Monsoon Influence on the Island Mass Effect Around the Maldives and Sri Lanka

Abstract

The monsoon circulation in the Northern Indian Ocean (NIO) is unique since it develops in response to the bi-annual reversing monsoonal winds, with the ocean currents mirroring this change through directionality and intensity. The interaction between the reversing currents and topographic features have implications for the development of the Island Mass Effect (IME) in the NIO. The IME in the NIO is characterized by areas of high chlorophyll concentrations identified through remote sensing to be located around the Maldives and Sri Lanka in the NIO. The IME around the Maldives was observed to reverse between the monsoons to downstream of the incoming monsoonal current whilst a recirculation feature known as the Sri Lanka Dome (SLD) developed off the east coast of Sri Lanka during the Southwest Monsoon (SWM). To understand the physical mechanisms underlying this monsoonal variability of the IME, a numerical model based on the Regional Ocean Modeling System (ROMS) was implemented and validated. The model was able to simulate the regional circulation and was used to investigate the three-dimensional structure of the IME around the Maldives and Sri Lanka in terms of its temperature and velocity. Results revealed that downwelling processes were prevalent along the Maldives for both monsoon periods but was applicable only to latitudes above 4°N since that was the extent of the monsoon current influence. For the Maldives, atolls located south of 4°N, were influenced by the equatorial currents. Around Sri Lanka, upwelling processes were responsible for the IME during the SWM but with strong downwelling during the NEM. In addition, there were also regional differences in intra-seasonal variability for these processes. Overall, the strength of the IME processes was closely tied to the monsoon current intensity and was found to reach its peak when the monsoon currents were at the maximum.

Effect of ocean circulation and chlorophyll-a concentration on yellowfin tuna catch rates in Sri Lankan logline fishery

“…These monsoon currents reverse their direction twice a year and directly influence the seasonal circulation variability (Schott et al, 2009). NEM occurs from December to April and the SWM between June to October, along with two inter-monsoon periods in May and November (Su et al, 2021). During the NEM, currents flow from east to west (Figure 6).…”

Coral spawning patterns of Acropora across two Maldivian reef ecosystems

“…Multi-specific spawning events have also been observed to take place during monsoonal transition periods in Indonesia ( Wijayanti et al, 2019 ; Indrayanti et al, 2019 ). The bi-annual monsoon change strongly influences environmental parameters such as winds and currents in the North of the Maldives, in comparison to the South of the Maldives, which is less affected by monsoon changes and influenced by the equatorial currents ( Su, Wijeratne & Pattiaratchi, 2021 ). Given this study documented differences between two atolls in relatively close latitudinal proximity, the contrasting influence of the bi-annual monsoon change from the north and south of the Maldives highlights that further inter-atoll differences could be observed in spawning patterns across the North-to-South atoll chain of the Maldives and may be linked to the different effects of the monsoon, as well as intra-atoll variations further influenced by local environmental factors.…”

Anthropogenic nitrogen pollution threats and challenges to the health of South Asian coral reefs

“…The country experiences two monsoon seasons (southwest and northeast), has an average annual temperature of 30.7 °C and receives 2,124 mm of rainfall. The influence of the monsoon winds on local ocean circulation is significant and induces an appreciable island mass effect around the Maldives enhancing surface chlorophyll concentrations (Radice et al, 2019;Su et al, 2021). A recent numerical study of the island mass effect around the Maldives found both upwelling and downwelling favorable conditions during both monsoon seasons, but it was discovered that the seasonal development of downstream island wakes was limited to atolls above 4°N, with atolls further south being more heavily influenced by the zonal movement of equatorial currents (Su et al, 2021).…”