The Energetics of the 1998 Pacific Decadal Transition

Abstract To trace the energetics of the 1998 shift in Pacific decadal variability (PDV), monthly ocean assimilation, atmospheric reanalysis, and satellite-derived cloud cover and radiative data was analyzed. Data during 1980-2021 showed evidence of the Pacific Walker circulation abruptly forcing the equatorial Pacific Ocean into a persistently elevated energy state after 1998. This occurred as a step-like shift to stronger trade winds, positive SOI, and increased central equatorial Pacific (CEP) 500 mb subsidence (w500), accompanied by a significant 25% increase in mean equatorial Pacific oceanic available potential energy (OAPE). Covarying (|r| > 0.6) CEP w500, fractional cloud cover, top-of-atmosphere albedo, and surface short-wave absorbed radiative energy (SWARS) indicate a control of CEP Walker subsidence on CEP SWARS via a subsidence-albedo effect. Similarly correlated CEP SWARS and OAPE (r = 0.670), and a 71 exa Joule (eJ) increase in mean CEP SWARS after 1998 that is two orders of magnitude greater than the increase in mean OAPE (0.507 eJ), suggests the post-1998 strong Walker PDV phase was radiatively driven. Together, this is interpreted as evidence of a positive Walker radiative-dynamic feedback over the equatorial Pacific after 1998. The 1998 shift’s circumstances suggest the influence of persistent 1998-2001 La Niña conditions in tipping PDV from a climatically weak-Walker phase to a strong-Walker phase. Based on these results, and previous work that studied the 1976-1977 PDV transition, a preliminary hypothesis is proposed of PDV as a bi-stable radiative climate mechanism governed by Walker-driven subsidence-albedo variation and ENSO-related tipping events.