Abstract
AbstractObservations of oceanic transient tracers have indicated that the circulation in the Southern Ocean has changed in recent decades, potentially driven by changes in external climate forcing. Here, we use the CESM Large Ensemble to analyze changes in two oceanic tracers that are affected by ocean circulation: the partial pressure of chlorofluorocarbon‐12 (pCFC12) and the idealized model tracer Ideal Age (IAGE) over the 1991 to 2005 period. The small ensemble mean change in IAGE suggests that there has been very little externally forced change in Southern Ocean circulation over this period, in contrast to strong internal variability. Further, our analysis implies that real‐world observations of changes in pCFC12 may not be a robust way to characterize externally driven changes in Southern Ocean circulation because of the large internal variability in pCFC12 changes exhibited by the individual ensemble members.
Highlights
The circulation of the Southern Ocean sets the state for its important role in the climate system
Our analysis implies that real‐world observations of changes in pressure of chlorofluorocarbon‐12 (pCFC12) may not be a robust way to characterize externally driven changes in Southern Ocean circulation because of the large internal variability in pCFC12 changes exhibited by the individual ensemble members
We find that the forced changes in CFC‐derived estimates of Southern Ocean circulation are small relative to the spread across the ensemble, suggesting that internal variability plays a large role in observed changes
Summary
The circulation of the Southern Ocean sets the state for its important role in the climate system. Persistent westerly winds transport old circumpolar deep water (CDW) to the surface at the high latitudes and subduct Subantarctic Mode Water (SAMW) and intermediate water masses to the ocean interior at lower latitudes (Rintoul & Naveira Garabato, 2013) This meridional overturning circulation is tightly coupled to the fluxes of carbon and heat at the air‐sea interface (Lovenduski et al, 2007; Russell et al, 2006) and affected by the strength of the westerly winds (Hall & Visbeck, 2002; Lovenduski & Gruber, 2005). Ting and Holzer (2017) found coherent decadal changes in Southern Ocean ventilation, broadly consistent with those identified by Waugh et al (2013) These transient tracer studies indicate an acceleration in Southern Ocean meridional overturning from the 1990s to the 2000s
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