[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: 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: 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: 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:  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: 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: One component of the DoD HPC Challenge project on ``Basin-scale
Prediction with the HYbrid Coordinate Ocean Model" has been the
development of a high resolution, Pacific version of HYCOM. The
configuration for the Pacific (20° S - 66° N) has an equatorial
grid resolution of 1/12° and 20 vertical coordinate surfaces.
Surface forcing includes wind stress and speed, thermal forcing,
precipitation, relaxation to sea surface salinity and monthly varying
river runoff. Closed boundaries exist in the Indonesian Seas, the
Bering Strait and along 20° S. A 3° buffer zone is included
along 20° S with relaxation to monthly climatological temperature
and salinity. The model has been integrated with both monthly
climatological and 6-hourly interannual European Centre for Medium-Range
Weather Forecasts forcing. It accurately simulates the large-scale
circulation gyres and the major current systems. Near the western
boundary in the vicinity of Luzon Strait and Taiwan, model-data
comparisons of velocity as a function of depth indicate the
climatological forced simulation agrees closely with the observations.
The tropical current systems (South Equatorial Current, North Equatorial
Counter Current, Equatorial UnderCurrent (EUC), and subsurface,
off-equator Tsushiya jets) also compare very well against available
velocity observations. Interannual variability is clearly evident in the
simulation with 6-hourly forcing. In typical years, zonal velocity of
the EUC along the equator shoals and strengthens from west to east.
During the 1982-83 El Nino, the velocity core is essentially level with
uniform magnitude across the central Pacific Ocean. In addition,
Pacific HYCOM produces a coastally trapped wave generated by Hurricane
Juliette that propagates poleward along the Mexican coast, into and out
of the Gulf of California before continuing its poleward journey along
the west coast of the Baja California Peninsula. High correlations exist
between modeled and observed sea level along its path.
[show abstract][hide abstract] ABSTRACT: 1] An operational real-time eddy-resolving (1/16°) global ocean nowcast/forecast system is used to study the evolution of two coastally trapped waves generated by Hurricane Juliette along the Mexican West Coast. Results indicate that the first wave was generated along mainland Mexico and it propagated poleward as a free coastally trapped wave; it also generated anticyclonic eddies near Cabo Corrientes and the María Islands. Upon entering the Gulf of California the wave weakened cyclonic eddies and after reaching the shelf break north of Guaymas, it reversed direction and propagated southward along the east coast of the Baja California Peninsula (BCP). Next, the wave generated an anticyclonic eddy at Cabo San Lucas. Finally, the wave weakened while exiting the gulf and propagated northward along the BCP West Coast. The second coastally trapped wave was generated by Juliette's poleward winds along the BCP West Coast, but was subsequently greatly weakened by Juliette's equatorward winds.
Geophysical Research Letters - GEOPHYS RES LETT. 01/2002; 29(23).
[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: The interaction of cyclonic eddies with the Loop Current (LC) evolution is studied using the Navy Coastal Ocean Model, the Navy Layer Ocean Model, Topex/Poseidon altimetry, and AVHRR images. It is shown that the longest period registered between eddy shedding, from February 1998 to August 1999, is associated with the presence of a large cyclone that remains north of the LC during several months. Using numerical simulations, it is shown that large cyclones develop sporadically in the region and that they block the northward penetration of the LC. The LC leaks mass, momentum, and energy through a jet and small anticyclones moving along the slope of the West Florida Shelf, east of the cyclone. The process causes an enlargement of the period between eddy shedding.