Surface frontogenesis by surface heat fluxes in the upstream Kuroshio Extension region

Asymmetric air-sea heat flux response and ocean impact to synoptic-scale atmospheric disturbances observed at JKEO and KEO buoys

This study aims to identify patterns of surface heat fluxes, and corresponding surface ocean responses, associated with synoptic-scale atmospheric events and their modulation on seasonal time scales. In particular, northerly and southerly wind events associated with atmospheric disturbances were analyzed using high-temporal resolution time-series data from two moored buoys (JKEO: 2007–2010 and KEO: 2004–2019) north and south of the Kuroshio Extension current. Although each synoptic-scale wind event generally impacted both sites, the composite surface heat flux was larger at the northern site, especially for northerly events. Both types of wind events were observed throughout the year, with a minimum during June-July–August. Northerly wind events tended to be accompanied by lowered air-temperature, while southerly events tended to have elevated air-temperature relative to the previous three days. The resulting anomalous surface heat loss was asymmetric, with larger changes in northerly events compared to the southerly events. A large and significant ocean response of − 0.28 to − 0.46 K (p-value < 0.05) in SST was confirmed only for northerly events in spring–summer at the northern site, while smaller changes were found at the southern site. The results of this study suggest that sub-monthly air-sea interactions may affect seasonal variability and potentially climate change over longer timescales.

Maintenance of mid-latitude oceanic fronts by mesoscale eddies

Oceanic fronts associated with strong western boundary current extensions vent a vast amount of heat into the atmosphere, anchoring mid-latitude storm tracks and facilitating ocean carbon sequestration. However, it remains unclear how the surface heat reservoir is replenished by ocean processes to sustain the atmospheric heat uptake. Using high-resolution climate simulations, we find that the vertical heat transport by ocean mesoscale eddies acts as an important heat supplier to the surface ocean in frontal regions. This vertical eddy heat transport is not accounted for by the prevailing inviscid and adiabatic ocean dynamical theories such as baroclinic instability and frontogenesis but is tightly related to the atmospheric forcing. Strong surface cooling associated with intense winds in winter promotes turbulent mixing in the mixed layer, destructing the vertical shear of mesoscale eddies. The restoring of vertical shear induces an ageostrophic secondary circulation transporting heat from the subsurface to surface ocean.

Enhanced Warming and Intensification of the Kuroshio Extension, 1999–2013

The Pacific climate regime has anomalous warm and cool periods every decade associated with atmospheric circulation changes, which are known to have modulated the tropical and subtropical Pacific during the recent Pacific hiatus regime (1999–2013). However, the influence of the hiatus regime on the Kuroshio Extension (KE) remains unclear. Here, we show that the KE jet underwent enhanced warming (increased 1–1.5 °C), intensification (8–19%) and northward migration (0.5–1°). The KE jet became more perturbed in the upstream region (increased by 70%, west of 146°E) but became stable downstream (perturbation decreased 5–11%, east of 146°E). A poleward shift of the mid-latitude jet stream and weakened Aleutian Low (AL) contributed to the northward migration and intensification of the KE jet, respectively. The weakened AL was associated with negative wind stress curl (WSC) in the eastern Pacific, and this WSC generated an underlying positive sea surface height anomaly that propagated westward, intensifying the KE jet when it reached the KE region. Since the recent Pacific hiatus regime ended after 2013, these changes of the KE jet may reverse during the ongoing warming regime.