Questions relative to ITCZ migrations over the tropical Atlantic ocean, sea surface temperature and Senegal River runoff

Summary

The ITCZ (Intertropical Convergence Zone) is an important parameter for climatic studies in tropical areas, and meteorological satellite imagery provides an original way to follow its location. Using archive imagery covering the 1971–1987 period, we attempted to study further some of the relationships (suggested by former studies) between ITCZ locations (followed here over the Atlantic ocean at 28°W), and climate anomalies in the Sahel, an area affected by periodic drought for the last seventeen years. We also paid close attention to more frequently studied parameters, such as upper air data, wind at sea level, and sea surface temperature. As for relative drought estimates, we assumed that runoff from the Senegal River was representative of the sahelian area and we observed that its variations were consistent with the Lamb's rainfall index over the 1965–1987 period.

Since the onset of the rainy season for West Africa responds to wind changes, we assessed the link between ITCZ and wind at sea level and found the timing of northward ITCZ migration to be highly correlated (r=0.84) with the date of zonal wind stress intensification.

On a general point of view, the relationships we found between rainfall amount and ITCZ position anomalies (or SST anomalies) agree with known results of precedent works, though better fit is found with the seventies than the eighties. We think this discrepancy is due in part to the fact that the parameters studied were not identical and, perhaps to a possible change in climatic conditions (on a long term basis, the data show a continuous trend for less intense equatorial upwelling in the gulf of Guinea, and our time series covers a more recent period than referenced works).

With a closer look on the first half of the year, it appears that typical (wet/dry) schemes of the ITCZ migration can be evidenced more clearly, than in reporting the northernmost ITCZ location, that we found to be a less significant index: in other words, a sooner (respectively later) northward ITCZ migration corresponds to dry (respectively wet) episodes during the rainy season in sahelian areas. Hence, we propose the “speed” of ITCZ northwards movement as a parameterization of this event.

Moisture content of the lower troposphere revealed that steady anomalies of this parameter may last several years over sahelian areas. Taking into consideration the relative strength African tropical and easterly jets, some limited results were obtained, in regard of climatic anomalies.

As first conclusions, moisture transportation over sahelian area (associated with larger negative SST anomalies) is more efficient for wetter rainy season, than the intensity of convective process linked to higher local SST in the equatorial Atlantic area. In joining moisture analysis and ITZ migration (1980–1987 period), wetter rainy seasons were observed each time that positive humidity anomalies coincided with a later northward ITCZ migration (or greater northward ITCZ speed).