Gérard Eldin

Equatorial Kelvin and Rossby waves evidenced in the Pacific Ocean through Geosat sea level and surface current anomalies

Equatorial Kelvin and Rossby waves are comprehensively demonstrated over most of the equatorial Pacific basin, through their signatures in sea level and zonal surface geostrophic current anomalies. This was made possible with altimeter data pertaining to the first year of the Geosat (Geodetic Satellite) 17-day exact repeat orbit (November 8, 1986, to November 8, 1987). To this end, along-track corrected Geosat sea level anomalies (SLAs), relative to the time period of interest, were first smoothed using nonlinear and linear filters. The original 17-day time step was then reduced by combining all ascending and descending tracks within 10° longitudinal bands. Finally, SLAs were gridded onto a regular grid, and low-pass filters were applied in latitude and time in order to smooth out remaining high-frequency noise. Anomalies of zonal surface geostrophic current were calculated using the first and second derivatives of the SLA meridional gradient, off and on the equator, respectively. Sea level and surface current anomalies are validated in the western equatorial Pacific with in situ data gathered during seven hydrographic cruises at 165°E, and through expendable bathythermograph and mooring measurements. Following their chronological appearance along the 165°E meridian, the major low-frequency SLAs and zonal surface current anomalies are described and explained in terms of the equatorial wave theory. An equatorial downwelling Kelvin wave, known to be the main oceanic signal of the 1986-1987 El Nino, is generated in December 1986, concomitant with a strong westerly wind anomaly occurring west of the dateline. The associated propagating equatorial SLAs correspond to an elevation of 15 cm. Independent estimates of this Kelvin wave phase speed are obtained through time-lag correlation matrix analysis (2.82 ± 0.96 m s−1) and the least squares fit of the SLA meridional structures to theoretical Kelvin wave shape (2.26 ± 1.02 m s−1). Both estimates indicate that the Kelvin wave has the characteristic of a first baroclinic mode. An equatorial upwelling Kelvin wave is then detectable in June 1987. It is characterized by a 10-cm sea level drop, propagating only from the western to the central equatorial Pacific. A first meridional mode (m = 1) equatorial upwelling Rossby wave crossing the entire Pacific basin from March 1987 (eastern part) to September 1987 (western part) shows up in SLAs and zonal surface current anomalies. Such a Rossby wave corresponds to propagating sea level drops which are extreme (−12 cm) at about 4°N and 4°S latitudes. The consequences on zonal surface geostrophic current are very important since, in the case of the upwelling, it dramatically decreases the three major surface currents (the North and South Equatorial Countercurrents, and South Equatorial Current) by an amplitude similar to their mean annual velocity values. The least squares fit of the Rossby wave SLA meridional structures to its theoretical m = 1 form cogently suggests the dominance of the first baroclinic mode (c = 2.59 ± 0.65 m s−1). This dominance is corroborated by an estimate of the Rossby wave phase speed (1.02 ± 0.37 m s−1), which roughly corresponds to the theoretical phase speed (c/2m + 1) of the m = 1 equatorial Rossby wave. It is suggested that the equatorial upwelling Rossby wave is mostly due to a reflection of an equatorial upwelling Kelvin wave generated in January 1987 near the dateline. Whether or not the overall propagating features are part of the 1986–1987 El Nino or belong to the “normal” seasonal cycle cannot be decided in the absence of longer altimeter sea level time series.

A whirling ecosystem in the equatorial Atlantic

[1] The equatorial Pacific and Atlantic oceans exhibit remarkable meridional undulations in temperature and chlorophyll fronts visible from space over thousands of kilometers and often referred to as tropical instability waves. Here, we present new observations of an ecosystem ranging through three trophic levels: phytoplankton, zooplankton and small pelagic fish whirling within a tropical vortex of the Atlantic ocean and associated with such undulations. Cold, nutrient and biologically rich equatorial waters are advected northward and downward to form sharp fronts visible in all tracers and trophic levels. The equatorward recirculation experiences upwelling at depth, with the pycnocline and ecosystem progressively moving toward the surface to reconnect with the equatorial water mass. The observations thus indicate that it is a fully three-dimensional circulation that dominates the distribution of physical and biological tracers in the presence of tropical instabilities and maintains the cusp-like shapes of temperature and chlorophyll observed from space.

The coupled physical‐new production system in the equatorial Pacific during the 1992–1995 El Niño

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high-resolution Ocean General Circulation Model forced with satellite-derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September-October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate-depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the eastern edge of the warm pool marked by a sharp salinity front, also coincides with a “new production front”. Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh pool and oligotrophic region has an average new production of 0.9 mmol NO3 m−2 d−1, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3 m−2 d−1 and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink.

Annual Reversal of the Equatorial Intermediate Current in the Pacific: Observations and Model Diagnostics

Abstract A large reversal of zonal transport below the thermocline was observed over a period of 6 months in the western Pacific Ocean between 2°S and the equator [from 26.2 Sv (1 Sv ≡ 106 m3 s−1) eastward in October 1999 to 28.6 Sv westward in April 2000]. To document this reversal and assess its origin, an unprecedented collection of ADCP observations of zonal currents (2004–06), together with a realistic OGCM simulation of the tropical Pacific, was analyzed. The results of this study indicate that this reversal is the signature of intense annual variability in the subsurface zonal circulation at the equator, at the level of the Equatorial Intermediate Current (EIC) and the Lower Equatorial Intermediate Current (L-EIC). In this study, the EIC and the L-EIC are both shown to reverse seasonally to eastward currents in boreal spring (and winter for the L-EIC) over a large depth range extending from 300 m to at least 1200 m. The peak-to-peak amplitude of the annual cycle of subthermocline zonal currents at ...

Acoustic Doppler current profiling along the Pacific Equator from 95°W to 165°E

Acoustic Doppler current profiler and CTD data were collected in the equatorial Pacific, from 95W to 165E, during the ALIZE2 cruise, in January–March 1991. These data provide a quasi-synoptic picture of both currents and hydrology of this region. The Equatorial zonal and meridional current structures and associated density distribution are presented and compared with previous measurements. The Equatorial Undercurrent appears stronger and shallower than climatology in the eastern Pacific, but close to average in the west. Intense 20–30 day period oscillations are noticeable east of 140W. Wind measurements and dynamic height computations are used to estimate some terms of the zonal momentum balance along the Equator.