North Pacific Subtropical–Tropical Gyre Exchanges in the Thermocline: Simulations with Two Isopycnic OGCMs

Abstract

This study considers the advective transport in the thermocline between the subtropical and tropical Pacific using two different models: MICOM (fully isopycnal) and Poseidon (quasi-isopycnal). The annually averaged model climatologies are comparable to other model and observational results, with Poseidon showing relatively greater western boundary current exchange and MICOM having a more significant basin interior pathway. This difference is related to the different representations of the thermocline ridge associated with the ITCZ, and of the zonal current systems at 8°?10°N. In both models, the interior exchange pathway is highly seasonal, with pulsed southward flow occurring during the fall and early winter as the thermocline thickens. This is the time period when the thermocline is shallowest in the central Pacific associated with the maximum North Equatorial Countercurrent transport and when the Ekman pumping associated with positive wind stress curl is shifted northward in the Tropics. Idealized tracer fields show both the continuity and seasonality of the transport pathway between subtropics and the equator, with enhanced pumping of tracer into the equatorial Pacific at 140°W over a 6-month (fall/winter) period, followed by a quiescent (spring/summer) period. Both models suggest that not only is the interior flow across the potential vorticity maximum constrained to a relatively small zonal region but, also, that the shallow subtropical overturning cell is a localized process with significant zonal variation in intensity.