Arctic sea ice response to wind stress variations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jc002678, 2005 Arctic sea ice response to wind stress variations Eiji Watanabe and Hiroyasu Hasumi Center for Climate System Research, University of Tokyo, Chiba, Japan Received 20 August 2004; revised 30 May 2005; accepted 7 July 2005; published 12 November [1] Timescale of sea ice response to wind stress variations and the mechanisms controlling the timescale are investigated by a coupled sea ice ocean model. Wind stress variations account for a significant part of the changes of sea ice volume in the Arctic Ocean over the last several decades. The changes of sea ice volume associated with wind stress are mainly induced by changes of sea ice outflow from the Arctic Ocean. While outflow immediately responds to wind stress variations, responses of sea ice volume lag behind the changes of the outflow by several years. For example, when the model is driven by interannually varying wind stress with the other atmospheric forcing components given by climatology, sea ice outflow abruptly decreases in 1996 and remains almost constant for the following several years. However, the responding increase of sea ice volume is gradual and continues for several years. In order to clarify the timescale of sea ice response to an abrupt change of wind stress and the mechanisms controlling the timescale, the model is forced by two typical wind stress fields, which cause small and large outflow. Equilibrium annual mean ice thickness averaged over the Arctic Ocean is different by about 50 cm between these two wind-forcing fields. Starting from the equilibrium state obtained under one of these two fields, wind-forcing is switched to the other. Adjustment time of ice thickness is then defined as the year when annual mean ice thickness is adjusted to new equilibrium by 90% of the difference between the two equilibria. The timescale of the sea ice response and the mechanisms controlling the timescale are found to be different depending on whether the outflow increases or decreases. When the outflow increases, the timescale depends on the advection time of the sea ice flowing toward the exits. When the outflow decreases, the timescale depends on the thermodynamic growth rate of sea ice. The change in the late 1990s corresponds to the latter case. Citation: Watanabe, E., and H. Hasumi (2005), Arctic sea ice response to wind stress variations, J. Geophys. Res., 110,, doi: /2004jc Introduction [2] Observational and modeling studies have indicated rapid decrease of sea ice in the Arctic Ocean since the late 1980s. Parkinson et al. [1999] show a decreasing trend of 34,300 ± 3700 km 2 /yr ( 2.8%/decade) in the Arctic sea ice extent from 1978 to 1996 on the basis of the ice concentration data derived from the scanning multichannel microwave radiometer (SMMR) and the special sensor microwave imagers (SSM/I). Rothrock et al. [1999] compare the ice draft data acquired by the Scientific Ice Expeditions program between 1993 and 1997 with similar submarine data between 1958 and 1976, and report that the mean ice draft at the end of the melt season decreased by about 130 cm, from 310 cm before the 1970s to 180 cm in the 1990s. Rothrock et al. [2003] use a dynamic-thermodynamic sea ice model coupled to an ocean general circulation model (OGCM) to simulate the sea ice decrease. The model is forced by interannually varying atmospheric data. Daily Copyright 2005 by the American Geophysical Union /05/2004JC sea level pressure and surface air temperature are obtained from the National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) reanalysis data [Kalnay et al., 1996] between 1948 and 1978 and from the International Arctic Buoy Programme (IABP) between 1979 and The modeled ice thickness averaged over the Arctic Ocean decreases by about 80 cm from 1987 to From 1996 to 1999, the mean thickness shows a modest recovery. [3] Previous studies have revealed possible major causes for the decrease, such as the change of wind stress field that causes more sea ice outflow from the Arctic Ocean, the rise of surface air temperature and the increased inflow of the warm Atlantic water [Rothrock and Zhang, 2005; Köberle and Gerdes, 2003; Zhang et al., 1998]. Vinje et al. [1998] show an increase of the measured ice outflow through the Fram Strait in the early 1990s, which corresponds to about 30 cm change in mean thickness in the Arctic Ocean. They point out that the increase is caused by southward anomaly of wind stress over the Fram Strait. Makshtas et al. [2003] employ a dynamic-thermodynamic sea ice model forced by two surface air temperature data sets constructed from the NCEP/NCAR 1of12

2 reanalysis between 1958 and Interannually varying daily surface air temperature is given in one data set, while climatological daily data is given in the other. Wind stress and sea level pressure are interannually varying daily data, and all the other forcing components are climatologically averaged daily data. The interannually varying surface air temperature shows a positive trend over the entire Arctic Ocean, especially in the Beaufort and Chukchi Seas. In both cases, ice is thinner in the 1990s than before the 1970s in the entire Arctic Ocean. Comparison of the two simulation results suggests that the recent increase in surface air temperature accounts for 20% of the ice decrease. Zhang et al. [1998] use a dynamicthermodynamic sea ice model coupled to an OGCM. The model is forced by the IABP data. They suggest that there have been significant warming and salinification in the upper layer of the Arctic Ocean due to increasing inflow of the Atlantic water since the end of the 1980s and that the warming results in more oceanic heat flux to ice cover. [4] There are many studies which relate these factors to the Arctic Oscillation (AO [Thompson and Wallace, 1998]). Rigor et al. [2002] analyze the sea level pressure, surface air temperature and sea ice velocity data obtained from the IABP between 1979 and 1998, and indicate a close relationship between the sea ice variations in the Arctic Ocean and the AO. The sea level pressure over the central Arctic Ocean, especially in the Eurasian side, gets lower, and the anticyclone over the Beaufort Sea diminishes during the late 1980s and the early 1990s. These changes of sea level pressure correspond to the transition of the AO phase from negative to positive. The consequent change of wind stress field intensifies sea ice advection from the Siberian coast to the Greenland Sea through the Fram Strait. Regression of ice velocity to the winter AO index also shows strong Transpolar Drift and weak Beaufort Gyre. The surface air temperature over the entire Arctic Ocean increases in the same period, and it is attributed to enhanced inflow of warm air mass from midlatitudes, which is accompanied by negative anomaly of sea level pressure over the Arctic Ocean. Thus they indicate that the decrease of sea ice during the positive AO phase is caused by the increase of sea ice outflow from the Arctic Ocean and the decrease of net thermodynamic growth of sea ice. Some model studies exhibit similar results. Zhang et al. [2003] use an Arctic iceocean model forced by the NCEP/NCAR reanalysis to compare sea ice responses to two atmospheric forcing fields corresponding to positive and negative phases of the AO. During the positive phase of the AO, the Beaufort Gyre shrinks remarkably and the Transpolar Drift shifts correspondingly toward the Canada Basin, while the Beaufort Gyre becomes enlarged in the negative phase. When the phase of the AO changes from negative to positive, the simulated ice area and volume in the whole Arctic Ocean decreases by 3.2% and 8.8%, respectively. They also mention that the main factors for the reduction are change of wind stress field and increase of surface air temperature. [5] Considerable part of Arctic sea ice variations is accounted for as responses to atmospheric changes, and such responses of sea ice lag behind atmospheric variations. Kauker et al. [2003] calculate lagged regressions between sea ice concentration in each region of the Arctic Ocean and the AO index. The sea ice concentration data is obtained from the simulation in which a dynamic-thermodynamic sea ice model coupled to an OGCM is forced by the NCEP/ NCAR reanalysis from 1948 to The result shows that the time required for sea ice concentration to respond to the AO differs depending on areas. The highest correlation is found at the lag of 6 8 months in the Greenland Sea and of 2 3 years in the Barents Sea (the AO index precedes). When the observational ice concentration derived from the SMMR and the SSM/I is used instead of the simulation result, similar correlation between the concentration and the AO index is obtained. [6] Kwok et al. [2004] summarize the variation of ice outflow through the Fram Strait observed by an upward looking sonar and a synthetic aperture radar between 1978 and 2002, and suggest that there could be a time lag between the Northern Atlantic Oscillation index [Hurrell, 1995] and the outflow change. [7] Sea ice volume in the Arctic Ocean responds to changes in forcing such as wind stress, surface air temperature, and ocean heat flux, and timescale of response to each of them may be different. Actual response time could be interpreted as a combination of such timescales under changes in a single forcing component, though the combination might involve various interactive processes. Consequently, in order to clarify the mechanisms of sea ice variations, it is required to investigate the timescale of sea ice response to each of the forcing components. Krahmann and Visbeck [2003] investigate sea ice response to the Northern Annular Mode-like (NAM-like) wind forcing derived from the daily NCEP/NCAR reanalysis by using an ice-ocean general circulation model coupled to an atmospheric boundary layer model. The wind forcing is constructed by adding NAM-like wind anomaly patterns to the climatological field, and the anomaly modulates sinusoidally with 4 48 year periods. In each experiment the model is run over several forcing cycles to obtain a quasiequilibrium response of sea ice and upper ocean. The results show that the amplitude of ice volume change is the largest for the 20-year period forcing, while the spatial patterns of response of ice concentration and thickness are largely independent of the frequency of the NAM-like forcing. They assume that the response of sea ice volume cannot follow wind stress variations of short periods, and there may be negative feedback for variations of long periods. [8] However, the mechanisms controlling the timescale of sea ice response to wind stress variations are not clarified yet. In this study therefore, the timescale and the mechanisms are investigated by using a coupled ice-ocean model. For this purpose, the model is forced by two typical wind stress fields which cause small and large ice outflow from the Arctic Ocean one after the other. The typical wind stress fields are so chosen that they represent positive and negative index phases of the AO. [9] This paper is organized as follows. The model and the experimental design are outlined in section 2. The results of the experiments are described in section 3. Finally, summary and discussion are presented in section Model 2.1. Model Description [10] The coupled ice-ocean model used in this study is developed at Center for Climate System Research (CCSR), 2of12

3 Table 1. Thickness Categories a Category Category Lower Limit, cm a Category 1 corresponds to open water, so it has no thickness. the University of Tokyo. The sea ice model has both the thermodynamic and dynamic components. The thermodynamic part is the zero layer model of Semtner [1976]. In the dynamic part, equations for momentum, mass and concentration are taken from Mellor and Kantha [1989]. Internal ice stress is formulated by the elastic-viscous-plastic rheology [Hunke and Dukowicz, 1997]. A multithicknesscategory formulation [Bitz et al., 2001] is adopted, and subgrid-scale thickness distribution is represented by 8 categories (Table 1). [11] The ocean part is the CCSR Ocean Component Model (COCO) version 3.4 [Hasumi, 2002], which is a free surface OGCM formulated on the spherical coordinate system. The level 2 turbulence closure scheme of Mellor and Yamada [1982] is applied for determining the vertical diffusivity and viscosity at each level. The horizontal eddy diffusion and viscosity coefficients are m 2 s 1 and m 2 s 1, respectively. The model incorporates the isopycnal diffusion scheme [Cox, 1987] and the uniformly third-order polynomial interpolation algorithm (UTOPIA) [Hasumi and Suginohara, 1999] for tracer advection. The isopycnal diffusion coefficient is m 2 s Bathymetry and Boundary Conditions [12] The model domain is composed of the Arctic Ocean, the Greenland-Iceland-Norwegian Seas and the northern areas of the Pacific and the Atlantic (see the later horizontal maps for the model domain extent, as they are shown for the entire model domain). The model s spherical coordinate system is rotated so that the singular points are on the equator. The horizontal resolution is 1 in the rotated coordinate system, and there are 10 vertical levels between the surface and 500 m, which is the deepest model ocean floor. The vertical grid width varies from 5 m for the top level to 180 m for the bottom level. [13] The atmospheric forcing components other than freshwater flux and wind stress are obtained from the NCEP/NCAR reanalysis data. Sea surface freshwater flux is derived from the climatology of Röske [2001]. Wind stress is calculated from the NCEP/NCAR reanalysis sea level pressure by following the formulation adopted in the Arctic Ocean Model Intercomparison Project [Proshutinsky and Johnson, 1997]. The data frequency for all the forcing components is daily, whether they have interannual variations or not. Temperature and salinity at the margin of the model domain and at the depths deeper than 130 m in the Arctic Ocean are restored to the annual mean of the Polar Science Center Hydrographic Climatology (PHC) data [Steele et al., 2001]. The restoring time is 1 year. Since we intend to investigate direct response of sea ice to atmospheric changes, we don t take into account the influence of changes in the ocean below the mixed layer Experimental Design [14] The experiments are initiated by the annual mean temperature and salinity fields of the PHC data and no sea ice. First, we perform two experiments in order to investigate the contribution of wind stress to sea ice variations. In one case, the model is driven by the interannually varying atmospheric forcing from 1948 to This case is referred to as the all forcing varying (AFV) case. In the other case, only the wind stress has interannual variations, and the other atmospheric forcing components (surface air temperature, downward shortwave radiation, etc.) are given by daily climatology. This case is referred to as the wind stress varying (WSV) case. The climatology is calculated by averaging the data between 1948 and 2002 for each day. [15] Next, in order to investigate the timescale of sea ice response to an abrupt change of wind stress field, the model is forced by two typical daily wind stress fields one after the other. Since significant part of the changes of wind stress field in the Arctic Ocean is accounted for by the AO, the regression coefficients of wind stress to the AO index are added to and subtracted from the daily climatology to produce such typical fields. These wind stress fields corresponding to positive and negative AO phases are referred to as PAO and NAO, respectively. The AO index calculated by using the NCEP/NCAR sea level pressure data is shown in Figure 1. The model is forced by the NAO (PAO) for 20 years from the above mentioned initial condition until the annual mean sea ice thickness averaged in the Arctic Ocean reaches an equilibrium. Then, the wind stress field is switched to the PAO (NAO), and the model is integrated for 20 more years. The NAO to PAO case is referred to as NP, and the PAO to NAO case is referred to as PN, hereafter. 3. Results 3.1. Interannual Variations [16] The sea level pressure and the simulated ice fields in the AFV and WSV cases are depicted in Figure 2. The sea level pressure in the Arctic Ocean becomes higher and the anticyclone over the Beaufort Sea is enlarged from the early 1990s to the late 1990s. The associated variation in wind stress field induces wide changes in the spatial patterns of Figure 1. Twelve-month running mean AO index. 3of12

4 Figure 2. (a) Sea level pressure (contour interval is 1 hpa) and ice thickness (contour interval is 100 cm) and ice velocity in (b) the AFV case and (c) the WSV cases. The unit vector for the ice velocity is 10 cm s 1. Fields averaged for (left) and (right) are shown. ice velocity and thickness. Expansion of the Beaufort Gyre and increase of ice thickness in the central Arctic Ocean and the East Siberian Sea are recognized. Ice is thinner in the AFV case than in the WSV case over the entire Arctic Ocean in the early 1990s. The difference becomes larger in the late 1990s, while the spatial patterns of ice velocity in both cases are basically similar. [17] The time series of the annual mean ice thickness averaged in the Arctic Ocean for the AFV and WSV cases are shown in Figure 3. In the AFV case, the mean thickness decreases by about 60 cm from 1987 to 1995, and then increases by about 15 cm from 1995 to Although the magnitudes of the mean thickness changes are smaller than in the results of previous similar experiments [Rothrock et al., 2003; Haak et al., 2003], the phase of the changes is consistent with theirs. The amplitude of the mean thickness variation in the WSV case is smaller than that in the AFV case. For example, the decrease of the mean thickness is about 30 cm from 1987 to 1995 in the WSV case, which is a half of that in the AFV case. The result suggests that thermal forcing changes have significant contribution to ice volume decrease during this period. However, the 4of12

5 Figure 3. Time series of the annual mean ice thickness (cm) in the Arctic Ocean in the AFV case (solid line) and in the WSV case (dashed line). Figure 5. Annual mean of the ice thickness (contour interval is 100 cm) and the ice velocity for year 20 forced by (a) the PAO and (b) the NAO. The unit vector for the ice velocity is 10 cm s 1. variations in both cases are in phase, and the interannual variations of wind stress field also play a great role in the changes of ice volume in the Arctic Ocean. [18] One of the main factors for the ice volume changes is variations of ice outflow from the Arctic Ocean into the external seas. The time series of the net outflow for the AFV and WSV cases show that the net outflow is always positive and the phase of the outflow change is dominated by wind stress (Figure 4a). The net outflow in the AFV case is a little smaller than that in the WSV case because the outflow reflects the ice thickness over each strait. There is large difference in net thermodynamic growth of sea ice in the whole Arctic Ocean between the two cases (Figure 4b). Significantly lower net growth in the late 1990s in the AFV Figure 4. Time series of (a) the annual mean net ice outflow from the Arctic Ocean to the external regions (cm yr 1 ) and (b) the net ice growth averaged in the Arctic Ocean (cm yr 1 ) in the AFV case (solid line) and in the WSV case (dashed line). The unit of the outflow is converted to the equivalent change in mean thickness in the Arctic Ocean. About 45 cm yr 1 corresponds to 0.1 Sv. Table 2. Annual Mean of the Ice Thickness and the Net Ice Outflow in Year 20 Obtained by the PAO and the NAO Mean Thickness, cm Net Outflow, a Sv PAO (55) NAO (38) a Numbers in parentheses show the equivalent change in the mean thickness in the Arctic Ocean in cm yr 1. 5of12

6 Figure 6. Time series of the annual mean ice thickness in the Arctic Ocean in the NP case (solid line) and in the PN case (dashed line). case is supposed to induce the thinner ice and the consequent smaller outflow. The lower net growth in the AFV case may be caused by rise of surface air temperature. In this study, we don t investigate the effect of thermal forcing in order to focus on the impact of wind stress. The net outflow exhibits a positive trend from the late 1980s to the early 1990s when the ice volume decreases, and is low in the late 1990s when the ice volume increases. These variations of the net outflow are consistent with an observational estimate [Kwok et al., 2004]. [19] The ice volume does not immediately respond to rapid changes of outflow. While the outflow abruptly decreases in 1996 and remains almost constant for the following several years, the responding increase of ice volume is gradual and continues for several years (Figures 3 and 4a) Timescale of Sea Ice Response [20] The ice velocity and thickness fields in the equilibrium states obtained by the PAO and NAO are illustrated in Figure 5. When forced by the PAO, the Beaufort Gyre is small and ice currents are characterized by the Transpolar Drift from the Siberian coast toward the Canadian coast. Ice is especially thick along the northern coast of Canada and Greenland, and is very thin in the Siberian side of the Arctic Ocean. In contrast, when forced by the NAO, clockwise circulation extends over the whole Arctic Ocean. There is thick ice not only along the Canadian coast but also in the central Arctic Ocean and in the East Siberian Sea. The annual mean ice thickness averaged in the entire Arctic Ocean and the net ice outflow from the Arctic Ocean to the external seas in the equilibrium states are listed in Table 2. The mean thickness and the net outflow in the case forced by the PAO are smaller by 50 cm and larger by 0.04 Sv (1 Sv 10 6 m 3 s 1 ), respectively, than in the case forced by the NAO. In both cases, the net ice growth in the entire Arctic Ocean balances with the net outflow in the equilibrium states NP Case [21] When the wind stress field is switched, ice velocity field immediately responds to the wind stress change (not shown). In the NP case, the net outflow abruptly increases by imposing the PAO, and the mean thickness gradually decreases (Figures 6 and 7). Net annual thermodynamic growth of sea ice in the Arctic Ocean is always positive, so the decrease of the mean thickness is caused by excess ice outflow relative to the net growth. The inflow from the external seas into the Arctic Ocean is so small (about Sv) that it does not affect the mean thickness. The adjustment time of ice thickness and the ice velocity field averaged for 20 years are shown in Figure 8. We define the adjustment time in each grid as the year when the difference of ice thickness from the final state becomes less than a fixed value. The difference of the mean thickness in the Arctic Ocean between the initial and final states is 52 cm, so we set the fixed value to 5 cm, 10% of the mean thickness difference. While the thickness along the Siberian coast shortly attains an equilibrium state, the adjustment along the Canadian coast continues for a long time. [22] The time series of the mean thickness, the net outflow and the net growth in each of the regions indicated in Figure 8 are shown in Figure 9, where the net outflow is identical with divergence of ice mass in each region. In the East Siberian Sea (area 1), the net outflow suddenly increases in the first year and then decreases. After the third year, the net outflow remains constant and balances with the net growth. Correspondingly, the thickness immediately decreases and becomes constant before long. Because the East Siberian Sea is located in the origin of an ice pathway, the adjustment is fast. In the central Arctic Ocean (area 2), the thickness rapidly decreases since the increased net outflow exceeds the net growth. The adjustment is a little slower than that in the East Siberian Sea because larger ice inflow from the upstream regions than that in the equilibrium state obtained by the PAO prevents the net outflow from getting steady. [23] Advection time is roughly estimated from the ice velocity field. The distance between the area 1 and 2 along the ice pathway is about 1800 km, and the averaged velocity between the two regions is about 2 cm s 1. So, it takes about 3 years for the ice mass in the East Siberian Sea to Figure 7. Time series of the net ice outflow from the Arctic Ocean to the external seas and the net ice growth in the Arctic Ocean in the NP case and in the PN case. Units are cm yr 1. 6of12

7 Figure 8. Adjustment time of the ice thickness (year; see the text for definition) and the ice velocity field averaged for 20 years in the NP case. The unit vector for the ice velocity is 10 cm s 1. The plotted numbers indicate areas for later analyses. arrive at the central Arctic Ocean. The advection time between the Laptev Sea and the central Arctic Ocean is 4 or 5 years. After the ice mass in the upstream regions passes this area, the thickness change becomes small. In the Beaufort Sea (area 3), the thickness gradually decreases because the net outflow exceeds the net growth. As well as in the central Arctic Ocean, the net outflow is affected by larger inflow from the upstream regions (mainly along the Beaufort Gyre) than that in the equilibrium state obtained by the PAO. The thickness along the northern coast of Greenland (area 4) also increases for the initial several years and then gradually decreases. Ice mass concentrates into this area from various regions. The ice mass in the Siberian coast comes to this area earlier. It takes more than 10 years for the ice mass circulating in the Beaufort Gyre to arrive at this area because the ice velocity along the Gyre is small (less than 1 cm s 1 ). Outflow of these ice masses to the external seas cause the decrease of the mean thickness in the Arctic Ocean. Consequently, the response timescale in the NP case depends on the advection time of sea ice along its pathway toward the exits (mainly the Fram Strait). [24] Generally, decrease of mean thickness causes increase of net growth in the Arctic Ocean by enhancing upward heat flux from sea to air through ice. However, net growth depends not only on the mean thickness but also on spatial distribution of thickness. If thin ice area remarkably diminishes in spite of decrease of mean thickness, net growth may decrease. Actually, in the first year, the net growth decreases while the mean thickness decreases (Figures 6 and 7). Hence we inspect time evolution of the area fraction and the net growth for each thickness category in the Arctic Ocean (Figure 10). Note that we don t intend to discuss subgrid-scale distribution but to examine relationship between the thickness distribution and the net growth in the entire Arctic Ocean. The area fraction of open water (category 1) and thinner ice (categories 2 4) increases and that of thicker ice (category 5 8) decreases. Associated with these changes, which correspond to the decrease of the mean thickness, growth in open water (category 1) and thinner ice (categories 2 4) increases and melting of thicker ice (categories 5 8) decreases, so the net growth in the entire Arctic Ocean increases. The decrease of the net growth in the first year is due to just temporarily enhanced melting of thicker ice (category 7), which is not produced by thermodynamic growth but by a ridging process. Thus it is supported that the increase of the net growth primarily results from the decrease of the mean thickness. The decrease rate of the mean thickness is high in the early stage when the difference between the net growth and the net outflow is large, and then becomes gradually low as the difference is reduced. When the increased net growth balances with the net outflow, the mean thickness attains an equilibrium. It takes 2 years for the mean thickness to decrease by 50% of the difference (52 cm) between the initial and final states in the NP case. Then, it takes 9 years for 80% and 13 years for 90%. The mean thickness, the net outflow and the net growth averaged in Figure 9. Time series of (left) the ice thickness in cm (solid line) and (right) the net outflow (solid line) and the net growth (dashed line) in each of the areas indicated in Figure 8 for the NP case. The units of the net outflow and the net growth are converted to the equivalent change in mean thickness in each area (cm yr 1 ). 7of12

8 Figure 9 8of12

9 Figure 10. (a) Annual mean area fraction and (b) net ice growth (cm yr 1 ) in each thickness category in the entire Arctic Ocean in years 0 3 for the NP case. the entire Arctic Ocean are almost steady by year 20. In some subregions, however, these quantities still continue to change slightly even after 20 years PN Case [25] In the PN case, the mean thickness in the Arctic Ocean increases because the decrease rate of outflow is greater than that of net growth (Figures 6 and 7). The adjustment time of ice thickness is relatively uniform over the central Arctic Ocean and longer in the East Siberian Sea and the Laptev Sea (Figure 11). The time series of the mean thickness, the net outflow and the net growth in each of the regions indicated in Figure 11 are shown in Figure 12. In the central Arctic Ocean (area 5), the net outflow abruptly decreases in the first year and the thickness increases because most of the thermodynamically produced ice remains there without flowing to surrounding seas. The net outflow increases in the second year, and then does not change so much for the following years. Since the net growth almost balances with the net outflow, the thickness change becomes small. Along the Canadian coast (area 6), the net outflow decreases slightly, and then the thickness increases little by little since the net growth exceeds the net outflow. In the East Siberian Sea (area 7), the increase in thickness is very large because of the inflow from surrounding seas and the thermodynamic growth. Since the net outflow is almost constant except in the first year, the increase rate of the thickness is mainly controlled by the thermodynamic growth rate. Around the North Pole (area 8), the thickness increases because the net growth exceeds the decreasing net outflow. Thus, although there are some regions where inflow causes thickness increase, the increase of the mean thickness in the entire Arctic Ocean is mainly due to the thermodynamic growth. As well as in the NP case, the inflow from the external seas into the Arctic Ocean is so small (about Sv) that it does not affect the mean thickness. Figure 11. Same as Figure 8 but for the PN case. 9of12

10 Figure 12. Same as Figure 9 but for the PLMN case. 10 of 12

11 case. Then, it takes 10 years for 80% and 16 years for 90%. The response in the PN case is slower than in the NP case because the timescale of the thermodynamic adjustment in the PN case is longer than that of the advective adjustment in the NP case. Figure 13. Same as Figure 10 but for the PN case. [26] It is possible that spread of thin ice area causes increase of net growth even if mean thickness increases. However, in this case, increase of the mean thickness results in decrease of net growth by reducing upward heat flux from sea to air. Time evolution of the area fraction and the net growth for each thickness category in the entire Arctic Ocean is depicted in Figure 13. The area fraction of open water (category 1) and thinner ice (categories 2 4) decreases and that of thicker ice (categories 5 8) increases. These changes correspond to the increase of the mean thickness in the Arctic Ocean, and are reflected to the net growth changes. The growth in open water (category 1) and thinner ice (categories 2 4) decreases and the melting of thicker ice (categories 5 8) increases, so the net growth in the entire Arctic Ocean decreases. Hence it is supported that thin ice area does not expand and the decrease of the net growth primarily results from the increase of the mean thickness. The increase rate of the thickness is high in the early stage when the difference between the net growth and the net outflow is large, and then becomes gradually low as the difference is reduced. When the decreased net growth balances with the net outflow, the mean thickness reaches an equilibrium state. Thus the timescale of the thickening is controlled by thermodynamic growth rate in each region. It takes 4 years for the mean thickness to increase by 50% of the difference between the initial and final states in the PN 4. Summary and Discussion [27] The timescale of sea ice response to wind stress variations and the mechanisms controlling the timescale are investigated through numerical experiments. A coupled iceocean model is forced by two idealized wind stress fields corresponding to positive and negative AO phases one after the other. In both cases, ice velocity field immediately responds to the wind stress change, and the outflow from the Arctic Ocean abruptly varies within one or two years. As a result of the outflow variations, the ice volume in the Arctic Ocean changes. The response timescale and the mechanisms controlling the timescale are different depending on whether the outflow increases or decreases. When the outflow increases, the ice volume decreases since the net outflow exceeds the net thermodynamic growth. In this case, the timescale depends on the advection time of sea ice along its pathways. When the outflow decreases, the ice volume increases since the net growth exceeds the net outflow, and the timescale depends on the thermodynamic growth rate in each region. The switch of the wind stress field from a negative AO phase to a positive phase corresponds to the former case, and the switch from a positive phase to a negative phase corresponds to the latter case. [28] Krahmann and Visbeck [2003] perform the experiments where a sea ice model is driven by several idealized wind stress data sets varying with different periods. The amplitude of the ice volume change is the largest for the 20-year period forcing. The simulation results in this study also show that it takes about 20 years for the total ice volume in the entire Arctic Ocean to reach an equilibrium state, and we clarify the mechanisms controlling the timescale. [29] On the basis of this finding, the variations simulated in the WSV case are interpreted. In the late 1990s, while the outflow abruptly decreases from 1995 to 1996 and then remains almost constant for the following several years (Figure 4a), the annual mean ice thickness averaged in the Arctic Ocean continues to increase for 4 years from 1995 to 1999 (Figure 3). We can regard the change in the late 1990s as a response corresponding to the PN case, where the abruptly decreased outflow triggers the increase of the mean thickness. Thus the response timescale depends on the thermodynamic growth rate. However, it is unusual that the same wind stress field is maintained for several years like in the PN case, and the net growth fluctuates associated with spatial changes of ice velocity and thickness even if the mean thickness remains constant. For instance, the net growth becomes the minimum in 1996 and then increases in 1997 while the mean thickness continues to increase (Figures 3 and 4b). The minimum is derived from a temporal spread of melting region in the north of the Barents Sea (not shown). Therefore, when an accurate estimate of the response timescale in reality is intended, spatial change of net growth should also be taken into account. 11 of 12

12 [30] Surface air temperature gradually rises since the 1980s and continues to do so in the late 1990s [Comiso et al., 2003]. Because surface air temperature affects thermodynamic growth rate of sea ice, it is expected that thermal forcing plays some roles in timescale of sea ice change. Since we intend to focus on direct response of sea ice to wind stress variations in this study, the surface air temperature forcing is given by climatology in the experiments except for the AFV case. In future, the effect of thermal forcing on sea ice response is intended to be examined. [31] Whereas reduction of ice concentration in the late 1990s is reported from a satellite observation [Comiso et al., 2003], both AFV and WSV cases don t capture such a trend. One of the factors for this disagreement is supposed to be the restore of ocean temperature to climatology. Some observational studies indicate an increase of the warm Atlantic water inflow [Karcher et al., 2003; Zhang et al., 1998]. 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