Unravelling the roles of Indian Ocean Dipole and El-Nin tilde o on winter primary productivity over the Arabian Sea

The Northern Arabian Sea (NAS) is a highly productive basin in the Indian Ocean. The marked seasonal variation of surface winds has the major control on the oceanic primary productivity in the NAS. A regional physical and biogeochemical coupled model was used to examine the control of marine physical processes on the primary productivity in the NAS. It is found that the pure positive Indian Ocean Dipole (PPIOD) has positive anomalies, whereas a co-occurrence of El-Nin & SIM;o and positive IOD (CEPIOD) events leads to negative anomalies in winter-time Chlorophyll-a (Chl-a) concentration during 2000-2018. The CEPIOD events are charac-terized by weaker winter convective mixing than PPIOD events. The evolution of this discrepancy in the convective mixing process is sufficiently explained by the presence of weaker northeasterly (dry and cold) winds (< 2.4 m/s) and a lower net heat flux loss (<-47 W/m2) during CEPIOD as compared to the PPIOD events. Higher convective mixing results in intense convective cooling, a deeper mixed layer depth (MLD), increased nutrients' injection through entrainment, and a stronger winter bloom, whereas weak convective mixing causes warming and a shallow MLD, resulting in weak winter bloom. The higher-mixing and surface-cooling condition was observed more often in PPIOD than in CEPIOD years. These conditions promote vertical convective mixing that maintains nutrients' supply in the near-surface layers and supports primary productivity in PPIOD years.

Automated summary

The primary productivity in the Northern Arabian Sea is controlled by the seasonal variation of surface winds. Positive Indian Ocean Dipole events lead to increased Chlorophyll-a concentration in the winter, while the co-occurrence of El Niño and positive IOD events result in decreased concentration. Weaker winter convective mixing is observed during CEPIOD events compared to PPIOD events, which can be explained by the presence of weaker northeasterly winds and a lower net heat flux loss. Higher convective mixing promotes stronger winter bloom and supports primary productivity in PPIOD years.