A diagnostic model of the tropical circulation over the 0-30-m layer is derived by using quasi-linear and steady physics. The horizontal velocity is directly estimated from sea surface height (TOPEX/Poseidon), surface vector wind (SSM/I) and sea surface temperature (AVHRR ϩ in situ measurements). The absolute velocity is completed using the mean dynamic height inferred from the World Ocean Atlas (WOA). The central issue investigated in this study is the more accurate estimate of equatorial surface currents relative to prior satellitederived method. The model formulation combines geostrophic, Ekman, and Stommel shear dynamics, and a complementary term from surface buoyancy gradient. The field is compared with velocity observations from 15-m-depth buoy drifter and equatorial Tropical Ocean-Atmosphere (TAO) current meters. Correlations with TAO data on the equator are much higher in the eastern Pacific cold tongue than before. The mean field in the cold tongue is also much more accurate, now showing the equatorial minimum that splits the South Equatorial Current into northern and southern branches. The mean current strength is somewhat less than in drifter composites because the mean dynamic topography from WOA remains too smooth. However, the seasonal cycle and interannual variations are robust, especially anomalies on the order of 1 m s Ϫ1 during the 1997-98 ENSO. This direct method using satellite measurements provides surface current analyses for numerous research and operational applications.

Comparisons of OSCAR satellite-derived sea surface currents with in situ data from moored current meters, drifters, and shipboard current profilers indicate that OSCAR presently provides accurate time means of zonal and meridional currents, and in the near-equatorial region reasonably accurate time variability (correlation ϭ 0.5-0.8) of zonal currents at periods as short as 40 days and meridional wavelengths as short as 8°. At latitudes higher than 10°the zonal current correlation remains respectable, but OSCAR amplitudes diminish unrealistically. Variability of meridional currents is poorly reproduced, with severely diminished amplitudes and reduced correlations relative to those for zonal velocity on the equator. OSCAR's RMS differences from drifter velocities are very similar to those experienced by the ECCO (Estimating the Circulation and Climate of the Ocean) data-assimilating models, but OSCAR generally provides a larger ocean-correlated signal, which enhances its ratio of estimated signal over noise. Several opportunities exist for modest improvements in OSCAR fidelity even with presently available datasets.

[1] The coast of central Chile is characterized by intermittent low-level along-shore southerly wind periods, called coastal jets (CJs). In this study, we take advantage of long-term satellite data to document the CJs characteristics over 2000-2007 and investigate its impact on upwelling. The CJ structure has a core some 100 km from the shore and a cross-shore scale of $160 km, and it usually lasts for several days (3-10). Its period of occurrence ranges from weekly to a few months. On the basis of covariance analyses between wind stress and sea surface temperature (SST) anomalies, it is found that CJ activity is seasonally phase locked with SST, with a peak season in August-October. The statistically dominant forcing mechanisms of the SST cooling during CJ event is a combination of seaward advection of temperature resulting from Ekman transport, air-sea heat exchange, and Ekman-driven coastal divergence. However, case studies of two events suggest a significant sensitivity of the dominant upwelling forcing mechanisms to the background conditions. For instance, the upward Ekman pumping associated with cyclonic wind stress curl is enhanced for the event with the CJ located more to the south. Although there are limitations associated with both the formulation of the heat budget and the data sets, the results illustrate the complexity of the upwelling forcing mechanisms in this region and the need for realistic high-resolution forcing fluxes. A CJ activity index is also proposed that takes into account the coastal upwelling variability, which can be used for teleconnection studies.

[1] Climatological surface salinities and satellite-derived surface currents are combined to estimate the horizontal and vertical time mean and seasonal salinity divergence in the tropical ocean's surface layer. The mean salinity divergence has magnitude and spatial patterns matching major features of an estimate of evaporation minus precipitation averaged from several atmospheric general circulation model reanalyses and satellitederived products (spatial correlation of 0.63). Thus a substantial portion of the atmospheric freshwater flux appears to be balanced by ocean transports in a relatively thin (32.5 m) surface layer. Large regional discrepancies remain, however, and their magnitude is greater than the scatter among the various atmospheric forcing products used in this study. The present salinity divergence calculation is imprecise because the mean salinity gradients have been weakened by large-scale smoothing of sparse and unevenly sampled data. Further, other investigators show that higher-frequency ocean processes (e.g., vertical and horizontal mixing of salinity) are significant at least locally. Nevertheless, several areas of consistent disagreement between mean flow salinity divergence and atmospheric forcing, such as excess evaporation in subtropical gyres, are suggestive of errors in the forcing. In particular, the shortfall of salinity convergence balancing high precipitation in the western Pacific is roughly consistent with regional surface flux corrections required to bring ocean general circulation model integrations into long-term balance in this region. Thus this shortfall may reflect either overlarge precipitation estimates or a fundamental misunderstanding of how the ocean disperses this flux. Ultimately, further ocean physics must be addressed before firm conclusions can be drawn about the fraction of atmospheric freshwater input unaccounted for by the near-surface mean flow and seasonal salinity divergence. High-resolution space-based salinity measurements will be necessary to provide the temporal and spatial coverage necessary to develop ocean salinity transport calculations into useful constraints on ocean processes, surface hydrological forcing, and model simulations.

The relationships between tropical Atlantic Ocean surface currents and horizontal (mass) divergence, sea surface temperature (SST), and winds on monthly-to-annual time scales are described for the time period from 1993 through 2003. Surface horizontal mass divergence (upwelling) is calculated using surface currents estimated from satellite sea surface height, surface vector wind, and SST data with a quasi-linear, steadystate model. Geostrophic and Ekman dynamical contributions are considered. The satellite-derived surface currents match climatological drifter and ship-drift currents well, and divergence patterns are consistent with the annual north-south movement of the intertropical convergence zone (ITCZ) and equatorial cold tongue evolution. While the zonal velocity component is strongest, the meridional velocity component controls divergence along the equator and to the north beneath the ITCZ. Zonal velocity divergence is weaker but nonnegligible. Along the equator, a strong divergence (upwelling) season in the central/eastern equatorial Atlantic peaks in May while equatorial SST is cooling within the cold tongue. In addition, a secondary weaker and shorter equatorial divergence occurs in November also coincident with a slight SST cooling. The vertical transport at 30-m depth, averaged across the equatorial Atlantic Ocean between 2°S and 2°N for the record length, is 15(Ϯ6) ϫ 10 6 m 3 s Ϫ1 . Results are consistent with what is known about equatorial upwelling and cold tongue evolution and establish a new method for observing the tropical upper ocean relative to geostrophic and Ekman dynamics at spatial and temporal coverage characteristic of satellite-based observations.