[A-CG44] Kuroshio Large Meander

[ACG44-P04] Atmospheric response in the Kuroshio Large Meander area - preliminary report of KS-21-9 research cruise -

*Hatsumi Nishikawa¹, Eitarou Oka¹, Shusaku Sugimoto², Yoshimi Kawai³, Fumiaki Kobashi⁴ (1.Atmosphere and Ocean Research Institute, University of Tokyo, 2.Tohoku University, 3.Japan Agency for Marine-Earth Science and Technology, 4.Tokyo University of Marine Science and Technology)

Keywords:the Kuroshio large meander, Cold water pool, Air-sea interaction, Radiosonde observation

The current Kuroshio Large Meander (LM) that started in August 2017 is the second longest event in the observed record, next to the event in 1975-1980 (4 years and 8 months). In the LM period, it is well known that a cold water pool is present in the onshore region between the Kuroshio and the southern coast of the Tokai district. While the westward Kuroshio bifurcation flow causing coastal warming along the southern coast of the Kanto-Tokai district is also found during the LM period. Previous studies reported that these characteristic ocean structures such as the cold water pool and the coastal warming can affect the overlying atmosphere, e.g., southward shift of extratropical cyclone tracks in winter (Nakamura et al. 2012; Hayasaki et al. 2013), extreme hot summer of Kanto-Tokai district (Sugimoto et al., 2020; 2021), and so on. However, previous studies have analyzed using reanalysis data, satellite measurement data, and numerical experiments, and the “actual” vertical structure of the atmosphere over the Kuroshio LM area is not well understood due to the lack of direct observation data. In May 2021, we carried out intensive atmosphere-ocean observation in the Kuroshio LM area to capture the detailed atmospheric and oceanic structures during the LM period.
The results by using radiosonde observations show a clear contrast of atmospheric structure over the Kuroshio and the cold water pool, for example, atmospheric mixing layer over the Kuroshio and the cold pool was 600-800 m and 200-300 m, respectively. The atmospheric response to a strong SST front (3 K/100 km) during the Kuroshio crossing was also observed, and a strong horizontal temperature gradient of 3 K/100 km was observed up to about 800 m above the surface.
A comparison of air temperature between the Meso-Scale Model (MSM) provided by the Japan Meteorological Agency and our observation showed that the observed air temperature was 0.5-1 K lower than that of MSM near the surface over the cold water pool. SST based on the MGD-SST used in the MSM was 0.5-1 K higher than the observed SST in the cold water pool. The air temperature difference near the surface between the MSM and observation might be due to the high SST bias of the MGD-SST.
In my presentation, I will show the detailed atmospheric response over the Kuroshio LM area, including a comparison with continuous meteorological observations such as ceilometers.