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Michael Winton, Stephen M. Griffies, Bonita L. Samuels, Jorge L. Sarmiento, and Thomas L. Frölicher

Abstract

The influence of changing ocean currents on climate change is evaluated by comparing an earth system model’s response to increased CO2 with and without an ocean circulation response. Inhibiting the ocean circulation response, by specifying a seasonally varying preindustrial climatology of currents, has a much larger influence on the heat storage pattern than on the carbon storage pattern. The heat storage pattern without circulation changes resembles carbon storage (either with or without circulation changes) more than it resembles the heat storage when currents are allowed to respond. This is shown to be due to the larger magnitude of the redistribution transport—the change in transport due to circulation anomalies acting on control climate gradients—for heat than for carbon. The net ocean heat and carbon uptake are slightly reduced when currents are allowed to respond. Hence, ocean circulation changes potentially act to warm the surface climate. However, the impact of the reduced carbon uptake on radiative forcing is estimated to be small while the redistribution heat transport shifts ocean heat uptake from low to high latitudes, increasing its cooling power. Consequently, global surface warming is significantly reduced by circulation changes. Circulation changes also shift the pattern of warming from broad Northern Hemisphere amplification to a more structured pattern with reduced warming at subpolar latitudes in both hemispheres and enhanced warming near the equator.

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Stephanie M. Downes, Andrew McC. Hogg, Stephen M. Griffies, and Bonita L. Samuels

Abstract

Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely distributed Antarctic Bottom Water. Here two distinct geothermal heat flux datasets are tested under different experimental designs in a fully coupled model that mimics the control run of a typical Coupled Model Intercomparison Project (CMIP) climate model. The regional analysis herein reveals that bottom temperature and transport changes, due to the inclusion of geothermal heating, are propagated throughout the water column, most prominently in the Southern Ocean, with the background density structure and major circulation pathways acting as drivers of these changes. While geothermal heating enhances Southern Ocean abyssal overturning circulation by 20%–50%, upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by 3–5 Sv (1 Sv = 106 m3 s−1). The transient responses in regional bottom temperature increases exceed 0.1°C. The large-scale features that are shown to transport anomalies far from their geothermal source all exist in the Southern Ocean. Such features include steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. Recently the Southern Ocean has been identified as a prime region for deep ocean warming; geothermal heating should be included in climate models to ensure accurate representation of these abyssal temperature changes.

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Colm Sweeney, Anand Gnanadesikan, Stephen M. Griffies, Matthew J. Harrison, Anthony J. Rosati, and Bonita L. Samuels

Abstract

The impact of changes in shortwave radiation penetration depth on the global ocean circulation and heat transport is studied using the GFDL Modular Ocean Model (MOM4) with two independent parameterizations that use ocean color to estimate the penetration depth of shortwave radiation. Ten to eighteen percent increases in the depth of 1% downwelling surface irradiance levels results in an increase in mixed layer depths of 3–20 m in the subtropical and tropical regions with no change at higher latitudes. While 1D models have predicted that sea surface temperatures at the equator would decrease with deeper penetration of solar irradiance, this study shows a warming, resulting in a 10% decrease in the required restoring heat flux needed to maintain climatological sea surface temperatures in the eastern equatorial Atlantic and Pacific Oceans. The decrease in the restoring heat flux is attributed to a slowdown in heat transport (5%) from the Tropics and an increase in the temperature of submixed layer waters being transported into the equatorial regions. Calculations were made using a simple relationship between mixed layer depth and meridional mass transport. When compared with model diagnostics, these calculations suggest that the slowdown in heat transport is primarily due to off-equatorial increases in mixed layer depths. At higher latitudes (5°–40°), higher restoring heat fluxes are needed to maintain sea surface temperatures because of deeper mixed layers and an increase in storage of heat below the mixed layer. This study offers a way to evaluate the changes in irradiance penetration depths in coupled ocean–atmosphere GCMs and the potential effect that large-scale changes in chlorophyll a concentrations will have on ocean circulation.

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Jaime B. Palter, Stephen M. Griffies, Bonita L. Samuels, Eric D. Galbraith, Anand Gnanadesikan, and Andreas Klocker

Abstract

Despite slow rates of ocean mixing, observational and modeling studies suggest that buoyancy is redistributed to all depths of the ocean on surprisingly short interannual to decadal time scales. The mechanisms responsible for this redistribution remain poorly understood. This work uses an Earth system model to evaluate the global steady-state ocean buoyancy (and related steric sea level) budget, its interannual variability, and its transient response to a doubling of CO2 over 70 years, with a focus on the deep ocean. At steady state, the simple view of vertical advective–diffusive balance for the deep ocean holds at low to midlatitudes. At higher latitudes, the balance depends on a myriad of additional terms, namely mesoscale and submesoscale advection, convection and overflows from marginal seas, and terms related to the nonlinear equation of state. These high-latitude processes rapidly communicate anomalies in surface buoyancy forcing to the deep ocean locally; the deep, high-latitude changes then influence the large-scale advection of buoyancy to create transient deep buoyancy anomalies at lower latitudes. Following a doubling of atmospheric carbon dioxide concentrations, the high-latitude buoyancy sinks are suppressed by a slowdown in convection and reduced dense water formation. This change is accompanied by a slowing of both upper and lower cells of the global meridional overturning circulation, reducing the supply of dense water to low latitudes beneath the pycnocline and the commensurate flow of light waters to high latitudes above the pycnocline. By this mechanism, changes in high-latitude buoyancy are communicated to the global deep ocean on relatively fast advective time scales.

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John P. Krasting, Ronald J. Stouffer, Stephen M. Griffies, Robert W. Hallberg, Sergey L. Malyshev, Bonita L. Samuels, and Lori T. Sentman

Abstract

Oceanic heat uptake (OHU) is a significant source of uncertainty in both the transient and equilibrium responses to increasing the planetary radiative forcing. OHU differs among climate models and is related in part to their representation of vertical and lateral mixing. This study examines the role of ocean model formulation—specifically the choice of the vertical coordinate and the strength of the background diapycnal diffusivity K d—in the millennial-scale near-equilibrium climate response to a quadrupling of atmospheric CO2. Using two fully coupled Earth system models (ESMs) with nearly identical atmosphere, land, sea ice, and biogeochemical components, it is possible to independently configure their ocean model components with different formulations and produce similar near-equilibrium climate responses. The SST responses are similar between the two models (r 2 = 0.75, global average ~4.3°C) despite their initial preindustrial climate mean states differing by 0.4°C globally. The surface and interior responses of temperature and salinity are also similar between the two models. However, the Atlantic meridional overturning circulation (AMOC) responses are different between the two models, and the associated differences in ventilation and deep-water formation have an impact on the accumulation of dissolved inorganic carbon in the ocean interior. A parameter sensitivity analysis demonstrates that increasing the amount of K d produces very different near-equilibrium climate responses within a given model. These results suggest that the impact of the ocean vertical coordinate on the climate response is small relative to the representation of subgrid-scale mixing.

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John P. Dunne, Jasmin G. John, Alistair J. Adcroft, Stephen M. Griffies, Robert W. Hallberg, Elena Shevliakova, Ronald J. Stouffer, William Cooke, Krista A. Dunne, Matthew J. Harrison, John P. Krasting, Sergey L. Malyshev, P. C. D. Milly, Peter J. Phillipps, Lori T. Sentman, Bonita L. Samuels, Michael J. Spelman, Michael Winton, Andrew T. Wittenberg, and Niki Zadeh

Abstract

The physical climate formulation and simulation characteristics of two new global coupled carbon–climate Earth System Models, ESM2M and ESM2G, are described. These models demonstrate similar climate fidelity as the Geophysical Fluid Dynamics Laboratory’s previous Climate Model version 2.1 (CM2.1) while incorporating explicit and consistent carbon dynamics. The two models differ exclusively in the physical ocean component; ESM2M uses Modular Ocean Model version 4p1 with vertical pressure layers while ESM2G uses Generalized Ocean Layer Dynamics with a bulk mixed layer and interior isopycnal layers. Differences in the ocean mean state include the thermocline depth being relatively deep in ESM2M and relatively shallow in ESM2G compared to observations. The crucial role of ocean dynamics on climate variability is highlighted in El Niño–Southern Oscillation being overly strong in ESM2M and overly weak in ESM2G relative to observations. Thus, while ESM2G might better represent climate changes relating to total heat content variability given its lack of long-term drift, gyre circulation, and ventilation in the North Pacific, tropical Atlantic, and Indian Oceans, and depth structure in the overturning and abyssal flows, ESM2M might better represent climate changes relating to surface circulation given its superior surface temperature, salinity, and height patterns, tropical Pacific circulation and variability, and Southern Ocean dynamics. The overall assessment is that neither model is fundamentally superior to the other, and that both models achieve sufficient fidelity to allow meaningful climate and earth system modeling applications. This affords the ability to assess the role of ocean configuration on earth system interactions in the context of two state-of-the-art coupled carbon–climate models.

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Stephen M. Griffies, Michael Winton, Leo J. Donner, Larry W. Horowitz, Stephanie M. Downes, Riccardo Farneti, Anand Gnanadesikan, William J. Hurlin, Hyun-Chul Lee, Zhi Liang, Jaime B. Palter, Bonita L. Samuels, Andrew T. Wittenberg, Bruce L. Wyman, Jianjun Yin, and Niki Zadeh

Abstract

This paper documents time mean simulation characteristics from the ocean and sea ice components in a new coupled climate model developed at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL). The GFDL Climate Model version 3 (CM3) is formulated with effectively the same ocean and sea ice components as the earlier CM2.1 yet with extensive developments made to the atmosphere and land model components. Both CM2.1 and CM3 show stable mean climate indices, such as large-scale circulation and sea surface temperatures (SSTs). There are notable improvements in the CM3 climate simulation relative to CM2.1, including a modified SST bias pattern and reduced biases in the Arctic sea ice cover. The authors anticipate SST differences between CM2.1 and CM3 in lower latitudes through analysis of the atmospheric fluxes at the ocean surface in corresponding Atmospheric Model Intercomparison Project (AMIP) simulations. In contrast, SST changes in the high latitudes are dominated by ocean and sea ice effects absent in AMIP simulations. The ocean interior simulation in CM3 is generally warmer than in CM2.1, which adversely impacts the interior biases.

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Anand Gnanadesikan, Keith W. Dixon, Stephen M. Griffies, V. Balaji, Marcelo Barreiro, J. Anthony Beesley, William F. Cooke, Thomas L. Delworth, Rudiger Gerdes, Matthew J. Harrison, Isaac M. Held, William J. Hurlin, Hyun-Chul Lee, Zhi Liang, Giang Nong, Ronald C. Pacanowski, Anthony Rosati, Joellen Russell, Bonita L. Samuels, Qian Song, Michael J. Spelman, Ronald J. Stouffer, Colm O. Sweeney, Gabriel Vecchi, Michael Winton, Andrew T. Wittenberg, Fanrong Zeng, Rong Zhang, and John P. Dunne

Abstract

The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.

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