Shifting Mode Of El Ni?o And Its Teleconnection To East Asian And Arctic Summer Climate Anomalies In The Context Of Global Change

In the past three decades,the central Pacific(CP)type of El Ni?o has appeared more and more intensely and frequently under the global warming background.However,there are some controversies about the modal characteristic of CP El Ni?o.Meanwhile,accompanied with the shifts in mode of El Ni?o,the East Asian summer monsoonal precipitation and Arctic summer climate anomalies become very different and their inter-relationships have not been fully understood.This study is aimed at the linkages between the spatial-temporal characteristics of tropical Pacific sea surface temperature(SST)and the corresponding climate anomalies over the East Asian and Arctic Ocean during boreal summer,with an emphasis on the interannual time scales.Several major findings are summarized as follows:1.Reinspecting two types of El Ni?o:A new pair of Ni?o indices for improving real-time ENSO monitoring is proposed.We first reinspect the dominant modes of different ENSO types from the perspectives of monthly mean and seasonality using a combined technique referred as RC-REOF.Several features have been revealed.(1)The explained variances of eastern Pacific(EP)ENSO and CP ENSO are comparable and in the ranges of 33-43%and 23-28%,respectively.(2)The feature is more in line with the frequent occurrence of CP ENSO compared to the result from orthogonal EOF analysis in which El Ni?o Modoki only explains a smaller variance of 11-12%.(3)Both special patterns of EP ENSO and CP ENSO are equatorial asymmetry that is often overlooked previously by the traditional Ni?o indices.Accordingly,a new pair of Ni?o indices referred to as Ni?o3b and Ni?o4b are proposed,which are superior in the following respects:(1)simple calculation,(2)robust and stable relationship with the EP/CP ENSO modes,(3)more representative for Pacific decadal signals,(4)easier to distinguish ENSO types,and(5)without restriction by orthogonality,etc.Hence Ni?o3b and Ni?o4b are potentially useful for scientific research and real-time monitoring of the two types of ENSO.Besides,index Ni?o3.4b is also introduced to describe the hybrid ENSO events,with relatively equal covariance to EP ENSO and CP ENSO and with larger covariance in decadal time scale than index Ni?o3.4.2.Phsical mechanism for the CP El Ni?o weakens East Asian monsoonal precipitation during mid-summer is proposed.It is found that the interannual variations of East Asian summer monsoon(EASM)and the resultant precipitation anomalies over the Yangtze River valley(YRV)are closely linked to the CP El Ni?o during mid-summer.The drivers and mechanisms of weakened monsoon and precipitation over East Asia and YRV during mid-summer are revealed by performing a comparative analysis via regressions/correlations with the CP El Ni?o and EASM indices.Comparative results shown that the tropospheric warming induced by the CP El Ni?o over the western North Pacific(WNP)contributes to the enhanced land-ocean thermal contrast between East Asia and WNP,which inturn contributes to the decreased YRV-precipitation through weakening the EASM circulations(i.e.,the South Asian high,the WNP subtropical high,and the low-level anticyclone over the South Chia Sea and the Philippine Sea,etc.)and the associated moisture transport.The CP El Ni?o and EASM indices are also tightly associated with the Pacific-Japan or East Asian-Pacific teleconnection,indicating a possible role of internal dynamics in the interannual variability of EASM precipitation.Meanwhile,significant CP La Nina-like pattern could be reproduced directly by regressing SST anomalies onto the EASM index,further confirming that the CP El Ni?o indeed plays an important role in weakening the EASM and precipitaions.3.Shifting El Ni?o inhibits summer Arctic warming and Arctic sea ice melting over the Canada Basin is proposed.Summer Arctic climate changes include not only the changes in climate trends and mean states but also the strong interannual variations.Although it is known that tropical-extratropical teleconnection is sensitive to changes in the face of El Ni?o,whether Arctic climate variability is linked to the shift in El Ni?o,in particular on interannual timescales,still remains unclear.Here,we demonstrate a long-range linkage between CP El Ni?o and summer Arctic climate.Observations show that the CP warming(CPW)related to CP El Ni?o events deepens the Arctic polar vortex(APV)and strengthens the circumpolar westerly wind(CWW),thereby contributing to summer Arctic cooling and sea ice increase.Atmospheric model experiments generally capture the observed responses of Arctic circulation and robust surface cooling.We suggest that(1)summer Arctic sea ice loss would be more intense without considering the inhibited effect of CP El Ni?o under the global warming condition;and(2)identification of the Equator-Arctic teleconnection,via the"atmospheric bridge" and "oceanic memory",potentially contributes to improving the skill of predicting Arctic climate.