Danabasoglu Gokhan

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Response of the North Atlantic thermohaline circulation and ventilation to increasing carbon dioxide in CCSM3

(American Meteorological Society, 2006-06-01) Bryan, Frank O. ; Danabasoglu, Gokhan ; Nakashiki, Norikazu ; Yoshida, Yoshikatsu ; Kim, Dong-Hoon ; Tsutsui, Junichi ; Doney, Scott C.

The response of the North Atlantic thermohaline circulation to idealized climate forcing of 1% per year compound increase in CO2 is examined in three configurations of the Community Climate System Model version 3 that differ in their component model resolutions. The strength of the Atlantic overturning circulation declines at a rate of 22%–26% of the corresponding control experiment maximum overturning per century in response to the increase in CO2. The mean meridional overturning and its variability on decadal time scales in the control experiments, the rate of decrease in the transient forcing experiments, and the rate of recovery in periods of CO2 stabilization all increase with increasing component model resolution. By examining the changes in ocean surface forcing with increasing CO2 in the framework of the water-mass transformation function, we show that the decline in the overturning is driven by decreasing density of the subpolar North Atlantic due to increasing surface heat fluxes. While there is an intensification of the hydrologic cycle in response to increasing CO2, the net effect of changes in surface freshwater fluxes on those density classes that are involved in deep-water formation is to increase their density; that is, changes in surface freshwater fluxes act to maintain a stronger overturning circulation. The differences in the control experiment overturning strength and the response to increasing CO2 are well predicted by the corresponding differences in the water-mass transformation rate. Reduction of meridional heat transport and enhancement of meridional salt transport from mid- to high latitudes with increasing CO2 also act to strengthen the overturning circulation. Analysis of the trends in an ideal age tracer provides a direct measure of changes in ocean ventilation time scale in response to increasing CO2. In the subpolar North Atlantic south of the Greenland–Scotland ridge system, there is a significant increase in subsurface ages as open-ocean deep convection is diminished and ventilation switches to a predominance of overflow waters. In middle and low latitudes there is a decrease in age within and just below the thermocline in response to a decrease in the upwelling of old deep waters. However, when considering ventilation within isopycnal layers, age increases for layers in and below the thermocline due to the deepening of isopycnals in response to global warming.
Variability of the Atlantic meridional overturning circulation in CCSM4

(American Meteorological Society, 2012-08-01) Danabasoglu, Gokhan ; Yeager, Stephen G. ; Kwon, Young-Oh ; Tribbia, Joseph J. ; Phillips, Adam S. ; Hurrell, James W.

Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 1° horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50–200-yr range, contrasting sharply with the multidecadal variability seen in the T85 × 1 resolution CCSM3 present-day control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.
Impacts of Arctic sea ice on cold season atmospheric variability and trends estimated from observations and a multimodel large ensemble

(American Meteorological Society, 2021-09-24) Liang, Yu-Chiao ; Frankignoul, Claude ; Kwon, Young-Oh ; Gastineau, Guillaume ; Manzini, Elisa ; Danabasoglu, Gokhan ; Suo, Lingling ; Yeager, Stephen G. ; Gao, Yongqi ; Attema, Jisk J. ; Cherchi, Annalisa ; Ghosh, Rohit ; Matei, Daniela ; Mecking, Jennifer V. ; Tian, Tian ; Zhang, Ying

To examine the atmospheric responses to Arctic sea ice variability in the Northern Hemisphere cold season (from October to the following March), this study uses a coordinated set of large-ensemble experiments of nine atmospheric general circulation models (AGCMs) forced with observed daily varying sea ice, sea surface temperature, and radiative forcings prescribed during the 1979–2014 period, together with a parallel set of experiments where Arctic sea ice is substituted by its climatology. The simulations of the former set reproduce the near-surface temperature trends in reanalysis data, with similar amplitude, and their multimodel ensemble mean (MMEM) shows decreasing sea level pressure over much of the polar cap and Eurasia in boreal autumn. The MMEM difference between the two experiments allows isolating the effects of Arctic sea ice loss, which explain a large portion of the Arctic warming trends in the lower troposphere and drive a small but statistically significant weakening of the wintertime Arctic Oscillation. The observed interannual covariability between sea ice extent in the Barents–Kara Seas and lagged atmospheric circulation is distinguished from the effects of confounding factors based on multiple regression, and quantitatively compared to the covariability in MMEMs. The interannual sea ice decline followed by a negative North Atlantic Oscillation–like anomaly found in observations is also seen in the MMEM differences, with consistent spatial structure but much smaller amplitude. This result suggests that the sea ice impacts on trends and interannual atmospheric variability simulated by AGCMs could be underestimated, but caution is needed because internal atmospheric variability may have affected the observed relationship.
Mechanisms governing interannual variability of upper-ocean temperature in a global ocean hindcast simulation

(American Meteorological Society, 2007-07) Doney, Scott C. ; Yeager, Stephen G. ; Danabasoglu, Gokhan ; Large, William G. ; McWilliams, James C.

The interannual variability in upper-ocean (0–400 m) temperature and governing mechanisms for the period 1968–97 are quantified from a global ocean hindcast simulation driven by atmospheric reanalysis and satellite data products. The unconstrained simulation exhibits considerable skill in replicating the observed interannual variability in vertically integrated heat content estimated from hydrographic data and monthly satellite sea surface temperature and sea surface height data. Globally, the most significant interannual variability modes arise from El Niño–Southern Oscillation and the Indian Ocean zonal mode, with substantial extension beyond the Tropics into the midlatitudes. In the well-stratified Tropics and subtropics, net annual heat storage variability is driven predominately by the convergence of the advective heat transport, mostly reflecting velocity anomalies times the mean temperature field. Vertical velocity variability is caused by remote wind forcing, and subsurface temperature anomalies are governed mostly by isopycnal displacements (heave). The dynamics at mid- to high latitudes are qualitatively different and vary regionally. Interannual temperature variability is more coherent with depth because of deep winter mixing and variations in western boundary currents and the Antarctic Circumpolar Current that span the upper thermocline. Net annual heat storage variability is forced by a mixture of local air–sea heat fluxes and the convergence of the advective heat transport, the latter resulting from both velocity and temperature anomalies. Also, density-compensated temperature changes on isopycnal surfaces (spice) are quantitatively significant.
Improving oceanic overflow representation in climate models : the Gravity Current Entrainment Climate Process Team

(American Meteorological Society, 2009-05) Legg, Sonya ; Ezer, Tal ; Jackson, Laura ; Briegleb, Bruce P. ; Danabasoglu, Gokhan ; Large, William G. ; Wu, Wanli ; Chang, Yeon ; Ozgokmen, Tamay M. ; Peters, Hartmut ; Xu, Xiaobiao ; Chassignet, Eric P. ; Gordon, Arnold L. ; Griffies, Stephen M. ; Hallberg, Robert ; Price, James F. ; Riemenschneider, Ulrike ; Yang, Jiayan

Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model developers with those conducting observational, numerical, and laboratory process studies of overflows. Here, the organization of the Climate Process Team is described, and a few of the successes and lessons learned during this collaboration are highlighted, with some emphasis on the well-observed Mediterranean overflow. The Climate Process Team has developed several different overflow parameterizations, which are examined in a hierarchy of ocean models, from comparatively well-resolved regional models to the largest-scale global climate models.
Local and downstream relationships between Labrador Sea Water volume and North Atlantic meridional overturning circulation variability

(American Meteorological Society, 2019-07-11) Li, Feili ; Lozier, M. Susan ; Danabasoglu, Gokhan ; Holliday, Naomi Penny ; Kwon, Young-Oh ; Romanou, Anastasia ; Yeager, Stephen G. ; Zhang, Rong

While it has generally been understood that the production of Labrador Sea Water (LSW) impacts the Atlantic meridional overturning circulation (MOC), this relationship has not been explored extensively or validated against observations. To explore this relationship, a suite of global ocean–sea ice models forced by the same interannually varying atmospheric dataset, varying in resolution from non-eddy-permitting to eddy-permitting (1°–1/4°), is analyzed to investigate the local and downstream relationships between LSW formation and the MOC on interannual to decadal time scales. While all models display a strong relationship between changes in the LSW volume and the MOC in the Labrador Sea, this relationship degrades considerably downstream of the Labrador Sea. In particular, there is no consistent pattern among the models in the North Atlantic subtropical basin over interannual to decadal time scales. Furthermore, the strong response of the MOC in the Labrador Sea to LSW volume changes in that basin may be biased by the overproduction of LSW in many models compared to observations. This analysis shows that changes in LSW volume in the Labrador Sea cannot be clearly and consistently linked to a coherent MOC response across latitudes over interannual to decadal time scales in ocean hindcast simulations of the last half century. Similarly, no coherent relationships are identified between the MOC and the Labrador Sea mixed layer depth or the density of newly formed LSW across latitudes or across models over interannual to decadal time scales.
Climate Process Team on internal wave–driven ocean mixing

(American Meteorological Society, 2017-12-01) MacKinnon, Jennifer A. ; Zhao, Zhongxiang ; Whalen, Caitlin B. ; Waterhouse, Amy F. ; Trossman, David S. ; Sun, Oliver M. ; St. Laurent, Louis C. ; Simmons, Harper L. ; Polzin, Kurt L. ; Pinkel, Robert ; Pickering, Andrew I. ; Norton, Nancy J. ; Nash, Jonathan D. ; Musgrave, Ruth C. ; Merchant, Lynne M. ; Melet, Angelique ; Mater, Benjamin D. ; Legg, Sonya ; Large, William G. ; Kunze, Eric ; Klymak, Jody M. ; Jochum, Markus ; Jayne, Steven R. ; Hallberg, Robert ; Griffies, Stephen M. ; Diggs, Stephen ; Danabasoglu, Gokhan ; Chassignet, Eric P. ; Buijsman, Maarten C. ; Bryan, Frank O. ; Briegleb, Bruce P. ; Barna, Andrew ; Arbic, Brian K. ; Ansong, Joseph ; Alford, Matthew H.

Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatiotemporal patterns of mixing are largely driven by the geography of generation, propagation, and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last 5 years and under the auspices of U.S. Climate Variability and Predictability Program (CLIVAR), a National Science Foundation (NSF)- and National Oceanic and Atmospheric Administration (NOAA)-supported Climate Process Team has been engaged in developing, implementing, and testing dynamics-based parameterizations for internal wave–driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here, we review recent progress, describe the tools developed, and discuss future directions.
A review of the role of the Atlantic meridional overturning circulation in Atlantic multidecadal variability and associated climate impacts

(American Geophysical Union, 2019-04-29) Zhang, Rong ; Sutton, Rowan ; Danabasoglu, Gokhan ; Kwon, Young-Oh ; Marsh, Robert ; Yeager, Stephen G. ; Amrhein, Daniel E. ; Little, Christopher M.

By synthesizing recent studies employing a wide range of approaches (modern observations, paleo reconstructions, and climate model simulations), this paper provides a comprehensive review of the linkage between multidecadal Atlantic Meridional Overturning Circulation (AMOC) variability and Atlantic Multidecadal Variability (AMV) and associated climate impacts. There is strong observational and modeling evidence that multidecadal AMOC variability is a crucial driver of the observed AMV and associated climate impacts and an important source of enhanced decadal predictability and prediction skill. The AMOC‐AMV linkage is consistent with observed key elements of AMV. Furthermore, this synthesis also points to a leading role of the AMOC in a range of AMV‐related climate phenomena having enormous societal and economic implications, for example, Intertropical Convergence Zone shifts; Sahel and Indian monsoons; Atlantic hurricanes; El Niño–Southern Oscillation; Pacific Decadal Variability; North Atlantic Oscillation; climate over Europe, North America, and Asia; Arctic sea ice and surface air temperature; and hemispheric‐scale surface temperature. Paleoclimate evidence indicates that a similar linkage between multidecadal AMOC variability and AMV and many associated climate impacts may also have existed in the preindustrial era, that AMV has enhanced multidecadal power significantly above a red noise background, and that AMV is not primarily driven by external forcing. The role of the AMOC in AMV and associated climate impacts has been underestimated in most state‐of‐the‐art climate models, posing significant challenges but also great opportunities for substantial future improvements in understanding and predicting AMV and associated climate impacts.
Quantification of the arctic sea ice-driven atmospheric circulation variability in coordinated large ensemble simulations

(American Geophysical Union, 2019-12-26) Liang, Yu‐Chiao ; Kwon, Young-Oh ; Frankignoul, Claude ; Danabasoglu, Gokhan ; Yeager, Stephen G. ; Cherchi, Annalisa ; Gao, Yongqi ; Gastineau, Guillaume ; Ghosh, Rohit ; Matei, Daniela ; Mecking, Jennifer V. ; Peano, Daniele ; Suo, Lingling ; Tian, Tian

A coordinated set of large ensemble atmosphere‐only simulations is used to investigate the impacts of observed Arctic sea ice‐driven variability (SIDV) on the atmospheric circulation during 1979–2014. The experimental protocol permits separating Arctic SIDV from internal variability and variability driven by other forcings including sea surface temperature and greenhouse gases. The geographic pattern of SIDV is consistent across seven participating models, but its magnitude strongly depends on ensemble size. Based on 130 members, winter SIDV is ~0.18 hPa2 for Arctic‐averaged sea level pressure (~1.5% of the total variance), and ~0.35 K2 for surface air temperature (~21%) at interannual and longer timescales. The results suggest that more than 100 (40) members are needed to separate Arctic SIDV from other components for dynamical (thermodynamical) variables, and insufficient ensemble size always leads to overestimation of SIDV. Nevertheless, SIDV is 0.75–1.5 times as large as the variability driven by other forcings over northern Eurasia and Arctic.
Ocean climate observing requirements in support of climate research and climate information

(Frontiers Media, 2019-07-31) Stammer, Detlef ; Bracco, Annalisa ; AchutaRao, Krishna ; Beal, Lisa M. ; Bindoff, Nathaniel L. ; Braconnot, Pascale ; Cai, Wenju ; Chen, Dake ; Collins, Matthew ; Danabasoglu, Gokhan ; Dewitte, Boris ; Farneti, Riccardo ; Fox-Kemper, Baylor ; Fyfe, John ; Griffies, Stephen M. ; Jayne, Steven R. ; Lazar, Alban ; Lengaigne, Matthieu ; Lin, Xiaopei ; Marsland, Simon ; Minobe, Shoshiro ; Monteiro, Pedro M. S. ; Robinson, Walter ; Roxy, Mathew Koll ; Rykaczewski, Ryan R. ; Speich, Sabrina ; Smith, Inga J. ; Solomon, Amy ; Storto, Andrea ; Takahashi, Ken ; Toniazzo, Thomas ; Vialard, Jérôme

Natural variability and change of the Earth’s climate have significant global societal impacts. With its large heat and carbon capacity and relatively slow dynamics, the ocean plays an integral role in climate, and provides an important source of predictability at seasonal and longer timescales. In addition, the ocean provides the slowly evolving lower boundary to the atmosphere, driving, and modifying atmospheric weather. Understanding and monitoring ocean climate variability and change, to constrain and initialize models as well as identify model biases for improved climate hindcasting and prediction, requires a scale-sensitive, and long-term observing system. A climate observing system has requirements that significantly differ from, and sometimes are orthogonal to, those of other applications. In general terms, they can be summarized by the simultaneous need for both large spatial and long temporal coverage, and by the accuracy and stability required for detecting the local climate signals. This paper reviews the requirements of a climate observing system in terms of space and time scales, and revisits the question of which parameters such a system should encompass to meet future strategic goals of the World Climate Research Program (WCRP), with emphasis on ocean and sea-ice covered areas. It considers global as well as regional aspects that should be accounted for in designing observing systems in individual basins. Furthermore, the paper discusses which data-driven products are required to meet WCRP research and modeling needs, and ways to obtain them through data synthesis and assimilation approaches. Finally, it addresses the need for scientific capacity building and international collaboration in support of the collection of high-quality measurements over the large spatial scales and long time-scales required for climate research, bridging the scientific rational to the required resources for implementation.
Biogeochemical protocols and diagnostics for the CMIP6 Ocean Model Intercomparison Project (OMIP)

(Copernicus Publications on behalf of the European Geosciences Union, 2017-06-09) Orr, James C. ; Najjar, Raymond G. ; Aumont, Olivier ; Bopp, Laurent ; Bullister, John L. ; Danabasoglu, Gokhan ; Doney, Scott C. ; Dunne, John P. ; Dutay, Jean-Claude ; Graven, Heather ; Griffies, Stephen M. ; John, Jasmin G. ; Joos, Fortunat ; Levin, Ingeborg ; Lindsay, Keith ; Matear, Richard J. ; McKinley, Galen A. ; Mouchet, Anne ; Oschlies, Andreas ; Romanou, Anastasia ; Schlitzer, Reiner ; Tagliabue, Alessandro ; Tanhua, Toste ; Yool, Andrew

The Ocean Model Intercomparison Project (OMIP) focuses on the physics and biogeochemistry of the ocean component of Earth system models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). OMIP aims to provide standard protocols and diagnostics for ocean models, while offering a forum to promote their common assessment and improvement. It also offers to compare solutions of the same ocean models when forced with reanalysis data (OMIP simulations) vs. when integrated within fully coupled Earth system models (CMIP6). Here we detail simulation protocols and diagnostics for OMIP's biogeochemical and inert chemical tracers. These passive-tracer simulations will be coupled to ocean circulation models, initialized with observational data or output from a model spin-up, and forced by repeating the 1948–2009 surface fluxes of heat, fresh water, and momentum. These so-called OMIP-BGC simulations include three inert chemical tracers (CFC-11, CFC-12, SF6) and biogeochemical tracers (e.g., dissolved inorganic carbon, carbon isotopes, alkalinity, nutrients, and oxygen). Modelers will use their preferred prognostic BGC model but should follow common guidelines for gas exchange and carbonate chemistry. Simulations include both natural and total carbon tracers. The required forced simulation (omip1) will be initialized with gridded observational climatologies. An optional forced simulation (omip1-spunup) will be initialized instead with BGC fields from a long model spin-up, preferably for 2000 years or more, and forced by repeating the same 62-year meteorological forcing. That optional run will also include abiotic tracers of total dissolved inorganic carbon and radiocarbon, CTabio and 14CTabio, to assess deep-ocean ventilation and distinguish the role of physics vs. biology. These simulations will be forced by observed atmospheric histories of the three inert gases and CO2 as well as carbon isotope ratios of CO2. OMIP-BGC simulation protocols are founded on those from previous phases of the Ocean Carbon-Cycle Model Intercomparison Project. They have been merged and updated to reflect improvements concerning gas exchange, carbonate chemistry, and new data for initial conditions and atmospheric gas histories. Code is provided to facilitate their implementation.
The Community Climate System Model version 4

(American Meteorological Society, 2011-10-01) Gent, Peter R. ; Danabasoglu, Gokhan ; Donner, Leo J. ; Holland, Marika M. ; Hunke, Elizabeth C. ; Jayne, Steven R. ; Lawrence, David M. ; Neale, Richard B. ; Rasch, Philip J. ; Vertenstein, Mariana ; Worley, Patrick H. ; Yang, Zong-Liang ; Zhang, Minghua

The fourth version of the Community Climate System Model (CCSM4) was recently completed and released to the climate community. This paper describes developments to all CCSM components, and documents fully coupled preindustrial control runs compared to the previous version, CCSM3. Using the standard atmosphere and land resolution of 1° results in the sea surface temperature biases in the major upwelling regions being comparable to the 1.4°-resolution CCSM3. Two changes to the deep convection scheme in the atmosphere component result in CCSM4 producing El Niño–Southern Oscillation variability with a much more realistic frequency distribution than in CCSM3, although the amplitude is too large compared to observations. These changes also improve the Madden–Julian oscillation and the frequency distribution of tropical precipitation. A new overflow parameterization in the ocean component leads to an improved simulation of the Gulf Stream path and the North Atlantic Ocean meridional overturning circulation. Changes to the CCSM4 land component lead to a much improved annual cycle of water storage, especially in the tropics. The CCSM4 sea ice component uses much more realistic albedos than CCSM3, and for several reasons the Arctic sea ice concentration is improved in CCSM4. An ensemble of twentieth-century simulations produces a good match to the observed September Arctic sea ice extent from 1979 to 2005. The CCSM4 ensemble mean increase in globally averaged surface temperature between 1850 and 2005 is larger than the observed increase by about 0.4°C. This is consistent with the fact that CCSM4 does not include a representation of the indirect effects of aerosols, although other factors may come into play. The CCSM4 still has significant biases, such as the mean precipitation distribution in the tropical Pacific Ocean, too much low cloud in the Arctic, and the latitudinal distributions of shortwave and longwave cloud forcings.
The CCSM4 ocean component

(American Meteorological Society, 2012-03-01) Danabasoglu, Gokhan ; Bates, Susan C. ; Briegleb, Bruce P. ; Jayne, Steven R. ; Jochum, Markus ; Large, William G. ; Peacock, Synte ; Yeager, Stephen G.

The ocean component of the Community Climate System Model version 4 (CCSM4) is described, and its solutions from the twentieth-century (20C) simulations are documented in comparison with observations and those of CCSM3. The improvements to the ocean model physical processes include new parameterizations to represent previously missing physics and modifications of existing parameterizations to incorporate recent new developments. In comparison with CCSM3, the new solutions show some significant improvements that can be attributed to these model changes. These include a better equatorial current structure, a sharper thermocline, and elimination of the cold bias of the equatorial cold tongue all in the Pacific Ocean; reduced sea surface temperature (SST) and salinity biases along the North Atlantic Current path; and much smaller potential temperature and salinity biases in the near-surface Pacific Ocean. Other improvements include a global-mean SST that is more consistent with the present-day observations due to a different spinup procedure from that used in CCSM3. Despite these improvements, many of the biases present in CCSM3 still exist in CCSM4. A major concern continues to be the substantial heat content loss in the ocean during the preindustrial control simulation from which the 20C cases start. This heat loss largely reflects the top of the atmospheric model heat loss rate in the coupled system, and it essentially determines the abyssal ocean potential temperature biases in the 20C simulations. There is also a deep salty bias in all basins. As a result of this latter bias in the deep North Atlantic, the parameterized overflow waters cannot penetrate much deeper than in CCSM3.
Ocean chlorofluorocarbon and heat uptake during the twentieth century in the CCSM3

(American Meteorological Society, 2006-06-01) Gent, Peter R. ; Bryan, Frank O. ; Danabasoglu, Gokhan ; Lindsay, Keith ; Tsumune, Daisuke ; Hecht, Matthew W. ; Doney, Scott C.

An ensemble of nine simulations for the climate of the twentieth century has been run using the Community Climate System Model version 3 (CCSM3). Three of these runs also simulate the uptake of chlorofluorocarbon-11 (CFC-11) into the ocean using the protocol from the Ocean Carbon Model Intercomparison Project (OCMIP). Comparison with ocean observations taken between 1980 and 2000 shows that the global CFC-11 uptake is simulated very well. However, there are regional biases, and these are used to identify where too much deep-water formation is occurring in the CCSM3. The differences between the three runs simulating CFC-11 uptake are also briefly documented. The variability in ocean heat content in the 1870 control runs is shown to be only a little smaller than estimates using ocean observations. The ocean heat uptake between 1957 and 1996 in the ensemble is compared to the recent observational estimates of the secular trend. The trend in ocean heat uptake is considerably larger than the natural variability in the 1870 control runs. The heat uptake down to 300 m between 1957 and 1996 varies by a factor of 2 across the ensemble. Some possible reasons for this large spread are discussed. There is much less spread in the heat uptake down to 3 km. On average, the CCSM3 twentieth-century ensemble runs take up 25% more heat than the recent estimate from ocean observations. Possible explanations for this are that the model heat uptake is calculated over the whole ocean, and not just in the regions where there are many observations and that there is no parameterization of the indirect effects of aerosols in CCSM3.
The impact of oceanic near-inertial waves on climate

(American Meteorological Society, 2013-05-01) Jochum, Markus ; Briegleb, Bruce P. ; Danabasoglu, Gokhan ; Large, William G. ; Norton, Nancy J. ; Jayne, Steven R. ; Alford, Matthew H. ; Bryan, Frank O.

The Community Climate System Model, version 4 (CCSM4) is used to assess the climate impact of wind-generated near-inertial waves (NIWs). Even with high-frequency coupling, CCSM4 underestimates the strength of NIWs, so that a parameterization for NIWs is developed and included into CCSM4. Numerous assumptions enter this parameterization, the core of which is that the NIW velocity signal is detected during the model integration, and amplified in the shear computation of the ocean surface boundary layer module. It is found that NIWs deepen the ocean mixed layer by up to 30%, but they contribute little to the ventilation and mixing of the ocean below the thermocline. However, the deepening of the tropical mixed layer by NIWs leads to a change in tropical sea surface temperature and precipitation. Atmospheric teleconnections then change the global sea level pressure fields so that the midlatitude westerlies become weaker. Unfortunately, the magnitude of the real air-sea flux of NIW energy is poorly constrained by observations; this makes the quantitative assessment of their climate impact rather uncertain. Thus, a major result of the present study is that because of its importance for global climate the uncertainty in the observed tropical NIW energy has to be reduced.
Should AMOC observations continue: how and why?

(Royal Society, 2023-10-23) Frajka-Williams, Eleanor ; Foukal, Nicholas P. ; Danabasoglu, Gokhan

The Atlantic meridional overturning circulation (AMOC) is a large-scale circulation pattern responsible for northward heat transport in the Atlantic and is associated with climate variations on a wide range of time scales. Observing the time-varying AMOC has fundamentally changed our understanding of the large-scale ocean circulation and its interaction with the climate system, as well as identified shortcomings in numerical simulations. With a wide range of gains already achieved, some now ask whether AMOC observations should continue. A measured approach is required for a future observing system that addresses identified gaps in understanding, accounts for shortcomings in observing methods and maximizes the potential to guide improvements in ocean and climate models. Here, we outline a perspective on future AMOC observing and steps that the community should consider to move forward.
Atlantic meridional overturning circulation: Observed transport and variability

(Frontiers Media, 2019-06-07) Frajka-Williams, Eleanor ; Ansorge, Isabelle ; Baehr, Johanna ; Bryden, Harry L. ; Chidichimo, Maria Paz ; Cunningham, Stuart A. ; Danabasoglu, Gokhan ; Dong, Shenfu ; Donohue, Kathleen A. ; Elipot, Shane ; Heimbach, Patrick ; Holliday, Naomi Penny ; Hummels, Rebecca ; Jackson, Laura C. ; Karstensen, Johannes ; Lankhorst, Matthias ; Le Bras, Isabela A. ; Lozier, M. Susan ; McDonagh, Elaine L. ; Meinen, Christopher S. ; Mercier, Herlé ; Moat, Bengamin I. ; Perez, Renellys ; Piecuch, Christopher G. ; Rhein, Monika ; Srokosz, Meric ; Trenberth, Kevin E. ; Bacon, Sheldon ; Forget, Gael ; Goni, Gustavo J. ; Kieke, Dagmar ; Koelling, Jannes ; Lamont, Tarron ; McCarthy, Gerard D. ; Mertens, Christian ; Send, Uwe ; Smeed, David A. ; Speich, Sabrina ; van den Berg, Marcel ; Volkov, Denis L. ; Wilson, Christopher G.

The Atlantic Meridional Overturning Circulation (AMOC) extends from the Southern Ocean to the northern North Atlantic, transporting heat northwards throughout the South and North Atlantic, and sinking carbon and nutrients into the deep ocean. Climate models indicate that changes to the AMOC both herald and drive climate shifts. Intensive trans-basin AMOC observational systems have been put in place to continuously monitor meridional volume transport variability, and in some cases, heat, freshwater and carbon transport. These observational programs have been used to diagnose the magnitude and origins of transport variability, and to investigate impacts of variability on essential climate variables such as sea surface temperature, ocean heat content and coastal sea level. AMOC observing approaches vary between the different systems, ranging from trans-basin arrays (OSNAP, RAPID 26°N, 11°S, SAMBA 34.5°S) to arrays concentrating on western boundaries (e.g., RAPID WAVE, MOVE 16°N). In this paper, we outline the different approaches (aims, strengths and limitations) and summarize the key results to date. We also discuss alternate approaches for capturing AMOC variability including direct estimates (e.g., using sea level, bottom pressure, and hydrography from autonomous profiling floats), indirect estimates applying budgetary approaches, state estimates or ocean reanalyses, and proxies. Based on the existing observations and their results, and the potential of new observational and formal synthesis approaches, we make suggestions as to how to evaluate a comprehensive, future-proof observational network of the AMOC to deepen our understanding of the AMOC and its role in global climate.

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