David Adamec

The Seasonal Upwelling in the Gulf of Guinea Due to Remote Forcing

Abstract A linear model on an equatorial β plane is integrated over a 120-day period in a basin that approximates the tropical Atlantic Ocean. An increase in the westward wind stress of 0.025 N m−2 in the western Atlantic excites an equatorially trapped Kelvin wave that propagates eastward along the equator, moves poleward at the eastern boundary, and produces upwelling throughout the Gulf of Guinea. Cases that study the effects of nonlinearities and the inclusion of a northward wind stress are included. Nonlinearities are shown to have the effect of amplifying the effects of the Kelvin wave and prolonging the upwelling event. The inclusion of a southerly wind stress in the eastern basin provides a secondary mechanism for upwelling south of the equator along the eastern basin. Local winds. cannot account for the seasonal upwelling in the Gulf of Guinea. The simple baroclinic ocean model is integrated from rest. The effects of mean currents and bottom topography are not considered in detail.

Correlations between surface chlorophyll and sea surface height in the tropical Pacific during the 1997–1999 El Niño‐Southern Oscillation event

Correlations between Sea-viewing Wide Field-of-view Sensor (SeaWiFS) surface chlorophyll and TOPEX sea surface height (SSH) are examined in the tropical Pacific (30°S-30°N) using empirical orthogonal function (EOF) analysis both separately and jointly on the two fields. This analysis is done on data from September 1997, the start of data from the SeaWiFS satellite, through December 1999, a time period dominated by the El Nino-Southern Oscillation. Four distinct biological responses are observed. The dominant response is a symmetric off-equatorial chlorophyll increase during La Nina that extends between 2° and 18° latitude from the eastern Pacific to the date line. The chlorophyll mode is tightly correlated to SSH, suggesting that the chlorophyll increase is a result of the shoaling thermocline, which increases the surface nutrient supply. The better known equatorial decrease in chlorophyll during El Nino is seen in a separate EOF mode. Using acoustic Doppler current profiler data from the Tropical Atmosphere-Ocean/Triton array, it is shown that the cessation of the El Nino equatorial chlorophyll minimum is tied to the recommencement of the iron-rich Equatorial Undercurrent which occurs several months prior to the termination of the El Nino. There is an off-equatorial bloom during the peak of the El Nino between 120°W-180°W and 8°N-15°N. This bloom occurs within the area covered by the previously mentioned La Nina bloom, but it is more localized, and its fluctuations appear correlated with changes in the North Equatorial Counter Current. The shoaling thermocline in the western warm pool during El Nino results in a chlorophyll bloom that extends from the Philippines to 155°E between 0°N and 15°N. This bloom terminates in unison with the end of the El Nino when elevated SSH is reestablished in the western basin.

Dynamics of the seasonal variations in the Indian Ocean from TOPEX/POSEIDON sea surface height and an ocean model

Observations from the TOPEX/POSEIDON (T/P) altimeter reveal that the sea level in the Indian Ocean varies strongly with season. Surface undulations propagate eastward along the equator and westward outside the equatorial waveguide. The forcing mechanisms responsible for such variations are investigated in this paper by analyzing T/P data and surface wind-stress forcing, and by using a nonlinear, 2.5-layer, reduced gravity model. Both the data and the model show that Kelvin and Rossby waves forced by the Indian Ocean monsoon are primarily responsible for the seasonal sea-level change in the tropical and the subtropical Indian Ocean.

Modulation of the seasonal signal of the Kuroshio Extension during 1994 from satellite data

Interannual modulation of the seasonal signal in the Kuroshio Extension region is studied using TOPEX altimeter and advanced very high resolution radiometer (AVHRR) data. Meridional profiles of the surface eddy kinetic energy indicate a significant drop in eddy energy levels beginning early in 1994 and persisting until the summer of 1995. The low eddy energy levels are most apparent during the summer and fall 1994. The contribution of the convergence of Reynolds stresses to the zonal momentum budget, -(u′v′¯)y, is much weaker in 1994 than in either 1993 or 1995, especially during summer. The low values of Reynolds stresses lead the low eddy kinetic energy values by about a season, consistent with the characteristic timescale of the eddy energy, which is about 90–110 days in this region. AVHRR-derived estimates of sea surface temperature indicate that the subpolar gyre was cooler during summer 1994 relative to the summers of 1993 and 1995. Calculations of the baroclinic zonally symmetric circulation indicate that the Kuroshio Extension was frontolytic during summer 1994, which could account for the observed drop in eddy kinetic energy. In particular, convergence of heat due to eddy flow appears to play a crucial role in the interannual modulation of the seasonal signal of the Kuroshio Extension.

The Time-varying Characteristics of the Meridional Ekman Heat Transport for the World Ocean

Abstract The time-varying meridional Ekman heat transport for the World Ocean is calculated for the 30-year period 1960–1989 using wind stress and sea surface temperature data from the Comprehensive Ocean–Atmosphere Data Set The average monthly heat transports are dominated by poleward transport of heat in the tropical Pacific and Atlantic oceans. In the Indian Ocean, the strong monsoonal flow during summer is responsible for a large equatorward transport. If seasonal effects are removed, the tropical Pacific displays the most variance in Ekman heat transport and the Atlantic the least. Two sets of empirical orthogonal functions (EOFs) are computed to investigate the nonseasonal variability of the Ekman transport The first set, which uses the eigenvector decomposition of the covariance matrix, emphasizes covariability in the worlds tropical oceans. The time series of the EOF amplitudes contain a clear signal that can be directly related to El Nino. The El Nino signal is manifest as an increase in the equ...

Meridional Ekman Heat Transport: Estimates from Satellite Data

Abstract Analyses of satellite-derived SSM/I winds and AVHRR sea surface temperatures are used to compute weekly estimates of global meridional ocean Ekman heat transport for the 4-year period 1987–1991. The heat transport is consistently poleward throughout the year over the Atlantic and much of the Pacific between 30°S and 30°N and equatorward at higher latitudes. The zonally integrated Ekman heat transport in the Pacific was weak and equatorward at 10°N in September 1989 and 1990, whereas in other years it is poleward throughout the year. In the Indian Ocean, equatorward heat transport was strongest in Northern Hemisphere summer 1990. The weekly time series provides better temporal resolution than previous studies that at best used monthly averages. The higher-frequency variations are explored through rotated empirical orthogonal functions (REOFs) of nonseasonal heat transport anomalies. The REOFs show large-scale coherence across the tropical and subtropical Pacific and Indian Oceans. The first REOF h...

Assessing the Potential to Derive Air-Sea Freshwater Fluxes from Aquarius-Like Observations of Surface Salinity

A state‐of‐the‐art numerical model is used to investigate the possibility of determining freshwater flux fields from temporal changes in sea‐surface salinity (SSS), a goal of the satellite salinity‐measuring mission, Aquarius/SAC‐D. Because the estimated advective temporal scale is usually longer than the Aquarius/SAC‐D revisit time, the possibility of producing freshwater flux estimates from temporal salinity changes is first examined by using a correlation analysis. For the mean seasonal cycle, the patterns of the correlations between the freshwater fluxes and surface salinity temporal tendencies are mainly zonally oriented, and are highest where the local precipitation is also relatively high. Nonseasonal (deviations from the monthly mean) correlations are highest along mid‐latitude storm tracks and are relatively small in the tropics. The complex correlation patterns presented here suggest that a global retrieval of the difference between evaporation and precipitation (E–P) from salinity changes requires more complex techniques than a simple consideration of a local balance with surface forcing.

Eddy flow characteristics and mean flow interactions in the North Pacific

The strength, shape, orientation, mean flow interactions, and propagation characteristics of the time-varying (eddy) flow determined from TOPEX/Poseidon altimeter data in the North Pacific are investigated using equivalent barotropic diagnostic calculations as presented by Hoskins et al. [1983]. Separate calculations for low- and high-pass flow fields, distinguished by periods greater than and less than 6 months, respectively, are performed and revealed a fundamental difference between the eddy characteristics of the two. The areas with largest values of surface eddy kinetic energy occur in the vicinity of the Kuroshio Extension and the subtropical front. Near the Kuroshio Extension the low-pass surface eddy kinetic energy is greater than the high-pass energy, but in the subtropical frontal area the high-pass eddy flow accounts for more energy than the low-pass flow. For both areas the values of eddy kinetic energy are larger in summer than during winter. Low-pass eddy flow tends to be aligned zonally, and the high-pass eddy flow is aligned meridionally. Near the subtropical front the ratio of major to minor axis length is typically ∼1.5∶1, whereas near the Kuroshio Extension that ratio increases to values near 2:1. Along the southern boundary of the subtropical gyre the high-pass eddy flow tends to propagate eastward relative to the mean flow, tending to decrease the gyre strength. However, the low-pass eddy flow propagates westward relative to the mean flow and tends to increase the gyre strength. An additional computation of surface heat transport is used to deduce vertical propagation tendencies of the eddy flow and indicates that the low-pass flow field transports more heat than the high-pass flow with the largest eddy transports occurring near the coast of Honshu. The sign of the transport indicates a preference for upward energy propagation of the eddy flow there. Many of the characteristics, such as the orientation and propagation tendencies of the eddy flow, are similar to eddy flow in the atmosphere and accentuate a similarity between the general circulations of the atmosphere and ocean.

Western Boundary Current Separation Sensitivity Studies Using a Quasigeostrophic Ocean Model

Abstract The sensitivity of the separation of the western boundary current of an idealized single gyre circulation to the specification of various model parameters is investigated through a series of quasigeostrophic simulations. The model parameters considered are the value of β, bottom topography, lateral boundary conditions, deformation radius of the first baroclinic mode, horizontal friction, and model resolution. Changes in these model parameters affect western boundary current separation characteristics, but those parameter changes also produced changes in several measures of the global flow field such as level of mean and eddy kinetic energies. A number of flow field statistics are correlated with the average separation point, and it was found that total average baroclinic kinetic energy is the most highly correlated variable to the average separation point, with higher levels of kinetic energy being correlated with more poleward penetration. In the immediate vicinity of the separation point, the a...

Improving Short-term Climate Forecasts with Satellite Observations

Understanding and predicting seasonal-to-interannual climate variations is a central goal within U.S. climate research. At NASAs Global Modeling and Assimilation Office, we are developing a coupled model forecast system to optimize the use of existing and planned satellite data, together with in situ observations, for experimental predictions of short-term climate variations. Our focus is on using satellite data to initialize the ocean and land surface, the slower components of the climate system that have the potential memory to enhance climate prediction. The longer temporal scales of the ocean and land surface are the key sources of memory in the coupled climate system that promise skill in predicting short-term climate variability. The focus within the GMAO is on optimizing the use of satellite altimeter data and evaluating the impact of planned satellite observations such as surface salinity data from Aquarius. Predictability experiments and hindcast tests that use observed precipitation to precondition the soil moisture distribution have also been conducted. The results indicate that the key to summertime precipitation forecasts over transition zones between dry and humid areas in tropical and mid-latitude regions (such as the central U.S.) lies in the initialization of soil moisture (1). Given these results, the GMAO system also focuses on the initialization of the land surface model, with developments undertaken for assimilation of soil moisture estimates from AMSR-E. III. THE FORECAST SYSTEM