Atmospheric intraseasonal variability and the seasonal cycle over tropical Indo-Pacific region

This study investigates the seasonal variation of the tropical intraseasonal variability (ISV) of the deep convection over the Indo-Pacific region. The observational part of the analysis is based on NOAA OLR, NCEP reanalysis of the 850 hPa zonal wind and Reynolds SST for the 1979-2005 period. The ISV is estimated all along the time series by using a complex demodulation approach. The ISV has a strong seasonal cycle and shows maximum amplitude over monsoon regions off the equator. Monthly intraseasonal perturbations patterns of convection, low-level zonal wind and SST are well collocated and suggest mainly a dynamical response and SST feedback to deep convection triggered over warm water.

Over the central Bay of Bengal, the ISV appears and is maximal in May in relation with “bogus” monsoon onsets. For the Arabian Sea region, the ISV is for most years a single event in June that corresponds to the Indian monsoon onset and vanished afterward. The maximum amplitude for these two regions is reached before the deepening of the ocean mixed layer (as described by a mixed layer depth climatology) by the strong monsoon low-level jet. For other monsoon regions such as north of Australia and western Indian Ocean during boreal winter or over the South China Sea during boreal summer, the mixed layer depth remains relatively small and the ISV is distributed all along the monsoon season without any clear seasonal maximum. These results reinforce the hypothesis that the ISV could be mainly thermodynamically driven with a strong influence of air-sea coupling.

The impact of the SST on the ISV of the convection is further examined by performing the same seasonal ISV diagnostic for two ensembles of 9 simulations using the LMD GCM forced with SST from the Tropical Rainfall Measuring Mission's (TRMM) Microwave Imager (TMI) between 1998 and 2003 (and Reynolds SST poleward of 40°). The first ensemble is made with actual TMI SST dataset and the second one with the same SST time series filtered to retain periods larger than 90 days. Also, this seasonal ISV diagnostic is repeated for a coupled version of the LMD-GCM in order to inspect in more detail the link between the ocean mixed layer variability and the seasonal characteristics of the ISV of the convection.