75 research outputs found
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Global ocean heat transport dominated by heat export from the tropical Pacific
Heat redistribution is one of the main mechanisms by which oceans regulate Earthâs climate. Analyses of ocean heat transport tend to emphasize global-scale seawater pathways and concepts such as the great ocean conveyor belt. However it is the divergence or convergence of heat transport within an oceanic region, rather than the origin or destination of seawater transiting through that region, which is most immediately relevant to Earthâs heat budget. Here we use a recent gridded estimate of ocean heat transport to reveal the net effect on Earthâs heat budget, the âeffectiveâ ocean heat transport, by removing internal ocean heat loops that have obscured the interpretation of measurements. The result demonstrates the overwhelming predominance of the tropical Pacific which exports four times as much heat as is imported in the Atlantic and Arctic. It also highlights the unique ability of the Atlantic and Indian Oceans to transport heat across the Equator â Northward and Southward, respectively. However effective inter-ocean heat transports are smaller than expected, suggesting that global-scale seawater pathways only play a minor role in Earthâs heat budget
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On the observability of turbulent transport rates by Argo: supporting evidence from an inversion experiment
Although estimation of turbulent transport parameters using inverse methods is not new, there is little evaluation of the method in the literature. Here, it is shown that extended observation of the broad scale hydrography by Argo provides a path to improved estimates of regional turbulent transport rates. Results from a 20 year ocean state estimate produced with the ECCO v4 non-linear inverse modeling framework provide supporting evidence. Turbulent transport parameter maps are estimated under the constraints of fitting the extensive collection of Argo profiles collected through 2011. The adjusted parameters dramatically reduce misfits to in situ profiles as compared with earlier ECCO solutions. They also yield a clear reduction in the model drift away from observations over multi-century long simulations, both for assimilated variables (temperature and salinity) and independent variables (bio-geochemical tracers). Despite the minimal constraints imposed specifically on the estimated parameters, their geography is physically plausible and exhibits close connections with the upper ocean ocean stratification as observed by Argo. The estimated parameter adjustments furthermore have first order impacts on upper-ocean stratification and mixed layer depths over 20 years. These results identify the constraint of fitting Argo profiles as an effective observational basis for regional turbulent transport rates. Uncertainties and further improvements of the method are discussed
A global glacial ocean state estimate constrained by upper-ocean temperature proxies
Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 8059-8079, doi:10.1175/JCLI-D-17-0769.1.We use the method of least squares with Lagrange multipliers to fit an ocean general circulation model to the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) estimate of near sea surface temperature (NSST) at the Last Glacial Maximum (LGM; circa 23â19 thousand years ago). Compared to a modern simulation, the resulting global, last-glacial ocean state estimate, which fits the MARGO data within uncertainties in a free-running coupled oceanâsea ice simulation, has global-mean NSSTs that are 2°C lower and greater sea ice extent in all seasons in both the Northern and Southern Hemispheres. Increased brine rejection by sea ice formation in the Southern Ocean contributes to a stronger abyssal stratification set principally by salinity, qualitatively consistent with pore fluid measurements. The upper cell of the glacial Atlantic overturning circulation is deeper and stronger. Dye release experiments show similar distributions of Southern Ocean source waters in the glacial and modern western Atlantic, suggesting that LGM NSST data do not require a major reorganization of abyssal water masses. Outstanding challenges in reconstructing LGM ocean conditions include reducing effects from model biases and finding computationally efficient ways to incorporate abyssal tracers in global circulation inversions. Progress will be aided by the development of coupled oceanâatmosphereâice inverse models, by improving high-latitude model processes that connect the upper and abyssal oceans, and by the collection of additional paleoclimate observations.DEA was supported by a NSF Graduate Research Fellowship
and NSF Grant OCE-1060735. OM acknowledges
support from the NSF. GF was supported by
NASA Award 1553749 and Simons Foundation Award
549931
Cloud-based solutions for distributed climate modeling
ECCO in the cloud - overviewA new, cloud-based framework for climate modeling is introduced allowing to run climate models at the âclick of a buttonâ. The framework aims to simplify dissemination of climate models, increase transparency of modeling activities, expand their user base, and facilitate broader research collaboration.NASA Physical Oceanograph
ECCO Version 4 Release 3
This note provides a brief synopsis of ECCO Version 4 Release 3.This note provides a brief synopsis of ECCO Version 4 Release 3, an updated edition to the global ocean state estimate described by Forget et al. (2015b, 2016), covering the period 1992-2015.JPL/Caltech and NASA Physical Oceanograph
Timescales and regions of the sensitivity of Atlantic meridional volume and heat transport: Toward observing system design
A dual (adjoint) model is used to explore elements of the oceanic state influencing the meridional volume and heat transports (MVT and MHT) in the sub-tropical North Atlantic so as to understand their variability and to provide the elements of useful observational program design. Focus is on the effect of temperature (and salinity) perturbations. On short timescales (months), as expected, the greatest sensitivities are to local disturbances, but as the timescales extend back to a decade and longer, the region of influence expands to occupy much of the Atlantic basin and significant areas of the global ocean, although the influence of any specific point or small area tends to be quite weak. The propagation of information in the dual solution is a clear manifestation of oceanic teleconnections. It takes place through identifiable âdualâ Kelvin, Rossby, and continental shelf-waves with an interpretable physics, in particular in terms of dual expressions of barotropic and baroclinic adjustment processes. Among the notable features are the relatively fast timescales of influence (albeit weak in amplitude) between 26°N and the tropical Pacific and Indian Ocean, the absence of dominance of the sub-polar North Atlantic, significant connections to the Agulhas leakage region in the southeast Atlantic on timescales of 5â10 years, and the marked sensitivity propagation of Doppler-shifted Rossby waves in the Southern Ocean on timescales of a decade and beyond. Regional, as well as time-dependent, differences between MVT and MHT sensitivities highlight the lack of a simple correspondence between their variability. Some implications for observing systems for the purpose of climate science are discussed.National Oceanographic Partnership Program (U.S.) (ââEstimating the Circulation and Climate of the Oceanââ (ECCO) and the ââAtlantic MOC Observing System Studies Using Adjoint Modelsââ projects)National Science Foundation (U.S.) (NSF Collaboration in Mathematics and Geoscience (CMG) project ââUncertainty Quanti- fication in Geophysical State Estimationââ
Comparison of MERRA-2 and ECCO-V4 Ocean Surface Heat Fluxes: Consequences of Different Forcing Feedbacks on Ocean Circulation and Implications for Climate Data Assimilation
Forcing ocean models with reanalysis data is a common practice in ocean modeling. As part of this practice, prescribed atmospheric state variables and interactive ocean SST (Sea Surface Temperature) are used to calculate fluxes between the ocean and the atmosphere. When forcing an ocean model with reanalysis fields, errors in the reanalysis data, errors in the ocean model and errors in the forcing formulation will generate a different solution compared to other ocean reanalysis solutions (which also have their own errors). As a first step towards a consistent coupled ocean-atmosphere reanalysis, we compare surface heat fluxes from a state-of-the-art atmospheric reanalysis, the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), to heat fluxes from a state-of-the-art oceanic reanalysis, the Estimating the Circulation and Climate of the Ocean Version 4, Release 2 (ECCO-v4). Then, we investigate the errors associated with the MITgcm (Massachusetts Institute of Technology general circulation model) ocean model in its ECCO-v4 ocean reanalysis configuration (1992-2011) when it is forced with MERRA- 2 atmospheric reanalysis fields instead of with the ECCO-v4 adjoint optimized ERA-interim state variables. This is done by forcing ECCO-v4 ocean with and without feedbacks from MERRA-2 related to turbulent fluxes of heat and moisture and the outgoing long wave radiation. In addition, we introduce an intermediate forcing method that includes only the feedback from the interactive outgoing long wave radiation. The resulting ocean circulation is compared with ECCO-v4 reanalysis and in-situ observations. We show that, without feedbacks, imbalances in the energy and the hydrological cycles of MERRA-2 (which are directly related to the fact it was created without interactive ocean) result in considerable SST drifts and a large reduction in sea level. The bulk formulae and interactive outgoing long wave radiation, although providing air-sea feedbacks and reducing model-data misfit, strongly relax the ocean to observed SST and may result in unwanted features such as large change in the water budget. These features have implications in a desired forcing recipe to be used. The results strongly and unambiguously argue for next generation data assimilation climate studies to involve fully coupled systems
Heat distribution in the Southeast Pacific is only weakly sensitive to high-latitude heat flux and wind stress.
The Southern Ocean features regionallyâvarying ventilation pathways that transport heat and carbon from the surface ocean to the interior thermocline on timescales of decades to centuries, but the factors that control the distribution of heat along these pathways are not well understood. In this study, we use a global ocean state estimate (ECCOv4) to (1) define the recently ventilated interior Pacific (RVP) using numerical passive tracer experiments over a 10âyear period and (2) use an adjoint approach to calculate the sensitivities of the RVP heat content (RVPh) to changes in net heat flux and wind stress. We find that RVPh is most sensitive to local heat flux and wind stress anomalies north of the sea surface height contours that delineate the Antarctic Circumpolar Current, with especially high sensitivities over the South Pacific Gyre. Surprisingly, RVPh is not especially sensitive to changes at higher latitudes.
We perform a set of step response experiments over the South Pacific Gyre, the subduction region, and the highâlatitude SO. In consistency with the adjoint sensitivity fields, RVPh is most sensitive to wind stress curl over the subtropical gyre, which alter isopycnal heave, and it is only weakly sensitive to changes at higher latitudes. Our results suggest that despite the localized nature of mode water subduction hotspots, changes in basinâscale pressure gradients are an important controlling factor on RVPh. Because basinâscale wind stress is expected to change in the coming decades to centuries, our results may have implications for climate, via the atmosphere/ocean partitioning of heat
Recent wind-driven variability in Atlantic water mass distribution and meridional overturning circulation
Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 633-647, doi:10.1175/JPO-D-16-0089.1.Interannual variability in the volumetric water mass distribution within the North Atlantic Subtropical Gyre is described in relation to variability in the Atlantic meridional overturning circulation. The relative roles of diabatic and adiabatic processes in the volume and heat budgets of the subtropical gyre are investigated by projecting data into temperature coordinates as volumes of water using an Argo-based climatology and an ocean state estimate (ECCO version 4). This highlights that variations in the subtropical gyre volume budget are predominantly set by transport divergence in the gyre. A strong correlation between the volume anomaly due to transport divergence and the variability of both thermocline depth and Ekman pumping over the gyre suggests that wind-driven heave drives transport anomalies at the gyre boundaries. This wind-driven heaving contributes significantly to variations in the heat content of the gyre, as do anomalies in the airâsea fluxes. The analysis presented suggests that wind forcing plays an important role in driving interannual variability in the Atlantic meridional overturning circulation and that this variability can be unraveled from spatially distributed hydrographic observations using the framework presented here.DGE was supported by a Natural
Environment Research Council studentship award
at the University of Southampton. JMTâs contribution
was supported by the U.S. National Science Foundation
(Grant OCE-1332667). GFâs contribution was
supported by the U.S. National Science Foundation
through Grant OCE-0961713 and by the U.S. National
Oceanic and Atmospheric Administration through
Grant NA10OAR4310135. The contributions of JDZ
and AJGN were supported by the NERC Grant ââClimate
scale analysis of air and water massesââ (NE/
K012932/1). ACNG gratefully acknowledges support
from the Leverhulme Trust, the Royal Society, and the
Wolfson Foundation. LY was supported by NASA
Ocean Vector Wind Science Team (OVWST) activities
under Grant NNA10AO86G
ECCO Version 4: Second Release
The purpose of this note is twofold: (1) document the second release of ECCO version 4 state estimates (ECCO v4-r2); (2) provide a citable identifier to distinguish it from the previous release (ECCO v4-r1; Forget et al 2015).Major support for this work was provided through NASAâs Physical Oceanography Program. The bulk of the calculations was performed on the NASA Advanced Supercomputing (NAS) divisionâs Pleiades supercomputer at NASA/ARC
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