Relationship between temperature and volume variations of the North Pacific Central Mode Water contrasting with Subtropical Mode Water

Abstract Relationships among the year-to-year variations in the volume, potential density, potential temperature, and salinity of the North Pacific Central Mode Water (CMW) are investigated via the Grid Point Values of the Monthly Objective Analysis using the Argo data. The CMW temperature and salinity vary synchronously with volume, whereas the CMW density shows variations that are negatively correlated with volume. That is, when the CMW volume is larger than normal, the mean CMW temperature and salinity are higher than normal, and when the CMW volume is smaller, the opposite is true. The relationships among the CMW properties and volume shown here have not been noted before. The CMW volume is primarily controlled by the depth of the upper boundary of the CMW layer, but the lower boundary does not have a significant influence on the volume variation. When the volume is larger, the upper boundary of the CMW layer becomes shallower to enclose warmer and lighter water in the upper part of the CMW layer, and when the volume is smaller, the opposite occurs. This CMW volume variation reflects the mixed layer properties in the probable formation region of the CMW in late winter. When the CMW volume is greater than normal, the net surface heat flux in the region is greater than normal, resulting in southward migration of the southern boundary of the CMW formation region, thus heaving the upper boundary of the CMW layer; during small CMW volume years, the opposite occurs.