Evaluating CFSv2 Simulations for the Phase-locked Intraseasonal Variation of the Asian Summer Monsoon

Many previous observational studies have shown that the intraseasonal oscillation superimposed on the slower seasonal cycle tends to be phase-locked to the latter, which can largely control the onset/break date of local rainy season over much of the Asian monsoon region. This study investigates how well the climatological intraseasonal variation, as well as seasonal cycle, is simulated in CFSv2. The climatological annual cycle of CFSv2 is derived from the last 25 years of a 30-year simulation starting from the ocean-atmosphere CFSR state on November 1, 1980 and is decomposed into the climatological slow seasonal cycle and fast intraseasonal component (covering timescales from 7.5 to 90 days). It is seen that the simulated slow seasonal cycles of outgoing longwave radiation (OLR) are in good agreement with the observational ones in both Indian (80°E-100°E) and East Asian (120°E-140°E) monsoon regions after the CFSv2 model biases (overestimated OLR mainly due to less total clouds) are removed. The northward propagation of the fast intraseasonal variations (both wet and dry phases), a key feature of summer monsoon in both regions, is also well reproduced. In the Indian monsoon region, however, CFSv2 shows a warm SST bias (greater than 1°C between 0° and 10°N) in the seasonal cycle till mid May, responsible for earlier monsoon onset and long lasting stronger convections in the simulation. The resulting shortwave radiative heat flux reduction gradually leads to negative SST anomalies and then the following long lasting dry phase at the same latitude whereas both the continuing shortwave radiation and diminished latent heat fluxes result in positive SST anomalies in the north of convections, accounting for the northward propagation of intraseasonal variations. More severe air-sea interaction due to initially warm SST bias, in turn, induces a cold SST bias in the seasonal cycle starting from early fall in the Indian monsoon region. On the other hand, in the East Asian monsoon region, later monsoon onset and much weaker air-sea coupling than those in observations arise from a cold SST bias in CFSv2 around 25°N till early June, which turns into a warm SST bias after summer monsoon. These findings suggest that reducing climatological SST bias in CFSv2 may be a primary factor for better Asian summer monsoon prediction.