Semyon A. Grodsky, and James A. Carton

(MsWord document rainfall_2.doc  is here).
Department of Meteorology, University of Maryland, College Park, MD 20742, USA

E-mail: senya@ocean2.umd.edu

Abstract. The winds of the tropical Atlantic are known to have strong fluctuations on periods of a few days. These African Waves result from instabilities in the tropical winds crossing Northwest Africa and in some cases develop into the tropical storms and hurricanes of the western Atlantic [Yanai and Mukarami, 1970; Viltard et al., 1997; Diedhiou et al., 1998]. Here we use new surface wind observations made available by the QuickSCAT satellite and rainfall estimates from the Tropical Rainfall Measuring Mission (TRMM) to reexamine fluctuations in the synoptic meteorology of tropical West Africa and the Atlantic. We find the additional presence in surface winds and rainfall of strong quasi-two week disturbances in northern spring and summer. These disturbances appear to result from a cycle of continental heating, convection, and cooling that gives rise to fluctuations in the monsoon system.


Intensive study of synoptic-scale disturbances over central and West Africa and the tropical Atlantic has revealed the existence of two types of westward propagating wave-like fluctuations. The first are the African Waves with zonal wavelengths of 2,500-3,000 km, periods of 3-5 days, and westward speeds of 9-10 m/s and which are most evident in the meridional component of winds [Carlson, 1969; Viltard et al., 1997]. The second type has longer 6-9 day periods, correspondingly longer 6,000 km zonal wavelengths, and higher, 11 m/s, westward speeds [Yanai and Mukarami, 1970]. These latter wave-like fluctuations have their strongest expressions in zonal surface wind and cloud cover in the latitude band 100-200N[Oubuih et al., 1999]. Recently Grodsky and Carton [2001] have used daily data of the year 2000 provided by the SeaWinds scatterometer aboard QuickSCAT satellite [Spencer et al., 2000] and Tropical rainfall estimates from the Tropical Rainfall Measuring Mission satellite microwave sensor suite [Kummerow et al., 1998] to report on a third class of intraseasonal disturbances in the winds of the eastern tropical Atlantic. They occur closer to the equator, develop inside and close to the ITCZ area during April - June, and seem do not propagate westward. The spatial extent of the wind oscillations changes with season and is closely related to the meridional shift of the ITCZ. Janicot and Sultan [2001] have also observed a quasi-periodic signal of about 15 days in the rainfall and wind fields over West Africa based on a longer 1968-1990 data set. The presence of the quasi-biweekly fluctuations in the reanalysis zonal winds over the eastern tropical Atlantic and Northwest Africa have been also noted by Viltard et al. [1997]. Moreover, the existence of bi-weekly oscillations in the tropical wind system is not unique to Northwest Africa as very similar oscillation was found by Krishnamurti and Bhalme [1976] in the Indian monsoon. These studies provide further evidence that quasi- biweekly period is an important time scale of oscillation in the monsoon-like system that develops due to its own inherent dynamics.


It is evident from Figure 1 that the pattern of winds and rainfall can undergo rather rapid changes. The 12-day period beginning May 17 illustrates a reversal of the normal trade winds between 20N and 80N and east of 250W. This change brings moist maritime air eastward onto the areas surrounding the Gulf of Guinea resulting in intermittent heavy rainfall at least a month prior to the annual rainy season. The eastward shift of convection causes a corresponding reduction in rainfall in the western side of the basin (May 22 panel). During the succeeding week (late May- first days of June) the winds return to their normal configuration, while convection shifts westward onto the ocean and towards eastern Brazil (not shown in Figure 1). The trade winds reversal seems to be the result of interactions between the continental hydrologic cycle, summertime heating, and the tropical trade winds.
Figure 1. Two-day average surface winds and rainfall. Rates exceeding 0.5 mm/hr are shaded. This pattern has switched back by 30 May.

The sequence of wind, rainfall, and land heating cycles leading to development of the biweekly oscillation is further illustrated by the scheme presented in Figure 2. During normal (eastward) trade winds the convection is shifted seaward and sunshine over land increases the land temperature that lowers atmospheric pressure (upper panel). Decline in air pressure over land reverses wind direction that shifts convection towards the land bringing in maritime moisture, rainfall and cloudiness (lower panel). This decreases surface temperature over the land and restores (increases) atmospheric pressure, and the trade winds switch to normal direction shown in upper panel. Grodsky and Carton [2001] have shown using the NCEP/NCAR reanalysis of Kalnay et al. [1996] that variations of the land temperature, sea-land pressure difference, and rainfall over the land occur in close correspondence with wind oscillations within the ITCZ west off the African coast. 
Figure 2. The scheme of trade winds direction and convective clouds during the normal easterly winds (upper panel) and trade winds reversal (bottom panel).

The wind oscillation changes wind speed and direction that, in turn, shifts the area of intense convection in zonal direction. Here we illustrate the impact of these processes on air-sea fluxes using daily NCEP/NCAR reanalysis data. The air-sea fluxes will be estimated west off African coast over the area bounded by 40N-60N, 200W-100W. The largest impact is expected on the Short Wave Radiation (SWR) as it depends strongly on cloud coverage. Thus, the SWR must be maximal during the normal easterly trade winds when the convection shifts seaward towards the west. As trade winds reverse and convection shifts towards the African coast, the SWR becomes smaller due to shadowing effect of clouds. The Latent Heat Flux (LHFL) is expected to change in a more complicated way as it depends on both, wind speed and relative humidity. If we disregard the effect of relative humidity, the LHFL simply follows the changes in wind increasing as wind speed rises regardless of wind direction.

Figure 3 presents the components of the air-sea surface net heat flux along with 3-day average zonal wind. The flux data are 3 - 60 day band passed and shown during April - June when a succession of 5 bi-weekly wind oscillations is evident. Main contribution to net heat flux change comes from variation in SWR and LHFL, as was supposed earlier. Both components, LHFL and SWR, vary about ±20 W/m2. Their average values are <LHFL>=78 W/m2 and <SWR>=205 W/m2. Intraseasonal variation of the LHFL displays correlation with zonal wind speed modulus (Figure 4, upper panel). This correlation is obvious during May when the strongest oscillation occurred. The correlation isn?t unique because of impact of air humidity.
Figure 3. Three day mean zonal wind, U, and 3 - 60 day band passed latent heat flux, LHFL, sensible heat flux, SHFL, long wave radiation, LWR, and short wave radiation, SWR. All values are averaged over 4N - 6N, 20W - 10W.

Short wave radiation is expected to vary with phase opposite to that of the zonal wind. This regularity is observed in Figure 4 (bottom panel) beginning 16-th of May. Before this date, the relationship between SWR and zonal wind is less definite. Possible reason for that may relate to insufficient accuracy of the cloud parameterization used by the reanalysis model.
Figure 4. (Upper panel). Latent heat flux, LHFL, (3 - 60 day band passed, bold line) and 3-day mean zonal wind modulus, |U|, (thin line). (Bottom panel). Short wave radiation, SWR, (3 - 60 day band passed, bold line) and 3 -day mean zonal wind, U, (thin line). All values are averaged over 4N - 6N, 20W - 10W.


This paper presents observational evidence of the existence of quasi - biweekly oscillations in the tropical wind system over the eastern Atlantic. Oscillatory regime related to feedbacks within the hydrologic cycle is not unique to Northwest Africa. Lau and Bua [1998], for example, present modeling evidence of a similar phenomenon occurring in the region of East Asia/Indochina. These studies provide further evidence of strong interactions between continental and maritime atmospheric processes and the hydrologic cycle.

It is much probable that zonal wind oscillations in the tropics might force an oceanic response. The biweekly oscillations strongly modify winds in vicinity of the ITCZ that may influence the wind-driven circulation of the tropical ocean. Note also that during the trade winds reversal an upwelling favorite wind westerly blows along the northern coast of the Gulf of Guinea. All these issues are awaiting future studies.


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