Extensive Marine Heatwaves at the Sea Surface in the Northwestern Pacific Ocean in Summer 2021

Abstract

In July–August 2021, intense marine heatwaves (MHWs) occurred at the sea surface over extensive areas of the northwestern Pacific Ocean, including the entire Sea of Japan and part of the Sea of Okhotsk. In extent and intensity, these MHWs were the largest since 1982, when satellite measurements of global sea surface temperatures started. The MHWs in summer 2021 were observed at the sea surface and occurred concomitantly with a stable shallow oceanic surface boundary layer. The distribution of the MHWs was strongly related to heat fluxes at the sea surface, indicating that the MHWs were generated mainly by atmospheric forcing. The MHWs started to develop after around 10 July, concurrent with an extreme northward shift of the atmospheric westerly jet. The MHWs developed rapidly under an atmospheric high-pressure system near the sea surface, associated with a northwestward expansion of the North Pacific Subtropical High. The MHWs exhibited peaks around 30 July to 1 August. Subsequently, following the southward displacement of the westerly jet, the MHWs weakened and then shrank abruptly, synchronously with rapid deepening of the oceanic surface boundary layer. By 18 August, the MHWs had disappeared.

Unprecedented Outbreak of Harmful Algae in Pacific Coastal Waters off Southeast Hokkaido, Japan, during Late Summer 2021 after Record-Breaking Marine Heatwaves

“…A mixed layer depth of 10 m was also related to the right peak of the frequency distribution of high SCCs as a function of mixed layer depth (Figure 7c). The period of the most extensive and intense marine heatwaves ("MHWs period") is based on Kuroda and Setou [14] and is indicated by a double-headed arrow.…”

“…In mid-September 2021, about a month after the subsidence of the most intense and extensive marine heatwaves over the north-western Pacific Ocean [14], an unprecedented large-scale outbreak of harmful algae occurred in Pacific coastal waters off the south-eastern coast of Hokkaido, Japan. There had been no previous reports of catastrophic disasters attributed to HABs over these coastal waters, although there were a few reports of occurrence of harmful algae in a limited area of this region during the 1970-1980s [15].…”

“…SCCs were based on near-realtime L4 products. (e) Time series of daily sea surface temperatures (pink line) and vertical eddy diffusivity on the second top layer of the model (green line) that were averaged over the focal region.The period of the most extensive and intense marine heatwaves ("MHWs period") is based on Kuroda and Setou[14] and is indicated by a double-headed arrow.…”

Summer Marine Heatwaves in the Kuroshio-Oyashio Extension Region

“…Further analysis found that the reduction of cloud cover in the KOE was primarily explained by the anomalously strengthened North Pacific High (NPH) system (Figure 9a), which suppressed the convective activity, manifested as positive OLR and negative precipitation anomalies (Figure 9b,c). Similarly, Kuroda and Setou [19] found that MHW in the Northwest Pacific in summer 2021 was related to air-sea heat fluxes, and the northwestern shift of the NPH was an important factor. However, besides the NPH system, some other factors were still worthy of consideration.…”

“…Many MHWs, with diverse physical drivers, have been recorded over the global ocean [5,6]. The five major West Boundary Currents (WBCs) and their extension regions, the Oyashio region during summer in 2010-2016 were attributed to increased anticyclonic eddy activity; Kuroda and Setou [19] reported that a MHW occurred in July-August 2021 over the Northwest Pacific, including the entire Sea of Japan and part of the Sea of Okhotsk, which was mainly caused by anomalous surface heat flux forced by the atmospheric conditions.…”