Biological dipole mode indices: New parameters to characterize the physical and biological processes of the Indian Ocean Dipole event

Abstract

The Indian Ocean Dipole (IOD) is a basin-wide ocean–atmosphere phenomenon that has profound impacts on the global climate, land, and ocean. The Dipole Mode Index (DMI), which is defined as the difference of the sea surface temperature (SST) anomaly in the east IOD zone and the west IOD zone has long been used to characterize and quantify the strength of IOD events. In this study, we propose biological dipole mode indices (BDMIs) based on the dipolar observations of chlorophyll-a (Chl-a) anomalies (difference and relative difference) in the east and west IOD zones during the IOD event. The two BDMIs, which are based on Chl-a difference and relative difference, not only represent the dipolar biological activities in the Equatorial Indian Ocean, but also reflect the thermocline dynamics in the east IOD zone and west IOD zone. The in situ measurements in the east and west IOD zones show the clear linkage between the BDMIs and the dynamics of the 20°C isothermal depth. This linkage is attributed to the changes of the nutrient supplies driven by the various ocean physical processes in the IOD event, thus the BDMIs could also act as the surrogate for the thermocline dynamics in the two IOD zones. The BDMIs from satellite ocean color observations show that they can identify and characterize all the major IOD events in the last two and half decades. The SST-based DMI and Chl-a-based BDMIs may depict some different aspects of the IOD events (e.g., surface versus subsurface properties). The performance comparison between the two BDMIs and DMI also shows that the BDMIs and traditional DMI can effectively detect IOD signal for the major IOD events. Indeed, the BDMIs and DMI are complementary for characterizing the IOD events, and the combination of these indices can provide a better understanding of the atmosphere and ocean processes for both surface and subsurface, as well as biological processes in the Equatorial Indian Ocean.