20 - An experiment in monitoring cross-equatorial airflow at low level over Kenya and rainfall of western India during the northern summers

Summary

The identification of the core of the major low-level air current of the northern summer monsoon at a topographically locked position over eastern Africa has led to experiments in monitoring the airflow and relating its pulsations to the rainfall of parts of western India.

Using five-day overlapping means, it is found that pulsations in the airflow across eastern Africa near the Equator are reflected in the rainfall of western Maharashtra Province, sometimes with a lag of a few days.

The mean airflow in July has also been compared with the mean July rainfall of western Maharashtra Province and, when two-year overlapping means are used, a pronounced lag of one year is evident. It is demonstrated that this lag might be usefully exploited in experimental work towards the development of long-range rainfall forecasting techniques.

Introduction

A special feature of the low-level airflow over eastern Africa and the western Indian Ocean during the northern summer is that it is organized into a relatively narrow high-speed transequatorial current in the western periphery of the monsoon regime. The flow is strongest where the current is blocked or guided by high ground, and is weakest in the vicinity of the oceanic Equator. The current is characterized by a system of daily low-level jet streams, sufficiently persistent to show up markedly in monthly-averaged wind data.

The characteristics of the major current and the daily low-level jet streams have been described in detail by Findlater (1966, 1967, 1969a, b, 1970, 1971a, b, 1972, 1974, 1977b), but the general form of the current at the 1 km level in July can be seen in Fig. 20.1.