Where and how long ago was water in the western North Atlantic ventilated? Maximum entropy inversions of bottle data from WOCE line A20

Mark Holzer,François W Primeau,Samar Khatiwala,William M Smethie

doi:10.1029/2009jc005750

Abstract

A maximum entropy (ME) method is used to deconvolve tracer data for the joint distribution of locations and times since last ventilation. The deconvolutions utilize World Ocean Circulation Experiment line A20 repeat hydrography for CFC‐11, potential temperature, salinity, oxygen, and phosphate, as well as Global Ocean Data Analysis Project (GLODAP) radiocarbon data, combined with surface boundary conditions derived from the atmospheric history of CFC‐11 and the World Ocean Atlas 2005 and GLODAP databases. Because of the limited number of available tracers the deconvolutions are highly underdetermined, leading to large entropic uncertainties, which are quantified using the information entropy of relative to a prior distribution. Additional uncertainties resulting from data sparsity are estimated using a Monte Carlo approach and found to be of secondary importance. The ME deconvolutions objectively identify key water mass formation regions and quantify the local fraction of water of age τ or older last ventilated in each region. Ideal mean age and radiocarbon age are also estimated but found to have large entropic uncertainties that can be attributed to uncertainties in the partitioning of a given water parcel according to where it was last ventilated. Labrador/Irminger seawater (L water) is determined to be mostly less than ∼40 a old in the vicinity of the deep western boundary current (DWBC) at the northern end of A20 but several decades older where the DWBC recrosses the section further south, pointing to the importance of mixing via a multitude of eddy‐diffusive paths. Overflow water lies primarily below L water with young waters (τ ≲ 40 a) at middepth in the northern part of A20 and waters as old as ∼600 a below ∼3500 m.

Highlights

[2] Water mass properties such as salinity, S, temperature, T, and the concentrations of tracers of atmospheric origin are determined in the mixed layer through the interplay of air‐ sea exchange processes and the ocean circulation
[4] The compositions of the North Atlantic Deep Water (NADW) components have been determined by classical temperature/salinity analysis [e.g., Swift, 1984; Rudels et al, 2002] and the circulation pathways have been delineated by measuring the distribution of chlorofluorocarbons (CFCs) [e.g., Weiss et al, 1985; Smethie et al, 2000] and tritium [e.g., Jenkins and Rhines, 1980; Doney and Jenkins, 1994], which are incorporated in the NADW at its surface source regions [e.g., Östlund and Rooth, 1990; Smethie, 1993]
Inclusion of CFC‐11 removes this feature except for the very small blips visible in the 20.8°N distributions for tn < 20 a between 1000 and 2000 m depths. (The sharper blips visible below 3500 m in the 39.9°N distributions for tn < 20 a are due to Nordic/Barents Sea Water.) Overall, the transit time distributions (TTDs) again show the pattern of young water in the northern part of the section, with older waters in the southern part that we described in terms of water mass fractions

Summary

Introduction

[2] Water mass properties such as salinity, S, temperature, T, and the concentrations of tracers of atmospheric origin are determined in the mixed layer through the interplay of air‐ sea exchange processes and the ocean circulation. North Atlantic Deep Water (NADW) forms from dense waters originating in the Arctic Ocean [Mauritzen, 1996; Rudels et al, 2002] and in the Greenland, Iceland, and Norwegian Seas (the Nordic Seas) [Swift et al, 1980], as well as from water undergoing deep wintertime convection in the Labrador [Lazier, 1973; Talley and McCartney, 1982; Lilly et al, 1999; Pickart et al, 1996; Stramma et al, 2004] and Irminger [Pickart et al, 2003] Seas. [4] The compositions of the NADW components have been determined by classical temperature/salinity analysis [e.g., Swift, 1984; Rudels et al, 2002] and the circulation pathways have been delineated by measuring the distribution of chlorofluorocarbons (CFCs) [e.g., Weiss et al, 1985; Smethie et al, 2000] and tritium [e.g., Jenkins and Rhines, 1980; Doney and Jenkins, 1994], which are incorporated in the NADW at its surface source regions [e.g., Östlund and Rooth, 1990; Smethie, 1993]

Methods

Results

Conclusion