Impacts of the Tropical Pacific Cold Tongue Mode on ENSO Diversity Under Global Warming

Abstract

The causes of ENSO diversity, although being of great interest in recent research, do not have a consistent explanation. This study provides a possible mechanism focused on the background change of the tropical Pacific as a response to global warming. The second empirical orthogonal function mode of the sea surface temperature anomalies (SSTA) in the tropical Pacific, namely the cold tongue mode (CTM), represents the background change of the tropical Pacific under global warming. Using composite analysis with surface observations and subsurface ocean assimilation data sets, we find ENSO spatial structure diversity is closely associated with the CTM. A positive CTM tends to cool the SST in the eastern equatorial Pacific and warm the SST outside, as well as widen (narrow) zonal and meridional scales for El Niño (La Niña), and vice versa. Particularly in the positive CTM phase, the air‐sea action center of El Niño moves west, resembling the spatial pattern of CP‐El Niño. This westward shift of center is related to the weakened Bjerknes feedback (BF) intensity by the CTM. By suppressing the SSTA growth of El Niño in the eastern equatorial Pacific, the CTM contributes to more frequent occurrence of CP‐El Niño under global warming.

Long‐Term Trend of the Tropical Pacific Trade Winds Under Global Warming and Its Causes

“…This may be related to the modulation of the tropical Pacific mean state by the greater occurrence of EM events (e.g., McPhaden et al, ). However, much debate remains about the relationship and interaction between the recent higher occurrence of EM events and the tropical Pacific mean state under global warming (e.g., Lemmon & Karnauskas, , Li et al, , McPhaden et al, , Yeh et al, ).…”

“…The second EOF mode is identified as a cold tongue mode (hereafter CTM; Figure ; Lemmon & Karnauskas, ; Li et al, ; Li et al, , ; Zhang et al, ). Note that the CTM is independent of the IPO, with completely different spatial and temporal characteristics, as well as a different physical mechanism (Li et al, ). Although the increased occurrence of EM event can produce a cooling in the eastern equatorial Pacific, its contribution to the CTM may not be dominant.…”

“…Although there is uncertainty at the beginning of the century due to the lower observational density in the earlier part of the record (e.g., Compo et al, ), the strengthening trend of Pacific Walker circulation or trades since 1950 is a robust feature in multiple data sets (e.g., L'Heureux et al, ) and its associated increasing trend of zonal SST gradient over the equatorial Pacific since 1900 has also been found in multiple observational data sets (e.g., Cane et al, , Li et al, , Solomon & Newman, ). Particularly, many studies have reported that the intensification of the tropical Pacific trade winds over the past two decades is the strongest in the past century (e.g., Boisséson et al, ; England et al, ; L'Heureux et al, ; Takahashi & Watanabe, ).…”

“…They further pointed out that the intensification of the Walker circulation is associated with the La Niña‐like pattern of the long‐term trend of tropical Pacific SSTA, with greater cooling SSTA in the eastern tropical Pacific than in the west. Although there are large discrepancies between the various estimates of long‐term change in the tropical Pacific under global warming (e.g., Collins et al, , Vecchi et al, ), many observations have confirmed that the long‐term trend of tropical Pacific SSTA is characterized by cooling in the eastern equatorial Pacific and warming elsewhere in the tropical Pacific (i.e., a La Niña‐like pattern; e.g., Cane et al, , Coats & Karnauskas, , Compo & Sardeshmukh, , Funk & Hoell, , Karnauskas et al, , Li et al, , Li et al, , Solomon & Newman, , Zhang et al, ). Furthermore, the accelerating equatorial undercurrent, accelerating subtropical cells in the tropical Pacific, and the subsurface cooling in the eastern equatorial Pacific associated with the La Niña‐like pattern have also been found in multiple observational data sets (e.g., Drenkard & Karnauskas, , Li et al, , Yang et al, ).…”

“…Although there are large discrepancies between the various estimates of long‐term change in the tropical Pacific under global warming (e.g., Collins et al, , Vecchi et al, ), many observations have confirmed that the long‐term trend of tropical Pacific SSTA is characterized by cooling in the eastern equatorial Pacific and warming elsewhere in the tropical Pacific (i.e., a La Niña‐like pattern; e.g., Cane et al, , Coats & Karnauskas, , Compo & Sardeshmukh, , Funk & Hoell, , Karnauskas et al, , Li et al, , Li et al, , Solomon & Newman, , Zhang et al, ). Furthermore, the accelerating equatorial undercurrent, accelerating subtropical cells in the tropical Pacific, and the subsurface cooling in the eastern equatorial Pacific associated with the La Niña‐like pattern have also been found in multiple observational data sets (e.g., Drenkard & Karnauskas, , Li et al, , Yang et al, ). In particular, the La Niña‐like pattern in the tropical Pacific is dominated by ocean dynamical process, whereby vigorous upwelling of cold water in the eastern equatorial region causes the slower warming in the eastern Pacific than in the western Pacific (e.g., Cane et al, ; Clement et al, ; DiNezio et al, ; Karnauskas et al, ; Li et al, , ; Seager & Murtugudde, ; Sun & Liu, ; Zhang et al, ).…”