NAO influence on net sea ice production and exchanges in the. Arctic region

Transcription

1 NAO influence on net sea ice production and exchanges in the Arctic region Aixue Hu National Center for Atmospheric Research, Boulder, CO 835 Claes Rooth Rosenstiel School of Marine and Atmospheric Sciences, Miami, FL and Rainer Bleck Los Alamos National Laboratory, Los Alamos, NM February 18, 23 Submitted to Journal of Climate Corresponding author: Aixue Hu, National Center for Atmospheric Research, PO Box 3, Boulder, CO 837, 1

2 Aixue Hu February 18, 23 2 Abstract The variability of the net sea ice production and the sea ice exchanges between the Arctic and its adjacent seas are studied, using a coupled sea ice-ocean general circulation model. The wind driven divergence (or ice flux export) is the major factor controlling the net sea ice production in the Arctic region since a thinning ice cover suppresses the development of thermal insulation during the ice formation process. The North Atlantic Oscillation (NAO) related atmospheric circulation changes can significantly modify the sea ice export and net production, especially in the thin ice zone (such as the Eurasian coastal region).

3 Aixue Hu February 18, Introduction Understanding the variability of the sea ice export from the Arctic to the North Atlantic marginal seas and variability of the net sea ice production in the Arctic region is an essential requirement for prediction of the high latitude climate evolution. Variations in sea ice exchanges between the Arctic and its adjacent seas will change the surface buoyancy forcing in the latter, thus impacting the variability of dense water formation in these seas and, indirectly, the strength of the oceanic Meridional Overturning Circulation (MOC). The Great Salinity Anomaly (GSA) in late 196s (documented by Dickson et al., 1988) is believed to be caused by an anomalous ice flux from the Arctic into Greenland-Iceland-Norwegian Seas (GIN Seas, see, e.g. Aagaard and Carmack, 1989, Walsh and Chapman, 199, Häkkinen, 1993). The resulting anomalous fresh water flux traveled through the subpolar gyre in the North Atlantic from late 196s to early 198s, reducing the intensity of the deep convective processes in the entire northern North Atlantic (Rudels, 1995). The focus of this study is to explore the effects of decadal and interdecadal timescale atmospheric circulation changes on the net sea ice production in different sub-basins in the Arctic and on the exchanges of sea ice between the Arctic and its adjacent seas, using the North Atlantic Oscillation (NAO) index as an indicator of the atmospheric circulation variations. The NAO index, which measures the combined pressure anomaly contrasts between the Icelandic Low and Azore High, characterizes a recurrent mode of atmospheric variability in the North Atlantic and Arctic regions which explains about 1/3 of the interannual variance in winter sea level pressure (Hurrell, 1995). The atmospheric circulation in the Arctic is highly influenced by the NAO (Dickson et al., 2). During a positive phase of the NAO index, one observes strong westerlies at midlatitudes over North Atlantic, increased North Atlantic water inflow to the Arctic, more ice export out of the Arctic through Fram Strait, discharge of the Russian rivers into a region extending beyond the Eurasian basin, and increased rainfall along southeast Greenland. During a negative

4 Aixue Hu February 18, 23 2 phase of the NAO index, there are weak mid-latitude westerlies, less North Atlantic inflow to the Arctic, less ice export, and the discharge of the Russian rivers mainly enters the Eurasian basin. The variability of the net sea ice production inside the Arctic affects the interaction between the mixed layer and the Atlantic layer, modifying the water mass properties of the Arctic upper ocean. A higher net sea ice production will lead to a deepened mixed layer due to the salt ejection during the ice freezing period. It is thus likely to enhance the interaction of the Atlantic layer water and the surface water in the Arctic. On the other hand, this strengthened interaction results in a greater heat flux entering the mixed layer (Note: the Atlantic layer water is warmer than the mixed layer water, the latter is close to freezing point of sea water during freezing season.), weakening the local ice freezing process. This is called freeze-melting process by Lemke (1987). Because of the difficulties of measuring it, reliable direct observations of the sea ice efflux through Fram Strait are only available in recent years (Vinje et al., 1998). Indirect estimates of ice area flux using satellite data are available for a longer time period (Kwok and Rothrock, 1999). However the estimation of the ice efflux from satellite observations is very difficult sheltered by the lack of ice thickness measurement. For climate diagnosis purposes, the time series of observed sea ice efflux are still too short to allow strong conclusions about long term variations. A numerical modeling approach, therefore, is an important alternate method of estimating decadal and interdecadal time scale variability of the sea ice efflux at Fram Strait. That task is approached here in a set of response experiments, broadly structured around the observed NAO related forcing variability. The system behavior is first explored under sustained high and low NAO index conditions (including the averaged seasonality), whereupon decadal time scale switches are applied between the high and low NAO forcing states. The paper is orgnized as follows: Section 2 briefly describes the coupled ice-ocean general circulation model, atmospheric forcing functions and the experiment design; The mean state of sea ice volume,

5 Aixue Hu February 18, 23 3 transport and budget is reported in section 3; The response of sea ice condition to the idealized NAO transients is given in section 4; The main conclusions are presented in section 5. 2 Model and experiments 2.1 The model The model used here was developed, and described in detail by Hu (1998, 21). It combines the Miami Isopycnic Coordinate Ocean Model (MICOM, Bleck et al., 1989, 1992; Sun et al., 1999) with the Elastic-viscos-plastic (EVP) dynamic sea ice model of Hunke and Dukowicz (1997) and a simple thermodynamic sea ice model of Semtner (1976). The model domain is meridionally truncated at 6 N (Figure 1) where a buffer zone is used, in which the temperature and salinity of sea water are relaxed back to the PHC monthly climatology (Steele et al., 21). (A substantial insensitivity of the basin scale ventilation/overturning process to the choice of truncation latitude has been demonstrated in MICOM based experiments by Paiva (1999).) The Bering Strait and Canadian Archipelago are closed. However, the Bering Strait inflow is parameterized as a virtual (negative) surface salinity flux mimicking the influx of.8 Sv (1 Sv = 1 6 m 3 /s) with salinity of 32.5 ppt (Aagaard et al., 1991). The oceanic initial condition is derived from the PHC data of Steele et al. (21). 2.2 Experiment design and atmospheric forcings Guided by the observed spectral characteristics of NAO index variability (Hurrell and van Loon, 1997, Cook et al., 1998, Appenzeller et al., 1998) which exhibit multi-year episodes of alternate high and low NAO conditions, the decision was made to first determine the sea ice response to sustained conditions on century time scales in either forcing mode, and then to explore the transient responses

6 Aixue Hu February 18, 23 4 to forcing mode switching. The motivation behind this decision is based on the lack of reliable longterm observation. The best available observed atmospheric data the NCEP/NCAR reanalysis products, covering our interest region, is only about 5+ years, which is still short to resolve the interdecadal timescale NAO variation. Luckily, data in these 5+ years cover the extreme high and low NAO years, thus provide us a way to distinguish the mean atmospheric circulation in these extreme conditions. Thus, idealized high and low NAO forcing climatologies were accordingly constructed based on this reanalysis products from 1948 to 1998 in the form of composite monthly means of the individual high and low NAO episodes. A climatic mean annual cycle was analogously defined for the total data set. This produced a set of three forcing climatologies, called high and low NAO climatologies and the mean climatology, which were then applied to the coupled ice-ocean model. The high NAO index climatology is defined here as the mean annual cycle of years 1973, 1981, 1983, 1989, 199, 1992 to 1995 whose winter NAO index was higher than +2 standard deviations based on the definition of winter (December to March) NAO index (Hurrell, 1995). The low NAO index climatology is defined as the mean annual cycle of years 1955, 1962 to 1965, 1969, 1977, 1979 and 1996 (NAO index lower than 2 standard deviations). It should be pointed out that, while the years are chosen on the basis of wintertime (December to March) NAO conditions, all 12 months of the respective years are used to derive the composite forcing functions. As illustrated in Figure 3, significant weakening of the Beaufort High and strengthening of the Aleusian and Icelandic Lows is observed in high NAO condition. The surface air temperature (SAT) difference between high and low NAO conditions indicates a 1 to 2 C warming in the Arctic region and a 3 C warming at east of Greenland. The modification of the Arctic sea ice export and net sea ice production due to these changes in surface atmospheric condition in our model solution will be addressed later in this paper.

7 Aixue Hu February 18, 23 5 To understand the mean state of the Arctic sea ice under idealized NAO conditions, quasi-equilibrium states of sea ice in the model are found from three century-long integrations forced by different idealized NAO conditions, namely the high (low) NAO condition and climatic mean condition (Figure 2, left panel). Hereafter, these three experiments are referenced as the high NAO case, climatic case and low NAO case, respectively. The responses of the Arctic ice flux and net sea ice production to idealized NAO transients on decadal and interdecadal timescales are explored in three experiments. A 1-year (7-year) period, according to the spectral analysis of the observed NAO index is chosen to represent the idealized NAO transient variability on decadal (interdecadal) timescale. The first pair of the transient experiments are designed to study the sea ice response to interdecadal timescale variations of the NAO conditions and to test sensitivity of this response to the initial state of NAO conditions. Integrations of the coupled ice-ocean model for this pair of experiments start with sustained high (low) NAO condition for 35 years, then with low (high) NAO condition for another 35 years. Henceforth, these two experiments are referenced as Case 4 and Case 5. The design of the third transient experiment, Case 6, is intended to mimick the low frequency variations of the winter NAO index in recent several decades as shown in Hurrell (1995, his Figure 1A), in which a 1-year oscillation of the NAO condition is superimposed on a 7-year period background NAO variation. Integration for this experiment begin with a background of low NAO condition (the mean winter NAO index is about -1.5 standard deviation). During this period, the NAO conditions oscillate between the extreme low NAO to climate mean for every 5 years. In the second half of this experiment, the NAO conditions change from extreme high NAO to climate mean for every 5 years with a background high NAO conditions (mean winter NAO index is 1.5 standard deviation). Detailed variation of the NAO index for these three cases is shown in Figure 2 (right panel, solid line for Case 4, dotted line for Case 5, and dashed line for Case 6).

8 Aixue Hu February 18, 23 6 It should be pointed out that all of the idealized experiments discussed in this paper start from a 3-year spinup run, forced by the mean climatology. 2.3 Diagnostic approach Since the NAO-related variations of SLP and SAT are not uniform horizontally in the Arctic region, it is not realistic to expect a uniform variation of the net sea ice production and exchanges under different NAO conditions in this region. A reasonable diagnostic choice is to divide the ice covered part of the model domain into several sub-domains in order to study the regional variability of the sea ice. As illustrated in Figure 1, the sub-domains are the Canadian Basin, Eurasian Basin, Eurasian Coast Basin (Siberian to Alaskan coast), Barents Sea (including the Barents Sea and Kara Sea), GIN Seas and Baffin Bay. Model outputs for each subdomain include fractional ice cover, compactness, and magnitudes of ice flux to (from) neighboring domains. The first five sub-domains are our main focus and the sea ice variability in the Baffin Bay is briefly discussed in this paper. 3 Ice volume, transport and budget with sustained NAO conditions 3.1 Sea ice volume Variations in sea ice volume represent changes in fresh water stored in the solid state in the Arctic. A decrease in ice volume in the Arctic basically accompanies a decrease in sea ice extent, thickness and compactness, and a possible increase in sea ice export. Thus, the total sea ice volume is as sensitive to atmospheric condition changes as the other sea ice properties. Model solutions show that the winter maximum sea ice volume is reduced by 4% (or 91 km 3 ) in high NAO phase from

9 Aixue Hu February 18, 23 7 low NAO phase, and the summer minimum ice volume is reduced by 14% (or 1455 km 3 ). In the annual mean, the reduction in sea ice volume is 7.3% (or 193 km 3 ). This seasonal variations of the sea ice volume qualitatively agrees well with the result from a stand alone sea ice model with more complicated ice thermodynamics (Hilmer and Lemke, 2). Variations of the sea ice storage in the Arctic can be considered as two parts: changes in ice volume and in export of sea ice. The later is closely related to the net sea ice production in the Arctic. As to be illustrated in next sub-section, the differences in sea ice export through Fram Strait between high and low NAO phase is about 322 km 3, a number about 3 times higher than the mean annual reduction of total sea ice volume. Therefore a higher rate of net sea ice production is expected during high NAO winters which will be addressed in sub-sections 3.3 and 3.4. On a sub-basin scale, the major changes in sea ice volume occurs inside of the Arctic basin since the ice volume in those marginal sea basins (the Barents Sea, GIN Seas and Baffin Bay) is only about 15% of the total ice volume in winter. During summer, all of the ice melts in these marginal sea regions in our model solutions, except in the GIN Seas for the high NAO case where approximately 1% of the ice cover is maintained because of sea ice export from the Arctic to the GIN Seas. In general, the sea ice volume peaks in late April and reaches the minimum in early September in our model solution. Changes of the sea ice volume from its maximum to minimum can be expressed as follows: V t = M + E (1) where V t represents the changes of ice volume with time, M is the rate of ice melting and E is the rate of sea ice export. Our diagnostic indicates that it is the local melting process who plays the dominant role on the variations in sea ice volume from its winter maximum to summer minimum in the Arctic. The sea ice export induced ice volume variation is only 1.7% in high NAO phase (total changes in sea ice volume: 9223 km 3 ), 5.4% in climatic phase (total changes in sea ice volume: 8924

10 Aixue Hu February 18, 23 8 km 3 ), and 3.6% in low NAO phase (total changes in sea ice volume: 8656 km 3 ). This model result implies that the sea ice volume variation is more sensitive to the atmospheric temperature changes than circulation changes, agreeing with Hilmer and Lemke (2). Interestingly, the net sea ice melting inside the Arctic basin is actually 11 km 3 higher in the low NAO case although the air is in general colder in low NAO condition relative to high NAO condition. Regionally, this higher sea ice melting in low NAO summer mainly occurs in the Eurasian coastal basin (23 km 3 higher). In the Canadian and Eurasian Basins, the net summer melting in low NAO summers is actually 8 km 3 and 4 km 3 lower than in high NAO summers. The higher rate of summer ice melting in the Eurasian coastal region in low NAO condition is because the available sea ice for melting is higher in low NAO condition than in high NAO condition. As discussed by Hu et at. (22), 73% of the Eurasian coastal region is ice covered by the end of the summer season in the low NAO case comparing to only 17% in the high NAO case. The lower rate of sea ice export from the Eurasian coastal region to the interior Arctic results in a higher sea ice volume (and a larger sea ice extent) in low NAO condition in the Eurasian coastal region. The signature of the different rate of sea ice melting can be found in the mixed layer salinity field. In Canadian basin, the mixed layer is about.5 to.18 ppt fresher in high NAO years as a result of the higher ice melting, agreeing well with the recent observations which show similar trend of the salinity variation (Carmack el al., 1995, McPhee et al., 1998, Macdonald et al., 1999). However, in the Eurasian coastal region, the lower rate of ice melting and a higher rate of sea ice export (see next section) cause a slightly salty mixed layer in high NAO summers. This saltier mixed layer agrees with the recent observations (Carmack et. al., 1997).

11 Aixue Hu February 18, Inter-basin sea ice exchanges Annual mean state Recent in situ and remote observations indicate that sea ice export from the Arctic to the Greenland Sea via Fram Strait exhibits significant interannual and decadal timescale variability (Vinje et al., 1998; Kwok and Rothrock, 1999; Dickson et al., 2). Since 197s, this flux has varied from a minimum of 18 km 3 per year in 1987 to a maximum of 47 km 3 per year in This large variability in sea ice flux may influence the strength of deep convection in the Northern North Atlantic marginal seas through modifying the surface buoyancy fluxes. This makes it potentially important to the global scale oceanic MOC, which plays a key role in the global meridional heat transport. In the model solutions, the annual sea ice export varies from 2255 km 3 (.725 Sv) under low NAO condition to 5474 km 3 (.176 Sv) under high NAO condition. Under climatic mean condition, model produced annual ice efflux is 317 km 3 (.97 Sv). The efflux in high NAO years is about 17% higher than the observed estimate of 47 km 3 in (Vinje et al., 1998) which is a typical high NAO year whose index is 3.96 (data from winter.html). The closest observed estimate of ice efflux for low NAO years is 21 km 3 (Vinje and Finnekasa, 1986) in 1979 (NAO index is -2.25). The model solution is about 7% higher. The annual ice efflux for the climatic case is close to observational estimates (such as 2843 km 3, Vinje et al., 1998; 31 km 3, Aagaard and Carmack, 1989) and other model estimates (such as 287 km 3, Harder et al., 1998). The higher sea ice export in the model solution may be caused by the overestimation of the wind forcing in Fram Strait region in the NCEP/NCAR reanalysis.

12 Aixue Hu February 18, Seasonal variations The seasonal variations of the sea ice efflux at Fram Strait (Figure 4, upper left panel) show that the ice efflux in high NAO years is higher than it is in low NAO years in all seasons. In winter (October March), the ice efflux accounts for about 74%, 78%, and 84% of the annual efflux for the high NAO, climatic, and low NAO cases, respectively. In the same season, the ice flowing through Fram Strait is about 71 cm thicker and the ice velocity is about 1.36 cm/s faster in the high NAO case than in the low NAO case when averaged over the width of Fram Strait (Figure 4, mid-left and lower left panels). The comparison of the winter mean sea ice flow pattern and thickness between high and low NAO years shows that driven by stronger wind, the thinner ice at Eurasian coastal region first flows towards the central Arctic, then towards the Fram Strait in high NAO winters (Figure 5, upper left panel). The convergence effect of this flow pattern on sea ice in central Arctic results in a thicker ice flow at Fram Strait in high NAO winters. In low NAO winter, the thinner ice at Eurasian coastal region flows with a slower transpolar stream directly toward Fram Strait (Figure 5, upper right panel). The convergence effect is small, resulting a thinner, slower ice flow. In summer (April September), the sea ice export only accounts for 16% of the annual efflux in the low NAO case, being essentially zero from July to September. Sea ice velocity fields indicate that ice flowing along the north Greenland coast turns towards the interior of the Arctic before reaching Fram Strait during these three months (July to September) in low NAO years. On the other hand, there is strong ice flow relative to the low NAO case along the north coast of the Greenland in the high NAO case which brings thicker ice from the Canadian basin to Fram Strait, hence enhancing the ice efflux (Figure 5, lower left panel). This ice flow pattern is also the cause of the weak seasonal ice thickness variations at Fram Strait in the high NAO case. The seasonal variation of the sea ice exchanges between the Arctic and the Barents Sea in the model solutions is asymmetric, leading to a net annual export from the latter. The net sea ice flux

13 Aixue Hu February 18, is from the Barents Sea to the Arctic in winter (November to April), and from the Arctic to the Barents Sea in summer (May to October). The summer ice outflow from the Arctic to the Barents Sea is only about 28%, 25% and 24% of the winter inflow for the high NAO, climatic, and low NAO cases. The changes in wind pattern from winter to summer cause changes in ice flow direction. A comparison of the seasonal variations of the mean sea ice thickness and ice velocity suggests that the variation of sea ice velocity is the dominant factor affecting the sea ice flux between the Arctic and the Barents Sea (Figure 4, mid-right and lower right panels) since the mean thickness of the flowing ice is thinner in high NAO years than in low NAO years. From the sea ice velocity field, ice flows into the Arctic on both side of Franz Josef Land in high NAO winters and only on the east side in low NAO winters (see Figure 5, upper two panels). The wider flow width causes a higher sea ice flux from Barents Sea into Arctic in high NAO years. 3.3 Sea ice exchange between sub-basins inside the Arctic In general, the strength and direction of the wind determine the direction and volume of the sea ice exchanges between sub-basins. Inside the Arctic, variations of the wind pattern under different NAO conditions lead to changes of the ice flow, resulting in a modified sea ice exchanges. The seasonal variation of the sea ice exchange between the Canadian basin and Eurasian basin (Figure 6, upper left panel) shows that, except in January through March, the ice is always moving from the Canadian basin to the Eurasian basin with a maximum flux of Sv in May in the high NAO case. In the other two cases, the sea ice transports from the Canadian Basin to the Eurasian basin also reach their maximum in May, however the absolute value is only 6% of the high NAO case. There are net ice volume fluxes from the Eurasian basin to the Canadian basin during September and from November to February in the low NAO and climatic cases. Mean monthly sea level pressure fields from the three climatologies indicate that a zonally extended

14 Aixue Hu February 18, high ridge sitting along the West Siberian Bering Strait Alaska results a trans-arctic wind pattern from the Eurasian coast to Canadian coast. Driven by this wind, ice flows from the Eurasian Basin into the Canadian basin in all cases. In May, the center of the Beaufort High move to the northwest of the Canadian coast. The geostrophic wind blows from the Bering Strait Alaska towards Fram Strait, driving a high ice volume flux from the Canadian basin to the Eurasian basin, eventually this ice exits at Fram Strait. Between the Eurasian coast and Canadian basins (Figure 6, upper right panel), there are only four months of the year that ice flows from the Canadian to the Eurasian coast basin with a mean of Sv in the high NAO case, however, seven months in the low NAO case with a mean transport of Sv. This ends up in a net ice volume flux ( Sv) from the Canadian basin to the Eurasian coast basin in the low NAO case and a net ice volume flux of Sv from the Eurasian coast basin to the Canadian basin in the high NAO case. Analysis of the SLP and ice velocity reveals that a formation of a wind-driven anticyclonic ice flow pattern is essential for the ice export from the Canadian basin into the Eurasian coast basin. On the other hand, a cyclonic ice flow pattern results in a ice flow from the Eurasian coast to Canadian basin. As shown in Figure 6, the major season for ice flowing from the Eurasian coastal zone to the Canadian basin in high NAO case is from May to July, which accounts for 71% of the total volume transport. In this season, sea ice in the Eurasian coast basin changes from the maximum extent, thickness and compactness towards the minimum. This higher rate of the ice efflux from the Eurasian coast basin to the Canadian basin makes these changes faster, especially by reducing sea ice extent (by about 8%) and compactness (from.965 to.629). In contrast to this, the reduction of the ice extent during the same season in the low NAO case is only 2% and the ice compactness varies from.97 to.718. Therefore, variations in sea ice efflux can significantly influence the basic sea ice properties (Hu et al., 22).

15 Aixue Hu February 18, Ice flux exchanges between the Eurasian coastal region and the Eurasian basin is dominated by the ice flow from the former to the later (about 1 months in a year). Peak values appear from November to May, driven by a transpolar transport. From July (June for the high NAO case) to August, the pattern of ice flow changes. Ice flows in the direction opposite to the winter season, resulting in an ice influx into the Eurasian coast basin. The mean transport of sea ice from the Eurasian coast basin to Eurasian basin is Sv during October to May in the high NAO case, about 57% higher than in the low NAO case. In summary, the transport of sea ice from the Eurasian coastal basin to the Canadian and Eurasian basins and from the Canadian basin to the Eurasian basin is the highest in the high NAO case and lowest in the low NAO case. These differences are mainly driven by the NAO-related wind changes. The different rates of sea ice exchange between the sub-basins, in turn, influence the local sea ice production which is addressed in next sub-section. 3.4 Sea ice budget in each sub-basin Figure 7 shows the annual mean net sea ice production in each sub-basin and the net exchange of the sea ice flux between sub-basins (unit: 1 3 Sv). In the Eurasian coast basin (Siberian west Alaskan coast), the net sea ice production in the high NAO case (left panel) is about 1 times higher than in the low NAO case (right panel), and twice as much as in the climatic case (central panel). This higher net sea ice production is driven by the higher rate of sea ice advection, caused by favorable winds blowing from the Siberian to the Canadian side of the Arctic, from the Eurasian coast basin to the Canadian ( Sv) and Eurasian ( Sv) basins, and further to the GIN Seas. As demonstrated by Hu et al. (22), this higher rate of ice advection, especially during winter season, lead to an ice about half meter thinner in the high NAO case than in the low NAO case. As a result, the insolation effect of the ice cover in this region is weakened which leads to a

16 Aixue Hu February 18, higher sensible, latent heat loss from ocean to air and a higher rate of outgoing longwave radiation. Therefore the net sea ice production is increased. On the other hand, a strong anticyclonic wind pattern in the low NAO case leads to a net sea ice flux of Sv from the Canadian basin to the Eurasian coast basin. This influx into the Eurasian coast basin primarily occurs in late summer to early winter as shown in Figure 6, leading to a thicker, closely pacted ice than those in high NAO case. The insulation effect of ice is increased and the heat loss from ocean to air is decreased, resulting in a less net sea ice production in winter season. In the Canadian basin, the net sea ice production is Sv in the high NAO case, Sv in the climatic case, and Sv in the low NAO case. The higher net sea ice production in the high NAO case is mainly due to the higher sea ice divergence. The sea ice flux from the Canadian to the Eurasian basin in the high NAO case is Sv, more than three times that in the low NAO case, and more than twice that in the climatic case. This higher rate of ice divergence results in a lower ice compactness in early winter season, leading to more open water area exposing to the cold air and increasing the heat release from ocean to air. Model results show that the outgoing longwave radiation in high NAO early winter season (October to January) is about 6% (1.8 W/m 2 ) higher than in low NAO one and the sensible heat flux is also 3.5% (.4 W/m 2 higher in the same season in high NAO condition. Thus, the early winter ice freezing process is enhanced by this increased heat loss. Model results also indicate that 77% of the annual sea ice production difference between the high and low NAO cases is generated in early winter season. In the Eurasian basin, more than 5% of the ice efflux (54.3%, 56.2%, and 56.3% in the high NAO, climatic, and low NAO cases, respectively) to the GIN Seas comes from the other three basins (named the Canadian basin, Eurasian coast basin, and Barents Sea). The local net sea ice production in this basin contributes Sv in the high NAO case, Sv in the

17 Aixue Hu February 18, climatic case, and Sv in the low NAO case to the sea ice efflux to the GIN Seas. The high rate of net sea ice production in this basin is also resulting from a high heat release from ocean to air in high NAO winter. The increase of the outgoing longwave radiation and the sensible heat flux reaches about 1% (3 W/m 2 ) and 6% (2.7 W/m 2 ) higher in high NAO early winter season relative to low NAO one, respectively. This higher heat loss is due to lower ice compactness and thinner ice in this basin, which are caused by the high rate sea ice divergence in high NAO years. Therefore, it is clear that the divergence of the sea ice is the most important driving force for increasing the net sea ice production. For example, as ice efflux in the high NAO case climbs to twice the value of the low NAO case, so does the net sea ice production. In the Barents Sea, the mean direction of the sea ice flow is to the Eurasian basin. The mean volume fluxes are Sv in the high NAO case, Sv in the climatic case, and Sv in the low NAO case. The net sea ice production is the same as the ice effluxes. As a result, the mixed layer is about.5 ppt saltier in high NAO winter. The Greenland Sea acts as a sink of the net sea ice produced in the Arctic region. On annual average, the mixed layer of Greenland Sea is about.4 ppt fresher in high NAO condition than in low NAO condition. 4 Response of the sea ice exchange to NAO transients 4.1 Response of Fram Strait efflux to NAO fluctuations The time variation of the Fram Strait sea ice volume flux has important effects on the deep convective activities in the GIN Seas and further in the Labrador Sea. The observed sea ice export at Fram Strait has been linked to the NAO inter-annual variations by Dickson et al. (2). Their

18 Aixue Hu February 18, Figure 15 basically shows that the export of sea ice through Fram Strait varies with the NAO index. In general, the ice efflux is higher during higher NAO index years and lower during lower NAO index years. As discussed earlier, variations in sea ice export is related to the wind and ice thickness. However, the changes in wind forcing are the major contributor to the sea ice efflux variation (Walsh et al., 1996, Hilmer and Jung, 2). This figure also suggests that the changes of the ice efflux occur at the same pace as the changes of the NAO index since The range of the sea ice efflux variation is about 22 km 3 based on the estimates of Vinje et al. (1986) and Vinje et al. (1998) (Alekseev et al., 1997). Figure 8 is the modeled sea ice efflux (upper part) and the NAO index used to force the coupled ice-ocean model. Although there is an interannual variation in the ice efflux during multi-year high or low NAO period, the amplitude of this interannual variation is much less than the changes of the efflux between high and low NAO years (Case 4 and Case 5). When the NAO index oscillates on a shorter (1 years) or longer (7 years) time scale, the ice efflux oscillates with the NAO index simultaneously. The variation of the observed, and estimated sea ice efflux is generally consistent with our model result, but the amplitude of the modeled efflux variation is roughly 32 km 3, about 45% higher than the observed estimates. One may also have noticed that in Case 6, the response of sea ice export to the NAO transient during the first 35 years and second 35 years is different although the amplitude variations of the NAO index are almost symmetric (from -3.3 to.21 or from.21 to +3.31, see Figure 8). The changes in sea ice export between the low NAO and climatic cases are only half of those between the climatic and high NAO cases. This difference mainly results from the difference in the wind strength between different NAO phases. The wind speed difference in the north Fram Strait region between low NAO and climatic years is about 2 to 4 cm/s weaker than that between high NAO and climatic years. This implies that the NAO induced surface wind anomaly at Fram Strait region is not in proportion to the variations of the NAO index. In the other word, the NAO variation may

19 Aixue Hu February 18, not be the only mechanism controlling the ice efflux at Fram Strait. Other forcing mechanisms, such as winter atmospheric planetary wave (Cavalieri and Häkkinen, 21, Cavalieri, 22), may also play important roles on Fram Strait ice export. Since there is no significant time lag between the forcing changes and the ice efflux, the consistency between the observed and modeled efflux estimates suggests that the direct regional atmospheric circulation changes are the basic driving force behind the variations of the sea ice efflux. Thus, no matter how long the high or low NAO index dominated period has lasted, when the NAO index changes, the ice efflux at Fram Strait changes due to the changes of the wind pattern. The amplitude of the efflux change is determined by the amplitude of the wind changes in the Arctic and at Fram Strait. As mentioned by Dickson et al. (2): the winter NAO index explains about 63% of the variance in the annual efflux of ice since 1976 (their Figure 17b). Interestingly, there is an ice efflux dip between year 56 and 6 in Case 6. The efflux varies from 5123 km 3 in year 56 to 377 km 3 in year 58 during this high NAO condition. The seasonal variation of this efflux shows that the difference occurs mainly in the winter season (October to March) which accounts for about 87.4% of the difference (Figure 9). Horizontally, the mean ice thickness distribution from October to March shows that the ice is more than 4 cm thinner in the southern Eurasian basin, and close to 1 m thinner on the west side of Fram Strait in year 58 than in year 56. The difference in ice velocity is small between these two years. Therefore, the thinner ice is the major reason for causing the decrease of the efflux in year 58. The ML salinity field shows that the ML in year 58 is about.625 ppt saltier than in year 56 in Fram Strait and over a large part of the Eurasian basin.on average, mixed layer in large part of this basin is more than 25 m deeper in year 58 than in year 56. This saltier and deeper ML indicates a stronger ML entrainment process. Since the temperatures of the layers below the ML are higher than the ML temperature in the ice covered areas, the higher rate of entrainment adds heat to the

20 Aixue Hu February 18, ML, resulting in thinner ice in year 58. The annual mean entrainment heat flux in the ice covered area is about.4 W/m 2 higher in year 58 than in year 56, which further demonstrates the effect of entrainment heat flux. 4.2 Influence of NAO transients on sea ice volume on sub-basin scales The influence of the NAO transients on sea ice volume basically represents the combined effect of NAO transients on ice thickness, compactness and extent. In general, the total sea ice volume in our model solution follows the variation of the NAO index, with a higher ice volume in lower NAO index years and a lower ice volume in higher NAO years. On the sub-basin scale, the relative importance of the variability in ice thickness, compactness and extent to the ice volume variation is different in different sub-basins. In general, the ice volume in the high NAO index years is higher in the Canadian basin and GIN Seas, and lower in the other basins, caused mainly by the wind-driven redistribution of ice mass. In the Canadian basin, ice thickness plays the dominate role on the ice volume variability (correlation greater than.78 in all cases). The compactness is secondary important. There is almost no contribution from the ice extent for the ice volume variation in this basin since the variability of the sea ice extent is very small in all cases. In the Eurasian basin, the variations of the sea ice thickness and compactness are almost equally important to the ice volume. Their correlation with ice volume is higher than.8 in all cases. The sea ice extent is also important for Case 5 and Case 6. In the GIN Seas, sea ice extent is the most important factor controlling the ice volume variations in all cases. The ice compactness plays a secondary role. In the Barents Sea, the ice extent and thickness are important.

21 Aixue Hu February 18, On the other hand, the contributions of ice thickness, compactness and extent are of comparable important to the ice volume variation in the Eurasian coast basin and in the Baffin Bay. In these two basins, the sea ice conditions almost exactly vary with the NAO index oscillation. 4.3 Effects of NAO transients on sub-basin sea ice exchange It is not surprising that the variations of sea ice fluxes between sub-basins also very closely follow the oscillations of the atmospheric forcing. The adjustments of ice flux exchanges between subbasins are nearly completed immediately after the changes in the atmospheric forcing. When the NAO index varies from near zero (.21) to a positive extreme (3.33), the variation in amplitude of the sea ice exchange between the Canadian and Eurasian basins and the ice efflux through Fram Strait is twice as large as that when the NAO index varies from a negative extreme (-3.3) to near zero (.21) (Figure 1). This indicates that the change of the wind field in the Arctic and Fram Strait is not in exact proportion to the amplitude change of the NAO index. The amplitude variation of ice flux between the Eurasian coast basin and Canadian basin is almost symmetric with the NAO index variation, and the amplitude variation of ice flux between the Eurasian coast basin and Eurasian basin is only slightly different. The Eurasian coast basin is a thinner ice zone and much more easily influenced by the changes of the wind pattern. The sea ice flux between the Eurasian basin and the Barents Sea also varies with the NAO index; however, the interannual variation of this sea ice flux seems as important as the longer time scale variability. 5 Conclusions In general, this work suggests that the rate of wind-driven sea ice export (or divergence) is the major factor controlling the rate of net sea ice production in the Arctic region. The sea ice thickness,

22 Aixue Hu February 18, 23 2 compactness and extent also play important role on the net sea ice production. An increased sea ice export causes a thinning in the ice cover (thinner ice and lower compactness), suppressing the development of thermal insulation during the ice formation process, leading to a higher rate of oceanic heat release which contributes significantly to the increase in net sea ice production. A good example of this mechanism is the Eurasian coast basin whose net sea ice production is 1 times larger in high NAO condition than in low NAO condition. The sea ice exchanges between the Arctic and the marginal seas and the ice exchanges between the sub-basins inside the Arctic display significant variations under different NAO conditions. The sea ice export from the Arctic to the Greenland Sea is higher for all seasons in high NAO years than in low NAO years. On annual average, this sea ice efflux in the high NAO phase is more than twice as large as in the low NAO phase, agreeing well with observations (Maslanik et al., 1998). The variations of ice efflux follow the NAO variations without a noticeable time lag. Our model result suggests the observed variations of the sea ice efflux through Fram Strait appear to be basically driven by the NAO-related atmospheric anomalies in the Arctic and the Fram Strait region. This agrees well with the available observations since 1977, summarized by Dickson et al. (2). However, the model solution from Himler and Jung (2) shows a sea ice export at Fram Strait in the winter season (DJFM) is correlated with NAO variations of the same winter season since 1978, but not for the period They suggest that the displacement of the NAO s centers might be the cause for the breaking down in the teleconnection between the NAO and sea ice export. As shown in section 2, the atmospheric forcings used in this research basically represent the atmospheric circulation patterns in 196s and 199s. In addition, the model solution shows significant correlation between the changes in NAO index (idealized variation) and sea ice export on decadal and interdecadal time scale. This may indicate the needs to reexamine Himler and Jung s work by using this fully coupled sea ice-ocean model forced by low-pass filtered atmospheric parameters or

23 Aixue Hu February 18, a fully coupled AOGCM in the future. In our model solution, thicker ice with higher velocity in the North Fram Strait region is responsible for the higher sea ice efflux in high NAO years. In winter, the thinner ice in the Eurasian coastal region is first advected towards the central Arctic, then towards Fram Strait. The convergence effect causes an increase in ice thickness there in the high NAO phase. In the low NAO phase, ice flows anticyclonically along the shelf break. When this ice reaches Fram Strait, it is thinner than the ice coming from the central Arctic in high NAO years. In summer, an atmospheric circulation change from a weak anticyclonic to a cyclonic dominated pattern in the Arctic drives the thick ice along North Greenland coast to Fram Strait in high NAO years, resulting in a higher sea ice export. Interestingly, model solutions suggest a net annual sea ice flux from the Barents Sea into the Arctic in all phases of the NAO. In winter months, ice flows from the Barents Sea into the Arctic driven by both winds and currents. In summer, changes in atmospheric wind patterns result in a smaller amount of ice being advected from the Arctic into the Barents Sea. Variations of this ice flux follows the NAO index in the same way as the ice export through Fram Strait. This Barents Sea ice flux contributes about 15% of the total ice export at Fram Strait. The volume transport of the sea ice from the Barents Sea to the Arctic in the high NAO phase is approximately twice as large as in the low NAO phase. Variations of the atmospheric circulation also drive an anomalous sea ice exchange between the sub-basins inside the Arctic Ocean. The total amount of ice advected from the Siberian coast to the Canadian and Eurasian basin is about 1 times higher in high NAO years than in low NAO years, resulting in a high rate of net sea ice production in the Siberian coast region during high NAO years. The net sea ice production in the Eurasian Basin in high NAO years is more than twice as much as that in low NAO years. In the Canadian basin, the differences in net sea ice production are relatively small between high and low NAO years (25% higher in the high NAO years) relative

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