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A Destabilizing Thermohaline Circulation–Atmosphere–Sea Ice

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A Destabilizing Thermohaline Circulation–Atmosphere–Sea Ice 642 JOURNAL OF CLIMATE VOLUME 12 NOTES AND CORRESPONDENCE A Destabilizing Thermohaline Circulation±Atmosphere±Sea Ice Feedback STEVEN R. JAYNE MIT±WHOI Joint Program in Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts JOCHEM MAROTZKE Center for Global Change Science, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 18 November 1996 and 9 March 1998 ABSTRACT Some of the interactions and feedbacks between the atmosphere, thermohaline circulation, and sea ice are illustrated using a simple process model. A simpli®ed version of the annual-mean coupled ocean±atmosphere box model of Nakamura, Stone, and Marotzke is modi®ed to include a parameterization of sea ice. The model includes the thermodynamic effects of sea ice and allows for variable coverage. It is found that the addition of sea ice introduces feedbacks that have a destabilizing in¯uence on the thermohaline circulation: Sea ice insulates the ocean from the atmosphere, creating colder air temperatures at high latitudes, which cause larger atmospheric eddy heat and moisture transports and weaker oceanic heat transports. These in turn lead to thicker ice coverage and hence establish a positive feedback. The results indicate that generally in colder climates, the presence of sea ice may lead to a signi®cant destabilization of the thermohaline circulation. Brine rejection by sea ice plays no important role in this model's dynamics. The net destabilizing effect of sea ice in this model is the result of two positive feedbacks and one negative feedback and is shown to be model dependent. To date, the destabilizing feedback between atmospheric and oceanic heat ¯uxes, mediated by sea ice, has largely been neglected in conceptual studies of thermohaline circulation stability, but it warrants further investigation in more realistic models. 1. Introduction in its sensitivity to changes in the forcing (e.g., Naka- mura et al. 1994, hereafter NSM). The thermohaline circulation (THC) and its sensitiv- Since Stommel (1961) it has been known that models ity to perturbations have become central points in the of the THC can exhibit at least two steady modes. One debate about climate variability. An understanding of is expressed as sinking at high latitudes and upwelling the response of the system to changes in its forcing is at low latitudes and is qualitatively the regime that the essential for trustworthy climate predictions. However, North Atlantic exhibits today. But other modes exist the coupled ocean and atmosphere system is extremely without convection in the North Atlantic, and these complex with numerous nonlinear feedbacks that make modes compete to determine the overall pattern of the data analysis and computer modeling dif®cult. State-of- THC. For example, the THC in coupled general cir- the-art models of the fully coupled climate system are culation models (Manabe and Stouffer 1988) can exhibit very complex and often do not allow a simple insight multiple equilibria either with sinking of salty water in into the system's dynamics. Furthermore, they have of- the North Atlantic (the current mode) or with the sinking ten required order 1 adjustments to the modeled ¯uxes turned off. Manabe and Stouffer (1995) and Schiller et between the ocean and atmosphere to maintain a stable al. (1997) have also found that the sinking mode can climate representative of the current conditions (e.g., be turned off temporarily by high meltwater runoff. Manabe and Stouffer 1988). Therefore, they are likely Many investigators have chosen to investigate the dy- to distort the physics of the climate system, particularly namics of the coupled system with simpli®ed models to clarify the role of the feedbacks in the system. Box models, perhaps the simplest models of all, have been used to identify and analyze feedbacks between the at- Corresponding author address: Steven Jayne, Woods Hole Ocean- ographic Institution, Clark 3, MS 21, Woods Hole, MA 02543-1541. mospheric eddy transports and the THC (NSM; Mar- E-mail: [email protected] otzke and Stone 1995; Tang and Weaver 1995; Marotzke q 1999 American Meteorological Society Unauthenticated | Downloaded 09/27/21 08:08 PM UTC FEBRUARY 1999 NOTES AND CORRESPONDENCE 643 1996). The idealized models have not, however, in- cluded the effects of sea ice. Recently, Nakamura (1996) investigated the feedbacks owing to ice albedo and run- off from land on the stability of the THC and found the primary feedback, the storage of precipitation in the polar latitudes as ice, to be stabilizing for the climate system. Since much of the direct forcing for the THC takes place at high latitudes, sea ice probably plays a signif- icant role in the climate system; however, the exact na- ture of its interaction with the THC is unknown. Sea ice directly affects the density of the ocean in two ways. First, when sea ice forms, it excludes brine that can elevate the salinity and hence increase the density of FIG. 1. Vertical cross section of the model, with ¯ux arrows given the water beneath it. However, sea ice also decreases in the positive direction. density because it insulates the warmer ocean from cool- ing by the overlying cold atmosphere. It is not obvious climate without any sea ice to a threshold at which sea which of these two effects is dominant and how they ice forms and temperature gradients must change. What interact with other parts of the system. The effects of then are the pure sea ice effects on the THC's stability sea ice coupled to the THC have been investigated by under the two scenarios as opposed to those compound- box models (Yang and Neelin 1997b), two-dimensional ed by externally forced atmospheric cooling and reduc- domains (Yang and Neelin 1993; Yang and Neelin tion in moisture content? 1997a), and idealized three-dimensional models (Zhang We address this question in a similar manner as NSM, et al. 1995). Yang and Neelin (1993) found that brine Marotzke (1996), and Rahmstorf (1995), who analyzed rejection is an important dynamical effect and may lead the impact of atmospheric feedbacks, by tuning an ice- to periodic behavior in the model. While Zhang et al. free and an ice-admitting model to the same THC state. (1995) also found periodic behavior, they pointed to the In contrast, Lohmann and Gerdes (1998) only investi- insulating effect of ice on the water below as the source gated the impact of sea ice on THC±atmosphere inter- of the oscillations. However, none of these investiga- actions but not on the overall model stability. Hence, tions have included a dynamic atmosphere in their mod- they were unable to say whether the presence of sea ice els. Recently, Lohmann and Gerdes (1998) used an was or was not a stabilizing in¯uence. Here we use the ocean general circulation model coupled to a one-di- much cruder box model to focus on a complementary mensional atmospheric energy balance model and a question: Does the presence of sea ice stabilize or de- thermodynamic sea-ice model to investigate the stability stabilize the THC? More precisely, we try to mimic the of the THC. Their main ®nding was that when sea ice comparison between a colder mean climate against one was present, the response of atmospheric heat ¯ux to warm enough that, under the same meridional temper- sea surface temperature changes stabilized the THC, in ature gradient, sea ice would not appear. We investigate contrast to the ®ndings of ice-free models where at- the behavior of the ocean±atmosphere±sea ice system mospheric heat transport changes destabilize the THC by combining the coupled atmosphere±ocean model of (NSM; Marotzke 1996). NSM with a simple model of the two principal effects As our summary shows, the published literature on of sea ice: brine rejection and insulation. Our model is THC±sea ice interaction is small, and the only paper we very simple, but serves to identify some of the funda- are aware of that describes THC±atmosphere±sea ice mental feedbacks related to sea ice and the role of sea interaction has just recently been published (Lohmann ice in the climate system. and Gerdes 1998). On the other hand, there have been numerous publications on THC±atmosphere interac- tions, as partially summarized in Marotzke (1996) and 2. Model Rahmstorf et al. (1996). It is unclear which conclusions A thermodynamic parameterization of sea ice is added drawn from the many ``ice free'' models carry over to to a model with the same geometry and physics as that the case including sea ice. Moreover, the question arises of NSM except that the parameterizations for the heat whether the presence of sea ice would stabilize or de- and moisture ¯uxes in the atmosphere have been sim- stabilize the THC or, conversely, whether the absence pli®ed. The model represents an ocean basin with three of ice in models arti®cially destabilizes or stabilizes. boxes: the sea ice by a single box partially overlaying Not only is this relevant for the interpretation of pre- a portion of the northern ocean box and the atmosphere vious model results, but it also bears directly on ice with two boxes (Fig. 1). The upper boxes represent the effects on the THC's stability in colder or warmer cli- zonally averaged atmosphere between the equator and mates. With the THC and meridional temperature gra- 358N and 358N and the North Pole. The ocean's three dients ®xed, global cooling could lead from a warm boxes span an area of 608 of longitude between 10.448 Unauthenticated | Downloaded 09/27/21 08:08 PM UTC 644 JOURNAL OF CLIMATE VOLUME 12 and 358N and 358 and 758N. The surface areas of the Closely following the derivation of Ahmad et al.
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