Effective generation mechanisms of tropical instability waves as represented by high-resolution coupled atmosphere-ocean prediction experiments.

Cusp-shaped fluctuations of the sea surface temperature (SST) front in the tropical Pacific, now known as tropical instability waves (TIWs), were discovered by remote sensing in the 1970s. Their discovery was followed by both theoretical and analytical studies, which, along with in situ observations, identified several possible generation mechanisms. Although modeling studies have shown that TIWs strongly influence the heat budget, their influence on local variations of realistically initialized predictions is not yet understood. We here evaluate a series of medium-range (up to ~ 10 days) coupled atmosphere-ocean predictions by a coupled model with different horizontal resolutions. Observational SST, surface wind stress, heat flux, and pressure data showed that representation of temporally and spatially local variations was improved by resolving fine-scale SST variations around the initialized coarse-scale SST front fluctuations of TIWs. Our study thus demonstrates the advantage of using high-resolution coupled models for medium-range predictions. In addition, analysis of TIW energetics showed two dominant sources of energy to anticyclonic eddies: barotropic instability between equatorial zonal currents and baroclinic instability due to intense density fronts. In turn, the eddy circulation strengthened both instabilities in the resolved simulations. This revealed feedback process refines our understanding of the generation mechanisms of TIWs.

Northward shift of the Kuroshio Extension during 1993-2021.

The Kuroshio Extension (KE) flows eastward at the northern boundary of the North Pacific subtropical gyre. By transporting large amounts of seawater with heat, the KE contributes significantly to the formation of sea surface temperature (SST) fields. Recently, poleward shifts of major ocean gyres in the world ocean, including the North Pacific subtropical gyre, have been highlighted based on basin-scale changes in SST and sea surface height (SSH) distributions. However, a detailed investigation of the long-term meridional KE movement has not been presented. Investigation of KE path changes helps provide insights into long-term changes in the physical fields in the western North Pacific. In this study, we identified the KE path from satellite-derived SSH and surface current velocity data using a front identification method and showed that the KE migrated northward by approximately 210 km during 1993-2021. We further explored the cause of the northward KE shift based on atmospheric reanalysis data and numerical experiments using a high-resolution ocean general circulation model. It was revealed that the northward KE shift is mostly caused by the trend of wind stress curl in the North Pacific during 1993-2021.

Cold- versus warm-season-forced variability of the Kuroshio and North Pacific subtropical mode water.

The ocean responds to atmospheric variations. Changes in sea surface winds, surface air temperature, and surface air humidity cause upper ocean variability by modulating air-sea momentum and heat exchanges. Upper ocean variability in the mid-latitudes on inter-annual and longer timescales has previously been considered to be attributable to atmospheric variations in the cold season, because atmospheric forcing is stronger in the cold season than in the warm season. However, this idea has not been sufficiently confirmed yet. Although the ocean model is a useful tool to evaluate the impact of the atmospheric forcing in each season, there are no past studies having examined ocean model responses respectively to the cold- and warm-season atmospheric forcing. In this study, we performed numerical experiments with an eddy-resolving ocean general circulation model and investigated oceanic responses to cold- and warm-season atmospheric forcing, focusing on the Kuroshio and North Pacific subtropical mode water (STMW) in the western mid-latitude North Pacific. We found that temporal variations of net Kuroshio transport and STMW distribution/temperature are dominantly controlled by atmospheric forcing in the cold season. These results suggest that cold-season atmospheric variations are key to obtaining insights into large-scale upper ocean variability in the North Pacific subtropical gyre.