Intra-seasonal sea level variability along the west coast of India

Graduation date: 2014The importance of local versus distant forcing is studied for the wind driven intra-\ud seasonal (30-120 day) sea level anomaly (SLA) variations along the west coast of\ud India. Three locations on the continental slope are chosen to represent the west\ud coast. Two more locations, one east of Sri-Lanka and another on the south east\ud coast of India are chosen to investigate their remote contributions to the intra-\ud seasonal SLA variability along the west coast of India. Significant correlations\ud of altimeter derived SLA on the west coast are found with the SLA east of Sri-\ud Lanka and SLA as far as Sumatra and the Equator, with increased lags. These\ud correlations are consistent with remote forcing from the Equator in the form of\ud reflected Rossby waves originating along the Sumatra coast. Correlations of SLA\ud on the west coast with the SLA on the south-east coast of India are significant, but\ud lower in magnitude than signals arriving from Sumatra. This suggest that signals\ud on the south-east coast of India (generated locally or along the wave guide between there and the Equator) are significantly weakened as they propagate around Sri-Lanka, the tip of India, in comparison to signals propagating westward from the coast of Sumatra.\ud \ud The highest correlations between SLA on the west coast and winds are found\ud with the winds from the southern tip of India, suggesting the importance of remotewind forcing from south of India. This supports the contribution of the coastal trapped wave signals to the west coast of India, although lags between winds at tip of India and SLA on the west coast are longer than expected (6-7 days). An idealised model is used to explore the possibility that the apparent 6-7 day lag in simple correlation is caused by the arrival and combination of several signals, which were generated simultaneously at different locations by the large scale winds.\ud \ud Extending the 2-point correlations, multivariate linear regression models and\ud coherence calculations identify the remote winds from south of India and the sea\ud level anomalies east of Sri-Lanka as the major contributors to intra-seasonal SLA\ud variability on the west coast of India, with a minor contribution from the south-\ud east coast of India. Use of correlation techniques is complicated by the fact that\ud winds at the bottom of India are correlated with winds throughout the basin-scale\ud system. Some clarification is provided by coherence and phase calculations, which demonstrate the importance of the 40-60 day band in the intra-seasonal (30-120 day) period. Use of the 40-60 day band pass filter eliminates an unrealistically long (5 day) lag between local winds and sea level response on the west coast of India. Hovmoller diagrams help to illustrate the propagation of signals to the west coast of India for two different pathways: A Rossby wave pathway from Sumatra and an East coast pathway from the south-east coast of India. These pathways are consistent with the above statistical analysis