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F. Krikken
and
W. Hazeleger

Abstract

The large decrease in Arctic sea ice in recent years has triggered a strong interest in Arctic sea ice predictions on seasonal-to-decadal time scales. Hence, it is important to understand physical processes that provide enhanced predictability beyond persistence of sea ice anomalies. This study analyzes the natural variability of Arctic sea ice from an energy budget perspective, using 15 climate models from phase 5 of CMIP (CMIP5), and compares these results to reanalysis data. The authors quantify the persistence of sea ice anomalies and the cross correlation with the surface and top-of-atmosphere energy budget components. The Arctic energy balance components primarily indicate the important role of the seasonal ice–albedo feedback, through which sea ice anomalies in the melt season reemerge in the growth season. This is a robust anomaly reemergence mechanism among all 15 climate models. The role of the ocean lies mainly in storing heat content anomalies in spring and releasing them in autumn. Ocean heat flux variations play only a minor role. Confirming a previous (observational) study, the authors demonstrate that there is no direct atmospheric response of clouds to spring sea ice anomalies, but a delayed response is evident in autumn. Hence, there is no cloud–ice feedback in late spring and summer, but there is a cloud–ice feedback in autumn, which strengthens the ice–albedo feedback. Anomalies in insolation are positively correlated with sea ice variability. This is primarily a result of reduced multiple reflection of insolation due to an albedo decrease. This effect counteracts the ice-albedo effect up to 50%. ERA-Interim and Ocean Reanalysis System 4 (ORAS4) confirm the main findings from the climate models.

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W. Hazeleger
and
S. S. Drijfhout

Abstract

Subduction is the process by which fluid transfers from the mixed layer to the interior of the ocean. South of the Gulf Stream extension, 18° mode water is formed in a region of high subduction rates. In this region there is high mesoscale eddy activity. In the present study the role of eddies in modifying the large-scale subduction into mode water is investigated. An eddy-resolving isopycnic ocean model with mixed layer physics included and coupled to an atmospheric anomaly model is used for this purpose. The geometry and forcing of the model are idealized.

Annual mean subduction rates into mode water up to 200 m yr−1 are found south of the Gulf Stream extension. The eddy contribution to the annual subduction is estimated by comparing the annual mean subduction rates, obtained from monthly mean quantities, with the total subduction rates (i.e., eddy plus mean contributions). The latter are determined by integrating the detrainment rates over the period when fluid is irreversibly detrained. Eddy subduction rates up to 100 m yr−1 are found. The high-frequency variability enhances the annual mean subduction by almost a factor 2.

The subduction is compared to annual net detrainment. The latter is related to the so-called shallow Ekman overturning. The eddy contribution to this overturning is determined by calculating divergence of the eddy transports in mode water as well as comparing results from the eddy-resolving version of the model with results from a coarse resolution version of the model. The results show an eddy-induced enhancement of the Ekman overturning. Conclusions are drawn with regard to parameterization of eddy subduction and eddy-induced changes of the Ekman overturning.

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W. Hazeleger
and
S. S. Drijfhout

Abstract

The response of mode water formation to typical atmospheric forcing anomalies is studied as a possible mechanism for generating the observed interannual to decadal variability in mode water. An isopycnal model of the North Atlantic subtropical gyre, coupled to a mixed layer model, is used for this purpose. Geometry and forcing are idealized. The control run shows that mode water is a well-ventilated water mass. Formation rates up to 200 m yr−1 are found at the outcrop of the mode water layer. In a series of experiments the sensitivity to the position of the anomalous forcing and to the timescale of the forcing is examined. The anomalous forcing has a dipole pattern that mimics the spatial structure of the North Atlantic oscillation.

Anomalous cooling induces a positive thickness anomaly in the mode water layer at the center of the gyre and a negative anomaly at the eastern side of the gyre. The response to anomalous heat flux forcing appears to be sensitive to the position of the forcing anomaly with respect to the formation region of mode water. The formation and attenuation of the positive thickness anomaly turns out to be mainly controlled by entrainment and detrainment from the mixed layer. In the model, it takes five years to attenuate the thickness anomaly. Enhanced wind forcing generates westward propagating thickness anomalies. Adjustment takes place by long baroclinic waves. The center of the gyre, where dominant mode water variability is observed, appears to be relatively unaffected by anomalous wind forcing.

It is concluded that variability in mode water formation of the observed amplitude and timescale can be generated in the model by heat loss variations of the observed amplitude. The response to a series of heat loss events is determined by a storage mechanism by which consecutive cold winters, despite interrupting warm winters, can induce prolonged thickness anomalies in the mode water layer.

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W. Hazeleger
and
S. S. Drijfhout

Abstract

Substantial interannual to decadal variability is observed in the properties of subtropical mode water of the North Atlantic. In this study the response of mode water to stochastic atmospheric forcing is investigated in a numerical model.

In a series of experiments the response is studied to different components of stochastic atmospheric forcing, such as wind stress, freshwater flux, and heat flux. The numerical model consists of an isopycnal ocean model with explicit mixed layer physics. The stochastic forcing is superimposed on the climatological forcing. The stochastic forcing function has an idealized form, but the amplitude, the spatial, and the temporal variability are based on observations. When a stochastic heat flux is applied, an atmospheric anomaly model is coupled to the ocean model. The geometry of the model is idealized and mimics the subtropical gyre of the North Atlantic.

The stochastic wind stress forcing excites an internal mode in the mode water layer of the model. The response is characterized by the propagation of baroclinic waves. The spectrum of the response to stochastic freshwater flux is red.

In the coupled model the stochastic heat flux forcing generates variability characterized by a dipole pattern in the mode water. The spectrum of the response is red and dominates the response to the stochastic wind stress and freshwater flux. The response is damped by an atmospheric feedback that consists of anomalous heat fluxes, depending on the SST anomalies generated by the stochastic forcing itself.

Only stochastic heat flux forcing can generate mode water variability of the observed amplitude. A preferred timescale in mode water variability should be contained in the forcing itself or it may result from modes that could not be simulated by the present model.

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S. S. Drijfhout
and
W. Hazeleger

Abstract

Signal-to-noise patterns for the meridional overturning circulation (MOC) have been calculated for an ensemble of greenhouse scenario runs. The greenhouse-forced signal has been defined as the linear trend in ensemble-mean MOC, after year 2000. It consists of an overall decrease and shoaling of the MOC, with maximum amplitudes of 10 Sv (Sv ≡ 106 m3 s−1) per century. In each member the internal variability is defined as the anomaly with respect to the ensemble-mean signal. The interannual variability of the MOC is dominated by a monopole with a maximum amplitude of 2 Sv at 40°N. This variability appears to be driven by the North Atlantic Oscillation (NAO), mainly through NAO-induced variations in the wind field.

The signal-to-noise ratio was estimated for various time spans, all starting in 1950 or later. Different noise estimates were made, both with and without intra-annual variability, relevant for episodic and continuous monitoring, respectively, and with and without an estimate of the observational error. Detection of a greenhouse-forced MOC signal on the basis of episodic measurements is impossible before 2055. With continuous monitoring, detection becomes possible after 35 years of observation. The main motivation for calculating signal-to-noise ratios and detection times is their usefulness for local monitoring strategies and detection methods. The two-dimensional pattern of detection times of a MOC change supports the rationale for deploying a sustained monitoring array on at 26°N.

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C. A. Severijns
and
W. Hazeleger

Abstract

An efficient method to optimize the parameter values of the subgrid parameterizations of an atmospheric general circulation model is described. The method is based on the downhill simplex minimization of a cost function computed from the difference between simulated and observed fields. It is used to find optimal values of the radiation and cloud-related parameters. The model error is reduced significantly within a limited number of iterations (about 250) of short integrations (5 yr). The method appears to be robust and finds the global minimum of the cost function. The radiation budget of the model improves considerably without violating the already well simulated general circulation. Different aspects of the general circulation, such as the Hadley and Walker cells improve, although they are not incorporated into the cost function. It is concluded that the method can be used to efficiently determine optimal parameters for general circulation models even when the model behavior has a strong nonlinear dependence on these parameters.

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J. Donners
,
S. S. Drijfhout
, and
W. Hazeleger

Abstract

The transformation of water masses induced by air–sea fluxes in the South Atlantic Ocean is calculated with a global ocean model, Ocean Circulation and Climate Advanced Modeling (OCCAM), and has been compared with several observational datasets. Air–sea interaction supplies buoyancy to the ocean at almost all density levels. The uncertainty of the estimates of water mass transformations is at least 10 Sv (Sv ≡ 106 m3 s−1), largely caused by the uncertainties in heat fluxes. Further analysis of the buoyancy budget of the mixed layer in the OCCAM model shows that diffusion extracts buoyancy from the water column at all densities. In agreement with observations, water mass formation of surface water by air–sea interaction is completely balanced by consumption from diffusion. There is a large interocean exchange with the Indian and Pacific Oceans. Intermediate water is imported from the Pacific, and light surface water is imported from the Indian Ocean. South Atlantic Central Water and denser water masses are exported to the Indian Ocean. The air–sea formation rate is only a qualitative estimate of the sum of subduction and interocean exchange. Subduction generates teleconnections between the South Atlantic and remote areas where these water masses reemerge in the mixed layer. Therefore, the subduction is analyzed with a Lagrangian trajectory analysis. Surface water obducts in the South Atlantic, while all other water masses experience net subduction. The subducted Antarctic Intermediate Water and Subantarctic Mode Water reemerge mainly in the Antarctic Circumpolar Current farther downstream. Lighter waters reemerge in the eastern tropical Atlantic. As a result, the extratropical South Atlantic has a strong link with the tropical Atlantic basin and only a weak direct link with the extratropical North Atlantic. The impact of the South Atlantic on the upper branch of the thermohaline circulation is indirect: water is significantly transformed by air–sea fluxes and mixing in the South Atlantic, but most of it reemerges and subducts again farther downstream.

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E. van der Swaluw
,
S. S. Drijfhout
, and
W. Hazeleger

Abstract

The mechanisms for Bjerknes compensation of heat transport variations through the atmosphere and ocean on decadal time scales are investigated, using data output from a preindustrial control run of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It has recently been shown that Bjerknes compensation occurs on decadal time scales in a long preindustrial control run of HadCM3. This result is elaborated on by performing lead/lag correlations of the atmospheric and oceanic heat transports. By using statistical analysis, Bjerknes compensation is observed on decadal time scales at latitudes between 50° and 80°N. A maximum compensation rate of ∼55% occurs at 70°N. At this latitude, the correlation rate peaks when the ocean leads the atmosphere by one year. The mechanisms by which Bjerknes compensation occurs at this latitude are investigated. Anomalies in oceanic heat transport appear to be associated with variations in the strength of the Atlantic meridional overturning circulation (MOC). The associated sea surface temperature (SST) anomalies are in general too weak to assert a significant impact on the atmosphere. At 70°N, however, such SST anomalies are a prelude to the transition from sea ice coverage to open water after which the associated changes in heat exchange with the atmosphere are strong enough to force an atmospheric response. Because of the presence of a strong MOC component in the Atlantic Ocean, this interaction is confined to the region where the northeast Atlantic and Arctic Oceans connect. The atmospheric response to increased (decreased) heating from below is a decreased (increased) poleward temperature gradient, leading to a decreased (increased) heat transport by baroclinic eddies. The anomalous thermal low that is set up by heating from the ocean is associated with anomalous advection of cold air from the Greenland landmass.

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E. C. van der Linden
,
R. Bintanja
,
W. Hazeleger
, and
C. A. Katsman

Abstract

Century-scale global near-surface temperature trends in response to rising greenhouse gas concentrations in climate models vary by almost a factor of 2, with greatest intermodel spread in the Arctic region where sea ice is a key climate component. Three factors contribute to the intermodel spread: 1) model formulation, 2) control climate state, and 3) internal climate variability. This study focuses on the influence of Arctic sea ice in the control climate on the intermodel spread in warming, using idealized 1% yr−1 CO2 increase simulations of 33 state-of-the-art global climate models, and combining sea ice–temperature relations on local to large spatial scales. On the Arctic mean scale, the spread in temperature trends is only weakly related to ice volume or area in the control climate, and is probably not dominated by internal variability. This suggests that other processes, such as ocean heat transport and meteorological conditions, play a more important role in the spread of long-term Arctic warming than control sea ice conditions. However, on a local scale, sea ice–warming relations show that in regions with more sea ice, models generally simulate more warming in winter and less warming in summer. The local winter warming is clearly related to control sea ice and universal among models, whereas summer sea ice–warming relations are more diverse, and are probably dominated by differences in model formulation. To obtain a more realistic representation of Arctic warming, it is recommended to simulate control sea ice conditions in climate models so that the spatial pattern is correct.

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M. F. de Jong
,
S. S. Drijfhout
,
W. Hazeleger
,
H. M. van Aken
, and
C. A. Severijns

Abstract

The performance of coupled climate models (CCMs) in simulating the hydrographic structure and variability of the northwestern North Atlantic Ocean, in particular the Labrador and Irminger Seas, has been assessed. This area plays an important role in the meridional overturning circulation. Hydrographic properties of the preindustrial run of eight CCMs used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) are compared with observations from the World Ocean Circulation Experiment Repeat section 7 (WOCE AR7). The mean and standard deviation of 20 yr of simulated data are compared in three layers, representing the surface waters, intermediate waters, and deep waters. Two models simulate an extremely cold, fresh surface layer with model biases down to −1.7 psu and −4.0°C, much larger than the observed ranges of variability. The intermediate and deep layers are generally too warm and saline, with biases up to 0.7 psu and 2.8°C. An analysis of the maximum mixed layer depth shows that the low surface salinity is related to a convective regime restricted to the upper 500 dbar. Thus, intermediate water formed by convection is partly replaced by warmer water from the south. Model biases seem to be caused by the coupling to the atmospheric component of the CCM. Model drift during long spinup periods allows the initially small biases in water mass characteristics to become significant. Biases that develop in the control run are carried over to the twentieth-century runs, which are initialized from the control runs.

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