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- Author or Editor: Joël Picaut x
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Abstract
Several mechanisms have been proposed to explain the coastal and equatorial upwelling in the eastern Atlantic (Guinea Gulf). The most controversial is the mechanism of remote wind forcing in the western equatorial Atlantic suggested by Moore et al. (1978). Most of the possible explanations for the upwelling and their relative importance are discussed in view of recent observations.
Detailed analysis of daily sea surface temperature (SST) collected at 16 coastal stations along the northern coast of the Guinea Gulf reveals that the upwelling event propagates westward along this coast at a mean speed of 0.7 m s−1. Similar analysis of historical monthly mean SST data shows that the coastal upwelling event propagates poleward from 1°S to at least 13°S at the same phase speed. Furthermore, the Northern Hemisphere and Southern Hemisphere coastal upwelling signals seem to start at the same time from the equator. The same kind of analysis applied to hydrographic data from a station situated 41 km off Abidjan, reveals an upward phase propagation of the upwelling event at 7 m day−1 from 300 m to the surface. These results and those of Servain et al. (1982) suggest that remote wind forcing west of the Gulf of Guinea is an important factor affecting the temperature in the Gulf.
Abstract
Several mechanisms have been proposed to explain the coastal and equatorial upwelling in the eastern Atlantic (Guinea Gulf). The most controversial is the mechanism of remote wind forcing in the western equatorial Atlantic suggested by Moore et al. (1978). Most of the possible explanations for the upwelling and their relative importance are discussed in view of recent observations.
Detailed analysis of daily sea surface temperature (SST) collected at 16 coastal stations along the northern coast of the Guinea Gulf reveals that the upwelling event propagates westward along this coast at a mean speed of 0.7 m s−1. Similar analysis of historical monthly mean SST data shows that the coastal upwelling event propagates poleward from 1°S to at least 13°S at the same phase speed. Furthermore, the Northern Hemisphere and Southern Hemisphere coastal upwelling signals seem to start at the same time from the equator. The same kind of analysis applied to hydrographic data from a station situated 41 km off Abidjan, reveals an upward phase propagation of the upwelling event at 7 m day−1 from 300 m to the surface. These results and those of Servain et al. (1982) suggest that remote wind forcing west of the Gulf of Guinea is an important factor affecting the temperature in the Gulf.
Abstract
A numerical model incorporating a single baroclinic mode and realistic coastline geometry is used to analyze the linear, dynamic response to estimates of the seasonal wind field over the tropical Atlantic Ocean. The forced periodic response consists of a spatially dependent combination of a locally forced response, Kelvin waves, Rossby waves and multiple wave reflections. The seasonal displacements of the model pycnocline are compared with observed dynamic height. Annual and semiannual fluctuations dominate the seasonal signal throughout the basin. In general, the distribution of amplitude and phase are similar for annual changes in dynamic height and pycnocline depth. Major features of the seasonal response are reproduced, e.g., east-west changes in pycnocline depth about a nodal point at the equator, the seasonal pycnocline movement along the northern and southern coast of the Guinea Gulf, and a significant changes of phase in the ocean variability north and south of the ITCZ. The relative importance between local and remote forcing is determined for several parts of the model basin. The wind-driven annual signal in the idealized Gulf of Guinea is due to equatorial zonal wind stress fluctuations west of the Gulf. The semi-annual response in the Gulf of Guinea is a result of zonal and meridional wind stress fluctuations in the eastern half of the tropical Atlantic. The seasonal response in the western equatorial and northernmost parts of the model basin are primarily local.
Abstract
A numerical model incorporating a single baroclinic mode and realistic coastline geometry is used to analyze the linear, dynamic response to estimates of the seasonal wind field over the tropical Atlantic Ocean. The forced periodic response consists of a spatially dependent combination of a locally forced response, Kelvin waves, Rossby waves and multiple wave reflections. The seasonal displacements of the model pycnocline are compared with observed dynamic height. Annual and semiannual fluctuations dominate the seasonal signal throughout the basin. In general, the distribution of amplitude and phase are similar for annual changes in dynamic height and pycnocline depth. Major features of the seasonal response are reproduced, e.g., east-west changes in pycnocline depth about a nodal point at the equator, the seasonal pycnocline movement along the northern and southern coast of the Guinea Gulf, and a significant changes of phase in the ocean variability north and south of the ITCZ. The relative importance between local and remote forcing is determined for several parts of the model basin. The wind-driven annual signal in the idealized Gulf of Guinea is due to equatorial zonal wind stress fluctuations west of the Gulf. The semi-annual response in the Gulf of Guinea is a result of zonal and meridional wind stress fluctuations in the eastern half of the tropical Atlantic. The seasonal response in the western equatorial and northernmost parts of the model basin are primarily local.
Abstract
Long time series of sea level and sea surface temperatures measured at different coastal stations along the northern coast of the Gulf of Guinea are analyzed statistically. The results indicate that pronounced fortnightly oscillations in sea level are composed of two waves: one is the lunar fortnightly tide Mf (13.661-day period) which has a constant phase all along the coast. The other wave has a period of 14.765 days which is the period of the luni-solar fortnightly tide Msf; this wave propagates westward along the east-west oriented coastline with a mean phase speed of 53 cm s−1 and a wavelength of 675 km. These waves have important effects on the thermal structure and give rise to strong vertical oscillations of the subsurface isotherms throughout the year. The sea surface temperature, however, has pronounced oscillations around the Msf frequency during the upwelling season (June–September) only. The 14.765-day wave is of tidal origin and is due to a nonlinear interaction of the M2 and S2 (barotropic or baroclinic) tides but the generation mechanism is obscure.
Abstract
Long time series of sea level and sea surface temperatures measured at different coastal stations along the northern coast of the Gulf of Guinea are analyzed statistically. The results indicate that pronounced fortnightly oscillations in sea level are composed of two waves: one is the lunar fortnightly tide Mf (13.661-day period) which has a constant phase all along the coast. The other wave has a period of 14.765 days which is the period of the luni-solar fortnightly tide Msf; this wave propagates westward along the east-west oriented coastline with a mean phase speed of 53 cm s−1 and a wavelength of 675 km. These waves have important effects on the thermal structure and give rise to strong vertical oscillations of the subsurface isotherms throughout the year. The sea surface temperature, however, has pronounced oscillations around the Msf frequency during the upwelling season (June–September) only. The 14.765-day wave is of tidal origin and is due to a nonlinear interaction of the M2 and S2 (barotropic or baroclinic) tides but the generation mechanism is obscure.
Abstract
Several studies using sea level observations and coupled models have shown that heat buildup in the western equatorial Pacific is a necessary condition for a major El Niño to develop. However, none of these studies has considered the potential influence of the vertical salinity stratification on the heat buildup and thus on El Niño. In the warm pool, this stratification results in the presence of a barrier layer that controls the base of the ocean mixed layer. Analyses of in situ and TOPEX/Poseidon data, associated with indirect estimates of the vertical salinity stratification, reveal the concomitant presence of heat buildup and a significant barrier layer in the western equatorial Pacific. This relationship occurs during periods of about one year prior to the mature phase of El Niño events over the period 1993–2002. Analyses from a coupled ocean–atmosphere general circulation model suggest that this relationship is statistically robust. The ability of the coupled model to reproduce a realistic El Niño together with heat buildup, westerly wind bursts, and a salinity barrier layer suggests further investigations of the nature of this relationship. In order to remove the barrier layer, modifications to the vertical ocean mixing scheme are applied in the equatorial warm pool and during the 1-yr period of the heat buildup. At the bottom of the ocean mixed layer, the heat buildup is locally attenuated, as expected from switching on the entrainment cooling. At the surface, the coupled response over the warm pool increases the fetch of westerly winds and favors the displacement of the atmospheric deep convection toward the central equatorial Pacific. These westerly winds generate a series of downwelling equatorial Kelvin waves whose associated eastward currents drain the heat buildup toward the eastern Pacific Ocean. The overall reduction of the heat buildup before the onset of El Niño results in the failure of El Niño. These coupled model analyses confirm that the buildup is a necessary condition for El Niño development and show that the barrier layer in the western equatorial Pacific is important for maintaining the heat buildup.
Abstract
Several studies using sea level observations and coupled models have shown that heat buildup in the western equatorial Pacific is a necessary condition for a major El Niño to develop. However, none of these studies has considered the potential influence of the vertical salinity stratification on the heat buildup and thus on El Niño. In the warm pool, this stratification results in the presence of a barrier layer that controls the base of the ocean mixed layer. Analyses of in situ and TOPEX/Poseidon data, associated with indirect estimates of the vertical salinity stratification, reveal the concomitant presence of heat buildup and a significant barrier layer in the western equatorial Pacific. This relationship occurs during periods of about one year prior to the mature phase of El Niño events over the period 1993–2002. Analyses from a coupled ocean–atmosphere general circulation model suggest that this relationship is statistically robust. The ability of the coupled model to reproduce a realistic El Niño together with heat buildup, westerly wind bursts, and a salinity barrier layer suggests further investigations of the nature of this relationship. In order to remove the barrier layer, modifications to the vertical ocean mixing scheme are applied in the equatorial warm pool and during the 1-yr period of the heat buildup. At the bottom of the ocean mixed layer, the heat buildup is locally attenuated, as expected from switching on the entrainment cooling. At the surface, the coupled response over the warm pool increases the fetch of westerly winds and favors the displacement of the atmospheric deep convection toward the central equatorial Pacific. These westerly winds generate a series of downwelling equatorial Kelvin waves whose associated eastward currents drain the heat buildup toward the eastern Pacific Ocean. The overall reduction of the heat buildup before the onset of El Niño results in the failure of El Niño. These coupled model analyses confirm that the buildup is a necessary condition for El Niño development and show that the barrier layer in the western equatorial Pacific is important for maintaining the heat buildup.
Abstract
An analysis of sea-surface temperature (SST) and surface winds in selected areas of the tropical Atlantic indicates that the nonseasonal variability of SST in the eastern equatorial Atlantic (Gulf of Guinea) is highly correlated with the nonseasonal variability of the zonal wind stress in the western equatorial Atlantic. A negative (positive) anomaly of the zonal wind stress near the north Brazilian coast is followed by a positive (negative) SST anomaly in the Gulf of Guinea about one month later. Furthermore, the correlation between the local wind stress anomaly and SST anomaly in the Gulf of Guinea is considerably smaller. These preliminary results indicate that remote forcing in the western equatorial Atlantic Ocean is an important factor affecting the eastern equatorial Atlantic sea-surface temperature. Recent equatorial theories are consistent with these observations.
Abstract
An analysis of sea-surface temperature (SST) and surface winds in selected areas of the tropical Atlantic indicates that the nonseasonal variability of SST in the eastern equatorial Atlantic (Gulf of Guinea) is highly correlated with the nonseasonal variability of the zonal wind stress in the western equatorial Atlantic. A negative (positive) anomaly of the zonal wind stress near the north Brazilian coast is followed by a positive (negative) SST anomaly in the Gulf of Guinea about one month later. Furthermore, the correlation between the local wind stress anomaly and SST anomaly in the Gulf of Guinea is considerably smaller. These preliminary results indicate that remote forcing in the western equatorial Atlantic Ocean is an important factor affecting the eastern equatorial Atlantic sea-surface temperature. Recent equatorial theories are consistent with these observations.
