Water Mass Formation in an Isopycnal Model of the North Pacific

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Abstract
An isopycnal model coupled with a mixed layer model is used to study transformation and formation rates in the North Pacific. When annual formation rates are averaged over the entire North Pacific, a large peak in water mass formation is found at a density of approximately σθ = 26 kg m−3. This peak in formation rate corresponds to the formation of North Pacific Central Mode Water (CMW) in the model. No corresponding peaks in formation rate are found at the densities of Subtropical Mode Water (STMW; σθ ∼25.4 kg m−3) or Eastern Subtropical Mode Water (ESMW; σθ ∼24–25.4 kg m−3) when averaged over the entire model basin. However, when calculated locally, enhanced formation rates are found at the densities of these mode water masses. The formation of each of the three types of North Pacific mode water in the model occurs because of different circumstances. As expected, STMW formation is dependent on the strong cooling and resultant deep mixed layers over the Kuroshio Current region. However, formation rates in the STMW formation region (west of the date line) imply that most of the thickness maximum formed there each winter is subsequently reentrained into the mixed layer during the next winter where it is further cooled, preconditioning it to become denser varieties of STMW farther east. Similarly, preconditioning west of the formation region is important in CMW formation. Only 33% of the STMW escapes reentrainment the next winter. The STMW signature (minimum in vertical stratification) remains in the region in the next winter owing to a tight recirculation that carries the mode water south before the mixed layer deepens again the next winter. The renewal times calculated from the model are 1.5–5.5 yr for STMW and approximately 10–14 yr for CMW. The ESMW formation is due to a band of weak positive formation combined with a wide layer outcrop. The only region where forcing by the atmosphere can directly influence an isopycnal layer is where the isopycnal layer outcrops into the mixed layer. The wide layer outcrop at ESMW densities (∼30°N, 140°W) is at least partially due to weak summer heating in the southeastern part of the formation region. Only 13% of the ESMW volume escapes reentrainment by the mixed layer in the succeeding year contributing to a renewal time of only 1–2 yr.

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