Understanding the predictability of East Asian summer monsoon from the reproduction of land-sea thermal contrast change in AMIP-type simulation

Previous studies on the predictability of East Asian summer monsoon circulation based on SST-constrained AMIP-type simulation show that this phenomenon is reproduced with lower skill than other monsoon patterns. The authors examine the reason in terms of the predictability of land-sea thermal contrast change. In the observation, a stronger monsoon circulation is dominated by a tropospheric warming over East Asian continent and a cooling over the tropical western Pacific and North Pacific, indicating an enhancement of the summertime “warmer-land-colder-ocean” mean state. The tropospheric cooling over the tropical western Pacific and North Pacific, and the tropospheric warming over East Asian continent are reproducible in AMIP-type simulations, although there are biases over both North Pacific and East Asia. The tropospheric temperature responses in the model indicate a reasonable predictability of the meridional land-sea thermal contrast; the zonal land-sea thermal contrast change is also predictable but shows bias over the region north to 25°N of North Pacific. The reproducibility of the meridional thermal contrast is higher than that of the zonal thermal contrast. An examination of the predictability of two commonly-used monsoon indices reveals far different skills. The index defined as zonal wind shear between 850 hPa and 200 hPa averaged over East Asia is highly predictable. The skill comes from the predictability of the meridional land-sea thermal contrast. Although the zonal thermal contrast change is mostly predictable except for the biases over North Pacific, the monsoon index defined as zonal Sea Level Pressure (SLP) difference across East Asian continent and North Pacific is unpredictable. The low skill is related to the index definition, which attaches more importance to the land SLP change. The limitation of the index in measuring the land SLP change reduces the model skill. Although regional features of monsoon precipitation changes remain a challenge for current climate models, the predictable land-sea thermal contrast change sheds light on monsoon circulation prediction.