Response of ocean surface waves to the co-occurrence of Boreal Summer Intra-Seasonal Oscillation and El Nino Southern Oscillation

Abstract

Studies related to the combined effect of different atmospheric oscillations on the ocean surface waves are limited. The present study focuses on the change in wave climatology due to the combined effect of Boreal Summer Intra-Seasonal Oscillation (BSISO) and El Nino Southern Oscillation (ENSO) using 40 years of reanalysis data on the Asian Summer Monsoon Region (ASMR). Composite analysis of surface wind, significant wave height, wind-sea, swell and mean wave period are analysed corresponding to different ENSO-BSISO phase combinations. The study showed noteworthy differences in wave parameters when ENSO-BSISO phases are analysed together. In El Nino-BSISO combined phase, enhancement of north-easterly wind causes the advancement in the reversal of wind direction (north-easterly to monsoon enhanced south-westerly) and disrupted propagation of positive wind and wave anomalies. Conversely, during La Nina-BSISO co-occurrence, south-westerlies are enhanced and as a result, the negative north-easterly anomalies are disrupted. In all the ENSO-BSISO combined phases, maximum wave height variability occurs at the South China Sea owing to the combined action of the north Indian Ocean (NIO) and Western North Pacific (WNP) surface wind forcing. High fluctuations in Tm over the NIO and WNP are observed during different ENSO-BSISO phase combinations. The phase relationship of Hs with Tm and the significant height of wind-sea (Hsw) and swell (Hss) are assessed to understand the propagation of swells. Due to the effect of multiple atmospheric perturbations, significant changes in Hs occur over the coastal regions of the NIO.