The role of ENSO in the Tropical Intra-seasonal Oscillation

Abstract:

Deep convection over the tropics initiates over the Indian Ocean and propagates into the Pacific on intra-seasonal (30-90 days) timescales. This so-called Tropical Intra-Seasonal Oscillations (TISO) impacts on global weather and climate, such as El Nino/Southern Oscillation (ENSO), extreme weather in northern hemisphere, Asian and Australian monsoon, so on. However, the individual TISO events are unpredictable beyond 20 days. In the Pacific, on the other hand, ENSO occurs on intra-annual (2-7 years) timescales with significant global socio-economic and environmental impact, and has a predictability of 8-10 months prior to its mature stage. Here we use NCAR Community Climate System Model (CCSM4) forecasts and observational estimates to show the roles of ENSO in the TISO. This study shows that mixed layer depth in the southwestern Indian Ocean, which is known to be one of the TISO onset regions, deepens (shoals) during El-Nino (La-Nina) through the low frequency modulation. The deeper (shallower) mixed layer reduces (enhanced) TISO-associated Sea surface temperature (SST) and wind variability, which leads to less (more) favorable conditions for TISO onset over the Indian Ocean.