![]() indicates a decrease in the age of SAMW along section I08 (90☎ between 20 and 35°S), the Álvarez et al. also used TTD analysis together with repeat CFC measurements to examine changes in the mean age in the subtropical Indian Ocean (section I05 32°S). As shown in figure 2 b, their change in lower CDW along the Prime Meridian is very similar to that for CDW in section A16. performed a similar TTD analysis using measurements of CFC-12 and SF 6 made in the Weddell Sea 10 times between 19, and showed large increases in the age of both intermediate (Warm Deep Water and lower CDW) and deep (Weddell Sea Bottom Water and Weddell Sea Deep Water) waters. Contours show potential density referenced to the sea surface σ 0 (kg m −3), acronyms correspond to water masses defined in the text, and the latitudes of the climatological polar front (PF) and subantarctic front (SAF) are marked on ( b– d). Locations of the sections are shown in figure 2 a. ![]() The change in mean age is estimated from CFC-12 measurements using TTDs with Δ/ Γ=1.0. Vertical cross sections of the change in mean age between WOCE and repeat cruises for ( a) P06 sampled in 19, ( b) P16 sampled in 19, ( c) P18 sampled in 19 and ( d) A16 sampled in 19. These water-mass averages reveal consistency among the sections, which were first sampled on different dates between 19 and re-sampled between 20, with a decrease in the SAMW age (of around 15–25% per decade) and an increase in CDW age (covering a larger range of 20–40% per decade).įigure 1. The change in age averaged over the SAMW (26.6≤ σ 0≤27.0 kg m −3) and subpolar CDW (27.2≤ σ 0≤27.6 kg m −3) water masses (expressed as percentage change per decade) is shown in figure 2 b. As illustrated in figure 1, this analysis showed decreases in the mean age of subtropical thermocline and increases in subpolar waters (see figure 2 a for a map showing the sections). estimated the change in mean age between the late 1980s or early 1990s and the mid- or late 2000s for four meridional sections (in the southern Pacific, Indian and Atlantic oceans) and a zonal section across the subtropical South Pacific. Then the mean age Γ can be estimated independently from the CFC measurements for each cruise (WOCE and repeat), and the difference in these estimates is the change in mean age between cruises. In this approach, the TTD is assumed to be an inverse Gaussian distribution with a specified width/mean ratio (Δ/ Γ). Several recent studies have used transit time distribution (TTD) theory together with these repeat CFC measurements to estimate the change in the ‘mean age’ (the mean transit time since water has last contact with the surface). Measurements of CFCs have been made in the southern oceans over the past three decades, primarily as part of the World Ocean Circulation Experiment (WOCE) in the 1990s and the CLImate VARiability and predictability (CLIVAR) and CO 2 Repeat Hydrography Program in the mid- and late 2000s. The cause of these observed changes in water mass ages is examined in §4 by comparisons with the observed changes in the wind stress and strength of the subtropical gyres, and analysis of the age in Community Climate System Model version 4 (CCSM4) perturbation experiments where the zonal wind stress is increased. These analyses show large-scale coherent changes in the ventilation, with a decrease in the water mass age of subtropical Subantarctic Mode Waters (SAMW) and an increase in the age of Circumpolar Deep Waters (CDW). In the next section, we review several recent studies that have used CFC measurements made over the past three decades to examine changes in the ventilation of the southern oceans. Oceanic measurements of chlorofluorocarbons (CFCs) can, as their atmospheric concentrations have increased rapidly from the 1930s to the mid-1990s and they are conserved within the oceans, be used to constrain the rates and pathways of ocean ventilation. Here, we examine changes in the ventilation of the southern oceans using ocean measurements of transient tracers, and perturbation experiments in a climate model where the zonal wind stress is increased. There is, however, debate over the sensitivity of the southern ocean circulation to decadal changes in wind stresses. This wind stress has strengthened over recent decades, primarily as a consequence of Antarctic stratospheric ozone depletion, and studies suggest that this will have caused changes in the ocean's overturning circulation and the oceanic uptake of heat and carbon. The transport of surface waters into the interior (‘ventilation’) of the southern oceans is driven primarily by the stress due to the overlying westerly winds.
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