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The Effect of Atlantic Nino on the Summer Monsoon Rainfall Anomalies in Sri Lanka

Author(s): Malinda Millangoda*

Sri Lankan climate is influenced by temperature patterns in the Indian Ocean as well as the Pacific Ocean. El-Nino Southern Oscillation (ENSO) is one of the global scale climate phenomena that have significant influence on the year to year variability of the monsoon over South-Asia. There have been numerous studies which explores the connection between the Indian Summer Monsoon (ISM) rainfall and the Atlantic Niño. However, the teleconnections of the Atlantic Ocean with the rainfall of Sri Lanka are not extensively studied. Considering the rainfall over Sri Lanka, Atlantic Niño and the South-west monsoon (summer monsoon) peaks during the June-July-August (JJA) period. Therefore, in this study the connections of the Atlantic Ocean, specifically the Atlantic Niño with regards to the ATL3 region rainfall during June-July-August (JJA) periods were considered. It was found that the JJA rainfall anomaly had significant correlations with the SSTA of ATL3 region (Atlantic Niño/Niña). In this study it was revealed that the Atlantic Niño has a significant positive correlation with rainfall over Sri Lanka with the Niño (positive phase) resulting in increasing JJA seasonal rainfalls over Sri Lanka while the Niña (negative phase) reduces the rainfall. Then the potential mechanisms of how the Atlantic Niño/Niña is linked to the seasonal rainfall JJA were studied. In doing so, Relative Humidity (RH) at lower levels of the atmosphere, Outgoing Long-wave Radiation (OLR), divergence at different levels, zonal and meridional wind components at different levels of the atmosphere, moisture flux and moisture flux divergence, streamline analysis were extensively studied. In this analysis, it was revealed that low pressure areas associated with the SSTA anomaly over the tropical Atlantic Ocean during the Niño phase has drawn the zonal winds at lower-levels of the atmosphere towards the Atlantic Ocean which has strengthened the latter part of the cross-equatorial flow prevalent during the south-west monsoon period. Cross-equatorial flow plays a vital role during the south-west monsoon period. In addition to that, the extra water vapor that is evaporated during the Niño phase is then transported over the North African continent to the Indian Ocean by the strong westerly zonal wind anomaly prevalent over the Atlantic Ocean. This moisture is then fed to the westerly flowing upper part of the cross-equatorial flow which will further enhance the rainfall over south-western part of Sri Lanka. This wind formation also has resulted in keeping the Inter Tropical Convergence Zone (ITCZ) or the Monsoon Trough (MT) over Sri Lanka for an extended period.

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