Seasonal and intraseasonal oscillations in a monsoonal estuary

Estuaries on the Indian subcontinent are influenced by the Asian Summer Monsoon and show strong seasonality. As a result, these estuaries are often referred to as monsoonal estuaries (Vijith et al., 2009).  The seasonal cycle is superposed with intra-seasonal oscillations (ISO) with periods ranging from 10 to 90 days. Due to the lack of high-resolution data in Indian estuaries, even while the seasonal cycle of monsoonal estuaries is comparatively well understood, the intra-seasonal variability has not yet been examined. The active-break monsoon spells drive quasi-biweekly oscillations (10--20 days) within the ISO, and oscillations of 30–60 days are present due to northward-moving cloud bands.  In this study, we use high-resolution salinity, temperature and sea level measurements from the Cochin estuary, located on the southwestern coastal plain of India, to investigate the ISO-related variability. The spectral analysis of the sub-tidal signals of salinity, temperature and sea level shows that ISO exists year round (December 2019 to May 2021). During the dry season (December to April), the salinity was on an average of 30 PSU, and the amplitude of ISO was from 3 to 5 PSU. In the wet monsoon season, the amplitude of ISO varies around 5 to 10 PSU. The temperature, sea level, and precipitation spectrum also exhibited similar patterns in the wet season. The coherence of salinity with sea level and precipitation is also quantified. 

 

Reference

Vijith, V., Sundar, D., Shetye, S.R., 2009. Time-dependence of salinity in monsoonal estuaries. Estuar. Coast. Shelf Sci. 85, 601-608. . http://dx.doi.org/10.1016/j.ecss.2009.10.003