Oral

IAMAS

JMP10 - Ocean-Atmosphere Mechanisms of Climate Variability, Change and Predictability

Interactions of Recent Extreme Marine Heatwaves around Japan with Record Atmospheric Heatwaves

Interactions of extreme marine heatwaves around Japan with record-setting atmospheric heatwaves are compared between the 2023 and 2024 summers. In the 2023 summer sea-surface temperature (SST) and subsurface ocean temperatures around northern Japan were unprecedentedly high. This was especially the case off the east coast of northern Japan, where cool Oyashio water was replaced with much warmer water due to a striking poleward meander of the Kuroshio Extension (KE) that persisted from early spring. Locally, sub-surface water temperature anomaly reached as much as +10��C, and the corresponding SST anomaly exceeded +4��C, suggesting that the marine heatwave acted to sustain the atmospheric heatwave. Anomalous upward sensible and latent heat fluxes from the warmed sea surface were indicative of local oceanic forcing on the overlying atmosphere. Specifically, the pronounced warm SST anomaly reduced the near-surface stratification to maintain unfavorable conditions for low-level cloud formation. In fact, the reduction of satellite-measured low-cloud fraction reached an unprecedented level, which in turn led to increased surface insolation for further SST warming in midsummer as positive feedback. In addition, daily sea breeze was less effective in cooling the coastal regions, and increased moisture in the warmed lower troposphere contributed to the enhanced surface downward infrared radiation even under the reduced cloudiness. This enhanced greenhouse effect acted not only as positive feedback on the warm SST anomalies but also as a contributor to the extreme warmth over northern Japan landmass. In the 2024 summer, the pronounced poleward KE meander still continued, causing another extreme marine heatwave. Though constructive, however, its impact on another hot summer over northern Japan was not as dominant as in the 2023 summer. This is because the upper-level subtropical jetstream and associated surface Baiu front were closer to northern Japan, and thus increased fraction of middle and high clouds made the SST-albedo feedback of low clouds less effective. Rather, the 2024 summer was characterized by another extreme marine heatwave to the southwest of Japan, including Okinawa/Amami islands. Over that region, air temperature averaged for July-August-September was record high in the lower troposphere with the strongest anomaly at the surface. Concurremtly, temperature in the ocean mixed layer was also record high since 1993, and significantly warm anomalies extended well below the mixed layer. Anomalous turbulent sensible heat flux (SHF) was upward, indicating that the extremely warm ocean directly heated the overlying atmosphere. The extremely high SST was then reinforced under enhanced insolation and reduced near-surface wind speeds, while the above-normal subsurface water temperatures might have underpinned the positive SST anomalies, which appeared to influence typhoon behavior in August.