A Comparative Study on the Dominant Factors Responsible for the Weaker-than-expected El Ni

A Comparative Study on the Dominant Factors Responsible for the Weaker-than-expected El Ni～

no Event in 2014

LI Jianying1,3,LIU Boqi1,4,LI Jiandong1,and MAO Jiangyu?1,2 【摘 要】ABSTRACT：

【期刊名称】大气科学进展（英文版） 【年(卷),期】2015(032)010 【总页数】10

【关键词】Key words：El Ni～no event,westerly wind burst,instability of coupled ocean–atmosphere system,positive Bjerknes feedback

Anomalous warming occurred in the equatorial central-eastern Pacific in early May 2014,attracting much attention to the possible occurrence of an extreme El Ni～no event that year because of its similarity to the situation in early 1997.However,the subsequent variation in sea surface temperature anomalies(SSTAs)during summer 2014 in the tropical Pacific was evidently different to that in 1997,but somewhat similar to the situation of the 1990 aborted El Ni～no event.Based on NCEP(National Centers for Environmental Prediction)oceanic and atmospheric reanalysis data,the physical processes responsible for the strength of El Ni～no events are examined by comparing the dominant factors in 2014 in terms of the preceding instability of the coupled ocean–atmosphere system and westerly wind bursts(WWBs)with those

in 1997 and 1990,separately.Although the unstable ocean–atmosphere system formed over the tropical Pacific in the preceding winter of 2014,the strength of the preceding instability was relatively weak.Weak oceanic eastward-propagating downwelling Kelvin waves were forced by the weak WWBs over the equatorial western Pacific in March 2014,as in February 1990.The consequent positive upper-oceanic heat content anomalies in the spring of 2014 induced only weak positive SSTAs in the central-eastern Pacific–unfavorable for the subsequent generation of summertime WWB sequences.Moreover,the equatorial western Pacific was not cooled, indicating the absence of positive Bjerknes feedback in early summer 2014.Therefore,the development of El Ni～no was suspended in summer 2014.

1.Introduction

It is well known that El Ni～no refers to anomalous warm episodes during which sea surface temperatures(SSTs)in the central and eastern tropical Pacific are warmer than normal (Philander,1985).El Ni～no is also considered as the warm phase of the El Ni～no–Southern Oscillation(ENSO)cycle in the coupled atmosphere–ocean system.As a dominant variability in the air–sea interacting system on the interannual timescale,ENSO significantly affects the tropical and extratropical climate(Yang,1996;Webster et al.,1998;Wang, 2000;Wang,2000;Mao and Wu,2007;Sun and Yang,2007; Zheng et al.,2009).Therefore,it is of great

importance to monitor and forecast the evolution of ENSO events. In early May 2014,anomalous warming was observed in the equatorial central-eastern

Pacific(National

Aeronautics

and

Space

Administration,2014)digital dash,and the pattern and magnitude of satellite-based sea surface height anomalies(SSHAs)bore a close resemblance to those in May 1997. Since the 1997–98 El Ni～no was the strongest warm episode among the ENSO cycles in the 20th century,the similar SSHAs and predictions derived from some climate models (Tollefson,2014)suggested that a super El Ni～no event would possibly develop during subsequent months in 2014.However,the warming remained in an unexpectedly neutral state at the end of August 2014(Australian Government Bureau of Meteorology,2014),rather different from the situation in August 1997 when a mature El Ni～no had already occurred. Although the latest climate model results predicted

that

the

2014

El

Ni～no

would

subsequently

resurge(Zastrow,2014), it is still unclear as to why the equatorial central-eastern Pacific warming suspended that summer.Actually,the strength of an El Ni～no event depends mainly on two dominant factors:the preceding instability of the tropical Pacific ocean–atmosphere coupled system(Wyrtki,1975,1985;Jin,1997;Fedorov et al.,2015;Lai et al.,2015)and the westerly wind bursts(WWBs)over the equatorial western-central Pacific.