Asymmetric relation between the position of Atlantic rain band and anomalous sea surface temperature

What is it about?

The trade winds from the north and south meet over or around the equator and produce an almost continuous rain band called the Inter-tropical Convergence Zone (ITCZ). The position of the ITCZ moves in the north-south direction depending on the season. In a given season, once in a while, the ITCZ can arrive earlier or later than usual. Such anomalous position of ITCZ in the months 0f March-April-May has been shown to lead to anomalous sea surface temperatures in the south tropical Atlantic in the following months of June-July-August. These anomalous sea surface temperatures and its associated anomalous changes in winds and water temperatures below the ocean surface are all part of a phenomenon called the Atlantic zonal mode (AZM). The previous studies showed that when the ITCZ during March-April-May is positioned anomalously north, it tends to lead to a cold AZM in the following summer and vice versa, implying a symmetric relation between the two. However, in this study, we show that the relation is not symmetric and explain that the reason is inherent in the way the ITCZ responds to the movement of the sun. We have also observed that the physical mechanisms causing cold AZM events share many common features and are less diverse compared to that of the warm AZM events.

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Why is it important?

The variability of the AZM is shown to be of relevance to several phenomena of importance around the globe, such as El Nino - Southern Oscillation and Indian summer monsoon. The results can enhance our understanding of the AZM as well as that of chronic model biases and can contribute eventually to the predictability of the said phenomena.

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