Role of Pacific trade winds in driving ocean temperatures during the recent slowdown and projections under a wind trend reversal

Abstract

Interannual to decadal variability in the Pacific Ocean is a prominent feature of Earth's climate system, with global teleconnections. Recent studies have identified Pacific decadal variability as a major driver of periods of rapid and slower global mean surface air temperature change. Here, we use an eddy-permitting global ocean model to investigate the role of the observed 1992-2011 trade wind intensification and concurrent trends in surface atmospheric variables over the Pacific associated with the negative phase of the Interdecadal Pacific Oscillation (IPO) in driving ocean circulation and heat content changes. We find a strengthening of the Equatorial Undercurrent (EUC) in response to strengthened winds, which brings cooler water to the surface of the eastern Pacific and an increase in the Pacific shallow overturning cells (PSOC), which in turn drives additional heat into the subsurface western Pacific. The wind acceleration also results in an increase in the strength and subsequent heat transport of the Indonesian throughflow (ITF), which transports some of the additional heat from the western Pacific into the Indian Ocean. The circulation changes result in warming of the subsurface western Pacific, cooling of the upper eastern Pacific Ocean and warming of the subsurface Indian Ocean, with an overall increase in Indo-Pacific heat content. Further experiments impose a symmetric reversal of the atmospheric state to examine how the ocean would behave if the winds (and other atmospheric variables) were to revert to their initial state. This mimics a return to the neutral phase of the IPO, characterised by a weakening of the Pacific trade winds. In response we find a slowdown of the EUC and the PSOC, which results in a return to climatological SST conditions in the western and eastern Pacific. The ITF also slows towards its original strength. However, the subsurface temperature, heat content and ITF responses are not symmetric due to an overall increase in the surface heat flux into the ocean associated with the cooler surface of the Pacific. There may also be irreversible heat transport across the thermocline via diapycnal mixing, further contributing to this asymmetry. The net result of the experiment is that the Indo-Pacific subsurface ocean is warmer than it was in its initial state.

A Recent Shift Toward an El Niño‐Like Ocean State in the Tropical Pacific and the Resumption of Ocean Warming

“…The simulated trends are similar to those found in the observations (Figure b); however, the simulated tropical thermocline is a little shallower than that indicated by observational evidence. This discrepancy may be due to difficulties in the model with handling the vertical mixing processes (Maher et al, ; Moum et al, ). The temperature trends of all the data sets demonstrate a weakening of heat convergence in the WTP thermocline and a warming of the upper CETP, as opposed to the La Niña‐like trends identified during the recent hiatus period (England et al, ; Nieves et al, ).…”

“…Conversely, the temperature trends during the 2003–2010 period are well matched by a recent opposing trend due to the reversed trend of the wind patterns on a decadal time scale. These decadal temperature variations are also consistent with changes in the strength of the EUC that, in general, bring cooler subsurface water from the western to the eastern tropical Pacific in the upper layer (Maher et al, ). During the 2003–2010 period, the strengthened trade winds induced the SSL rise in the WTP due to convergences of the heat and mass, which generate a strengthening of the EUC and the equatorial upwelling (Figure f).…”

“…Since the beginning of the 21st century, there has been tremendous public and scientific interest in a pause in global mean surface temperature warming known as the recent hiatus (England et al, ; Kosaka & Xie, ; Lee et al, ; Maher et al, ). Various mechanisms have been proposed for this hiatus; however, most studies support the possible importance of internal climate variability, including the Interdecadal Pacific Oscillation (IPO; Power et al, ) or the Pacific Decadal Oscillation (PDO; Mantua et al, ), in modulating global warming.…”

“…Various mechanisms have been proposed for this hiatus; however, most studies support the possible importance of internal climate variability, including the Interdecadal Pacific Oscillation (IPO; Power et al, ) or the Pacific Decadal Oscillation (PDO; Mantua et al, ), in modulating global warming. Recent analyses of observations and climate models revealed that the recent hiatus period was closely linked to the negative phase of the IPO, which manifests as a low‐frequency La Niña‐like pattern, with enhanced trade winds, warming of the western Pacific subsurface, strengthening of the Equatorial Undercurrent (EUC), and cooling of the eastern Pacific surface (England et al, ; Maher et al, , ; Meehl & Arblaster, ; Meehl et al, ; Nieves et al, ).…”

“…Recently, Meehl et al () predicted a transition in the IPO from negative to positive, with a resumption of global warming, in a decadal prediction for the period of 2015–2019 that was initialized in 2013. Indeed, the PDO (highly correlated with the IPO) has been shifting toward a positive peak since approximately 2011 (Maher et al, ). With the PDO transitioning back to positive, previous studies suggest that an El Niño‐like state should exist, with a weakening of the trade winds and associated changes in the ocean circulations.…”