Olivier Arzel

Increased variability of the Arctic summer ice extent in a warmer climate

Simulations performed with general circulation models and a model of intermediate complexity show that the variability of the September sea ice extent in the Arctic of the 21st century increases first when the mean extent decreases from present-day values. A maximum of the variance is found when the mean September ice extent is around 3 million km(2). For lower extents, the variance declines with the mean extent. The behavior is clearly different in Antarctica where the variance always decreases as the mean ice extent decreases, following roughly a square-root law compatible with very simple geometric arguments. Several mechanisms are responsible for the non-linear behavior of the Arctic. However, the strong interhemispheric contrast suggests that the difference in geometrical setting, with an open ocean in the south and a semi-closed basin in the north, plays a significant role. Citation: Goosse, H., O. Arzel, C. M. Bitz, A. de Montety, and M. Vancoppenolle (2009), Increased variability of the Arctic summer ice extent in a warmer climate, Geophys. Res. Lett., 36, L23702, doi: 10.1029/2009GL040546.

The Different Nature of the Interdecadal Variability of the Thermohaline Circulation under Mixed and Flux Boundary Conditions

The differences between the interdecadal variability under mixed and constant flux boundary conditions are investigated using a coarse-resolution ocean model in an idealized flat-bottom single-hemisphere basin. Objective features are determined that allow one type of oscillation to be distinguished versus the other. First, by performing a linear stability analysis of the steady state obtained under restoring boundary conditions, it is shown that the interdecadal variability under constant flux and mixed boundary conditions arises, respectively, from the instability of a linear mode around the mean stratification and circulation and from departure from the initial state. Based on the budgets of density variance, it is shown next that the two types of oscillations have different energy sources: Under the constant-flux boundary condition (the thermal mode), the downgradient meridional eddy heat flux in the western boundary current regions sustains interdecadal variability, whereas under mixed boundary conditions (the salinity mode), a positive feedback between convective adjustment and restoring surface heat flux is at the heart of the existence of the decadal oscillation. Furthermore, the positive correlations between temperature and salinity anomalies in the forcing layer are shown to dominate the forcing of density variance. In addition, the vertical structure of perturbations reveals vertical phase lags at different depths in all tracer fields under constant flux, while under mixed boundary conditions only the temperature anomalies show a strong dipolar structure. The authors propose that these differences will allow one to identify which type of oscillation, if any, is at play in the more exhaustive climate models.

The Role of Oceanic Heat Transport and Wind Stress Forcing in Abrupt Millennial-Scale Climate Transitions

The last glacial period was punctuated by rapid climate shifts, known as Dansgaard‐Oeschger events, with strong imprint in the North Atlantic sector suggesting that they were linked with the Atlantic meridional overturning circulation. Here an idealized single-hemisphere three-dimensional ocean‐atmosphere‐sea ice coupled model is used to explore the possible origin of the instability driving these abrupt events and to provide a plausible explanation for the relative stability of the Holocene. Focusing on the physics of noise-free millennial oscillations under steady external (solar) forcing, it was shown that cold climates become unstable, that is, exhibit abrupt millennial-scale transitions, for significantly lower freshwater fluxes than warm climates, in agreement with previous studies making use of zonally averaged coupled models. This fundamental difference is a direct consequence of the weaker stratification of the glacial ocean, mainly caused by upper-ocean cooling. Using a two-hemisphere configuration of a coupled climate model of intermediate complexity, it is shown that this result is robust to the added presence of a bottom water mass of southern origin. The analysis reveals that under particular conditions, a pronounced interdecadal variability develops during warm interstadials. While the nature of the instability driving the millennial oscillations is identical to that found in ocean models under mixed boundary conditions, these interstadial‐interdecadal oscillations share the same characteristics as those previously found in ocean models forced by fixed surface fluxes. The wind stress forcing is shown to profoundly affect both the properties and bifurcation structure of thermohaline millennial oscillations across a wide range of variation of freshwater forcing. In particular, it is shown that the wind stress forcing favors the maintenance of thermally direct meridional overturning circulations during the cold stadial phases of Dansgaard‐Oeschger cycles.

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

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.

Multidecadal Variability of the Overturning Circulation in Presence of Eddy Turbulence

At low-resolution, idealized ocean circulation models forced by prescribed differential surface heat fluxes show spontaneous multidecadal variability depending critically on eddy diffusivity coefficients. The existence of this critical threshold in the range of observational estimates legitimates some doubt on the relevance of such intrinsic oscillations in the real ocean. Through a series of numerical simulations with increasing resolution up to eddy-resolving ones (10 km) and various diapycnal diffusivities, this multidecadal variability proves a generic ubiquitous feature, at least in model versions with a flat bottom. The mean circulation largely changes in the process of refining the horizontal grid (along with the associated implicit viscosity and diffusivity), and the spatial structure of the variability is largely modified, but there is no clear influence of the resolution on the main oscillation period. The interdecadal variability appears even more robust to low vertical diffusivity and overturning when mesoscale eddies are resolved. The mechanism previously proposed for these oscillations, involving westward-propagating baroclinically unstable Rossby waves in the subpolar region and its feedback on the mean circulation, appears unaffected by mesoscale turbulence and is simply displaced following the polar front.

On the origin of interdecadal oscillations in a coupled ocean–atmosphere model

Interdecadal oscillations are analysed in a coupled ocean–atmosphere model made of a planetary geostrophic ocean model within an idealized geometry, coupled to a zonally-averaged tropospheric atmosphere model. The interdecadal variability that arises spontaneously in this coupled system is caused by intrinsic ocean dynamics, the coupled air-sea feedbacks being not essential. The spatial pattern of the variability bears some resemblance with observations and results obtained with atmosphere-ocean general circulation models (AOGCMs) as well as simpler climate models: large and quasi-stationary upper ocean temperature-dominated density anomalies are found in the north-western part of the ocean basin along with weaker, westward propagating anomalies in the remaining interior. The basic physical mechanism that lies at the heart of the existence of the interdecadal mode is a large-scale baroclinic instability of the oceanic mean flow in the vicinity of the western boundary, characteristic of ocean models forced by constant surface fluxes. Freshwater feedbacks associated with the hydrological cycle are found to have only a modest influence on the interdecadal mode. The presence of a periodic channel mimicking the Antarctic Circumpolar Current at high southern latitudes prevents the oceanic baroclinic instability to occur in the Southern Hemisphere.

The Impact of Wind Stress Feedback on the Stability of the Atlantic Meridional Overturning Circulation

Recent results based on models using prescribed surface wind stress forcing have suggested that the net freshwater transport Σ by the Atlantic meridional overturning circulation (MOC) into the Atlantic basin is a good indicator of the multiple-equilibria regime. By means of a coupled climate model of intermediate complexity, this study shows that this scalar Σ cannot capture the connection between the properties of the steady state and the impact of the wind stress feedback on the evolution of perturbations. This implies that, when interpreting the observed value of Σ, the position of the present-day climate is systematically biased toward the multiple-equilibria regime. The results show, however, that the stabilizing influence of the wind stress feedback on the MOC is restricted to a narrow window of freshwater fluxes, located in the vicinity of the state characterized by a zero freshwater flux divergence over the Atlantic basin. If the position of the present-day climate is farther away from this state, then wind stress feedbacks are unable to exert a persistent effect on the modern MOC. This is because the stabilizing influence of the shallow reverse cell situated south of the equator during the off state rapidly dominates over the destabilizing influence of the wind stress feedback when the freshwater forcing gets stronger. Under glacial climate conditions by contrast, a weaker sensitivity with an opposite effect is found. This is ultimately due to the relatively large sea ice extent of the glacial climate, which implies that, during the off state, the horizontal redistribution of fresh waters by the subpolar gyre does not favor the development of a thermally direct MOC as opposed to the modern case.

Wind-stress feedback amplification of abrupt millennial-scale climate changes

The influence of changes in surface wind-stress on the properties (amplitude and period) and domain of existence of thermohaline millennial oscillations is studied by means of a coupled model of intermediate complexity set up in an idealized spherical sector geometry of the Atlantic basin. Using the atmospheric CO2 concentration as the control parameter, bifurcation diagrams of the model are built to show that the influence of wind-stress changes on glacial abrupt variability is threefold. First, millennial-scale oscillations are significantly amplified through wind-feedback-induced changes in both northern sea ice export and oceanic heat transport. Changes in surface wind-stress more than double the amplitude of the strong warming events that punctuate glacial abrupt variability obtained under prescribed winds in the model. Second, the average duration of both stadials and interstadials is significantly lengthened and the temporal structure of observed variability is better captured under interactive winds. Third, the generation of millennial-scale oscillations is shown to occur for significantly colder climates when wind-stress feedback is enabled. This behaviour results from the strengthening of the negative temperature-advection feedback associated with stronger northward oceanic heat transport under interactive winds.

The Internal Generation of the Atlantic Ocean Interdecadal Variability

AbstractNumerical simulations of a realistic ocean general circulation model forced by prescribed surface fluxes are used to study the origin and structure of intrinsic interdecadal variability of ...

North Atlantic Ocean Internal Decadal Variability: Role of the Mean State and Ocean‐Atmosphere Coupling

The origin of the decadal variability in the North Atlantic Ocean is investigated in a series of coupled and ocean‐only numerical experiments. Two versions of the IPSL‐CM5A model are considered, differing only by their atmospheric horizontal resolution (3.75° × 1.87° and 2.5° × 1.25°). When the ocean model is forced by the climatological surface fluxes from the low atmospheric resolution coupled model version, a 20‐year variability emerges, similar to the variability found in the coupled simulation. Such decadal variability is consistent with a large‐scale baroclinic instability of the mean flow in the west European basin. Increasing the atmospheric resolution leads to a more intense Icelandic low, which intensifies the western subpolar gyre, and warms the eastern North Atlantic subpolar gyre region. The mean state changes nearly vanish the associated internal oceanic variability under the corresponding climatological surface fluxes. Increasing the atmospheric resolution also produces a slightly weaker atmospheric stochastic forcing. Both the mean state and atmospheric variability changes are consistent with the decreasing amplitude of the variability in the coupled model. For both model versions, the amplitude of the internal oceanic variability is strongly enhanced in the presence of atmospheric stochastic forcing. Air‐sea coupling on the other hand has a moderate influence on the amplitude of the variability only in the low‐resolution model version, where the North Atlantic oceanic variability at 20 years increases by 23% due to coupling. The coupling effect is therefore modest and sensitive to the atmospheric horizontal resolution.