[Show abstract][Hide abstract] ABSTRACT: The temporal variability of volume transport from the North Pacific Ocean to the East China Sea (ECS) through Kerama Gap (between Okinawa Island and Miyakojima Island - a part of Ryukyu Islands Arc) is investigated using a 20-year global HYbrid Coordinate Ocean Model (HYCOM) reanalysis with the Navy Coupled Ocean Data Assimilation from 1993 to 2012. The HYCOM mean transport is 2.1 Sv (positive into the ECS, 1 Sv = 106 m3/s) from June 2009 to June 2011, in good agreement with the observed 2.0 Sv transport during the same period. This is similar to the 20-year mean Kerama Gap transport of 1.95 ± 4.0 Sv. The 20-year monthly mean volume transport (transport seasonal cycle) is maximum in October (3.0 Sv) and minimum in November (0.5 Sv). The annual variation component (345-400 days), mesoscale eddy component (70-345 days), and Kuroshio meander component (< 70 days) are separated to determine their contributions to the transport seasonal cycle. The annual variation component has a close relation with the local wind field and increases (decreases) transport into the ECS through Kerama Gap in summer (winter). Most of the variations in the transport seasonal cycle come from the mesoscale eddy component. The impinging mesoscale eddies increase the transport into the ECS during January, February, May, and October, and decrease it in March, April, November, and December, but have little effect in summer (June-September). The Kuroshio meander components cause smaller transport variations in summer than in winter.
[Show abstract][Hide abstract] ABSTRACT: Tides in global HYCOM have been assessed and validated in the global ocean but this assessment has concentrated mostly on deeper waters and not on the performance of the tides in shelf seas. Accurate reproduction of shelf tides requires both accurate tides at the shelf edge and a good representation of complex shallow water tidal processes, including topography, influencing the propagation and superposition of tidal waves within the shallow shelf seas. We provide a first order assessment of shelf sea tides in global HYCOM by comparing sea surface tidal heights in shelf seas to the accurate altimeter-constrained models TPXO and GOT. We also compare the sea surface temperatures and the position of mixing fronts on the North West European shelf in HYCOM with those computed from the International Council for the Exploration of the Sea (ICES) database.
[Show abstract][Hide abstract] ABSTRACT: High-resolution global ocean models forced by atmospheric fields and tides are beginning to display realistic internal gravity wave spectra, especially as model resolution increases. This paper examines internal waves in global simulations with 0.08° and 0.04° (~8 and 4 km) horizontal resolutions, respectively. Frequency spectra of internal wave horizontal kinetic energy in the North Pacific lie closer to observations in the 0.04° simulation than in the 0.08° simulation. The horizontal wave number and frequency (K-ω) kinetic energy spectra contain peaks in the semidiurnal tidal band and near-inertial band, along with a broadband frequency continuum aligned along the linear dispersion relations of low-vertical-mode internal waves. Spectral kinetic energy transfers describe the rate at which nonlinear mechanisms remove or supply kinetic energy in specific K-ω ranges. Energy is transferred out of low-mode inertial and semidiurnal internal waves into a broad continuum of higher-frequency and higher-wave number internal waves.
No preview · Article · Apr 2015 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: The US Navy's operational global ocean nowcast/forecast system is presently comprised of the 0.08° HYbrid Coordinate Ocean Model (HYCOM) and the Navy Coupled Ocean Data Assimilation (NCODA). Its high horizontal resolution and adaptive vertical coordinate system make it capable of producing nowcasts (current state) and forecasts of oceanic "weather," which includes three-dimensional ocean temperature, salinity, and current structure; surface mixed layer depth; and the location of mesoscale features such as eddies, meandering currents, and fronts. It runs daily at the Naval Oceanographic Office and provides seven-day forecasts that support fleet operations, provide boundary conditions to higher resolution regional models, and are available to the community. Using a data-assimilative hindcast and series of 14-day forecasts for 2012, the system is shown to have forecast skill of the oceanic mesoscale out to about 10 days for the Gulf Stream region and to 14+ days for the global ocean and other selected subregions. Forecast skill is sensitive to the type of atmospheric forcing (i.e., operational vs. analysis quality). Subsurface temperature bias is small (< 0.25°C) and root mean square error peaks at the depth range of the mixed layer and thermocline. Coupled to the Community Ice CodE (CICE) on the same grid, the HYCOM/CICE/NCODA system (initially restricted to the Arctic) provides sea ice nowcasts and forecasts. Ice edge location errors are improved from the previous sea ice prediction system but are limited in part by the accuracy of the satellite observations it assimilates.
[Show abstract][Hide abstract] ABSTRACT: Atmospheric forcing, which is known to have a strong influence on surface ocean dynamics and production, is typically not
considered in studies of the deep sea. Our observations and models demonstrate an unexpected influence of surface-generated
mesoscale eddies in the transport of hydrothermal vent efflux and of vent larvae away from the northern East Pacific Rise.
Transport by these deep-reaching eddies provides a mechanism for spreading the hydrothermal chemical and heat flux into the
deep-ocean interior and for dispersing propagules hundreds of kilometers between isolated and ephemeral communities. Because
the eddies interacting with the East Pacific Rise are formed seasonally and are sensitive to phenomena such as El Niño, they
have the potential to introduce seasonal to interannual atmospheric variations into the deep sea.
[Show abstract][Hide abstract] ABSTRACT: The 7 years (2003-2009) of output from a regional version of the Hybrid Coordinate Ocean Model (HYCOM) nested in global HYCOM are used to study the seasonal and interannual variability of the salinity in the northern Gulf of California (NGOC). Previous studies illustrate that the NGOC is characterized by an annual evaporation of ~0.9 m/yr. This evaporation generates high-sea-surface-salinity (SSS) water, which reaches a maxima in the NGOC (>37) and decreases to ~35 toward the entrance of the Gulf of California. The NGOC surface water is interannually modulated by fluctuations in evaporation and by fluctuations in the low-salinity water transported into the region by poleward eastern boundary currents. The fluctuations in the transport of low-salinity water are linked to the arrival of equatorially originated coastally trapped waves. The crucial role of the transport of low-salinity water for the interannual variability of SSS is illustrated by the 2006 and 2008 fall seasons, which include the lowest simulated salinity of the period 2003-2009. The lowest salinity in 2006 and 2008 cannot be explained solely by evaporation because 2006 was characterized by the largest evaporation of the period 2003-2009. However, the presence of the lowest-salinity environment can be attributed to the evaporation in conjunction with the largest upper ocean transport of low-salinity water carried to the NGOC by the 2006 and 2008 coastally trapped waves intensified poleward eastern boundary currents.
[Show abstract][Hide abstract] ABSTRACT: The HYbrid Coordinate Ocean Model (HYCOM) has been configured for the Gulf of California (GOC) at 1/12° and 1/25° horizontal grid resolution and has been nested inside a basin-scale 1/12° Pacific version of HYCOM. The nested GOC models are used to study the upper-ocean GOC response to Hurricane Juliette. The model results indicate that Juliette’s winds forced strong poleward coastal baroclinic currents (meridional velocity >60 cm/s) along the southwestern coast of the GOC. That reversed the well-observed mean equatorward currents along the southeastern coast of the Baja California Peninsula. These Juliette-induced currents forced a transport variation of >0.2 Sv along the entrance of the GOC. In addition, Juliette’s winds increased the mixed layer depth (from ∼5 m to ∼40 m) and induced strong upwelling (vertical velocity >30 m/day) along the southeastern coast of the Baja California Peninsula. The model simulated upwelling is corroborated by model independent analysis of SeaWiFS chlorophyll-a satellite measurements. During its early stage Juliette generated a coastally trapped wave (CTW) along mainland Mexico. After its generation the CTW propagated poleward along the coasts of the mainland and GOC, where it reached the shelf break between the 28°N and 29°N and it reversed the direction and propagated equatorward along the western coast of the GOC. Next, the CTW propagated to the southwestern coast of the GOC, where it partially modulated the intensity of the Juliette-generated coastal upwelling.
[Show abstract][Hide abstract] ABSTRACT: Ocean models run with atmospheric forcing but without ocean data assimilation are useful in studies of ocean model dynamics and simulation skill. Models that give realistic simulations with accurate dynamics, when run without data assimilation, are essential for eddy-resolving ocean prediction because of the multiple roles that ocean models must play in ocean nowcasting and forecasting, including dynamical interpolation during data assimilation, representing sparsely observed subsurface ocean features from the mixed layer depth to abyssal currents, converting atmospheric forcing into ocean responses, imposing topographic and geometric constraints, performing ocean forecasts, and providing boundary and initial conditions to nested regional and coastal models. A wide range of ocean dynamics contribute to these different roles. Here we focus on evaluating the dynamics of mid-latitude ocean currents simulated by state-of-the- art, eddy-resolving ocean general circulation models (OGCMs) with high vertical resolution, using the Gulf Stream as an example.
[Show abstract][Hide abstract] ABSTRACT: The Hybrid Coordinate Ocean Model (HYCOM) is used to forecast the three-dimensional structure in various parts of the world ocean (the Gulf of Mexico, the northern Gulf of Mexico, the Persian Gulf, the Gulf of California, and the Hawaii region). The horizontal resolution varies between them, but the coarsest is ~3.5 km. Vertical resolution varies from 20 to 32 layers. Lateral boundary forcing is supplied by global or basin- scale versions of HYCOM, and surface wind and heat flux forcing from the Navy Operational Global Atmospheric Prediction System (NOGAPS). The lateral boundary conditions use a "buffer area" for the baroclinic mode where the fine-grid solution is relaxed towards the outer coarse grid solution. The method of Characteristics is used for the barotropic mode. Most of the models assimilate ocean observations via the Navy Coupled Ocean Data Assimilation (NCODA) system. The primary observations include satellite-derived sea surface height and temperature as well as Argo profile data. The NCODA configuration used here is based on multi-variant optimal interpolation and uses the Cooper-Haines (1983) technique for downward projection of surface observations. The forecast length varies but is typically between 3-7 days. The value-added of downscaling to higher resolution is demonstrated through various model-data comparisons, particularly data that was withheld from the data assimilation system. In the Gulf of Mexico, Loop Current Rings and (some) cyclonic rings compare favorably to independently derived thermal fronts measured with multi-channel SST's (MCSST). The northern Gulf of Mexico domain, which represents a triple- nested system, is used to generate ensembles to examine the variance associated with errors in the initial state, surface wind forcing, etc. Near-surface current patterns in the Persian Gulf are compared to drouged drifters. Several of the circulation features in the Hawaii area are compared to observations collected during a recent Navy exercise. The sea level height in the Gulf of California agrees very well with the height measured by coastal tide guage stations. This domain was also used to exhaustively investigate the sensitivity of the lateral boundary condition parameters. The general circulation features in these regions are discussed, as well as technical aspects of the assimilation and validation.
[Show abstract][Hide abstract] ABSTRACT: An operational version of the Navy Layered Ocean Model is used to study the generation of a coastally trapped wave forced by a strong and intermittent wind event at the Northern Bight of Panamá. This study identifies the winds at the Northern Bight of Panamá as a new source for the generation of coastally trapped waves along the west coast of the North American continent. The results indicate that after its generation, the wave propagated poleward increasing the sea level > 10 cm, producing surface currents > 50 cm/s, and traveling > 1200 km. The generation and existence of the coastally trapped wave and the model results are validated with sea surface height coastal tide gauge observations.
[Show abstract][Hide abstract] ABSTRACT: Ocean color and sea surface temperature satellite-observations show the existence of a series of anticyclonic eddies along the axis of the southern Gulf of California (SGOC). To investigate the summer generation of these eddies, a regional version of the HYbrid Coordinate Ocean Model (HYCOM) has been configured for the GOC and has been nested inside the global model. A Suite of experiments, using the nested GOC model, was developed and used to isolate the effects of the local wind and the effects of the oceanic remote forcing On the generation of the SGOC eddies The results indicate that the local wind is not essential for the generation of these eddies rather it is the oceanic remote forcing. In the SGOC the monthly variability of the currents and sea surface height is mainly due to the deterministic near-coastal poleward eastern boundary currents (PEBC). The interaction of the PEBC with the topographic irregularities (the capes at Topolobampo and Cabo Lobos and the ridges extending...
[Show abstract][Hide abstract] ABSTRACT: The HYbrid Coordinate Ocean Model (HYCOM) has been configured for the Gulf of Mexico (GOM) at 1/25° horizontal grid resolution and has been nested inside a basin-scale 1/12° Atlantic version of HYCOM. The 1/25° nested GOM model is used to study temperature variations, current patterns, transport variations, and two coastal-trapped waves (CTWs) generated by Hurricane Ivan during mid September 2004. The model results indicate that the winds generated by Ivan: (1) induced a transport variation of approximately 2 Sv/day along the Yucatan Channel, (2) enhanced the oceanic mixing lowering the sea surface temperature more than 3 °C along Ivan’s path, (3) produced a thermocline vertical velocity of >100 m/day, and (4) generated a westward transport of ∼8 Sv along the northern coast of the GOM that was redirected by the Louisiana coastline inducing a southward transport of ∼6 Sv. Throughout its passage over the Caribbean Sea Ivan generated first a CTW along the south east coast of Cuba. After its generation this wave propagated along the coast and partially propagated along the western tip of the Cuban Island and continued its propagation along the northern coast of the Island. The model existence of CTWs along the coast of Cuba is reported for the first time. Later on, over the Florida–Alabama–Mississippi–Louisiana coast, Ivan’s westward winds drove a model oceanic onshore transport and generated a strong coastal convergence. The convergence raised the sea surface height ∼90 cm generating a second CTW, which is characterized by alongshore and cross-shore scales of ∼700 and ∼80 km, respectively. The CTW current pattern includes westward surface currents of more than 2.0 m/s. After its generation, the wave weakened rapidly due to Ivan’s eastward winds, however a fraction of the CTW propagated to the west and was measured by a tide gauge at Galveston, Texas. The descriptions, hypothesis, and discussions presented in this study are based on model results and those results are compared and validated with sea surface height coastal tide gauge observations and sea surface temperature buoy observations.
[Show abstract][Hide abstract] ABSTRACT:  TOPEX/Poseidon and ERS-2 satellite altimeter observations and the 1/16 degree Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) show the existence of anticyclonic eddies in the Cabo Corrientes - Maria Islands region off the Mexican West Coast. Analysis of the results demonstrates that: (1) The Cabo Corrientes - Maria Islands region is characterized by mean poleward coastal currents, driven by local wind forcing. (2) The local currents are intensified by the arrival of baroclinic down welling coastally trapped waves (CTWs), generated in the equatorial Pacific. (3) Anticyclonic eddies are generated as the intensified local currents pass cape-like features in the coastline or shelf-break geometry. (4) From I 979 to 2001 the CTWs generated an average of 2.35 (2.5) Cabo Corrientes (Maria Islands) anticyclonic eddies per year. (5) The formation of eddies carries interannually. increasing (decreasing) during El Nino (La Nina) years. Comparison of a variety of numerical simulations. which include different dynamics and%or different wind forcing and/or different topographic effects, suggests that bottom topography. local wind, and baroclinic instabilities are not essential for the eddy generation. It is (a) the capes at Cabo Corrientes and the Maria Islands and (b) the strong transient events associated with the CTWs that are essential to the formation of these newly recognized eddies
[Show abstract][Hide abstract] ABSTRACT: The variability of the Eastern Pacific Warm Pool (EPWP) is studied analyzing observations from the Modular Ocean Data Assimilation System, results from the HYbrid Coordinate Ocean Model (HYCOM), and sea surface height anomaly data. Results indicate that the EPWP strengthens and weakens, but remains throughout the year. The monthly variability of the EPWP is primarily forced by the surface heat fluxes (SHF), which generate the EPWP's May (January) maximum (minimum) coverage of ~4,000,000 (~30,000) km2 and the July mid-summer minimum of ~2,300,000 km2. In addition to the SHF the interannual variability of the EPWP is influenced by the coastal-warm-water advected poleward by the Costa Rica Coastal Current and interannual coastally-trapped- waves (CTWs). During its poleward propagation the interannual CTWs generate westward propagating long Rossby waves that advect the coastal-warm-water offshore, contributing to the westward strengthening of the EPWP. Furthermore, it is shown that the dimensions of the EPWP increase (decrease) during El Niño (La Niña) years.
[Show abstract][Hide abstract] ABSTRACT: TOPEX/Poseidon satellite altimeter observations and the Naval Research Laboratory Layered Ocean Model simulations show interannual variability in the number intensity of Tehuantepec eddies off the Mexican southwest coast. Analysis of the results illustrates that downwelling coastally trapped waves, which are generated in the equatorial Pacific, play a crucial role in the modulation and generation of Tehuantepec eddies and a paradigm in which the generation and modulation of Tehuantepec eddies is not exclusively explained in terms of the strong and intermittent Tehuantepec wind events. In fact, the results show anticyclonic eddy formation during periods of calm Tehuantepec winds. That is specifically exemplified by the formation of two anticyclonic Tehuantepec eddies during a five-month period of weak Gulf of Tehuantepec winds during summer of 1997. Furthermore, the satellite-observed and NLOM-simulated proliferation of Tehuantepec eddies during El Nino years is explained by the corresponding increase in downwelling coastally trapped waves and a lack of increases in the number and strength of Tehuantepec wind events during El Nino years.
[Show abstract][Hide abstract] ABSTRACT: Hydrographic and current measurements in the Northern Gulf of California from December 1994 through March 1995 show conditions that are markedly different from those accepted as typical for winter. A rapid subsurface warm water intrusion (∼0.5 °C warmer) was observed, which reduced the stratification and stopped, and eventually reversed the typical anticyclonic winter circulation. An empirical orthogonal function analysis of the mean vertical structure of temperature and salinity of historical hydrographic data reveals that similar conditions occurred during March 1973 (warm water intrusion and weakening of stratification). In this study, it is proposed that these anomalous conditions are due to a combination of local and remote forcings: extensive water-mass formation in the Upper Gulf of California and the arrival of an intraseasonal downwelling coastally trapped wave to the Northern Gulf of California.
Full-text · Article · Jan 2006 · Continental Shelf Research
[Show abstract][Hide abstract] ABSTRACT: Analysis of observations of the Loop Current (LC) northward penetration and LC area, from satellite sea surface temperature, Topex/Poseidon (TP) sea surface height anomaly, and results from numerical simulations shows that when a relatively large cyclone remains north of the LC, the shedding period between two consecutive eddies may increase. It is shown that the interaction between the LC and the cyclone produces leakage of mass from the current and pushes the LC towards the West Florida Shelf escarpment, where mass is also redistributed due to the generation of a pressure gradient and a jet along the shelf edge. This process delays the northward penetration of the LC and the enlargement of its area, increasing the time between eddy shedding. This happened in 1998, when the largest registered period between eddy shedding since 1973 occurred, and the largest cyclone of the TP era was north of the LC.
[Show abstract][Hide abstract] ABSTRACT: We analyze sea level and transports in the Gulf of California from two large-scale, high-resolution numerical models: the Naval Research Laboratory Layered Ocean Model and the Hybrid Coordinate Ocean Model. Sea level from both models correlates well with sea surface height from a nearby TOPEX track and subsurface pressure measured in the northern gulf during 1997-1998. An 8-year mean (1993-2000) section of the modeled meridional velocity at the mouth of the gulf is in good qualitative agreement with observations, showing a localized near-surface outflow on the Baja California side. The 8-year mean, laterally integrated modeled near-surface transport at the mouth of the gulf shows an opposite direction for both models. During the 1997-1998 El Niño, however, both models show a similar structure with increased inflow in the upper 100 m and an increased outflow in the underlying layer to about 500 m depth. The modeled surface transport also shows increased inflow in the northern gulf. The modeled incoming surface transport from both models during El Niño is negatively correlated to near-bottom currents measured at a sill in the northern gulf. This measured near-bottom current has previously been shown to constitute part of the strong, localized incoming flow from the Pacific Ocean. The negative correlation strongly suggests that the anomalous incoming surface flow at the mouth of the gulf inhibited the normal near-surface outflow of the northern Gulf of California water. This, in turn, inhibited the normal deeper inflow, entering the northern gulf from the Pacific Ocean. These anomalies apparently increased the residence time of the waters in the northern gulf. They are consistent with anomalously high near-bottom salinities found during 1997-1998. Hydrographic data for July 1997 also show an anomalous low salinity surface water, which is most likely associated with advection of tropical waters from the Pacific Ocean.
Preview · Article · Nov 2005 · Journal of Geophysical Research Atmospheres