Decade-to-decade changes in the sea surface temperature (SST) of the Indian Ocean (IO) have been implicated in a variety of worldwide climatic fluctuations, including variations in Indian monsoon rainfall, changes in the frequency of cyclones in the Arabian Sea, and changes in drought frequency in the US, the Mediterranean, and parts of Africa. Averaged over the basin, IO SSTs have been increasing since the 1950s, often at a faster rate than SSTs in other tropical oceans, and have continued to increase even during the recent global warming hiatus. The goal of this research is to understand the mechanisms that produce decadal variability of IO SSTs and accompanying variations of sea level pressure (SLP), surface winds, and other meteorological conditions. A particular goal is to identify the separate roles of remote influences from the Pacific sector, and coupled ocean-atmosphere dynamics internal to the IO sector, in generating decadal IO SST variability. The Pacific influence is associated with the interdecadal Pacific oscillation (IPO), a slowly varying pattern of SST anomalies with some resemblance to the more rapidly evolving SSTs of El Nino/Southern Oscillation events. Previous work suggests that the positive phase of the IPO, when SSTs warm over the central equatorial Pacific, can produce uniform warming over the IO. The PI hypothesizes that the warming is caused by strengthening of the Walker circulation, which enhances subsidence over the IO and increases surface insolation by reducing cloudiness. But the more recent observational record shows a continued increase in mean IO SSTs even as the IPO has transitioned to its negative phase, so factors other than the IPO must also play a role in basin-wide IO warming and cooling. While the Pacific influence is linked to uniform SST anomalies, the IO basin contains a second prominent decadal variability mode which is largely independent of the IPO. This mode is an east-west pattern in which warm SSTs in the western half of the basin are accompanied by cold anomalies off the coast of Indonesia, referred to as the Indian Ocean dipole (IOD). The PI hypothesizes that the IOD is driven by coupled atmosphere-ocean dynamics in which subsurface heat transport plays a key role. In addition to attribution of IO variability in terms of internal dynamics and external influences from the Pacific, the project also seeks to understand the role of external forcing, including greenhouse gas-induced warming, solar variability, and anthropogenic aerosols. A variety of statical techniques are applied to diagnose the mechanisms of IO variability in observations, reanalysis products, and climate model simulations. In addition, specialized climate model experiments are performed in which atmosphere-ocean interactions are enabled only in the IO sector to determine the extent to which IO-sector dynamics can produce decadal variability. Complementary experiments are used to determine the extent to which decadal variability in the IO sector is due to coupled atmosphere-ocean dynamics external to the basin. Further climate model experiments examine the role of external forcing by imposing changes in greenhouse gases, aerosols, and solar irradiance.
The work is of societal as well as scientific interest given that that conditions in the Indian Ocean affect the weather and climate of the adjacent land region, which is home to perhaps a third of the world's population, much of it in developing countries with heightened vulnerability. Also, as noted above, there is evidence that the decadal variability of IO SSTs are linked to a variety of climatic impacts worldwide, including some that affect the US. In addition, the project would support and train a postdoc, thereby providing workforce development in this research area.
