Can Southern Ocean Eddy Effects Be Parameterized in Climate Models?

Abstract Present-day control and 1% yr−1 increasing carbon dioxide runs have been made using two versions of the Community Climate System Model, version 3.5. One uses the standard versions of the ocean and sea ice components where the horizontal resolution is 1° and the effects of mesoscale eddies are parameterized, and the second uses a resolution of 1/10° where the eddies are resolved. This is the first time the parameterization has been tested in a climate change run compared to an eddy-resolving run. The comparison is made not straightforward by the fact that the two control run climates are not the same, especially in their sea ice distributions. The focus is on the Antarctic Circumpolar Current region, where the effects of eddies are of leading order. The conclusions are that many of the differences in the two carbon dioxide transient forcing runs can be explained by the different control run sea ice distributions around Antarctica, but there are some quantitative differences in the meridional overturning circulation, poleward heat transport, and zonally averaged heat uptake when the eddies are parameterized rather than resolved.

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Variability of the Atlantic Meridional Overturning Circulation in CCSM4

Journal of Climate ◽

10.1175/jcli-d-11-00463.1 ◽

2012 ◽

Vol 25 (15) ◽

pp. 5153-5172 ◽

Cited By ~ 93

Author(s):

Gokhan Danabasoglu ◽

Steve G. Yeager ◽

Young-Oh Kwon ◽

Joseph J. Tribbia ◽

Adam S. Phillips ◽

...

Keyword(s):

Low Frequency ◽

Surface Flux ◽

Atlantic Meridional Overturning Circulation ◽

Horizontal Resolution ◽

Meridional Overturning Circulation ◽

Positive Density ◽

Overturning Circulation ◽

Climate System Model ◽

Control Simulation ◽

Meridional Overturning

Abstract 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.

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Deep-water formation and Arctic circulation under sea-ice retreat: a comparison between CMIP6 models

10.5194/egusphere-egu21-12155 ◽

2021 ◽

Author(s):

Anais Bretones ◽

Kerim Hestnes Nisancioglu ◽

Mari Fjalstad Jensen

Keyword(s):

Sea Ice ◽

Climate Models ◽

Mixed Layer Depth ◽

Meridional Overturning Circulation ◽

The Arctic ◽

Overturning Circulation ◽

Ice Retreat ◽

Meridional Overturning ◽

Sea Ice Edge ◽

Sea Ice Retreat

<div> <div> <div> <div> <p>While a rapid sea-ice retreat in the Arctic has become ubiquitous, the potential weakening of the Atlantic Meridional Overturning circulation (AMOC), in response to rising greenhouse gases, is still under debate. Although climate models predict a weakening of the AMOC, observations are so far inconclusive. It has been suggested that the strength and vertical extent of the AMOC responds to sea-ice retreat, as deep mixing occurs in open-ocean areas close to the sea-ice edge. Here, we investigate this hypothesis by looking at the Arctic tidional Overturning Circulation (ArMOC) and mixed-layer depth in several CMIP6 models forced with the SSP5- 8.5 scenario. For every models we find a decoupling of the ArMOC with the AMOC: while the AMOC weakens during the 21st century, the ArMOC is enhanced.</p> </div> </div> </div> </div>

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AMOC stability and diverging response to Arctic sea ice decline in two climate models

Journal of Climate ◽

10.1175/jcli-d-20-0572.1 ◽

2021 ◽

pp. 1-47

Author(s):

Hui Li ◽

Alexey Fedorov ◽

Wei Liu

Keyword(s):

Sea Ice ◽

Climate Models ◽

Horizontal Resolution ◽

Arctic Sea Ice ◽

Meridional Overturning Circulation ◽

Quasi Equilibrium ◽

Freshwater Transport ◽

Sea Ice Decline ◽

Basin Scale ◽

Arctic Sea

AbstractThis study compares the impacts of Arctic sea ice decline on the Atlantic Meridional Overturning Circulation (AMOC) in two configurations of the Community Earth System Model (CESM) with different horizontal resolution. In a suite of model experiments we impose radiative imbalance at the ice surface, replicating a loss of sea ice cover comparable to the observed during 1979-2014, and find dramatic differences in the AMOC response between the two models. In the lower-resolution configuration, the AMOC weakens by about one third over the first 100 years, approaching a new quasi-equilibrium. By contrast, in the higher-resolution configuration, the AMOC weakens by ~10% during the first 20-30 years followed by a full recovery driven by invigorated deep water formation in the Labrador Sea and adjacent regions. We investigate these differences using a diagnostic AMOC stability indicator, which reflects the AMOC freshwater transport in and out of the basin and hence the strength of the basin-scale salt-advection feedback. This indicator suggests that the AMOC in the lower-resolution model is less stable and more sensitive to surface perturbations, as confirmed by hosing experiments mimicking Arctic freshening due to sea ice decline. Differences between the models’ mean states, including the Atlantic mean surface freshwater fluxes, control the differences in AMOC stability. Our results demonstrate that the AMOC stability indicator is indeed useful for evaluating AMOC sensitivity to perturbations. Finally, we emphasize that, despite the differences in the long-term adjustment, both models simulate a multi-decadal AMOC weakening caused by Arctic sea ice decline, relevant to climate change.

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Quantifying Agulhas Leakage in a High-Resolution Climate Model

Journal of Climate ◽

10.1175/jcli-d-15-0568.1 ◽

2016 ◽

Vol 29 (19) ◽

pp. 6881-6892 ◽

Cited By ~ 10

Author(s):

Yu Cheng ◽

Dian Putrasahan ◽

Lisa Beal ◽

Ben Kirtman

Keyword(s):

Time Scales ◽

Climate Models ◽

Climate Model ◽

Critical Role ◽

Meridional Overturning Circulation ◽

Climate System ◽

Climate System Model ◽

Mean Fields ◽

Meridional Overturning ◽

The Indian Ocean

Abstract The leakage of warm and salty water from the Indian Ocean via the Agulhas system into the South Atlantic may play a critical role in climate variability by modulating the buoyancy fluxes associated with the meridional overturning circulation (MOC). New climate models, such as the Community Climate System Model, version 3.5 (CCSM3.5), are now able to resolve the Agulhas retroflection and constrain the inertially choked Agulhas leakage to more realistic values. These ocean-eddy-resolving climate models are poised to bolster understanding of the sensitivity and influence of Agulhas leakage in the coupled climate system. Here, a strategy is devised to quantify Agulhas leakage in CCSM3.5 by applying an offline Lagrangian particle-tracking approach, finding a mean interbasin transport of 11.2 Sv (1 Sv ≡ 106 m3 s−1). It is shown that monthly mean outputs can be used to produce a reliable time series of Agulhas leakage variability on longer-than-seasonal time scales (correlation coefficient r = 0.88; p < 0.01) by comparing to a parallel simulation that archives daily mean fields every 5 days. The results show that Agulhas leakage variability at longer-than-seasonal time scales is less sensitive to the temporal resolution of the velocity fields than is the mean leakage transport.

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Climate Sensitivity of Moderate- and Low-Resolution Versions of CCSM3 to Preindustrial Forcings

Journal of Climate ◽

10.1175/jcli3754.1 ◽

2006 ◽

Vol 19 (11) ◽

pp. 2567-2583 ◽

Cited By ~ 66

Author(s):

Bette L. Otto-Bliesner ◽

Robert Tomas ◽

Esther C. Brady ◽

Caspar Ammann ◽

Zav Kothavala ◽

...

Keyword(s):

Sea Ice ◽

North Atlantic ◽

Meridional Overturning Circulation ◽

Low Resolution ◽

Climate System Model ◽

Enso Variability ◽

Proxy Records ◽

The North ◽

Northern Latitudes ◽

Circumpolar Current

Abstract Preindustrial (PI) simulations of the Community Climate System Model version 3 (CCSM3) at two resolutions, a moderate and a low resolution, are described and compared to the standard controls for present-day (PD) simulations. Because of computational efficiency, the moderate- and low-resolution versions of CCSM3 may be appropriate for climate change studies requiring simulations of the order of hundreds to thousands of years. The PI simulations provide the basis for comparison for proxy records that represent average late Holocene conditions. When forced with PI trace gases, aerosols, and solar irradiance estimates, both resolutions have a global cooling of 1.2°–1.3°C, increased sea ice in both hemispheres, and less precipitation near the equator and at midlatitudes as compared to simulations using PD forcing. The response to PI forcings differs in the two resolutions for North Atlantic meridional overturning circulation (MOC), the Antarctic Circumpolar Current (ACC), and ENSO. The moderate-resolution CCSM3 has enhanced ACC, North Atlantic MOC, and tropical Pacific ENSO variability for PI forcings as compared to PD. The low-resolution CCSM3 with more extensive sea ice and colder climate at high northern latitudes in the PD simulation shows less sensitivity of the North Atlantic MOC to PI forcing. ENSO variability and the strength of the ACC do not increase with PI forcing in the low-resolution CCSM3.

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Abrupt CO2 release to the atmosphere under glacial and early interglacial climate conditions

Science ◽

10.1126/science.aay8178 ◽

2020 ◽

Vol 369 (6506) ◽

pp. 1000-1005

Author(s):

C. Nehrbass-Ahles ◽

J. Shin ◽

J. Schmitt ◽

B. Bereiter ◽

F. Joos ◽

...

Keyword(s):

Carbon Dioxide ◽

Intertropical Convergence Zone ◽

Meridional Overturning Circulation ◽

Before Present ◽

Freshwater Input ◽

Warm Climate ◽

Climate Conditions ◽

Carbon Dioxide Release ◽

Meridional Overturning ◽

Co2 Release

Pulse-like carbon dioxide release to the atmosphere on centennial time scales has only been identified for the most recent glacial and deglacial periods and is thought to be absent during warmer climate conditions. Here, we present a high-resolution carbon dioxide record from 330,000 to 450,000 years before present, revealing pronounced carbon dioxide jumps (CDJ) under cold and warm climate conditions. CDJ come in two varieties that we attribute to invigoration or weakening of the Atlantic meridional overturning circulation (AMOC) and associated northward and southward shifts of the intertropical convergence zone, respectively. We find that CDJ are pervasive features of the carbon cycle that can occur during interglacial climate conditions if land ice masses are sufficiently extended to be able to disturb the AMOC by freshwater input.

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Reduced Stability of the Atlantic Meridional Overturning Circulation due to Wind Stress Feedback during Glacial Times

Journal of Climate ◽

10.1175/2008jcli2291.1 ◽

2008 ◽

Vol 21 (23) ◽

pp. 6260-6282 ◽

Cited By ~ 14

Author(s):

Olivier Arzel ◽

Matthew H. England ◽

Willem P. Sijp

Keyword(s):

Sea Ice ◽

Wind Stress ◽

Surface Wind ◽

Meridional Overturning Circulation ◽

Nonlinear Dependence ◽

Upper Ocean ◽

Overturning Circulation ◽

Climate Conditions ◽

Meridional Overturning ◽

The Impact

Abstract A previous study by Mikolajewicz suggested that the wind stress feedback stabilizes the Atlantic thermohaline circulation. This result was obtained under modern climate conditions, for which the presence of the massive continental ice sheets characteristic of glacial times is missing. Here a coupled ocean–atmosphere–sea ice model of intermediate complexity, set up in an idealized spherical sector geometry of the Atlantic basin, is used to show that, under glacial climate conditions, wind stress feedback actually reduces the stability of the meridional overturning circulation (MOC). The analysis reveals that the influence of the wind stress feedback on the glacial MOC response to an external source of freshwater applied at high northern latitudes is controlled by the following two distinct processes: 1) the interactions between the wind field and the sea ice export in the Northern Hemisphere (NH), and 2) the northward Ekman transport in the tropics and upward Ekman pumping in the core of the NH subpolar gyre. The former dominates the response of the coupled system; it delays the recovery of the MOC, and in some cases even stabilizes collapsed MOC states achieved during the hosing period. The latter plays a minor role and mitigates the impact of the former process by reducing the upper-ocean freshening in deep-water formation regions. Hence, the wind stress feedback delays the recovery of the glacial MOC, which is the opposite of what occurs under modern climate conditions. Close to the critical transition threshold beyond which the circulation collapses, the glacial MOC appears to be very sensitive to changes in surface wind stress forcing and exhibits, in the aftermath of the freshwater pulse, a nonlinear dependence upon the wind stress feedback magnitude: a complete and irreversible MOC shutdown occurs only for intermediate wind stress feedback magnitudes. This behavior results from the competitive effects of processes 1 and 2 on the midlatitude upper-ocean salinity during the shutdown phase of the MOC. The mechanisms presented here may be relevant to the large meltwater pulses that punctuated the last glacial period.

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Projections of Cold Air Outbreaks In CMIP6 Earth System Models

10.21203/rs.3.rs-697105/v1 ◽

2021 ◽

Author(s):

Erik T. Smith ◽

Scott Sheridan

Keyword(s):

North Atlantic ◽

Climate Models ◽

Coupled Model ◽

Meridional Overturning Circulation ◽

Cold Air ◽

Complex Processes ◽

The North ◽

Meridional Overturning ◽

Earth System Models ◽

Cold Air Outbreaks

Abstract Historical and future simulated temperature data from five climate models in the Coupled Model Intercomparing Project Phase 6 (CMIP6) are used to understand how climate change might alter cold air outbreaks (CAOs) in the future. Three different Shared Socioeconomic Pathways (SSPs), SSP 1 – 2.6, SSP 2 – 4.5, and SSP 5 – 8.5 are examined to identify potential fluctuations in CAOs across the globe between 2015 and 2054. Though CAOs may remain persistent or even increase in some regions through 2040, all five climate models show CAOs disappearing by 2054 based on current climate percentiles. Climate models were able to accurately simulate the spatial distribution and trends of historical CAOs, but there were large errors in the simulated interannual frequency of CAOs in the North Atlantic and North Pacific. Fluctuations in complex processes, such as Atlantic Meridional Overturning Circulation, may be contributing to each model’s inability to simulate historical CAOs in these regions.

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Impact of Agulhas Leakage on the Atlantic Overturning Circulation in the CCSM4

Journal of Climate ◽

10.1175/jcli-d-12-00714.1 ◽

2014 ◽

Vol 27 (1) ◽

pp. 101-110 ◽

Cited By ~ 17

Author(s):

Wilbert Weijer ◽

Erik van Sebille

Keyword(s):

North Atlantic ◽

South Atlantic ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Climate System Model ◽

The North ◽

Salinity Variability ◽

Meridional Overturning ◽

The North Atlantic ◽

The Impact

Abstract The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30–40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of the AMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on the AMOC. Salinity biases leave the South Atlantic and Indian Oceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed.

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Southern Ocean Heat Uptake, Redistribution, and Storage in a Warming Climate: The Role of Meridional Overturning Circulation

Journal of Climate ◽

10.1175/jcli-d-17-0761.1 ◽

2018 ◽

Vol 31 (12) ◽

pp. 4727-4743 ◽

Cited By ~ 24

Author(s):

Wei Liu ◽

Jian Lu ◽

Shang-Ping Xie ◽

Alexey Fedorov

Keyword(s):

Southern Ocean ◽

Heat Transport ◽

Climate Models ◽

Climate Model ◽

Meridional Overturning Circulation ◽

The Other ◽

Anthropogenic Heat ◽

Overturning Circulation ◽

Ocean Heat Uptake ◽

Meridional Overturning

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60°S and is stored around 45°S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60°S coupled to a surface heat flux increase. In contrast, at 45°S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46°S at a rate of 0.07 ZJ yr−1 (° lat)−1 (1 ZJ = 1021 J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42°S at a rate of 0.30 ZJ yr−1 (° lat)−1, accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

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