Evolution of Intermediate Water Masses Based on Argo Float Displacements

F. Sévellec,A. Colin de Verdiére,M. Ollitrault

doi:10.1175/jpo-d-16-0182.1

Abstract

AbstractThe evolution and dispersion of intermediate water masses in the ocean interior is studied. To this purpose, an empirical statistical model of Lagrangian tracers at a constant depth level is developed. The model follows the transfer operator based on 10-day deep displacements of Argo floats at ~1000 m depth. An asymptotic analysis of the model shows the existence of 10 principal stationary points (the 10 locations attract asymptotically 97% of the tracers). It takes ~1000 years to reach this asymptotic regime relevant for estimating the stationary points. For Lagrangian floats, the concept of attractor needs to be generalized in a statistical sense (versus deterministic), except for a few places in the ocean. In this new framework, a tracer has a likelihood to reach the stationary points, rather than a certainty to reach a single stationary point. The empirical statistical model is used to describe the fate of three intermediate water masses: North Pacific Intermediate Water (NPIW), Mediterranean Water (MW), and Antarctic Intermediate Water (AAIW). These experiments show a dramatic difference in the long-time behavior of NPIW, MW, and AAIW. In the permanent regime, the NPIW concentrates locally (in the Kuroshio recirculation) and the MW remains mainly regional (concentrated in the subtropical gyre of the North Atlantic), whereas the AAIW spreads globally (well mixed throughout the entire ocean).

Highlights

Water mass can be defined as a consistent and continuous volume of ocean water with fairly constant properties in terms of temperature and salinity, and can be identified through a classical u–S diagram
We test how MW spreads in the ocean when a continuous source of water is imposed at the Strait of Gibraltar. This experiment shows that on interannual time scales the MW remains in the neighborhood of the Strait of Gibraltar with a nonnegligible fraction moving northward in the Bay of Biscay [Fig. 11b(1)], consistently with observations on the salt tongue of Mediterranean Water into the North Atlantic through Lagrangian float analyses (Spall et al 1993; Bower et al 1997, 2002)
With a vertical equilibrium location around ;1000 m, they contribute to the upper limb of the global ocean circulation

Summary

Introduction

Water mass can be defined as a consistent and continuous volume of ocean water with fairly constant properties in terms of temperature and salinity, and can be identified through a classical u–S diagram. The vertical displacement of water parcels can be estimated by Tstd/›zT, where Tstd is the time standard deviation of the potential temperature around 1000 m measured by Argo floats and ›zT is the mean vertical gradient of potential temperature around 1000 m given by the climatology of the World Ocean Atlas 2009 (Locarnini et al 2010).

Results

Conclusion