Global Contraction of Antarctic Bottom Water between the 1980s and 2000s, by S. G. Purkey & G. C. Johnson, J. Climate, 25 (2012)

Purkey, Sarah G., Gregory C. Johnson, 2012: Global Contraction of Antarctic Bottom Water between the 1980s and 2000s*. J. Climate, 25, 5830–5844.

doi: http://dx.doi.org/10.1175/JCLI-D-11-00612.1

Global Contraction of Antarctic Bottom Water between the 1980s and 2000s*

Sarah G. Purkey and Gregory C. Johnson

School of Oceanography, University of Washington, and NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Abstract

A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the meridional overturning circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW. Rates of change in AABW-related circulation are estimated in most of the world’s deep-ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water below θ = 0°C at a rate of −8.2 (±2.6) × 10⁶ m³ s⁻¹. This bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near-surface recovery, apparently through a southward surge of Circumpolar Deep Water from the north. To the north, smaller losses of bottom waters are seen along three of the four main northward outflow routes of AABW. Volume and heat budgets below deep, cold θ surfaces within the Brazil and Pacific basins are not in steady state. The observed changes in volume and heat of the coldest waters within these basins could be accounted for by small decreases to the volume transport or small increases to θ of their inflows, or fractional increases in deep mixing. The budget calculations and global contraction pattern are consistent with a global-scale slowdown of the bottom, southern limb of the MOC.

Keywords: Southern Ocean, Meridional overturning circulation, Abyssal circulation, Bottom currents/bottom water, Thermohaline circulation, Climate variability

Received: October 20, 2011; Accepted: January 19, 2012

* Pacific Marine Environmental Laboratory Contribution Number 3771.

Corresponding author address: Sarah G. Purkey, School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195-7940. E-mail: sarah.purkey@noaa.gov