The influence of sea ice on ocean heat uptake in response to increasing CO 2

Transcription

1 For JCLI CCSM Special Issue The influence of sea ice on ocean heat uptake in response to increasing CO C. M. Bitz, P.R. Gent, R. A. Woodgate, A. Hall, M. M. Holland, and R. Lindsay Abstract.. Introduction XXX Decide whether to use deep in OHU The uptake of heat by the deep ocean influences the pattern of surface warming as well as the rate of climate change resulting from increasing greenhouse gases. Modeling studies show that the polar and subpolar regions regulate the most important mechanisms that give rise to increasing deep ocean heat uptake in global warming scenarios [Gregory, ;?]. Although sea ice is known to influence the oceanic stability and circulation beneath it [e.g.,??aagaard and Carmack, 994], deep ocean heat uptake has not yet been related to changes in sea ice. Instead sea ice is well known for it contribution to global warming through ice-albedo feedback??. This study investigates the potential for processes asociated with sea ice to amplify both the surface warming over sea ice and the rate of ocean heat Polar Science Center, University of Washington National Center for Atmospheric Research UCLA uptake. The rate of warming in response to increasing greenhouse gases in models is often highly asymmetric between hemispheres. Manabe et al. [99] attributed the relatively slow warming in the southern hemisphere to the deep vertical mixing of heat in the Southern Ocean, in addition to the greater fraction of ocean. Manabe et al. also noted mixing in the Southern Ocean weakened with increasing CO forcing. Gregory [] found that weakened vertical mixing in turn reduced upward diffusion of heat along isopycnals below the mixed layer in the Southern Ocean, which causes considerable warming at depth near Antarctica.? verified the results of Gregory [] for the Southern Ocean using the adjoint of an ocean general circulation model. It is noteworthy that Manabe et al. [99], Gregory [], and? did not related the high ocean heat uptake in the Southern Ocean to changes in sea ice. One of the goals of this study is to determine the extent to which the Southern Ocean warming depends on sea ice changes, and how this influences the overlying atmosphere.

2 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY A second goal of this study is to investigate the cause of warming below the surface in the Arctic Ocean from increasing greenhouse gases that is seen in many models [e.g., Manabe et al., 99; Gregory, ; Raper et al., ;?]. This warming is likely to related the increase in northward ocean heat transport that occurs north of approximately 6 N in most models [see Fig. 8 in Holland and Bitz, ]. Strengthened incursions of Atlantic flow into the Arctic Ocean below the surface were also found by? in a model forced with increasing CO. Our study investigates the potential for sea ice to influence such heat transport and warming in the Arctic Ocean. In this study we identify the climate change caused by reducing the summer sea ice to an extent that is characteristic of a global warming scenario. We initiate this climate change by artificially reducing the sea ice albedo. We present results from this experiment as well as our analysis of the response to increasing CO at the rate of % per year.. Experiments We use version of the Community Climate System Model (CCSM) [Collins et al., ] for a series of experiments to investigate the influence of sea ice on climate sensitivity and ocean heat uptake in global warming. The sea ice component of this model is described in detail by Holland et al. [] and Briegleb et al. [4]. To isolate the climate change forced by a reduction in summer sea ice coverage we artificially and abruptly reduce the albedo of sea ice (both bare and snow covered). The albedo of ice and snow on land is unchanged. The albedo of sea ice in CCSM is decomposed into two spectral bands, denoted visible and infrared, for wavelengths above and below 7nm. The albedo of each band is a parameterized as a function of snow depth, sea ice thickness, and surface temperature. In our integration, we reduce the visible (infrared) albedo when the sea ice snow covered by.8 (.) and when the sea ice is bare by. (.6). Reducing the albedo causes the sea ice to melt earlier and faster, which in turn causes the area of seasonal ice to expand and the mean ice thickness and concentration to decline. The albedo is lowered to produce approximately the same minimum summertime ice cover that is reached at equilibrium when CO is twice the 99s level. Reducing the albedo accomplishes this without creating an imbalance in the surface fluxes, which is is not the case if the sea ice cover is prescribed. Moreover, reducing the albedo does not alter any feedbacks associated with the sea ice, except to the extent that feedbacks depend on the basic state of the sea ice. The amount the albedo must be reduced was determined from a series of experiments with a slab ocean model, rather than the full ocean general circulation model, so an equilibrium was reached in a only a few decades. We then carried out an integration with a full ocean general circulation model where we reduced the ice albedo at the start and integrated the model for years. Recently Hansen and Nazarenko [4] estimated the effect of increasing soot on sea ice by decreasing the sea ice albedo, but only by a few percent. They found that for a given change in radiative forcing (the net radiation at the top of the model atmosphere), reducing the sea ice albedo is about twice as effective as increasing CO in altering global surface air temperature. Our forcing is much larger than theirs and is not intended to represent the effects of soot. Instead we attempted to create a reduction in sea ice cover comparable to that which occurs in response to the radiative forcing of doubling CO. In addition our experiment is conducted with an ocean general circulation model, while Hansen and Nazarenko [4] used a slab ocean model with diffusive mixing of heat into the deep ocean. Our albedo reduction experiment is compared to a control integration with 99s forcing conditions [Collins et al., ]. Additionally, the response to reducing the sea ice albedo is compared to the response to a pair (two ensemble members) of integrations with increasing CO. In the latter case, CO was increased at the rate of % per year for 6 years, which we refer to as the CO ramp run. Our analysis presented here includes only output from the first 8 years, although we did analyze the full 6 year record. CO doubles in year 7 of the CO ramp runs, so most of our discussion is confined to years 6-8, which we refer to as the time of CO doubling. We present results from the same period of the integration that was forced by reducing the sea ice albedo, that is an average of 6-8 years after the albedo was abruptly reduced. To avoid confusing long term transients due to spin-up with the transient response to the forcing, we compare output from the forced runs and the controls in the same years since the forced runs were branched from their respective control.. Results.. -m temperature In response to reducing the sea ice albedo, the change in -m air temperature, shown in Fig. a, has a strong asymmetry when averaged over years 6 8 of the integration. The warming is about C over sea ice in the Arctic, but just. C over sea ice in the Antarctic. The Southern Hemisphere has about equal areas of significant cooling and significant warming. The asymmetry of the warming in response to reducing the sea ice albedo is even greater than the asymmetry of the warming that results in the CO ramp run (see Fig. b) at the time of CO doubling (average of years 6 8 in the CO ramp run). One might imagine that the warming asymmetry in

3 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY (a). (b) 4... Figure. Change in -m surface temperature in C resulting from lowering sea ice albedo (a) and doubling CO (b). In (a), the grey dots mark gridcells with significant warming or cooling. In (b), grey X s mark gridcells with significant cooling. Nearly all other gridcells in (b) have significant warming. Fig. a and b might be due, at least in part, to an asymmetry in the feedbacks of the system. However, it is well known that the surface warming is far more symmetric in models with surface mixed layer physics in place of a full ocean general circulation model [?]. The equilibrium warming in response to doubling CO in CCSM using a simple slab ocean formulation [see Kiehl et al., ] is quite symmetric. The same is true for the response to reducing the sea ice albedo in CCSM with a slab ocean (not shown). Therefore if feedbacks (unrelated to deep ocean heat uptake) explain part of the asymmetry, the feedbacks must necesarily involve a deep and/or dynamical ocean. An alternative to this idea is the possibility that increased deep ocean heat uptake is driven, at least in part, by the reduction of sea ice coverage. If the latter is true, then sea ice influences both climate sensitivity and deep ocean heat uptake. The slight, but significant, cooling north of the Ross Sea in Fig b is noteworthy. Most climate models exhibit a local warming minimum in the Southern Ocean in response to increasing greenhouse gases [?]. The local minimum results from reduced convection, so less heat is mixed upwards from below the mixed layer [Manabe et al., 99]. Convection is thought to weaken because surface heat loss is reduced when the overlying atmosphere warms. The warming in the Southern Ocean, and hence the degree of asymmetry globally, in models has been shown to depend on ocean model mixing schemes [Wiebe and Weaver, 999; Hirst et al., 996; Flato and Boer, ]. If convection is also weaker in the case where the sea ice albedo is lower, we must question this assumption because there is virtually no surface air warming in sea ice-free portions of the Southern Hemisphere in Fig. a. A question along these lines was posed by Gregory [] and remains unanswered: Is the reduction in surface heat loss a consequence or a cause, of the reduction in convection? Before addressing these specific questions, we describe the sea ice response to the two types of forcing. (XXX RON SUGGESTS MOVING THIS TO THE INTRO - too repetitive).. Sea ice Figures and show the change in the annual mean ice thickness and ice concentration in each hemisphere. Here and henceforth, Change refers to the average for years 6 8 after abrupty reducing the sea ice albedo or after beginning to increase CO. Compared to doubling CO, reducing the sea ice albedo produces about 6% of the reduction in ice thickness in the northern hemisphere but nearly an equivalent reduction in ice thickness in the southern hemisphere. The ice thins most where it was initially the thickest in the control integration (not shown here, but the Arctic mean thickness is shown in Fig of Holland et al., this issue). This fundamental characteristic of ice thinning in response to climate forcing was explained by Bitz and Roe [4]. (XXX) HOW CLOSE TO SAME SUMMER MIN DID I GET? The change in ice concentration is considerably greater in response to doubling CO than reducing the albedo in either hemisphere. We believe this is because reducing the albedo only directly contributes to reducing the ice coverage when there is sunlight. Thus any climate change that occurs during winter is a result of feedbacks in the climate system. For example, the surface is warmer in winter because the ice is thinner and less insulating and from additional heat stored in the upper ocean from solar radiation absorbed durring summer.

4 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY 4 Figure. Change in annual mean ice thickness in m (upper) and ice concentration in % (lower) resulting from reducing sea ice albedo (left) and from doubling CO (right) for the northern hemisphere.

5 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY Figure. As in Fig. but for the southern hemisphere.

6 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY S S N 6N 6S S N 6N S S N 6N 6S S N 6N Figure 4. The change in zonal mean ocean potential temperature in C (upper) and ocean ideal age in yr (lower) resulting from reducing sea ice albedo (left) and from doubling CO (right). Indeed most of the warming in Fig. a occurs primarily durring the cold season, although the radiative forcing occurs in summer. The winter warming only indirectly contributes to reducing the ice coverage in winter. In contrast, increasing CO yields a direct radiative forcing year round, in addition to these and other indirect effects during winter... Global ocean heat uptake Figure 4a shows the zonal mean potential temperature change in the global oceans in response to reducing the sea ice albedo. Like the surface air temperature change, the upper ocean temperature change also has a strong asymmetry. In mid-latitudes of the southern hemisphere, the upper ocean cools by about /4 C. In contrast in mid-latitudes of the northern hemisphere, the upper ocean warms by about / C. The greatest warming is in the Arctic at about.7 C just beneath the mixed layer. Perhaps most interesting is the large warming centered at about km depth in the Southern Ocean, with a maximum of.6 C. In response to doubling CO, the potential temperature change in CCSM (Fig. 4b) has features in common with other models [e.g., Manabe et al., 99; Gregory, ; Raper et al., ; Flato and Boer, ]. Deep ocean mixing in the Southern Ocean delays warming of the mixed layer of the Southern Ocean compared to elsewhere, so the upper ocean there has warmed relative little by the time of CO doubling. In addition, weakened convection eliminates a source of heat to the mixed layer from below [Manabe et al., 99]. Warming below km is greatest in the Southern Ocean, which has been attributed to reduced upward diffusion along isopycnal surfaces below the mixed layer [Gregory, ]. The warming in the upper km but below the mixed layer in the tropics is due to reduced upwelling strength, as the meridional overturning circulation (MOC) is weaker throughout most of the world ocean in the warmer climate [Manabe et al., 99]. The very intense warming in the Arctic is typical of models that employ the Gent-McWilliams mixing scheme [Wiebe and Weaver, 999; Gent and Danabasoglu, 4]. Comparing the potential temperature change in response to reducing the sea ice albedo and doubling CO (Fig.??a and b), we see many features in common below about km depth. However the deep ocean warming in the Southern Ocean is greater in the former. The upper ocean warms much less from reducing the sea ice albedo, as the climate forcing is only imposed over sea ice. Nonetheless, the pattern of warming just below the surface in the Arctic Ocean is quite similar in either case, although the magnitude is much smaller in response to reducing the sea ice albedo. The relative magnitude of the warming asymmetry in the ocean in

7 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY 7 response to these two cases is consistent with the magnitude of sea ice thinning. Recall that the northern hemisphere sea ice thickness change is considerably larger in response to doubling CO than reducing the sea ice albedo (Fig. ), while the sea ice thicness change is more comparable in the southern hemisphere (Fig. ). In either experiment, the considerable warming at depth in the Southern Ocean is not within reach of Antarctic sea ice, where it might enhance melting. The Arctic is quite the opposite. Temperature increases in the Arctic are much nearer to the surface and even at the surface in the Barents Sea in the CO ramp run, at locations where sea ice is present in the 99s control run. PW.... 6S S N 6N Latitude Figure. The change in zonal mean northward heat transport in the global oceans resulting from doubling CO (solid) and lowering sea ice albedo (dashed). W m 6S S N 6N Latitude Figure 6. The change in zonal mean surface heat flux in the global oceans resulting from doubling CO and lowering sea ice albedo (dashed) Evidence of reduced deep water production in the mid to high latitudes is seen from the change in ideal age shown in Figs. 4c and d. Ideal age is a tracer of the time since sea water has been at the surface Bryan et al. []. A relative increase in ideal age at depth is an indication of reduced deep water formation. The increase in ideal age from 7N (Fig. 4d in particular) is evidence of a weakening of the thermohaline circulation, which is confirmed by a reduction in the meridional streamfunction in the North Atlantic [Bryan et al., ]. The ideal age increase around Antarctica indicates a reduction in vertical mixing and deep water formation, or more specifically bottom water formation. There is a shallow region hugging the Antarctic continent where the water is younger in Figs. 4c and d. Reducing sea ice albedo produces younger water in the upper ocean at about 6-7 S too. These regions with younger water do not span the whole latitude circle, rather they are relatively localized but large enough in magnitude to affect the zonal mean. (Section.4 includes more detail about horizontal patterns of ideal age in the Southern Ocean.) The upper ocean of the tropics appears slightly less old as a result of weakened upwelling there. These changes in vertical advection and mixing that are inferred from the ideal age diagnostic fit well with the interpretation of changes in circulation drawn from ocean temperature in Figs. 4a and b. Another area that deserves further scrutiny is the upper km of the Arctic Ocean in Fig. 4. A question arises from these model results: Could it be that the increase in temperature and decrease in ideal age in the Arctic is connected to the increase in horizontal heat transport that, as mentioned in the Introduction, occurs in most global climate models when forced with increasing CO? Indeed, the zonal mean northward heat transport in the global oceans (Fig. ) (the combined Eulerian mean plus eddy or bolus heat transport) increases north of about 6 N in both of our experiments. Figure 6 shows the change in zonal mean heat flux with the sign convention such that a positive change is an increase in ocean heating, or a decrease in heat loss as is the case in the polar regions. Interestingly Fig. 6 indicates that in the Arctic, north of about 7 N, surface heat loss is greater, which is not too surprising as the sea ice cover diminishes a great deal there. In section. we further analyze these changes in the Arctic. In the southern hemisphere, we see the surface heat loss is reduced in the Southern Ocean with either forcing (Fig. 6), while horizontal heat transport is nearly unchanged south of 6S. The substantial increase in northward heat tranport north of about 6S serves to remove heat from about 6 S and deposit it at about 4 S. The large reduction in surface heat loss of W m along the Antarctic coast in Fig 6 occurs in several small, polynya-like, areas on the coast of Antarctica, where the sea ice concentration actually increases in the wintertime. The increase in sea ice concentration occurs at such small scale that it is difficult to see in Fig, particularly as the figure shows an annual mean and not just a winter mean. These very significant changes in ocean heat uptake small amplitude below about m and extending down very deep in the Southern Ocean and large amplitude but in a relatively shallow layer in the Arctic appear in the vicinity of sea ice cover. Figures 4 6 show that the changes are roughly the same in the polar ocean when the forcing is either from ramping CO or reducing sea ice albedo. It is curious that the relative

8 7. 8 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY 8 depth m depth m ocean warming and change in ideal age in the Southern Ocean is much greater from lowering the albedo and in the Arctic Ocean is much greater from ramping CO. We attempt to explain this apparent difference in the next two sections (XXX) S 7S 6S S 4S S 7S 6S S 4S Figure 7. Zonal mean potential temperature in C and potential density countours in kg m in the control integration from 4-8 S..4. Ocean heat uptake in the Southern Ocean Now we focus on the problem of heat uptake by the Southern Ocean. In section., we found that the deep ocean warmed in the southern hemisphere in response to either ramping CO or reducing the sea ice albedo. Here we diagnose the cause of this warming. As discussed in Gregory [], the warming below the mixed layer in the Southern Ocean in response to increasing CO is dominated by reduced convection and isopycnal diffusion. The reduction in convection is inferred from the increased buoyancy in surface waters that results from the reduced surface heat loss (see Fig. 6) and the increased ice age below the mixed layer in the Southern Ocean. The vertical temperature profile in the Southern Ocean exhibits a temperature maximum at about m (see Fig. 7). A loss of convection reduces entrainment of water from this warmer layer into the mixed layer, so the temperature maximum intensifies further. This in turn decreases the temperature gradient along isopycnal surfaces below the mixed layer (see Fig. 7), which then gives rise to reduced isopycnal diffusion. Hence reduced convection lessens a sink of heat from the m deep layer and reduced isopycnal diffusion lessens a sink of heat from even greater depths. Figure 8a and c show the change in temperature and ideal age at 8 m depth around Antarctica in response to reducing the sea ice albedo. The water bordering Antarctica is considerably warmer and older in three somewhat distinct patches in each ocean sector (Atlantic, Indian, and Pacific), with the greatest increase in temperature and age occurring in the Atlantic sector. The water is cooler and younger in a narrow swath just to the north of the warmer and older patch in the Atlantic sector and in three small spots along the Antarctic continent between the warmer-older patches. The relationship between age and temperature is strongest in regions that are ice covered in winter. The changes in temperature and ideal age at 8 m in response to ramping CO (see Fig. 8b and d) corresponds well with the response to reducing the sea ice albedo (see Fig. 8a and c). However the warming is greater outside of the Southern Ocean in response to ramping CO. The areas in Fig. 8b that are younger also tend to be more extreme than in Fig. 8d. The cooler younger swath in the Atlantic in either experiment is indicative of a local increase in convection, which may result from a reduction in sea ice melt (not shown) at the outer margins of the ice cover, as the ice thins and less ice is transported there. However, generally the match between reduced surface buoyancy forcing and the areas of warmer-older water are much better than the match between the areas of increased surface buoyancy forcing and areas of cooler younger water (not shown). Nonetheless if we interpret the changes in ideal age as mostly driven by changes in convection, two results give us some confidence that changes in the sea ice are involved in this process: (i) The largest changes are almost entirely within the average July-August sea ice cover and (ii) The spatial pattern is remarkably similar in response to either type of forcing in Fig. 8, albeit with somewhat greater amplitude for the reduced sea ice albedo experiment. The pattern of temperature change in Fig. 8b and d reflects the changes in convection as well as the vertical temperature profile. Areas where the vertical temperature profile peaks at this depth become warmer if mixing is reduced. This is the case in the warmer-older patches in the Atlantic and Indian sectors. In the Pacific sector, something strange is happening that needs further analysis (XXX). Isopycnals slope upwards to the south (Fig. 7) and

9 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY 9 Figure 8. Change in potential temperature in C (top) and ocean ideal age in yr (bottom) at 8 m depths in response lowering sea ice albedo (left) and doubling CO (right). the temperature along these isopycnals decreases southwards at all depths and upwards between - m depth. Isopycnal diffusion is reduced by either decreasing the southward temperature gradient or the upwards temperature gradient. Reduced mixing influences the vertical and horizontal temperature gradients, but most importantly it decreases the horizontal gradient a great deal at roughly S, on the northern edge of the warm patches in each ocean sector along Antarctica (see Figs. 8b and d). Following the same procedure used by Gregory [], we estimate (not shown) a substantial reduction in the upward heat flux from isopycnal diffusion results from this reduced horizontal temperature gradient. Lowering the sea ice albedo is particularly effective at reducing the horizontal temperature gradient. Thus, the upward isopycnal diffusion is reduced considerably more in the lowered sea ice albedo experiment than in the CO ramp run. However, ice-albedo feedback, which amplifies the polar response for either type of forcing, contributes to reducing the horizontal temperature gradient, and hence ice-albedo feedback influences climate sensitivity and ocean heat uptake. Figure 9 shows the resulting temperature change and ideal age at. km. The warming is greatest in the vicinity that is thought to be occupied by Antarctic Bottom Water (AABW) formed through dense water production off the ice shelves and in sea ice polynyas (ref XXX)... Horizontal Heat transport into the Arctic We have already argued that Figs and 6 indicate that the warming in the Arctic is advective. Now we aim to show what drives this advection. We analyze the CO ramp run because the Arctic Ocean warming is much larger in response to doubling CO than reducing the sea ice albedo. We assume this is the case because the sea ice change is much larger from doubling CO.

10 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY Figure 9. As in Fig. 8 but for. km depth.

11 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY (XXX WHAT ABOUT THE WINDS?) Northward heat transport, such as shown in Fig, is not ideally suited to show changes in heat transport into the Arctic from the northern North Atlantic. Because it is averaged along latitude circles, the heat transport at say 8N includes not only northward heat transport from the GIN seas but also northward heat transport from the Beaufort Sea. A better projection is shown in Fig. c along the line from Iceland to the Siberian Coast. Figure a shows the overturning circulation for the 99s control simulation along this line. About Sv of the Atlantic overturning circulation penetrates the GIN seas, of which about. Sv reaches into the Arctic Ocean. Heat transport (Fig c associated with this overturning, is about. PW in the GIN Seas. In the CO ramping experiment, the overturning circulation into the Arctic Ocean in the upper km has an increasing trend for at least 6 years. Figure shows the overturning anomaly in three successive year intervals. The Atlantic overturning circulation south of Iceland weakens over time and affects the higher latitudes, but mostly only below about km in the Arctic Ocean. We interpret Fig to indicate that the Atlantic thermohaline circulation weakens put penetrates further into the Arctic. Associated with the strengthened overturning in the Arctic, we see heat transport increasing by about.4 PW into the Arctic Ocean at the time of CO doubling (Fig. a). The heat transport change may be broken into independents components due to the change in temperature of the water being advected and the change in current velocity: (vt) v T + vt, assuming v T is small. These components for the 4-6 year interval of CO ramping experiments, contribute about / from the anomalous current velocity and about / from the anomalous temperature (see Fig. b). During the 6-8 year interval the components contribute about equal portions (not shown), as the anomalous temperature increases while the anomalous current velocity stays about the same. (say something about how the wind stress change is inappropriate to drive v XXX. Also Weibe and Weaver s estimate of advective timescale of change in heat transport.) If this additional.4 PW of heat at the time of CO doubling could reach the sea ice, it would melt the entire sea ice cover in less than a decade. Clearly some heat is stored in the ocean below the mixed layer, as seen in Fig. 4, and some of it is lost through open ocean and leads. (Rough estimates of how much would be nice here XXX) Nonetheless, the ocean-ice heat flux (heat derived from turbulent mixing near the surface) increases where sea ice has not yet melted away in the warmer climate. Figure a shows the change in oceanice flux in February is about W m higher near the North Pole and is in excess of W m along the ice edge in some regions. We chose the month of February in an attempt to eliminate as much as possible any change in heat storage via absorbed shortwave radiation the previous summer. Maykut and Untersteiner [97] showed that an increase in ocean-ice heat flux of this size could have a devastating effect on tsea ice cover. The dashed and solid lines in Fig. a indicate that the shift in the 8% sea ice concentration contour in February (often used to define the ice edge) correspond to the change in ocean-ice heating to some extent, although ice transport may also be a considerable factor [see Bitz et al., ]. PW. Siberian Shelf 4 GIN Fram Seas Strait Arctic Ocean..... Distance from North Pole 6 km Distance from North Pole 6 km Figure. Overturning circulation (a) in the 99s control and heat transport (b) along the transect shown in panel (c). Contour interval is. Sv (a). In the face of increased precipitation and runoff and weaker air-sea fluxes in a warmer climate in the subpolar North Atlantic Bryan et al. [], increased overturning and heat transport into the Arctic may seem surprising. However, more than a decade ago, Aagaard and Carmack [994] argued that ventilation in the Canadian basin could increase if ice production increased on the Siberian Shelf and in the Canadian Basin. By computing the buoyancy forcing, we further

12 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY.. 4 GIN Seas Fram Strait Arctic Ocean Siberian Shelf PW Distance from North Pole 6 km 4 GIN Seas Fram Strait Arctic Ocean Siberian Shelf PW Siberian Shelf 4 GIN Fram Seas Strait Arctic Ocean..... Distance from North Pole 6 km Figure. Change in overturning circulation in the CO ramping experiment along the transect shown in panel Fig. (c) in year intervals for year averages, beginning with years 4. Carbon dioxide levels reach twice the 99s level in the lowest panel. Contour intervals is. Sv (a) Distance from North Pole 6 km Figure. (a) Change in heat transport in the CO ramping experiment along the transect shown in panel Fig. (c) for the same three time periods shown in, years 4 (dashed), years 4 6 (dot-dash), and years 6 8 (solid). (b) Change in heat transport (solid) and its components v T (dot-dash) and vt (dashed) for years 4 6 of the CO ramp run, corresponds to overturning in middle panel of Fig..

13 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY argue that increasing ice production in the central Arctic also increases overturning and heat transport into the Arctic Ocean. First we show that ice production in the central Arctic increases as sea ice thins with increasing CO concentration. This unintuitive relationship follows from the inverse dependence of heat conduction (and hence sea ice growth) on ice thickness. The relationship breaks down where the ocean-ice heat flux dominates over the conductive flux in controlling growth, such as where solar absorption in the ocean is high or turbulent mixing brings heat to the ice in the GIN Seas and Southern Ocean. Therefore we only expect ice production to increase with increasing CO in the central Arctic until ice-free conditions prevail during summer and into the fall. Figure b shows ice production does indeed increase in the central Arctic when CO is increased. In fact the maximum increase in ice production occurs at about year in the CO ramping experiments (recall that doubling occurs in year 7). This corresponds roughly with the time of maximum increase in the overturning circulations shown in Fig.. Next we compute the buoyancy forcing due to the total freshwater supply to the ocean (negative for sea ice production and evaportion) with that due to heat loss through the surface of the ocean in the Arctic during winter. (I have had a miserable time making sense of this XXX). We are interested in the increase in buoyancy forcing as it relates to ocean heat uptake in the Arctic Ocean and heat exchange between ocean and sea ice that may further accelerate sea ice decay. The vertical projections of ocean heat transport into the Arctic show that heat transport increases near the surface in the GIN seas and in the vicinity of Fram Strait. A horizontal map (not shown) of heat transport in the upper ocean shows the increased heat transport follows a path that is primarily east of Svalbard and into the Barents Sea. This path is well matched with the region where the ice edge retreats most in winter and surface air warming exceeds 6 C on average in Dec-Feb at XCO - more than anywhere else on the globe. Figure. Change in ocean-ice flux in W m for February (a) and annual ice production (excludes ice melt) in cm (b) resulting from doubling CO. 4. Conclusions Two very significant changes in ocean heat uptake occur in the vicinity of sea ice cover: a deep warmings in the Antarctic from about m and extending down several kilometers and large amplitude warming in the Arctic mostly below the surface but in a relatively shallow layer. These characteristics of the warming in the high latitude oceans occur when the forcing is either from ramping CO or lowering sea ice albedo. Changes in the Arctic Ocean are due to an enhanced incursion of heat from the northern North Atlantic that is driven by an increase in convection along the Siberian

14 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY 4 shelf. This convection is driven by an increase in wintertime ice production along the shelf, as the ice cover changes from perennial to firstyear (thinner ice grows faster). Increased convection is local to the central Arctic only. In contrast, convection is reduced in the northern North Atlantic south of the Scotland Ridge owing to higher precip and runoff. While the meridional overturning circulation south of the Scotland Ridge is weaker, overturning strengthens into the Arctic Ocean with sinking along the Siberian shelf and in the Canadian Basin. Eventually when future CO levels exceed a doubling, less and less sea ice grows in winter. At some point, the overturning circulation into the Arctic ceases and the enhanced northward heat transport into the Arctic slowly declines again. The Antarctic, which is covered by firstyear ice at present, is already in the mode where any increase in CO reduces ice production and convection is reduced under Antarctic sea ice right away when sea ice begins to thin. Convection appears to be slightly enhanced at the wintertime sea ice edge in the Atlantic sector as less ice is transported to the outer margins in the warmer climate. Below about m these changes in convection give rise to a weakened horizontal temperature gradient in the Southern Ocean, which significantly reduces isopycnal diffusion of heat upwards around Antarctica. The same mechanisms that give rise to the ocean warming at either pole occur in response to lowering the albedo of sea ice... Our results show that sea ice contributes strongly to the ocean warming. Often it is assumed climate sensitivity and deep ocean heat uptake are unrelated??. Recently, the efficiency of ocean heat uptake in models was found to be higher in models with higher climate sensitivity [Raper et al., ]. Acknowledgments. We thank Computational facilities have been provided by the National Center for Atmospheric Research (NCAR). NCAR is supported by the National Science Foundation. Two of the model integrations were performed by CRIEPI using the Earth Simulator through the international research consortium of CRIEPI, NCAR and LANL under the Project for Sustainable Coexistence of Human Nature and the Earth of the Japanese Ministry of Education, Culture, Sports, Science and Technology. References Aagaard, K. and E. C. Carmack, 994: The Arctic Ocean and climate: A perspective. in O. M. Johannessen, R. D. Muench and J. E. Overland, editors, The Geophysics of Sea Ice, pp.. The polar oceans and their role in shaping the global environment, Goephys. Monograph 8, American Geophysical Union. Bitz, C. M., M. M. Holland, E. C. Hunke and R. E. 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15 submitted to JOURNAL OF CLIMATE: BITZ, GENT, WOODGATE, HALL, HOLLAND, and LINDSAY Peter R. Gent and Marika M. Holland, National Center for Atmospheric Research, P.O. Box, Boulder, CO 8XXXX Alex Hall, UCLA This preprint was prepared with AGU s LATEX macros v., with the extension package AGU ++ by P. W. Daly, version.6b from 999/8/9.