Role of Seasonal Transitions and the Westerly Jet in the Interannual Variabiligy of the East Asian Summer Monsoon

A recent hypothesis holds that variations in the East Asian Summer Monsoon can best be explained by changes in the timing and duration of the seasonal stages (Pre-Meiyu, Meiyu, and Midsummer) and that these timings and durations are controlled by the meridional position of the mid-latitude westerly jet. We examine this hypothesis with the help of the first mode of East Asian rainfall, which is better known as the characteristic "tripole" pattern. We use self-organizing maps of precipitation to objectively identify the onsets and durations of the seasonal stages over a climatological year. These are linked very closely to the meridional transitions of the westerly jet but lag 3 days behind, which indicates a fairly stable relationship between the jet's north-south positioning and the seasonal stage of the East Asian Summer Monsoon. In terms of the interannual variability, one phase of the tripole pattern - associated with more rainfall over central eastern China but reduced rainfall over northeastern and southeastern China - is associated with a longer Meiyu stage and shorter Midsummer stage (and vice versa for the other phase). For this phase, both the onset and termination of the Meiyu are delayed, but the end of the Meiyu is delayed by even more than its onset, resulting in a net lengthening; while the end of Midsummer stage comes even earlier and is accompanied by an earlier southward shift of the jet. Finally, we characterize the `tripole' mode of interannual variability using various fields, including the frequency and spatial distribution of extratropical cyclones.