17:15 〜 18:45
[ACG32-P10] Warm conveyor belt and warm current causing the retreat of sea ice along the coast of the Sea of Okhotsk
キーワード：ウォームコンベアベルト、大気の川、宗谷暖流、熱フラックス
Sea ice in the mid- and high-latitude oceans plays a crucial role in the Earth's climate system. In the Sea of Okhotsk, sea ice extends from November to June, reaching the coast of Hokkaido from January to April. Coastal sea ice is not only a tourism resource but also significantly influences the weather and climate along the coastal areas of the Sea of Okhotsk. In addition to many previous studies on the sea ice variability in the Pan-Okhotsk region, research focusing on observed sea ice variations along the coast of Hokkaido have also been conducted extensively. According to the development of high-resolution reanalysis data, detailed spatial and temporal behavior of atmospheric phenomena and the associated sea ice variability in the Sea of Okhotsk can be investigated in recent years. Kamae et al. (2023) investigated synoptic-scale features related to reduction events in the sea ice concentration (SIC) in the Sea of Okhotsk on subweekly-to-weekly timescales using daily high-resolution reanalysis data. They revealed that the southeasterly wind anomalies over the Sea of Okhotsk, associated with the eastward-moving extratropical cyclones from northern Japan to the North Pacific, were important factors for to the SIC reduction events. In the current study, we apply a similar approach to the early spring season in the southern part of the Sea of Okhotsk, revealing atmospheric and oceanic phenomena leading to rapid decreases in SIC in this region.
As an example of rapid reduction events of SIC over the southern Sea of Okhotsk, SIC of 62.7% was rapidly decreased to 8.0% in 11 days between in March 2004. Between 1993 and 2019, we detected 7 rapid reduction events. Most of the rapid reduction events were accompanied with warm conveyor belt (WCB) passages over the northern Japan and the southern Sea of Okhotsk. Composited atmospheric fields at the timing of WCB passages exhibits a meridionally elongated warm band over the East China Sea, Japan, and southern Sea of Okhotsk. This warm band is formed along cold fronts associated with eastward-moving extratropical cyclones. These warm flows in the 6 events are also diagnosed as atmospheric rivers (ARs) because the strong, warm and moist winds developed along the cold fronts are also the key features of the ARs. The warm air moved northeastward along the cold front, resulting in a sharp temperature rise over the southern Sea of Okhotsk. In addition to the extratropical cyclone, high pressure anomaly can also be found over the subtropical western North Pacific. This high pressure anomaly suggests an enhanced southwesterly low level jet along isobars between the extratropical cyclone and the anomalous high pressure.
The strong southwesterly winds should have significant effects to SIC reduction in the southern Sea of Okhotsk. Composited SIC was decreased along the Hokkaido coast (especially over the northern part) during the WCB-related events. Sea ice velocity was consistently northeastward, suggesting a contribution of sea ice drift to the coastal SIC reduction. However, negative SIC tendency was broadly found during the WCB events, suggesting an important effect of sea ice melting. We also investigated sea water velocity and found a tendency of Soya Current enhancement during the WCB events. This enhanced current should have strong effects to the SIC reduction via transporting relatively warm sea water from the Sea of Japan to southern Sea of Okhotsk.
As an example of rapid reduction events of SIC over the southern Sea of Okhotsk, SIC of 62.7% was rapidly decreased to 8.0% in 11 days between in March 2004. Between 1993 and 2019, we detected 7 rapid reduction events. Most of the rapid reduction events were accompanied with warm conveyor belt (WCB) passages over the northern Japan and the southern Sea of Okhotsk. Composited atmospheric fields at the timing of WCB passages exhibits a meridionally elongated warm band over the East China Sea, Japan, and southern Sea of Okhotsk. This warm band is formed along cold fronts associated with eastward-moving extratropical cyclones. These warm flows in the 6 events are also diagnosed as atmospheric rivers (ARs) because the strong, warm and moist winds developed along the cold fronts are also the key features of the ARs. The warm air moved northeastward along the cold front, resulting in a sharp temperature rise over the southern Sea of Okhotsk. In addition to the extratropical cyclone, high pressure anomaly can also be found over the subtropical western North Pacific. This high pressure anomaly suggests an enhanced southwesterly low level jet along isobars between the extratropical cyclone and the anomalous high pressure.
The strong southwesterly winds should have significant effects to SIC reduction in the southern Sea of Okhotsk. Composited SIC was decreased along the Hokkaido coast (especially over the northern part) during the WCB-related events. Sea ice velocity was consistently northeastward, suggesting a contribution of sea ice drift to the coastal SIC reduction. However, negative SIC tendency was broadly found during the WCB events, suggesting an important effect of sea ice melting. We also investigated sea water velocity and found a tendency of Soya Current enhancement during the WCB events. This enhanced current should have strong effects to the SIC reduction via transporting relatively warm sea water from the Sea of Japan to southern Sea of Okhotsk.