The Role of Ocean Dynamical Thermostat in Delaying the El Niño–Like Response over the Equatorial Pacific to Climate Warming

Abstract

The role of ocean dynamics in the response of the equatorial Pacific Ocean to climate warming is investigated using both an atmosphere–ocean coupled climate system and its ocean component. Results show that the initial response (fast pattern) to an uniform heating imposed on the ocean is a warming centered to the west of the date line owing to the conventional ocean dynamical thermostat (ODT) mechanism in the eastern equatorial Pacific—a cooling effect arising from the up-gradient upwelling. In time, the warming pattern gradually propagates eastward, becoming more El Niño–like (slow pattern). The transition from the fast to the slow pattern likely results from 1) the gradual warming of the equatorial thermocline temperature, which is associated with the arrival of the relatively warmer extratropical waters advected along the subsurface branch of the subtropical cells (STCs), and 2) the reduction of the STC strength itself. A mixed layer heat budget analysis finds that it is the total ocean dynamical effect rather than the conventional ODT that holds the key for understanding the pattern of the SST in the equatorial Pacific and that the surface heat flux works mainly to compensate the ocean dynamics. Further passive tracer experiments with the ocean component of the coupled system verify the role of the ocean dynamical processes in initiating a La Niña–like SST warming and in setting the pace of the transition to an El Niño–like warming and identify an oceanic origin for the slow eastern Pacific warming independent of the weakening trade wind.

The Active Role of the Ocean in the Temporal Evolution of Climate Sensitivity

“…Accordingly, shows much reduced warming in the tropics (El Niño‐like warming) and greater cooling in the NH high latitudes, in comparison to (Figures c and d). This active Pacific El Niño‐like warming pattern is shown to be a robust feature of global warming experiments in CESM, and partly due to the weakening of the shallow subtropical overturning cell (Luo et al, ). The widespread NH active SST cooling appears to be at odds with the large active heat uptake in the subpolar Atlantic, which would have caused anomalous warming there if acting in isolation.…”

Ocean Warming Pattern Effect On Global And Regional Climate Change

“…While mechanisms in favor of an El Niño‐like warming seem to dominate in models, the mean upwelling in the eastern two thirds of the equatorial Pacific damps the SST response to greenhouse warming, much as in the Southern Ocean. Seager et al () argued that this ocean upwelling damping limited the eastern Pacific warming in observations, although there are other ocean dynamic effects at work on equatorial warming (Coats & Karnauskas, ; Luo et al, , and references therein). Uncertainties in aerosol effect and internal variability further complicate the interpretation of the observed SST pattern.…”

LongRunMIP: Motivation and Design for a Large Collection of Millennial-Length AOGCM Simulations

“…A warming pattern reminiscent of the positive phase of ENSO and the interdecadal Pacific oscillation occurs throughout the Pacific basin ( Fig. 5b; Held et al 2010;Song and Zhang 2014;Andrews et al 2015;Luo et al 2017) but decays on centennial to millennial time scales (Figs. 5c,d), with a large model spread in time scales (not shown).…”