Two Types Of ENSO And Atmospheric Circulation Responses

Recent studies reavealed that two types of ENSO events were existed:the Eastern-Pacific (EP) and Central-Pacific (CP) type. Therefore, the study examined the monitoring ability of several current operational ENSO indices. The results indicated that single index could not distinguish the type of ENSO events as EP or CP from the historical ENSO events during 1950-2009. The Nino 3 index might only suit for monitoring the EP ENSO, while the Nino 4 index might for the CP type. Therefore, for the purpose of not only capturing the occurrence of ENSO event but also distinguishing its type, a new monitoring index group constructed by both Nino 3 and Nino 4 indices was proposed. Further analysis confirmed that this index group could monitor different types of historical ENSO events with different spatial distribution of sea surface temperature. And it had a better performance in determining some characteristics of those events, including the peak intensity, onset, decay, mature phase, and so on.On this basis, this paper analyzed the different responses of atmospheric temperature field, height field, wind field, circulation field and other characteristics during the peak period and in the process from the development of EP-/CP-ENSO events to the extinction. Short-term climatic characteristics modes of the two types of ENSO events were established, and they are explained with the 2009/2010 CP-E1 Nino event and 2010/2011 CP-La Nina event. The main conclusions include:1) A target group of indices combining both Nino 3 and Nino 4 indices to monitor different types of ENSO events is proposed, while United States, Japan and China National Climate Center are using single index, which can not define the incident type effectively at present. In addition, this target group can accurately determine the peak intensity, beginning and ending time, maturity period, and other features of ENSO events.2) From 1950 to 2009, ENSO events have occurred 36 times, including 19 El Nino events, and 17 La Nina events. The frequency of CP-E1 Nino, which is 11 times, is relatively higher than EP-E1 Nino; while for La Nina, frequency of EP-La Nina is higher happened 1 ltimes. In addition, interdecadal distribution shows that, there are more EP-ENSO events before 1995, while more CP-ENSO events after.3) The EP-ENSO events and CP-ENSO events defined by the Composite Index are much different from each other. There are significant differences between the responses on the atmospheric temperature field, height field, wind field, circulation field, et aL The difference between east part and west part of the heat source of sea surface causes heating zones change, leading to different atmospheric response. In the eastern equatorial Pacific near the Peruvian coast (Walker circulation in the sink area), circulation system showing opposite effects to most parts of Pacific is a landmark feature of CP-type event.4) Facts that average lifetime and intensity of EP-E1 Nino events and CP-La Nina events show a significant positive relationship. It's necessary to monitor and predict the two types of ENSO events separately, with establishing the short-term climatic characteristics mode for EP-/CP-ENSO events.