Impacts Of The Diversity In The Tropical Pacific Sea Surface Temperature Anomalies On Arctic Surface Air Temperature

In the context of global warming,Arctic has exhibited a significant warming trend,with a warming rate much above the Earth average,namely the “Arctic amplification”.Along with the melting of the sea ice,the rapid Arctic warming has a significant impact on the climate in the central latitudes of the Northern Hemisphere,especially in East Asia.Both from the scientific level and the strategic needs of the country,higher requirements for forecasting the changes of Arctic surface air temperature(SAT)are needed.Besides significant warming trend,Arctic SAT also shows a strong interannual variability,which is suggested to be related to the interannual oscillations of climate system.El Ni(?)oSouthern Oscillation(ENSO)acts as the most significant interannual oscillation signal in the global climate system,which is also one of the main sources of global interannual variation predictability.ENSO events can trigger a remote effect on the climate in high latitudes.Therefore,it is important to investigate the relationship between ENSO and Arctic in order to improve the prediction ability of Arctic SAT.However,with the rapid warming of Arctic,ENSO exhibits distinct diversity in spatiotemporal features such as amplitude,spatial modes and life cycle,with different climatic effects.The scientific community currently has a very limited understanding of the impacts of ENSO diversity on Arctic SAT.Based on this,this paper conducts a study on the impact of ENSO diversity on Arctic SAT,looking forward to improving the understanding of the climate impact difference of ENSO diversity and the changes in Arctic SAT.The main results are summarized as follows:(1)Arctic warming trend has significant seasonal lock.Autumn and winter show greatest warming rate.At the same time,both the surface and the upper troposphere over the Arctic show significant warming trend,and during other seasons except summer,the maximum warming rate concentrates near the ground.Among them,the warming trend in autumn shows significant periodic characteristics,while which in winter shows obvious linear warming trend.Both autumn and winter are accompanied by strong interannual oscillations.Through empirical orthogonal function(EOF)decomposition,the warming trend mode(EOF1)in winter is obtained and the SAT warming center is located in the area of Greenland and the Barents-Kara Seas;The second and third modes are dominated by interannual variability,showing the seesaw distribution of EurasiaNorth America and the reverse phase inside and outside the polar regions.The strong interannual oscillations and regional differences of Arctic SAT in winter indicate that excluding the long-term warming from greenhouse effects,the internal variation of the climate system including ENSO is able to cause the interannual SAT changes in the polar region.(2)In order to analyze the possible impact of the tropical Pacific sea surface temperature anomalies(SSTAs)on Arctic SAT,the response of Arctic SAT in winter during extreme El Ni(?)o events was analyzed.Since 1950,three extreme El Ni(?)o events have occurred in 1982-1983,1997-1998 and 2015-2016.The three events are manifested as significant positive SSTAs in the east tropical Pacific,and all three events are also accompanied by typical Pacific-North American(PNA)waves.However,Arctic SATs in the three events showed significant difference.In further analysis,the SSTA distributions of the three extreme El Ni(?)o events are different.Among them,the ENSO events in1982/83 and 1997/98 are usually classified to the so-called eastern Pacific(EP)ENSO events.However,the difference in their eastern sea surface temperature for these two events triggered different non-adiabatic thermal distributions,resulting in differences in the polarward wave fluxes and the opposite SAT distribution in the Arctic.The above work has confirmed the impacts of the tropical Pacific SSTAs on the Arctic region,pointed out that the diversity distribution of SSTAs can induce the diversity of the influence on Arctic SAT.Therefore,further dividing the diversity characteristics of the tropical Pacific SSTAs distribution is needed.(3)ENSO events are usually monitored by the tropical Pacific sea surface temperature anomaly(SSTA)patterns with dramatic impacts on the global climate.To explore the diversity of the tropical Pacific SSTA,a novel method combined EOF decomposition and the k-means clustering algorithm is carried out to classify SSTA patterns during 1950-2016.Meanwhile,the total distance variance and total silhouette value are introduced to determine an optimal number of distinguishable representative SSTA patterns.Ten SSTA patterns are obtained,which shows frequent basin-wide warming and extreme cold ENSO events in recent decades.It may be attributed to the changes in composition of the intrinsic modes along with the background mode of slowly increasing east–west SST gradient.The comparative analysis between periods 1950-1969 and 1997-2016 suggests that the two regimes of tropical Pacific SSTA,featured as extreme warm and moderate warm/extreme cold patterns respectively,become more distinct under recent global warming.(4)Including significant warming trend,the Arctic SAT changes also exhibit strong interannual variations in various fields,which is suggested to be related to ENSO events.Previous studies have demonstrated the different impacts on the Arctic of central Pacific(CP)and eastern Pacific(EP)ENSO events,and suggested these impacts are largely of opposite sign for ENSO warm and cold phases.Our results illustrate asymmetrical changes for the cold and warm ENSO events,especially for the La Ni(?)a events.Compared to the past frequent basin-wide cooling La Ni(?)a events,since the 1980 s the cooling center for the La Ni(?)a event has strengthened and moved westward along with the increasing frequency for the canonical and CP La Ni(?)a events.Contrary to the basinwide cooling and canonical La Ni(?)a events,the frequent CP La Ni(?)a events induce significant warming from the Beaufort Sea to Greenland via the convection center moving northward over the western Pacific.Observation analysis and numerical experiments both suggest that the changes in La Ni(?)a type may also accelerate Arctic warming.