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Intensification of the Atlantic Deep Circulation by the Canadian Archipelago Throughflow

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Intensification of the Atlantic Deep Circulation by the Canadian Archipelago Throughflow MAY 2005 KOMURO AND HASUMI 775 Intensification of the Atlantic Deep Circulation by the Canadian Archipelago Throughflow 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 ef- fect 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 © 2005 American Meteorological Society Unauthenticated | Downloaded 09/27/21 09:22 AM UTC JPO2709 776 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 35 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 circu- lation. 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.
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