Coupled Processes that Determine the Basic States of Tropical Indian Ocean SeaSurface-Temperature (SST) and Asian Monsoon Precipitation

University Of Hawaii, Honolulu

Investigators

Abstract

The overall goal of the proposed work is to identify the major causes for the persistent errors in simulating the Indian-Ocean monsoon in coupled models. This project will identify coupled air-sea interaction processes that are critical for shaping the basic state of monsoon precipitation in the Indian Ocean as well as investigate the sensitivity of the basic state to variations The research will help to train young climate scientists through the support of graduate students and a postdoctoral fellow. A coupled ocean-atmosphere model (the coupled atmosphere-ocean-sea ice model for the Earth Simulator, CFES) will be used as a tool to examine the following three hypotheses: (i) In the Indian-Ocean monsoon region as well as in the deep tropics, necessary conditions for the occurrence of deep convection are processes that moisten the free troposphere, e.g., the entrainment rate and fraction of convective precipitation exposed to evaporate. (ii) Positive precipitation errors in the southwestern Indian Ocean during the intermonsoons lead to weakened equatorial westerlies. The weak easterlies in turn weaken the Wyrtki Jets and, hence, the advection of warm water to the eastern Indian Ocean. Furthermore, wind curl on the southern flank of the weak westerlies deepens the thermocline in the southwestern Indian Ocean. The resulting changes in temperature structure lead to near-equatorial sea surface temperature (SST) anomalies that impact the monsoon in subsequent seasons, including erroneous dry conditions over South Asia during boreal summer. (iii) Freshwater influx due to precipitation and river run-off along with vertical mixing, coastal waves and horizontal transport stratifies the upper ocean over the northern Bay of Bengal and eastern Indian Ocean. In some instances, the salinity stratification is intense enough to create a temperature inversion, which inhibits cooling of SSTs during boreal winter and aids the maintenance of warm SSTs during the next boreal summer that is conducive for deep convection. Existing CFES simulations will be diagnosed and compared to observations. In addition, systematic experiments designed to isolate key processes and to explore the sensitivity of solutions to those processes, will be carried out. Having identified such processes, their parameterization in Community Climate System Model (CCSM4) will be examined. This effort will lead to insights that have the potential to lead to model improvements.