Temporal and Spatial Variability of the Lighter Variety of the North Pacific Central Mode Water Formation

Abstract:

Argo float data obtained from 2003 to 2013 in the North Pacific were analyzed to investigate temporal and spatial variability of the lighter variety of the North Pacific Central Mode Water (CMW) formation. The analysis clearly depicts a wide zonal band of deep winter mixed layer at 34N-39N from the east coast of Japan to the east of the dateline. This zonal band corresponds to the formation region of the lighter variety of CMW (L-CMW; Oka et al., 2011).

We examined inter-annual variations of winter mixed layer development and their main causes in the western, central, and eastern parts of the L-CMW formation region, respectively. In the eastern part of the L-CMW formation region, atmospheric forcing such as winter cooling and wind-induced mixing is an important factor of mixed layer deepening. On the other hand, in the western part of the L-CMW formation region, weakened stratification of subsurface associated with anti-cyclonic meso-scale eddies pinched off from the Kuroshio extension is more important. As a result, the L-CMW formation in each year is governed by combination of magnitude of atmospheric forcing in the eastern part and intensity of ocean subsurface stratification in the western part. Meanwhile, summer-time subsurface L-CMW in the subtropical gyre is distributed more widely and thickly when the mixed layer in the previous winter is deeper in the eastern part of formation region. This may imply the subsurface L-CMW in the subtropical gyre is mainly subducted from the eastern part of the formation region.