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Comment on ‘Local interactions between the sea and the air at monthly and annual time scales’
Quarterly Journal of the Royal Meteorological Society
Abstract
A statistical analysis of wind, air, dew-point and sea-temperature records from all nine weather ships in the North Atlantic shows local variations between years which are highly significant when compared with variations within months. The fluctuations show a consistent pattern with a scale of more than 500 miles in the atmosphere and a persistence over several months. The horizontal extent of sea-surface temperature anomalies appears to be somewhat smaller, but they tend to last longer than air-temperature anomalies.
Short-period variations in the flux of latent and sensible heat are due predominantly to atmospheric variations, particularly in winter. The effect of sea-surface temperature anomalies is somewhat greater in summer, though it becomes significant only on the annual time scale.
... Comparisons of sampling and classical estimates of fluxes are given in many articles. Most consider temporal averaging (Kraus and Morrison 1966; Robinson 1966; Kondo 1972; Fissel et al. 1977; Esbensen and Reynolds 1981; Bortkovskiy 1983; Larin and Panin 1985; Hanawa and Toba 1987; Gulev and Ukrainsky 1989; Ledvina et al. 1993; Gulev 1994; Josey et al. 1995; Staneva et al. 1995; Zhang 1995). Some estimates indicate that higher fluxes are calculated with the sampling method. ...
Differences between `classical' and `sampling' estimates of mean climatological heat fluxes and their seasonal and interannual variability are considered on the basis of individual marine observations from the Comprehensive Ocean-Atmosphere Data Set. Calculations of fluxes were done for intramonthly averaging and for 1°-5° spatial averaging. Sampling estimates give in general 10% to 60% higher values of fluxes than do classical estimates. Spatial averaging has a larger effect than temporal averaging in the Tropics and subtropics, and temporal averaging is more effective than spatial averaging in midlatitudes. The largest absolute differences between sampling and classical estimates of fluxes are observed in middle latitudes, where they are 15 to 20 W m2 for sensible heat flux and 50 to 70 W m2 for latent heat flux. Differences between sampling and classical estimates can change the annual cycle of sea-air fluxes. There is a secular tendency of increasing `sampling- to-classical' ratios of 1% to 5% decade1 over the North Atlantic. Relationships between sampling-to-classical ratios and parameters of the sea-air interface, the number of observations, and the spatial arrangement of samples are considered. Climatologically significant differences between sampling and classical estimates are analyzed in terms of the contribution from different covariances between individual variables. The influence of different parameterizations of the transfer coefficients on sampling minus classical differences is considered. Parameterizations that indicate growing transfer coefficients with wind speed give the larger sampling minus classical differences in comparison with those based on either constant or decreasing with wind coefficients. Nevertheless, over the North Atlantic midlatitudes, all parameterizations indicate significant sampling minus classical differences of about several tens of watts per square meter. The importance of differences between sampling and classical estimates for the evaluation of meridional heat transport shows that differences between sampling and classical estimates can lead to 0.5-1-PW differences in meridional heat transport estimates.
... From a different point of view, Namias (1959) hypothesized that the mid-ocean blocking ridge could be a fundamental unstable mode of the coupled ocean/ atmosphere interacting system which derives its energy from thermal energy anomalies contained in the upper ocean. However, it seems clear from the work of Clark (1967) and Kraus and Morrison (1966) that the fluctuating ocean temperature does not necessarily play an active role in thermal energy exchange on time scales less than a year. Furthermore, mid-ocean blocking ridge development has been duplicated in numerical simulations of the January climatic state (e.g., Mintz, 1968; Rountree, 1972 ) by treating the ocean as a passive infinite heat source that gives up thermal energy to the atmosphere on demand. ...
Monthly mean atmospheric data taken over the North Pacific during the period 1950 - 70 are used to investigate blocking ridge activity over the central ocean. The blocking ridge is observed to be a finite-amplitude, quasi-stationary long wave, most often centered over the North Pacific at 170W, superimposed upon the quasi-zonal mid-latitude westerlies. The development and behavior of this ridge are analyzed. The data suggest that this regional blocking activity owes its existence to the marine environment. To test this idea, appeal is made to some theoretical work by G. J. Haltiner. In addition to good scale agreement with observations, Haltiner's theory is able to explain both the seasonal and year-to-year variability in blocking activity in terms of corresponding fluctuations in sensible heat transfer and the strength of the mean westerly winds.
... In contrast to the wind spectrum, which is highest at wavenumber 1, the maximum variance of the simulated SST occurs at wavenumbers 2 and 3. This corresponds roughly to the observed scale of the dominant SST anomaly patterns, which are typically several thousand kilometers in diameter, and is also consistent with the observation that the dominant scales of the SST anomalies appear to be somewhat smaller than the scales of air temperature or sea-level pressure anomalies (e.g. Kraus & Morrison, 1966; Davis, 1976). Although our ocean-atmosphere model is admittedly highly simplified, the main features of the SST spectral response to short time scale weather forcing appear to be reproduced reasonably well in the numerical experiments. ...
The concept of stochastic climate models developed in Part I of this series (Hasselmann, 1976) is applied to the investigation of the low frequency variability of the upper ocean. It is shown that large-scale, long-time sea surface temperature (SST) anomalies may be explained naturally as the response of the oceanic surface layers to short-time-scale atmospheric forcing. The white-noise spectrum of the atmospheric input produces a red response spectrum, with most of the variance concentrated in very long periods. Without stabilizing negative feedback, the oceanic response would be nonstationary, the total SST variance growing indefinitely with time. With negative feedback, the response is asymptotically stationary. These effects are illustrated through numerical experiments with a very simple ocean-atmosphere model. The model reproduces the principal features and orders of magnitude of the observed SST anomalies in mid-latitudes. Independent support of the stochastic forcing model is provided by direct comparisons of observed sensible and latent heat flux spectra with SST anomaly spectra, and also by the structure of the cross correlation functions of atmospheric surface pressure and SST anomaly patterns. The numerical model is further used to simulate anomalies in the near-surface thermocline through Ekman pumping driven by the curl of the wind stress. The results suggest that short-time-scale atmospheric forcing should be regarded as a possible candidate for the origin of large-scale, low-period variability in the seasonal thermocline.
Winter conditions impose dramatic constraints on temperate, boreal, and polar ecosystems, and shape the abiotic and biotic interactions underpinning these systems. At high latitudes, winter can last longer than the growing season and may have a disproportionately large impact on organisms and ecosystems. Even so, our understanding of the ecological implications of winter is often lacking. Indeed, even what exactly defines winter is currently unclear, and boundaries that delineate this season are blurred across marine, freshwater, and terrestrial realms and fields of biology. Here, we discuss the complexity of defining winter, and highlight the importance of maintaining the capacity to test hypotheses across seasons, realms, and domains of life. We then outline questions drawn from diverse fields of research that address current gaps in our understanding of winter ecology and how winter influences multiple levels of biological organization, from individuals to ecosystems. Finally, we highlight the potential consequences of changes to both the length and severity of winter due to climate change, and discuss the role winter may play in mediating ecosystem function in the future.
The turbulent surface fluxes of heat and momentum from a high-resolution atmospheric model are presented and assessed. The errors in computing the fluxes from monthly mean atmospheric model data are calculated, and the consequences for coupled ocean models are discussed.
After the first — mostly short-lived — experiments to install a network of climatological stations during the 17th century (Academy of Florence 1652, Royal Society London 1662) and, at an international base, during the 18th century (Societas Meteorologica Palatina, Mannheim 1781), governmental station networks were founded around 1850 in many countries, but only after 1900 in many others and in some (Yemen, Ethiopia, Tibet, Nepal) even around 1940 or 1950 (Antarctica 1957). Their observations should be representative for larger areas — but this is only true for atmospheric pressure, while precipitation (P) data, temperature extremes and surface winds are too often locally distorted and represent points instead of areas. In large plains, as well as at the ocean, the wind as a vector is representative and informative. Some of the most important quantities — e.g. actual evaporation (E) or evapotranspiration, net radiation — are quite difficult to measure with sufficient accuracy; they are usually derived from empirical formulae. Representative values are especially needed for the hydrological cycle, e.g. lake levels (specially in areas with internal drainage) and runoff (which represents P-E including storage terms).
Sea salt aerosol (SSA) exerts a major influence over a broad reach of
geophysics. It is important to the physics and chemistry of the marine
atmosphere and to marine geochemistry and biogeochemistry generally. It
affects visibility, remote sensing, atmospheric chemistry, and air
quality. Sea salt aerosol particles interact with other atmospheric
gaseous and aerosol constituents by acting as sinks for condensable
gases and suppressing new particle formation, thus influencing the size
distribution of these other aerosols and more broadly influencing the
geochemical cycles of substances with which they interact. As the key
aerosol constituent over much of Earth's surface at present, and all the
more so in pre-industrial times, SSA is central to description of
Earth's aerosol burden.
The role of advection, evaporation, and upwelling in determining the sea-surface temperature of the North Atlantic and the North Sea is studied by establishing time series of year-to-year variability for the period 1948 to 1974 and by comparing these with wind strength indices derived from the pressure field. For the open ocean, secular temperature changes appear to be primarily advected and determined by variations in the North Atlantic Current. An increase in the flow of the Gulf Stream results in warmer waters in the vicinity of the Sargasso Sea and cooling of the surface waters at higher latitudes. Over the European shelf and in the North Sea, on the other hand, the surface temperatures appear to be determined to a large extent by direct heat exchange with the atmosphere, mediated primarily by the meridional component of the surface winds.
Gridded fields of TOPEX/Poseidon sea level height (SLH) from 1993 to 1998 and National Centers for Environmental Prediction sea surface temperature (SST) and meridional surface wind (MSW) anomalies from 1970 to 1998 are used to examine coupled Rossby waves in the Indian Ocean from 10°S to 30°S. Time-longitude diagrams of monthly SLH, SST, and MSW anomalies yield significant peak spectral energy density in propagation wavenumber-frequency spectra for westward propagating waves of >2 yr period and >4000 km wavelength. Subsequent low-pass filtering of SLH, SST, and MSW anomalies for these interannual timescales >2 yr finds them propagating westward over the Indian Ocean in fixed phase with one another at speeds significantly less (0.04-0.07 m s-1) than first-mode baroclinic Rossby waves taking 3 to 4 years to cross the basin. These coupled Rossby waves display weak beta refraction patterns in all three variables. Significant squared coherence between interannual SLH and SST (SST and MSW) anomalies yield phase differences ranging from 0° to 45° (150° to 180°). Warm SST anomalies overlie high SLH anomalies, suggesting that pycnocline depth anomalies associated with the Rossby waves modify vertical mixing processes to maintain SST anomalies against dissipation. Warm SST anomalies are associated with outgoing latent heat flux anomalies in the eastern and central ocean, indicating that the ocean is capable of forcing the overlying atmosphere. Poleward MSW anomalies occur directly over warm SST anomalies, suggesting that anomalous planetary vorticity advection balances anomalous low-level convergence in response to SST-induced midtroposphere convection. These inferred thermodynamic processes allow a simple analytical model of coupled Rossby waves to be constructed that yields much slower westward phase speeds than for free Rossby waves as observed. Maintenance of wave amplitude against dissipation occurs for coupled waves that travel westward and poleward, as observed.
The effect of the sensible heating by the oceans on atmospheric circulation is dealt with in terms of the atmospheric available potential energy (APE) generated by such heating. By using the exact expression for the generation of atmospheric APE one can assess what fraction of this sensible heating is actually effective in producing atmospheric APE. A measure of this efficiency (termed the efficiency factor) was studied. The study showed considerable time and spatial variations in the field of the efficiency factor. It was also shown that many of the oceanic regions that are known to be strong sources of sensible heating produce but little atmospheric APE. Low latitude regions (the Caribbean and Gulf of Mexico especially) are associated with higher efficiencies and hence are most effective in producing available potential energy.
By computing two extra terms, the contributions of eight selected oceanic regions to the global APE were also studied for the three Decembers of 1964, 1965, 1966. The overall mean December contribution of these regions to the global APE turned out to be significant in 1964 but dropped considerably in 1965 and 1966.
The spatial and time variations of the APE contribution field were studied by the empirical orthogonal functions technique.
Autumn-winter temperature and precipitation records at 34 stations over
New Zealand from 1982 to 1995 are found by empirical orthogonal function
(EOF) analysis to fluctuate together with 3-6-yr quasi periodicity
similar to that associated with the Antarctic Circumpolar Wave (ACW),
which propagates slowly eastward past New Zealand in its global traverse
around the Southern Ocean. By allowing these EOF time sequences to
represent New Zealand temperature and precipitation indices, both the
positive temperature index related to warm sea surface temperature (SST)
anomalies around New Zealand and the positive precipitation index
related to warm (cool) SST anomalies north and east (south and west) of
New Zealand are found. These warm (cool) SST anomalies are associated
with poleward (equatorward) meridional surface wind (MSW) anomalies, the
same as observed in association with the ACW. When warm (cool) SST and
poleward (equatorward) MSW anomalies are located north (south) of New
Zealand, then anomalous low-level wind convergence occurs over New
Zealand, and when they are located east (west) of New Zealand, then
anomalous cyclonicity occurs over New Zealand, both during years of
anomalously high autumn-winter precipitation over New Zealand. Regular
eastward propagation of the ACW past New Zealand suggests that covarying
SST and MSW anomalies (and New Zealand autumn-winter temperature and
precipitation) can be predicted 1-2 yr into the future. The authors test
for this by utilizing the eastward propagation of the ACW contained in
the dominant extended EOF mode of SST anomalies upstream from New
Zealand to predict SST indices in the western South Pacific that are
linked statistically to New Zealand temperature and precipitation
indices. At 0-yr lead, this statistical climate prediction system
nowcasts the observed sign of New Zealand temperature (precipitation)
indices 12 (12) years out of the 14-yr record, explaining 50% (62%) of
the interannual variance for each index. At 1-yr lead, it hindcasts the
observed sign of New Zealand temperature (precipitation) indices 12 (13)
years out the 14-yr record, explaining 24% (74%) of the interannual
variance. At 2-yr lead, hindcasting is insignificant. This hindcast
skill at 1-yr lead suggests that prediction of interannual climate
variability over New Zealand may depend more upon predicting the
amplitude and phase of the ACW than upon predicting it for tropical
ENSO.
Daily fields obtained from a 17-yr atmospheric GCM simulation are used to study the surface sensible and latent heat flux variability and its relationship to the sea level pressure (SLP) field. The fluxes are analyzed over the North Pacific and Atlantic Oceans during winter. The leading mode of interannual SLP variability consists of a single center associated with the Aleutian low in the Pacific, and a dipole pattern associated with the Icelandic low and Azores high in the Atlantic. The surface flux anomalies are organized by the low-level atmospheric circulation associated with these modes in agreement with previous observational studies.The surface flux variability on all of the timescales examined, including intraseasonal, interannual, 3-10 day, and 10-30 day, is maximized along the north and west edges of both oceans and between Japan and the date line at 35°N in the Pacific. The intraseasonal variability is approximately 3-5 times larger than the interannual variability, with more than half of the total surface flux variability occuring on timescales of less than 1 month. Surface flux variability in the 3-10-day band is clearly associated with midlatitude synoptic storms. Composites indicate upward (downward) flux anomalies that exceed |30 W m2| occur to the west (east) of storms, which move eastward across the oceans at 10°-15° per day. The SLP and surface flux anomalies are also strong and coherent in the 10-30-day band but are located farther north, are broader in scale, and propagate 3-4 times more slowly eastward than the synoptic disturbances.The sensible and latent heat flux are proportional to the wind speed multiplied by the air-sea temperature and humidity difference, respectively. The anomalous wind speed has the greatest influence on surface flux anomalies in the subtropics and western Pacific, while the air temperature and moisture anomalies have the greatest impact in the northeast Pacific and north of 40°N in the Atlantic. The covariance between the wind speed and the air temperature or humidity anomalies, while generally small, is nonnegligible on synoptic timescales.
Starting from the occurrence of a zonal Walker circulation supplementing the meridional Hadley cell, the concept of ultralong waves with diagonal upper troughs extending far into the Tropics is stressed. The model of equatorial upwelling with air‐sea coupling is critically discussed and a modification proposed. An intensification of the subtropical jet at the peak of the 1972 El Niño (as suggested by Rowntree's model calculations) has been verified.
Empirical studies deal with the large interannual and interdecadal variations of the energy fluxes at the air‐sea interface; time variations of the oceanic evaporation and sensible heat flux are greater than expected. Using spectrum analysis, the occurrence of a 5‐year periodicity in equatorial rainfall of the Pacific and Indonesia is demonstrated, together with a marked phase shift along the NE coast of New Guinea.
A depth-integrated heat energy conservation equation is used with input data from three case studies of dissimilar meteorological forcing to provide values for the month-to-month change of mean monthly sea surface temperature anomalies. Evaluation of the model is made by comparison of predicted anomaly changes with those observed. The separate contributions of anomalous surface cooling and anomalous advection to the sea temperature anomaly change, give agreement of varying closeness: in all three studies best results are achieved by a combination of these processes; in each case inclusion of a horizontal diffusivity term produces deteriorations over sea temperature anomaly changes based on a combination of anomalous heat loss and advection terms. In regions of significant anomalous windstress curl, neither sea temperature anomaly changes produced in part by advection due to an alternative meridionally directed flow, nor changes otherwise consequent on the associated effects of induced vertical motion, are observed to be consistent with actual changes.
Fluctuations in heat energy transfer between ocean and atmosphere were studied by means of spectral analysis of time series. The hydrometeorological parameters which contribute to the interface heat transfer were analyzed similarly. Data consisted of three years of 3-hourly surface observations at North Atlantic stations Charlie and Delta. Frequency domain results were emphasized, but time domain statistics were also examined.
Time series of different length and timestep were analyzed. Time sequence of 4-month or 2-month autospectra gave the spectral field of a variable during the three years. Spectral fields of energy transfer were compared to spectral fields of the contributing variables.
Spectral properties of heat transfer in the time range studied (6 hours to 20 days) are mainly governed by fluctuations of corresponding vertical gradient. However, some secondary maxima in the transfer spectral fields are chiefly induced by wind fluctuations. The bulk of the heat transfer occurs in intermittent pulsations.
Anomalies of TOPEX/Poseidon sea level height (SLH), NCEP sea surface temperature (SST), and ECMWF meridional surface wind (MSW) and derived wind stress curl (WSC) are mapped biweekly over the global ocean from 60°S to 60°N for two years from 1993 to 1994. These anomalies (i.e., differences from the average over the two years) allow one cycle of biennial climate change to be examined, with the paucity of temporal degrees of freedom mitigated by greater numbers of spatial degrees of freedom. Time?longitude diagrams of SLH anomalies reveal biennial Rossby waves in the Pacific ocean with westward phase speeds in the extratropics (Tropics) faster (slower) than expected of free biennial Rossby waves. Complex empirical orthogonal function analysis of SLH, SST, MSW, and WSC anomalies finds the beta-refraction pattern of these biennial Rossby waves in all four variables over most of the eastern-central Pacific Ocean, suggesting that oceanic Rossby waves there are coupled with the overlying atmosphere. The spatial phase relationship between SLH and SST anomaly patterns is consistent with an upper-ocean heat budget where the anomalous meridional heat advection by geostrophic flows associated with biennial Rossby waves is balanced by anomalous heat loss to the atmosphere. The spatial phase relationship between SST and MSW anomaly patterns is consistent with lower-tropospheric heat and vorticity budgets, where MSW anomalies arise from a balance between planetary vorticity advection and SST-induced low-level convergence. Using these basic heat and vorticity balances for upper ocean and lower troposphere, an analytic coupled model is constructed where the subsequent feedback by WSC anomalies upon the free biennial Rossby waves (via Ekman pumping) is found to be positive, causing wave amplitudes to grow or be maintained against dissipation, and producing westward phase speeds faster (slower) than expected of the free Rossby wave speeds in the extratropics (Tropics), as observed.
In deriving the surface latent heat flux with the bulk formula for the thermal forcing of some ocean circulation models, two approximations are commonly made to bypass the use of atmospheric humidity in the formula. The first assumes a constant relative humidity, and the second supposes that the sea-air humidity difference varies linearly with the saturation humidity at sea surface temperature. Using climatological fields derived from the Marine Deck and long time series from ocean weather stations, the errors introduced by these two assumptions are examined. It is shown that the errors reach above 100 W/sq m over western boundary currents and 50 W/sq m over the tropical ocean. The two approximations also introduce erroneous seasonal and spatial variabilities with magnitudes over 50 percent of the observed variabilities.
There has been a recent and encouraging re-emphasis on investigations which involve some phase of the terrestrial heat budget, and the steadily accumulating number of investigations on the subject has brought proof that many meteorological and climatological problems can be wholly or partially solved through a consideration of energy transformation processes. In nearly all cases the purpose of the inquiries has been to ascertain the order of magnitude of one or more of the following components of the heat budget:
The relation between the vertical distributions of mean temperature in buoyant convection over a horizontal surface as observed in the atmosphere and in the laboratory is examined. Some measurements by Webb, obtained in conditions of strong instability, are analysed and shown to be consistent with three distinct regions of flow, distinguished by their heat‐transfer characteristics. They are: a region of forced convection with temperature gradient inversely proportional to height (negative Richardson number less than 0·02);
a region of mixed (or free) convection with temperature gradient inversely proportional to the 4/3‐power of the height (negative Richardson number between 0·03 and 1·0) and
a region of natural convection with temperature gradient inversely proportional to the square of the height (negative Richardson number greater than 1·0).
Reasons for the existence and properties of the last two regions are discussed briefly.