The Delayed Impact Of The Wintertime Arctic Oscillattion/North Atlantic Oscillation On The Climate Over East Asia And The Changing Interannual Relationship Between Them

The Arctic Oscillation(AO)/ North Atlantic Oscillation(NAO)is the dominant mode of atmospheric variability over the extratropics/ North Atlantic of the Northern Hemisphere(NH).The NAO and AO have almost identical spatial structures over the North Atlantic,making time series of these two phenomena highly correlated with each other.The AO/NAO exhibit strong seasonal variabilities with the largest variabilities occurring in the winter.The wintertime AO/NAO can exert substantial influence on the simulteanous weather and climate over large areas of the extratropics in the NH.Meanwhile,the wintertime AO/NAO has a strong forcing effects on the SST over the North Atlantic,and the resultant SST anomalies over the North Atlantic may lead to weather and climate anomalies during the subsequent periods.Focusing on the theme of delayed impact of the wintertime AO/NAO on SST and atmosphere,the beyond-season and beyond-month influence of the wintertime AO/NAO on climate over East Asia,as well as the changing interannual relationship between them are investigated in this study,respectively.Firstly,we search the possibility of the beyond-season influence of the wintertime AO/NAO on the subsequent summertime SST over the Atlantic,and investigate the physical process for the wintertime AO/NAO on the subsequent summertime SST over the tropical Atlantic.Then,we explore the mechanism of enhanced interannual relationship between the wintertime AO/NAO and the subsequent summertime climate over East Asia.Addtionally,we find the significant impact of the December NAO on the following February East Asian trough,and analyze the related air-sea interaction process and the mechanism of changing relationship between them.The main conclusions are summarized as follows:(1)The mechanism of the delayed impact of the wintertime AO/NAO on the subsequent summertime SST over the tropical AtlanticAfter the middle 1990 s,the negative(positive)phase of the wintertime AO/NAOfavor the positive(negative)SST anomalies over the whole tropical Atlantic,while the impact of the wintertime AO/NAO on the SST over the tropical Atlantic is insignificant or confined to the area north of the equator before the 1990 s.Since the middle 1990 s,the negative AO/NAO-related warming process of the tropical Atlantic consist of three stages: the pronounced increase of SST over the latitudes around the20°N and 10°S during December-the following January,the pronounced increase of SST over the tropical northwestern Atlantic(0-10°N,30-60°W)during the following February-March,and the persistent warming of the North Atlantic after the following April via the feedback of SST and thermocline.During December-the following January,the increase of SST over the latitudes around the 20°N and 10°S are attributed to significantly positive latent heat flux anomalies,which is induced by anomalous large scale atmospheric circulation associated with negative AO/NAO.During the following February-March,the pronounced increase of SST over the tropical northwestern Atlantic is attributed to increasing downward short wave flux,and this is associated with enhanced sinking motion.At this time,the wintertime AO/NAO exert an indirect impact on the increase of SST over the tropical northwestern Atlantic.(2)The enhanced interannual relationship between the wintertime AO and the subsequent summertime climate over the East AsiaA regime shift of the winter AO index was detected around 1988,with mean AO indices of-0.69 and 0.10 for 1958-1987 and 1988-2014,respectively,and their differences passed the 99% confidence level.After the regime shift and especially since the early 1990 s,negative(positive)winter AO indices corresponds with positive(negative)height anomalies over central East Asia(CEA)during the following summer.Meanwhile,the westerly jet stream tends to be northward(southward)and the East Asian summer monsoon(EASM)is strong(weak),which contributes to the warm(cold)anomalies over central China and the significantly enhanced(reduced)rainfall over northeast China.The enhanced connection of the winter AO and the following summer height anomalies over CEA is largely due to the enhanced connection of the winter AO and the following Indian summer monsoon(ISM)rainfall during the same period.The negative(positive)winter AO favors increasing(decreasing)rainfall over the ISM region,which triggers a positive(negative)circumglobal teleconnection(CGT),and results in significantly positive(negative)height anomalies over CEA.One possible way the winter AO could influence the following ISM rainfall is that the negative(positive)winter AO memories will be maintained by the positive(negative)sea surface temperature(SST)anomalies over the tropical Atlantic of the following summer,which are responsible for the La Ni?a(El Ni?o)developing phase and increases(decreases)the ISM rainfall.(3)Impact of the December NAO on the following February East Asian troughDuring winter,the December NAO has an impact on the following February East Asian trough(EAT),and a significant positive correlation exists between them.It is shown that the circulation anomalies affected by the December NAO for December and for the following January are primarily confined to the Euro-Atlantic sector while they extend to East Asia during the following February,and this is related to anomalous wave trains originating from the southwestern Atlantic and spreading to the northeastern Atlantic,northern Europe,western Siberia,and East Asia.When the NAO is positive phase in December,the SST tripole pattern is forced by persistence positive NAO from December to the following January,contributing to pronounced positive SST anomalies in mid-latitude areas of the North Atlantic during the following February.The pronounced positive SST anomalies found during this period can generate feedback for atmospheric anomalies,and the westerly winds are enhanced(reduced)to the north(south)side of the positive SST anomalies,which result from strengthened(weakened)baroclinicity there.In addition,the Rossby wave source(RWS)over the northeastern Atlantic shows a positive anomaly,establishing a link between the positive SST anomalies in mid-latitude areas of the North Atlantic and the deepened EAT downstream.(4)The changing interannual relationship between the December North Atlantic Oscillation and the following February East Asian troughThe relationship between the December NAO and the following February EAT issignificantly enhanced after the late 1980 s compared with the period before the late1980 s.The changing relationship mainly results from the enhanced relationship between the December NAO and the following February North Atlantic mid-latitudes' sea surface temperature(SST)anomalies(NAMA)during the same period.During the period after the late 1980 s,accompanying with the enhanced interannual variability of the December NAO,the persistent positive(negative)NAO pattern from December to the following January contributes to a positive(negative)NAMA,which reaches its maximum magnitude in the following February and excites an anomalous wave train along the North Atlantic and northern Eurasia,and significantly impacts the EAT.During the period before the late 1980 s,the positive(negative)NAO pattern during December cannot persist into the following January,and the related positive(negative)NAMA is insignificant during the following February,causing the response of the simultaneous EAT to be insignificant as well.Moreover,there exists a significant impact of the December NAO on the December-January NAMA after the late 1980 s,while the December-January NAMA is relatively less affected by the December NAO before the late 1980 s.As a result,the simultaneous response of the atmospheric circulation anomalies to the December-January NAMA are evident before the late1980 s,and the positive(negative)NAMA can excite an anomalous wave train along the North Atlantic and northern Eurasia and significantly deepen(shallow)the downstream EAT.By contrast,after involving a feature of atmosphere forcing of SST,the simultaneous feedback of the December-January NAMA on EAT is significantly decreased after the 1980 s.