9:45 AM - 10:00 AM
[ACG49-04] Enhancement of air-sea moisture flux in the Kuroshio and the Gulf Stream and its impacts on the atmosphere
Keywords:moisture (latent heat) flux, western boundary currents, Kuroshio, Gulf Stream, meander
1 Introduction
Western boundary currents represented by the Kuroshio and the Gulf Stream import large amounts of heat and seawater from the subtropic to the subarctic region. Recent studies have shown that higher sea surface temperature (SST) along the current path plays a decisive role in the intensive precipitation in the surrounding area (e.g., Sasaki et al., 2012; Manda et al., 2014; Sekizawa et al., 2019). However, although the air-sea moisture (latent heat) flux caused by the wind turbulence in the western boundary currents is an essential process that is the starting point for the water vapor supply from the ocean to the atmosphere, the actual situation and the details of its impact on the atmosphere remain unclear. Our group has clarified that the moisture flux was significantly higher than the surrounding area in the Kuroshio current path by direct observation of turbulent fluxes south of Shikoku during the rainy season (Tomita, 2023). As this observation was only conducted once, the factors behind this flux enhancement, its spatiotemporal universality, and its impacts on the weather above are not well understood. Also, as a recent important extreme phenomenon, the Kuroshio, which usually flows away to the east off the Boso Peninsula, flowed north to the Sanriku offshore area, meandering in a reverse U-shape in 2023-2024.
We focus on the enhancement of the moisture flux in the current paths of both the Kuroshio and the Gulf Stream, and aim to elucidate the factors involved, the characteristics of its horizontal distribution and seasonal variation, and its impacts on the atmosphere. In addition, we try to clarify the impacts of the abnormal northward shift of the Kuroshio in 2023-2024.
2 Methodology
In order to analyze the spatio-temporal changes of the moisture flux in the Kuroshio and the Gulf Stream regions and their relationships with wind and precipitation, data of atmospheric conditions, SST, and the Kuroshio current-path were obtained from MSM-GPV (JMA), OISST (NOAA) and Quick Bulletin of Ocean Conditions (JCG), respectively for the Kuroshio region. On the other hand, for the Gulf Stream region, atmospheric conditions and SST data were obtained from ERA5 (ECMWF) and the current path of the Gulf Stream was estimated using the satellite-derived sea surface heights (AVISO). Latent heat flux was calculated by applying the representative bulk method: the COARE 3.5 algorithm (Fairall et al., 2003; Edson et al., 2013), and its uncertainty was confirmed to be free of any essential defects by comparing with direct observations of turbulent fluxes during the R/V Shinsei-maru cruise conducted in June 2024.
3 Result and discussion
Enhancement of latent heat flux was found in each season along both the Kuroshio and the Gulf Stream. The large humidity difference between the sea surface and the atmosphere induced by higher SST in the currents mainly contributed to the flux enhancement. During the abnormal northward shift of the Kuroshio in 2023 and 2024, unprecedented rise of the sea surface temperature caused the locally and extremely high latent heat flux in the Kuroshio Extension off Sanriku.
Precipitation was also heavier than surrounding regions in the Kuroshio and the Gulf Stream where the enhancement of the moisture flux was found to promote evaporation from the sea surface and to increase precipitation through the accumulation of large amounts of water vapor due to horizontal convergence. In 2023 and 2024, when the Kuroshio was unusually diverted northward, and westerly winds were dominant off Sanriku, strong divergence of horizontal winds was distributed on the western side of the ridge of the meander, as indicated by previous studies, whereas strong convergence of horizontal winds, which is not usually seen, was found on the eastern side. This is presumably due to the vertical mixing mechanism over the front of the Kuroshio, which has shifted northward to a latitude where westerly winds are strong and the Kuroshio meanders sharply, and it indicates the possibility that precipitation is enhanced locally on the east side of the meander rather than on the west side.
Comparison of the Kuroshio and the Gulf Stream in terms of the enhancement of the moisture flux revealed that the seasonal change, the horizontal distribution, mechanisms, and impacts on precipitation are substantially similar, which indicated the universality of the moisture flux enhancement in the western boundary currents. However, the larger variability of the Kuroshio path than the Gulf Stream is highly likely to induce enhancement of latent heat flux anomaly, horizontal convergence, and precipitation such as the case in 2023 and 2024 when the Kuroshio flowed northward abnormally.
Western boundary currents represented by the Kuroshio and the Gulf Stream import large amounts of heat and seawater from the subtropic to the subarctic region. Recent studies have shown that higher sea surface temperature (SST) along the current path plays a decisive role in the intensive precipitation in the surrounding area (e.g., Sasaki et al., 2012; Manda et al., 2014; Sekizawa et al., 2019). However, although the air-sea moisture (latent heat) flux caused by the wind turbulence in the western boundary currents is an essential process that is the starting point for the water vapor supply from the ocean to the atmosphere, the actual situation and the details of its impact on the atmosphere remain unclear. Our group has clarified that the moisture flux was significantly higher than the surrounding area in the Kuroshio current path by direct observation of turbulent fluxes south of Shikoku during the rainy season (Tomita, 2023). As this observation was only conducted once, the factors behind this flux enhancement, its spatiotemporal universality, and its impacts on the weather above are not well understood. Also, as a recent important extreme phenomenon, the Kuroshio, which usually flows away to the east off the Boso Peninsula, flowed north to the Sanriku offshore area, meandering in a reverse U-shape in 2023-2024.
We focus on the enhancement of the moisture flux in the current paths of both the Kuroshio and the Gulf Stream, and aim to elucidate the factors involved, the characteristics of its horizontal distribution and seasonal variation, and its impacts on the atmosphere. In addition, we try to clarify the impacts of the abnormal northward shift of the Kuroshio in 2023-2024.
2 Methodology
In order to analyze the spatio-temporal changes of the moisture flux in the Kuroshio and the Gulf Stream regions and their relationships with wind and precipitation, data of atmospheric conditions, SST, and the Kuroshio current-path were obtained from MSM-GPV (JMA), OISST (NOAA) and Quick Bulletin of Ocean Conditions (JCG), respectively for the Kuroshio region. On the other hand, for the Gulf Stream region, atmospheric conditions and SST data were obtained from ERA5 (ECMWF) and the current path of the Gulf Stream was estimated using the satellite-derived sea surface heights (AVISO). Latent heat flux was calculated by applying the representative bulk method: the COARE 3.5 algorithm (Fairall et al., 2003; Edson et al., 2013), and its uncertainty was confirmed to be free of any essential defects by comparing with direct observations of turbulent fluxes during the R/V Shinsei-maru cruise conducted in June 2024.
3 Result and discussion
Enhancement of latent heat flux was found in each season along both the Kuroshio and the Gulf Stream. The large humidity difference between the sea surface and the atmosphere induced by higher SST in the currents mainly contributed to the flux enhancement. During the abnormal northward shift of the Kuroshio in 2023 and 2024, unprecedented rise of the sea surface temperature caused the locally and extremely high latent heat flux in the Kuroshio Extension off Sanriku.
Precipitation was also heavier than surrounding regions in the Kuroshio and the Gulf Stream where the enhancement of the moisture flux was found to promote evaporation from the sea surface and to increase precipitation through the accumulation of large amounts of water vapor due to horizontal convergence. In 2023 and 2024, when the Kuroshio was unusually diverted northward, and westerly winds were dominant off Sanriku, strong divergence of horizontal winds was distributed on the western side of the ridge of the meander, as indicated by previous studies, whereas strong convergence of horizontal winds, which is not usually seen, was found on the eastern side. This is presumably due to the vertical mixing mechanism over the front of the Kuroshio, which has shifted northward to a latitude where westerly winds are strong and the Kuroshio meanders sharply, and it indicates the possibility that precipitation is enhanced locally on the east side of the meander rather than on the west side.
Comparison of the Kuroshio and the Gulf Stream in terms of the enhancement of the moisture flux revealed that the seasonal change, the horizontal distribution, mechanisms, and impacts on precipitation are substantially similar, which indicated the universality of the moisture flux enhancement in the western boundary currents. However, the larger variability of the Kuroshio path than the Gulf Stream is highly likely to induce enhancement of latent heat flux anomaly, horizontal convergence, and precipitation such as the case in 2023 and 2024 when the Kuroshio flowed northward abnormally.