East Asian Winter Air Temperature And Its Association With The Mid-high Latitude Atmosphere Circulation Anomalies And The External Forcing

The variations of winter mean surface air temperature (SAT) over East Asia were analyzed by applying the empirical orthogonal function (EOF) analysis to both the NCEP/NCAR and the JRA-25monthly reanalysis data for the period of1979-2011. And by using NCEP/NCAR daily reanalysis data, the intraseasonal variability of East Asian winter temperature and the variations of extreme low (high) temperature over East Asia are investigated. This study will focus on the possible links between East Asian winter SAT and atmosphere circulation, and the relationship of the East Asian winter SAT with the early autumn external forcing factors (such as sea surface temperature, sea ice and snow cover, etc.). The main conclusions are summarized as follows:(1) Variations of East Asian winter SAT were revealed by both the NCEP/NCAR and JRA-25reanalysis data are consistent. Two dominant modes were clarified, with the leading EOF mode depicting a pan-East Asia temperature variation and the second EOF mode describing a meridional dipole between the northern and southern parts of East Asia. And the second leading EOF exhibits a notable interdecadal shift in the late1990s. Namely, the northern (southern) part of East Asia tends to be cooler (warmer) during the recent three decades.(2) From the analysis, we find that during the positive phase of PC1, positive SLP anomalies occur over the Pacific Northwest, while the negative SLP anomalies appear in Eurasia region with an anomalous center near to the Tajmyr Peninsula. The center of geopotential height anomalies in the northern regions of Northern Eurasia, Western Europe and East Asia are positive-negative-positive, separately. And the East Asian westerly jet tends to be weakened. Situations are opposite during the negative phase of PC1. Additionally, the previous autumn sea surface temperature (SST) anomalies over the East Arctic ocean and the Middle East Pacific Ocean, and the sea ice concentration (SIC) anomalies over the Barents-Kara-Laptev seas, as well as snow cover anomalies over northern Eurasia and Tibet Plateau, are closely linked to PC1. And these forcing factors may have important impact on the interannual variation of East Asian winter SAT.(3) The circulation system over the mid-high latitudes of Eurasia and East Asia has a close relationship with the interdecadal variation of PC2. These can be reflected from the distribution differences of circulation field between the periods of1997-2011and1979-1996(the former minus the latter). Differences of the winter mean SLP show a negative (positive) anomaly over the southern (northern) Eurasia. And the500hPa height differences exhibited a clear wave-train pattern, with anticyclonic anomalies over the northern Eurasia and cyclonic anomalies over the Baikal Lake. In addition, compared to the period of1979-1996, East Asian westerly jet tends to be intensified and located northward during the period of1997-2011. Additionally, the previous autumn SST and SIC could be the influencing factors of interdecadal shift of the East Asian winter SAT in the mid or late1990s. We can see that the previous autumn SST differences between the period of1997-2011and1979-1996show positive SST anomalies over the Arctic Ocean, North Atlantic Ocean, North India Ocean and the Northwest Pacific Ocean, and negative SIC anomalies over the northern Eurasia marginal seas and the Beaufort Sea. Particularly, the decadal shifts of the SST over the North Atlantic and the SIC over the Arctic are both well consistent with that of the winter SAT over East Asia.(4) The frequency change of East Asian winter extreme temperature has a close relationship with winter atmospheric circulation over the mid-high latitudes and previous autumn external forcing factors. The relations are shown below:When the frequency of extreme low temperature event is on the high side, it is evident that positive SLP anomalies occurred in northern Eurasia, and there was signification negative SLP over northwest Pacific Ocean. The500hPa height differences exhibited a clear wave-train pattern named EUP, with anticyclonic anomalies over the northern Eurasia and cyclonic anomalies over the Baikal Lake. And the East Asian westerly jet tended to be intensified and located southward. Further study shows that, the interannual variation of the frequency of East Asian winter extreme low temperature event is related with the SST anomalies over the East Arctic Ocean and Northwest Pacific Ocean, same as the SIC anomalies over Barents-Kara seas.When the frequency of extreme high temperature event is on the high side, the pressure difference between the land and sea, northerly anomaly, East Asian trough and East Asian upper-tropospheric jet would weaken. In addition, the interannual variation of the frequency of East Asian winter extreme high temperature event is related with the SST anomalies over the North Atlantic Ocean and Northwest Pacific Ocean, same as the SIC anomalies over Laptev seas and the snow cover anomalies over the northern Eurasia marginal and the Tibet Plateau.