum of species to the static baited gears. An overall consistency by latitude was found in the composition of the net catches. Despite possible indications of faunal boundaries at around 1100 m and 2100 m, the salient feature of an analysis of catch composition by sounding was the generally steady faunal change to a depth of about 3000 m. The only obvious discontinuity was evident at this level, beyond which the fish fauna was more uniform. This lack of marked species assemblages, characteristic of other regions, may result from the high primary productivity in the area. Features distinguishing the fish fauna of the area from that of the non-upwelling temperate western North Atlantic were found to be the smaller mean size of the dominant species, the abundance of bathygadine macrourids and the evident lack of the ‘bigger-deeper’ phenomenon. The relative density of fishes was estimated and shown to decrease by 2 orders of magnitude from the upper slope to 2000 m soundings. Below this the decline was less marked and relative densities of around 0.5 fish/1000 m2 were maintained to at least 4000 m soundings.The abundance of slope fishes in the area was tentatively estimated to be greater than in the non-upwelling temperate western North Atlantic. Few species were abundant; only 25% were represented by more than 17 specimens. Yet the dominant family, the Macrouridae, was represented by 48% of the specimens and 18%of the species. A total of 14 species from 6 families, represented by 60 or more specimens, were examined in detail to provide information on vertical distribution, population structure, breeding biology and feeding. Niche availability and resource partitioning among co-occurring species of macrourids are briefly discussed. Station and species data are given, together with a note on the likely conspecificity of Coryphaenoides colon with C. zaniophorus.
 
Article
We describe and analyze observations of the water exchange through the Strait of Gibraltar. The primary observations were taken in April 1986 and included data from moored recording instruments at four locations in the strait, together with an intensive survey by ship. These measurements included extensive CTD profiling, acoustic Doppler current profiling, numerous profiles from expendable instruments, and imaging using a high-frequency echo sounder.The analysis focuses on the internal hydraulics of the Strait and in particular the presence of hydraulic controls and their influence on the exchange. Our observations during April 1986 show that the maximal exchange condition, in which a subcritical flow is bounded by supercritical flow at both ends of the Strait, did occur, although with various subtleties not explicitly incorporated in our previous theoretical developments.The Atlantic water, moving east along the surface, encounters a control at Tarifa Narrows in the eastern part of the Strait. The Mediterranean water passes through controls both at Camarinal Sill and at Spartel Sill, further west. At our westernmost mooring at Spartel, the outflowing Mediterranean water is continuously supercritical, with little tidal variability. The control at Camarinal Sill is periodically lost due to tidal action and reappears on a reverse flow during a falling spring tide. Tangier Basin, bounded by Camarinal and Spartel Sills, acts as an internal reservoir for the outflowing Mediterranean water, the interface rising and falling through each tidal cycle. During spring tides, the Tangier Basin interface rises high enough to flood the control at Camarinl Sill and produces a reverse flow of the lower layer. When control is lost at Camarinal Sill a travelling bore is released. This bore may modify the location of the control acting on the surface layer in Tarifa Narrows. The Atlantic water enters the Alboran Sea as a jet which may have a well-defined northern boundary where it separates from the European coast; this boundary may be identified by the presence of a slick. Although the internal jump west of Camarinal Sill is almost certainly an area of intense mixing, frictional effects do not appear to be dominant in the subcritical portion of the flow.
 
Article
The ‘biological pump’ mediates flux of carbon to the interior of the ocean by interctions between the components of the vertically-structured pelagic ecosystem of the photic zone. Chlorophyll profiles are not a simple indicator of autotrophic biomass or production, because of non-linearities in the physiology of cells and preferential vertical distribution of taxa. Profiles of numbers or biomass of heterotrophs do not correspond with profiles of consumption, because of depth-selection (taxa, seasons) for reasons unconnected with feeding. Depths of highest plant biomass, chlorophyll and growth rate coincide when these depths are shallow, but become progressively separated in profiles where they are deeper — so that highest growth rate lies progressively shallower than the chloropyll maximum. It is still uncertain how plant biomass is distributed in deep profiles.Depths of greatest heterotroph biomass (mesozooplankton) are usually close to depths of fastest plant growth rate, and thus lie shallower than the chlorophyll maximum in profiles where this itself is deep. This correlation is functional, and relates to the role of heterotrophs in excreting metabolic wastes (especially ammonia), which may fuel a significant component of integrated algal production, especially in the oligotrophic ocean.Some, but not all faecal material from mesozooplankton of the photic zone appears in vertical flux below the pycnocine, depending on the size of the source organisms, and the degree of vertical mixing above the pycnocline. Diel, but probably not seasonal, vertical migration is significant in the vertical flux of dissolved nitrogen.Regional generalisations of the vertical relations of the main components of the ‘biological pump’ now appear within reach, and an approach is suggested.
 
Article
Warm and saline waters enter the Nordic Seas from the south as part of the warm-to-cold water transformation of the thermohaline circulation of the northern North Atlantic. One explanation for the origin of the Nordic Seas Inflow is a “shallow source hypothesis” under which the Inflow waters are a modification of upper ocean subtropical waters. Warm waters from the subtropical gyre are carried to the eastern North Atlantic by the North Atlantic Current and branch northwards, joined by poleward upper thermocline flow along the upper continental slope, to provide the Nordic Seas Inflow. Along this pathway the upper water column is progressively cooled and freshened by winter convection, the subpolar mode water transformation process, and this sets the Inflow characteristics.
 
Article
Profiles of the radioisotopes 210Pb and 137Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of 210Pbex (unsupported) in core top sediments increased with water depth from 25 dpm g−1 on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g−1 at the deep-sea station SB. The high 210Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective 210Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, 137Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced 137Cs fallout.The mean sedimentation rates estimated by the 210Pbex inventory method, rather than using 210Pbex profiles, ranged from 0.06 g cm−2 y−1 to 0.14 g cm−2 y−1. The average value of the rates in SB cores was calculated to be 0.11 g cm−2 y−1, which was similar to that calculated under the assumption that the age of the 137Cs peak corresponds to its maximum fallout year in 1963.Although 137Cs inventories represented one tenth of the anthropogenic fallout of 137Cs until 1997, they correlated with the increase in 210Pbex inventory. This suggests that the scavenging of refractory 137Cs as well as 210Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site.
 
Article
Sex ratios and reproductive activity of benthic copepod assemblages were investigated at the bathyal site (depth 1430 m) in Sagami Bay, central Japan. The ratio of adult females to adult males was approximately 3.5:1, significantly different from 1:1, although this parameter did not show a seasonal pattern. On the other hand, the percentage of ovigerous females among adult females and the ratio of nauplii to total copepods appeared to fluctuate seasonally in 1997 and 1998. Statistical tests, however, could not detect significant difference in either parameter. We discuss the possibility that the reproductive activity of copepods was enhanced by the increased supply of fresh phytodetritus to the sea floor.
 
Article
Comprehensive seasonal cycles of production and consumption in the pelagial require the ocean to be partitioned. This can be done rationally at two levels: into four primary ecological domains (three oceanic and one coastal), or about fifty biogeochemical provinces. The domains differ in their characteristic seasonal cycles of stability, nutrient supply and illumination, while provinces are defined by ocean currents, fronts, topography and recurrent features in the sea surface chlorophyll field.For each of these compartments, seasonal cycles of photic depth, primary production and accumulation (or loss) of algal biomass were obtained from the climatological CZCS chlorophyll field and other data and these, together with mixed layer depths, rendered characteristic seasonal cycles of production and consumption, which can be grouped into eight models: i — polar irradiance-mediated production peak; ii — nutrient-limited spring production peak; iii — winter-spring production with nutrient limitation; iv — small amplitude response to trade wind seasonality; v — large amplitude response to monsoon reversal; vi — canonical spring-fall blooms of mid-latitude continental shelves; vii — topography-forced summer production; viii — intermittent production at coastal divergences.For higher latitudes, these models suggest that the observed late-summer ‘blooms’ result not from a renewal of primary production rate, but from a relaxation of grazing pressure; in mid-latitudes, the observed ‘winter’ bloom represents chlorophyll accumulation at a season when loss terms are apparently smaller than during the period of peak primary production rate which occurs later, in spring. Where an episodic seasonal increase in rate of primary production occurs, as in the Arabian Sea, algal biomass accumulation may brief, lasting only until consumption is fully re-established. Only in the low latitude oligotrophic ocean are production and consumption perennially and closely coupled.
 
Article
Daytime zooplankton samples collected at a fixed station position in St Helena Bay during a 27-day time series in March–April 1987 were used to describe the variability in the vertical distribution, abundance and population age structure of Calanoides carinatus in response to upwelling-related processes, and to provide an estimate of their production.The vertical distribution of C. carinatus was characterized by ontogenetic layering of copepodites and adults. The thermocline delimited the maximum depth of young copepodites (CI–CIII). Older stages avoided low-oxygen (<1ml O2l−1) bottom water. C. carinatus was usually not associated with the chlorophyll a maximum. The spatial segregation of young and older stages is briefly discussed in relation to differential feeding habits and diel vertical migration behaviour.The demographic structure of the C. carinatus population showed evidence of a stable age distribution of copepodites and adults during the first of two upwelling cycles observed during the study. Overall mean abundance was 360 animals m−3. However, during the second upwelling cycle their mean abundance was reduced to 183 animals m−3 and adults dominated the population by 54%. These changes in abundance and age structure are discussed in relation to upwelling-induced advective processes. Surface-dwelling young copepodites (CI–CIII) are thought to be transported away from the study site, while re-seeding of the reduced nearshore population probably took place through advection of diapausal pre-adults (CV) in the upwelled water.Daily production of juvenile C. carinatus was estimated at 0.7 mg C m−3d−1 which, combined with an egg production of 1.0mg C m−3d−1 by addult females, is equivalent to 2% of the observed daily primary production. The mean P:B ratio for the copepodite stages was 0.167d−1. The role of this dominant copepod and the mesozooplankton in the carbon budget in St Helena Bay is discussed. Consumption by mesozooplankters was estimated at 22% of the daily primary production indicating that there was a considerable imbalance between primary producers and production by these heterotrophs.
 
Article
In December 1883 the U.S. Navy Hydrographic Office began to publish monthly Pilot Charts. These showed the positions and drifts of abandoned derelict sailing vessels and other danger to navigation in the North Atlantic. During the years 1887–1893 a total of 1,628 derelict sightings were made. The repeated sightings of 200 derelicts identified by name gave the first large-scale and long-term measurement of ocean trajectories. A new chart of these trajectories between 1883 and 1902 is presented showing features of the general ocean circulation and early evidence of the variability of ocean currents.
 
Article
The Benguela upwelling system in the subtropical Southeast Atlantic is subject to dramatic interannual fluctuations sometimes termed ‘Benguela Niño’ events. The South Atlantic Anticyclone (SAA) is assumed to be the responsible climatic ‘activity centre’ for the south-east trade winds driving the upwelling processes along the Namibian and South-west African coasts. Air temperature and humidity signals of this region, modulated by upwelling-controlled sea surface temperature (SST), are carried by the trades towards St. Helena Island. The island’s 1893–1999 century-long monthly weather records of temperature, pressure and rainfall have been assembled and homogenized. They exhibit trends for decreasing precipitation (10 mm/100 year), increasing air temperature (0.9 °C/100 year), and decreasing air pressure (0.6 hPa/100 year). Their first empirical orthogonal eigenfunction (EOF) covers 46% of the total variance; its associated temporal coefficient is proposed as a ‘St. Helena Island Climate Index (HIX)’. Austral winter HIX has a 42% correlation with a remotely sensed SST-derived Benguela upwelling index, called intense Benguela upwelling, for the time period 1982–1999. Not yet identified Benguela Niños (1895, 1905, 1912, 1916, 1946) and years of strong Benguela upwelling (1911, 1922, 1967, 1976) are newly suggested by the HIX.
 
Article
The Law of the Minimum was originally formulated by Justus von Liebig, as one of the 50 interlinked laws concerned with agriculture. The original writings of J. von Liebig often were misinterpreted by his successors. Brandt (1899) took this one law out of its context and proposed that limitation by nitrogen is a dominant factor in plankton ecology, far beyond its original application to agriculture. This was opposed by Nathansohn (1908) who suggested instead a dynamic balance of growth and loss terms. Towards validating, or eventually falsifying Brandt's hypothesis, Atkins, Harvey, Cooper and others developed the chemical methods necessary for re-defining ocean nutrient cycling and growth limitation. The major exception to these modern perspectives was the Antarctic Paradox of high nutrients and low chlorophyll which inspired Gran, Atkins, Harvey and Cooper to pioneer the concept of iron limitation. An exhaustive overview is given of efforts to define Fe in seawater and its controlling effect on in situ plankton growth, for the 1920–1984 period. Somewhat parallel work in the laboratory on single species of algae in chelation-controlled media has provided much insight, but is sketched only briefly. Martin and contemporaries developed the chemical methods necessary for defining the ocean chemistry of Fe and its role for in situ growth. These developments are sketched for the 1982–1991 period. Once again the Law of the Minimum and associated bold hypotheses served, albeit briefly, to bring a nutrient element in the forefront of research. This, and the recent awareness of CO2 as rate limiting factor, underline the conclusion that advances in sciences often hinge on advances in technology, confirming Kuhn (1962). In this case the new analytical techniques developed by Atkins, Harvey, Cooper, Martin and their associates have proven revolutionary for plankton ecology. Some observations in plankton ecology may be reminiscent of the agricultural Law of the Minimum, but this would not warrant its direct application, beyond its original context and agriculture, to plankton ecology. Rather the net rate of increase of phytoplankton is the dynamic balance of multiple growth and loss terms, together also determining the biomass at given time and space.
 
Article
During the 1920s and 1930s, there was a dramatic warming of the northern North Atlantic Ocean. Warmer-than-normal sea temperatures, reduced sea ice conditions and enhanced Atlantic inflow in northern regions continued through to the 1950s and 1960s, with the timing of the decline to colder temperatures varying with location. Ecosystem changes associated with the warm period included a general northward movement of fish. Boreal species of fish such as cod, haddock and herring expanded farther north while colder-water species such as capelin and polar cod retreated northward. The maximum recorded movement involved cod, which spread approximately 1200 km northward along West Greenland. Migration patterns of “warmer water” species also changed with earlier arrivals and later departures. New spawning sites were observed farther north for several species or stocks while for others the relative contribution from northern spawning sites increased. Some southern species of fish that were unknown in northern areas prior to the warming event became occasional, and in some cases, frequent visitors. Higher recruitment and growth led to increased biomass of important commercial species such as cod and herring in many regions of the northern North Atlantic. Benthos associated with Atlantic waters spread northward off Western Svalbard and eastward into the eastern Barents Sea. Based on increased phytoplankton and zooplankton production in several areas, it is argued that bottom-up processes were the primary cause of these changes. The warming in the 1920s and 1930s is considered to constitute the most significant regime shift experienced in the North Atlantic in the 20th century.
 
Article
Copepodites of Neocalanus plumchrus dominate the macrozooplankton biomass of the subarctic Pacific during spring. Living specimens have patches of bright red-orange pigment. During May of 1984 it was found sharing dominance with another, related species with differently placed patches of more distinctly red pigment. This is an undescribed species, which is designated here as Neocalanus flemingeri sp. nov. In the Gulf of Alaska it is smaller than N. plumchrus, but in the Bering Sea where food supplies are greater it reaches dimensions as large or larger. The morphologies of the two species are distinct, and previous confusion in the taxonomy of these forms was caused by differences in their life histories.
 
Article
The mixed layer of the ocean and the processes therein affect the ocean’s biological production, the exchanges with the atmosphere, and the water modification processes important in a climate change perspective. To provide a better understanding of the variability in this system, this paper presents time series of the mixed layer properties depth, temperature, salinity, and oxygen from Ocean Weather Station M (OWSM; 66° N,2° E) as well as spatial climatologies for the Norwegian Sea. The importance of underlying mechanisms such as atmospheric fluxes, advective signals, and dynamic control of isopycnal surfaces are addressed. In the region around OWSM in the Norwegian Atlantic Current (NwAC) the mixed layer depth varies between ∼20 m in summer and ∼300 m in winter. The depth of the wintertime mixing here is ultimately restrained by the interface between the Atlantic Water (AW) and the underlying water mass, and in general, the whole column of AW is found to be mixed during winter. In the Lofoten Basin the mean wintertime mixed layer reaches a depth of ∼600 m, while the AW fills the basin to a mean depth of ∼800 m. The temperature of the mixed layer at OWSM in general varies between 12 °C in summer and 6 °C in winter. Atmospheric heating controls the summer temperatures while the winter temperatures are governed by the advection of heat in the NwAC. Episodic lateral Ekman transports of coastal water facilitated by the shallow summer mixed layer is found important for the seasonal salinity cycle and freshening of the northward flowing AW. Atmospheric freshwater fluxes have no significant influence on the salinity of the AW in the area. Oxygen shows a clear annual cycle with highest values in May–June and lowest in August–September. Interannual variability of mixed layer oxygen does not appear to be linked to variations in any of the physical properties of the mixed layer.
 
Article
An examination of large archives (1950–1997) of the oceanographic and atmospheric data from the northwestern North Pacific Subtropical Gyre has revealed clear linkages between atmospheric forcing factors, physical processes and biological events. Large changes in the winter and spring biomass of phytoplankton and macroplankton observed over annual, decadal and inter-decadal time scales could clearly be attributed to climate-related changes in oceanographic processes. Interannual changes in the intensity of the winter-time East Asian Monsoon had a significant impact on the extent of convective overturning, on nitrate inputs into the euphotic zone and the concentrations of chlorophyll a in winter and during the following spring. A prolonged period of deeper winter mixed layers observed from the mid-1970s to the mid-1980s led to a sizeable increase in winter mixed-layer nitrate concentrations. This change resulted in a decrease in winter-time phytoplankton biomass. Spring-time chlorophyll a, in contrast, showed a steady increase during this period. The decline in winter phytoplankton biomass could be attributed to the depths of mixed layer. A deeper mixed layer prevents phytoplankton from remaining in the euphotic zone for long enough to photosynthesize and grow, leaving substantial amounts of nutrients unutilised. However, as a result of stratification of the water column in spring following each of these winters, phytoplankton could take advantage of the enhanced ambient concentrations of nutrients and increase its biomass. Another noteworthy observation for the period from the mid-1970s to the early 1980s is that the western subtropical gyre progressively became phosphate limited. The period of diminishing mixed-layer phosphate concentrations was observed in our study area from the early 1990s onwards was consistent with recent observations at Station ALOHA in the eastern subtropical gyre.
 
Article
The Labrador Sea has exhibited significant temperature and salinity variations over the past five decades. The whole basin was extremely warm and salty between the mid-1960s and early 1970s, and fresh and cold between the late 1980s and mid-1990s. The full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s. The freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the Labrador Sea sustained throughout the subsequent years. It took only a decade for the whole water column to lose most of its excessive freshwater, reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s. If the recent tendencies persist, the basin’s storages of salt and heat will fairly soon, likely by 2008, exceed their historic highs.
 
Article
We studied the effects of low-frequency climate change on the reproductive performance of 11 species of marine bird in the southern California Current system, 1969–1997. Reproductive performance of Brown Pelican (Pelecanus occidentalis) and Double-crested Cormorant (Phalacrocrax auritus) in southern California demonstrated an increase in the 1970s and early 1980s, attributable to recovery from organochlorine contamination (primarily DDE). Brandt's Cormorant (Phalacrocorax penicillatus) in central California was the only species to demonstrate a secular increase in performance through time, a pattern that remains unexplained. Ashy Storm-petrel (Oceanodroma homochroa) and Pelagic Cormorant (Phalacrocorax pelagicus) demonstrated curvilinear patterns of change, with decreasing reproductive performance in the past decade. All other species including Western Gull (Larus occidentalis), Pigeon Guillemot (Cepphus columba), Xantus's Murrelet (Synthiloboramphus hypoleucus), Common Murre (Uria aalge), Cassin's Auklet (Ptychoramphus aleuticus) and Rhinoceros Auklet (Cerorhinca monocerata) showed diminishing reproductive performance through time. Patterns of change for the murre and auklets were not significant, presumably because of a lack of reproductive variation for these species, which display a conservative breeding effort (i.e. single-egg clutches). Changes in the birds' abilities to provision young and maintain chick survival during May–July each year appeared most closely related to overall changes in reproductive performance. Dietary change indicated a decline in use of juvenile rockfish (Sebastes spp.) by marine birds in central California. There was also significant interannual variability in consumption of juvenile rockfish and the euphausiid Thysanoessa spinifera. Patterns of change in marine bird reproductive performance were generally concordant between southern and central California after considering the period of recovery for Brown Pelican and Double-crested Cormorant. The decline in reproductive performance and changes in diet composition do not appear directly related to the polarity reversal of the Pacific Decadal Oscillation in 1976/1977. Instead, reproductive performance and dietary characteristics indicate substantial change in the late 1980s, suggesting another regime-shift at that time.
 
Article
It is now widely accepted that a climatic regime shift transpired in the North Pacific Ocean in the winter of 1976–77. This regime shift has had far reaching consequences for the large marine ecosystems of the North Pacific. Despite the strength and scope of the changes initiated by the shift, it was 10–15 years before it was fully recognized. Subsequent research has suggested that this event was not unique in the historical record but merely the latest in a succession of climatic regime shifts. In this study, we assembled 100 environmental time series, 31 climatic and 69 biological, to determine if there is evidence for common regime signals in the 1965–1997 period of record. Our analysis reproduces previously documented features of the 1977 regime shift, and identifies a further shift in 1989 in some components of the North Pacific ecosystem. The 1989 changes were neither as pervasive as the 1977 changes nor did they signal a simple return to pre-1977 conditions. A notable feature of the 1989 regime shift is the relative clarity that is found in biological records, which contrasts with the relative lack of clear changes expressed by indices of Pacific climate. Thus, the large marine ecosystems of the North Pacific and Bering Sea appear to filter climate variability strongly, and respond nonlinearly to environmental forcing. We conclude that monitoring North Pacific and Bering Sea ecosystems may allow for an earlier identification of regime shifts than is possible from monitoring climate data alone.
 
Article
The results are presented from three hydrogrpahic surveys in April 1979 of a 40 × 50 km region of the Celtic Sea, centred at 7°W and 51°N, using a towed undulating sensor system. In the days between Surveys 1 and 2, the seasonal thermocline was established, with surface to bottom temperature differences reaching 1.5°C, the average surface chlorophyll a level increased from ∼ 1 to ∼ 5.5 mg m⁻³ due mainly to the growth of diatoms, and the surface nitrate concentration decreased from 6 to 1 μM. The third survey was carried out after a further two days and, although surface properties changed little, there was a general deepening of the mixed layer due to stronger winds, and a further increase in the standing stock of phytoplankton.
 
Article
Observations of surface oceanographic and meteorological fields collected during the First GARP Global Experiment (FGGE) in the equatorial Atlantic Ocean have been combined and averaged by month onto a 2° × 2° grid. Monthly distributions of sea-surface temperature, wind speed and direction, air temperature, specific humidity and cloud cover have been generated for the period from December 1978 through November 1979. Net short wave and long wave radiation, and sensible and latent heat flux distributions have been generated from the surface data using the bulk aerodynamic formulas. In 1979, large-scale patterns of all the climatic and heat budget variables are very similar to distributions determined from long-term climatological averages. Positive anomalies of SST in the region of the equatorial cold water tongue represent one region of systematic differences between the 1979 and climatological distributions. The positive SST anomalies are, in general, coincident with negative anomalies in the net oceanic heat gain. Negative heat gain anomalies are primarily caused by positive wind speed anomalies, through increased latent and sensible heat fluxes.
 
Article
Transient tracer data (tritium, CFC11 and CFC12) from the southern, central and northwestern Weddell Sea collected during Polarstern cruises ANT III-3, ANT V-2/3/4 and during Andenes cruise NARE 85 are presented and discussed in the context of hydrographic observations. A kinematic, time-dependent, multi-box model is used to estimate mean residence times and formation rates of several water masses observed in the Weddell Sea.Ice Shelf Water is marked by higher tritium and lower CFC concentrations compared to surface waters. The tracer signature of Ice Shelf Water can only be explained by assuming that its source water mass, Western Shelf Water, has characteristics different from those of surface waters. Using the transient nature of tritium and the CFCs, the mean residence time of Western Shelf Water on the shelf is estimated to be approximately 5 years. Ice Shelf Water is renewed on a time scale of about 14 years from Western Shelf Water by interaction of this water mass with glacial ice underneath the Filchner-Ronne Ice shelf. The Ice Shelf Water signature can be traced across the sill of the Filchner Depression and down the continental slope of the southern Weddell Sea. On the continental slope, new Weddell Sea Bottom Water is formed by entrainment of Weddell Deep Water and Weddell Sea Deep Water into the Ice Shelf Water plume. In the northwestern Weddell Sea, new Weddell Sea Bottom Water is observed in two narrow, deep boundary currents flowing along the base of the continental slope. Classically defined Weddell Sea Bottom Water (θ ≤ −0.7°C) and Weddell Sea Deep Water (−0.7°C ≤ θ ≤ 0°C) are ventilated from the deeper of these boundary currents by lateral spreading and mixing. Model-based estimates yield a total formation rate of 3.5Sv for new Weddell Sea Bottom Water (θ = −1.0°C) and a formation rate of at least 11Sv for Antarctic Bottom Water (θ = −0.5°C).
 
Article
A climatic regime shift, an abrupt change from cooling to warming in the Japan/East Sea (JES), particularly in the Tsushima warm current (TWC) region, occurred in the late 1980s. The ecosystem of the JES responded strongly to the changing thermal regime. Many, but not all biological components of the ecosystem, spanning from plankton to predatory fishes, and including both warm-water pelagic and cold-water demersal species responded to this late 1980s climatic regime shift in the JES.Diatom abundance (cell number) in spring from a monitoring line located in the central part of JES showed decadal variations with a step change from positive to negative anomalies in 1991. Zooplankton biomass in spring and autumn was high in the 1970s, declined during the 1980s, and returned to higher, but quite variable levels during the 1990s. Japanese sardine catch increased after 1974 to its peak level in 1989 and then declined dramatically to 1974 levels by 1997 with step changes in 1979 and 1994. Conversely, catches of other small pelagic species such as Japanese anchovy and common squid, and several higher-trophic fishes, such as yellowtail and tunas increased markedly in the 1990s compared to the early-mid 1980s. Step changes were detected in these pelagic species during 1989–1992. Catch of demersal species (crab, pink shrimp, Pacific cod and walleye pollock) were high during most of the 1970–1980s, but declined at various times in the late 1980s to generally low catches in the 1990s. Detailed analysis of the demersal fish assemblage composition, abundance and distribution indicated a shift in the late 1980s with several years lag in the time of change. Cold-water species (e.g., walleye pollock, Pacific cod) decreased in abundance and the regions in which their abundances remained high became greatly reduced in extent. Conversely, warm-water species (e.g., pointhead flounder, shotted halibut) increased in abundance and/or extended their spatial range (as indicated by trawl catch) during the warm 1990s. A principal component analysis for pelagic and demersal fish assemblages, suggested decadal variation patterns with a step change during 1986–1988. Abundance changes were identified not only in the plankton, but also in small pelagic fishes, and in predatory fishes. These changes were reflected in fish community indicators, and suggest an ecosystem regime shift occurred in the TWC region as a result of the late 1980s climatic regime shift. A hypothesis on the ecological response process to the late 1980s climatic regime shift was proposed.
 
Article
Fish scales were used to investigate the interannual variability in chum salmon growth rates at specific ages in relation to climatic/environmental changes during the 1980s–1990s. Scales were obtained from adult salmon returning to the east coast of Korea between 1984 and 1998. Assuming proportionality between scale size increments and fish length, distances between scale annuli were regarded as the growth conditions in different habitat areas with respect to the life stages of chum salmon. In estuarine and coastal areas, growth rates of fingerling salmon were higher in the 1990s than in the 1980s. Zooplankton abundance off the east coast of Korea increased after the late 1980s, which may have provided favorable growth conditions for young salmon in the 1990s. Growth of juvenile chum salmon during the first summer (Okhotsk Sea) was relatively stable, and neither SST nor zooplankton biomass fluctuated significantly during the study period. However, in the Bering Sea, salmon growth rates between age-2 and age-4 (i.e. ocean-phase immature salmon) were higher in the 1980s than in the 1990s. Variability in salmon growth in the Bering Sea was correlated to zooplankton biomass. These results suggest that the climate regime shift of 1988/1989 in the subarctic North Pacific affected salmon growth mediated by changes of zooplankton biomass, revealing a bottom-up process.
 
Article
Satellite-derived sea surface temperatures illustrate the variability of the path of the Tsushima Current in the Sea of Japan. In the spring of 1981 the Tsushima Current did not split as it left the Korea Strait and flowed into the Sea of Japan, which is contrary to the historical concept of branching. Warm water remained along Honshu, the main island of Japan, making a strong front oriented in an east-west direction. Hydrographic data confirm that this spring condition lasted through to the fall of 1981. On the other hand, during the springs of 1982 and 1983 the branching is evident from satellite images: one branch flowed northward along the east coast of Korea, and the other flowed eastward along Honshu of Japan.
 
Article
We use hourly sea level records from southern California to the Aleutian Islands to examine low-frequency variability in the northeast Pacific during the 1982–83 and 1997–98 El Niño events. Comparison between events is facilitated by long-term records for Yakutat, Tofino, Neah Bay, and San Francisco which appear to be representative of distinct coastal oceanic regimes. Residual (tide-removed) time series reveal that the anomalously high sea levels of the 1997–98 El Niño persisted from the spring of 1997 to the spring of 1998, and this event was more protracted than the 1982–83 El Niño. During both major events, the annual (Sa), semiannual (Ssa), and synoptic-scale sea level constituents were amplified relative to non-El Niño years, with synoptic fluctuations in sea level height and sea level pressure for mid-latitude sites Tofino and Neah Bay strongly correlated through the inverted barometer response. The spectral anomaly coefficient, SjR(ω)=[Rj(ω)−1]/[Rj(ω)+1] (where is the spectral ratio between the El Niño spectrum, Sj(ω), and the time-averaged non-El Niño spectrum, , for tide gauge site “j”) indicates that both events gave rise to enhanced sea level variability at periods of days to weeks because of passing atmospheric systems. However, the 1982–83 El Niño generated greater signal variance at seasonal-scales (periods of months) while the 1997–98 El Niño generated greater variance at synoptic scales (periods of days). Findings further indicate that poleward oceanic propagation of the 1982–83 and 1997–98 events terminated near central California, and that much of the enhanced sea level variability observed at mid to high latitudes was associated with atmospheric teleconnection responses that gave rise to amplified variations in coastal sea level pressure and/or surface wind stress.
 
Article
Nine hydrographic (temperature, salinity, and in some cases oxygen) and four zonal velocity sections near the equator along 95°W in 1980–1984 are presented and discussed. Perturbations associated with the 1982–1983 El Niño form the locus for the discussion. This event was sampled by sections in November 1982 and in March, May, November and December 1983.The upper ocean response in November 1982 was a downward displacement of isotherms above 400m on the equator. The Equatorial Undercurrent (EUC) was deep and strong for this season. The vertical displacement perturbations extended to at least 5°S and were more uniform with depth off the equator.In May 1983 the largest vertical displacements were observed. The EUC was replaced by a westward jet at about 75m and the Equatorial 13°C Water thermostad has disappeared. Local estimates of the zonal pressure gradient suggest that between 110°W and 95°W this slope had reversed in May 1983 and was, at least in part, responsible for the observed westward jet. Local winds were westerly and forced the observed eastward surface current.By November 1983, surface conditions were fairly normal. However, the deeper stratification remained anomalous; the Equatorial 13°C Water thermostad was still missing. The vertical profile of vertical displacements from a mean suggested a complicated vertical structure near the equator. In April 1984 conditions had returned to near normal.
 
Article
Isotherm vertical displacements within the thermocline and surface currents were investigated in the tropical Atlantic Ocean from 12°N to 12°S in 1982–1984, the period of the FOCAL-SEQUAL experiment. The study is based on a numerical simulation of an oceanic general circulation model tuned for the study of the equatorial regions, and on the analysis of the large scale thermocline displacements and currents using observed temperature profiles. Ground truth is provided by temperature and currents from moorings, records from inverted echo sounders and tide gauges as well as from drifting buoys. Comparison of the analysis with the ground truth shows that some important aspects of the low frequency variability are “captured” by the analysis when the data base is large enough.On large scales, the simulation generally resembles the analysis. Along the equator, the upwelling signal propagates eastward. The seasonal set-up of the westerly winds is associated with large westward currents, and a following overshoot of the zonal dynamic topography. Otherwise, the zonal dynamic topography is in near-equilibrium with the winds. The North Equatorial Countercurrent is portrayed comparably in the analysis and the simulation, where, after starting as a narrow eastward flow near 5°N, it extends northward through the northern summer. Interannual variations are found both in the analysis and the simulation. In particular, the thermocline flattened early in 1984.However, the simulation differs in significant respects from the real world: the equatorial undercurrent is too weak in the east and the model produces too much variability south of the equator. The 20°C isotherm is too shallow above the core of the thermocline, and the surface layer is too stratified. Because the surface layer is where the wind stress, main forcing of the model is applied, major effort will have to be devoted to parameterizing the near-surface downward mixing of momentum, heat and fresh water.
 
Article
Particulate organic carbon (POC) is vertically transported to the oceanic interior by aggregates and their ballasts, mainly CaCO3 and biogenic opal, with a smaller role for lithogenic aerosols through the mesopelagic zone. Diel migrating zooplankton communities effect vertical transport and remineralization of POC in the upper layers of the ocean. Below 1.5 km, the presence of zooplankton is reduced and thus the aggregates travel mainly by gravitational transport. We normalized the fluxes of POC, CaCO3, and biogenic opal from data published on samples collected at 134 globally distributed, bottom-tethered, time-series sediment trap (TS-trap) stations to annual mole fluxes at the mesopelagic/bathypelagic boundary (m/b) at 2 km and defined them as Fm/bCorg, Fm/bCinorg, and Fm/bSibio. Using this global data set, we investigated (1) the geographic contrasts of POC export at m/b and (2) the supply rate of ∑CO2 to the world mesopelagic water column. Fm/bCorg varies from 25 (Pacific Warm Pool) to 605 (divergent Arabian Sea) mmolC m−2 yr−1; Fm/bCinorg varies from >8 (high latitude Polar Oceans) or 15 (Pacific Warm Pool) to 459 (divergent Arabian Sea) mmolC m−2 yr−1; and Fm/bSibio, the most spatially/temporally variable flux, ranges from 6 (North Atlantic Drift) to 1118 (Pacific Subarctic Gyre) mmolSi m−2 yr−1. The oceanic region exhibiting the highest POC flux over a significantly large region is the area of the North Pacific Boreal Gyres where the average Fm/bCorg = 213, Fm/bCinorg = 126, and Fm/bSibio = 578 mmol m−2 yr−1. Fm/bCorg and Fm/bCinorg are particularly high in large upwelling margins, including the divergent Arabian Sea and off Cape Verde. One of the data sets showing the lowest flux over a significant region/basin is Fm/bCorg = 39, Fm/bCinorg = 69, and Fm/bSibio = 22 mmol m−2 yr−1 in the North Pacific subtropical/tropical gyres; Pan-Atlantic average fluxes are similar except Fm/bSibio fluxes are even lower. Where Corg/Cinorg and Sibio/Cinorg are <1 defines the “Carbonate Ocean”, and where these ratios are ⩾1 defines the “Silica Ocean”. The Carbonate Ocean occupies about 80% of the present world pelagic ocean between the two major oceanographic fronts, the North Pacific Polar Front and the Antarctic Polar Front, and the Silica Ocean is found on the polar sides of these fronts. The total global annual fluxes of Fm/bCorg, Fm/bCinorg, and Fm/bSibio at m/b calculated by parameterizations of the export flux data from 134 stations are surprisingly similar; 36.2, 33.8, and 34.6 teramol yr−1 (120, 112, and 114 mmol m−2 yr−1), respectively, resulting in a near uniform binary ratio between the above three elements of about one. The global ternary % ratios estimated from 152 TS-trap samples of the three elements are 35:32:33. From our global Fm/bCorg and a published model estimate of the global export production, we estimate the regeneration rate of CO2 through the mesopelagic zone by the biological pump is 441 teramolC yr−1. Based on our global Fm/bCinorg and recently estimated global primary production of PIC, 36–86 teramolC yr−1 of PIC is assumed to be dissolved within the upper 2 km of the water column.
 
Article
Stage durations of third and fourth copepodites of Neocalanus plumchrus Marukawa were determined from stage frequency changes from 2 through 21 May 1984 at 50°N, 145°W. Median durations were approximately 24.0 days for C3 and 24.8 days for C4. Advance by the population to later stages was compared with developmental progress indicated by (1) direct molting rate determinations and (2) frequency of molt cycle phases based on tooth development. All indicators showed that the population was developing and growing steadily. The growth rate for C4, determined from Growth Rate = [1n(Weight of oldest C4) - 1n(Weight of new C4)]/Stage Duration was 0.048 per day. Carbon determinations for C5 collected from the field on several dates implied growth of 0.15 per day. DAGG and WALSER (1987) have shown that phytoplankton rations consumed by this species during the same cruise were less than metabolic requirements. Therefore, nutrition for the substantial growth rates observed by our methods must come from sources other than phytoplankton.
 
Results of principal component analysis showing the loadings of the different components of the four dominant multi-variate environmental factors and the proportion of the observed variance explained by each factor. Those variables with large loadings (>0.5) are in bold font. 
Time series of abundance of 18 focal species during the early (1985–1994) and late (1997–2006) periods, compared to the long-term mean (solid line) ±1 SD (dashed lines): (a–d) species for which no patterns were discernible, (e–j) those for which ''Year " was an important variable, (k–n) those for which PC1 was important, and (o–r) those for which a combination of factors were important. Years when species were not sighted are highlighted with an asterisk (Ã).  
Generalized additive models (GAM) relating seabird density (number km À2 ) to the survey year 1985–2006. The best-fit loess function (thick line), the 95% confidence intervals (fine lines), and the distribution of surveys (vertical ticks) are shown. Shown are species that occurred in >75% of cruises: (A) Those with highly significant (p < 0.003) trends (cassin's auklet, western gull, black-footed albatross); (B) those with significant (p < 0.05) trends (rhinoceros auklet, ashy storm-petrel, pigeon guillemot).  
Humpback whale observations during NMFS-RRS surveys in May–June 1985–2006, showing the occurrence (% of survey years observed) and the number of sightings and individuals recorded during the early and late time periods of the study. Whale abundance (mean, minimum, maximum, CV), calculated as the number of whales seen per 100 km 2 of survey area, is reported for both time periods. 
Article
To characterize the environmental factors affecting seabird population trends in the central portion of the California current system (CCS), we analyzed standardized vessel-based surveys collected during the late spring (May–June) upwelling season over 22 yr (1985–2006). We tested the working hypothesis that population trends are related to species-specific foraging ecology, and predicted that temporal variation in population size should be most extreme in diving species with higher energy expenditure during foraging. We related variation in individual species abundance (number km−2) to seasonally lagged (late winter, early spring, late spring) and concurrent ocean conditions, and to long-term trends (using a proxy variable: year) during a multi-decadal period of major fluctuations in the El Niño-Southern oscillation (ENSO) and the Pacific decadal oscillation (PDO). We considered both remote (Multivariate ENSO Index, PDO) and local (coastal upwelling indices and sea-surface temperature) environmental variables as proxies for ocean productivity and prey availability. We also related seabird trends to those of potentially major trophic competitors, humpback (Megaptera novaeangliae) and blue (Balaenoptera musculus) whales, which increased in number 4–5-fold midway during our study. Cyclical oscillations in seabird abundance were apparent in the black-footed albatross (Phoebastria nigripes), and decreasing trends were documented for ashy storm-petrel (Oceanodroma homochroa), pigeon guillemot (Cepphus columbus), rhinoceros auklet (Cerorhinca monocerata), Cassin’s auklet (Ptychoramphus aleuticus), and western gull (Larus occidentalis); the sooty shearwater (Puffinus griseus), exhibited a marked decline before signs of recovery at the end of the study period. The abundance of nine other focal species varied with ocean conditions, but without decadal or long-term trends. Six of these species have the largest global populations in the CCS, and four are highly energetic, diving foragers. Furthermore, three of the diving species trends were negatively correlated with the abundance of humpback whales in the study area, a direct competitor for the same prey. Therefore, on the basis of literature reviewed, we hypothesize that the seabirds were affected by the decreasing carrying capacity of the CCS, over-exploitation of some prey stocks and interference competition from the previously exploited, but now increasing, baleen whale populations. Overall, our study highlights the complexity of the ecological factors driving seabird population trends in the highly variable and rapidly changing CCS ecosystem.
 
Article
A brief review of the meteorological setting, hydrography and the circulation in the Levantine Basin of the Eastern Mediterranean is given. The recent high resolution data obtained in POEM coordinated experiments of 1985–1986 are then used to optimally estimate the circulation in the basin in two different seasons and to describe the water mass distributions. Some of the features observed during the experiments support the historical knowledge on the locations of sub-basin scale gyres and the general circulation, in addition to which some new features are established. Details of the circulation such as the intensity, the multiple scales and the three dimensional structure of the various vortices and the Central Levantine Basin Current are displayed extensively. A variety of sub-basin, meso- and sub-mesoscale vortices occur with highly assymmetric (baroclinic) vertical structures. Some eddies split into multiple centres with depth, interpreted as indicating possible coalescences. A number of long-lived eddies were persistent in both surveys. The sub-surface Atlantic Water (AW) is advected by and entrapped within the eddy field. The Levantine Intermediate Water (LIW) at intermediate depths is shown to be maintained throughout the year in the northeastern sector of the Levantine Basin and along the periphery of the Rhodes gyre. The Levantine Intermediate Water also has a patchy distribution, owing to the advection and trapping by the eddy field which it helps to generate through adjustment processes.
 
Article
GEOSAT sea level anomalies, XBT temperature profiles and Levitus climatologies were assimilated monthly during 1987 into a non-linear primitive equation model of the tropical Atlantic ocean. Assimilating GEOSAT sea level anomalies increased the variability of the north equatorial currents when it was too weak without assimilation. The assimilated run simulates both the 1987 to 1988 sea level rise in the equatorial band and lowering in the tropics. The comparison with Reynolds satellite SST data is useful to identify the upwelling areas as the zones of highest error for the model SST. The SST warming of early 1988 is little changed by the assimilation. It confirms that this warming was determined more by the wind forcing than by the internal dynamics. As previously deduced from inverted echo sounders, we find that the North Equatorial Counter Current (NECC) increased its transport during 1987. The assimilation reveals in addition that the NECC formed a strong gyre with the North Equatorial Current (NEC). Consistently with previous analysis, this strengthening of the gyre was a result of the anomaly in ITCZ position and intensity. The heat gain by the Equatorial Under Current (EUC) was reduced because of this northward drift of the NECC. The South equatorial Current (SEC) was strong in spring and summer, moving warm surface water to the west. The eastward transport of the underlying EUC was thus reduced, leading to a deficit of warm water in the Gulf of Guinea in 1987. Comparisons of the assimilated northward heat transport (NHT) with climatological data estimates principally suggests that there was a heat gain of up to 1 petaWatt in the northern tropics (3°N–18°N) in 1987. The assimilated NHT was nil at 25°N and negative within 5°S–15°S. The 1987–88 warm events in the tropical Atlantic ocean presents a temporal progression from the north to the south. The assimilation presents an increased transport of warm water by the EUC after September 1987. It confirms the positive impact of the mass redistribution on the subsurface warming of the Gulf of Guinea. The other significant impact in 1988 is an increase of the gyre formed by the SEC and the South Equatorial Counter Current (SECC) and an inhibition of the coastal upwelling off Namibia.
 
Article
GEOSAT sea level anomalies, XBT temperature profiles and Levitus (1982) climatologies are assimilated monthly during 1987 into a non-linear primitive equation model of the tropical Atlantic ocean. The model has a resolution of 1° longitude × 1/3° latitude × 20 vertical levels extending from 50°S to 50°N. The model physics are complex, including in particular an imbedded 1D-mixed layer model and a 3D-Richardson number dependent mixing under the mixed layer. The model is forced monthly by ship winds and Oberhüber (1988) climatologies of heat and fresh water fluxes. A 2° longitude × 1° latitude gridded optimal interpolation analysis of GEOSAT sea level anomalies is assimilated between 20°S and 20°N at each grid point of the model. Validated TOGA XBT profiles are also assimilated between 30°S and 30°N at their exact location. Decorrelation in space and time is assumed for the error structures. Data weights are taken to vary with data accuracy and with a proxy function of the model representativity.
 
Article
Horizontal distribution patterns of zooplankton biomass and the stock-forming zooplankton taxa are presented and discussed for the entire North Sea in connection with hydrographic and climatological conditions and the general current regime.The winter distribution pattern of the various zooplankton taxa can be divided roughly into three categories: 1.1. Species associated with water masses of Atlantic origin, whose southernmost boundary of distribution is generally the Dogger Bank.2.2. Species occurring primarily in the central North Sea, concentrated mostly in the area between 56°-58°N and 2°–5°E.3.3. Neritic forms, with large stocks in the Continental Coastal Water, but which can also be found in relatively large numbers in the mixed oceanic/coastal water off the Scottish northeast coast where they are subject to advection in the Scottish Coastal Water and the Dooley Current.It was generally found that the finer the taxonomic analysis of the plankton, the better it could be related to the horizontal distribution patterns of the North Sea current regime.During both the winter cruise and the late spring cruise, the largest planktonic biomasses were found in the central and northern North Sea. However, in late spring the influence of the shelf edge was evident from the incoming Calanus stocks, whereas in late winter the greatest biomasses stemmed from the Dogger Bank and the Skagerrak regions. At this time, the Dogger Bank was already a reproductive centre for various meroplanktic and holoplanktic organisms, which apparently provided the trophic basis for large populations of predators such as Sagitta spp., amphipods and Aglantha digitale. The large biomass in the Skagerrak was mainly based on various Atlantophilic forms such as Calanus finmarchicus, Metridia spp. and diverse mesopelagic copepods as well as euphausiids and chaetognaths.
 
Article
The overall background to the U.K. BOFS (Biogeochemical Ocean Flux Study) Project, designed to investigate oceanic carbon flux processes throughout the water column, is briefly described together with the strategy for the 1990 BOFS Spring Bloom Experiment. The Experiment involved two ships and was carried out in the northeast Atlantic between 46–50°N, 14–22°W in the period 18 April – 25 June 1990 with the objective of monitoring and quantifying the major carbon flux changes associated with the succession of the spring bloom. Sampling was carried out over a 7 week period adjacent to a Lagrangian buoy drogued at 30m. The spatial fields of the major variables were characterized from box grid surveys around the position of the marker drogue at the beginning and end of the time series observations with the time series hydrographical changes being related to features observed in the spatial surveys.
 
Article
A time-series particle flux study has been conducted at two sites near the Aleutian Islands: Station AB in the Bering Sea for nine years (1990–1999) and Station SA in the central subarctic Pacific for ten years (1990–2000). Significant variability in seasonal and inter-annual particle flux was observed at both stations. The annual primary flux maxima that occurred during spring through summer tended to vary considerably in their timing and magnitude, whereas the secondary maxima in fall tended to be fairly consistent of timing and flux levels. Biogenic opal contributed to a major portion of the time-series fluxes. Especially diatoms such as Neodenticula seminae are important in the biogenic opal fluxes and drive bulk of the biological system. Calcium carbonate fluxes were comprised of six species of planktonic foraminifera and mainly two species of coccolithophores. At Station AB, the fall CaCO3 flux maxima were higher than those of spring, whereas the magnitudes of the CaCO3 flux maxima were similar during both spring and fall at Station SA. The seasonal change in CaCO3 flux was significantly different from that of biogenic opal. The planktonic foraminiferal contribution was greater than that of coccoliths to the total CaCO3 flux.
 
Article
Meridional sections of upper ocean zonal currents, potential temperature, and salinity are estimated at ten longitudes from 143°E to 95°W using Conductivity–Temperature–Depth and Acoustic Doppler Current Profiler data from 172 synoptic sections taken in the tropical Pacific between 138°E and 86°W, mostly in the 1990s. Data reduction is carried out in a potential isopycnal and mixed layer framework to preserve a sharp pycnocline, a mixed layer, water property extrema, and velocity extrema. Mean zonal currents, potential temperatures, and salinities are produced at each longitude. The seasonal cycles of these fields are also estimated, as well as a simple El Niño Southern Oscillation (ENSO) cycle. Zonal sections along the equator are also presented. Properties of the near-equatorial zonal currents, including transports, temperatures, and salinities, are estimated separately from the synoptic sections. The seasonal cycles of these quantities and their correlations with the Southern Oscillation Index are investigated. The work is distinguished from most existing literature in that direct estimates of zonal velocity are combined with contemporaneous temperature and salinity data, allowing trans-Pacific estimates of near-equatorial current transports and properties, including those of the northern branch of the South Equatorial Current, the New Guinea Coastal Undercurrent, and the Equatorial Undercurrent.
 
Article
Seasonal and interannual change in mesozooplankton community structure in the offshore Tsushima Current area of the Japan/East Sea was studied in relation to climatic events and temporal variability of the upper water column environment from 1991 to 1999. We observed a clear seasonal succession in zooplankton community structure from a cold-water copepod-dominated community in winter and spring to a gelatinous, carnivorous and warm-water copepod-dominated community in summer and autumn. The mean abundance (inds. m−3) of the spring community was 3–4-fold higher than that of the other season. The spring community structure varied considerably between years: the community characterized by the summer–autumn type zooplankton assemblage appeared in 1991–1993 and 1998, while the community characterized by high abundance of cold-water copepods appeared in the mid 1990s. Time series profiles of water density and nutrients showed the thickness of the surface warm Tsushima Current and the cold subsurface water increased and decreased, respectively, limiting nutrient supply to the surface water in 1992 and 1998. These results suggest that a thick, warm surface layer might reduce the reproductive success and survival ratio of the cold-water copepods both directly and indirectly, by hindering their upward migration to the surface where food is available, and by limiting phytoplankton growth due to nutrient depletion, respectively. A Monsoon Index (MOI) showed weaker winter wind stress in 1992 and 1998, which might have attenuated formation of the cold subsurface layer in the northern Japan/East Sea and been responsible for surface warming of the study area. Since 1992 and 1998 were El Niño years, this study revealed that ENSO related climatic variability on an interannual time scale considerably influenced the lower trophic level ecosystem in the Japan/East Sea.
 
Article
A synoptic, hydrographic data set comprising 32 full depth CTD casts and 2500 CTD/SeaSoar profiles to 500 m is used to describe the θ/S properties and circulation of Central Water east of the mid-Atlantic Ridge and between 39°N and 54°N. Eastward transport of 20 × 106 m3 s−1 in the North Atlantic Current turns entirely northwards to the west of 54°N, 20°W. This transport consists in the upper layers of Western North Atlantic Water freshened at temperatures below 10°C by mixing with SubArctic Intermediate Water. Northern and Southern branches of the North Atlantic Current are well defined and both turn northwards west of 20°W. A further 10 × 106 m3 s−1 of Eastern North Atlantic Water forms and recirculates anticyclonically to the west of Spain south of the North Atlantic Current and north of 40°N. Eastern North Atlantic Water is most weakly stratified east of 20°W and there is clear correlation between weakly stratified pycnostads and positive salinity anomalies relative to Western North Atlantic Water. Thus Eastern North Atlantic Water is a winter Mode Water in which strong winter cooling has increased the density and hence also the salinity anomaly at a given temperature. Near the southern entrance to the Rockall Trough there is evidence that salinities are also increased by Mediterranean Water influence. Circulation south of the North Atlantic Current is complex. There is no evidence for direct ventilation southwards across 40°N where water properties (θ/S, potential vorticity and CFC-113) and historical data all indicate westward ventilation east of 24°W, with weak southward ventilation occurring further west, in the vicinity of the Azores. The circulation pattern suggested is remarkably similar to that proposed by Helland-Hansen and Nansen in 1926 (The eastern North Atlantic, Geophysiske Publicajoner, 4, 1–76), with anticyclonic circulation of colder Eastern North Atlantic Water north of 40°N meeting warmer water from south of 40°N circulating cyclonically north of the Azores Current. The distribution of pycnostads and θ/S properties between 20°W and 35°W north of the Azores indicates alternate bands of Western and Eastern North Atlantic Water moving eastward and westward respectively, including evidence for westward motion immediately south of the Southern branch of the North Atlantic Current, possibly by westward propagation of anticyclonic eddies containing deep pycnostads.
 
Long-lived eddy genesis and propagation. (a) Number of newly generated eddies. (b) Propagation velocity vectors of both cyclonic and anticyclonic eddies. (c) Mean radius of long-lived eddies. (d) Zonal average of radii (left) and propagation speeds (right) of long-lived eddies (green lines), with red lines indicating the Rossby radius of deformation (left) and propagation speed of first baroclinic mode Rossby waves (right).  
Long-lived eddy frequency (left) and polarity (right). See text for definitions.  
Eddy statistics in the four major EBUS. Numbers in brackets in the fourth column denote the number of eddies generated each week.
Article
Eddy activity in the four major eastern boundary upwelling systems (EBUS) is investigated using 15 years of satellite altimetry data. Based on the analysis of more than 4000 long-lived eddy trajectories in every EBUS, we show that mesoscale structures are mainly generated along the continental coasts and south of the main archipelagos and propagate westward with velocities increasing toward the equator. These mesoscale eddies, having radii of 70–160 km, are then frequently observed along the coastal transition zones and frontal regions and some large oceanic areas are preferentially populated by cyclonic or anticyclonic eddies. Temporal variations of the number of newly-formed eddies and the associated eddy activity index, defined as the mean eddy energy density, are finally examined at seasonal and interannual scales. The strongest seasonal (interannual, respectively) variations are observed in the California (Benguela) upwelling systems. The proposed indices also exhibit contrasted long-term trends in each EBUS, which suggests that eddy activity might be sensitive to a warming climate.
 
Article
A 12-year hindcast of the physical state of the North Indian Ocean has been carried out for the period 1993–2004, using a data-assimilative, primitive equation, multi-level circulation model that assimilated altimetric sea surface height anomalies and weekly MCSST, and was driven by 6-h ECMWF winds. This period encompasses the anomalous wind events in 1994 and 1997–1998 that led to anomalous oceanic state including anomalous sea surface heights and sea surface temperatures, especially in the equatorial regions. Since the in situ database in the Indian Ocean is rather sparse, the hindcast provides an alternative means of examining the state of the ocean, including its interior, during these anomalous years, as well as normal years during this period. By comparison with observations available during this period, it is shown that the model possesses reasonable skill to be useful in the description of various events in the North Indian Ocean. In this paper, we examine the circulation and its variability over the 12 years of the hindcast. We discuss equatorial events, as well as events in the Bay of Bengal and the Arabian Sea, including the Somali Current system. The heat and mass fluxes are examined. Finally, the hindcast is repeated with QuikSCAT wind stress fields available from July 1999 onwards. Comparison of the 2000–2004 hindcasts forced by ECMWF winds and QS wind stresses shows that in the former, the currents and fluxes are underestimated by 20–30%, but the circulation patterns are roughly similar.
 
Article
An area some 120×70 km off the eastern coast of Oman (containing the UK JGOFS Arabesque station) was intensively studied over a 17 day period in August 1994, with the objective of determining the relationships between the biological populations, the oxygen minimum layer and the dynamic hydrography of the euphotic zone. The outer margin of the area was delimited by a rectangle of 15 full depth CTD casts and the hydrography was further defined by two Seasoar surveys within the box. Midwater trawls were used to sample the populations at three stations, oceanic, slope and shelf edge respectively. Day and night samples of macroplankton and micronekton were taken at each station to determine the extent of diel vertical migration and the effect of the hypoxic region on these migrations. Concurrent ADCP data were used to follow the migrations and spatial changes in real time. Despite the limited area studied the patterns of upwelling and their temporal and spatial changes were complex. Coastal upwelling was observed directly only at the southwestern edge of the area during both Seasoar surveys. Persian Gulf Water was a consistent but spatially discrete feature of the region at depths of 200–300 m. Arabian Sea Surface Water was present at the eastern margin of the first survey. Between these two water masses was a large area with small horizontal gradients and variable silicate and chlorophyll levels. Satellite data suggest that this water may have been advected as a filament from a more northerly coastal source. Very marked changes took place in the hydrography and in the phytoplankton composition and abundance at the reference station at 19°N 59°E over the 16 day period between visits. The highest biomass of plankton and micronekton (expressed as wet volume or as carbon) occurred in the upper 100 m, closely correlated with the relatively high oxygen levels at these depths. Gelatinous animals predominated in these layers, with additional swarms of swimming crabs. Quite large populations of myctophid and photichthyid fishes and of decapod crustaceans were present below the oxycline by day. Most of these migrated into the surface layers at night, leaving minimal biomass behind, with the result that the ADCP backscatter data from beneath the oxycline at night were often below instrument resolution. Daytime ADCP data, on the other hand, showed multiple fine layering, some of which correlated with salinity differences. At the base of the oxygen minimum layer there was a large increase in biomass, marking the presence of a more typical bathypelagic fauna.
 
Article
Phytoplankton communities, production rates and chlorophyll levels, together with zooplankton communities and biomass, were studied in relation to the hydrological properties in the euphotic zone (upper 100 m) in the Cretan Sea and the Straits of the Cretan Arc. The data were collected during four seasonal cruises undertaken from March 1994 to January 1995.The area studied is characterised by low nutrient concentrations, low 14C fixation rates, and impoverished phytoplankton and zooplankton standing stocks. Seasonal fluctuations in phytoplankton densities, chlorophyll standing stock and phytoplankton production are significant; maxima occur in spring and winter and minima in summer and autumn. Zooplankton also shows a clear seasonal pattern, with highest abundances occurring in autumn–winter, and smallest populations in spring–summer. During summer and early autumn, the phytoplankton distribution is determined by the vertical structure of the water column.Concentrations of all nutrients are very low in the surface waters, but increase at the deep chlorophyll maximum (DCM) layer, which ranges in depth from about 75–100 m. Chlorophyll-a concentrations in the DCM vary from 0.22–0.49 mg m−3, whilst the surface values range from 0.03–0.06 mg m−3. Maxima of phytoplankton, in terms of cell populations, are also encountered at average depths of 50–75 m, and do not always coincide with chlorophyll maxima. Primary production peaks usually occur within the upper layers of the euphotic zone.There is a seasonal succession of phytoplankton and zooplankton species. Diatoms and ‘others’ (comprising mainly cryptophytes and rhodophytes) dominate in winter and spring and are replaced by dinoflagellates in summer and coccolithophores in autumn. Copepods always dominate the mesozooplankton assemblages, contributing approximately 70% of total mesozooplankton abundance, and chaetognaths are the second most abundant group.
 
Article
Four seasonal oceanographic cruises were carried out in the Eastern Mediterranean Sea, within the framework of the CEC/MAST-MTP Project PELAGOS, during 1994–1995. The surveys covered the South Aegean Sea and the adjacent open sea regions (southeastern Ionian, northwestern Levantine). Analysis of CTD data revealed that a multiscaled circulation pattern prevails in the area. It differs from the circulations detected during the 1986–87, thus indicating interannual variability. Cyclonic and anticyclonic gyres and eddies are interconnected by currents and jets variable in space and time. Most of the features are persistent, others seem transitional or recurrent. The hydrological structure is also complex and apart from the upper layer does not present basinwide any significant seasonality. Dynamical and hydrological regimes are variable in the upper and intermediate layers at the Straits of the Cretan Arc, while the deep regime seems rather constant. Topographic control is evident on the flows through the straits. The new very dense deep water mass, namely the Cretan Deep Water (CDW) and a well-defined intermediate layer of minimum temperature and salinity, the so-called Transition Mediterranean Water (TMW), consists the new important structural elements of the South Aegean Sea. The CDW outflows towards the deep and bottom layers of the Eastern Mediterranean, thus considerably contributing to the formation of the new, denser Deep and Bottom Water of the Eastern Mediterranean, which sinks and displaces the Eastern Mediterranean Deep Water of Adriatic origin in the adjacent sea regions outside the Aegean Sea.
 
Article
From late 1995 through early 2001, three major interannual climate events occurred in the tropical Pacific; the 1995–97 La Niña (LN), 1997–98 El Niño (EN), and 1998–2001 LN. We analyze atmospheric and upper oceanic anomalies in the northeast Pacific (NEP) during these events, and compare them to anomalies both elsewhere in the north and tropical Pacific, and to typical EN and LN anomaly patterns. The atmospheric and oceanic anomalies varied strongly on intraseasonal and interannual scales. During the 1995–97 LN and 1997–98 EN, the Northeast Pacific was dominated by negative SLP and cyclonic wind anomalies, and by upper ocean temperature and sea surface height (SSH) anomalies. The latter were positive along the North American west coast and in the NEP thermal anomaly pool (between Hawaii, Vancouver Island, and Baja California), and negative in the central north Pacific. This atmospheric/oceanic anomaly pattern is typical of EN. An eastward shift in the atmospheric teleconnection from east Asia created EN-like anomalies in the NEP during the 1995–97 LN, well before the 1997–98 EN had begun. The persistence of negative sea-level pressure (SLP) and cyclonic wind anomalies in the NEP during the 1997–98 EN intensified pre-existing upper oceanic anomalies. Atmospheric anomalies were shifted eastward during late 1996–early 1998, leading to a similar onshore shift of oceanic anomalies. This produced exceptionally strong positive upper ocean temperature and SSH anomalies along the west coast during the 1997–98 EN, and explains the unusual coastal occurrences of several species of large pelagic warm-water fishes. The growth and eastward shift of these pre-existing anomalies does not appear to have been linked to tropical Pacific EN anomalies until late 1997, when a clear atmospheric teleconnection between the two regions developed. Prior to this, remote atmospheric impacts on the NEP were primarily from east Asia. As the 1998–2001 LN developed, NEP anomalies began reversing toward the typical LN pattern. This led to predominantly negative SLP and cyclonic wind anomalies in the NEP, and upper ocean temperature and SSH anomalies that were mainly negative along the west coast and positive in the central north Pacific. The persistence of these anomalies into mid-2001, and a number of concurrent biological changes in the NEP, suggest that a decadal climate shift may have occurred in late 1998.During 1995–2001, NEP oceanic anomalies tracked the overlying atmospheric anomalies, as indicated by the maintenance of a characteristic spatial relationship between these anomalies. In particular, wind stress curl and SSH anomalies in the NEP maintained an inverse relationship that strengthened and shifted eastward toward the west coast during late 1996–early 1998. This consistent relationship indicates that anomalous Ekman transport driven by regional atmospheric forcing was an important contributor to temperature and SSH anomalies in the NEP and CCS during the 1997–98 EN. Other studies have shown that coastal propagations originating from the tropical Pacific also may have contributed to coastal NEP anomalies during this EN. Our results indicate that at least some of this coastal anomaly signal may have been generated by regional atmospheric forcing within the NEP.
 
Article
The Northern Humboldt Current Ecosystem is one of the most productive in the world in terms of fish production. Its location near to the equator permits strong upwelling under relatively low winds, thus creating optimal conditions for the development of plankton communities. These communities ultimately support abundant populations of grazing fish such as the Peruvian anchoveta, Engraulis ringens. The ecosystem is also subject to strong inter-annual environmental variability associated with the El Niño Southern Oscillation (ENSO), which has major effects on nutrient structure, primary production, and higher trophic levels. Here our objective is to model the contributions of several external drivers (i.e. reconstructed phytoplankton changes, fish immigration, and fishing rate) and internal control mechanisms (i.e. predator-prey) to ecosystem dynamics over an ENSO cycle. Steady-state models and time-series data from the Instituto del Mar del Perú (IMARPE) from 1995 to 2004 provide the base data for simulations conducted with the program Ecopath with Ecosim. In simulations all three external drivers contribute to ecosystem dynamics. Changes in phytoplankton quantity and composition (i.e. contribution of diatoms and dino- and silicoflagellates), as affected by upwelling intensity, were important in dynamics of the El Niño of 1997–98 and the subsequent 3 years. The expansion and immigration of mesopelagic fish populations during El Niño was important for dynamics in following years. Fishing rate changes were the most important of the three external drivers tested, helping to explain observed dynamics throughout the modeled period, and particularly during the post-El Niño period. Internal control settings show a mix of predator–prey control settings; however a “wasp-waist” control of the ecosystem by small pelagic fish is not supported.
 
Article
We collected mesozooplankton samples in the upper 100 m in spring or early summer each year between 1995 and 2000 along a section from Hamilton Bank (Labrador) to Cape Desolation (Greenland), and along additional sections in spring 1997 and early summer 1995. The North Atlantic waters of the central basin were characterised by the presence of the copepods Calanus finmarchicus, Euchaeta norvegica and Scolecithrocella minor and euphausiids. Calanus glacialis, Calanus hyperboreus and Pseudocalanus spp. were associated with the Arctic waters over the shelves. Amongst the other enumerated groups larvaceans were concentrated over the shelves and around the margins. Amphipods, pteropods and the copepods Oithona spp. and Oncaea spp. showed no definable relationships with water masses or bathymetry, while the diel migrant ostracods and chaetognaths were confined to deep water. Metrida longa, also a strong diel migrant, and Microcalanus spp., a mainly deep water species and possible diel migrant, were both sometimes quite abundant on the shelves as well as in the central basin, consistent with their likely Arctic origins.
 
Article
In summer 1996, a tracer release experiment using sulphur hexafluoride (SF6) was launched in the intermediate-depth waters of the central Greenland Sea (GS), to study the mixing and ventilation processes in the region and its role in the northern limb of the Atlantic overturning circulation. Here we describe the hydrographic context of the experiment, the methods adopted and the results from the monitoring of the horizontal tracer spread for the 1996–2002 period documented by ∼10 shipboard surveys. The tracer marked “Greenland Sea Arctic Intermediate Water” (GSAIW). This was redistributed in the gyre by variable winter convection penetrating only to mid-depths, reaching at most 1800 m depth during the strongest event observed in 2002.
 
Article
Repeated hydrographic casts, mooring time series and satellite sea surface temperature collected during the CANALES experiment (1996–98) are used to describe the thermohaline circulation in the Balearic Channels (western Mediterranean) and to analyze its variability. Mass transports are estimated by inverse calculations. The role played by each channel in the meridional water exchange is clarified: the Ibiza Channel funnels southward cool, saline, northern waters whereas the Mallorca Channel appears as the preferred route for the northward progression of warm, fresh, southern waters. A neat interannual trend is revealed by the continuous decrease of the amount of Western Mediterranean Intermediate Waters (WIW) brought by the Northern Current, reflecting the increase in temperature of the winter mixed layer in the northern Mediterranean that occurred each year between 1996 and 1998. A clear seasonal signal was also seen in the transport of the Northern Current which decreased from 1 to 1.4 Sv in winter to < 0.5 Sv in summer. The current intensified again in fall. A number of mesoscale eddies, from 20 to 70 km in size, most of them anticyclonic vortex eddies were brought by the unstable Northern Current, these eddies strongly perturbed the water exchange in the Ibiza Channel forcing retroflections of northern waters back to the north-east into the Balearic Current. These eddies either stayed stalled for several months in the Gulf of Valencia to the north of the channel, or were slowly funnelled southward through the channel narrows. A decreasing trend was observed in the mesoscale activity of the Northern Current between 1996 and 1998. Conversely, large, anticyclonic eddies, 150-km diameter, progressively invaded the Algerian Basin to the south of the channels in 1997–98 and forcing northward inflows (up to 0.75 Sv) of fresh and warm waters of Atlantic origin (AW) into the Mallorca Channel. The marked interannual differences observed in both northern and southern eddy activity may be linked to the interannual variability of the large scale thermohaline circulation.
 
Top-cited authors
Nathan J. Mantua
  • National Oceanic and Atmospheric Administration
Steven Hare
  • The Pacific Community
Paul Wassmann
  • UiT - The Arctic University of Norway
Gregory Beaugrand
  • French National Centre for Scientific Research
E.D. Barton
  • Spanish National Research Council