Figure 7 – Somali current

It is observed that there is an east-west atmospheric circulation over the tropical oceanic regions. Such circulation in Pacific Ocean is generally called walker cell. However, many scientists use the term ‘walker cell’ for all east-west circulations in different oceans. Walker cell is associated with southern oscillation and its strength fluctuates with that of Southern Oscillation Index (SOI). With a high positive SOI, there would be a zone of low atmospheric pressure over Australia and Indonesian archipelago. The rising air from this region deflects in upper atmosphere in both directions towards Africa and South America. In Indian Ocean, the air descends down at high pressure zone from where surface winds blow as Southwest monsoon towards Indian sub-continent in summers. During La-Nina Indian ocean branch of walker cell get strengthen and surface winds are more intense. La-Nina condition is generally associated with good monsoon.

During appearance of El-Nino or negative SOI, the ascending branch of the walker cell shifts to the central regions of the Pacific Ocean from west pacific region (Figure 8). In result, the Indian Ocean cell shifts towards east. The surface winds or Southwest monsoon winds are weaker than normal conditions.