Ocean & Fishery Resources

Interannual Variability of North Pacific Eastern Subtropical Mode Water Formation in the 1990s Derived from a 4-Dimensional Variational Ocean Data Assimilation Experiment

The interannual variability of eastern subtropical mode water (ESTMW) formation in the North Pacific is examined using a newly-derived ocean reanalysis dataset constructed by a 4-dimensional variational data assimilation experiment covering the decade of the 1990s. The interannual variation of the volume of newly formed ESTMW (Figure. 1) is consistent with the interannual variations of three physical factors in the surface layer: 1) a convergence in the transport of surface saline water induced by Ekman flow in the vicinity of the formation region, 2) thermal stratification in the preconditioning phase in association with the insolation anomaly induced largely by low-level cloud coverage, and 3) wintertime surface cooling in the eastern subtropics (Table 1).

SST, PDO

Figure 1. (a) Time series of the ESTMW volume from our results (solid line) and minimum and maximum value in the WOA data (dash lines). (b) Newly formed volume of ESTMW in each year (winter), defined as the maximum value in the winter minus the minimum value in the preceding autumn as calculated from the time series of total volume shown in panel (a).


Table 1. Anomalies in the production of new ESTMW (+(-) denotes high (low)), relative contributions of : (1) salt convergence anomalies (+ denotes positive), (2) temperature stratification anomalies (+ denotes weak stratification), and (3) net wintertime surface heat flux anomalies (+ denotes strong cooling).

SST, PDO