Formation Mechanisms for North Pacific Central and Eastern Subtropical Mode Waters

Abstract

The effects of one-dimensional processes on the formation of deep mixed layers in the central mode water (CMW) and eastern subtropical mode water (ESMW) formation regions of the North Pacific have been analyzed using a mixed layer model. By running the model with various combinations of initial (August) background stratification and forcing fields (heat flux, E − P, and wind stress), and comparing the resultant March mixed layer depths, the relative importance of these effects on creating deep mixed layers was diagnosed. Model results suggest that the contributions of evaporation minus precipitation and wind mixing to mixed layer depth in both the CMW and the ESMW formation regions are negligible. In the ESMW formation region (centered at approximately 30°N, 140°W), the initial stratification is very important in determining where deep mixed layers form. Summer heating is quite weak in this region, resulting in a weak (or even nonexistent) seasonal pycnocline at the end of the summer at about 30°N. It is this lack of shallow seasonal stratification that allows a local maximum of winter mixed layer depth even though the wintertime cooling is much weaker than other regions of locally deep mixed layers. In the CMW formation region (approximately 40°N between 170°E and 160°W), in contrast to the ESMW formation region, wintertime cooling is strong enough to erode through the shallow seasonal pycnocline. In the region of deepest mixed layers in the CMW region, the deeper stratification (150–400 m) is quite weak. Once the seasonal pycnocline has been eroded away, the lack of deeper stratification becomes important in allowing the mixing to penetrate further.