SUB-TROPICAL ANTICYCLONES OVER THE SOUTH WEST INDIAN AND SOUTH ATLANTIC OCEANS AND THEIR INFLUENCE ON ETHIOPIAN WEATHER DURING KIREMT SEASON

ABSTRACT

Zonal and Meridional Moisture Flux up to the 850 mb level had been computed for the months June to September during the years 1982-94 and the interval 10oW to 70o E and 20o S to 35o N over the South West Indian and South East Atlantic Oceans using the NOAA (National Oceanic and Atmospheric Administration). Reanalysis of Data; the data was made and year-to-year variability of the moisture flux was investigated for the months June to September. The computed low level moisture flux considered grid wise in the neighborhood of Ethiopia, for carrying out comparison between the moisture coming from the Atlantic and the Indian Oceans. The behaviour of the computed cross equatorial moisture flux from the Indian Ocean during drought years was assessed and its relation with the El-Nino Southern Oscillation Phenomena was investigated by using Linear Regression Analysis between the Southern Oscillation Index (SOI) of the ENSO Teleconnection and the computed low level moisture flux. Year to year variabilities in the moisture flux from the Indian and Atlantic Oceans was studied. Variabilities of different parameters namely, anomalies in Mean Sea Level Pressure, 850 mb Geo-Potential Height, Sea Surface Temperature, Specific humidity and Zonal and Meridional wind over the Sub-tropical Anticyclones of South East Atlantic and South West Indian Oceans were considered for the months June to September during the years 1982-94 vis-a-vis the characteristic behavior of the Kiremt rainfall of June to September during drought and non-drought years. The study shows that the moisture source for Ethiopia during Kiremt season is both the Indian Ocean and the Atlantic Ocean. However, grid wise computations show that the moisture coming from the Indian Ocean could be two to three times as large as that coming from the Atlantic Ocean. The computations done indicate that low level cross equatorial moisture flux from the Indian Ocean during the months of June, July and August constitutes about 20 percent of that crossing the entire equator during these months and closely agrees well with the findings of other people (Boogard and Rao, 1984), thus indicating its importance during the Northern Hemisphere Summer months, as it is related with the presence of the Low Level Jet. The study of annual variabilities shows that greater westerly moisture flux from the Atlantic Ocean is observed in years of strong Tropical Easterly Jets over Ethiopia. Moisture coming from the Indian Ocean is greatly and negatively affected during El Nino years, where as moisture coming from the Atlantic Ocean could be greatly affected by Sea Surface Temperature Anomaly over the Tropical Atlantic.

Note: The report is available at the Central Technical Library of the ESTC.

(Report 1999)

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