Impact Of Tropical Indian-Pacific Ocean Warming On Decadal Change Of Northern Hemisphere Winter Climate

The sea surface temperature (SST) anomaly in the tropical oceans is an important thermal force that can affect climate anomalies over many regions of globe. The observational analysis suggests that the SST of the tropical Indian-Pacific oceans showed a pronounced decadal warming since the late1970s. Understanding the impact and mechanism of the warming on decadal change of Northern Hemisphere (NH) winter climate is of great scientific significance to understand and forecast decadal climate change. The impact of tropical Indian-Pacific oceans warming on decadal change of global climte change is still an open question. In this study, with the ERA-40and NCEP reanalysis data and a series of atmospheric general circulation model (CCM3/NCAR) numerical simulations, some key issues associated with the impact and mechanism of the Indian-Pacific ocean warming on decadal change of NH winter climate are investigated, in which the impact on decadal change of winter atmospheric general circulation in different regions of NH, the winter snow depth over Tibetan Plateau (TP) and the East Asian winter precipitation, and NH storm tracks is particularly emphasized. The main conclusions are as follows:(1) Since the1980s, the impacts of the tropical Indian-Pacific ocean warming on the decadal change of winter climate over northern Atlantic, northern Pacific, Europe, East Asia, and northern America are quite different. The tropical Indian Ocean (TIO) warming is the principal contributor to a decadal positive-phase North Atlantic oscillation (NAO) anomaly, while the tropical central-eastern Pacific (TCEP) warming is concurrent with the anomalies associated with the Pacific-North American (PNA) pattern. The tropical Pacific warming is closely associated with the northern Pacific decadal cooling, and their interaction determines the decadal anomaly of the PNA pattern.The Pacific ocean-atmosphere system coherently experienced a significant abrupt change around1976/77, which caused a warming in the tropical central-to-eastern Pacific, and a cooling in the midlatitude North Pacific, together with a strengthened Aleutian low with its position shifting southeastward, and a PNA pattern in its positive phase. Accompanied with the Pacific Decadal Oscillation (PDO), the TIO also showed a significant decadal warming. A series of atmospheric general circulation model (CCM3) experiments with prescribed SST are conducted to examine the impact of the Indian-Pacific SST change on the northern Hemisphere wintertime atmospheric circulation. The results show that the TIO SST and the tropical Pacific SST act on the decadal change of northern Atlantic, northern Pacific, Europe, East Asia, and northern America winter climate quite differently. The TIO warming is the principal contributor to a decadal positive-phase NAO anomaly over the North Atlantic, but the TCEP warming can not induce the NAO positive phase. The TIO warming gives rise to an anticyclonic anomaly over the midlatitude North Pacific and a positive-phase NAO anomaly over the North Atlantic that extends to the Europe. The TCEP warming is a key factor to cause the decadal PNA pattern, while the impact of independent northern Pacific cooling on PNA is not remarkable. The TIO warming causes a negative PNA pattern, along with a weakened East Asian jet. To East Asia, both the impact of TIO and TCEP SST change are important. The NAO positive phase caused by TIO warming and PNA positive pattern caused by Pacific SST change have great effect on decadal change of Canadian and American winter climate. Such a consistent effect on the Northern Hemisphere atmospheric circulations is closely associated with the changes in the high-level jet stream and the diabatic heating caused by the precipitation. (2) The tropical Indian Ocean warming can cause more moisture supply over southeast Tibetan Plateau (TP) and a deeper India-Burma trough, therefore leading to a significant decadal increase of snow depth over eastern Tibetan Plateau during the1980s-1990s. The decadal TIO warming is in favor of more winter precipitation in South China.The snow depth over eastern TP from the late1970s to the end of the1990s shows a pronounced increase, while the amount of winter precipitation over East Asia increased in the corresponding period. Uncoupled atmospheric general circulation model (CCM3) experiments with prescribed SST are conducted to examine the impact of different tropical oceanic basins (the Indian Ocean versus the tropical Pacific Ocean) on the wintertime snow depth and precipitation variability over TP as well as in South China. The results show that the TIO warming played a significant role in the increase of the snow depth over eastern TP from the1980s to the end of the1990s, and also has contributed to the increase of wintertime rainfall in the South China during the same period. Such an impact of TIO warming is in contrast to the impact of the tropical western Pacific warming, and is also different from the tropical central and eastern Pacific warming. It is found that the increased winter snow depth over the eastern TP after the mid-1970s is concurrent with a deeper India-Burma trough, that is beneficial to more cold air flowing southward to TP due to the warming of TIO. Additional factors for the excessive snowfall depth include more moisture supply associated with the intensification of the southerly flow over the Bay of Bengal and an increase of humidity over the Indian Ocean.(3) The Indian-western Pacific Ocean (IWP) warming is a key factor to cause decadal northward shift of the North Atlantic storm track since the1980s.The ERA-40reanalysis data shows that an increased transient eddy (TE) activity north of the climatological north Atlantic storm track axis but a reduced TE activity south of it. With40-yr integration output of two atmospheric general circulation models (GAMIL/IAP and HadAM3/UKMO) forced with identical prescribed seasonally-varying sea surface temperature, this study examines the effect of the observed IWP warming on the Northern Hemisphere storm tracks (NHSTs). Both models indicate that the observed IWP warming tends to cause both the North Pacific storm track (NPST) and the North Atlantic storm track (NAST) to move northward. The IWP warming can excite a wavelike circum-global teleconnection in the geopotential height that gives rise to an anticyclonic anomaly over the midlatitude North Pacific and a positive-phase NAO anomaly over the North Atlantic. These geopotential height anomalies tend to enhance upper-level zonal westerly winds north of the climatological jet axes and increase low-level baroclinicity and eddy growth rates, thus favoring transient eddy more active north of the climatological storm track axes, responsible for the northward shift of the both storm tracks. The IWP warming-induced northward shift of the NAST is quite similar to the observed, suggesting that the IWP warming can be a key factor to cause decadal northward shift of the NAST since the1980s. However, the IWP warming-induced northward shift of the NPST is completely opposite to the observed, implying that the observed southward shift of the NPST since the1980s would be primarily attributed to other reasons, although the IWP warming can have a cancelling effect against those reasons.