Intensification of the Atlantic Deep Circulation by the Canadian Archipelago Throughflow

Home , Fram Strait, Labrador Sea, West Greenland Current

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YOSHIKI KOMURO AND HIROYASU HASUMI Center for Climate System Research, University of Tokyo, Tokyo, Japan

(Manuscript received 20 February 2004, in final form 4 October 2004)

ABSTRACT

Low-salinity water export through the Canadian Archipelago is one of the main components of the freshwater budget in the Arctic Ocean. Nevertheless, the Canadian Archipelago is closed in most global ocean models. How it is that deep-water formation at high latitudes of the Northern Hemisphere depends on the opening and closing of the Canadian Archipelago is investigated. An ice–ocean coupled model, whose horizontal resolution is 1°, is used without restoring surface salinity to observed data. When the Canadian Archipelago is open, the Atlantic deep circulation strengthens by 21%. This enhancement is caused by intensification of deep-water formation in the northern North Atlantic Ocean. Surface salinity in these regions is affected by the East Greenland Current, which flows from the Fram Strait and increases its salinity when the Canadian Archipelago is opened. The low-salinity flow through the Canadian Archipelago affects surface salinity only in the western part of the Labrador Sea. A cyclonic circulation in the Labrador Sea plays an important role in limiting the direct impact of the Canadian Archipelago throughflow. Con- sequently, the deep-water formation there is intensified and the Atlantic deep circulation is strengthened. Thus, it is suggested that the Canadian Archipelago throughflow does not weaken the Atlantic deep circulation by the freshening of the Labrador Sea but strengthens it by the salinity increase in the Fram Strait.

1. Introduction there are the main components of NADW, which flows southward in the deep layer of the Atlantic Ocean Since the thermohaline circulation transports a vast (Dickson and Brown 1994; Schmitz 1996). amount of mass and heat, it is considered to control not Seawater and sea ice are exchanged between the only the ocean itself but also the global state of the Arctic Ocean and the adjacent seas—namely, the GIN climate. Therefore, to understand what determines the Seas, the Labrador Sea, and the Pacific Ocean. An ob- pattern and strength of the oceanic thermohaline circu- served estimate indicates that the contributions of these lation is very important for understanding the global exchanges to the freshwater budget of the Arctic Ocean climate. In the present state of the ocean, water loses are comparable to or larger than that of the sea surface buoyancy and sinks at high latitudes, and gains buoy- freshwater flux (Aagaard and Carmack 1989). The con- ancy and upwells at lower latitudes. Deep convection tribution of sea ice is especially large at the Fram Strait. and dense water flow from continental shelves or over Regarding seawater, the relatively fresh halocline wa- sills are important for the former process (e.g., Kill- ter, which originates in the waters from the Pacific worth 1983), and diapycnal mixing in the interior ocean Ocean and rivers and is affected by melting of sea ice is important for the latter process (e.g., Munk and (e.g., Bauch et al. 1995), mainly contributes to the fresh- Wunsch 1998). water budget. As for the Pacific Ocean water through In this study, we focus on the high latitudes of the the Bering Strait, observations suggest that major part Northern Hemisphere, where North Atlantic Deep goes out of the Arctic Ocean through the Canadian Water (NADW) forms. In this region, open ocean con- Archipelago and the rest flows to the GIN Seas through vection takes place in the Greenland–Iceland– the Fram Strait (Jones et al. 1998, 2003). The outflow of Norwegian Seas (GIN Seas) and the Labrador Sea relatively fresh seawater and the sea-ice export are im- (Marshall and Schott 1999). The deep waters formed portant factors that control stability of water columns in the GIN Seas and the Labrador Sea, because their effect on salinity is confined to the upper layer. Conse- Corresponding author address: Yoshiki Komuro, Center for Cli- mate System Research, University of Tokyo, 4-6-1 Komaba, Me- quently, they are thought to affect the deep-water for- guro-ku, Tokyo 153-8904, Japan. mation there. E-mail: [email protected] The channels in the Canadian Archipelago and the

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Bering Strait are so narrow that they are not resolved in vection in the Labrador Sea by the formation of the many coarse-resolution global models. Especially, the vertical stratification and the doming of the isopycnals Canadian Archipelago is closed in most cases (e.g., En- (Marshall and Schott 1999). If a more realistic flow gland 1993; Manabe and Stouffer 1995; Gent et al. 1998; system in the Labrador Sea is reproduced by using a Hewitt et al. 2003; Komuro and Hasumi 2003). How- higher-resolution model, the flow possibly affects the ever, previous studies show that transports of seawater deep-water formation there, and the response of the and sea ice through these straits significantly affect the strength of the Atlantic deep circulation could be dif- strength of the Atlantic deep circulation—namely, the ferent. upper-meridional overturning cell in the Atlantic Based on the consideration mentioned above, the Ocean. Studies that investigate the effect of the inflow purpose of this study is to investigate the effects of the to the Arctic Ocean through the Bering Strait by using flow through the Canadian Archipelago on the Atlantic numerical models (Goosse et al. 1997a; Hasumi 2002; deep circulation by using an OGCM whose resolution is Wadley and Bigg 2002) show that the low-salinity water higher than the previous studies. that flows into the Arctic Ocean through the Bering The paper is organized as follows. The model, bound- Strait reduces NADW formation and the resulting At- ary condition, and experimental settings are described lantic deep circulation. in section 2. Results are presented in section 3. Last, On the other hand, only a few studies have examined summary and discussions are presented in section 4. the role of the outflow from the Arctic Ocean through the Canadian Archipelago in the Atlantic deep circulation. Goosse et al. (1997b) conduct numerical experi- 2. Model and forcing ments in which the Canadian Archipelago is open and a. Ice–ocean coupled model closed by using an ocean general circulation model (OGCM) and show that the Atlantic deep circulation An ice–ocean coupled model, which has been used in weakens by about 6% when the Canadian Archipelago a study on the global thermohaline circulation (e.g., is opened. They employ sea surface salinity (SSS) re- Oka and Hasumi 2004), is employed in this study. The storing to observed values. The restoring boundary oceanic component of the ice–ocean coupled model is condition is helpful for reproducing realistic SSS and the Center for Climate System Research (CCSR) resultant deep-water formation. However, the salt flux Ocean Component Model version 3 (COCO3). applied by the restoring boundary condition artificially COCO3 is an OGCM based on the primitive equations reduce the difference in SSS and freshwater transport on a spherical coordinate. Sea surface height is pre- caused by the opening/closing of the Canadian Archi- dicted by the method of Killworth et al. (1991). The pelago, and thus leads to underestimating its effect on model domain is global, and the topography is made the Atlantic deep circulation. Additionally, the result- from a 5-min earth topography dataset (ETOPO5) (Na- ant surface salt fluxes given by the restoring boundary tional Geophysical Data Center 1988). In a standard condition are generally not the same among the experi- setting, there is a channel of 100-m depth at the Cana- ments. The difference in the salt fluxes makes it harder dian Archipelago, which connects the Arctic Ocean and to distinguish changes induced by processes in the the Baffin Bay. The geometry at high latitudes in the ocean among the experiments from those caused by the Northern Hemisphere is shown in Fig. 1. Rotation of salt fluxes. Thus, the restoring boundary condition is the spherical coordinate system is applied, and the not suitable in this case. Wadley and Bigg (2002, here- poles are placed on Greenland and Antarctica. The ver- inafter WB02) perform the same experiments without tical coordinate system is a hybrid of ␴ and z, where ␴ SSS restoring. The Atlantic deep circulation weakens means normalized depth. The horizontal resolution is by about 20% in their study. However, the reasons are 1° both in the zonal and the meridional direction of the not so simple as in the case of the Bering Strait. When rotated coordinate system. The resolution is high for the Canadian Archipelago is opened in WB02, the flow global models integrated for a thousand years or more. through there decreases SSS in the downstream Labra- There are 40 vertical levels; ␴ coordinate is applied to dor Sea, and the deep-water formation there is de- the uppermost five levels between the free surface and pressed. On the other hand, salinity of the flow through 50 m below the mean surface level, and z coordinate is the Fram Strait increases, and SSS in the downstream applied to the remaining. The thickness of the levels in GIN Seas also increases, resulting in enhancement of z coordinate region varies from 20 to 200 m. The scheme the deep-water formation there. Since the former has a for prediction of tracers includes the uniformly third- larger impact than the latter, the Atlantic deep circula- order polynomial interpolation algorithm (UTOPIA; tion weakens in WB02. However, the resolution of their Leonard et al. 1993) for advection, the isopycnal diffu- model does not seem to be high enough to reproduce sion (M. D. Cox 1987, unpublished manuscript), and the cyclonic circulation in the Labrador Sea, which is the isopycnal layer thickness diffusion (Gent et al. formed by the West Greenland Current, the Labrador 1995). The isopycnal diffusion coefficient is 1.0 ϫ 103 Current, and a branch of the North Atlantic Current. m2 sϪ1 and the thickness diffusion coefficient is 3.0 ϫ This flow system is intricately linked to the deep con- 102 m2 sϪ1. The level-2 turbulence closure model of

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The turning angle for ice–ocean drag (Hibler 1979) is set to be zero. Salinity of sea ice is fixed at 5 psu.

b. Surface boundary condition The surface boundary conditions are derived from a climatology based on European Centre for Medium- Range Weather Forecasts (ECMWF) reanalysis (Röske 2001). The momentum flux is given at the surface. The heat flux is diagnosed from the shortwave and longwave radiative flux given by the dataset and the sensitive and latent heat flux derived by use of the bulk formulas of Kara et al. (2000). For the freshwater flux, precipitation (P) minus evaporation (E) plus river run- off (R) is given. Restoring of SSS to observed values is not employed. As for R, there are large differences among different datasets. For example, the annual- mean R integrated in the Arctic Ocean is 0.100 Sv (1 Sv ϵ 106 m3 sϪ1) for the current climatology. The observational estimates range from 0.076 to 0.10 Sv (Baum- FIG. 1. The geometry in high latitudes in the Northern gartner and Reichel 1975; Aagaard and Carmack 1989; Hemisphere, and geographic names mentioned in the text. Perry et al. 1996). The Atlantic deep circulation is significantly influenced by this range of uncertainty in R (Oka and Hasumi 2004). It is quite likely that R esti- Mellor and Yamada (1982) is employed for diagnosing mated by a numerical model has a large error, because the vertical diffusivity and vertical viscosity. The back- Ϫ it depends on the uncertainty of the model both in P Ϫ ground vertical diffusion coefficient is 0.1 ϫ 10 4 Ϫ E on the land and in land processes such as routing of m2 s 1 at the surface and gradually increases up to 3.0 Ϫ Ϫ rivers and prediction (or diagnosis) of water budget of ϫ 10 4 m2 s 1 with depth. The background vertical vis- Ϫ Ϫ the land. Thus, we employ the observation-based cosity coefficient is 1.0 ϫ 10 4 m2 s 1 except the upper- Ϫ Ϫ dataset of Perry et al. (1996), for which integrated R most 100 m where the coefficient is 1.0 ϫ 10 3 m2 s 1. Ϫ into the Arctic Ocean is 0.076 Sv. Around Greenland The horizontal eddy viscosity is 3.0 ϫ 104 m2 s 1.A and Antarctica, we give additional freshwater input bottom boundary layer (BBL) parameterization (Na- uniformly in time and space, since this dataset does not kano and Suginohara 2002) is incorporated in order to include the ice and water input from the ice sheets on represent flows of dense water along slopes. The setting Greenland and Antarctica. The amount of the freshwa- of the BBL parameterization is the same as Nakano ter input is determined on the observational estimates and Suginohara (2002): it is only applied to high lati- of the mass balance: it is 0.084 Sv from Antarctica (Ja- tudes (to the north of 49°N except for the Pacific cobs et al. 1992) and 0.009 Sv from Greenland (case Ocean, and to the south of 54°S). The Rayleigh drag pr_capil of Janssens and Huybrechts 2000). The glo- coefficient is the same value as the Coriolis parameter bally integrated P Ϫ E ϩ R given in this way is not above 2000 m and zero below 2000 m. The BBL thick- exactly zero in general. Since the oceanic volume in the ness is set to 100 m. The vertical diffusion coefficient model must remain constant in order to conduct a long- and viscosity coefficient at the top of the BBL and the Ϫ Ϫ Ϫ term integration, P Ϫ E ϩ R is modified uniformly in level just above are 1.0 ϫ 10 4 m2 s 1 and 3.0 ϫ 10 4 Ϫ space so that the globally integrated value becomes m2 s 1, respectively. zero. The settings of the sea-ice model follow those of Ko- muro and Hasumi (2003). The sea-ice component of the c. Experimental settings model includes both dynamics and thermodynamics. The thermodynamic part is the zero-layer model of Two cases are performed: one is the case in which the Semtner (1976). In the dynamic part, the momentum Canadian Archipelago is open, and the other is the case equation and the equations for mass and concentration in which the Canadian Archipelago is closed. These are taken from Mellor and Kantha (1989). Harmonic cases are hereinafter referred to as OPEN and CLOSED, and biharmonic terms are adopted in the latter two respectively, and collectively called the standard cases. equations. The coefficients for the harmonic and bihar- The other settings are the same in the two cases. The monic diffusion are 1.0 ϫ 104 m2 sϪ1 and 1.0 ϫ 1012 initial condition is Polar Science Center Hydrographic m4 sϪ1, respectively. Treatment for lateral melting and Climatology (PHC; Steele et al. 2001) for temperature freezing also follows the method of Mellor and Kantha and salinity and a state of rest. The model is integrated (1989). Internal ice stress is formulated by the elastic– for 1000 yr with the acceleration method of Bryan visco–plastic rheology (Hunke and Dukowicz 1997). (1984) for the momentum equations. For OPEN, the

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FIG. 2. Time series of the annual-mean NADW transport at the equator for OPEN.

NADW transport at the equator, which is an indicator of the strength of the Atlantic deep circulation, does FIG. 3. The area where the salinity is restored to the observed not seem to have any trend in the last 500 yr of the value in CLOSED-SR and OPEN-SR (shown by shade). integration (Fig. 2). Thus the Atlantic deep circulation is considered to reach the steady state. The results averaged for the last 10 yr are used for the analysis. tends too much in the Labrador Sea, the extent in the Two additional cases are also performed. These cases GIN Seas and the distribution of thickness in the Arctic are for validating the inference that the salinity at the Ocean are realistic. For OPEN, the amount of the sea- Fram Strait controls the deep-water formation in the ice transport through the Fram Strait to the GIN Seas is Ϫ northern North Atlantic, which will be presented and 2560 km3 yr 1 in sea-ice volume. The sea-ice export discussed in section 3 based on the results of OPEN and through the Canadian Archipelago from the Arctic Ϫ CLOSED. In these additional experiments, salinity is Ocean is 790 km3 yr 1. These values are slightly differ- restored to the PHC climatology in all the levels at the ent from observational estimates of 0.1 Sv or about Ϫ Fram Strait. The region for the salinity restore is des- 3600 km3 yr 1 in sea-ice volume for the Fram Strait and ignated by the shaded area in Fig. 3. The time constant a negligible amount for the Canadian Archipelago (Aa- for the restoring is 15 days. The Canadian Archipelago gaard and Carmack 1989). The freshwater budget for is open in one case (hereinafter called OPEN-SR), and the Arctic Ocean is summarized in Table 2, where the closed in the other case (hereinafter called CLOSED- reference salinity is set to the mean salinity in the Arc- SR). The initial conditions for OPEN-SR and tic Ocean for each case. The total influence of sea-ice CLOSED-SR are taken from the state of the 900th year and seawater transport on the Arctic Ocean salinity is of OPEN and CLOSED, respectively. The duration of reasonably represented. Freshwater is imported from the integration is 100 yr, and the results averaged for the Bering Strait and exported through the other straits the last ten years are used for the analysis. The settings and channels. Note that other observational estimates for all the cases are summarized in Table 1. of the sea-ice transport through the Fram Strait are scattered in a wide range, from 1530 to 2846 km3 yrϪ1 (Vinje et al. 1998; Martin and Wadhams 1999; Kwok et 3. Results al. 2004). Thus, the sea-ice transport at the Fram Strait in this study is in the range of the observational esti- a. Performance of the model mates. The zonally averaged potential temperature and sa- Figure 6 depicts the zonally integrated meridional linity in the Atlantic Ocean for OPEN and the PHC overturning streamfunction in the Atlantic Ocean for climatology are compared in Fig. 4. In the model re- OPEN. The NADW transport at the equator is 12.0 Sv, sults, the vertical stratification in the Southern Ocean and the Antarctic Bottom Water (AABW) transport at and the Arctic Ocean is not well reproduced, and the the equator is 4.9 Sv. An observational estimation of salinity of NADW is higher than the reality. Neverthe- these amounts is 14 and 4 Sv, respectively (Schmitz less, potential temperature and salinity are in good 1995). Although the NADW transport is slightly agreement with the climatology as a whole, especially smaller and the AABW transport is slightly larger in in the deep layer at middle and low latitudes. OPEN compared with the real ocean, the pattern of the Figure 5 shows sea-ice thickness in the Northern deep circulation, two deep circulation cells in the deep Hemisphere (NH) in February. Although sea ice ex- Atlantic Ocean, is reproduced.

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TABLE 1. List of experiments. Salinity restoring Integration time Abbreviation Canadian Archipelago at Fram Strait Initial condition (yr) OPEN Open No PHC 1000 CLOSED Closed No PHC 1000 OPEN-SR Open Yes OPEN (900th yr) 100 CLOSED-SR Closed Yes CLOSED (900th yr) 100

Apart from some shortcomings, the model reason- range of Ϯ5%, which is sufficiently small compared ably reproduces the thermohaline circulation and the with the difference between OPEN and CLOSED. associated oceanic structure in the Atlantic Ocean, and With respect to the freshwater budget of the Arctic the freshwater budget of the Arctic Ocean is compa- Ocean (Table 2), freshwater flows into the Arctic rable to the observational estimates. Thus, we believe Ocean through the Bering Strait in both cases, and it is that the model is good enough for performing the cur- consistent with the observation. By opening the Cana- rent impact study on the Atlantic deep circulation. dian Archipelago, freshwater export decreases through the Fram Strait and increases through the Canadian b. Comparison of standard cases Archipelago. These results suggest that a significant part of the low-salinity water, whose main component is The results for OPEN and CLOSED are now com- the water from the Bering Strait, flows through the pared. The zonally integrated meridional overturning Canadian Archipelago. A passive tracer is used in order streamfunction in the Atlantic Ocean for CLOSED is to confirm this. The value of the passive tracer is set to shown in Fig. 7. The NADW transport at the equator is unity at the Bering Strait, and no sink exists. Since sea- 9.9 Sv, so it increases by 21% when the Canadian Ar- water always flows from the Pacific Ocean at the Bering chipelago is opened. Note that the variability of the Strait as in the real ocean, it represents the distribution annual-mean NADW transport for the last 100 yr of the of the water from the Pacific Ocean. The passive tracer integration for both OPEN and CLOSED is within the is put into the model for the last 100 yr of the integra-

FIG. 4. Zonally averaged potential temperature and salinity in the Atlantic Ocean: (a) PHC potential temperature, (b) PHC salinity, (c) OPEN potential temperature, and (d) OPEN salinity. Contour interval is 1°C for potential temperature and 0.1 psu for salinity.

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Fram Strait decreases, although the total export from the Arctic Ocean remains almost the same. It is expected that the deep-water formation in the Labrador Sea be suppressed in OPEN because of the low-salinity water inflow through the Canadian Archi- pelago, as is the case in the previous studies. However, the deep convection in the northern North Atlantic Ocean occurs more actively in OPEN than in CLOSED (Fig. 9). Figure 10 shows zonally integrated meridional overturning streamfunction whose vertical coordinate ␴ is 2 (potential density referenced to 2000 dbar). The amount of the deep water formed and flowing out of the GIN Seas (65°–80°N), which is obtained from the maximum value of the streamfunction at 65°N, is about 6 Sv for both OPEN and CLOSED. These results indi- cate that the change in the GIN Seas by opening the Canadian Archipelago is not responsible for the increase in the NADW transport. It is accounted for by the activation of the deep-water formation in the other region, namely, the northern North Atlantic. To con- FIG. 5. Sea-ice thickness in Feb in the NH for OPEN. Contour interval is 0.5 m. firm this, the amount of the deep-water formation in the northern North Atlantic is estimated by the NADW transport at the equator minus the outflow of the deep tion. The distribution of the passive tracer in the last water from the GIN Seas. The result is about 6 Sv for year of the integration at 100-m depth is shown in Fig. OPEN and 4 Sv for CLOSED. 8. Its concentration in OPEN is lower in the GIN Seas It seems that the increase in the deep-water forma- and higher in the Labrador Sea compared with tion in the northern North Atlantic is against expecta- CLOSED. We define transport of the passive tracer tion, since the low-salinity water flows through the Ca- through a specific section, F ,as nadian Archipelago into the Labrador Sea and the trc deep-water formation region (to the south of the Lab- rador Sea in this model). However, the model results ϭ ͵␷ Ftrc Ctrc dA, show that it is not true that these regions are under the direct influence of the Canadian Archipelago through- ␷ where is cross-sectional velocity and Ctrc is concen- flow. The distribution of the passive tracer correspond- tration of the passive tracer. The tracer is exported only ing to the water from the Pacific Ocean for OPEN (Fig.

to the GIN Seas in CLOSED: Ftrc through the Fram 8a) shows that the tracer concentration is lower in the Strait plus the Barents Sea Opening is 0.74 Sv. In eastern Labrador Sea than in the western Labrador OPEN, by contrast, about a half of the water from the Sea. The SSS for OPEN (Fig. 11a) is higher in the east- Pacific Ocean flows through the Canadian Archipelago: ern Labrador Sea than in the western Labrador Sea,

Ftrc through the Fram Strait plus the Barents Sea Open- hence it indicates a similar result. This SSS distribution ing and the Canadian Archipelago are 0.43 and 0.37 Sv, is consistent with that of the PHC (figure is not shown). respectively. Thus, the Pacific water export through the The SSS difference between OPEN and CLOSED (Fig.

TABLE 2. The freshwater budget of the Arctic Ocean for the standard cases. An observational estimation (Aagaard and Carmack 1989) is also shown. Each item is indicated by freshwater amount that has equivalent effect on seawater of reference salinity. The reference salinity for OPEN and CLOSED is set to the mean salinity in the Arctic Ocean for each case. Units are cubic kilometers per year (positive value means freshening of the Arctic Ocean), except for reference salinity, which is expressed in practical salinity units (psu). OPEN CLOSED Obs estimate (km3 yrϪ1) (km3 yrϪ1) (km3 yrϪ1) Reference salinity (psu) 34.69 34.64 34.8 Bering Strait Water import ϩ1550 ϩ1400 ϩ1670 Fram Strait Water export Ϫ1060 Ϫ2260 Ϫ820 Ice export Ϫ1970 Ϫ2100 Ϫ2790 Barents Sea Opening Water import Ϫ770 Ϫ830 Ϫ540 Ice export Ϫ120 Ϫ190 Negligible Canadian Archipelago Water export Ϫ1030 — Ϫ920 Ice export Ϫ610 — Negligible

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FIG. 6. Zonally integrated meridional overturning streamfunction in the Atlantic Ocean for OPEN. Contour interval is 2 Sv.

11b) shows that the SSS for OPEN is lower than that Sea, where the deep convection occurs in OPEN, are for CLOSED in the Baffin Bay. The lower SSS is not in the downstream of the flow from the Canadian caused by the low-salinity flow from the Canadian Ar- Archipelago but the downstream of the flow from the chipelago. Nevertheless, in the Labrador Sea, the SSS Fram Strait. for OPEN is lower in the western region but higher in Figure 13 depicts salinity sections in the upper 500 m the eastern region compared with CLOSED. The low- at the Fram Strait for OPEN, CLOSED, and the PHC salinity water from the Canadian Archipelago affects climatology. In the model, the flow is southward on the the Labrador Sea SSS only in the western part. entire section in about the upper 200 m, and the mass The different behavior between the eastern and west- transport integrated at each level is southward in about ern Labrador Sea can be explained by the flow scheme the upper 500 m. The southward flow corresponds to in this region. In the Labrador Sea, there is a cyclonic the East Greenland Current. In the real ocean, there circulation near the surface (e.g., Schmitz 1996; Mar- also is the northward-flowing West Spitsbergen Current shall and Schott 1999; Fratantoni 2001). This flow is in the eastern part of the Fram Strait. However, it is not formed by the West Greenland Current, which flows found in the upper layer of the model. It is a typical northward in the eastern Labrador Sea, and the Labra- shortcoming in models whose resolution is 1° or coarser dor Current, which flows southward in the western (Oka and Hasumi 2004, manuscript submitted to J. Labrador Sea. The West Greenland Current is down- Geophys. Res., hereinafter OH2). Thus, the salinity in stream of the East Greenland Current, which flows the upper layer at the Fram Strait in the model repre- southward along the east coast of Greenland and is sents the salinity of the East Greenland Current. The connected to the West Greenland Current. Horizontal salinity in the upper layer, especially the upper 300 m, velocity field averaged in the upper 1070 m (the upper- at the Fram Strait for OPEN is higher than that for most 17 levels) for OPEN (Fig. 12) shows that the CLOSED. This is consistent with the decrease in the model roughly reproduces such a flow system, although low-salinity water that flows through the Fram Strait. it is somewhat weak in the northern part of the Labra- Note that the salinity even for OPEN is lower than the dor Sea. The Irminger Sea and the eastern Labrador PHC climatology.

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FIG. 7. Same as Fig. 6 but for CLOSED.

From these analyses, the following scenario is sug- c. Results of additional cases gested. When the Canadian Archipelago is opened, a significant part of the low-salinity water, the main com- If our scenario presented in the previous section is ponent of which is the water from the Pacific Ocean true, the intensity of the Atlantic deep circulation is through the Bering Strait, flows through the Canadian relatively insensitive to the configuration of the Cana- Archipelago. It causes the salinity to decrease in the dian Archipelago when the salinity at the Fram Strait is Baffin Bay and the western Labrador Sea, which are fixed. We analyze the results of the additional cases located in the downstream of the Canadian Archi- (OPEN-SR and CLOSED-SR) and check whether the pelago throughflow. On the other hand, the salinity of idea is true. It should be pointed out that the salinity the East Greenland Current, which flows through the restore causes to increase the salinity of the Canadian Fram Strait, is increased, resulting in increase in the Archipelago throughflow by about 0.14 psu and to de- salinity in the downstream region, namely, the Irminger crease the sea-ice export through the Canadian Archi- Sea and the eastern Labrador Sea. The increase in pelago by 5% in OPEN-SR compared with OPEN. For salinity leads to vigorous deep-water formation in the freshwater budget of the Arctic Ocean, these these regions. The deep-water formation region in the changes correspond to about 10% decrease in the fresh- GIN Seas remains nearly unchanged, since the East water export there. However, enough freshwater is ex- Greenland Current does not directly influence there. ported compared with the observational estimate even Consequently, the Atlantic deep circulation is inten- in OPEN-SR, and thus the comparison between sified. This scenario means that the salinity of the OPEN-SR and CLOSED-SR is valid. East Greenland Current (and thus the West Greenland Figure 14 shows the stream functions of the zonally Current) has a larger impact on the strength of the integrated meridional overturning circulation in the At- Atlantic deep circulation than the salinity of the Cana- lantic Ocean for OPEN-SR and CLOSED-SR. The At- dian Archipelago throughflow and the western Labra- lantic deep circulation transport at the equator for the dor Sea. additional cases does not strongly depend on whether

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FIG. 8. Distribution of the passive tracer at 100-m depth in the last year of the integration (100 yr after the tracer is put into the model). Contour interval is 0.1. Regions where the concentration exceeds 0.2 are shaded: (a) OPEN and (b) CLOSED. FIG. 9. The deepest convection depth (over 500 m) in winter (Dec–Mar): (a) OPEN, (b) CLOSED. the Canadian Archipelago is open: 13.4 and 14.6 Sv for OPEN-SR and CLOSED-SR, respectively. These re- in the transport compared with the corresponding stan- sults are summarized in Table 3 with those of the standard cases is 4.7 Sv for CLOSED-SR, larger than 1.4 Sv dard cases. In both the cases, vigorous deep convection for OPEN-SR. It is also consistent with the lower sa- takes place in the northern North Atlantic Ocean (fig- linity at the Fram Strait for CLOSED than that for ure is not shown). OPEN. The amount of deep water formed in the north- The increases in the Atlantic deep circulation trans- ern North Atlantic Ocean, which is estimated by the port for the additional cases are consistent with the same way as that for the standard cases, is about 8 Sv lower salinity at the Fram Strait for the standard cases for OPEN-SR and about 9 Sv for CLOSED-SR. These than that for the PHC climatology (Fig. 13), since the values are larger than the corresponding values for salinity restore to the PHC climatology at the the Fram OPEN and CLOSED (about 6 Sv and about 4 Sv, re- Strait tends to increase the salinity there. The increase spectively). Thus, these results of the additional cases

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Current is restored to the same salinity as CLOSED- SR. This may be the reason why the deep-water formation in OPEN-SR is slightly weak compared with CLOSED-SR.

4. Summary and discussion

The effect of seawater exchange through the Cana- dian Archipelago on NADW formation is investigated in this study. An ice–ocean coupled model, whose horizontal resolution is 1°, is used without restoring SSS to observed data. Two cases are performed as standard. The Canadian Archipelago is open in one case and is closed in the other case. When the Canadian Archipelago is opened, the At- lantic deep circulation originated in the NADW formation at northern high latitudes strengthens. This enhancement is caused by increase in the deep water formed in the northern North Atlantic Ocean. The salinity and flow scheme are closely analyzed in order to understand the increase. In the model, low-salinity water flows into the Arctic Ocean through the Bering Strait as in reality. When the Canadian Archipelago is opened, a part of the low-salinity water flows there toward the Labrador Sea. However, it affects only the salinity in the western Labrador Sea. On the other hand, the low-salinity water that flows through the Fram Strait decreases, and the salinity of the East Greenland Current, which flows there, increases. The increase in salinity leads to activation of the deep-water ␴ FIG. 10. Zonally integrated 2 density meridional streamfunc- formation in the Irminger Sea and the Labrador Sea tion in the Atlantic Ocean. The reference level is 2000 dbar. Con- where the East Greenland Current and the West tour interval is 1 Sv: (a) OPEN, (b) CLOSED. Greenland Current flow. Thus, it is suggested that the salinity of the water flowing out of the Arctic Ocean support the idea that the NADW formation in this through the Fram Strait has a larger influence on the model is relatively independent on whether the Cana- deep-water formation in the northern North Atlantic dian Archipelago is open or not. than that through the Canadian Archipelago. It should be pointed out that almost all the water Two additional cases are conducted in order to con- from the Arctic Ocean toward the GIN Seas and the firm this idea. In the additional cases, the experiments northern North Atlantic flows through the Fram Strait start from the 900th year of the standard cases, and are in CLOSED-SR. In OPEN-SR, on the other hand, a integrated for 100 yr with restoring salinity to the PHC part of the water flows there and the rest flows through climatology at the Fram Strait. The restoring results in the Canadian Archipelago. Salinity is restored to the increase in the salinity at the Fram Strait. Whether the observed value (and increased) only at the Fram Strait. Canadian Archipelago is open or not does not have a Thus, most of the water exported from the Arctic large effect on the formation. These results support the Ocean is increased in salinity in CLOSED-SR, while idea that the most important change when the Cana- only a part of the water is increased in salinity in dian Archipelago is opened is the increase in salinity at OPEN-SR. The difference in the increase in salinity the Fram Strait and consequent enhancement of probably makes the salinity of the midlatitude North NADW formation. Atlantic (30°–50°N) averaged over the upper 230 m The result of this study is opposite to that of WB02. (the uppermost nine levels) lower in OPEN-SR than in In WB02, the Atlantic deep circulation weakens when CLOSED-SR by 0.12 psu. In the standard cases, the the Canadian Archipelago is opened. Contrary to this difference in the midlatitude upper layer salinity is less study, the deep-water formation in the Labrador Sea in significant: the corresponding mean salinity is lower in WB02 decreases, and it is the main reason why the OPEN than in CLOSED by only 0.05 psu. In OPEN- Atlantic deep circulation weakens. One of the possible SR, the slightly fresher water there is advected to the reasons for the difference is the horizontal grid size in deep-water formation region, while the East Greenland the Labrador Sea. In this study, the cyclonic circulation

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FIG. 11. (a) SSS for OPEN. Contour interval is 0.5 psu. (b) SSS difference between OPEN and CLOSED [(OPEN) Ϫ (CLOSED)]. Positive value means higher SSS for OPEN. The area where the value is negative is shaded. Contour interval is 0.5 psu. in the Labrador Sea, which is also found in reality, al- Sea (Fig. 3b in WB02), the cyclonic circulation does not lows the vigorous convection in the northern North At- seem to be reproduced sufficiently in WB02. Because lantic when the Canadian Archipelago is open. The grid of the unrealistic flow scheme, the low-salinity flow size in the zonal direction for this study is about 100 km. through the Canadian Archipelago would decrease the By contrast, the zonal grid size in the Labrador Sea for SSS in the whole Labrador Sea, and consequently, WB02 is about 200ϳ300 km (Fig. 2a in WB02). Based weaken the deep-water formation there. Thus, the re- on this grid size and the fact that the SSS in the eastern producibility of the flow scheme in the Labrador Sea Labrador Sea is similar to that in the western Labrador has a large impact on the sensitivity of the deep-water

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of the Irminger Sea. The deep convection in WB02 occurs in the interior of the Labrador Sea when the Canadian Archipelago is closed, although it moves to the south end of the Labrador Sea by the Canadian Archipelago opening. The deep convection could move to the central Labrador Sea if the cyclonic circulation in this study, which is somewhat weak and vague in the northern part of the Labrador Sea, sufficiently intruded to the north. Even in that case, it would be unlikely that the influence of the Canadian Archipelago throughflow overwhelms that of the East Greenland Current in the central Labrador Sea. Since the SSS does not vary much in the central Labrador Sea and increases in the eastern Labrador Sea when the Canadian Archipelago is opened in this study, it is likely that the deep convection in the central Labrador Sea shifts eastward and is activated, or stays there and remains active. The result of this study seems to be valid in that sense. Another shortcoming in this study is too much sea- ice transport through the Canadian Archipelago. Ob- FIG. 12. Mean velocity in the upper 1070 m (the uppermost 17 servations show that there is almost no sea-ice transport layers) for OPEN in GIN Seas and northern North Atlantic (Aagaard and Carmack 1989). By contrast, the sea-ice Ocean. transport through there from the Arctic Ocean is 790 km3 yrϪ1 in sea-ice volume for OPEN in this study. The contribution of the sea-ice transport to the freshwater formation in the Labrador Sea to the Canadian Archi- budget of the Arctic Ocean is about two-thirds of that pelago throughflow. Apart from the effect of the Ca- of the seawater through there, and is unable to be ne- nadian Archipelago throughflow, it also suggests in glected. In fact, in terms of the freshwater budget of the general that the strength of the Atlantic deep circula- Arctic Ocean, the total effect of the volume and sea-ice tion in OGCMs depends on the horizontal resolution in transport through the Canadian Archipelago in this the Labrador Sea, as is the case for the horizontal reso- study is nearly twice as large as that in reality. One of lution at the Fram Strait (OH2). the reasons for this overestimation is the too broad Another reason may be the difference in the regions channel at the Canadian Archipelago, which is caused where the deep convection occurs. It occurs at the south by the horizontal grid size of about 100 km. The trans- end of the Labrador Sea and in the Irminger Sea in this ported sea ice melts in the downstream so that the study. In the real ocean, deep convection is considered freshening effect of the flow through the Canadian Ar- to take place in the central Labrador Sea (e.g., Marshall chipelago is also overestimated. Since the freshening and Schott 1999; Lavender and Davis 2002), although itself has only a limited impact on the deep-water for- some recent studies (Pickart et al. 2003a, b) argue that mation in the Labrador Sea and the northern North deep convection also occurs in the southwestern region Atlantic Ocean in this study, this overestimation itself

FIG. 13. Salinity section in the upper 500 m at the Fram Strait for OPEN, CLOSED, and the PHC climatology. GL and SB in the figures are abbreviations of Greenland and Spitsbergen, respectively. Contour interval is 0.5 psu. Regions where the salinity exceeds 34 psu are shaded.

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TABLE 3. Transport of the Atlantic deep circulation at equator in each case. Canadian Archipelago Open Closed Restoring at No OPEN: 12.0 Sv CLOSED: 9.9 Sv Fram Strait Yes OPEN-SR: 13.4 Sv CLOSED-SR: 14.6 Sv

GIN Seas SSS. When the Canadian Archipelago is opened, SSS increases throughout the GIN Seas in WB02, whereas in this study SSS increases only where the East Greenland Current flows. Since the water from the Arctic Ocean is spread over the western part of the GIN Seas surface layer in reality (e.g., Hansen and Østerhus 2000), the two models deviate from the reality to opposite extremes. It is also pointed out that thermal coupling between given atmospheric condition and SST is stronger in this model than in WB02. In WB02, the thermal coupling coefficient is globally set to 15 W mϪ2 KϪ1. The coupling coefficient is not explicitly given in this study, but the equivalent thermal coupling coefficient can be calculated by following the method of Haney (1971). The annual mean coefficient averaged in the GIN Seas and the northern North Atlantic to the north of 50°Nis33WmϪ2 KϪ1. Since stronger thermal coupling tends to more easily cease deep-water formation and weaken the stability of the thermohaline circulation (e.g., Zhang et al. 1993), the Atlantic deep circulation in our model may be more sensitive to the change in the geography than that in WB02, although FIG. 14. Same as Fig. 6 but for (a) OPEN-SR and (b) the thermal coupling in this study is weaker than an CLOSED-SR. estimation from commonly used bulk parameterization formulas in Rahmstorf and Willebrand (1995) and would not largely affect the results. Note, however, that those in some studies on the thermohaline circulation the sea-ice transport through the Canadian Archi- (e.g., England 1993; Hasumi and Suginohara 1999). If it pelago leads to greater increase in salinity at the Fram is weaker than the present setting, the increase in the Strait. If all the sea ice through the Canadian Archi- transport of the Atlantic deep circulation by opening pelago went out through the Fram Strait instead, or if the Canadian Archipelago may become smaller, since all the sea ice through the Canadian Archipelago the Atlantic deep circulation would be more stable than melted in the Arctic Ocean and the melt water de- that in this study. The weak coupling, however, would creased the salinity at the Fram Strait, the difference of not cause the transport to decrease. If the thermal cou- the freshwater transport at the Fram Strait (Table 2) pling in WB02 is stronger, the decrease in the transport between the two standard cases would decrease by of the Atlantic deep circulation by opening the Cana- 46%. Thus, the impact of the Canadian Archipelago dian Archipelago may become larger, but the transport opening may be overestimated in that sense, although would not increase. Thus, our relatively strong thermal the conclusion is qualitatively robust. coupling and the difference in strength of thermal cou- It should be noted that the response in the GIN Seas pling between this study and WB02 do not seem to to the Canadian Archipelago opening also differs be- affect our conclusion, at least qualitatively. tween this study and WB02, although the difference is Higher resolution for both the model and the clima- of minor importance on the strength of the Atlantic tology for the surface boundary condition may correct deep circulation. The amount of the deep water in the shortcomings of the present study’s model. Regions WB02 that flows out of the GIN Seas to the Atlantic where deep convection occurs are determined by fea- Ocean is 2.7 Sv when the Canadian Archipelago is tures such as surface boundary condition, geometry, closed and 5.1 Sv when it is open. The outflow from the and flow scheme. If these features are well represented GIN Seas in this study does not increase by opening the in a higher-resolution model, deep convection may also Canadian Archipelago. The different responses in the occur in more realistic regions. For instance, the cy- GIN Seas may be due to the different influence on the clonic flow scheme in the Labrador Sea would be im-

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