The Persistence of Winter Sea Surface Temperature in the North Atlantic

International audienc

Diurnal cycle and seasonal evolution of the West African monsoon in thesouthern coastal region

International audienceThe representation of the diurnal cycle is an identified problem for the West African Monsoon forecasts, inparticular for the intraseasonal variability : models are known for their poor representation of clouds, whichhas a strong impact on solar radiation and surface energy balance, and therefore on the diurnal cycle in theatmospheric boundary layer. Since the latter is connected to the triggering of convection, this flaw leads to anunrealistic representation of humidity gradient between the Gulf of Guinea and the Sahel, moisture transportand precipitation. In this study, the Guinean Coastal Rainfall is analysed from the end of the oceanic phaseuntil the beginning of the Sahelian phase of the monsoon in 2008-2015, with reanalyses (ECMWF ERA5) andsatellite observations for clouds (MSG), precipitation (TRMM B42) and surface wind (ASCAT). The sea breeze/ land breeze alternation and its connection with the low-level wind divergence and surface temperature gradientwere found to strongly dominate the diurnal signal. Reanalyses and observations were then compared to betterunderstand the poor representation of precipitation in the mod

Sea surface temperature impact on diurnal cycle and seasonal evolution of the Guinea Coast Rainfall in boreal spring and summer

International audienceERA5 reanalyses and observations of convective clouds and precipitation are used over the northern Gulf of Guinea between 7W and 3E to study the influence of ocean surface temperature and land-sea temperature gradient on Guinea Coast Rainfall (GCR) in boreal spring and summer. Seasonal composites are calculated around two dates indexing the onset ( T ref ) and demise ( T end ) of the GCR: T ref corresponds to the emergence of the equatorial upwelling in boreal spring, which “pushes” the zonal precipitation belt northward against the Guinea coast. T end characterizes the emergence of the coastal upwelling in July, which is known to coincide with the beginning of the “little dry season” that lasts until September. Along the Guinea Coast, the diurnal cycle of the air-sea temperature gradient controls precipitation through the land-sea breeze, which explains why precipitation reaches its maximum around noon over the ocean, and in the late afternoon over the continent. The emergence of the Guinea Coast upwelling in July induces a weakening of southerlies on a seasonal scale, and a weaker land breeze on a diurnal scale: it induces a decrease in the convergence of humidity transport across the coast and in coastal oceanic precipitation. Therefore, the GCR is seasonally controlled by the latitude of the maximum tropospheric water vapor content and the annual cycle of the West African Monsoon, but the ocean surface temperature is responsible for the abruptness of its onset via the intensification of the equatorial upwelling around the end of May, and possibly of its demise as well via the emergence of the coastal upwelling by early July

Diurnal cycle and seasonal evolution of the West African monsoon in thesouthern coastal region

International audienceThe representation of the diurnal cycle is an identified problem for the West African Monsoon forecasts, inparticular for the intraseasonal variability : models are known for their poor representation of clouds, whichhas a strong impact on solar radiation and surface energy balance, and therefore on the diurnal cycle in theatmospheric boundary layer. Since the latter is connected to the triggering of convection, this flaw leads to anunrealistic representation of humidity gradient between the Gulf of Guinea and the Sahel, moisture transportand precipitation. In this study, the Guinean Coastal Rainfall is analysed from the end of the oceanic phaseuntil the beginning of the Sahelian phase of the monsoon in 2008-2015, with reanalyses (ECMWF ERA5) andsatellite observations for clouds (MSG), precipitation (TRMM B42) and surface wind (ASCAT). The sea breeze/ land breeze alternation and its connection with the low-level wind divergence and surface temperature gradientwere found to strongly dominate the diurnal signal. Reanalyses and observations were then compared to betterunderstand the poor representation of precipitation in the mod

Sea surface temperature impact on diurnal cycle and seasonal evolution of the Guinea Coast Rainfall in boreal spring and summer

International audienceERA5 reanalyses and observations of convective clouds and precipitation are used over the northern Gulf of Guinea between 7W and 3E to study the influence of ocean surface temperature and land-sea temperature gradient on Guinea Coast Rainfall (GCR) in boreal spring and summer. Seasonal composites are calculated around two dates indexing the onset ( T ref ) and demise ( T end ) of the GCR: T ref corresponds to the emergence of the equatorial upwelling in boreal spring, which “pushes” the zonal precipitation belt northward against the Guinea coast. T end characterizes the emergence of the coastal upwelling in July, which is known to coincide with the beginning of the “little dry season” that lasts until September. Along the Guinea Coast, the diurnal cycle of the air-sea temperature gradient controls precipitation through the land-sea breeze, which explains why precipitation reaches its maximum around noon over the ocean, and in the late afternoon over the continent. The emergence of the Guinea Coast upwelling in July induces a weakening of southerlies on a seasonal scale, and a weaker land breeze on a diurnal scale: it induces a decrease in the convergence of humidity transport across the coast and in coastal oceanic precipitation. Therefore, the GCR is seasonally controlled by the latitude of the maximum tropospheric water vapor content and the annual cycle of the West African Monsoon, but the ocean surface temperature is responsible for the abruptness of its onset via the intensification of the equatorial upwelling around the end of May, and possibly of its demise as well via the emergence of the coastal upwelling by early July

Influence océanique du golfe de Guinée sur la mousson en Afrique de l'Ouest

La mousson africaine démarre chaque année à la fin du printemps boréal, lorsqu un fort contraste thermique se développe entre le Sahara surchauffé et le golfe de Guinée qui se refroidit en surface. De nombreuses études montrent le rôle majeur des températures de surface de l'océan dans le système de mousson. Cette thèse vise à explorer les mécanismes d'interactions océan-atmosphère agissant sur les précipitations côtières de la mousson africaine au printemps. L'étude s'appuie à la fois sur des mesures in situ et satellites, et sur des données de modèle. La saison de mousson de l'année 2006 a été analysée grâce au grand nombre de données rassemblées cette année-là par le programme AMMA (Analyse Multidisciplinaire de la Mousson Africaine). Il a ainsi été mis en évidence que le refroidissement de la surface de l'océan crée un front océanique à l'équateur qui engendre une accélération du vent au nord de l'équateur semblant favoriser l'activité convective le long de la côte africaine. L'étape suivante a été d'étendre l'étude à une période de dix ans (2000-2009) à l'aide des mesures satellites et des données de réanalyses. Les phénomènes observés pour la saison de mousson 2006 ont été retrouvés et les analyses réalisées ont mis en avant un mécanisme liant les coups de vents dans l'Est de l'Atlantique équatorial aux précipitations côtières. Ainsi, un coup de vent entraîne un refroidissement de la surface de l'océan à l'équateur. En retour, ce refroidissement allié à l'accélération du vent au nord de l'équateur, agit sur la circulation atmosphérique de basse couche dans le golfe de Guinée. L'activité convective est alors favorisée et les précipitations augmentent à la côte.The West African monsoon starts each year at the end of boreal spring, when a strong thermal contrast develops between the warmer Sahara region and the cooler Gulf of Guinea. Many studies show that sea surface temperatures play a key role in the monsoon system. The aim of this thesis is to explore the air-sea interactions occurring in this region, and the mechanisms through which they may impact the coastal rainfall of the monsoon during spring. The study is based on in situ and satellite measurements as well as model data. The 2006 monsoon season is analysed thanks to the vast set of observations collected by the AMMA (African Monsoon Multidisciplinary Analyses) project during this year. Results show that a cooling of the sea surface creates an oceanic front at the equator. This strengthens the wind north of the equator and seems to favour the convective activity along the African coast. The next step is to extend this study to a ten-year period (2000-2009) with satellite measurements and reanalyses data. Results are consistent with the process observed in the 2006 case study, and the statistical analyses show a link between the wind burst in the East Equatorial Atlantic and the coastal rainfall. In this context, a wind burst generates a cooling of the sea surface at the equator. This cooling combined with the wind strengthening north of the equator, in turn, impacts the low-level atmospheric circulation in the Gulf of Guinea. This enhances convective activity and increases precipitation along the coast.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Role of air-sea interactions on the coastal rainfall in the Gulf of Guinea during boreal spring

International audienceThe role of air-sea interactions in the boreal spring precipitation of the West African monsoon is explored through the wind variability in the Gulf of Guinea. Satellite measurements and reanalyses data are used to describe the atmosphere and the sea surface in the Gulf of Guinea from 2000 to 2009. Previous results showed a statistical link between the strengthening of southerlies between the Equator and the Guinean coast, and precipitation along the coast. In this study, linear regressions are first performed in May-June (2000-2009) to investigate the mechanisms at stake : an equatorial SST cooling strengthens the wind north of the equator, via the SST front located along 1°N. This wind acceleration intensifies the low atmospheric local circulation, which components are surface southerlies, coastal convergence, low atmosphere southward return flow, and subsidence over the Gulf of Guinea. When this circulation is stronger than normal, it brings more humidity toward the coast, which triggers deeper atmospheric convection and increases the coastal rainfall . In addition, an abrupt change in the surface wind pattern is observed between April and July. Composites are used to analyse temporal and spatial variations of the SST, surface wind speed and humidity, in surface as well as in altitude. A clear transition is observed during the spring season, when the wind strengthens between the equator and 5°N, which generally occurs at the end of May. Eventually, this study emphasizes very clearly the importance of the intraseasonal variability in the seasonal evolution and setting of the guinean coastal rainfall

Role of air-sea interactions on the coastal rainfall in the Gulf of Guinea during boreal spring

International audienceThe role of air-sea interactions in the boreal spring precipitation of the West African monsoon is explored through the wind variability in the Gulf of Guinea. Satellite measurements and reanalyses data are used to describe the atmosphere and the sea surface in the Gulf of Guinea from 2000 to 2009. Previous results showed a statistical link between the strengthening of southerlies between the Equator and the Guinean coast, and precipitation along the coast. In this study, linear regressions are first performed in May-June (2000-2009) to investigate the mechanisms at stake : an equatorial SST cooling strengthens the wind north of the equator, via the SST front located along 1°N. This wind acceleration intensifies the low atmospheric local circulation, which components are surface southerlies, coastal convergence, low atmosphere southward return flow, and subsidence over the Gulf of Guinea. When this circulation is stronger than normal, it brings more humidity toward the coast, which triggers deeper atmospheric convection and increases the coastal rainfall . In addition, an abrupt change in the surface wind pattern is observed between April and July. Composites are used to analyse temporal and spatial variations of the SST, surface wind speed and humidity, in surface as well as in altitude. A clear transition is observed during the spring season, when the wind strengthens between the equator and 5°N, which generally occurs at the end of May. Eventually, this study emphasizes very clearly the importance of the intraseasonal variability in the seasonal evolution and setting of the guinean coastal rainfall

On decadal-scale ocean-atmosphere interactions in the extended ECHAM1/LSG climate simulation

International audienceThe last 810 years of a control integration with the ECHAM1/LSG coupled model are used to clarify the nature of the ocean-atmosphere interactions at low frequencies in the North Atlantic and the North Pacific. To a first approximation, the atmosphere acts as a white noise forcing and the ocean responds as a passive integrator. The sea surface temperature (SST) variability primarily results from short time scale fluctuations in surface heat exchanges and Ekman currents, and the former also damp the SST anomalies after they are generated. The thermocline variability is primarily driven by Ekman pumping. Because the heat, momentum, and vorticity fluxes at the sea surface are correlated in space and time, the SST variability is directly linked to that in the ocean interior. The SST is also modulated by the wind-driven geostrophic fluctuations, resulting in persistent correlation with the thermocline changes and a slight low-frequency redness of the SST spectra. The main dynamics are similar in the two oceans, although in the North Pacific the SST variability is more strongly influenced by advection changes and the oceanic time scales are larger. A maximum covariance analysis based on singular value decomposition in lead and lag conditions indicates that some of the main modes of atmospheric variability in the two oceans are sustained by a very weak positive feedback between the atmosphere, SST, and the strength of the subtropical and subpolar gyres. In addition, in the North Atlantic the main surface pressure mode has a small quasi-oscillatory component at 6-year period, and advective resonance occurs for SST around 10-year period, both periods being also singled out by multichannel singular spectrum analysis. The ocean-atmosphere coupling is however much too weak to redden the tropospheric spectra or create anything more than tiny spectral peaks, so that the atmospheric and oceanic variability is dominated in both ocean sectors by the one-way interactions