The Relationship Between Arctic Synoptic-scale Warming And Tropical Ocean And Mid-Latitude Atmosphere

In late December 2015,due to an Atlantic storm moving into the Arctic region,the Arctic winter daily air temperature broke the melting point;in mid-March 2022,the Arctic air temperature is close to 0 ℃ again.Furthermore,such Arctic synoptic-scale warming events have occurred many times in history at the polar region.At the moment,academic research on Arctic warming mainly focuses on long-term temperature increase in the Arctic;however,less attention has been paid to Arctic synoptic-scale warming events.Using multiple sets of reanalysis datasets and the model data of Coupled Model Intercomparison Project phase-6(CMIP6)models,this doctoral thesis used linear regression,composite analysis and sliding correlation methods to analyze the synoptic-scale and interannual-scale atmospheric circulation characteristics of the Arctic winter Rapid Tropospheric Daily Warming(referred to as RTDW;i.e.,synopticscale warming)event,with a focus on the change in the influence of the atmospheric and oceanic main interannual modes,such as Arctic Oscillation(AO),North Atlantic Oscillation(NAO),El Nino-Southern Oscillation(ENSO),and Central Pacific El NinoSouthern Oscillation(CP ENSO),on the occurrence frequency of RTDW events,as well as the related possible mechanism.The main conclusions are as follows:1.From 1950 to 2018,RTDW events were defined as an increase in Arctic winter daily near-surface air temperature(i.e.,the air temperature difference between adjacent days)that exceeded a certain threshold;they can be classified into Pacific RTDW(PRTDW)events and Atlantic RTDW(A-RTDW)events,with warming energy coming from the Pacific and Atlantic oceans,respectively.The key synoptic-scale process of the RTDW event occurrence:there were southerly anomalies in the middle troposphere,which could lead the cyclone into the Arctic to trigger the occurrence of the RTDW event.2.From the North Pacific to the Arctic,an AO-related northwest-southeast dipole pattern induced southerly wind anomalies at the mid-troposphere,steering the cyclone into the Arctic to trigger the occurrence of P-RTDW event.Since the early 1980s,the correlation between AO and the occurrence frequency of P-RTDW events has strengthened,which is attributed to the intensified Pacific storm track(PST)activity intensity.Since this period,the stronger PST has induced stronger synoptic-scale eddy feedbacks to low-frequency flows;meanwhile,the stronger synoptic-scale eddy feedbacks could provide a favorable condition for the formation of such a dipole pattern associated with the AO,bolstering the connection between AO and P-RTDW events.Prior to this period,however,the eddy feedback is weak due to the weak PST intensity;thus it is unable to provide the favorable conditions.3.Around the mid-1980s,there is an interdecadal change in the effect of NAO on the occurrence frequency of A-RTDW events.Specifically,before the mid-1980s,in the positive(negative)phase of NAO,500 hPa southerly(northerly)wind anomalies were over the North Atlantic(NA),increasing(decreasing)the occurrence frequency of A-RTDW events by leading the Atlantic storms into(away from)the Arctic;thus,NAO could influence the occurrence frequency of A-RTDW events.However,since the mid-1980s,the relationship between the NAO and the occurrence frequency of ARTDW events has weakened;in the positive(negative)phase of NAO during this period,such 500 hPa southerly(northerly)wind anomalies over the NA disappeared due to enhanced Atlantic storm track(AST)activity intensity;the enhanced AST induced an enhanced NAO-related cyclone through the interactions between synopticscale eddy and mean flow,causing the southerly and northerly wind anomalies disappearing over the NA.4.Around the late 1970s,the ENSO’s impact on the occurrence frequency of PRTDW existed an interdecadal change,possibly attributed to changes in ENSO intensity.Because of the stronger ENSO intensity since the late 1970s,ENSO has induced a stronger Rossby wave;then,El Ni?o(La Ni?a)could deepen(weaken)the Aleutian low and strengthen northerly(southerly)wind anomalies over the North Pacific,thereby decreasing(increasing)the occurrence frequency of P-RTDW events.In contrast,due to the weaker ENSO intensity prior to the late 1970s,ENSO had no apparent direct impact on the occurrence frequency of P-RTDW events in this period.These findings will provide a potential relation between the equator and the Arctic to improve the prediction skill of extreme Arctic synoptic-scale warming events.By analyzing CMIP6 models,this doctoral thesis confirmed that the potential relation may be strengthened under the global warming scenario.5.Before the mid-1980s,Central Pacific El Nino(La Nina)events weakened(strengthened)the Iceland Low(IL);the resulting anomalous northerly(southerly)wind at the east of the IL prevented(favored)the A-RTDW event occurrence by leading Atlantic storms away from(into)the Arctic.As a result of the CP ENSO/IL teleconnection,the CP ENSO could influence the occurrence frequency of the ARTDW event.In contrast,this effect is no longer present after the mid-1980s.Prior to the mid-1980s,the CP ENSO could influence the polar vortex via planetary wave propagation upwards into the stratosphere,resulting in the CP ENSO/IL teleconnection;thus,the CP ENSO and A-RTDW could be connected.However,after the 1980s,the CP ENSO related with planetary wave could not propagate upwards into the stratosphere,possibly due to the strengthened climatological polar vortex;thus,the ENSO/IL teleconnection vanished,resulting in CP ENSO having no effect on the occurrence frequency of A-RTDW.