The North Pacific pacemaker effect on historical ENSO and its mechanisms

Dillon J Amaya, University of Colorado, Boulder, United States, Yu Kosaka, University of Tokyo, La Jolla, CA, United States, Wenyu Zhou, Pacific Northwest National Laboratory, Richland, WA, United States, Yu Zhang, Ocean University of China, Qingdao, China; Scripps Institution of Oceanography, La Jolla, CA, United States, Shang-Ping Xie, University of California San Diego, Scripps Institution of Oceanography, La Jolla, United States and Arthur J Miller, University of California San Diego, La Jolla, CA, United States
Abstract:
Studies have indicated that North Pacific sea surface temperature (SST) variability can significantly modulate the El Niño-Southern Oscillation (ENSO), but there has been little effort to put extratropical-tropical interactions into the context of historical events. To quantify the role of the North Pacific in pacing the timing and magnitude of observed ENSO, we use a fully-coupled climate model to produce an ensemble of North Pacific Ocean-Global Atmosphere (nPOGA) SST pacemaker simulations. In nPOGA, SST anomalies are restored back to observations in the North Pacific (>15˚N), but are free to evolve throughout the rest of the globe. We find that the North Pacific SST has significantly influenced observed ENSO variability, accounting for approximately 15% of the total variance in boreal fall and winter. The connection between the North and tropical Pacific arises from two physical pathways: 1. A Wind-Evaporation-SST (WES) propagating mechanism, and 2. A Gill-like atmospheric response associated with anomalous deep convection in boreal summer and fall, which we refer to as the Summer Deep Convection (SDC) response. The SDC response accounts for 25% of the observed zonal wind variability around the equatorial dateline. On an event-by-event basis, nPOGA most closely reproduces the 2014-2015 and the 2015-2016 El Niños. In particular, we show that the 2015 Pacific Meridional Mode event increased wind forcing along the equator by 20%, potentially contributing to the extreme nature of the 2015-2016 El Niño. Our results illustrate the significant role of extratropical noise in pacing the initiation and magnitude of ENSO events and may improve the predictability of ENSO on seasonal timescales.
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