Oceanic Mechanisms of Tropical Pacific Climate Variability Involving the Subtropical-Tropical Cells (STCs)

Tropical Pacific decadal variability (TPDV) plays an important role in the global climate, as evident from its influence on the recent slowdown of the global surface temperature trend. Decadal variations of tropical Pacific background conditions also affect the amplitude, frequency, and spatial pattern of El Niño Southern Oscillation (ENSO) events on inter-annual timescales, whose global impacts can be sensitive to the distinctive evolution and structure of individual events. For both of these reasons it is important to assess the predictability of TPDV through a better understanding of its underlying mechanisms.
Despite extensive research, however, a clear understanding of those mechanisms remains elusive. Some studies have suggested that TPDV may originate purely by chance, while others have emphasized the role of slow oceanic adjustment processes and their possible feedbacks on the atmosphere. Such processes involve upper-oceanic meridional overturning circulations in both hemispheres known as Subtropical-Tropical Cells (STCs), as well as oceanic Rossby waves that mediate the STC adjustment and the evolution and structure of tropical Pacific heat content. A connection between STC strength and equatorial SST anomalies has been shown in several studies using relatively short observational and model datasets. However, many questions remain unanswered.
The primary goal of the proposed research is to elucidate the role of oceanic dynamical processes in TPDV using a combination of observations, oceanic and coupled atmosphere-ocean reanalysis products, and model outputs from the Climate Model Inter-comparison Project phase 6 (CMIP6). Our specific objectives are 1) to examine the role of the northern and southern STCs and oceanic Rossby waves in altering the tropical upper-oceanic heat content and tropical SSTs at decadal timescales, and to assess the nature of the wind anomalies forcing these processes; 2) to evaluate the representation of TPDV in pre-industrial and historical simulations of the CMIP6 models; and 3) to use the CMIP6 future scenario simulations to examine projected changes in TPDV and its associated processes.
The proposed research will directly address the first priority area of the competition: “Investigation of mechanisms that govern variability of the coupled climate system and its predictability on the interannual to multi-decadal timescales within long-term observation and/or model data (such as, CMIP6)” by examining the mechanisms of decadal/multi-decadal variability in the tropical Pacific, the low-frequency modulation of interannual variability, and the degree of predictability associated with those mechanisms.