scholarly journals The Effect of Localized Mixing on the Ocean Circulation and Time-Dependent Climate Change

2006 ◽  
Vol 36 (1) ◽  
pp. 140-160 ◽  
Author(s):  
Oleg A. Saenko
Keyword(s):  
Ocean Circulation ◽  
Climate Models ◽  
Vertical Mixing ◽  
Mean Diffusivity ◽  
Deep Ocean ◽  
Heat Diffusion ◽  
Western Boundary ◽  
Diffusivity Coefficient ◽  
Mean Values ◽  
Vertical Diffusivity

Abstract Observations indicate that intense mixing in the ocean is localized above complex topography and near the boundaries. Model experiments presented here illustrate that accounting for this fact can be important. In particular, it is found that in the case of localized mixing, the rate of overturning circulation is proportional to the net rate of generation of potential energy by the vertical mixing, linked to the net downward heat diffusion, rather than to the value of the mean vertical diffusivity coefficient. Furthermore, it is shown that two climate models, having the same vertical profile of diffusivity but differing in their distribution (horizontally uniform versus topography/boundary intensified) can simulate significantly different meridional oceanic circulations, vertical heat transfers, and responses of simulated climate to atmospheric CO2 increase. This is found for relatively large [O(1.0 cm2 s−1)] horizontal-mean values of vertical diffusivity in the pycnocline. However, in cases of relatively small [O(0.1 cm2 s−1)] mean diffusivity in the pycnocline, the simulated integral quantities such as meridional mass and heat transports do not depend much on the details of the mixing distribution. Even so, it is found that the deep western boundary currents are more localized near the boundaries in the case of topography/boundary-intensified mixing; also, the stratification in the deep ocean is set through the localized regions of intense vertical mixing. In addition, it is shown that reconciling the observed basin-mean values of diffusivity in the abyssal ocean of O(10 cm2 s−1) with realistic stratification can be problematic, unless the regions of enhanced vertical mixing are localized.

scholarly journals Changes in ocean circulation and carbon storage are decoupled from air-sea CO<sub>2</sub> fluxes

2011 ◽  
Vol 8 (2) ◽  
pp. 505-513 ◽  
Author(s):  
I. Marinov ◽  
A. Gnanadesikan
Keyword(s):  
Carbon Dioxide ◽  
Carbon Storage ◽  
Ocean Circulation ◽  
Vertical Mixing ◽  
Deep Ocean ◽  
High Latitudes ◽  
Poor Indicator ◽  
Ocean Carbon ◽  
Mean Circulation ◽  
Residual Mean Circulation

Abstract. The spatial distribution of the air-sea flux of carbon dioxide is a poor indicator of the underlying ocean circulation and of ocean carbon storage. The weak dependence on circulation arises because mixing-driven changes in solubility-driven and biologically-driven air-sea fluxes largely cancel out. This cancellation occurs because mixing driven increases in the poleward residual mean circulation result in more transport of both remineralized nutrients and heat from low to high latitudes. By contrast, increasing vertical mixing decreases the storage associated with both the biological and solubility pumps, as it decreases remineralized carbon storage in the deep ocean and warms the ocean as a whole.

scholarly journals Spatial and Temporal Scales of Sverdrup Balance*

2014 ◽  
Vol 44 (10) ◽  
pp. 2644-2660 ◽  
Author(s):  
Matthew D. Thomas ◽  
Agatha M. De Boer ◽  
Helen L. Johnson ◽  
David P. Stevens
Keyword(s):  
Ocean Circulation ◽  
Climate Model ◽  
Spatial Scales ◽  
Deep Ocean ◽  
Western Boundary ◽  
Meridional Transport ◽  
Temporal Scales ◽  
State Estimate ◽  
Spatial And Temporal Scales ◽  
Sverdrup Balance

Abstract Sverdrup balance underlies much of the theory of ocean circulation and provides a potential tool for describing the interior ocean transport from only the wind stress. Using both a model state estimate and an eddy-permitting coupled climate model, this study assesses to what extent and over what spatial and temporal scales Sverdrup balance describes the meridional transport. The authors find that Sverdrup balance holds to first order in the interior subtropical ocean when considered at spatial scales greater than approximately 5°. Outside the subtropics, in western boundary currents and at short spatial scales, significant departures occur due to failures in both the assumptions that there is a level of no motion at some depth and that the vorticity equation is linear. Despite the ocean transport adjustment occurring on time scales consistent with the basin-crossing times for Rossby waves, as predicted by theory, Sverdrup balance gives a useful measure of the subtropical circulation after only a few years. This is because the interannual transport variability is small compared to the mean transports. The vorticity input to the deep ocean by the interaction between deep currents and topography is found to be very large in both models. These deep transports, however, are separated from upper-layer transports that are in Sverdrup balance when considered over large scales.

scholarly journals Changes in ocean circulation and carbon storage are decoupled from air-sea CO<sub>2</sub> fluxes

2010 ◽  
Vol 7 (5) ◽  
pp. 7985-8000
Author(s):  
I. Marinov ◽  
A. Gnanadesikan
Keyword(s):  
Carbon Dioxide ◽  
Carbon Storage ◽  
Ocean Circulation ◽  
Vertical Mixing ◽  
Deep Ocean ◽  
High Latitudes ◽  
Poor Indicator ◽  
Ocean Carbon ◽  
Mean Circulation ◽  
Residual Mean Circulation

Abstract. The spatial distribution of the air-sea flux of carbon dioxide is a poor indicator of the underlying ocean circulation and of ocean carbon storage. The weak dependence on circulation arises because mixing-driven changes in solubility-driven and biologically-driven air-sea fluxes largely cancel out. This cancellation occurs because mixing driven increases in the poleward residual mean circulation results in more transport of both remineralized nutrients and heat from low to high latitudes. By contrast, increasing vertical mixing decreases the storage associated with both the biological and solubility pumps, as it decreases remineralized carbon storage in the deep ocean and warms the ocean as a whole.

scholarly journals Wave–turbulence interaction-induced vertical mixing and its effects in ocean and climate models

2016 ◽  
Vol 374 (2065) ◽  
pp. 20150201 ◽  
Author(s):  
Fangli Qiao ◽  
Yeli Yuan ◽  
Jia Deng ◽  
Dejun Dai ◽  
Zhenya Song
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
General Circulation ◽  
Climate Models ◽  
Theoretical Calculations ◽  
Critical Role ◽  
Vertical Mixing ◽  
General Circulation Models ◽  
Wave Turbulence ◽  
Coupled Models

Heated from above, the oceans are stably stratified. Therefore, the performance of general ocean circulation models and climate studies through coupled atmosphere–ocean models depends critically on vertical mixing of energy and momentum in the water column. Many of the traditional general circulation models are based on total kinetic energy (TKE), in which the roles of waves are averaged out. Although theoretical calculations suggest that waves could greatly enhance coexisting turbulence, no field measurements on turbulence have ever validated this mechanism directly. To address this problem, a specially designed field experiment has been conducted. The experimental results indicate that the wave–turbulence interaction-induced enhancement of the background turbulence is indeed the predominant mechanism for turbulence generation and enhancement. Based on this understanding, we propose a new parametrization for vertical mixing as an additive part to the traditional TKE approach. This new result reconfirmed the past theoretical model that had been tested and validated in numerical model experiments and field observations. It firmly establishes the critical role of wave–turbulence interaction effects in both general ocean circulation models and atmosphere–ocean coupled models, which could greatly improve the understanding of the sea surface temperature and water column properties distributions, and hence model-based climate forecasting capability.

scholarly journals LongRunMIP: Motivation and Design for a Large Collection of Millennial-Length AOGCM Simulations

2019 ◽  
Vol 100 (12) ◽  
pp. 2551-2570 ◽  
Author(s):  
Maria Rugenstein ◽  
Jonah Bloch-Johnson ◽  
Ayako Abe-Ouchi ◽  
Timothy Andrews ◽  
Urs Beyerle ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Land Surface ◽  
Radiative Forcing ◽  
Climate Models ◽  
Deep Ocean ◽  
Internal Variability ◽  
Ocean Temperature ◽  
Intercomparison Project ◽  
Cloud Radiative Effects ◽  
Co2 Forcing

Abstract We present a model intercomparison project, LongRunMIP, the first collection of millennial-length (1,000+ years) simulations of complex coupled climate models with a representation of ocean, atmosphere, sea ice, and land surface, and their interactions. Standard model simulations are generally only a few hundred years long. However, modeling the long-term equilibration in response to radiative forcing perturbation is important for understanding many climate phenomena, such as the evolution of ocean circulation, time- and temperature-dependent feedbacks, and the differentiation of forced signal and internal variability. The aim of LongRunMIP is to facilitate research into these questions by serving as an archive for simulations that capture as much of this equilibration as possible. The only requirement to participate in LongRunMIP is to contribute a simulation with elevated, constant CO2 forcing that lasts at least 1,000 years. LongRunMIP is an MIP of opportunity in that the simulations were mostly performed prior to the conception of the archive without an agreed-upon set of experiments. For most models, the archive contains a preindustrial control simulation and simulations with an idealized (typically abrupt) CO2 forcing. We collect 2D surface and top-of-atmosphere fields and 3D ocean temperature and salinity fields. Here, we document the collection of simulations and discuss initial results, including the evolution of surface and deep ocean temperature and cloud radiative effects. As of October 2019, the collection includes 50 simulations of 15 models by 10 modeling centers. The data of LongRunMIP are publicly available. We encourage submissions of more simulations in the future.

scholarly journals Poleward shift in the Southern Hemisphere westerly winds synchronous with the deglacial rise in CO2

2021 ◽  
Author(s):  
William Gray ◽  
Casimir de Lavergne ◽  
Robert Jnglin Wills ◽  
Laurie Menviel ◽  
Paul Spence ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
Climate Models ◽  
Deep Ocean ◽  
Last Deglaciation ◽  
Westerly Winds ◽  
Poleward Shift ◽  
Deep Ocean Circulation ◽  
Southern Hemisphere Westerly Winds

Abstract The Southern Hemisphere westerly winds strongly influence deep ocean circulation and carbon storage1. While the westerlies are hypothesised to play a key role in regulating atmospheric CO2 over glacial-interglacial cycles2–4, past changes in their position and strength remain poorly constrained5–7. Here, we use a compilation of planktic foraminiferal δ18O from across the Southern Ocean and constraints from an ensemble of climate models to reconstruct changes in the westerlies over the last deglaciation. We find a 4.7° (2.9-6.9°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (about 20,000 years ago) relative to the mid-Holocene (about 6,000 years ago). Our reconstruction shows that the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO2. Experiments with a 0.25° resolution ocean-sea-ice-carbon model demonstrate that shifting the westerlies equatorward substantially reduces the overturning rate of the abyssal ocean, leading to a suppression of CO2 outgassing from the Southern Ocean. Our results establish a central role for the westerly winds in driving the deglacial CO2 rise, and suggest natural CO2 outgassing from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming8–10.

scholarly journals Geothermal heating, diapycnal mixing and the abyssal circulation

2008 ◽  
Vol 5 (3) ◽  
pp. 281-325 ◽  
Author(s):  
J. Emile-Geay ◽  
G. Madec
Keyword(s):  
Heat Flux ◽  
Ocean Circulation ◽  
Scaling Law ◽  
Vertical Mixing ◽  
Circulation Model ◽  
Deep Ocean ◽  
Moderate Increase ◽  
Diapycnal Mixing ◽  
Geothermal Heating ◽  
Abyssal Circulation

Abstract. The dynamical role of geothermal heating in abyssal circulation is reconsidered using three independent methods. First, we show that a uniform geothermal heat flux close to the observed average (86.4 mW m−2) supplies as much heat to the abyss as diapycnal mixing with a rate of ~1 cm2 s−1. A simple scaling law, based upon a purely advective balance, indicates that such a heat flux is able to generate a deep circulation of order 5 Sv (1 Sv ≡ 106 m3 s−1) associated with the Antarctic Bottom Water mass (AABW). Its intensity is inversely proportional to the strength of deep temperature gradients. Second, this order of magnitude is confirmed by the density-binning method (Walin, 1982) applied to the observed thermohaline structure of Levitus (1998). Additionally, the method allows to investigate the effect of realistic spatial variations of the flux obtained from heatflow measurements and classical theories of lithospheric cooling. It is found that a uniform heatflow forces a transformation of about 6 SV at σ4=45.90, consistent with the previous estimate. The result is very similar for a realistic heatflow, albeit shifted towards slightly lighter density classes. Third, we use a general ocean circulation model in global configuration to perform three sets of experiments: (1) a thermally homogenous abyssal ocean with and without uniform geothermal heating; (2) a more stratified abyssal ocean subject to (i) no geothermal heating, (ii) a constant heat flux of 86.4 mW m−2, (iii) a realistic, spatially varying heat flux of identical global average; (3) experiments (i) and (iii) with enhanced vertical mixing at depth. It is found, for strong vertical mixing rates, that geothermal heating enhances the AABW cell by about 15% (1.5 Sv) and heats up the last 2000 m by 0.3°, reaching a maximum of 0.5° in the deep North Pacific. Its impact is even stronger in a weakly diffusive deep ocean. The spatial distribution of the heat flux acts to enhance this temperature rise at mid-depth and reduce it at great depth, producing a more moderate increase in overturning than in the uniform case. The three approaches converge to the conclusion that geothermal heating is an important actor of abyssal dynamics, and should no longer be neglected.

scholarly journals An evaluation of the performance of vertical mixing parameterizations for tidal mixing in the Regional Ocean Modeling System (ROMS)

2019 ◽  
Vol 6 (1) ◽  
Author(s):  
Robin Robertson ◽  
Changming Dong
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
Climate Models ◽  
Energy Transport ◽  
Vertical Mixing ◽  
Internal Tides ◽  
Ocean Modeling ◽  
Tidal Mixing ◽  
Ocean Energy ◽  
Regional Ocean Modeling System

AbstractVertical mixing is important in the ocean for maintaining its stratification, redistributing temperature and salinity, distributing nutrients and pollutants, and the energy cascade. It plays a key role in ocean energy transport, climate change, and marine ecosystems. Getting the mixing right in ocean circulation and climate models is critical in reproducing ocean and climate physics. Ocean models, like the Regional Ocean Modeling System (Rutgers ROMS 3.4), provide several options for determining vertical mixing through the vertical mixing parameterization schemes. To evaluate which of these methods best reproduces realistic vertical mixing by internal tides, simulations of baroclinic tides generated by a seamount were performed using seven different vertical mixing parameterizations: Mellor-Yamada 2.5 (MY), Large-McWilliams-Doney’s Kpp (LMD), Nakanishi-Niino’s modification of Mellor-Yamada (NN), and four versions of Generic Length Scale (GLS). The GLS versions in ROMS 3.4 severely overmixed the water column within a day and were not considered realistic. We suspect that a coding error has been introduced for it. We focused on the performance of the MY, LMD, and NN vertical mixing parameterizations. LMD was found to overmix the water column. The performance of MY and NN were nearly equivalent and both well reproduced the observed velocity and diffusivity fields. NN performed slightly better by having a lower rms for M2 and K1, less benthic mixing, more mid-water column mixing, less overmixing, and fewer extremely high diffusivities (> 1 m2 s−1).

Modeling Water-Mass Distributions in the Modern and LGM Ocean: Circulation Change and Isopycnal and Diapycnal Mixing

2021 ◽  
Vol 51 (5) ◽  
pp. 1523-1538
Author(s):  
C. S. Jones ◽  
Ryan P. Abernathey
Keyword(s):  
Ocean Circulation ◽  
Water Mass ◽  
Vertical Mixing ◽  
Deep Ocean ◽  
Atlantic Water ◽  
Ocean Water ◽  
Mass Distributions ◽  
Mixing Coefficient ◽  
Deep Ocean Water ◽  
Isopycnal Mixing

AbstractPaleoproxy observations suggest that deep-ocean water-mass distributions were different at the Last Glacial Maximum than they are today. However, even modern deep-ocean water-mass distributions are not completely explained by observations of the modern ocean circulation. This paper investigates two processes that influence deep-ocean water-mass distributions: 1) interior downwelling caused by vertical mixing that increases in the downward direction and 2) isopycnal mixing. Passive tracers are used to assess how changes in the circulation and in the isopycnal-mixing coefficient impact deep-ocean water-mass distributions in an idealized two-basin model. We compare two circulations, one in which the upper cell of the overturning reaches to 4000-m depth and one in which it shoals to 2500-m depth. Previous work suggests that in the latter case the upper cell and the abyssal cell of the overturning are separate structures. Nonetheless, high concentrations of North Atlantic Water (NAW) are found in our model’s abyssal cell: these tracers are advected into the abyssal cell by interior downwelling caused by our vertical mixing profile, which increases in the downward direction. Further experiments suggest that the NAW concentration in the deep South Atlantic Ocean and in the deep Pacific Ocean is influenced by the isopycnal-mixing coefficient in the top 2000 m of the Southern Ocean. Both the strength and the vertical profile of isopycnal mixing are important for setting deep-ocean tracer concentrations. A 1D advection–diffusion model elucidates how NAW concentration depends on advective and diffusive processes.

scholarly journals Ocean Heat Uptake Processes: A Model Intercomparison

2015 ◽  
Vol 28 (2) ◽  
pp. 887-908 ◽  
Author(s):  
Eleftheria Exarchou ◽  
Till Kuhlbrodt ◽  
Jonathan M. Gregory ◽  
Robin S. Smith
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Vertical Mixing ◽  
Deep Ocean ◽  
Global Climate Models ◽  
Quasi Equilibrium ◽  
Mixing Processes ◽  
Ocean Heat Uptake ◽  
Heat Uptake ◽  
The Tropics

Abstract The quasi-equilibrium heat balances, as well as the responses to 4 × CO2 perturbation, are compared among three global climate models with the aim to identify and explain intermodel differences in ocean heat uptake (OHU) processes. It is found that, in quasi equilibrium, convective and mixed layer processes, as well as eddy-related processes, cause cooling of the subsurface ocean. The cooling is balanced by warming caused by advective and diapycnally diffusive processes. It is also found that in the CO2-perturbed climates the largest contribution to OHU comes from changes in vertical mixing processes and the mean circulation, particularly in the extratropics, caused both by changes in wind forcing and by changes in high-latitude buoyancy forcing. There is a substantial warming in the tropics: a significant part of which occurs because of changes in horizontal advection in extratropics. Diapycnal diffusion makes only a weak contribution to the OHU, mainly in the tropics, because of increased stratification. There are important qualitative differences in the contribution of eddy-induced advection and isopycnal diffusion to the OHU among the models. The former is related to the different values of the coefficients used in the corresponding scheme. The latter is related to the different tapering formulations of the isopycnal diffusion scheme. These differences affect the OHU in the deep ocean, which is substantial in two of the models, with the dominant region of deep warming being the Southern Ocean. However, most of the OHU takes place above 2000 m, and the three models are quantitatively similar in their global OHU efficiency and its breakdown among processes and as a function of latitude.

Sign in / Sign up

Export Citation Format

Share Document