Effects of locally enhanced vertical diffusivity over rough bathymetry on the world ocean circulation

Hasumi, Hiroyasu; Suginohara, Nobuo

doi:10.1029/1999jc900191

Cited by 83 publications

(58 citation statements)

References 26 publications

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“…The model incorporates a nonlinear free surface and includes an empirical parameterization of bottom-enhanced vertical mixing described in ref. 46 with a low background diffusivity of 0.1 cm 2 s Ϫ1. Isopycnal Redi stirring (34) is set to 1.0 ϫ 10 7 cm 2 s Ϫ1 , whereas the constant Gent-McWilliams thickness diffusivity (33) is 0.25 ϫ 10 7 cm 2 s Ϫ1 .…”

Section: Methodsmentioning

confidence: 99%

On the stability of the Atlantic meridional overturning circulation

Hofmann

Rahmstorf²

2009

Proc. Natl. Acad. Sci. U.S.A.

115

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One of the most important large-scale ocean current systems for Earth's climate is the Atlantic meridional overturning circulation (AMOC). Here we review its stability properties and present new model simulations to study the AMOC's hysteresis response to freshwater perturbations. We employ seven different versions of an Ocean General Circulation Model by using a highly accurate tracer advection scheme, which minimizes the problem of numerical diffusion. We find that a characteristic freshwater hysteresis also exists in the predominantly wind-driven, low-diffusion limit of the AMOC. However, the shape of the hysteresis changes, indicating that a convective instability rather than the advective Stommel feedback plays a dominant role. We show that model errors in the mean climate can make the hysteresis disappear, and we investigate how model innovations over the past two decades, like new parameterizations and mixing schemes, affect the AMOC stability. Finally, we discuss evidence that current climate models systematically overestimate the stability of the AMOC.climate change ͉ ocean circulation ͉ thermohaline circulation ͉ tipping points ͉ ocean modeling T he Atlantic meridional overturning circulation (AMOC) comprises a northward near-surface flow from the tip of South Africa via the Benguela Current, the Gulf Stream, and the North Atlantic (NA) Current right up into the Arctic Ocean. The water sinks in deep-water formation regions in the Nordic and Labrador Seas and returns to the south at depths of 2,000-3,000 m. The large heat transport of this current system [Ϸ10 15 W (1, 2)] has a major effect on northern hemisphere climate (3). Paleoclimatic evidence points to instabilities in this current system and demonstrates that large and abrupt shifts in Atlantic ocean circulation have repeatedly occurred (e.g., during the last Glacial) and were associated with large and abrupt changes in surface climate (4).Global temperatures are projected to increase by up to 6.4°C by the year 2100 (5), accompanied by a more vigorous hydrological cycle leading to a stronger net precipitation over the northern Atlantic and increased river inflow as well as meltwater influx from a shrinking Greenland Ice Sheet. Moreover, higher sea surface temperatures will further decrease the density of waters in the Nordic Seas, inhibiting deep-water formation. The expected climate change has raised concerns about the future fate of the AMOC, and the International Panel on Climate Change (IPCC) (5) concluded that there is an up to 10% probability that the AMOC ''will undergo an abrupt transition during the course of the 21st century''. The impacts of such a transition would likely be severe (6).Research over the past decades, starting with Stommel's seminal 1961 paper (7), has found interesting stability properties of the AMOC, reviewed, e.g., in ref. 8. In brief, the AMOC, at least in models, has a bistable regime in its parameter space, where deep-water formation can be ''on'' (as in present climate) or ''off'', depending only on initial cond...

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Section: Methodsmentioning

confidence: 99%

On the stability of the Atlantic meridional overturning circulation

Hofmann

Rahmstorf²

2009

Proc. Natl. Acad. Sci. U.S.A.

115

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show abstract

“…The model includes an empirical parameterization of bottom-enhanced vertical mixing similar to that of ref. 36, with a vertical background diffusivity of 0.1 cm 2 s Ϫ1 . Isopycnic Redi stirring was set to 1.25 ϫ 10 7 cm 2 s Ϫ1 , whereas thickness diffusivities vary between 0.275 and 0.55 ϫ 10 7 cm 2 s Ϫ1 .…”

Section: Methodsmentioning

confidence: 99%

Oceanic acidification affects marine carbon pump and triggers extended marine oxygen holes

Hofmann

Schellnhuber

2009

Proc. Natl. Acad. Sci. U.S.A.

179

150

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Rising atmospheric CO2 levels will not only drive future global mean temperatures toward values unprecedented during the whole Quaternary but will also lead to massive acidification of sea water. This constitutes by itself an anthropogenic planetary-scale perturbation that could significantly modify oceanic biogeochemical fluxes and severely damage marine biota. As a step toward the quantification of such potential impacts, we present here a simulation-model-based assessment of the respective consequences of a business-as-usual fossil-fuel-burning scenario where a total of 4,075 Petagrams of carbon is released into the atmosphere during the current millennium. In our scenario, the atmospheric pCO 2 level peaks at Ϸ1,750 atm in the year 2200 while the sea-surface pH value drops by >0.7 units on global average, inhibiting the growth of marine calcifying organisms. The study focuses on quantifying 3 major concomitant effects. The first one is a significant (climate-stabilizing) negative feedback on rising pCO 2 levels as caused by the attenuation of biogenic calcification. The second one is related to the biological carbon pump. Because mineral ballast, notably CaCO 3, is found to play a dominant role in carrying organic matter through the water column, a reduction of its export fluxes weakens the strength of the biological carbon pump. There is, however, a third effect with severe consequences: Because organic matter is oxidized in shallow waters when mineral-ballast fluxes weaken, oxygen holes (hypoxic zones) start to expand considerably in the oceans in our model world-with potentially harmful impacts on a variety of marine ecosystems.climate change ͉ ocean carbon cycle ͉ ocean carbon sink ͉ tipping points

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“…Several studies have examined the effects of locally enhanced vertical diffusivity over rough bathymetry in ocean models (e.g., Hasumi and Suginohara 1999;Saenko and Merryfield 2005). Unlike these previous studies, we make no attempt to examine the effects of contrasts in vertical mixing that may exist between basins in the real ocean.…”

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confidence: 97%

Sensitivity of the Atlantic Thermohaline Circulation and Its Stability to Basin-Scale Variations in Vertical Mixing

Sijp

England

2006

Journal of Climate

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This study shows that a reduction in vertical mixing applied inside the Atlantic basin can drastically increase North Atlantic Deep Water (NADW) stability with respect to freshwater perturbations applied to the North Atlantic. This is contrary to the notion that the stability of the ocean's thermohaline circulation simply scales with vertical mixing rates. An Antarctic Intermediate Water (AAIW) reverse cell, reliant upon upwelling of cold AAIW into the Atlantic thermocline, is found to be associated with stable states where NADW is collapsed. Transitions between NADW "on" and "off" states are characterized by interhemispheric competition between this AAIW cell and the NADW cell. In contrast to the AAIW reverse cell, NADW eventually upwells outside the Atlantic basin and is thus not as sensitive to changes in vertical mixing within the Atlantic. A reduction in vertical mixing in the Atlantic weakens the AAIW reverse cell, resulting in an enhanced stability of NADW formation. The results also suggest that the AAIW reverse cell is responsible for the stability of NADW collapsed states, and thereby plays a key role in maintaining multiple equilibria in the climate system. A global increase of vertical mixing in the model results in significantly enhanced NADW stability, as found in previous studies. However, an enhancement of vertical mixing applied only inside the Atlantic Ocean results in a reduction of NADW stability. It is concluded that the stability of NADW formation to freshwater perturbations depends critically on the basin-scale distribution of vertical mixing in the world's oceans.

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Effects of locally enhanced vertical diffusivity over rough bathymetry on the world ocean circulation

Cited by 83 publications

References 26 publications

On the stability of the Atlantic meridional overturning circulation

On the stability of the Atlantic meridional overturning circulation

Oceanic acidification affects marine carbon pump and triggers extended marine oxygen holes

Sensitivity of the Atlantic Thermohaline Circulation and Its Stability to Basin-Scale Variations in Vertical Mixing

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