Tropospheric winds in the North Pacific.
Climatological January values are depicted for sea level pressure (hPa, a), surface winds (m/s, a), geopotential height at 300 hPa (m, b) and winds at 300 hPa (m/s, b). Panels c and d depict the seasonal cycle of the northwestern component of tropospheric winds in Japan (NW-WIND, m/s) and of the Pacific Zonal Wind Index (P-WIND, m/s), respectively. NW-WIND is here defined as the projection of the horizontal two-dimensional wind onto a unit vector in the northwest/southeast direction (i.e. cos(45°)·uwind −sin(45°)·vwind). P-WIND is defined as the mean zonal wind along the parallel 35°N between longitudes 140°E and 240°E (see the horizontal green line in panels a and b).