Pascal Roucou

Influence of the Mediterranean Sea on the West African monsoon: Intraseasonal variability in numerical simulations

[1] The influence of May to September sea surface temperature (SST) anomalies in the Mediterranean Sea on the West African monsoon is investigated, analyzing the outputs of numerical sensitivity experiments performed using three atmospheric general circulation models (Action de Recherche Petite Echelle Grande Echelle, European/Hamburg, and University of California, Los Angeles) in the framework of the African Monsoon Multidisciplinary Analysis project. The precipitation and atmospheric dynamics response to the SST forcing is explored, in terms of intraseasonal variability, evaluating the results from the individual models and from the multimodel mean. A positive precipitation response to warmer than average conditions in the Mediterranean Sea is found in the Sudano-Sahelian belt in August-September. The proposed dynamic mechanism underlying the Mediterranean action on the West African monsoon is based on the modifications produced by the SST forcing in the moisture content in the lower troposphere. A warmer eastern Mediterranean in August-September feeds the lower troposphere with additional moisture, with a consequent reinforcement of northerly moisture transport toward the Sahel. Furthermore, warmer SST is linked to a strengthening of the Saharan heat low and to an enhancement of the moist static energy meridional gradient over West Africa, favoring the northward displacement of the monsoonal front.

Weather Regimes in the Euro-Atlantic and Mediterranean Sector, and Relationship with West African Rainfall over the 1989–2008 Period from a Self-Organizing Maps Approach

AbstractWeather regimes (WRs) have been defined over the Euro-Mediterranean region (15°–70°N, 60°W–60°E) from May to October using the daily sea level pressure, 700-hPa geopotential height, and specific humidity from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA)-Interim over the 1989–2008 period. Computations are based on a neural network classification technique referred to as self-organizing maps, and the WRs produced can be used by the scientific community for comparison with other periods, projection onto model outputs, seasonal prediction, or teleconnection studies. The article particularly examines the relationship between WRs and West Africa (WA) rainfall, and the study’s results suggest that changes in particular WR frequencies can account for a part of the WA’s interannual rainfall variability. Thus, during anomalous wet (dry) years in WA rainfall, both more occurrences of WRs related to the negative (positive) summer North Atlantic Oscillation (NAO)–like pattern and fe...

VERTICAL MOTION CHANGES RELATED TO NORTH‐EAST BRAZIL RAINFALL VARIABILITY: A GCM SIMULATION

The atmospheric structure over north-east Brazil during anomalous rainfall years is studied in the 11 levels of the outputs of the Laboratoire de Meteorologie Dynamique atmospheric general circulation model (LMD AGCM). Seven 19-year simulations were performed using observed sea-surface temperature (SST) corresponding to the period 1970– 1988. The ensemble mean is calculated for each month of the period, leading to an ensemble-averaged simulation. The simulated March-April rainfall is in good agreement with observations. Correlations of simulated rainfall and three SST indices relative to the equatorial Pacific and northern and southern parts of the Atlantic Ocean exhibit stronger relationships in the simulation than in the observations. This is particularly true with the SST gradient in the Atlantic (Atlantic dipole). Analyses on 200;hPa velocity potential, vertical velocity, and vertical integral of the zonal component of mass flux are performed for years of abnormal rainfall and positive/negative SST anomalies in the Pacific and Atlantic oceans in March–April during the rainy season over the Nordeste region. The results at 200 hPa show a convergence anomaly over Nordeste and a divergence anomaly over the Pacific concomitant with dry seasons associated with warm SST anomalies in the Pacific and warm (cold) waters in the North (South) Atlantic. During drought years convection inside the ITCZ indicated by the vertical velocity exhibits a displacement of the convection zone corresponding to a northward migration of the ITCZ. The east–west circulation depicted by the zonal divergent mass flux shows subsiding motion over Nordeste and ascending motion over the Pacific in drought years, accompanied by warm waters in the eastern Pacific and warm/cold waters in northern/southern Atlantic. Rainfall variability of the Nordeste rainfall is linked mainly to vertical motion and SST variability through the migration of the ITCZ and the east–west circulation.

Global equatorial variability of 850 and 200 hPa zonal winds from rawinsondes between 1963 and 1989

The longitude-height-time variability of 3-month averaged zonal wind anomalies at 850 and 200 hPa over the equatorial area (5{degrees}N-5{degrees}S) is analyzed using a three-dimensional dataset constructed from rawinsonde data (1963-1989). The first mode, closely related to the Southern Oscillation Index, suggests a strong vertical coupling associated with a horizontal out-of-phase pattern between the central/western Pacific and the remainder of the equatorial belt. The vertical coupling appears to be phase-locked to the annual cycle with strongest intensities found over South America and near the maritime continent early in the calendar year and over the Pacific basin and Africa during the second half of the year. This mode of variability can be viewed as a standing pattern superimposed with an eastward-migrating component, coherent with the annual cycle. This westerly moving mode originates near the maritime continent during the northern autumn, and tends to precede El Nino/Southern Oscillation events in the central and eastern Pacific Ocean. Variance is mainly concentrated in the 3-8 year low-frequency time scale throughout the tropics, and in the 2-3 year quasi-biennal band from 110{degrees}-120{degrees}E to 180{degrees}. 24 refs., 4 figs.

Réponse à Marcel Leroux

• Tout article scientifique, surtout de synthèse, nécessite de rappeler les travaux antérieurs ayant vraiment traité le sujet, en l’occurrence les relations entre anomalies de température de surface de la mer (TSM) et anomalies pluviométriques autour du bassin Atlantique. Sur cet aspect, les références annexées à l’article remontent aux années 77-78 (Hastenrath et Heller, 1977 ; Lamb, 1978 a et b), soit dix, dix-sept et dix-huit ans avant les écrits de M. Leroux rappelés dans sa lettre (Leroux, 1988, 1995 et 1996), écrits qui, de plus, ne traitent pas directement de ces questions.