Influence Of The Boreal Summer Intraseasonal Oscillation On Extreme Temperature In Northern Hemisphere

The impact of the boreal summer intraseasonal oscillation(BSISO)on extreme hot and cool events was investigated,by analyzing the observed and reanalysis data for the period from 1983 to 2012.It is found that the frequency of the extreme temperature events in mid-high latitudes is significantly modulated by the BSISO convection in the tropics,with a 3-9-day lag.During Phases 1 and 2 when the BSISO positive rainfall anomaly is primarily located over a northwest—southeast oriented belt extended from India to Maritime Continent and a negative rainfall anomaly appears in western North Pacific,the frequency of extreme hot events during active BSISO increases by 40%compared to that during inactive BSISO in Phases 1 and 2.Most pronounced increase of frequency of extreme temperature events appears in North Pacific north of 40N,North America,and Polar region.In contrast,more frequent extreme cool events happen during Phases 5 and 6 over these regions.Two physical mechanisms are primarily responsible for the change of the frequency of the extreme temperature events.Firstly,in the phase1 and phase2,an upper-tropospheric Rossby wave train(due to the wave energy propagation)is generated in response to a negative heating anomaly over tropical western north Pacific.This wave train consists of a strong high pressure anomaly center northeast of Japan,a weak low pressure anomaly center over Alaska,and a strong high pressure anomaly center over the western coast of United States.Easterly anomalies to the south of the two strong mid-latitude high pressure centers weaken the climatological Subtropical Westerly Jet along 40°N,which induces anomalous subsidence and wanning in North Pacific north of 40°N.Secondly,an enhanced monsoonal heating over South Asia and East Asia sets up a transverse monsoonal circulation,with large scale ascending(descending)anomalies over tropical Indian Ocean(Pacific).Both the processes favor more frequent extreme hot events in middle and high latitude northern hemisphere.In phase 5 and phase 6,the situation is mainly opposite to phase 1 and phase 2.An anomalous atmospheric general circulation model is used to confirm the modulation of the tropical heating effect on the mid-high latitudes extreme temperature events.