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David Ferreira and Claude Frankignoul

Abstract

To study the transient atmospheric response to midlatitude SST anomalies, a three-layer quasigeostrophic (QG) model coupled to a slab oceanic mixed layer in the North Atlantic is used. As diagnosed from a coupled run in perpetual winter conditions, the first two modes of SST variability are linked to the model North Atlantic Oscillation (NAO) and eastern Atlantic pattern (EAP), respectively, the dominant atmospheric modes in the Atlantic sector. The two SST anomaly patterns are then prescribed as fixed anomalous boundary conditions for the model atmosphere, and its transient responses are established from a large ensemble of simulations.

In both cases, the tendency of the air–sea heat fluxes to damp the SST anomalies results in an anomalous diabatic heating of the atmosphere that, in turn, forces a baroclinic response, as predicted by linear theory. This initial response rapidly modifies the transient eddy activity and thus the convergence of eddy momentum and heat fluxes. The latter transforms the baroclinic response into a growing barotropic one that resembles the atmospheric mode that had created the SST anomaly in the coupled run and is thus associated with a positive feedback. The total adjustment time is as long as 3–4 months for the NAO-like response and 1–2 months for the EAP-like one. The positive feedback, in both cases, is dependent on the polarity of the SST anomaly, but is stronger in the NAO case, thereby contributing to its predominance at low frequency in the coupled system. However, the feedback is too weak to lead to an instability of the atmospheric modes and primarily results in an increase of their amplitude and persistence and a weakening of the heat flux damping of the SST anomaly.

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David Ferreira and Claude Frankignoul

Abstract

The transient atmospheric response to interactive SST anomalies in the midlatitudes is investigated using a three-layer QG model coupled in perpetual winter conditions to a slab oceanic mixed layer in the North Atlantic. The SST anomalies are diagnosed from a coupled run and prescribed as initial conditions, but are free to evolve. The initial evolution of the atmospheric response is similar to that obtained with a prescribed SST anomaly, starting as a quasi-linear baroclinic and then quickly evolving into a growing equivalent barotropic one. Because of the heat flux damping, the SST anomaly amplitude slowly decreases, albeit with little change in pattern. Correspondingly, the atmospheric response only increases until it reaches a maximum amplitude after about 1–3.5 months, depending on the SST anomaly considered. The response is similar to that at equilibrium in the fixed SST case, but it is 1.5–2 times smaller, and then slowly decays away.

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David Ferreira, John Marshall, and Patrick Heimbach

Abstract

A global ocean circulation model is formulated in terms of the “residual mean” and used to study eddy–mean flow interaction. Adjoint techniques are used to compute the three-dimensional eddy stress field that minimizes the departure of the coarse-resolution model from climatological observations of temperature. The resulting 3D maps of eddy stress and residual-mean circulation yield a wealth of information about the role of eddies in large-scale ocean circulation. In eddy-rich regions such as the Southern Ocean, the Kuroshio, and the Gulf Stream, eddy stresses have an amplitude comparable to the wind stress, of order 0.2 N m−2, and carry momentum from the surface down to the bottom, where they are balanced by mountain form drag. From the optimized eddy stress, 3D maps of horizontal eddy diffusivity κ are inferred. The diffusivities have a well-defined large-scale structure whose prominent features are 1) large values of κ (up to 4000 m2 s−1) in the western boundary currents and on the equatorial flank of the Antarctic Circumpolar Current and 2) a surface intensification of κ, suggestive of a dependence on the stratification N2. It is shown that implementation of an eddy parameterization scheme in which the eddy diffusivity has an N2 dependence significantly improves the climatology of the ocean model state relative to that obtained using a spatially uniform diffusivity.

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David Ferreira, John Marshall, and Brian Rose

Abstract

Multiple equilibria in a coupled ocean–atmosphere–sea ice general circulation model (GCM) of an aquaplanet with many degrees of freedom are studied. Three different stable states are found for exactly the same set of parameters and external forcings: a cold state in which a polar sea ice cap extends into the midlatitudes; a warm state, which is ice free; and a completely sea ice–covered “snowball” state. Although low-order energy balance models of the climate are known to exhibit intransitivity (i.e., more than one climate state for a given set of governing equations), the results reported here are the first to demonstrate that this is a property of a complex coupled climate model with a consistent set of equations representing the 3D dynamics of the ocean and atmosphere. The coupled model notably includes atmospheric synoptic systems, large-scale circulation of the ocean, a fully active hydrological cycle, sea ice, and a seasonal cycle. There are no flux adjustments, with the system being solely forced by incoming solar radiation at the top of the atmosphere.

It is demonstrated that the multiple equilibria owe their existence to the presence of meridional structure in ocean heat transport: namely, a large heat transport out of the tropics and a relatively weak high-latitude transport. The associated large midlatitude convergence of ocean heat transport leads to a preferred latitude at which the sea ice edge can rest. The mechanism operates in two very different ocean circulation regimes, suggesting that the stabilization of the large ice cap could be a robust feature of the climate system. Finally, the role of ocean heat convergence in permitting multiple equilibria is further explored in simpler models: an atmospheric GCM coupled to a slab mixed layer ocean and an energy balance model.

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David Ferreira, Claude Frankignoul, and John Marshall

Abstract

Midlatitude air–sea interactions are investigated by coupling a stochastically forced two-layer quasigeostrophic channel atmosphere to a simple ocean model. The stochastic forcing has a large-scale standing pattern to simulate the main modes of low-frequency atmospheric variability. When the atmosphere interacts with an oceanic mixed layer via surface heat exchanges, the white noise forcing generates an approximately red noise sea surface temperature (SST) response. As the SST adjusts to the air temperature changes at low frequency, thus decreasing the heat flux damping, the atmospheric spectra are slightly reddened, the power enhancement increasing with the zonal scale because of atmospheric dynamics. Decadal variability is enhanced by considering a first baroclinic oceanic mode that is forced by Ekman pumping and modulates the SST by entrainment and horizontal advection. The ocean interior is bounded at its eastern edge, and a radiation condition is used in the west. Primarily in wintertime conditions, a positive feedback takes place between the atmosphere and the ocean when the atmospheric response to the SST is equivalent barotropic. Then, the ocean interior modulates the SST in a way that leads to a reinforcement of its forcing by the wind stress, although the heat flux feedback is negative. The coupled mode propagates slowly westward with exponentially increasing amplitude, and it is fetch limited. The atmospheric and SST spectral power increase at all periods longer than 10 yr when the coupling with the ocean interior occurs by entrainment. When it occurs by advection, the power increase is primarily found at near-decadal periods, resulting in a slightly oscillatory behavior of the coupled system. Ocean dynamics thus leads to a small, but significant, long-term climate predictability, up to about 6 yr in advance in the entrainment case.

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Brian E. J. Rose and David Ferreira

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The authors study the role of ocean heat transport (OHT) in the maintenance of a warm, equable, ice-free climate. An ensemble of idealized aquaplanet GCM calculations is used to assess the equilibrium sensitivity of global mean surface temperature and its equator-to-pole gradient (ΔT) to variations in OHT, prescribed through a simple analytical formula representing export out of the tropics and poleward convergence. Low-latitude OHT warms the mid- to high latitudes without cooling the tropics; increases by 1°C and ΔT decreases by 2.6°C for every 0.5-PW increase in OHT across 30° latitude. This warming is relatively insensitive to the detailed meridional structure of OHT. It occurs in spite of near-perfect atmospheric compensation of large imposed variations in OHT: the total poleward heat transport is nearly fixed.

The warming results from a convective adjustment of the extratropical troposphere. Increased OHT drives a shift from large-scale to convective precipitation in the midlatitude storm tracks. Warming arises primarily from enhanced greenhouse trapping associated with convective moistening of the upper troposphere. Warming extends to the poles by atmospheric processes even in the absence of high-latitude OHT.

A new conceptual model for equable climates is proposed, in which OHT plays a key role by driving enhanced deep convection in the midlatitude storm tracks. In this view, the climatic impact of OHT depends on its effects on the greenhouse properties of the atmosphere, rather than its ability to increase the total poleward energy transport.

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David Ferreira, John Marshall, and Jean-Michel Campin

Abstract

A series of coupled atmosphere–ocean–ice aquaplanet experiments is described in which topological constraints on ocean circulation are introduced to study the role of ocean circulation on the mean climate of the coupled system. It is imagined that the earth is completely covered by an ocean of uniform depth except for the presence or absence of narrow barriers that extend from the bottom of the ocean to the sea surface. The following four configurations are described: Aqua (no land), Ridge (one barrier extends from pole to pole), Drake (one barrier extends from the North Pole to 35°S), and DDrake (two such barriers are set 90° apart and join at the North Pole, separating the ocean into a large basin and a small basin, connected to the south). On moving from Aqua to Ridge to Drake to DDrake, the energy transports in the equilibrium solutions become increasingly “realistic,” culminating in DDrake, which has an uncanny resemblance to the present climate. Remarkably, the zonal-average climates of Drake and DDrake are strikingly similar, exhibiting almost identical heat and freshwater transports, and meridional overturning circulations. However, Drake and DDrake differ dramatically in their regional climates. The small and large basins of DDrake exhibit distinctive Atlantic-like and Pacific-like characteristics, respectively: the small basin is warmer, saltier, and denser at the surface than the large basin, and is the main site of deep water formation with a deep overturning circulation and strong northward ocean heat transport. A sensitivity experiment with DDrake demonstrates that the salinity contrast between the two basins, and hence the localization of deep convection, results from a deficit of precipitation, rather than an excess of evaporation, over the small basin. It is argued that the width of the small basin relative to the zonal fetch of atmospheric precipitation is the key to understanding this salinity contrast.

Finally, it is argued that many gross features of the present climate are consequences of two topological asymmetries that have profound effects on ocean circulation: a meridional asymmetry (circumpolar flow in the Southern Hemisphere; blocked flow in the Northern Hemisphere) and a zonal asymmetry (a small basin and a large basin).

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Ryan Abernathey, John Marshall, and David Ferreira

Abstract

An eddy-resolving numerical model of a zonal flow, meant to resemble the Antarctic Circumpolar Current, is described and analyzed using the framework of J. Marshall and T. Radko. In addition to wind and buoyancy forcing at the surface, the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation (MOC) to exist at depth. The strength of the residual MOC is diagnosed for different strengths of surface wind stress. It is found that the eddy circulation largely compensates for the changes in Ekman circulation. The extent of the compensation and thus the sensitivity of the MOC to the winds depend on the surface boundary condition. A fixed-heat-flux surface boundary severely limits the ability of the MOC to change. An interactive heat flux leads to greater sensitivity. To explain the MOC sensitivity to the wind strength under the interactive heat flux, transformed Eulerian-mean theory is applied, in which the eddy diffusivity plays a central role in determining the eddy response. A scaling theory for the eddy diffusivity, based on the mechanical energy balance, is developed and tested; the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress. The MOC sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments.

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Mukund Gupta, John Marshall, and David Ferreira

Abstract

A coupled climate model with idealized representations of atmosphere, ocean, sea ice, and land is used to investigate transitions between global climate equilibria. The model supports the presence of climates with limited ice cover (Warm), a continuum of climates in which sea ice extends down into the midlatitudes and the tropics (Cold), together with a completely ice-covered earth (Snowball). Transitions between these states are triggered through volcanic eruptions, where the radiative effect of stratospheric sulfur emissions is idealized as an impulse reduction in incoming solar radiation. Snowball transitions starting from the Cold state are more favorable than from the Warm state, because less energy must be extracted from the system. However, even when starting from a Cold climate, Toba-like volcanic events (cooling of order −100 W m−2) must be sustained continuously for several decades to glaciate the entire planet. When the deep ocean is involved, the volcanic response is characterized by relaxation time scales spanning hundreds to thousands of years. If the interval between successive eruptions is significantly shorter (years to decades) than the ocean’s characteristic time scales, the cumulative cooling can build over time and initiate a state transition. The model exhibits a single hysteresis loop that connects all three climate equilibria. When starting from a Snowball, the model cannot access the Cold branch without first transitioning to an ice-free climate and completing the hysteresis loop. By contrast, a Cold state, when warmed, transitions to the Warm equilibrium without any hysteresis.

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Jake Aylmer, David Ferreira, and Daniel Feltham

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Climate-model biases in ocean heat transport (OHT) have been proposed as a major contributor to uncertainties in projections of sea ice extent. To better understand the impact of OHT on sea ice extent and compare it to that of atmospheric heat transport (AHT), an idealized, zonally averaged energy balance model (EBM) is developed. This is distinguished from previous EBM work by coupling a diffusive mixed layer OHT and a prescribed OHT contribution, with an atmospheric EBM and a reduced-complexity sea ice model. The ice-edge latitude is roughly linearly related to the convergence of each heat transport component, with different sensitivities depending on whether the ice cover is perennial or seasonal. In both regimes, Bjerknes compensation (BC) occurs such that the response of AHT partially offsets the impact of changing OHT. As a result, the effective sensitivity of ice-edge retreat to increasing OHT is only ~2/3 of the actual sensitivity (i.e., eliminating the BC effect). In the perennial regime, the sensitivity of the ice edge to OHT is about twice that to AHT, while in the seasonal regime they are similar. The ratio of sensitivities is, to leading order, determined by atmospheric longwave feedback parameters in the perennial regime. Here, there is no parameter range in which the ice edge is more sensitive to AHT than OHT.

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