10:45 AM - 11:00 AM
[AAS07-01] Meridional Migrations of the Tropics-midlatitude Boundary and the Jet Stream and Their Implications for Spring Rainfall in Japan
Keywords:tropics-midlatitude boundary, sea surface temperature front, westerly jet stream
In the tropical and midlatitude atmospheres, large-scale dynamics are governed by different dominant physical processes. The tropics is governed by the weak temperature gradient system where temperature gradient is constrained to be moderate, whereas the midlatitude area is governed by the quasi-geostrophic system where the Coriolis force and pressure gradient force are nearly balanced. Presumably, in order for these two different governing equations to be simultaneously valid in large scales, the boundary between these two regions must be connected by phenomena with small spatial scales.
Therefore, in this study, we investigate the atmospheric behavior at the tropics-midlatitude (TM) boundary in the Northern Hemisphere. In particular, we focus on the jet stream and precipitation. The 5,800 m height line at the 500 hPa level is defined as the TM boundary. This line serves as a proxy for the northern edge of the tropical region. Next, we focus on the strong wind axis of the westerly jet stream, which moves meridionally at mid-latitudes, because the jet stream can supply vortices with small spatial scales.
Calculations of the mean latitudinal distance between the jet stream and the TM boundary show that the jet stream flows near the TM boundary during most seasons. However, only in seasons when the sea surface temperature (SST) front exists near the TM boundary, the westerly jet stream is trapped over the SST front and temporarily leaves the tropical midlatitude boundary. In particular, in boreal spring, the degree to which the jet stream takes a northward flow path exhibits interannual variability, which triggers precipitation increases in Japan when the jet stream moves northward. The composite analysis shows that the jet stream takes a significantly northward path when upward sea surface heat fluxes, especially the latent heat flux, are anomalously strong.
Therefore, in this study, we investigate the atmospheric behavior at the tropics-midlatitude (TM) boundary in the Northern Hemisphere. In particular, we focus on the jet stream and precipitation. The 5,800 m height line at the 500 hPa level is defined as the TM boundary. This line serves as a proxy for the northern edge of the tropical region. Next, we focus on the strong wind axis of the westerly jet stream, which moves meridionally at mid-latitudes, because the jet stream can supply vortices with small spatial scales.
Calculations of the mean latitudinal distance between the jet stream and the TM boundary show that the jet stream flows near the TM boundary during most seasons. However, only in seasons when the sea surface temperature (SST) front exists near the TM boundary, the westerly jet stream is trapped over the SST front and temporarily leaves the tropical midlatitude boundary. In particular, in boreal spring, the degree to which the jet stream takes a northward flow path exhibits interannual variability, which triggers precipitation increases in Japan when the jet stream moves northward. The composite analysis shows that the jet stream takes a significantly northward path when upward sea surface heat fluxes, especially the latent heat flux, are anomalously strong.