Tidal and non-tidal sea level variations at two adjacent ports on the southwest coast of India

Abstract

Analysis of sea level data collected at two adjacent ports (Cochin and Beypore) on the southwest coast of India has been made to understand tidal and non-tidal variations. Amplitudes of the tidal harmonic constituents showed that M2 is maximum, followed by K1, for both the stations. A large spring-neap variation and monthly variation in the semi-diurnal forcing was seen at both the sites. The Form Numbers indicated that the tides are of a mixed and predominantly semi-diurnal type. Amplitudes of most of the tidal constituents are slightly larger at Beypore than at Cochin. The sea level variation is dominated by tidal signals at both the stations. Seasonal variation of the amplitudes of the most important harmonic constituents namely O1, K1, M2 and S2 are presented. The mean spring and neap tidal ranges exhibited a higher degree of variability (over the seasonal cycle) at Beypore than at Cochin. The annual cycles of non-tidal sea level at both the sites were similar, with most conspicuous changes during June and July at Beypore, caused by summer monsoonal river discharge. The annual cycles of atmospheric pressure at both the sites were remarkably similar. The atmospheric pressure is not an important controlling factor on sea level at both the sites. Analysis of the sea level series pertaining to the premonsoon season indicated that Cochin marginally leads Beypore, suggesting the propagation of coastal trapped waves from south to north. The cross-correlograms of the atmospheric pressure time series for the premonsoon and summer monsoon seasons between Cochin and Beypore suggest the mesoscale nature of the atmospheric pressure system.