Understanding the Driving Forces of the North Pacific Victoria Mode

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, the North Pacific Oscillation (NPO), anomalous ocean circulation in the Kuroshio-Oyashio Extension (KOE), and the cold tongue (CT) and warm pool (WP) El Nino-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-scale 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.