A comparison of North Pacific and North Atlantic subtropical mode waters in a climatologically-forced model

Abstract

Subtropical mode water (STMW), a water mass with homogeneous temperature and density and low potential vorticity, is formed in the subtropical gyres of both the North Pacific and North Atlantic Oceans. In the North Atlantic, this water mass is known as Eighteen Degree Water (EDW), while the corresponding water mass in the Pacific is identified as North Pacific Subtropical Mode Water (NPSTMW). This analysis compares properties of EDW with NPSTMW as well as the intrinsic oceanic variability of both, within the framework of a high resolution model with climatological atmospheric forcing. Interannual variability is evident in both volume and characteristics of EDW and NPSTMW, but the magnitude of variability is small, indicating that observed variability is largely forced by atmospheric changes. Volume of EDW is greater than volume of NPSTMW, but volume variability is similar in the two basins, and is dominated by the seasonal cycle in both cases. The most significant differences are found in the average ages of EDW compared to NPSTMW. Although the volume of STMW formed in each basin is similar, different circulation patterns lead to a higher average age in the North Atlantic, as EDW is more likely to be advected away from the formation region and remain subducted in the North Atlantic, while NPSTMW is more likely to be reventilated the following winter in the North Pacific.

Introduction

The subtropical Atlantic and Pacific Oceans have many similarities. In each, the most significant feature is the western boundary current. In the Atlantic, the Gulf Stream follows the coast of North America, and separates at Cape Hatteras, near 35°N. In the Pacific, the Kuroshio follows the coast of Japan and turns eastward in the Kuroshio Extension, at the Bōsō Peninsula, also near 35°N. After separation, both of these western boundary currents head eastward with jet-like structures, and both have southern subtropical recirculation gyres. In the winter, strong winds create a deep mixed layer, with the deepest regions just south of the jets. When restratification occurs in the spring, this deep mixed layer with uniform temperature and low potential vorticity is capped by the seasonal thermocline, preserving its homogeneity. This thermostad is the subtropical mode water (STMW).

In the Atlantic, this well-known water mass is called Eighteen Degree Water (EDW), and is easily identified by its consistent temperature (Worthington, 1959). Observations indicate that this water mass has temperature around 18 °C, density near 26.5 kg m⁻³, and low potential vorticity (Joyce et al., 2009, Talley and Raymer, 1982). In the Pacific, a similarly formed thermostad is named North Pacific Subtropical Mode Water (NPSTMW) (Masuzawa, 1969). Its temperature characteristics are not as consistent and are slightly cooler, with the temperature varying from 16° to 18 °C. Density is also lower, between 25.0 and 25.5 kg m⁻³, consistent with lower salinity in the Pacific Ocean (Hanawa and Suga, 1995, Suga et al., 2008). The low potential vorticity, however, is comparable (Hanawa and Talley, 2001). In both oceans, STMW represents a unified water mass, whose characteristics are important indicators of the ventilation of the upper ocean, the circulation and its structure, and the interaction with the atmosphere.

Variability of the volume of STMW has been documented through observations in both the Atlantic and the Pacific (Joyce et al., 2000, Oka, 2009, Suga and Hanawa, 1995, Suga et al., 1989, Talley and Raymer, 1982). In the Atlantic, there is a clear correlation between volume of EDW and the North Atlantic Oscillation (Joyce et al., 2000, Kwon and Riser, 2004). In the Pacific Ocean, it has been hypothesized that volume of NPSTMW is modulated by changes in monsoonal forcing (Suga and Hanawa, 1995, Qiu and Chen, 2006) or by changes in the Kuroshio Extension (Qiu and Chen, 2006, Qiu et al., 2006). However, these changes in the Kuroshio Extension are thought to be forced by changes in large-scale wind stress curl associated with the Pacific Decadal Oscillation (Qiu and Chen, 2005). As a result, in both basins, atmospheric variations dominate variability in ocean circulation. This analysis will elucidate differences in intrinsic oceanic variability in the basins. Differences between the North Atlantic and North Pacific basins include the characteristics of the Kuroshio and the Gulf Stream, the width of the basins, and the formation of deep water in the North Atlantic that is not found in the North Pacific. By suppressing atmospheric variability, the ocean model run used in this analysis will help define differences in the variability of STMW due to sources other than the large-scale atmospheric changes. The model is forced with climatological atmospheric forcing, allowing analysis of the intrinsic variability of the nonlinear ocean system, rather than the variability arising from large scale changes in the atmospheric component of the coupled ocean–atmosphere system.

The goal of this paper is to compare and contrast the characteristics and variability of STMW in the North Pacific and North Atlantic oceans. The comparison is simplified by the fact that a single eddy-resolving integration is used to provide output for both regions. Comparisons with observations in each region are used to indicate how well model performs in some respects, but the main focus here is on comparisons between the variability and characteristics in the two separate basins.