Variability Of Surface Currents Research Articles

Central South Pacific thermocline water circulation from a high‐resolution ocean model validated against satellite data: Seasonal variability and El Niño 1997–1998 influence

The oceanic circulation in French Polynesia, central South Pacific, is investigated based on a high‐resolution model validated with satellite data. We focus on the upper 500 m, where ocean current variability has been barely documented due to a lack of in situ measurements. Seasonal and El Niño–Southern Oscillation 1997–1999 variability of surface and subsurface currents are characterized. In the north, the South Equatorial Current (SEC) follows a seasonal cycle with winter enhancement corresponding to trade wind strengthening. During summer, the SEC slows and the eastward South Equatorial Countercurrent (SECC) appears across the domain north of an upward Ekman pumping region related to the South Pacific Convergence Zone development in the area. Around the Marquesas Islands, a wind curl dipole is suspected to create westward and eastward (Marquesas Countercurrent, MCC) jets. The eastward Subtropical Countercurrent (STCC) is found south of 20°S. It is disturbed by eddies and broken into westward and eastward branches. During El Niño 1997–1998, due to the reversed winds in the equatorial band, the SEC weakened, while eastward countercurrents appeared. The SECC reinforced and moved northeastward in summer 1998. During La Niña 1998–1999, following the trade wind enhancement, the SEC strengthened. The countercurrents weakened or disappeared and the SECC moved southwestward in summer 1999. During both El Niño and La Niña events, the STCC eddy activity strengthened, while the MCC and associated westward jet were absent. These variations and their dynamical origins are discussed, and a coherent picture of the French Polynesian current system and its variations is proposed.

Robust of Doppler Centroid for Mapping Sea Surface Current by Using Radar Satellite Data

Problem statement: Sea surface current retrieving from Synthetic Apert ure Radar (SAR) is required standard methods due to the complexity of sea surface ocean imaging in SAR data. In this context, various analytical models have been develo ped which describe overall effects of sea surface roughness on the Doppler signal mechanisms. Nevertheless, such models are limited in the complexity of the sea surface current estimation that can be u sed. In fact, the resolution of the sea surface Dop pler velocity in azimuth direction is typically coarser as compared to the normalized radar cross section image. Approach: This study introduced a new method to retrieve sea surface current from RADARSAT-1 SAR Standard beam mode (S2) data. The method was based on the utilization of the Wavelength Diversity Ambiguity Resolving (WDAR) and Multi Look beat Frequency (MLBF) algorithms to remove Doppler centroid (f DC ) ambiguity. Results: The result showed that the proposed methods are able to correct Doppler centroid (f DC ) ambiguity and produced fine spatial sea surface current variations in S2 mode data. The current vel ocities were ranged between 0.18 and 0.78 m sec -1 with standard error of 0.11 m sec -1 . Conclusion: In conclusion, RADARSAT-1 SAR standard beam mode (S2) data can be utilized to retrieve real tim e sea surface current. Both WDAR and MLBF algorithms are able to provide accurately informati on on Doppler Centroid (fDC ) in which accurately real time sea surface current can be retrieved from SAR data.

Surface and Subsurface Geostrophic Current Variability in the Indian Ocean from Altimetry

Geostrophic currents were calculated at the surface and subsurface levels in the Indian Ocean using satellite altimetry data from TOPEX/POSEIDON and Jason-1, a geoid obtained from the GRACE mission, and historical hydrographic profiles from the World Ocean Atlas 2005. The results of this method were validated with currents measured using Acoustic Doppler Current Profilers moored along the equator at 77°E, 83°E, and 93°E. The measured and computed currents compared reasonably well with similar features present in both. The presence of the seasonal Wyrtki Jets and Equatorial Undercurrent and their variability from 2000–2001 was observed in the estimated currents.

Seasonal variation of surface currents in the southwestern Taiwan Strait observed with HF radar

Long-term measurements from a pair of High-Frequency radar systems deployed near the coast of southern Fujian Province showed that surface currents in the southwestern Taiwan Strait were composed mainly of the monsoon-driven, seasonal fluctuation of longshore current and a persistent northeastward background flow with speeds around 10 cm/s. Measurements from bottom-moored ADCPs further indicated that below the surface Ekman layer longshore currents also directed to the north all year round.

A high-resolution geophysical investigation of sediment distribution controlled by catchment size and tides in a multi-basin turbid outwash fjord: Simpson Bay, Prince William Sound, Alaska

Surficial sediment distribution within Simpson Bay is a function of antecedent bedrock and recently deposited glacial geology, as well as active physical processes both within Simpson Bay and Prince William Sound (PWS). Simpson Bay is a turbid, outwash fjord located in northeastern PWS, Alaska. Freshwater from heavy precipitation, and the melting of high alpine glaciers enter the bay through bay head rivers and small shoreline creeks. The catchment has a high watershed/basin surface area ratio (∼8:1), and easily erodible bedrock that contribute to high sediment loads. The system can be divided into three discrete basins, each with specific morphologic and circulatory characters. Side scan sonar, swath bathymetry, and seismic profiles reveal that bathymetric highs are areas of outcropping glacial surfaces. High backscatter coupled with surface grab samples reveal these surfaces to be composed of coarse sediment and bedrock outcrops. Bathymetric lows are areas of low backscatter, and grab samples reveal these areas to be ponded deposits of organic-rich estuarine muds. The data provide evidence of terminal morainal bank systems, and glacial grounding line deposits at the mouth of the bay and rocky outcrops were identified as subsurface extensions of aerial rocky promontories. Radioisotope analyses of short cores reveal that the bay has an average accumulation rate of approx. 0.5 cm year−1, but that this varies in function of the watershed/basin surface area ratios of the different basins. The interaction of tidal currents and sediment source drives sediment distribution in Simpson Bay. Hydrographic data reveal high spatial variability in surface and bottom currents throughout the bay. Subsurface currents are tide dominated, but generally weak (5–20 cm s−1), while faster currents are found along shorelines, outcrops, and bathymetric highs. Bathymetric data reveal steep slopes with little to no modern sediment throughout the bay, suggesting lack of deposition due to tidal currents.

Impact of resolving the diurnal cycle in an ocean–atmosphere GCM. Part 2: A diurnally coupled CGCM

The diurnal cycle is a fundamental time scale in the climate system, at which the upper ocean and atmosphere are routinely observed to vary. Current climate models, however, are not configured to resolve the diurnal cycle in the upper ocean or the interaction of the ocean and atmosphere on these time scales. This study examines the diurnal cycle of the tropical upper ocean and its climate impacts. In the present paper, the first of two, a high vertical resolution ocean general circulation model (OGCM), with modified physics, is developed which is able to resolve the diurnal cycle of sea surface temperature (SST) and current variability in the upper ocean. It is then validated against a satellite derived parameterization of diurnal SST variability and in-situ current observations. The model is then used to assess rectification of the intraseasonal SST response to the Madden–Julian oscillation (MJO) by the diurnal cycle of SST. Across the equatorial Indo-Pacific it is found that the diurnal cycle increases the intraseasonal SST response to the MJO by around 20%. In the Pacific, the diurnal cycle also modifies the exchange of momentum between equatorially divergent Ekman currents and the meridionally convergent geostrophic currents beneath, resulting in a 10% increase in the strength of the Ekman cells and equatorial upwelling. How the thermodynamic and dynamical impacts of the diurnal cycle effect the mean state, and variability, of the climate system cannot be fully investigated in the constrained design of ocean-only experiments presented here. The second part of this study, published separately, addresses the climate impacts of the diurnal cycle in the coupled system by coupling the OGCM developed here to an atmosphere general circulation model.

Application of a general electron emission equation to surface nonuniformity and current density variation

Electron emission nonuniformity is a cause of intrinsic emittance from the electron source, and is a consequence of work function variation due to crystal faces and coatings such as cesium, field enhancement effects due to surface structure, and temperature. Its investigation using particle-in-cell (PIC) codes such as MICHELLE is hampered due to the lack of an emission model that can treat thermal, field, and photoemission effects particularly in crossover regions where the canonical equations, e.g., the Fowler-Nordheim, Richardson-Laue-Dushman, and Fowler-Dubridge equations are compromised. A recently developed thermal-photo-field emission equation is used here to simulate the consequences of nonuniformity due to work function variation induced by coating variation. The analysis is performed both theoretically using simple models as well as using particle-in-cell codes (MICHELLE) to assess changes in current density and emittance. PIC simulations considering an idealized model of geometric effects and crystal face variation indicate that a flat, grainy surface causes the emittance to increase by a factor of 5 while the addition of hemispherical bumps causes the emittance to increase by an additional factor of 6 even though the current is but 10% larger.

Surface current and vertical thermal structure on the continental slope in Tosa Bay

In order to specify a vertical thermal structure related to surface current variation on the continental slope in Tosa Bay, Japan, we analyzed monthly regular hydrographic measurements in the years 1991–2004. Subsurface temperature below 200 m on the slope was found to vary synchronously with the vertical displacement of the main thermocline around 200 m. It is shown that the vertical-averaged temperature below 200 m is significantly correlated with an along-isobath/southwestward surface current velocity on the slope. This correlation indicates that when a strong (weak) southwestward surface current is observed, temperature below 200 m decreases (increases) simultaneously, that is, isotherms below the 200 m are displaced upward (downward) together with the main thermocline. Moreover, when the strong southwestward flow is detected, across-isobath isotherms around 200 m slope upward toward the offshore direction. Furthermore, it is suggested that as the Kuroshio axis moves offshore south of the bay, the southwestward flow tends to be weakened by the combined effect of other Kuroshio parameters such as transport and stream width as well as the Kuroshio axis position. As a result, it is inferred that the correlation between the surface current and subsurface temperature can be interpreted in terms of the formation and decay of an anticlockwise circulation interacting with a cold eddy.

Fine-resolution simulation of surface current and sea ice in the Arctic Mediterranean Seas

A fine-resolution model is developed for ocean circulation simulation in the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Chinese Academy of Sciences, and is applied to simulate surface current and sea ice variations in the Arctic Mediterranean Seas. A dynamic sea ice model in elastic-viscous-plastic rheology and a thermodynamic sea ice model are employed. A 200-year simulation is performed and a dimatological average of a 10-year period (141st–150th) is presented with focus on sea ice concentration and surface current variations in the Arctic Mediterranean Seas. The model is able to simulate well the East Greenland Current, Beaufort Gyre and the Transpolar Drift, but the simulated West Spitsbergen Current is small and weak. In the March climatology, the sea ice coverage can be simulated well except for a bit more ice in east of Spitsbergen Island. The result is also good for the September scenario except for less ice concentration east of Greenland and greater ice concentration near the ice margin. The extra ice east of Spitsbergen Island is caused by sea ice current convergence forced by atmospheric wind stress.

Intraseasonal variations of surface and subsurface currents off Java as simulated in a high‐resolution ocean general circulation model

A high‐resolution ocean general circulation model (OGCM) is used to explore dynamics of intraseasonal variability in surface and subsurface currents off Java. The results indicate that the surface current, the so‐called South Java Coastal Current (SJCC), is dominated by variations with a period of 90 days. In the subsurface current, which is referred to as the South Java Coastal Undercurrent (SJCU), 60‐day variations are the most prominent feature. A normal mode analysis demonstrates that the first baroclinic mode is the leading mode, which accounts for 70% of the total variance, whereas the second baroclinic mode explains 24% of the total variance. The 90‐day variations in the SJCC captured mostly by the first baroclinic mode are found to be primarily driven by winds. Those are associated with propagation of the first baroclinic Kelvin waves generated in the central equatorial Indian Ocean. On the other hand, the 60‐day variations in the SJCU enhanced by wind forcing over the eastern equatorial Indian Ocean off Sumatra are mostly captured by the second baroclinic mode.

The Mercator global ocean operational analysis system: Assessment and validation of an 11-year reanalysis

This paper presents Prototype Système 2 Global (PSY2G), the first Mercator global Ocean General Circulation Model (OGCM) to assimilate along-track sea level anomaly (SLA) satellite data. Based on a coarse resolution ocean model, this system was developed mainly for climatic purposes and will provide, for example, initial oceanic states for coupled ocean-atmosphere seasonal predictions. It has been operational since 3 September 2003 and produces an analysis and a two-week forecast for the global ocean every week. The PSY2G system uses an incremental assimilation scheme based on the Cooper and Haines [Cooper, M., Haines, K., 1996. Data assimilation with water property conservation. J. Geophys. Res., 101, 1059-1077.] lifting–lowering of isopycnals. The SLA increment is obtained using an optimal interpolation method then the correction is partitioned into baroclinic and barotropic contributions. The baroclinic ocean state correction consists of temperature, salinity and geostrophic velocity increments and the barotropic correction is a barotropic velocity increment. A reanalysis (1993–2003) was carried out that enabled the PSY2G system to perform its first operational cycle. All available SLA data sets (TOPEX/Poséïdon, ERS2, Geosat-Follow-On, Jason1 and Envisat) were assimilated for the 1993–2003 period. The major objective of this study is to assess the reanalysis from both an assimilation and a thermodynamic point of view in order to evaluate its realism, especially in the tropical band which is a key region for climatic studies. Although the system is also able to deliver forecasts, we have mainly focused on analysis. These results are useful because they give an a priori estimation of the qualities and capabilities of the operational ocean analysis system that has been implemented. In particular, the reanalysis identifies some regional biases in sea level variability such as near the Antarctic Circumpolar Current, in the eastern Equatorial Pacific and in the Norwegian Sea (generally less than 1 cm) with a small seasonal cycle. This is attributed to changes in mean circulation and vertical stratification caused by the assimilation methodology. But the model's low resolution, inaccurate physical parameterisations (especially for ocean–ice interactions) and surface atmospheric forcing also contribute to the occurrence of the SLA biases. A detailed analysis of the thermohaline structure of the ocean reveals that the isopycnal lifting–lowering tends to diffuse vertically the main thermocline. The impact on temperature is that the surface layer (0–200 m) becomes cooler whereas in deeper waters (from 500 to 1500 m), the ocean becomes slightly warmer. This is particularly true in the tropics, between 30°N and 30°S. However it can be demonstrated that the assimilation improves the variability in both surface currents and sub-surface temperature in the Equatorial Pacific Ocean.

Surface current variability in the Keum River Estuary (South Korea) during summer 2002 as observed by high-frequency radar and coastal monitoring buoy

High-frequency (HF) radar observations of surface currents were conducted for 3 months during summer 2002 in the Keum River estuary. A comparison between HF radar-derived currents and directly measured ones form a buoy showed that the regression slope is close to 1 and the correlation coefficient greater than 0.86, with an RMS difference less than 13 cm/s which is less than 17% of the tidal current. This fairly good agreement allows us to use HF radar observation in investigating the surface flow and circulation in this tidal-current-dominant coastal-plume area. To examine the spatial variation in tidal current characteristics, as well as currents associated with non-tidal forcing, the HF radar-derived currents were separated into tidal and sub-tidal frequency currents. The overall pattern of M 2-current ellipse distribution in the study area showed a counterclockwise rotation, with the offshore maximum current direction to the northeast. Eccentricity, the direction of maximum current, and the phase of net motion of the ellipse changed near the estuary mouth and near the gap of the Saemangeum reclamation tide dyke due to the complex coastal geometry and the out-flowing jet during the ebb period. The Eulerian mean current field in July was similar to the circulation patterns suggested in the previous studies using the CTD data and numerical models, implying that the region of northwestward flow may have developed by the rivers discharges. The short-term variation in the surface sub-tidal flow field was governed primarily by that of wind forcing and was responsible for the changes in the hydrographic conditions in the surface layer off the Keum River estuary mouth. The spatially averaged current, which is calculated after extracting the Eulerian mean currents from the sub-tidal flow, flowed almost to the right of wind during 22 days for an observer looking down-wind-stream, with the mean angle of 33°, indicating predominant Ekman drift. The sub-tidal current observed at the buoy showed that there is an almost unidirectional, persistent current irrespective of the variable wind forcing, producing strong Eulerian mean jet from the northern dyke gap. This current could be formed due to firstly the residual tidal currents generated by the open edges of the dyke and secondly the offshore discharge of the diluted water through the gap. However, when the daily river discharge increased greatly, the sub-tidal current speed at the buoy increased as much as 21 cm/s under the very strong southerly wind condition. This event is interpreted by relaxation process of the trapped coastal water by wind and the large river discharge.

High-resolution simulation of monsoon variability of the Indian Ocean currents

A numerical model of the Indian Ocean circulation is developed at the Institute of Numerical Mathematics, Russian Academy of Sciences, and the results of simulation of monsoon current variability are presented. The model resolution is 1/8° in longitude, 1/12° in latitude with 21 unevenly vertical levels. The model was run for 15 years using monthly mean atmospheric data from NCEP reanalysis. The annual cycle of surface and subsurface currents, temperature and salinity fields are analysed for the final 15th year. The model reproduces the Summer Monsoon and Winter Monsoon Currents, their annual cycles, space structures and magnitudes. The modelled Somali Current has a good resemblance to observations with model velocities exceeding 2 ms−1 and the transport about 70 Sv during the Summer Monsoon. The model results show that a turn of the Somali Current from the summer to winter direction is accompanied by the generation of anticyclonic eddies that drift westward by the β -effect and dissipate in the Somalia shore and the Gulf of Aden. The monsoon variability of equatorial surface currents and equatorial undercurrents is analysed. It is shown that these currents are mainly generated by zonal wind stress, which is dominated at the equator by a semi-annual harmonic. Therefore, the equatorial surface currents and the compensating equatorial subsurface undercurrents also change directions with a semi-annual periodicity. It is found that gradient currents caused by river run-off make an important contribution to the circulation in the Bay of Bengal. The main features of quasistationary vigorous eddies like Great Whirl, Socotra High, Lakshadweep High, and Lakshadweep Low are well reproduced.

High-resolution simulation of monsoon variability of the Indian Ocean currents

A numerical model of the Indian Ocean circulation is developed at the Institute of Numerical Mathematics, Russian Academy of Sciences, and the results of simulation of monsoon current variability are presented. The model resolution is 1/8° in longitude, 1/12° in latitude with 21 unevenly vertical levels. The model was run for 15 years using monthly mean atmospheric data from NCEP reanalysis. The annual cycle of surface and subsurface currents, temperature and salinity fields are analysed for the final 15th year. The model reproduces the Summer Monsoon and Winter Monsoon Currents, their annual cycles, space structures and magnitudes. The modelled Somali Current has a good resemblance to observations with model velocities exceeding 2 ms−1 and the transport about 70 Sv during the Summer Monsoon. The model results show that a turn of the Somali Current from the summer to winter direction is accompanied by the generation of anticyclonic eddies that drift westward by the β -effect and dissipate in the Somalia shore and the Gulf of Aden. The monsoon variability of equatorial surface currents and equatorial undercurrents is analysed. It is shown that these currents are mainly generated by zonal wind stress, which is dominated at the equator by a semi-annual harmonic. Therefore, the equatorial surface currents and the compensating equatorial subsurface undercurrents also change directions with a semi-annual periodicity. It is found that gradient currents caused by river run-off make an important contribution to the circulation in the Bay of Bengal. The main features of quasistationary vigorous eddies like Great Whirl, Socotra High, Lakshadweep High, and Lakshadweep Low are well reproduced.

Sea level and surface current variability in the Gulf of St Lawrence from satellite altimetry

Sea level and surface current variability in the Gulf of St Lawrence have been investigated primarily using seven years of TOPEX/Poseidon altimeter data. An orthogonal response analysis is used to derive an annual cycle from 1-s altimetric data along satellite ground tracks, while simultaneously removing aliased residual tides and dynamic signals at alias tide frequencies. An examination of tidal-frequency variability points to the need for a better tide model for detiding altimetric data in order to study shorter (than seasonal) period processes in the Gulf of St Lawrence. Annual sea level amplitudes and phase fields are constructed from the along-track analysis results using a linear interpolation procedure. The altimetric annual harmonic has a magnitude of 2-5 cm in amplitude and is highest in fall. The altimetric sea level results agree well with independent tide-gauge data at coastal stations and can be accounted for mainly by steric height. Geostrophic surface current anomalies derived from the altimetric annual sea level anomalies are then discussed in conjunction with numerical solutions from a regional hydrodynamic model. Interannual sea level change in the Gulf of St Lawrence is also investigated from both altimetry and tide-gauge data, indicating a prominent gulf-wide sea level drop around 1997, with exact timing dependent on location. The interannual sea level variability is thought to be primarily associated with the Labrador Current transport variability (via both the Strait of Belle Isle and Cabot Strait) and the south-north fluctuation of the Gulf Stream position (via Cabot Strait).

Seasonal current variability on the New Jersey inner shelf

The well‐sampled ocean off the coast of New Jersey provides a data‐rich environment in which to study ocean current variability over the inner shelf. Using a year‐long HF radar data set, complemented with in situ and meteorological observations, the annual‐ and seasonal‐scale variabilities are examined. The hydrographic variability of the inner shelf off New Jersey is largely bimodal between summer stratification and winter mixing. An annual oceanographic and atmospheric data set was separated into these two regimes. The influence of stratification is evident through a relatively steady current response strongly correlated with the wind during the stratified season and a more variable response less correlated with the wind during the mixed season. When the water column is mixed, the influence of the local topography on the surface current variability is dependent on the slope, with a tendency for the variability to be more aligned with steeper topography.

On the remote sensing of oceanic and atmospheric convection in the Greenland Sea by synthetic aperture radar

In this paper we discuss characteristic properties of radar signatures of oceanic and atmospheric convection features in the Greenland Sea. If the water surface is clean (no surface films or ice coverage), oceanic and atmospheric features can become visible in radar images via a modulation of the surface roughness, and their radar signatures can be very similar. For an unambiguous interpretation and for the retrieval of quantitative information on current and wind variations from radar imagery with such signatures, theoretical models of current and wind phenomena and their radar imaging mechanisms must be utilized. We demonstrate this approach with the analysis of some synthetic aperture radar (SAR) images acquired by the satellites ERS‐2 and RADARSAT‐1. In one case, an ERS‐2 SAR image and a RADARSAT‐1 ScanSAR image exhibit pronounced cell‐like signatures with length scales on the order of 10–20 km and modulation depths of about 5–6 dB and 9–10 dB, respectively. Simulations with a numerical SAR imaging model and various input current and wind fields reveal that the signatures in both images can be explained consistently by wind variations on the order of ±2.5 m/s, but not by surface current variations on realistic orders of magnitude. Accordingly, the observed features must be atmospheric convection cells. This is confirmed by visible typical cloud patterns in a NOAA AVHRR image of the test scenario. In another case, the presence of an oceanic convective chimney is obvious from in situ data, but no signatures of it are visible in an ERS‐2 SAR image. We show by numerical simulations with an oceanic convection model and our SAR imaging model that this is consistent with theoretical predictions, since the current gradients associated with the observed chimney are not sufficiently strong to give rise to significant signatures in an ERS‐2 SAR image under the given conditions. Further model results indicate that it should be generally difficult to observe oceanic convection features in the Greenland Sea with ERS‐2 or RADARSAT‐1 SAR, since their signatures resulting from pure wave‐current interaction will be too weak to become visible in the noisy SAR images in most cases. This situation will improve with the availability of future high‐resolution SARs such as RADARSAT‐2 SAR in fine resolution mode (2004) and TerraSAR‐X (2005), which will offer significantly reduced speckle noise fluctuations at comparable spatial resolutions and thus a much better visibility of small image intensity variations on spatial scales on the order of a few hundred meters.

Surface Water Mixing in the Solomon Sea as Documented by a High-Resolution Coral14C Record

A bimonthly coral-based record of the postbomb radiocarbon content of Solomon Sea surface waters is interpreted to reflect mixing of subtropical surface water and that advected in from the east by the equatorial branch of the South Equatorial Current (SEC). Annual mean D14C has a dynamic range of nearly 175‰, with a total range of nearly 200‰. Prebomb values average 256‰ and the annual mean postbomb maxima occurs in 1985 with a value of 1117%. Interannual variability in the record reflects surface current variations in

Tropical Atlantic surface current variability from 10 years of TOPEX/Poséïdon altimetry

10 years of surface geostrophic currents from TOPEX/Poséïdon altimetric data are used to describe the low frequency variability of the tropical Atlantic circulation through Empirical Orthogonal Function analysis. The seasonal variability clearly agrees with previous studies based on climatological data. It shows the tropical Atlantic response to seasonal fluctuations of the overlying wind system. More interesting is the capability, using altimetry, to reach for the first time on a basin scale the year‐to‐year variability from measurements. Abnormal events occur in 1996–1997 and in 2001 with different spatial scales regarding both large scale zonal distribution and regional variability located in the north‐western basin. A first attempt to link these events to climatic indexes (El Niño‐Southern Oscillation, North Atlantic Oscillation) is also evocated.

Yucatan Channel flow: Observations versus CLIPPER ATL6 and MERCATOR PAM models

The statistical properties of the flow structure in the Yucatan Channel's main section based on 2 years of continuous observations (August 1999 to June 2001) are compared with those simulated by the Océan Parallélisé (OPA) primitive equation model in two different configurations: the ATL6 configuration from the CLIPPER project (the whole Atlantic domain with a 1/6° resolution), and the Prototype Atlantique Méditerranée (PAM) configuration from the MERCATOR project (the North Atlantic domain with 1/12° resolution). While the observed 2‐year mean transport into the Gulf of Mexico is 23.06 Sv, ATL6 has a 5‐year mean of 27.49 Sv and PAM has a 3‐year mean of 29.06 Sv. Apart from this discrepancy in the mean transport both simulations are able to reproduce well, in structure and magnitude, details of the mean current and its variability. Features like the Yucatan Current core and the deep countercurrents in the Yucatan and Cuban sides are both well reproduced, as is the presence of a minimum surface current variability at the center of the channel. The two principal empirical modes of current variability, which are related to the passage of eddies through the channel, are also reproduced by the simulations. The observed and simulated transports above and below the 6°C isotherm show no coherency at any frequency except for a significant coherent peak at a period around 20 days, which we interpret as a first baroclinic Kelvin mode between the two layers. As in the observations, the shedding of eddies by the Loop Current in the simulations is preceded by periods of diminished negative horizontal vorticity flux into the Gulf of Mexico.