Understanding the Driving Forces of the North Pacific Victoria Mode

Abstract

The Victoria Mode (VM) is defined as the second empirical orthogonal function (EOF) of sea surface temperature anomalies (SSTAs) in the North Pacific. It is independent of the Pacific Decadal Oscillation (PDO) and represents the second climate mode of North Pacific variability. Using a first-order autoregressive model and forcing indices that track the Aleutian Low, North Pacific Oscillation (NPO), anomalous ocean circulation in the Kuroshio–Oyashio Extension (KOE), and the cold tongue (CT) and warm pool (WP) El Niño-Southern Oscillation (ENSO), this study reconstructs the North Pacific SSTAs and examines the relative contributions of different forcing mechanisms to the VM variance. It is found that the NPO and ENSO, especially the WP ENSO, are the dominant sources of VM variability, whereas the contributions of the Aleutian Low and zonal advection in the KOE region are relatively small. Further analysis shows that the forcing mechanisms acting on the VM are frequency dependent. At decadal and longer time scales, the VM variance comes mainly from the WP ENSO, whereas at interannual time scales, contributions from the NPO and CT ENSO are important. This study suggests that the VM is an oceanic response to the NPO forcing and atmospheric teleconnections excited by ENSO events.

Enhanced North Pacific Victoria mode in a warming climate

“…11b). This highlights the pivotal role of CP SST in driving the augmented lowfrequency VM variability, aligning with recent findings 35 that CP SST primarily contributes to VM variance on decadal and longer timescales. However, this doesn't discount the potential contributions of other processes to the enhanced VM variability.…”

“…7). These atmospheric changes, in turn, influence lowfrequency fluctuations in the VM/NPGO by modulating surface heat fluxes [33][34][35] . Significantly, there has been a noticeable escalation in the intensity and frequency of CP El Niño events since the 1980s to 1990s [36][37][38][39][40] , closely coinciding with the period when low-frequency VM variability began to rise.…”