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Le sous-courant équatorial et les échanges de masse et de chaleur associés dans le Pacifique tropical : variabilité, liens avec les événements El Niño-La Niña

Abstract : The Equatorial UnderCurrent (EUC) is part of the shallow subtropical/tropical meridional overturning cells (STCs/TCs), and by feeding the equatorial upwelling it may have strong influences on the eastern equatorial sea surface temperature (SST) and thus on ENSO (El Niño-Southern Oscillation) and its long-term variations. The EUC and the meridional overturning cells are studied using both in situ data, analytical and numerical models. Trajectories of water masses are calculated in a realistic ocean general circulation model (OGCM, forced by winds from NCEP reanalyses over 1948-1999 or from ERS satellites over 1992-1999). Their analysis reveals complex and asymmetric recharges/discharges of the equatorial band associated with the 1997-1998 El Niño-La Niña events. This lagrangian analysis also shows how the outcrop of cold water appearing during the rapid transition to La Niña in may 1998 was fed by STCs and the EUC. Then it is shown that transport, transport-weighted temperature and depth, and kinetic energy of the EUC over the entire meridional section, can be estimated by using TAO/TRITON moored data of current and temperature right at the equator, with their gaps carefully filled. Continuous time series at 170°W, 140°W and 110°W are constructed and show strong variations on seasonal to interannual time scales over 1980-2002. Their physical analysis reveals that the interannual variations in mass transport are a linear and quasi-stationary response to zonal wind stretch integrated zonally over the west and central Pacific. EUC temperature, which is important for the estimation of EUC heat transport, varies linearly with the difference of thermocline and EUC depths. The OGCM, validated especially with the EUC time series, is used to study over 1951-1999 the equatorial circulation associated with the EUC: pycnocline convergence, equatorial upwelling and surface divergence at 5°N and 5°S. Their interannual and long-term variations are all nearly equal (EUC is thus a good indicator of STCs strength), with some lags due to wave propagation. Such variations are mostly explained by linear ocean adjustment to zonal wind stretch integrated zonally over the equatorial Pacific, in agreement with linear theories. A decrease of about 30% in transport is thus seen over the last fifty years in the EUC and meridional overturning circulation, due to the overestimated decrease in NCEP easterlies. Analysis of the lags with equatorial SST, particularly the 5 months lead of upwelling and divergence over SST, reveals interesting cause to effect relationships, confirmed by heat budgets. The difference in temperature between divergence and convergence has the same variations as equatorial SST. A long-term decrease in EUC transport-weighted temperature of about 1°C is also seen, due to a shallower thermocline. For the heat budget of the eastern equatorial Pacific, EUC mass transport variations appear dominant on interannual timescale, but EUC transport-weighted temperature variations become also important on decadal timescale. For the heat budget of the equatorial band, both mass transport and temperature variations of meridional overturning are important. The links with theories of El Niño and decadal variability are finally discussed.
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Submitted on : Wednesday, July 21, 2004 - 5:38:36 AM
Last modification on : Tuesday, October 19, 2021 - 6:58:49 PM
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  • HAL Id : tel-00006355, version 1


Takeshi Izumo. Le sous-courant équatorial et les échanges de masse et de chaleur associés dans le Pacifique tropical : variabilité, liens avec les événements El Niño-La Niña. Océan, Atmosphère. Université Paul Sabatier - Toulouse III, 2003. Français. ⟨tel-00006355⟩



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