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Mio Terada, Shoshiro Minobe, and Curtis Deutsch

Abstract

The future change in equatorial upwelling between 1971–2000 and 2071–2100 is investigated using data from 24 coupled climate models. The multimodel ensemble (MME) mean exhibits substantial equatorial upwelling decrease in the eastern Pacific and weaker decrease in the western Atlantic Ocean. The MME mean of upwelling change and intermodel variation of that are decomposed into distinct isopycnal and diapycnal components. In the Pacific, the diapycnal upwelling decreases near the surface, associated with a weakened Ekman pumping. The isopycnal upwelling decreases at depths of 75–200 m around the core of the Equatorial Undercurrent (EUC) due to flattening of the density layer in which it flows. Both the weakened Ekman pumping and the EUC flattening are induced by the locally weakened trade wind over the eastern Pacific basin. In the equatorial Atlantic, both the change in MME mean and the intermodel variation of upwellings are significantly related to the weakened trade wind and enhanced stratification, although these drivers are not independent. The results for the Pacific Ocean imply that future reduction in upwelling may have impacts at different depths by different mechanisms. In particular, the rapid warming of sea surface temperature in the eastern Pacific basin may be mainly caused by the near-surface diapycnal upwelling reduction rather than isopycnal upwelling reduction associated EUC flattening, which is important at deeper levels.

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Klaus Keller, Curtis Deutsch, Matthew G. Hall, and David F. Bradford

Abstract

Many climate models predict that anthropogenic greenhouse gas emissions may cause a threshold response of the North Atlantic meridional overturning circulation (MOC). These model predictions are, however, uncertain. Reducing this uncertainty can have an economic value, because it would allow for the design of more efficient risk management strategies. Early information about the MOC sensitivity to anthropogenic forcing (i.e., information that arrives before the system is committed to a threshold response) could be especially valuable. Here the focus is on one particular kind of information: the detection of anthropogenic MOC changes. It is shown that an MOC observation system based on infrequent (decadal scale) hydrographic observations may well fail in the task of early MOC change detection. This is because this system observes too infrequently and the observation errors are too large. More frequent observations and reduced observation errors would result in earlier detection. It is also shown that the economic value of information associated with a confident and early prediction of an MOC threshold response could exceed the costs of typically implemented ocean observation systems by orders of magnitude. One open challenge is to identify a feasible observation system that would enable such a confident and early MOC prediction across the range of possible MOC responses.

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Eun Young Kwon, Curtis Deutsch, Shang-Ping Xie, Sunke Schmidtko, and Yang-Ki Cho

Abstract

The transport of dissolved oxygen (O2) from the surface ocean into the interior is a critical process sustaining aerobic life in mesopelagic ecosystems, but its rates and sensitivity to climate variations are poorly understood. Using a circulation model constrained to historical variability by assimilation of observations, the study shows that the North Pacific thermocline effectively takes up O2 primarily by expanding the area through which O2-rich mixed layer water is detrained into the thermocline. The outcrop area during the critical winter season varies in concert with the Pacific decadal oscillation (PDO). When the central North Pacific Ocean is in a cold phase, the winter outcrop window for the central mode water class (CMW; a neutral density range of γ = 25.6–26.6) expands southward, allowing more O2-rich surface water to enter the ocean’s interior. An increase in volume flux of water to the CMW density class is partly compensated by a reduced supply to the shallower densities of subtropical mode water (γ = 24.0–25.5). The thermocline has become better oxygenated since the 1980s partly because of strong O2 uptake. Positive O2 anomalies appear first near the outcrop and subsequently downstream in the subtropical gyre. In contrast to the O2 variations within the ventilated thermocline, observed O2 in intermediate water (density range of γ = 26.7–27.2) shows a declining trend over the past half century, a trend not explained by the open ocean water mass formation rate.

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Eun Young Kwon, Stephanie M. Downes, Jorge L. Sarmiento, Riccardo Farneti, and Curtis Deutsch

Abstract

A kinematic approach is used to diagnose the subduction rates of upper–Southern Ocean waters across seasonally migrating density outcrops at the base of the mixed layer. From an Eulerian viewpoint, the term representing the temporal change in the mixed layer depth (which is labeled as the temporal induction in this study; i.e., S temp = ∂h/∂t where h is the mixed layer thickness, and t is time) vanishes over several annual cycles. Following seasonally migrating density outcrops, however, the temporal induction is attributed partly to the temporal change in the mixed layer thickness averaged over a density outcrop following its seasonally varying position and partly to the lateral movement of the outcrop position intersecting the sloping mixed layer base. Neither the temporal induction following an outcrop nor its integral over the outcrop area vanishes over several annual cycles. Instead, the seasonal eddy subduction, which arises primarily because of the subannual correlations between the seasonal cycles of the mixed layer depth and the outcrop area, explains the key mechanism by which mode waters are transferred from the mixed layer to the underlying pycnocline. The time-mean exchange rate of waters across the base of the mixed layer is substantially different from the exchange rate of waters across the fixed winter mixed layer base in mode water density classes. Nearly 40% of the newly formed Southern Ocean mode waters appear to be diapycnally transformed within the seasonal pycnocline before either being subducted into the main pycnocline or entrained back to the mixed layer through lighter density classes.

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Annalisa Bracco, Matthew C. Long, Naomi M. Levine, R. Quinn Thomas, Curtis Deutsch, and Galen A. McKinley
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