The North Pacific Pacemaker Effect on Historical ENSO and Its Mechanisms

Abstract

Studies have indicated that North Pacific sea surface temperature (SST) variability can significantly modulate 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 date line. On an event-by-event basis, nPOGA most closely reproduces the 2014/15 and the 2015/16 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 2015/16 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 time scales.

Physical drivers of the summer 2019 North Pacific marine heatwave

“…3). This suggests that the 2019 PMM event may have produced an active SDC response during this time period, which would have the potential to reinforce the subtropical SSTAs through wind-evaporation-SST feedback 30,31 . However, we note that our North Pacific-only AGCM simulations cannot explicitly rule out the influence of local SST forcing on the SLPA structure seen in Fig.…”

“…In addition, recent work has focused on the influence of subtropical SSTAs associated with the Pacific Meridional Mode (PMM) 27,28 on the position of the mean intertropical convergence zone (ITCZ) in boreal summer. A PMM-driven shift of the ITCZ has been shown to produce a large-scale atmospheric circulation response that spans the breadth of the subtropics and projects on the North Pacific High 29,30 . This process, termed the summer deep convection (SDC) response, then projects onto mid-latitude SSTs through changes in surface heat fluxes.…”

“…3 and ref. 28,30 ). The northward shifted ITCZ is evident in the AGCM precipitation data and is qualitatively consistent with observations ( Supplementary Fig.…”

Quantitatively Isolating Extratropical Atmospheric Impact on the Tropical Pacific Interannual Variability in Coupled Climate Model

“…While in subPR, restoring walls are only placed from ocean depth of 50 m downward to the bottom. Here the ocean depth of 50 m is chosen because this is a typical value of the ocean mixed layer [47] and it is also consistent with the mixed layer depth in FOAM calculated from a 200-year simulation. Thus, subPR allows us to cut-off oceanic meridional teleconnections, which critically depend on ocean dynamics, but the coupled oceanatmosphere interactions can be reserved.…”

Understanding the development of systematic errors in the Asian Summer Monsoon

“…regional climate modelling with a range of lateral boundary locations (Levine and Martin, 2018;Karmacharya et al, 2015); global or regional relaxation experiments (Klinker et al, 1990;Rodriguez et al, 2017Rodriguez et al, , 2019; and "pacemaker" experiments (where a climate model is forced by observed sea surface temperature variations in a specific region, but allowed to evolve freely everywhere else; e.g., Deser et al 2017;Zhang et al, 2018;Amaya et al, 2019). Rodriguez and Milton (2019) describe analysis of the spin-up of the errors over the Asian monsoon region in initialized 15day atmosphere-only hindcasts.…”