The East Asian Winter Monsoon (EAWM)

The EAWM is one of the most energetic planetary scale circulation systems. Although the thermal direct circulation develops as a response to differential heating as is the case of the Asian summer monsoon, the two differ in more than one way. On the one hand, the heat source region of the EAWM is much closer to the equator, where the effect of the earth's rotation is diminished. On the other hand, the circulation of the Asian winter monsoon encompasses a larger meridional domain from southern tropics to northern extratropical latitudes, such that the tropical region has a strong interaction with the extratropical region and its baroclinic systems.

The midlatitude component of the EAWM is characterized by the cold-core Siberian-Mongolian high (SMH) at the surface whose variability affects all scales of the extratropical circulations. The SMH has been weakening in recent decades, which appears to correlate with the negative phase of NAO/AO due to increased warm air advection over the Eurasian continent and the resultant reduction in snow cover. However, it is not clear that this recent decrease in the EAWM intensity is unique in the most recent 400 years. Periodical cold air outbreaks that cause high-impact weather are associated with the intraseasonal and synoptic variation of the SMH, and they often continue as cold monsoonal surges into the tropics and affect the tropical component of the EAWM. There is some evidence that intraseasonal variability has decreased in 1990s although extreme weather events in the past few years counter this trend. An important mechanism for the intraseasonal and higher frequency enhancement of the SMH comes from upper level blocking ridges over the Atlantic and the Pacific. The Atlantic blocking triggers a Rossby wave train that has a downstream effect of enhancing the SMH. The Pacific blocking forces the SMH through slow retrogression of the blocking center.

The tropical component of the EAWM includes the northeasterly monsoon in the subtropics and the wet phase of the Maritime Continent monsoon. The region is situated bewteen the Asian and Australian summer monsoon and its annual cycle is characterized by asymmetries between boreal fall and boreal spring and also across the equator due to complex wind-terrain interactions. The Maritime Continent monsoon rainfall exhibits pronounced variability on all time scales from diurnal to interannual and longer, and is well correlated with ENSO during the dry and transition seasons. In this region the monsoon onset is substantially delayed during El Niño years, while the monsoon retreat is less impacted. Regional model simulations reveal increased monsoon rainfall intensities over orography during El Niño events, tied to strengthened diurnal land-sea and mountain breeze circulations, associated with weaker large-scale winds conditions during El Niño events. On intraseasonal and synoptic scales the region is heavily influenced by the MJO and cold surges, which can interact with each other as well as with in-situ synoptic systems such as the Borneo vortex, often leading to torrential rainfall, flash floods, and severe storms, including in one rare case a typhoon.