17:15 〜 19:15
[ACG36-P07] Impact of a marine heat wave on record hot summer in Okinawa and Amami of Japan in 2024
キーワード：海洋熱波、熱波、大気海洋相互作用、エルニーニョ・南方振動
Marine heatwave (MHW) events, which are characterised by anomalous high sea surface temperatures (SSTs) persisting from days to months, have attracted much attention recently because of various devastating impacts on marine ecosystems and fisheries. In addition, recent studies (Takemura et al. 2024; Sato et al. 2024) have suggested that unprecedented MHWs were likely to contribute locally to the record hot summer of northern Japan in 2023 along with large anomalies of atmospheric circulation. In summer 2024, a MHW occurred in the vicinity of Okinawa/Amami (OA), an island region in the southwestern Japan. Concurrently, the July-August-September (JAS)-mean surface air temperature (SAT) in OA based on weather station observations set a new highest record for JAS. One may wonder whether the MHW impacted on the record high summer SAT in OA as in 2023 summer over norther Japan. Still, much uncertainty still remains about oceanic influence on atmospheric heatwaves. Here, we examine a possible contribution of the oceanic processes to the record-high SAT in OA in JAS 2024 based on observation and reanalysis data.
Over OA air temperatures averaged in JAS 2024 were record high throughout the lower troposphere and the strongest anomalies were observed at the surface. At the same time, temperature in the ocean mixed layer were also higher than in any other years since 1993, and significantly warm anomalies further extended down to around the 200m depth well below the mixed layer. These characteristics are indicative of potential local impact of the warm ocean on the record high SATs in OA, in addition to significant contributions from large-scale atmospheric circulation anomalies.
In fact, anomalous turbulent sensible heat flux (SHF) was upward around OA, indicating that the extremely warm ocean directly heated the overlying atmosphere. A linear decomposition of the bulk formula of SHF has revealed that a contribution from positive SST anomalies was dominant in the anomalous upward SHF. The extremely warm SST anomalies were then reinforced by atmospheric forcing, such as enhanced insolation, suppressed evaporation and reduced near-surface wind speeds, while the above-normal subsurface water temperatures also might have underpinned the positive SST anomalies.
The anomalous warm water in OA beneath the ocean mixed layer may have been influenced by westward-propagating warm anomalies, which appears to be oceanic internal Rossby wave originating from the southwest of Japan around 20°-25°N in autumn 2023. The anomalies then moved westward for about a year until reaching the south of OA in summer and autumn 2024. Furthermore, our composite analysis suggests that such westward-propagating warm anomalies tend to reach the vicinity of OA in post El Niño-summer and autumn, similar to the conditions observed in 2024. It further suggests that combination of two delayed impacts of El Niño events could amplify positive anomalies of SAT and SST in OA in summer to autumn. Specifically, one is the well-known intensified westward expansion of the North Pacific Subtropical High due to the Indian Ocean capacitor effect (Xie et al. 2009), and the other is the westward-migrating warm anomalies described above.
Over OA air temperatures averaged in JAS 2024 were record high throughout the lower troposphere and the strongest anomalies were observed at the surface. At the same time, temperature in the ocean mixed layer were also higher than in any other years since 1993, and significantly warm anomalies further extended down to around the 200m depth well below the mixed layer. These characteristics are indicative of potential local impact of the warm ocean on the record high SATs in OA, in addition to significant contributions from large-scale atmospheric circulation anomalies.
In fact, anomalous turbulent sensible heat flux (SHF) was upward around OA, indicating that the extremely warm ocean directly heated the overlying atmosphere. A linear decomposition of the bulk formula of SHF has revealed that a contribution from positive SST anomalies was dominant in the anomalous upward SHF. The extremely warm SST anomalies were then reinforced by atmospheric forcing, such as enhanced insolation, suppressed evaporation and reduced near-surface wind speeds, while the above-normal subsurface water temperatures also might have underpinned the positive SST anomalies.
The anomalous warm water in OA beneath the ocean mixed layer may have been influenced by westward-propagating warm anomalies, which appears to be oceanic internal Rossby wave originating from the southwest of Japan around 20°-25°N in autumn 2023. The anomalies then moved westward for about a year until reaching the south of OA in summer and autumn 2024. Furthermore, our composite analysis suggests that such westward-propagating warm anomalies tend to reach the vicinity of OA in post El Niño-summer and autumn, similar to the conditions observed in 2024. It further suggests that combination of two delayed impacts of El Niño events could amplify positive anomalies of SAT and SST in OA in summer to autumn. Specifically, one is the well-known intensified westward expansion of the North Pacific Subtropical High due to the Indian Ocean capacitor effect (Xie et al. 2009), and the other is the westward-migrating warm anomalies described above.