[A-CG33] Multi-scale ocean-atmosphere interaction in the tropics

[ACG33-04] Pacific Meridional Mode-Equatorial Pacific Interaction Linked by Aleutian Low Variability

*Yu Zhang^1,2, Shi-Yun Yu¹, Dillon J. Amaya³, Yu Kosaka⁴, Malte F. Stuecker⁵, Jun-Chao Yang^1,2, Xiaopei Lin^1,2, Lei Fan¹ (1.Physical Oceanography Laboratory, Ocean Univ. of China, 2.Lanshao Laboratory, Qingdao, China, 3.Physical Science Laboratory, ESRL, NOAA, 4.RCAST, The Univ. of Tokyo, 5.Univ. of Hawaiʻi at Mānoa)

Keywords:Pacific meridional mode, Tropical-subtropical Pacific interaction

Dominant sea surface temperature (SST) variabilities in the tropical Pacific can be divided into three key regions—the equatorial, subtropical northeast, and subtropical southeast Pacific—with the first two focused on in this study. SST variability in the subtropical northeast Pacific is dominated by the Pacific Meridional Mode (PMM), which is by definition, largely independent of equatorial Pacific (EP) SST variability. Despite of this, the PMM can subsequently affect EP SST through multiple ocean-atmosphere processes. The affected EP variability can in turn induce sea level pressure (SLP) anomalies over the North Pacific, which force the PMM by altering the strength of the mean northeasterly trade winds. As a result, the PMM and EP variability form a two-way interaction. Previous studies mostly attributed the North Pacific SLP anomalies to the North Pacific Oscillation (NPO), a meridional dipole structure whose lobe over the subtropics can effectively affect the trade winds strength and thus the PMM. In contrast, a recent study ascribed the anomalies to the Aleutian Low (AL), mainly located over the North Pacific midlatitudes. Despite its location farther north than the subtropical lobe of the NPO, its-associated surface wind anomalies on its southern flank can still influence the trade winds strength, thereby the PMM. Therefore, a question is naturally raised: the PMM-EP interaction is linked by the EP-forced NPO or AL variability? To address this question, we quantify their relative importance in the interaction using tropical Pacific pacemaker experiments in which tropical Pacific SST anomalies are restored to observations. Consequently, ensemble mean of the initialized perturbed simulations enables to extract EP-forced NPO and AL variabilities. We find that EP-forced AL is tied to the PMM-EP interaction, while EP-forced NPO is too weak to force the PMM, thereby not in the interaction. Our study sheds light on the key role of EP-forced AL variability—instead of the traditional view on the importance of the NPO—in the PMM-EP interaction.