A Preliminary Research On Temporal-Spatial Characteristics And Physical Mechanisms Of The North Pacific Tripole Mode

A great number of studies have been performed to investigate the leading empirical orthogonal function (EOF) of SST anomalies (SSTA) in the North Pacific, which is defined as the Pacific Decadal Oscillation (PDO). In contrast, few studies have examined the second EOF mode of the North Pacific variability. In this paper, we performed EOF analysis of the monthly SSTA field over the North Pacific poleward of10°N. The structure of EOF1closely resembles the PDO pattern, and the PC1exhibits considerable variability on decadal scales. The EOF2exhibits a tripole pattern with a band of positive SSTA extending from the west coast of North America to the central tropical Pacific, a band of negative SSTA extending from the central North Pacific to the northwestern tropical Pacific, and another band of positive SSTA in the Pacific north of35°N. This mode will be referred to as the North Pacific tripole (NPT).The NPT is forced by the atmosphere through the North Pacific Oscillation (NPO) and are distinct from the PDO, which is linked to variations of the Aleutian Low. Lead-lag correlations between the NPT and NPO indices show that the highest correlation occurs when the NPO leads NPT by one month, indicating a significant influence of the atmosphere on the ocean. A significant fraction (-60%) of the NPT variability can be interpreted as the integrated response of the ocean to atmospheric forcing.The North Pacific Gyre Oscillation (NPGO) is defined as the EOF2of SSH anomalies (SSHA) in the central and eastern North Pacific (180°-110°W,25°-62°N). Because both the NPGO and VM are considered to represent the second mode of the North Pacific variability, they share some similarities:both modes are independent of the PDO, and both appear to be strongly related to the NPO forcing. However, there are in fact remarkable differences between the NPT and NPGO indices because The SSH and SST variations are not completely consistent. The damping timescale of the NPGO is about10months, while the damping timescale of the NPT is about4months, suggesting that the NPGO has greater low-frequency power than the NPT. The NPT may significantly influence the climatic variability of the North Pacific. For example, the NPT is associated with a tripole pattern of precipitation anomaly in the North Pacific. In addition, the NPT may significantly influence the surface temperature and precipitation over the North America. The NPT may also provide a precursory signal for El Nino Modoki.