Storm-track analysis is applied to the meridional winds at 10 m and 850 hPa for the winters of 1999–2006. The analysis is focused on the North Atlantic and North Pacific Ocean basins and the Southern Ocean spanning the region south of the Indian Ocean. The spatial patterns that emerge from the analysis of the 850-hPa winds are the typical free-tropospheric storm tracks. The spatial patterns that emerge from the analysis of the surface winds differ from the free-tropospheric storm tracks. The spatial differences between the surface and free-tropospheric storm tracks can be explained by the influence of the spatial variability in the instability of the atmospheric boundary layer. Strongly unstable boundary layers allow greater downward mixing of free-tropospheric momentum (momentum mixing), and this may be the cause of the stronger surface storm tracks in regions with greater instability in the time mean.

Principal component analysis suggests that the basin-scale variability that is reflected in the storm-track signature is the same for the free-tropospheric and surface winds. Separating the data based on the boundary layer stability shows that the surface storm track has a local maximum in the region of maximum instability, even when there is no local maximum in the free-tropospheric storm track above the region. The spatial patterns of the surface storm tracks suggest a positive feedback for storm development as follows: 1) an existing storm generates strong free-tropospheric wind variability, 2) the momentum mixing of the unstable boundary layers acts to increase the ocean–atmosphere energy fluxes, and 3) the fluxes precondition the lower atmosphere for subsequent storm development.

In the Northern Hemisphere midlatitude western boundary current (WBC) systems there is a complex interaction between dynamics and thermodynamics and between atmosphere and ocean. Their potential contribution to the climate system motivated major parallel field programs in both the North Pacific [Kuroshio Extension System Study (KESS)] and the North Atlantic [Climate Variability and Predictability (CLIVAR) Mode Water Dynamics Experiment (CLIMODE)], and preliminary observations and analyses from these programs highlight that complexity. The Gulf Stream (GS) in the North Atlantic and the Kuroshio Extension (KE) in the North Pacific have broad similarities, as subtropical gyre WBCs, but they also have significant differences, which affect the regional air–sea exchange processes and their larger-scale interactions.

The 15-yr satellite altimeter data record, which provides a rich source of information, is combined here with the longer historical record from in situ data to describe and compare the current systems. While many important similarities have been noted on the dynamic and thermodynamic aspects of the time-varying GS and KE, some not-so-subtle differences exist in current variability, mode water properties, and recirculation gyre structure. This paper provides a comprehensive comparison of these two current systems from both dynamical and thermodynamical perspectives with the goal of developing and evaluating hypotheses about the physics underlying the observed differences, and exploring the WBC's potential to influence midlatitude sea–air interaction. Differences between the GS and KE systems offer opportunities to compare the dominant processes and thereby to advance understanding of their role in the climate system.

Lagrangian estimates for ventilation rates in the Gulf Stream Extension using Argo and World Ocean Circulation Experiment/Atlantic Climate and Circulation Experiment (WOCE/ACCE) float data, scatterometer (QuikSCAT) wind stress satellite observations, and altimetric [Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO)] sea surface height (SSH) satellite observations from 2002 to 2006 are presented. Satellite winds and estimates of surface geostrophic currents allow the inclusion of the effects of currents on wind stress as well as their impact on the Ekman pumping. The presence of large surface geostrophic currents decreases the total Ekman pumping, contributing up to 20% where the Gulf Stream makes its two sharpest turns, and increases the total Ekman pumping by 10% or less everywhere else. The ageostrophic currents may be as large as 15% of the geostrophic currents, but only in proximity of the Gulf Stream.

Using currents and mixed layer depths (MLDs) that are either climatological or vary from year to year, obducted water tends to originate along the Gulf Stream, while subducted water tends to originate to its south. However, using time-varying MLDs for each year, subduction varies significantly, sometimes oppositely from obduction. The 18° Water (EDW) subducts in different locations and is distributed differently each year but tends to be located in the Sargasso Sea. Vertical pumping is the only dominant factor in ventilation closer to the coast where MLDs are shallower and lighter parcels are subducted. Vertical pumping contributes up to 20% of the several hundreds of ventilated meters per year around the Gulf Stream and less elsewhere. Using a temperature- or density-based criterion for estimating the MLDs, especially along the coasts and north of 45°N, obduction estimates differ by up to 25%. The horizontal and temporal structure of the MLDs is the primary factor that controls the tens of sverdrups of ventilation (and a few sverdrups of EDW subduction).

A major oceanographic field experiment is described, which is designed to observe, quantify, and understand the creation and dispersal of weakly stratified fluid known as "mode water" in the region of the Gulf Stream. Formed in the wintertime by convection driven by the most intense air-sea fluxes observed anywhere over the globe, the role of mode waters in the general circulation of the subtropical gyre and its biogeo-chemical cycles is also addressed. The experiment is known as the CLIVAR Mode Water Dynamic Experiment (CLIMODE). Here we review the scientific objectives of the experiment and present some preliminary results.

The roles of intraseasonal Kelvin waves and tropical instability waves (TIWs) in the intraseasonal and low-frequency mixed-layer temperature budget were examined in an isopycnal ocean model forced by QuikSCAT winds from 2000 to 2004. Correlations between temperature tendency and other terms of the intraseasonal budget compare well with previous results using Tropical Atmosphere Ocean (TAO) observations: the net heat flux has the largest correlation in the western Pacific and zonal advection has the largest correlation in the central Pacific. In the central Pacific, the intraseasonal variations in zonal advection were due to both the zonal background velocity acting on the Kelvin wave temperature anomaly and the Kelvin wave's anomalous velocity acting on the background temperature. In the eastern Pacific, three of the four temperature budget terms have comparable correlations. In particular, the vertical processes acting on the shallow thermocline cause large SST anomalies in phase with the intraseasonal thermocline anomalies.

On intraseasonal time scales, the influence of individual composite upwelling and downwelling Kelvin waves cancel each other. However, because the intraseasonal SST anomalies increase to the east, a zonal gradient of SST is generated that is in phase with intraseasonal zonal velocity. Consequently, heat advection by the Kelvin waves rectifies into lower frequencies in the eastern Pacific. Rectification resulting from TIWs was also seen. The prevalence of intraseasonal Kelvin waves and the zonal structure of intraseasonal SST from 2002 to early 2004 suggested that they might be important in setting the eastern Pacific SST on interannual time scales.

An ocean model was used to examine whether the scatterometer winds can improve the model performance both dynamically and thermodynamically. Comparisons were done using QuikSCAT and NCEP2 winds for both the mean and variability from 2000 to 2004. The comparisons showed that the model forced by QuikSCAT winds gives more realistic mean SST, 20°C isotherm depth (Z20), and latent heat flux than NCEP2 winds do. Sensitivity experiments indicated that QuikSCAT mean wind stress is important for the improved mean SST, Z20, and latent heat release to the atmosphere in the eastern Pacific. QuikSCAT wind speed, through its effect on the turbulent heat fluxes, is most important for the mean SST in the western Pacific. Finally, there were comparable correlations with observations of both SST and Z20 on the intra-seasonal time scale between the model forced with QuikSCAT winds and the model forced with NCEP2 winds.

Subsurface temperature data in the western North Atlantic Ocean are analyzed to study the variations in the heat content above a fixed isotherm and contributions from surface heat fluxes and oceanic processes. The study region is chosen based on the data density; its northern boundary shifts with the Gulf Stream position and its southern boundary shifts to contain constant volume. The temperature profiles are objectively mapped to a uniform grid (0.5° latitude and longitude, 10 m in depth, and 3 months in time).

The interannual variations in upper-ocean heat content show good agreement with the changes in the sea surface height from the Ocean Topography Experiment (TOPEX)/Poseidon altimeter; both indicate positive anomalies in 1994 and 1998–99 and negative anomalies in 1996–97. The interannual variations in surface heat fluxes cannot explain the changes in upper-ocean heat storage rate. On the contrary, a positive anomaly in heat released to the atmosphere corresponds to a positive upper-ocean heat content anomaly. The oceanic heat transport, mainly owing to the geostrophic advection, controls the interannual variations in heat storage rate, which suggests that geostrophic advection plays an important role in the air–sea heat exchange. The 18°C isotherm depth and layer thickness also show good correspondence to the upper-ocean heat content; a deep and thin 18°C layer corresponds to a positive heat content anomaly. The oceanic transport in each isotherm layer shows an annual cycle, converging heat in winter, and diverging in summer in a warm layer; it also shows interannual variations with the largest heat convergence occurring in even warmer layers during the period of large ocean-to-atmosphere flux.

A seasonal heat budget is based on observations that span the broad California Current (CC) region. Budget terms are estimated from satellite data (oceanic heat advection), repeat ship transects (heat storage rate), and the Comprehensive Ocean–Atmosphere Data Set (COADS) (surface heat flux). The balance between terms differs with distance from shore. Offshore, a local balance between the heat storage rate and net heat flux (Q₀) holds; the latter is dominated by its shortwave component Q_SW. Shoreward of 500 km, oceanic heat advection shifts the phase of the heat storage rate to earlier in the year and partially offsets an increase in Q₀ due to cloud clearing. During the summer maximum of Q₀, the 500-km-wide CC region loses heat to alongshore geostrophic transport, offshore Ekman transport, and, to a lesser degree, cross-shore geostrophic transport and eddy transport. The advective heat loss is neither uniform in space nor temporal phase; instead, the region of geostrophic and eddy heat loss expands cross shore with the annual widening of the California Current to 500 km. This expansion begins in spring with the onset of equatorward winds. A region of relatively positive wind stress curl widens at the same gradual rate as the CC, suggesting a coupling mechanism between the two.

A high resolution (0.08° at equator) HYbrid Coordinate Ocean Model (HYCOM) simulation is evaluated using observations for the period 1993–2003 for a western Pacific region containing the Kuroshio Extension (KE) (25–45°N and 135–180°E). Comparisons are made for the KE path and strength and for the upper ocean heat budget. The mean strength and path agree well with observations, except near the KE separation point, where the jet is as much as 2° of latitude too far south. However, the model path variations are considerably more energetic than observed, and this likely obscures the observed tendency for weaker meandering when the KE is strong. The model accurately reproduces seasonal variations of upper ocean heat content, but the long-period (about 10-yr) variations of heat content and KE strength clearly differ in the region upstream of 150°E. The long-period variations in model SSH do not show the same relationship to wind forcing that is seen in the observations and in a low-resolution ocean model simulation. The HYCOM upper ocean heat budget is similar to a diagnostic heat budget inferred from observations in that the dominant contribution is from lateral fluxes (advection); however, advection fluctuations (again primarily in the upstream region) are much larger in the model. This evaluation of HYCOM shows realistic mean quantities and realistic variations away from the separation region. However, an overly energetic jet upstream of 150°E can obscure the longer period variability and its contribution to the upper ocean heat budget.

We analyze a decade of sea surface height (SSH) measurements in the Southern Ocean from the TOPEX/Poseidon and ERS altimeters, with a focus on the variability at timescales <2 years. Among the different processes contributing to large-scale SSH variations, the barotropic response to the winds dominates poleward of 50°S, while thermosteric processes dominate equatorward, except for resonant basins for the barotropic modes and regions of intense eddy activity. A finite element barotropic model has been developed to analyze the vorticity budget. The SSH from the model agrees well with observations. The leading barotropic mode, which is annular and is confined near Antarctica, is responsible for most of the barotropic circumpolar transport. It is coherent with the zonally integrated eastward wind stress consistent with a free mode response. Although previously evidenced in bottom pressure data, this mode is only partially seen in altimeter data because of ice coverage. It nevertheless distinctly appears above the Pacific ridges where it expands meridionally up to midlatitudes. In the rest of the domain, several regions coherent with the local wind stress curl are found. These are regions isolated by f/H contours, mostly deep basins. An analysis of the vorticity budget shows that, generally, topographic Sverdrup balance is the leading process for periods ≥50 days, but in some regions (resonant basins), diffusive and nonstationary terms are important. A model experiment shows that transients redistribute energy along f/H waveguides, contributing to drain resonant regions, as was hypothesized in previous works.

Gridded hybrid turbulent heat flux fields were created by applying the state-of-the-art Coupled Ocean-Atmosphere Response Experiment (COARE) version 3.0 bulk algorithm to state variables (sea surface temperature, winds relative to currents, air temperature, and air specific humidity) derived from either numerical weather prediction (NWP) reanalysis (National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP-NCAR) reanalysis (NCEP1), NCEP reanalysis-2 (NCEP2), and 40-year European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA40)) or satellite sensors (QuikSCAT winds and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager microwave sea surface temperature). The most accurate source for each state variable was determined by comparing variables to tropical Pacific Tropical Atmosphere-Ocean (TAO) buoy observations for the years 2000–2001. The selected sources were as follows: QuikSCAT for winds relative to currents, ERA40 for air temperature and specific humidity, and TRMM Microwave Imager fusion product for sea surface temperature. Errors in latent and sensible heat fluxes to state variables were analyzed. Specific humidity errors contributed the most to errors in latent heat flux (LHF). Overall, the hybrid LHF product had a bias of –5.8 W m^-2 and a standard deviation of difference of 16.2 W m^-2, which is comparable to the accuracy of LHF derived from TAO measurements.

The differences between Tropical Atmosphere Ocean (TAO) anemometer and QuikSCAT scatterometer winds are analyzed over a period of 3 yr. Systematic differences are expected owing to ocean currents because the anemometer measures absolute air motion, whereas a radar measures the motion of the air relative to the ocean. Monthly averaged collocated wind differences (CWDs) are compared with available near-surface current data at 15-m depth from drifters, at 25-m depth from acoustic Doppler current profilers (ADCPs), and at 10-m depth from current meters and with geostrophic currents at the surface from the TOPEX/Poseidon radar altimeter. Because direct current observations are so sparse, comparisons are also made with climatological currents from these same sources. Zonal CWDs are in good agreement with the zonal current observations, particularly from 2°S to 2°N where there are strong currents and a robust seasonal cycle, with the altimeter-derived anomalous currents giving the best match. At higher latitudes there is qualitative agreement at buoys with relatively large currents. The overall variance of the zonal component of the CWDs is reduced by approximately 25% by subtracting an estimate of the zonal currents. The meridional CWDs are nearly as large as the zonal CWDs but are unpredictable. The mean CWDs show a robust divergence pattern about the equator, which is suggestive of Ekman currents, but with unexpectedly large magnitudes.

Coefficients for estimating climatological zonal surface currents from the altimeter at the TAO buoys are tabulated: the amplitudes and phases for the annual and semiannual harmonics, and a linear regression against the Southern Oscillation index, are combined with the mean from the drifter currents. Examples are shown of the application of these estimators to data from SeaWinds on the Midori satellite. These estimators are also useful for deriving air–sea fluxes from TAO winds.

A simple three-dimensional thermodynamic model is used to study the heat balance in the Gulf Stream region (30°–45°N, 40§–75°W) during the period from November 1992 to December 1999. The model is forced by surface heat fluxes derived from NCEP variables, with geostrophic surface velocity specified from sea surface height measurements from the TOPEX/Poseidon altimeter and Ekman transport specified from NCEP wind stress. The mixed layer temperature and mixed layer depth from the model show good agreement with the observations on seasonal and interannual time scales. Although the annual cycle of the upper-ocean heat content is underestimated, the agreement of the interannual variations in the heat content and the sea surface height are good; both are dominated by the large decrease from 1994 to 1997 and the increase afterward. As expected from previous studies, the surface heat flux dominates the seasonal and interannual variations in the mixed layer temperature. However, interannual variations in the upper-ocean heat content are dominated by the advection–diffusion term. Within the advection term itself, the largest variations are from the geostrophic advection anomaly. In the western Gulf Stream region the largest component of anomalous advection is the advection of the anomalous temperature by the mean current; elsewhere, the advection of the mean temperature by the anomalous current is also important. Other studies have shown that upper-ocean heat content is a more robust indicator of the potential contribution of the ocean to interannual heat flux anomalies than is sea surface temperature. The analysis here shows that the dominant term in interannual variations in heat content in the Gulf Stream region is anomalous advection by geostrophic currents. In fact, these ocean-forced variations in heat content appear to force air–sea fluxes: the surface heat flux anomalies in the western Gulf Stream region are negatively correlated with the anomalous upper-ocean heat content, that is, a large heat loss to the atmosphere corresponding to a positive heat content anomaly.

Satellite-based radars called scatterometers are yielding a wealth of new data about wind and ocean currents. In her Perspective, Kelly discusses the report by Chelton et al., in which scatterometer data are analyzed to show the importance of small-scale wind structures in interactions between wind and ocean. What is needed now, writes Kelly, is a sustained program of such observations to provide data for improved weather and climate prediction.

Part of the heat transported poleward from the Tropics by the ocean is stored near the energetic western boundary currents. These storage reservoirs provide a source of interannual to decadal climate fluctuations through their impact on the ocean–atmosphere heat fluxes. Changes in ocean heat storage result from the difference between surface fluxes and the convergence of oceanic heat transport. To estimate the heat budget for 26°–40°N, 140°E–180°, sea surface temperature and subsurface temperatures are assimilated into a one-dimensional model of the upper ocean that is forced by heat fluxes from the NCEP–NCAR reanalysis. Heat transport convergences are inferred as the residual of the heat budget for the period 1970–2000 using the "unknown control" from a Kalman filter/smoother technique. The estimates of heat transport convergence compare qualitatively with direct estimates from a three-dimensional model that uses geostrophic currents from the TOPEX/Poseidon radar altimeter for 1993–99; this period contains the largest lateral fluxes and the largest heat loss from the ocean in the 31-yr record. The analysis of the heat budget demonstrates that, on interannual to decadal time scales, the heat storage rate in the upper ocean is better correlated with lateral heat transport convergence than with surface fluxes. In addition, heat content and surface flux are negatively correlated, demonstrating the dominance of oceanic feedback over atmospheric forcing. The close relationship between heat content and surface fluxes suggests the possibility of predicting surface flux anomalies: there is a small but significant skill in predicting surface flux anomalies up to one year in advance using heat content. SST has no prediction skill.

Strong western boundary currents in the midlatitude oceans transport heat from the warm tropical regions to the cool subpolar regions and are responsible for much of the ocean's share of the mean heat transport. Recent studies of the upper ocean heat budget point to the importance of interannual-to-decadal heat transport and heat storage fluctuations by these current systems and their link to air-sea heat fluxes. An observed coherence between the North Atlantic and North Pacific wind forcing, heat storage, and heat fluxes suggests an annular coupled mode. Measurements of sea surface height (SSH) anomalies by the TOPEX/Poseidon radar altimeter have provided an unprecedented 10-year time series of variations in the western boundary currents. Currents derived from these SSH anomalies have been used in parallel studies of the upper ocean heat budget for the regions surrounding the Kuroshio Extension in the North Pacific and the Gulf Stream in the North Atlantic.

The heat budgets reveal several important results: 1) these regions store a large amount of heat for periods of several years, 2) heat storage is controlled by the transport of heat into the region by the currents, rather than by air-sea fluxes, 3) the currents that transport most of the heat are part of the large-scale geostrophic circulation, rather than locally forced Ekman currents, and 4) the surface fluxes are controlled by the ocean's heat storage, rather than the other way around. Surprisingly, a comparison of the 10-year heat budgets for the Pacific and the Atlantic show that the fluctuations in heat content and heat transport convergence are in phase, suggestive of repeat periods of about 6 years. This connection must be provided by the in-phase component in atmospheric circulation, the Northern Annular Mode (or Arctic Oscillation). Assuming that the relationships between ocean currents, heat transport, and heat storage are consistent over time with those revealed by the ten-year altimeter record, we examine heat storage estimated from the relatively sparse oceanic subsurface temperature data with estimates of winds and surface fluxes from the NCEP Reanalysis. The dominant modes of variability of both heat content and surface heat fluxes north of 3 ON have their largest values in the western boundary current regions. The relationships are consistent with wind-forced heat flux convergence in the western boundary currents, and surface heat flux anomalies forced by these convergences. The large component of this response that is correlated between the North Atlantic and the North Pacific constitutes the oceanic part of the Northern Annular Mode. These 10-year budgets used NCEP air-sea flux products. Recent analyses using scatterometer winds suggest that the air-sea coupling may be even greater than that which is estimated using numerical weather prediction (NWP) products, such as NCEP. NWP products neglect the effect of ocean currents on the relative wind speed needed for flux estimates; these currents have speeds up to 2 meters per second in these energetic ocean regions. In addition the commonly used SST products used by NWPs do not resolve the boundary currents well, so that the fluxes do not resolve the intense feedbacks possible in ocean frontal regions.

More than 6 years of measurements from the TOPEX/POSEIDON (T/P) altimeter are used to study the seasonal and interannual variations of the geostrophic velocity anomalies in the Middle Atlantic Bight region. Geostrophic velocities from T/P data are compared with the simultaneous low-pass filtered current meter data. The correlations are all above 95% significance for the three current meter observations at the 1000-m, 2210-m, and 2990-m isobaths. The seasonal mean geostrophic currents from 63°–75°W show coherent variations along isobaths, with seasonal reversals: toward the southwest during the winter and toward the northeast during the summer. The EOF analysis indicates that the seasonal reversals disappeared during 1996. This disruption is part of the intensification of the slope sea gyre and is related to the southward shift of the Gulf Stream, which acts as the boundary between the subpolar and subtropical gyres. The Gulf Stream moved farther south during 1996–1998. The variations in the Gulf Stream position may be caused by the wind stress/wind stress curl change.

An isopycnal model of the North Pacific is used to demonstrate that the seasonal cycle of heating and cooling and the resulting mixed layer depth entrainment and detrainment cycle play a role in the propagation of wind-driven Rossby waves. The model is forced by realistic winds and seasonal heat flux to examine the interaction of nearly annual wind-driven Rossby waves with the seasonal mixed layer cycle. Comparison among four model runs, one adiabatic (without diapycnal mixing or explicit mixed layer dynamics), one diabatic (with diapycnal mixing and explicit mixed layer dynamics), one with the seasonal cycle of heating only, and one with only variable winds suggests that mixed layer entrainment changes the structure of the response substantially, particularly at midlatitudes. Specifically, the mixed layer seasonal cycle works against Ekman pumping in the forcing of first-mode Rossby waves between 17° and 28°N. South of there the mixed layer seasonal cycle has little influence on the Rossby waves, while in the north, seasonal Rossby waves do not propagate. To examine the first baroclinic mode response in detail, a modal decomposition of the numerical model output is done. In addition, a comparison of the forcing by diapycnal pumping and Ekman pumping is done by a projection of Ekman pumping and diapycnal velocities on to the quasigeostrophic potential vorticity equation for each vertical mode. The first baroclinic mode's forcing is split between Ekman pumping and diapycnal velocity at midlatitudes, providing an explanation for the changes in the response when a seasonal mixed layer response is included. This is confirmed by doing a comparison of the modal decomposition in the four runs described above, and by calculation of the first baroclinic mode Rossby wave response using the one-dimensional Rossby wave equation.

Processes responsible for the seasonal and interannual variations of the sea surface temperature as well as of the heat content of the upper ocean (0–400 m) in the Kuroshio Extension region are examined from a 3D advection–diffusion model in finite elements, with an embedded bulk mixed layer. The geostrophic velocity is specified externally from TOPEX/Poseidon altimeter data, and Ekman velocity is specified from NCEP wind stress. The thermal field from the model shows good agreement with observations. While both atmospheric and oceanic processes are required to explain observed nonseasonal SST changes, the interannual heat storage rate is dominated by horizontal advection. In particular, the transition between an elongated and a contracted state of the Kuroshio caused by geostrophic advection has a clear signature in the SST. There is an indication that this process is accompanied by consistent changes in nonseasonal entrainment: when the Kuroshio is in an elongated state and warmer waters are present below the mixed layer, entrainment appears less efficient in exporting heat out of the mixed layer.

Westward propagating waves in the North Pacific Ocean from 10–16°N are overwhelmed by a zonally coherent response at the annual period, as observed in sea surface height (SSH) anomalies from the TOPEX/POSEIDON altimeter. SSH from a simple model of wind-forced Rossby waves and from seasonal heating are compared with observed SSH to understand the processes responsible for the observed signal. The seasonal heating cycle is out-of-phase and too weak to explain the SSH. The oceanic response to wind stress curl forcing more closely resembles the observations, but the response to NCEP Reanalysis winds does not show a strong annual cycle. Wind stress curl from the QuikSCAT/SeaWinds scatterometer has a strong and zonally coherent annual cycle that produces a corresonding strong annual signal in SSH. The model forced by scatterometer winds demonstrates that the response to Ekman pumping is the source of the strong annual cycle in the SSH.

Satellite-mounted radar scatterometers designed to quantify surface winds over the ocean actually measure the relative motion between the air and the ocean surface. Estimates of the wind stress from conventional surface wind measurements are usually derived neglecting ocean currents. However, when the relative motion is used, the differences in the estimated stress can be as large as 50% near the equator and may even reverse sign during an El Nino. This assertion is supported by the strong relationship between the surface currents measured by the Tropical Atmosphere–Ocean (TAO) array in the Pacific Ocean and the differences between the winds estimated from scatterometer data and those measured by TAO anemometers. The fact that the scatterometer measures relative motion, and not wind alone, makes scatterometer-derived stress a more accurate representation of the boundary condition needed for both atmospheric and oceanic models than stress fields derived neglecting ocean currents.

There was an opportunity to compare 10 months of collocated National Aeronautics and Space Administration scatterometer (NSCAT) wind vectors with those from the Tropical Atmosphere Ocean (TAO) buoy array, located in the tropical Pacific Ocean. Over 5500 data pairs, from nearly 70 buoys, were collocated in the calibration/validation effort for NSCAT. These data showed that the wind speeds produced from the NSCAT-1 model function were low by about 7%–8% compared with TAO buoy winds. The revised model function, NSCAT-2, produces wind speeds with a bias of about 1%. The scatterometer directions were within 20° (rms), meeting accuracy requirements, when compared to TAO data. The mean direction bias between the TAO and the NSCAT vectors (regardless of model function) is about 9° with the scatterometer winds to the right of the TAO winds, which may be due to swell. The statistics of the two datasets are discussed, using component biases in lieu of the speed bias, which is naturally skewed. Using ocean currents and buoy winds measured along the equator, it is shown that the scatterometer measures the wind relative to the moving ocean surface. In addition, a systematic effect of rain on the NSCAT wind retrievals is noted. In all analyses presented here, winds less than 3 m s^-1 are removed, due to the difficulty in making accurate low wind measurements.

The Pacific North Equatorial Countercurrent (NECC) is generally not well simulated in numerical models. In this study, the causes of this problem are investigated by comparing model solutions to observed NECC estimates. The ocean model is a general circulation model of intermediate complexity. Solutions are forced by climatological and interannual wind stresses, τ = (τ^x,τ^y), from Florida State University and the European Centre for Medium-Range Weather Forecasts. Estimates of the observed NECC structure and transport are prepared from expendable bathythermograph data and from the ocean analysis product of NOAA/National Centers for Environmental Prediction.

In solutions forced by climatological winds, the NECC develops a discontinuity in the central Pacific that is not present in the observations. The character of the error suggests that it arises from the near-equatorial (5°S–5°N) zonal wind stress, τ^x, being relatively too strong compared to the y derivative of the wind stress curl term, (curlτ)_y, associated with the intertropical convergence zone. This is confirmed in solutions forced by interannual winds, which exhibit a wide range of responses from being very similar to the observed NECC to being extremely poor, the latter occurring when near-equatorial τ^x is relatively too strong. Results show further that the model NECC transport is determined mainly by the strength of (curlτ)_y, but that its structure depends on near-equatorial τ^x; thus, NECC physics involves equatorial as well as Sverdrup dynamics. Only when the two forcing features are properly prescribed do solutions develop a NECC with both realistic spatial structure and transport.