Title: Drivers of Surface Chlorophyll Variability in the Tropical Indian Ocean: Observations, Future Projections, and the Role of the Indian Ocean Dipole

Time: June 4^th (Thursday) 9:30am-10:30am

Location: 13rd floor meeting room, Haozhiying Technology Tower

Presenter: Dr. Gayan Pathirana, Department of Oceanography and Marine Geology, Faculty of Fisheries and Marine Sciences & Technology, University of Ruhuna

Email: upgpathirana@fish.ruh.ac.lk

Abstract

Surface chlorophyll variability in the tropical Indian Ocean is governed by a complex interplay between regional ocean dynamics and large-scale climate variability, with significant implications for marine productivity and biogeochemical cycles. While long-term observations indicate a basin-scale decline in summer chlorophyll across the Northern Indian Ocean driven by enhanced upper-ocean stratification under global warming, recent evidence highlights pronounced regional heterogeneity, including a reversal toward increasing chlorophyll concentrations in recent years in areas around southern India and Sri Lanka. This emerging pattern is closely associated with physical changes such as thermocline shoaling and intensified wind stress curl, which enhance Ekman-driven upwelling and nutrient entrainment despite concurrent surface warming and weakened monsoonal winds, underscoring the limitations of current biogeochemical reanalysis products in resolving such variability. Superimposed on these regional processes, the Indian Ocean Dipole (IOD) plays a key role in modulating chlorophyll through a distinct dipole response, characterized by enhanced (suppressed) productivity in the southeastern (southwestern) Indian Ocean during positive IOD events, driven by anomalous wind-induced upwelling. Notably, this biological response exhibits asymmetry, with positive IOD phases exerting a stronger influence than negative phases. Future projections from Earth System Models suggest a strengthening of the IOD–chlorophyll relationship under greenhouse warming, whereby a shoaling thermocline and increased subsurface nutrient availability amplify surface chlorophyll responses despite a potential weakening of upwelling. Collectively, these findings emphasize that both regional-scale ocean dynamics and climate modes such as the IOD are critical in shaping present and future chlorophyll variability in the tropical Indian Ocean, highlighting the need for improved observational constraints and coupled modeling approaches to better understand and predict biophysical interactions in a changing climate.

About the presenter

Dr. Gayan Pathirana is a Senior Lecturer in the Department of Oceanography and Marine Geology at the Faculty of Fisheries and Marine Sciences & Technology, University of Ruhuna, Sri Lanka. He obtained his MSc from the State Key Laboratory of Tropical Oceanography at the South China Sea Institute of Oceanology (SCSIO), China, and his PhD from the Division of Environmental Science and Engineering at Pohang University of Science and Technology (POSTECH), South Korea. Dr. Pathirana is an active researcher and educator with expertise spanning climate science, air–sea interactions, ocean dynamics, and extreme weather events. His research primarily focuses on tropical air–sea interactions, biophysical processes in the Indian Ocean, and monsoon variability, with an emphasis on understanding their implications for regional climate and marine productivity.