Wind Stress Variations Research Articles

The wind work input into the global ocean revealed by a 17-year global HYbrid coordinate ocean model reanalysis

A 17-year (1999–2015) HYbrid Coordinate Ocean Model (HYCOM) reanalysis is used to calculate the wind work to the global oceanic circulation with a focus on the global integral of the wind work on the ageostrophic currents. While the wind work on the geostrophic currents estimate of the study is in good agreement with previous research, our estimation of the wind work on the ageostrophic currents (5.44 TW) is significantly larger than previous estimates. The two main reasons for this difference are: (1) the thickness of HYCOM's first vertical layer is 1 m and thus the surface ageostrophic currents are better represented than those in numerical models that use a thicker top layer; and (2) the reanalysis is forced with hourly surface stresses and thus contains wind work contributions from high frequency wind stress variations compared against previous estimates using daily wind stress.

Spatiotemporal dependence of Antarctic sea ice variability to dynamic and thermodynamic forcing: a coupled ocean–sea ice model study

Satellite-derived Antarctic sea ice extent has displayed a slight upward since 1979, but with strong temporal and regional variability—the drivers of which are poorly understood. Here, we conduct numerical experiments with a circum-Antarctic ocean–sea ice–ice shelf model driven by realistic atmospheric surface boundary conditions to examine the factors responsible for the temporal and spatial patterns in observed Antarctic sea ice variability. The model successfully reproduces observed seasonal and interannual variability in total sea ice extent and the temporal/spatial patterns of sea ice concentration and seasonality (days of advance and retreat and actual ice days) for 1979–2014. Sensitivity experiments are performed, in which the interannual variability in wind stress or thermodynamic surface forcing is ignored, to delineate their contributions to Antarctic sea ice fields. The results demonstrate that: (1) thermodynamic forcing plays a key role in driving interannual variability in sea ice extent and seasonality in most Antarctic sectors; (2) only in the Ross Sea the wind stress does become the main driver of sea ice extent variability; (3) thermodynamic forcing largely regulates interannual variability in the timing of sea ice advance, while wind stress largely controls the timing of the sea ice retreat; and (4) although both wind stress and thermodynamic forcing contribute to variability in total sea ice volume, the wind stress plays a dominant role in regulating sea ice volume variability in the near-coastal zone.

Eddy energy sources and mesoscale eddies in the Sea of Okhotsk

Based on an eddy-permitting ocean circulation model, the eddy kinetic energy (EKE) sources are studied in the Sea of Okhotsk. An analysis of the spatial distribution of the EKE showed that intense mesoscale variability occurs along the western boundary of the Sea of Okhotsk, where the East-Sakhalin Current extends. It was found a pronounced seasonally varying EKE with its maximal magnitudes in winter, and its minimal magnitudes in summer. An analysis of the EKE sources and the energy conversions showed that time-varying (turbulent) wind stress is a main contribution to mesoscale variability along the western boundary of the Sea of Okhotsk. The contribution of baroclinic instability to the generation of mesoscale variability predominates over that of barotropic instability along the western boundary of the Sea of Okhotsk. To demonstrate the mechanism of baroclinic instability, the circulation was considered along the western boundary of the Sea of Okhotsk from January to April 2005. An analysis of hydrological conditions showed outcropping isopycnals and being strong vertical shear of the along-shore velocity from January to May 2005. In April, mesoscale eddies are observed along the western boundary of the Sea of Okhotsk. It was established that seasonal variability of turbulent wind stress and the baroclinic instability of the East-Sakhalin Current are major reasons of mesoscale variability along the western boundary of the Sea of Okhotsk.

Wind Stress Mediated Variability of the Filchner Trough Overflow, Weddell Sea

AbstractThe Filchner Trough (FT) is a key site for exchange of water masses between the Weddell Sea continental shelf and the deep ocean. Cold and dense Ice Shelf Water (ISW), a precursor for Antarctic Bottom Water, flows north along the FT and overflows the Filchner Sill. Although access of warm water to the Weddell Sea continental shelf is limited due to the presence of the Antarctic Slope Front, southward transport of warm water is facilitated through the FT. We use moored current meters from the Filchner Sill region to show that the monthly scale variability of the ISW overflow is connected to the variability of the alongslope wind stress upstream. Periods with significant correlation between the wind and ISW overflow are characterized by (I) wind directed along the continental slope, (II) high ISW overflow speed, and (III) high variability in the 16–64 day period band for wind and current. We propose that a recirculation of the slope current, associated with the Antarctic Slope Front, may occur in the FT during periods of strong wind forcing, and that such recirculation could explain the correlation between the wind stress and the ISW overflow. We further show that an increased wind stress along the continental slope leads to increased current speed within the slope current and the Antarctic Coastal Current, with possible implications for the on‐shore heat transport.

Seasonality of Tides in Southeast Asian Waters

AbstractAn analysis of water level time series from 20 tide gauges in Southeast Asia finds that diurnal and semidiurnal astronomical tides exhibit strong seasonal variability of both amplitude and phase, which is not caused by known modulations of the astronomical tide-generating forces. Instead, it is found that the tidal properties are coherent with the western North Pacific monsoon index (WNPMI), indicating that monsoonal mechanisms are the likely cause. The study domain includes the Malacca Strait, Gulf of Thailand, the southern South China Sea, and Java Sea. The character of the geography and the tidal variability is different in each of these subregions. A new barotropic regional tide model is developed that incorporates the coupling between geostrophic currents, wind-driven (Ekman) currents, and tidal currents in the bottom boundary layer in order to examine the influence of these factors on tides. The dynamics thus preserve the frictional nonlinearities while neglecting advective nonlinearities and baroclinic tides, approximations that should be valid on the wide and shallow continental shelves in the study region. The model perturbation approach uses the climatological seasonal variability of wind stress and geostrophic currents, which are prescribed singly and in combination in the model, to explain the observed tidal variability. Results are most successful in the southern Gulf of Thailand and near Singapore, where it is found that the combined effect of geostrophic and Ekman currents shows increased skill in reproducing the tidal variability than individual models. Ambiguous results at other locations suggest more localized processes such as river runoff.

Evaluation of the zonal wind stress response to SST in the CMIP5 AMIP simulations

AbstractZonal wind stress plays an important role in the evolution of El Nino–Southern Oscillation (ENSO) events; however, a comprehensive comparison and analysis in terms of model performance and related bias in the interannual variability of zonal wind stress across the tropical Pacific has yet to be performed. In this study, the authors evaluate how well the individual atmospheric models participating in phase 5 of the Coupled Model Intercomparison Project simulate zonal wind stress. It is found that the wind stress anomalies simulated by the multi-model ensemble are weaker than those in the observation in both El Nino and La Nina events, with a larger bias in the former. Further analysis indicates that the bias associated with El Nino events may be mainly attributable to the weaker negative precipitation anomalies in the AMIP simulations, compared with observations, over the eastern Indian Ocean. Through the Gill-like responses in atmospheric circulation, the rainfall bias over the eastern Indian Ocea...

Reconstructing Tropical Pacific Sea Level Variability for the Period 1961–2002 Using a Linear Multimode Model

AbstractMonthly mean sea level anomalies in the tropical Pacific for the period 1961–2002 are reconstructed using a linear, multimode model driven by monthly mean wind stress anomalies from the NCEP/NCAR and ERA‐40 reanalysis products. Overall, the sea level anomalies reconstructed by both wind stress products agree well with the available tide gauge data, although with poor performance at Kanton Island in the western‐central equatorial Pacific and reduced amplitude at Christmas Island. The reduced performance is related to model error in locating the pivot point in sea level variability associated with the so‐called “tilt” mode. We present evidence that the pivot point was further west during the period 1993–2014 than during the period 1961–2002 and attribute this to a persistent upward trend in the zonal wind stress variance along the equator west of W throughout the period 1961–2014. Experiments driven by the zonal component of the wind stress alone reproduce much of the trend in sea level found in the experiments driven by both components of the wind stress. The experiments show an upward trend in sea level in the eastern tropical Pacific over the period 1961–2002, but with a much stronger upward trend when using the NCEP/NCAR product. We argue that the latter is related to an overly strong eastward trend in zonal wind stress in the eastern‐central Pacific that is believed to be a spurious feature of the NCEP/NCAR product.

The Effect of Alongcoast Advection on Pacific Northwest Shelf and Slope Water Properties in Relation to Upwelling Variability.

The Northern California Current System experiences highly variable seasonal upwelling in addition to larger basin‐scale variability, both of which can significantly affect its water chemistry. Salinity and temperature fields from a 7 year ROMS hindcast model of this region (43°N–50°N), along with extensive particle tracking, were used to study interannual variability in water properties over both the upper slope and the midshelf bottom. Variation in slope water properties was an order of magnitude smaller than on the shelf. Furthermore, the primary relationship between temperature and salinity anomalies in midshelf bottom water consisted of variation in density (cold/salty versus warm/fresh), nearly orthogonal to the anomalies along density levels (cold/fresh versus warm/salty) observed on the upper slope. These midshelf anomalies were well‐explained (R 2 = 0.6) by the combination of interannual variability in local and remote alongshore wind stress, and depth of the California Undercurrent (CUC) core. Lagrangian analysis of upper slope and midshelf bottom water shows that both are affected simultaneously by large‐scale alongcoast advection of water through the northern and southern boundaries. The amplitude of anomalies in bottom oxygen and dissolved inorganic carbon (DIC) on the shelf associated with upwelling variability are larger than those associated with typical variation in alongcoast advection, and are comparable to observed anomalies in this region. However, a large northern intrusion event in 2004 illustrates that particular, large‐scale alongcoast advection anomalies can be just as effective as upwelling variability in changing shelf water properties on the interannual scale.

A statistical significance test for sea-level variability

A statistical test is presented to address the null hypothesis that sea-level fluctuations in the open ocean are solely due to additive noise in the wind stress. The effect of high-frequency wind-stress variations can be represented as a correlated additive and multiplicative noise (CAM) stochastic model of sea-level variations. The main novel aspect of this article is the estimation of parameters in the discrete CAM model from time series of sea surface height observations. This leads to a statistical test, similar to the red noise [or AR(1)] test for sea surface temperature, which can be used to attribute specific sea-level variability to other effects than wind-stress noise. We demonstrate the performance of this test using altimeter data at several locations in the open ocean.

Spatiotemporal variation of vertical particle fluxes and modelled chlorophyll a standing stocks in the Benguela Upwelling System

Abstract Marine particle fluxes from high productive coastal upwelling systems return upwelled CO2 and nutrients to the deep ocean and sediments and have a substantial impact on the global carbon cycle. This study examines relations between production regimes on the shelf and over the continental margin of the Benguela Upwelling System (BUS) in the SE Atlantic Ocean. Data of composition and timing of vertical particle flux come from sediment trap time series (deployed intermittently between 1988 and 2014) in the regions Walvis Ridge, Walvis Bay, Luederitz and Orange River. We compare their seasonal variability to modelled patterns of chlorophyll concentrations in a 3-D ecosystem model. Both modelled seasonal chlorophyll a standing stocks and sampled particle flux patterns are highly correspondent with a bimodal seasonal cycle offshore the BUS. The material in the particle flux in offshore traps is dominantly carbonate (40–70%), and flux peaks in offshore particle flux originate from two independent events: in austral autumn thermocline shoaling and vertical mixing are decoupled from coastal upwelling, while fluxes in spring coincide with the upwelling season, indicated by slightly elevated biogenic opal values at some locations. Coastal particle fluxes are characterized by a trimodal pattern and are dominated by biogenic opal (22–35%) and organic matter (30–60%). The distinct seasonality in observed fluxes on the shelf is caused by high variability in production, sinking behaviour, wind stress, and hydrodynamic processes. We speculate that global warming will increase ocean stratification and alter coastal upwelling, so that consequences for primary production and particle flux in the BUS are inevitable.

Wind Forced Variability in Eddy Formation, Eddy Shedding, and the Separation of the East Australian Current

AbstractThe East Australian Current (EAC), like many other subtropical western boundary currents, is believed to be penetrating further poleward in recent decades. Previous observational and model studies have used steady state dynamics to relate changes in the westerly winds to changes in the separation behavior of the EAC. As yet, little work has been undertaken on the impact of forcing variability on the EAC and Tasman Sea circulation. Here using an eddy‐permitting regional ocean model, we present a suite of simulations forced by the same time‐mean fields, but with different atmospheric and remote ocean variability. These eddy‐permitting results demonstrate the nonlinear response of the EAC to variable, nonstationary inhomogeneous forcing. These simulations show an EAC with high intrinsic variability and stochastic eddy shedding. We show that wind stress variability on time scales shorter than 56 days leads to increases in eddy shedding rates and southward eddy propagation, producing an increased transport and southward reach of the mean EAC extension. We adopt an energetics framework that shows the EAC extension changes to be coincident with an increase in offshore, upstream eddy variance (via increased barotropic instability) and increase in subsurface mean kinetic energy along the length of the EAC. The response of EAC separation to regional variable wind stress has important implications for both past and future climate change studies.

Evaluation of Satellite and Reanalysis Wind Products with In Situ Wave Glider Wind Observations in the Southern Ocean

AbstractSurface ocean wind datasets are required to be of high spatial and temporal resolution and high precision to accurately force or be assimilated into coupled atmosphere–ocean numerical models and to understand ocean–atmospheric processes. In situ observed sea surface winds from the Southern Ocean are scarce and, consequently, the validity of simulation models is often questionable. Multiple wind data products were compared to the first known high-resolution in situ measurements of wind speed from Wave Glider (WG) deployments in the Southern Ocean with the intent to determine which blended satellite or reanalysis product best represents the magnitude and variability of the observed wind field. Results show that the ECMWF reanalysis product is the most accurate in representing the temporal variability of winds, exhibiting consistently higher correlation coefficients with in situ data across all wind speed categories. However, the NCEP–DOE AMIP-II Reanalysis product matches in situ trends of deviation from the mean and performs best in depicting the mean wind state, especially during high wind states. The ECMWF product also leads to smaller differences in wind speeds from the in situ data, while CFSv2 showed slightly higher biases and a greater RMSE. The SeaWinds (SW) product consistently performed poorly at representing the mean or wind stress variability compared to those observed by the WG. Overall, the study shows autonomous surface vehicles provide valuable observations by which to validate, understand, and potentially assist in correcting satellite/reanalysis products, particularly in remote regions, where few in situ estimates exist.

Numerical modeling of the autumnal thermal bar

Abstract The autumnal riverine thermal bar of Kamloops Lake has been simulated using atmospheric data from December 1, 2015, to January 4, 2016. The nonhydrostatic 2.5D mathematical model developed takes into account the diurnal variability of the heat fluxes and wind on the lake surface. The average values for shortwave and longwave radiation and latent and sensible heat fluxes were 19.7 W/m2, − 95.9 W/m2, − 11.8 W/m2, and − 32.0 W/m2 respectively. Analysis of the wind regime data showed prevailing easterly winds and maximum speed of 11 m/s on the 8th and 19th days. Numerical experiments with different boundary conditions at the lake surface were conducted to evaluate effects of variable heat flux and wind stress. The results of modeling demonstrated that the variable heat flux affects the process of thermal bar evolution, especially during the lengthy night cooling. However, the wind had the greatest impact on the behavior of the autumnal thermal bar: The easterly winds contributed to an earlier appearance of the thermal bar, but the strong winds generating the intensive circulations (the velocity of the upper lake flow increased to 6 cm/s) may destroy the thermal bar front.

The interannual sea level variability in the Indian Ocean as simulated by an Ocean General Circulation Model

ABSTRACTLimited coverage of tide gauge observations and limited length record of altimeter observations remain inadequate for the Indian Ocean sea level variability studies especially for longer periods. This study takes advantage of Ocean General Circulation Model (OGCM) simulations and ocean reanalysis to explore the interannual variability of sea level in the Indian Ocean. The model simulated sea level anomalies display good correlation with altimetry and reanalysis products. This consistency in model simulations with observations and reanalysis highlights the potential use of this OGCM in understanding the sea level variability. Analysis reveals that co‐occurrence years (years when Indian Ocean Dipole (IOD) and El Niño events co‐occur) contribute significantly towards the interannual variability in the Indian Ocean compared to pure IOD or El Niño events, and are characterized by persistent generation of open ocean downwelling, with maximum signals in the 10° to 5°S region. The OGCM experiments are carried out by modulating the forcing fields over the Pacific and the Indian Ocean to understand the physical mechanisms causing sea level variability. The experiments suggest that the remote forcing from the Pacific Ocean via Indonesian ThroughFlow (ITF) induce significant sea level changes in the southern Indian Ocean only with variability mostly in the decadal time scales and the northern and equatorial Indian Ocean are not affected by ITF. This study also shows that El Niño and IOD‐induced variations in zonal wind stress and wind stress curl mainly contribute the interannual sea level variations in the Indian Ocean. The forcing associated with the co‐occurrence years are multi‐phased with coastal and open ocean forcing. Whereas the pure IOD and El Nino forcing is mainly found only over the open ocean. The decadal variability in the ENSO is primarily responsible for the decadal sea level variability in the Indian Ocean by modulating ITF transport.

Biogeochemical modelling of dissolved oxygen in a changing ocean.

Secular decreases in dissolved oxygen concentration have been observed within the tropical oxygen minimum zones (OMZs) and at mid- to high latitudes over the last approximately 50 years. Earth system model projections indicate that a reduction in the oxygen inventory of the global ocean, termed ocean deoxygenation, is a likely consequence of on-going anthropogenic warming. Current models are, however, unable to consistently reproduce the observed trends and variability of recent decades, particularly within the established tropical OMZs. Here, we conduct a series of targeted hindcast model simulations using a state-of-the-art global ocean biogeochemistry model in order to explore and review biases in model distributions of oceanic oxygen. We show that the largest magnitude of uncertainty is entrained into ocean oxygen response patterns due to model parametrization of pCO2-sensitive C : N ratios in carbon fixation and imposed atmospheric forcing data. Inclusion of a pCO2-sensitive C : N ratio drives historical oxygen depletion within the ocean interior due to increased organic carbon export and subsequent remineralization. Atmospheric forcing is shown to influence simulated interannual variability in ocean oxygen, particularly due to differences in imposed variability of wind stress and heat fluxes.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

Interannual and Decadal Variability in Tropical Pacific Sea Level

A notable feature in the first 20-year satellite altimetry records is an anomalously fast sea level rise (SLR) in the western Pacific impacting island nations in this region. This observed trend is due to a combination of internal variability and external forcing. The dominant mode of dynamic sea level (DSL) variability in the tropical Pacific presents as an east-west see-saw pattern. To assess model skill in simulating this variability mode, we compare 38 Coupled Model Intercomparison Project Phase 5 (CMIP5) models with 23-year satellite data, 55-year reanalysis products, and 60–year sea level reconstruction. We find that models underestimate variance in the Pacific sea level see-saw, especially at decadal, and longer, time scales. The interannual underestimation is likely due to a relatively low variability in the tropical zonal wind stress. Decadal sea level variability may be influenced by additional factors, such as wind stress at higher latitudes, subtropical gyre position and strength, and eddy heat transport. The interannual variability of the Nino 3.4 index is better represented in CMIP5 models despite low tropical Pacific wind stress variability. However, as with sea level, variability in the Nino 3.4 index is underestimated on decadal time scales. Our results show that DSL should be considered, in addition to sea surface temperature (SST), when evaluating model performance in capturing Pacific variability, as it is directly related to heat content in the ocean column.

Mesoscale wind stress–SST coupling in the Kuroshio extension and its effect on the ocean

Mesoscale perturbations (with a size of 100–1000 km) of wind stress magnitude, divergence and curl in the Kuroshio Extension (KE) are observed to tightly link to those of sea surface temperature (SST), and downwind and crosswind SST gradients, respectively. Based on long-term satellite observational data, their empirical relationships are established, which are further used to represent mesoscale wind stress–SST coupling in an ocean model that is based on the Regional Oceanic Modelling Systems (ROMS). The strength of mesoscale perturbations of wind stress and SST is observed to display a consistent seasonal variability, with the maximum appeared in winter while the minimum appeared in summer. This seasonal variability characteristic is also successfully simulated by ROMS with high resolution. Through comparing two experiments with and without the mesoscale wind stress–SST coupling, it is found that the mesoscale wind stress perturbation (τ MS) has a negative feedback on SST perturbation (SSTMS). Analyses of sensitivity experiments suggest that the τ MS acts to inhibit SSTMS mainly by means of surface heat flux. The τ MS –SSTMS coupling also exerts influences on the ocean mean state and seasonal variability of SST in the KE. The effect of τ MS on the SST is distinct in autumn and winter when the mesoscale perturbations are most active. Analyses of sensitivity experiments demonstrate that the τ MS can affect the long term mean SST through either way of surface heat flux or momentum flux.

Spatio‐temporal variation in ocean current‐driven hatchling dispersion: Implications for the world's largest leatherback sea turtle nesting region

AbstractAimThe lives of juvenile leatherback turtles are amongst the most enigmatic of all marine mega‐vertebrates. For these cryptic organisms, ocean models provide important insights into their dispersion from natal sites. Here, corroborated by fisheries bycatch data, we simulate spatio‐temporal variation in hatchling dispersion patterns over five decades from the World's largest leatherback turtle nesting region.LocationEquatorial Central West Africa (3.5°N to −6°S) spanning the Gulf of Guinea in the North, Gabon and the Republic/Democratic Republic of the Congo in the South.ResultsDue to dynamic oceanic conditions at these equatorial latitudes, dispersion scenarios differed significantly: (1) along the north to south gradient of the study region, (2) seasonally and (3) between years. From rookeries to the north of the equator, simulated hatchling retention rates within the Gulf of Guinea were very high (>99%) after 6 months of drift, whilst south of the equator, retention rates were as low as c. 6% with the majority of simulated hatchlings dispersing west into the South Atlantic Ocean with the South Equatorial Current. Seasonal dispersion variability was driven by wind changes arising from the yearly north/southward migration of the intertropical convergence zone resulting in the increasing westerly dispersion of hatchlings throughout the hatching season. Annual variability in wind stress drove a long‐term trend for decreased retention within the Gulf of Guinea and increased westerly dispersion into habitats in the South Atlantic Ocean.Main conclusionsShifts in dispersion habitats arising from spatio‐temporal oceanic variability expose hatchlings to different environments and threats that will influence important life history attributes such as juvenile growth/survival rates; anticipated to impact the population dynamics and size/age structure of populations into adulthood. The impacts of local and dynamic oceanic conditions thus require careful considerations, such as subregional management, when managing marine populations of conservation concern.

Variability of the Wind Stress, the Field of Currents, Wind Stress Curl and Vorticity of Surface Currents in the North Atlantic

The role of the Atlantic Ocean in climate changes in the Atlantic-European region determines the importance of studying spatial-temporal variability of the field of currents and the wind field generating the circulation of water in the ocean. The analysis of literature shows that data on seasonal variability of currents in the North Atlantic are controversial and reliable information on the interannual variability of ocean circulation in some regions of the North Atlantic it is impossible to obtain due to the lack of long oceanographic observations. The purpose of this study was clarifying seasonal changes and analyzing of interannual changes of the response of circulation of the upper layer of the North Atlantic on atmospheric forcing (wind stress and wind stress curl) by homogeneous and long-term datasets of re-analysis. Features of seasonal variability of wind stress, velocities of currents and their vorticity in subtropical and subpolar areas of the North Atlantic are confirmed. To the north of 15-20° of north latitude, seasonal variability of wind stress, velocities of currents and their vorticity generally accounts for less than one third of their total variability, the remaining dispersion is due to interannual and decennial fluctuations. In the Subtropical Atlantic in January there was a trend toward increased seasonal variability of wind stress, velocities of currents and anticyclonic wind stress curl and vorticity of surface currents in the second half of the 20th century. In September, since the mid-1970s, there is the tendency towards the weakening of these values, which leads to a long-term increase in the amplitude of the annual cycle. In the Subpolar Atlantic since the late 1950s, during the months with extreme values in the annual cycle there is an increase in seasonal variability of wind stress, velocities of currents and cyclonic wind stress curl and vorticity of surface currents. While, a significant increase in the amplitude of the annual cycle it is not revealed.

Interannual variability of tropical Pacific sea level from 1993 to 2014

AbstractA multimode, linear reduced‐gravity model, driven by ERA‐Interim monthly mean wind stress anomalies, is used to investigate interannual variability in tropical Pacific sea level as seen in satellite altimeter data. The model output is fitted to the altimeter data along the equator, in order to derive the vertical profile for the model forcing, showing that a signature from modes higher than mode 6 cannot be extracted from the altimeter data. It is shown that the model has considerable skill at capturing interannual sea level variability both on and off the equator. The correlation between modeled and satellite‐derived sea level data exceeds 0.8 over a wide range of longitudes along the equator and readily captures the observed ENSO events. Overall, the combination of the first, second, third, and fifth modes can provide a robust estimate of the interannual sea level variability, the second mode being dominant. A remarkable feature of both the model and the altimeter data is the presence of a pivot point in the western Pacific on the equator. We show that the westward displacement of the pivot point from the center of the basin is strongly influenced by the fact that most of the wind stress variance is found in the western part of the basin. We also show that the Sverdrup transport is not fundamental to the dynamics of the recharge/discharge mechanism in our model, although the spatial structure of the wind forcing does play a role in setting the amplitude of the “warm water volume.”