Recent Changes in the Ventilation of the Southern Oceans

Abstract

Surface westerly winds in the Southern Hemisphere have intensified over the past few decades, primarily in response to the formation of the Antarctic ozone hole, and there is intense debate on the impact of this on the ocean's circulation and uptake and redistribution of atmospheric gases. We used measurements of chlorofluorocarbon-12 (CFC-12) made in the southern oceans in the early 1990s and mid-to late 2000s to examine changes in ocean ventilation. Our analysis of the CFC-12 data reveals a decrease in the age of subtropical subantarctic mode waters and an increase in the age of circumpolar deep waters, suggesting that the formation of the Antarctic ozone hole has caused large-scale coherent changes in the ventilation of the southern oceans.T he transport of surface waters into the interior ("ventilation") of the southern oceans plays an important role in global climate and the cycling of carbon, oxygen, and nutrients in the oceans (1-3). Over the past few decades, the southern oceans have warmed at roughly twice the rate of the global mean ocean (1), and around 40% of the anthropogenic carbon in the oceans entered south of 40°S (4). Southern ocean ventilation is driven primarily by the westerly winds (5), which have strengthened and shifted poleward over recent decades, primarily as a consequence of Antarctic stratospheric ozone depletion (6). Modeling studies suggest that this has caused changes in the ocean's overturning circulation (7-9), and carbon uptake (10, 11). However, the sensitivity of the southern ocean circulation and ventilation to decadal changes in wind stresses is under debate (12)(13)(14).Information on ventilation rates can be obtained from measurements of chlorofluorocarbons (CFCs). These compounds are conserved within the oceans, and their atmospheric concentrations have increased rapidly from when they were first produced in the 1930s until the mid1990s ( fig. S1). Because of this time dependence, CFC measurements can provide constraints on the rates and pathways of ocean transport (15, 16).We used CFC-12 measurements made along several sections in the southern oceans in the early 1990s as part of the World Ocean Circulation Experiment (WOCE) and resampled in mid-to late 2000s as part of the Climate Variability and Predictability (CLIVAR) and CO 2 repeat hydrography program (supplementary materials) to examine changes in the ventilation of the southern oceans over the past two decades.The measurements during WOCE show largest pCFC-12 at the surface, with values decreasing with depth along isopycnals (e.g., Fig. 1A). [We express the measurements as the partial pressure of CFC-12 ( pCFC-12), defined as the seawater concentration divided by the solubility (16). This enables direct comparison with the atmospheric history of CFC-12.] The repeat occupations 14 to 16 years later show large increases in pCFC-12 in the subtropical thermocline (~25°t o 45°S, 200 to 1000 m), which correspond to the waters formed by the transport of surface waters along surfaces of constant density (e.g., Fig. 1B). ...

Upwelling in the Southern Ocean

“…And although it is impossible to directly measure the strength of the upwelling, changes in the distribution of chlorofluorocarbons in the ocean suggest that upwelling may have increased in response to the wind increase. 9 In addition to those changes in the upper ocean, Antarctic bottom water-the deepest water mass in the Southern Ocean-has also warmed rapidly in recent decades. 10 It's not clear what's causing the abyssal warming, which is strongest in the south and concentrated along the northward pathways.…”

“…9,15 First, the delivery of more carbon-rich water to the surface increases the degassing of CO 2 . Second, the greater nutrient delivery and changed surfacewater properties alter the efficiency of the biological pump.…”

Decadal Patterns of Westerly Winds, Temperatures, Ocean Gyre Circulations and Fish Abundance: A Review

“…Several authors have indicated how wind oscillations may be altered as anthropogenic gases heat the earth's troposphere, or deplete ozone, and as atmospheric water vapor concentrations change [94][95][96][97][98]. For example, as the greenhouse gases CO 2 and CH 4 accumulate in the troposphere and trap heat, the stratosphere is cooled, intensifying the westerly winds at both poles [94].…”

“…For example, as the greenhouse gases CO 2 and CH 4 accumulate in the troposphere and trap heat, the stratosphere is cooled, intensifying the westerly winds at both poles [94]. Global warming due to greenhouse gases apparently intensifies the positive phase of the cycle [95][96][97][98]. The recent zonal phase, of the westerly winds in the northern hemisphere from the 1970s to the 1990s, was the strongest NAO oscillation in the recorded record [17].…”

Up and down

“…Likewise in the Southern Hemisphere, the ozone hole does not influence only the levels of incoming ultraviolet radiation. Together with greenhouse warming, it also alters the southern westerly wind systems 7 , summer climate 8 and the Southern Ocean's ability to take up trace gases from the atmosphere 9 .…”