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Gokhan Danabasoglu

Abstract

The time-mean wind-driven circulation of the uncoupled and coupled National Center for Atmospheric Research Climate System Model ocean model is investigated. Although the coupled surface wind stress and wind stress curl magnitudes are, in general, larger than the uncoupled distributions, the coupled wind stresses are realistic, given the substantial uncertainties in the observational estimates. The Sverdrup balance, which represents a simple dynamical relation, can indeed describe the model depth-integrated transports to a large degree and shows that the increase in the coupled ocean barotropic transports is mainly due to the larger wind stress curl of the coupled system. Another simple dynamical tool, the Ekman transport analysis, shows that the coupled ocean upwelling and downwelling velocities are, in general, larger than in uncoupled ocean, consistent with the larger wind stress curl. Both models have similar upper-ocean upwelling magnitudes in the equatorial Atlantic. In the equatorial Pacific, the coupled ocean upwelling is much larger. The coupled ocean surface currents, similarly stronger than in uncoupled ocean, differ from the uncoupled currents especially in the Nordic Seas. The Ekman transport contribution to the northward heat transport is significant in the tropical regions and in the Southern Hemisphere midlatitudes, and this transport is larger in coupled ocean than in uncoupled ocean.

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Gokhan Danabasoglu

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Multidecadal variability of the Atlantic meridional overturning circulation (MOC) is investigated diagnostically in the NCAR Community Climate System Model version 3 (CCSM3) present-day simulations, using the highest (T85 × 1) resolution version. This variability has a 21-yr period and is present in many other ocean fields in the North Atlantic. In MOC, the oscillation amplitude is about 4.5 Sv (1 Sv ≡ 106 m3 s−1), corresponding to 20% of the mean maximum MOC transport. The northward heat transport (NHT) variability has an amplitude of about 0.12 PW, representing 10% of the mean maximum NHT. In sea surface temperature (SST) and sea surface salinity (SSS), the peak-to-peak changes can be as large as 6°–7°C and 3 psu, respectively. The Labrador Sea region is identified as the deep-water formation (DWF) site associated with the MOC oscillations. In contrast with some previous studies, temperature and salinity contributions to the total density in this DWF region are almost equal and in phase. The heat and freshwater budget analyses performed for the DWF site indicate a complex relationship between the DWF, MOC, North Atlantic Oscillation (NAO), and subpolar gyre circulation anomalies. Their complicated interactions appear to be responsible for the maintenance of this multidecadal oscillation. In these interactions, the atmospheric variability associated with the model’s NAO plays a prominent role. In particular, the NAO modulates the subpolar gyre strength and contributes to the formation of the temperature and salinity anomalies that lead to positive/negative density anomalies at the DWF site. In addition, the wind stress curl anomalies occurring during the transition phase between the positive and negative NAO states produce fluctuations of the subtropical–subpolar gyre boundary, thus creating midlatitude SST and SSS anomalies. Comparisons with observations show that neither the pattern nor the magnitude of this dominant SST variability is realistic.

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Stephen Yeager and Gokhan Danabasoglu

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Surface forcing perturbation experiments are examined to identify the key forcing elements associated with late-twentieth-century interannual-to-decadal Atlantic circulation variability as simulated in an ocean–sea ice hindcast configuration of the Community Earth System Model, version 1 (CESM1). Buoyancy forcing accounts for most of the decadal variability in both the Atlantic meridional overturning circulation (AMOC) and the subpolar gyre circulation, and the key drivers of these basin-scale circulation changes are found to be the turbulent buoyancy fluxes: evaporation as well as the latent and sensible heat fluxes. These three fluxes account for almost all of the decadal AMOC variability in the North Atlantic, even when applied only over the Labrador Sea region. Year-to-year changes in surface momentum forcing explain most of the interannual AMOC variability at all latitudes as well as most of the decadal variability south of the equator. The observed strengthening of Southern Ocean westerly winds accounts for much of the simulated AMOC variability between 30°S and the equator but very little of the recent AMOC change in the North Atlantic. Ultimately, the strengthening of the North Atlantic overturning circulation between the 1970s and 1990s, which contributed to a pronounced SST increase at subpolar latitudes, is explained almost entirely by trends in the atmospheric surface state over the Labrador Sea.

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Stephen Yeager and Gokhan Danabasoglu

Abstract

The inclusion of parameterized Nordic Sea overflows in the ocean component of the Community Climate System Model version 4 (CCSM4) results in a much improved representation of the North Atlantic tracer and velocity distributions compared to a control CCSM4 simulation without this parameterization. As a consequence, the variability of the Atlantic meridional overturning circulation (AMOC) on decadal and longer time scales is generally lower, but the reduction is not uniform in latitude, depth, or frequency–space. While there is dramatically less variance in the overall AMOC maximum (at about 35°N), the reduction in AMOC variance at higher latitudes is more modest. Also, it is somewhat enhanced in the deep ocean and at low latitudes (south of about 30°N). The complexity of overturning response to overflows is related to the fact that, in both simulations, the AMOC spectrum varies substantially with latitude and depth, reflecting a variety of driving mechanisms that are impacted in different ways by the overflows. The usefulness of reducing AMOC to a single index is thus called into question. This study identifies two main improvements in the ocean mean state associated with the overflow parameterization that tend to damp AMOC variability: enhanced stratification in the Labrador Sea due to the injection of dense overflow waters and a deepening of the deep western boundary current. Direct driving of deep AMOC variance by overflow transport variations is found to be a second-order effect.

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Peter R. Gent and Gokhan Danabasoglu

Abstract

Ocean heat uptake and the thermohaline circulation are analyzed in present-day control, 1% increasing CO2, and doubled CO2 runs of the Community Climate System Model, version 2 (CCSM2). It is concluded that the observed 40-yr trend in the global heat content to 300 m, found by Levitus et al., is somewhat larger than the natural variability in the CCSM2 control run. The observed 40-yr trend in the global heat content down to a depth of 3 km is much closer to trends found in the control run and is not so clearly separated from the natural model variability. It is estimated that, in a 0.7% increasing CO2 scenario that approximates the effect of increasing greenhouse gases between 1958 and 1998, the CCSM2 40-yr trend in the global heat content to 300 m is about the same as the observed value. This gives support for the CCSM2 climate sensitivity, which is 2.2°C.

Both the maximum of the meridional overturning streamfunction and the vertical flow across 1-km depth between 60° and 65°N decrease monotonically during the 1% CO2 run. However, the reductions are quite modest, being 3 and 2 Sv, respectively, when CO2 has quadrupled. The reason for this is that the surface potential density in the northern North Atlantic decreases steadily throughout the 1% CO2 run. In the latter part of the doubled CO2 run, the meridional overturning streamfunction recovers in strength back toward its value in the control run, but the deep-water formation rate across 1-km depth between 60° and 65°N remains at 85% of the control run value. The maximum northward heat transport at 22°N is governed by the maximum of the overturning, but the transport poleward of 62°N appears to be independent of the deep-water formation rate.

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Gokhan Danabasoglu and James C. McWilliams

Abstract

The authors propose and assess principles for the design of an upper-ocean model (UOM) suitable for studies of large-scale oceanic variability over periods of a few months to many years. Its essential simplification when compared with a conventional full-depth model (FDM) is the specification of an abyssal climatology for material properties. Observational analyses of temperature and salinity fluctuations demonstrate their degree of confinement to the upper ocean. Two idealized models for diffusive penetration of tracer fluctuations and for wind-driven currents show that the UOM approximations are usually accurate for the phenomena of interest. A UOM for the oceanic general circulation is constructed, and its solutions are compared with those of an equilibrium FDM. From a stratified resting state, the UOM spins up to an equilibrium state over a period of about 30 yr. The UOM and FDM solutions agree well in both the mean state and short-term climate fluctuations, even for cases for which the model parameters and forcing are modestly inconsistent with the UOM’s abyssal climatology. A UOM can therefore be a useful, efficient tool for studies of coupled climate dynamics and sensitivity to forcing fields and model parameters, and for hypothesis testing about the roles of the abyssal ocean.

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Peter R. Gent and Gokhan Danabasoglu

Abstract

Results from two perturbation experiments using the Community Climate System Model version 4 where the Southern Hemisphere zonal wind stress is increased are described. It is shown that the ocean response is in accord with experiments using much-higher-resolution ocean models that do not use an eddy parameterization. The key to obtaining an appropriate response in the coarse-resolution climate model is to specify a variable coefficient in the Gent and McWilliams eddy parameterization, rather than a constant value. This result contrasts with several recent papers that have suggested that coarse-resolution climate models cannot obtain an appropriate response.

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Gokhan Danabasoglu and Peter R. Gent

Abstract

The equilibrium climate sensitivity of a climate model is usually defined as the globally averaged equilibrium surface temperature response to a doubling of carbon dioxide. This is virtually always estimated in a version with a slab model for the upper ocean. The question is whether this estimate is accurate for the full climate model version, which includes a full-depth ocean component. This question has been answered for the low-resolution version of the Community Climate System Model, version 3 (CCSM3). The answer is that the equilibrium climate sensitivity using the full-depth ocean model is 0.14°C higher than that using the slab ocean model, which is a small increase. In addition, these sensitivity estimates have a standard deviation of nearly 0.1°C because of interannual variability. These results indicate that the standard practice of using a slab ocean model does give a good estimate of the equilibrium climate sensitivity of the full CCSM3. Another question addressed is whether the effective climate sensitivity is an accurate estimate of the equilibrium climate sensitivity. Again the answer is yes, provided that at least 150 yr of data from the doubled carbon dioxide run are used.

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James C. McWilliams and Gokhan Danabasoglu

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Inspired by recent measurements of the eddy-induced Meridional Overturning Circulation in the tropical North Pacific Ocean by Roemmich and Gilson, the authors analyze an oceanic general circulation model for its Eulerian and eddy-induced Meridional Overturning Circulations throughout the Tropics. The model representation for the mesoscale eddy-induced circulation is the parameterization by Gent and McWilliams, and there are also rectified contributions to the time-mean overturning circulation due to seasonal and interannual fluctuations. The eddy-induced circulation is similar in all tropical basins. It has a strength of about 10% of the Eulerian (mainly Ekman) circulation, and its contribution to the meridional heat flux is a similar fraction. The pattern of the meridional streamfunction is one of double cells in the vertical and antisymmetry about the equator. Near the equator there is downwelling above the undercurrent and upwelling below, with the return circulations closed within the upper 250 m and ±5° latitude. Away from the equator in each basin, there are overturning cells with flow in the opposite directions to those nearest the equator, which reach deeper into and through the main pycnocline as well as poleward into the subtropics. Similar to the wind-driven Eulerian Meridional Overturning Circulation, the seasonal cycle in the eddy-induced circulation has a magnitude comparable to the time-mean circulation, although for an entirely different dynamical reason associated with seasonal changes in the buoyancy field that are primarily diabatic. There is also a circulation anomaly during the 1997/98 El Niño–Southern Oscillation event that nearly cancels the time-mean, counterrotating, eddy-induced cells nearest the equator and surface. The rather good agreement between the measurements and the model solution gives support to the theory underlying the parameterization of eddy-induced circulation, and it indicates that the associated eddy transport coefficients are larger in the Tropics (i.e., ∼2 × 103 m2 s−1) than in middle and high latitudes.

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Who M. Kim, Stephen Yeager, and Gokhan Danabasoglu

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The Great Salinity Anomaly (GSA) of the 1970s is the most pronounced decadal-scale low-salinity event observed in the subpolar North Atlantic (SPNA). Using various simulations with the Community Earth System Model, here we offer an alternative view on some aspects of the GSA. Specifically, we examine the relative roles of reduced surface heat flux associated with the negative phase of the North Atlantic Oscillation (NAO) and extreme Fram Strait sea ice export (FSSIE) in the late 1960s as possible drivers of the shutdown of Labrador Sea (LS) deep convection. Through composite analysis of a long control simulation, the individual oceanic impacts of extreme FSSIE and surface heat flux events in the LS are isolated. A dominant role for the surface heat flux events for the suppression of convection and freshening in the interior LS is found, while the FSSIE events play a surprisingly minor role. The interior freshening results from reduced mixing of fresher upper ocean with saltier deep ocean. In addition, we find that the downstream propagation of the freshwater anomaly across the SPNA is potentially induced by the persistent negative NAO forcing in the 1960s through an adjustment of thermohaline circulation, with the extreme FSSIE-induced low-salinity anomaly mostly remaining in the boundary currents in the western SPNA. Our results suggest a prominent driving role of the NAO-related heat flux forcing for key aspects of the observed GSA, including the shutdown of LS convection and transbasin propagation of low-salinity waters.

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