Formation mechanisms for North Pacific central and eastern subtropical mode waters

The effects of one-dimensional processes on the formation of deep mixed lay ers in the central mode water (CMW) and eastern subtropical mode water (ESM W) formation regions of the North Pacific have been analyzed using a mixed layer model. By running the model with various combinations of initial (Aug ust) background stratification and forcing fields (heat flux, E - P, and wi nd stress), and comparing the resultant March mixed layer depths, the relat ive importance of these effects on creating deep mixed layers was diagnosed . Model results suggest that the contributions of evaporation minus precipi tation and wind mixing to mixed layer depth in both the CMW and the ESMW fo rmation regions are negligible. In the ESMW formation region (centered at approximately 30 degrees N, 140 d egrees W), the initial stratification is very important in determining wher e deep mixed layers form. Summer heating is quite weak in this region, resu lting in a weak (or even nonexistent) seasonal pycnocline at the end of the summer at about 30 degrees N. It is this lack of shallow seasonal stratifi cation that allows a local maximum of winter mixed layer depth even though the wintertime cooling is much weaker than other regions of locally deep mi xed layers. In the CMW formation region (approximately 40 degrees N between 170 degrees E and 160 degrees 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 strati fication (150-400 m) is quite weak. Once the seasonal pycnocline has been e roded away, the lack of deeper stratification becomes important in allowing the mixing to penetrate further.