Multiple sea-ice states and abrupt MOC transitions in a general circulation ocean model
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- Ashkenazy, Y., Losch, M., Gildor, H. et al. Clim Dyn (2013) 40: 1803. doi:10.1007/s00382-012-1546-2
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Sea ice has been suggested, based on simple models, to play an important role in past glacial–interglacial oscillations via the so-called “sea-ice switch” mechanism. An important requirement for this mechanism is that multiple sea-ice extents exist under the same land ice configuration. This hypothesis of multiple sea-ice extents is tested with a state-of-the-art ocean general circulation model coupled to an atmospheric energy–moisture-balance model. The model includes a dynamic-thermodynamic sea-ice module, has a realistic ocean configuration and bathymetry, and is forced by annual mean forcing. Several runs with two different land ice distributions represent present-day and cold-climate conditions. In each case the ocean model is initiated with both ice-free and fully ice-covered states. We find that the present-day runs converge approximately to the same sea-ice state for the northern hemisphere while for the southern hemisphere a difference in sea-ice extent of about three degrees in latitude between the different runs is observed. The cold climate runs lead to meridional sea-ice extents that are different by up to four degrees in latitude in both hemispheres. While approaching the final states, the model exhibits abrupt transitions from extended sea-ice states and weak meridional overturning circulation, to less extended sea ice and stronger meridional overturning circulation, and vice versa. These transitions are linked to temperature changes in the North Atlantic high-latitude deep water. Such abrupt changes may be associated with Dansgaard–Oeschger events, as proposed by previous studies. Although multiple sea ice states have been observed, the difference between these states is not large enough to provide a strong support for the sea-ice-switch mechanism.
KeywordsSea iceGlacial-interglacial oscillationsMultiple sea-ice statesOceanic general circulation modelMITgcmEnergy moisture balance modelHysteresis
Over the last million years (the late Pleistocene), Earth’s climate has experienced dramatic glacial-interglacial oscillations (Imbrie et al. 1984; EPICA-Community-Members 2004) with well established characteristics. The ice-sheets grow slowly (during ∼90 kyr) and melt much more rapidly (during ∼10 kyr). The Northern Hemisphere (NH) maximum ice-volume during the last glacial maximum (LGM) was about 15 times larger than today’s (Mix et al. 2001), with 2–3 km thick ice covering Canada and the Northern U.S. (Peltier 1994), and sea level that was lower by ∼120 m. The global temperature during the LGM was about 6 °C lower compared to present day and glacial atmospheric CO2 concentration was lower by 80–100 ppm compared to interglacial times (Petit et al. 1999). LGM winds were much stronger (Ram and Koenig 1997) compared with today’s winds. The mechanisms underlying these massive changes are still not understood (e.g., Ghil 1994; Wunsch 2003).
Several studies have shown multiple sea-ice states using various models. Specifically, Langen and Alexeev (2004) used the community atmospheric model (CAM) (Holland et al. 2006a) coupled to a simple slab ocean model under aqua-planet and annual mean conditions, and demonstrated the existence of multiple states of sea-ice extent under the same parameters. The control parameter in their experiments was the oceanic “qflux” (i.e., prescribed flux representing ocean heat transport); three sea-ice extents were identified: (1) ice-free ocean, (2) intermediate sea-ice extent up to the high latitudes, and (3) extensive sea-ice extent (up to the mid-latitudes). Ferreira et al. (2011) used a coupled ocean-atmosphere version of the MITgcm (MITgcm Group 2010), but without sea-ice dynamics, in an aqua planet configuration and again identified three different states of sea ice: polar ice-cap extending to the mid-latitudes, ice free and snowball states. We take a complementary approach of using a full ocean general circulation model (GCM) with a dynamics-thermodynamic sea-ice component, coupled to a simple atmospheric model, and use realistic continental geometry and ocean bathymetry. Our simpler and computationally efficient GCM gives us larger flexibility in exploring the parameter space. In a different study, Marotzke and Botzet (2007) varied the solar constant in a coupled atmosphere–ocean GCM and showed that once the climate is sufficiently cold to enter a snowball state, a much larger radiation constant is needed to “escape” from such a state; this study thus showed multiple sea-ice states under the same solar radiation input. Recently, Abbot et al. (2011) suggested that multiple states of sea ice can arise due to the difference in albedo between dark, bare sea ice and bright, snow covered sea ice. Eisenman et al. (submitted) have used a fully coupled atmosphere-ocean GCM to study the DO events and demonstrated the possibility of two quasi-stable sea-ice states, associated with the stadial and interstadial phases of the DO events; the interstadial state converged to the stadial state after ∼700 hundreds years of simulations. Recent studies (Eisenman and Wettlaufer 2009; Lindsay and Zhang 2005; Overpeck et al. 2005; Serreze and Francis 2006; Holland et al. 2006b; Maslanik et al. 2007; Lenton et al. 2008; Merryfield et al. 2008) discussed the possibility of a tipping point in the Arctic sea ice cover (below which the Arctic will be ice free) and associated this point with hysteresis and multiple equilibria. However even more recent studies suggested that there is no tipping point in the Arctic sea-ice (e.g., Tietsche et al. 2011).
The main goal of this study is to test whether multiple states of sea ice exist under the same land ice cover in a realistic-geometry state-of-the-art ocean-ice model coupled to a simple atmospheric model. This goal is explored for both “present day” and for “cold” climates. We show that such multiple sea-ice states indeed exist in the model, although they are not as pronounced in the NH as predicted by the sea-ice switch mechanism. We note that the model used here, while using realistic geometry, still lacks many feedbacks and processes. We also examine rapid sea-ice changes in these model runs and consider their relevance to observed rapid climate change.
The paper is organized as follows: the model is described in Sect. 2, the experiments performed with the model are described in Sect. 3, followed by analysis of the meridional overturning circulation (MOC) and the sea-ice extent (Sect. 4); discussion and conclusions are presented in Sect. 5.
2 Model description and spinup
2.1 The oceanic model—MITgcm
The Massachusetts Institute of Technology ocean general circulation model (MITgcm) solves the primitive equations (Marshall et al. 1997a, b) and is used here in a global cubed-sphere configuration (Adcroft et al. 2004) with a lateral resolution of about 290 km (varying from 330 km resolution at the center of a cube-sphere face to 110 km at face corners). The ocean has 15 vertical levels, with thicknesses ranging from 50 m for the surface layer to 690 m for the bottom layer. We use the isopycnal eddy parametrization scheme of Gent and McWilliams (1990) and Redi (1982). The vertical background diffusion coefficient for both temperature and salinity is 3 × 10−5 m2/s, and the vertical viscosity is 10−3 m2/s. In addition, the k-profile parameterization (KPP, Large et al. 1994) scheme is used to simulate vertical mixing and deep convection processes.
2.2 The dynamic-thermodynamic sea-ice model
The sea-ice component of the MITgcm is used to simulate sea ice with a viscous-plastic rheology. Ice velocities advect effective ice thickness (volume), ice concentration and snow with a flux-limiting scheme. Ice formation and melting with zero-layer thermodynamics follows Semtner (1976) and Hibler (1980). The ice model exchanges heat and fresh water with the ocean and the atmosphere at each ocean time step. The load of the ice and snow depresses the sea-surface of the ocean to account for exact mass-balance (Campin et al. 2008). Further details of the model are described in Losch et al. (2010) and references therein.
2.3 The atmospheric energy–moisture-balance model
The atmospheric model is based on the energy moisture balance model (EMBM) of Fanning and Weaver (1996) and the atmospheric component of the UVic Earth System Climate Model (Weaver et al. 2001) as follows. Our EMBM consists of one vertical layer and a horizontal grid that coincides with the oceanic grid. Two prognostic variables, atmospheric temperature, Tair, and humidity, qair, are updated with a second order Adams–Bashforth scheme. Surface winds are prescribed and humidity is advected by these winds. Topographic effects on temperature and humidity are taken into account by assuming a prescribed lapse rate of 6 K/km. Atmospheric CO2 concentration is also taken into account.
The main difference from Weaver et al. (2001) is the treatment of surface albedo to include the effect of land ice albedo on short wave reflection. Over the ocean the albedo is set to a constant (0.07) while the sea-ice model computes the albedo over sea ice as a function of snow cover and temperature. Land surface is assumed to have no heat capacity, but spatially varying land albedos can be prescribed. The land albedo is set to that of land ice (0.6) over prescribed land ice cover. Shortwave radiation is scattered once while passing through the atmosphere, and is then reflected at the surface according to the albedo and scattered a second time on its way up through the atmosphere into space.
The atmospheric time step is set to 10 min, so that the atmosphere is stepped multiple times within a single ocean tracer time step of one day. The tracer acceleration method of (Bryan 1984) is used for efficiency, with a momentum time step of 20 min. This approach is not expected to lead to major biases in steady solutions with the time-independent forcing used here. The atmospheric model exchanges heat and fresh water with the surface at each ocean model time step. At the beginning of the ocean time step, the atmosphere computes heat and fresh water fluxes based on the ocean and ice state of the previous time step and averages them over the ocean time step while stepping the atmospheric variables forward in time. Then the sea ice and ocean models are stepped forward.
In addition to the “present-day” spinup run we performed similar spinup runs for the “cold-climate” setups described in Sect. 2.5. To achieve the cold conditions required for some of our numerical experiments we prescribed land-ice albedo over land at latitudes 40–90°N, sea-ice albedo of 1, and atmospheric CO2 level of 180 ppm. These values are not meant to be realistic, but are used to explore an extreme regime of parameter space.
2.5 The numerical experiment
“Present day” experiment: “present day” land ice and initial conditions of (1) no sea ice and (2) 10 m thick sea ice covering the entire ocean and a corresponding negative free surface anomaly to preserve the water content of the model (this is referred to below as the “all ice” initial state). Note that the model does not enter a snowball state in the last configuration, because of the relatively warm initial ocean temperatures.
“Cold climate 1” experiment: land ice albedo for latitudes 40–90°N, sea-ice albedo set to 1, atmospheric CO2 level of 180 ppm, and increased atmospheric albedo profile specified as function of latitude. Two initial conditions were again considered, (1) ice free ocean and (2) “all ice” initial state, and upper layer ocean (to a depth of 50 meters) that is 10 °C lower than that of the spinup run (but not lower than the ocean freezing temperature). The prescribed upper ocean cooling is meant to ensure convergence to a cold state if it exists.
“Cold climate 2” experiment: Same as the “cold climate 1” experiment but with a higher-yet atmospheric albedo profile (increase of ∼1 % compared to “cold climate 1”, equivalent to a decrease of ∼2 W/m2 in the incoming short-wave radiation), to yield an even colder climate (∼1 °C difference in mean ocean temperature).
3 Multiple sea-ice equilibria
3.1 “Present day” experiment
Higher humidity in the “all water” run is associated with higher atmospheric temperatures, following the Clausius–Clapeyron relation. Some regions, such as the western tropical Pacific, show higher humidity values for the “all water” run accompanied by a relatively small temperature difference in that region. This strong humidity response to a small temperature difference is due to the exponential dependence of moisture on temperature. The SSS differences between the “all water” and “all ice” runs may be mainly attributed to melting and formation of sea ice, as these occur in the high latitudes of both hemispheres.
3.2 “Cold climate 1” experiment
The sea-ice area maps of the two “cold climate 1” runs are presented in Fig. 6. The sea-ice extends further equatorward compared to the “present day” runs (Fig. 4); it reaches the northern part of Mediterranean Sea, covers extensive parts of the North Pacific, and reaches South America in the Southern Ocean. In addition, the “all ice” sea-ice clearly exceeds that of the “all water” run by 4° in latitude. The “cold climate 1” basic state thus supports multiple states of sea ice.
3.3 “Cold climate 2” experiment
3.4 Comparison between the experiments
The evolution of the North Atlantic (NA) maximum meridional overturning circulation (MOC), the NH and NA sea-ice extent, and the SH sea-ice extent are presented in Fig. 7. The extent of the sea ice is calculated as the latitude at which the zonal-mean sea-ice area fraction drops below 0.5. For the “present day” experiment there is a quick convergence to a single state of the MOC and NH sea ice while there are two distinct sea-ice states in the SH, with sea-ice extents that differ by about 3° in latitude.
The “cold climate 1” and “2” experiments both remain in very different quasi-equilibrium for some time, but then change into their steady states, yet in different ways. In both runs, the quasi-equilibrium states have distinct MOC amplitudes and corresponding different NH sea-ice states. The “all water” run is initially associated with the stronger MOC state and the “all ice” with the weaker MOC state. In the “cold climate 1” runs the weak MOC state jumps to the stronger MOC state after about 1,500 years of simulations, and simultaneously the NH sea ice edge moves northward toward the “all water” sea-ice extent. We did not observe significant further changes for the remaining 8,000 years of the simulations (not shown). In an opposite transition in the “cold climate 2” run, the stronger NA MOC state collapses to the weaker state after about 500 years of simulation; the NH sea-ice edge in the “all water” run simultaneously moves further southward. These abrupt transitions are further discussed in the next section. The model seems to support fairly long-lasting and significantly different multiple quasi-equilibria, and one wonders if some change in the model formulation could stabilize these quasi-equilibria so that they can last indefinitely.
The Southern-Ocean sea ice does not exhibit any abrupt transitions. The difference between the Southern Ocean sea-ice extent of the “all ice” run and the “all water” run in the three different experiments (“present-day” and “cold climate 1 and 2”) varies between three and five degrees latitude (Fig. 7), where a larger difference is observed in the coldest experiment (“cold climate 2”).
4 Meridional overturning circulation stability and NH sea-ice extent
The interaction of the MOC and sea-ice extent has been discussed in many previous studies (e.g., Manabe and Stouffer 1999; Kaspi et al. 2004; Timmermann et al. 2003; Gildor and Tziperman 2003; Wang and Mysak 2006; Loving and Vallis 2005; Colin de Verdiére and Te Raa 2010; Arzel et al. 2010, 2011). Freshening of the high-latitude NA creates a layer of light water that results in reduced formation of deep water and hence leads to an MOC shutdown and increased sea ice extent. When the MOC is restarted (Winton 1993), warm low latitude water reaches the high latitudes and thus reduces the sea ice extent.
We find that the transitions between the different MOC states are linked to changes in deep ocean temperatures, following the relaxation oscillation mechanism of Winton (1993) (see also Winton and Sarachik 1993; Ashkenazy and Tziperman 2007). In this mechanism deep ocean heat diffusion (i.e., parameterized eddy flux) from the low latitudes results in a warming of the deep high-latitude ocean (while the same eddies do not affect the surface ocean because it is strongly coupled to the atmosphere). This weakens vertical stratification in the high latitudes and eventually leads to restarted convection and an abrupt MOC increase.
In the “cold climate 2” “all water” run, there is a switch from a stronger MOC state to a weaker state (Fig. 7c). Prior to this transition (at t ≈ 4.52 kyr), the 50–70°N stratification (Fig. 8c) becomes stronger with time as the deep water cools, until the MOC switches to its weaker state. This transition is accompanied by a equatorward extension of sea ice (Fig. 7f). After the transition the stratification weakens within the 50–70°N band and the deep ocean warms. At the high latitudes (70–90°N) the surface layer warms (and thus gains buoyancy), and subsequently the deep water warms.
An opposite picture is seen between 65 and 80°N. As for the “cold climate 2” “all water” run, the surface water is colder and the deep water (of depth ∼2,000 m) is warmer after the MOC transition, consistent with the weaker MOC after the transition. The water becomes warmer for latitudes higher than ∼60°N. The picture for salinity is simpler (Fig. 12) where the salinity of the high latitudes of the NA under stronger MOC states is relatively high due to advection from low latitudes.
5 Discussion and conclusions
We explored multiple sea-ice states in a state-of-the-art ocean GCM for different basic states, including present-day like and colder climate conditions that were prescribed via the extent of land ice and atmospheric CO2. The GCM includes sea-ice dynamics and thermodynamics; it is coupled to an atmospheric energy and moisture balance model and has a realistic bathymetry and land configuration. For each cold and warm climate state, we perturbed the initial spun-up state twice by eliminating all sea ice (“all water”) and by prescribing a global initial sea ice cover (“all ice”) and ran these models into steady state. No significant NH multiple sea-ice states were observed in our model under present-day like conditions. However, when repeating the experiments under colder climate conditions, two distinct NH steady-state sea-ice states were found, in which the zonally averaged meridional sea-ice extent differs by a modest amount of about three degrees latitude. For the SH two sea-ice states that differed from each other by three to four degrees in latitudinal extent were observed for all experiments. Previous studies reported multiple states of sea ice such as a global sea ice cover, ice-free ocean and intermediate sea-ice cover. We show here that it is possible to obtain multiple states of sea ice that all correspond to an intermediate sea-ice cover and may be relevant to glacial climate dynamics.
While our results support the hypothesis of multiple sea-ice states (both in the NH and SH) under sufficiently cold conditions, the difference between the states, up to four degrees latitude, may be too small to support the sea-ice switch mechanism (Gildor and Tziperman 2000). However, the atmospheric model used here is simple and many feedbacks involving air-sea interaction are missing (e.g., the winds are constant in this model). It is possible, therefore, that with a more realistic atmospheric model, different multiple sea-ice states (more or less pronounced) may be observed. We used annual-averaged forcing, and multiple equilibria that exist under such conditions may disappear once seasonal forcing is introduced, due to the large seasonal cycle in sea ice extent. It is instructive, though, to first perform this study without a seasonal cycle as done here, before proceeding to the more realistic case.
We observed abrupt transitions between a warm state associated with a strong MOC and a small sea-ice cover, and a cold state with a weaker MOC and a larger sea-ice cover. The transitions are between quasi-steady states, although one could envision these states to be even more stable and longer-lasting in a different model configuration with different model parameters. Such transitions were previously suggested to be a possible mechanism for the climate signal of DO and Heinrich events (Kaspi et al. 2004; Dansgaard et al. 1989; Alley et al. 1993; Bond et al. 1992; Heinrich 1988). In particular, these studies showed that small MOC changes can lead to a finite sea-ice response, which then leads to a dramatic atmospheric temperature response, consistent with the proxy record of DO events (see also Li et al. 2005).
As mentioned in Sect. 4, the interaction between MOC and sea ice was discussed in many previous studies, mainly in relation to DO events. The results reported here are relevant to some of these studies. First, the steady states of the MOC and sea ice are stable after a transient period—we have extended the runs to cover a time period of 10 kyr and did not observe variations in the steady states. Our results are different from some of these previous studies that reported that the cold state is more unstable than the warm state, though the difference may be due to the simple atmospheric model and annual mean forcing used here. Second, as depicted in Fig. 7c, f, the cold state is not always unstable. We find that, before converging to the final states, the MOC switches from a strong to a weak state and the sea-ice cover becomes more extended at this transition.
There are at least two main mechanisms that are candidates for generating multiple sea-ice states. The first is the ice-albedo feedback, and the second is linked to MOC dynamics and multiple-equilibria. In studies that reported very different sea-ice states, for example, Marotzke and Botzet (2007) and Ferreira et al. (2011), these different states are mainly associated with the ice-albedo effect because for global scale sea-ice differences the ice-albedo effect is more important. Multiple sea-ice states that do not differ from each other on a global scale (such as those associated with DO events) are more likely linked to MOC dynamics. The NH multiple sea-ice states reported here are at least partially associated with MOC changes. It is interesting to note that multiple sea-ice states are observed here (although with small differences between them) even after the different MOC states have relaxed to almost the same state. In addition, we observed interesting multiple sea-ice states in the southern hemisphere, which warrant further investigation not possible here.
This work was supported by the Israel-US Binational Science foundation. ET was supported by the NSF climate dynamics program, grants ATM-0754332 and ATM-0902844 and thanks the Weizmann institute for its hospitality during parts of this work. We thank Ian Eisenman for helpful discussions and suggestions and André Paul for help with implementing the EMBM.