Although the onset and decay phases of ENSO typically occurring in boreal spring and summer play very important roles in forcing climate variability around the globe, the great majority of recent studies on ENSO diversity have focused on the peak phase in boreal winter. A team of scientists led by Sang-Ki Lee, a PhOD/CIMAS scientist, presented a new approach to efficiently summarize the diversity of ENSO during the onset, peak and decay phases as a whole in terms of two spatio-temporal orthogonal modes.
In a research article recently published online in the Geophysical Research Letters, a team of scientists from University of Miami/CIMAS, NOAA/AOML, University of Hawaii, NOAA/GFDL, and Hanyang University (in South Korea) presented a systematic exploration of differences in the spatio-temporal SST evolution among observed El Nino events. This inter-El Nino variability is captured by two leading orthogonal modes, which explain more than 60% of the inter-event variance (Figure 1). The first mode illustrates the extent to which warm SST anomalies in the eastern tropical Pacific (EP) persist into the boreal spring after the peak of El Nino. The analysis suggests that a strong El Nino event tends to persist into the boreal spring in the EP, whereas a weak El Nino favors a rapid development of cold SSTAs in the EP shortly after its peak. The second mode captures the transition and resurgence of El Nino in the following year. An early-onset El Nino tends to favor a transition to La Nina, whereas a late-onset El Nino tends to persist long enough to produce another El Nino event. The spatio-temporal evolution of several El Nino events during 1949-2013 can be efficiently summarized in terms of these two modes (Figure 2), which are not mutually exclusive, but exhibit distinctive coupled atmosphere-ocean dynamics.
Figure 1. Time-longitude plots of (a) composite mean (CM) and (b and c) the two leading inter-event EOFs of the tropical Pacific SST anomalies averaged between 5°S and 5°N, for 21 El Ninos during 1949–2013. (d and e) Same as b and c except that the two EOFs are rotated by 90º. Units are in ºC. The dashed gray boxes indicate Nino 3.4 in DJF (0,+1), Nino 3 (150°W–90°W and 5°S–5°N) in AMJ (+1), Niño 3 in AMJ (0), and Niño 3.4 in OND (+1).
Figure 2. (a) Normalized PC1 versus PC2 and (b) PC1+PC2 versus PC2-PC1 for all 21 El Nino events. The two digit numbers indicate the El Nino onset years.
Lee, S.-K. P. N. DiNezio, E.-S. Chung, S.-W. Yeh, A. T. Wittenberg, and C. Wang, 2015: Spring persistence, transition and resurgence of El Nino. Geophys. Res. Lett., doi:10.1002/2014GL062484 [pdf]