Yusuke Kawaguchi+∗ and Humio Mitsudera∗
+Department of Earth Environmental Science, Hokkaido University, Sapporo, Japan,
∗Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan, Emails: kawa-y@lowtem.hokudai.ac.jp, humiom@lowtem.hokudai.ac.jp
Abstract
In this paper, effect of along-shore (downwelling-favorable) wind on maximum salinity anomaly of Dense Shelf Water (DSW) is investigated by scale-based estimates and numerical model. First, the salinity anomaly is approximated by scaled salinity fluxes due to baroclinic eddies, along-shore cur-rent and Ekman compensation flow through the internal layer. The estimates show that the salinity anomaly remarkably decreases for large along-shore wind speed Ua. We found a critical wind speed Uc, where the wind-induced salinity export is comparable to that of baroclinic eddies; we estimated as Uc=5.4 ms−1 for the standard parameters. It is also revealed that the Ekman compensation flow has greater contribution to the salinity discharge than the along-shelf coastal current. Consequently, its time scale to reach the equilibrium anomaly is determined by the Ekman compensation flow as long as Ua > Uc. Further, we also carried out numerical calculations assuming the Sea of Okhotsk, in which an idealized model is imposed by realistic surface buoyancy forcing and along-shelf wind stress, based on ECMWF data. The simulated salinity shows a good agreement with the direct measurement of Shcherbina et al. (2003). Furthermore, we applied the theoretical estimates to the northwestern polynya (NWP) and the northern polynya (NP) in the Sea of Okhotsk in addition to numerical cal-culations. According to the results, the along-shore wind causes greater salinity decrease in NP than in NWP, whose variations substantially depends on the Aleutian Low activity.
Figure 1: AVHRR image of coastal polynyas and ice distribution in the Okhotsk Sea (taken on March 7 in 2004). Dark and light colors show thin and thick ice covers, respectively.
1 Introduction
Coastal polynyas often occur in polar continental shelf regions. Brine rejection accompanied by large amount of ice production in the coastal polynyas increases density of underlying water, producing dense
Water (Foster and Carmack, 1976; Carmack and Killworth, 1978; Foster and Middleton, 1980; Jacobs, 1986). Furthermore, many studies have suggested that the DSW plays an important role in maintaining the Arctic cold halocline (Aagaard et al., 1981; Melling and Lewis, 1982; Jones and Anderson, 1986;
Martin and Cavalieri, 1989; Cavalieri and Martin, 1994).
In the Sea of Okhotsk, strong northeast winds and cold temperature from northeastern Eurasia cause the growth of coastal polynyas in the pack ice and a large amount of sea ice production (Fig. 1) (Martin et al., 1998; Gladyshev et al., 2000; Ohshima et al., 2003). Accompanied by the large ice production, cold, oxygen-rich DSW is formed over the shelf. The DSW is transported in the southern part of the Okhotsk Sea and forms Okhotsk Sea Intermediate Water by a confluence of the Soya Warm Current Water (Watanabe and Wakatsuchi, 1998; Itoh et al., 2003; Gladyshev et al., 2003). Subsequently, it is diapycnally mixed with the East Kamchatka Water around the Kuril Island, contributing to the Oyashio Current Water. Thus, the DSW formed over the Okhotsk northern shelves is believed to be a ventilation source of the North Pacific Intermediate Water (Yasuda, 1997). Consequently, the DSW also brings out signals of climate change occurring in the Okhotsk Sea to the entire North Pacific. Itoh (2007) has reported rapid warming of the Okhotsk intermediate water based on historical data since 1950’s.
Nakanowatari et al. (2007) noted that the warming of the Okhotsk Sea is the greatest level in the North Pacific intermediate layer, and it spreads out in the subarctic gyre. They also suggest decreasing of sea ice formation in the Okhotsk Sea, mainly in NWP, as the most likely cause. The total amount of DSW production could be also changed by formation and modification processes, e.g., tidal mixing or entrainment/detrainment, even with the same amount of ice production (Nakamura et al., 2006).
Therefore, it is important to investigate the detailed processes of the DSW formation process over the northern shelf regions in the Sea of Okhotsk.
Shcherbina et al. (2003) presented direct measurements of DSW formation during 1999/2000 winter using two bottom moorings over the northwestern shelf in the Okhotsk Sea. A steady, near-linear salinity increase was observed at the inshore mooring over a month. The total salinity increase was 0.83 PSU for 35 days, corresponding to potential density increase of 0.68 kgm−3. Shcherbina et al. (2004) estimated the DSW export based on the moored velocity, and highlighted the importance of along-shelf advection to the observed salinity change. Simizu and Ohshima (2002) simulated remarkable along-shore coastal currents over the northwestern shelf by a barotropic numerical model. They interpreted the mechanism as the so-called arrested topographic waves (ATW) driven by the along-shore wind stress (Csanady, 1987).
Therefore, the effects of along-shore wind needs to be further discussed in terms of DSW formation over the Okhotsk shelves.
Generally, it is known that baroclinic eddies, developed at the edge of coastal polynya, effectively disperse the density anomaly through the cross-shore salinity transport (Gawarkiewicz and Chapman, 1995 and 1997; Chapman, 1999; Tanaka and Akitomo, 2000). On the other hand, Chapman (2000) investigated contribution of ambient currents with different along-shore velocities to the lateral DSW discharge and its influences on the total amount of DSW production, using an idealized numerical model.
He found that although there are almost no significant changes in the total DSW production, the ambient current advects the dense water downstream, and consequently reduces the maximum salinity anomaly.
Since the along-shore wind also yields the cross-shore Ekman current and its compensation flow (Carmack and Chapman, 2003; Yang, 2006), the dense water advected by the cross-shore circulation should be included in the lateral salinity flux from the polynya region for better estimate, in addition to the effects of the along-shore current discussed in Chapman (2000).
In this study, we investigate the effects of along-shore wind stress on the DSW formation, including the effect of the offshore salinity flux by the Ekman transport. First, dependency of equilibrium salinity anomaly on the along-shore wind speed is examined by an analytical approach. The analytically derived, salinity estimates are tested by idealized numerical experiments, forced by uniform buoyancy flux and along-shore wind stress at the surface. Additionally, we conducted idealized experiments assuming the Okhotsk coastal polynyas, which are forced by realistic wind stress obtained from ECMWF data and buoyancy forcing based on the surface heat budget. Here, the wind effect is also examined for NWP and NP, independently.
This paper is organized as follows: Section 2 describes model configurations. The results are discussed in Section 3. In section 3.1, theoretical estimates for the maximum salinity anomaly is presented and compared with a series of numerical calculations. In section 3.2, numerical experiments assuming the Okhotsk coastal polynyas are carried out, which are forced by a realistic along-shore wind stress and buoyancy forcing. Section 4 gives our conclusion.
Some figures are omitted from this article due to limited text length. Original version of this manuscript was submitted to JGR in July 2008 and now under review. See the figures in the original paper after published.
2.1 Ocean Model
The numerical model used in this study is the Princeton Ocean Model described by Blumberg and Mellor (1987). The model is a free-surface, primitive equation model that uses the hydrostatic and Boussinesq approximations for an incompressible fluid. The three-dimensional equations for conservation of mass, momentum, potential temperature, and salinity are solved using a finite differencing scheme, coupled with an equation of state. A level 2.5 turbulent closure scheme is embedded in the model to provide vertical mixing parameters. The horizontal viscosity and diffusivity coefficients are calculated by using the Smagorinsky diffusion formula (Smagorinsky, 1963).
The model domain is idealized for continental shelf in the Okhotsk Sea. A schematic drawing of the model is not presented here (see Fig. 2 of the original paper). Independent variables x, y and z denote along-shore, cross-shore, and vertical directions, respectively. The eastward and northward are positive in xand y directions, respectively. The model spans 200 km in the cross-shore direction for a standard case. Note that the along-shore distanceLx varies depending on the simulation (typically,Lx=400 km or 800 km). In this study, we designate the southward direction to the offshore. The bottom topography huniformly increases in the southward, given by
h= (h0−αy)
where h0 is the depth at the northern coast, αis the bottom slope iny direction andα=0.001. Here, =Lx/5, which is an along-shore length forhvarying near the western and the eastern boundaries.
In this study, we need to use a numerical model with a high resolution in the vertical and horizontal directions to represent eddies, the surface and bottom boundary layers. A baroclinic Rossby radiusR is roughly estimated to be 5 - 10 km from results of numerical experiments. Thus the horizontal grid size, 2 km, resolves features with the scale of deformation radius. The vertical distribution of 21 sigma levels is set along the water depths. The resolution becomes higher as the sigma reaches the surface and the bottom.
The ocean is assumed to begin at rest before the surface salinity flux is applied at time t =0. The model ocean initially has a homogeneous structure (potential temperature -1.8◦C and salinity 32.5 PSU) in all experiments. The Coriolis parameter f is set to be uniform and 1.3×10−4 s−1 throughout the present study. The condition of no heat and salt fluxes is applied at all boundaries. The surface salinity fluxFsis imposed as a buoyancy forcing in a range of offshore widthbof polynya from the coast. Fsand b are held constant (Fs =-1.0×10−5 PSU ms−1, b =15 km) in all experiments of section 3.1. Easterly wind stress (downwelling-favorable) is uniformly imposed throughout the study, given by
τa=ρaCaUa|Ua|, (2) where |Ua| (=
Ua2+Va2) is a magnitude of wind speed, Ua and Va are along-shore and cross-shore velocity components, respectively. The magnitude ofτa is the greatest aty=200 km (coastal boundary) and decreases offshore reaching zero aty=0.
2.2 Thermodynamical Polynya Model
The thermodynamical polynya model developed by Pease (1987) forces the numerical model in section 3.2 for an application to the Sea of Okhotsk, providing offshore widthbof polynya and surface buoyancy fluxFs. Fs andb are updated every 6 hours on the basis of surface heat balance, based on the ECMWF meteorological data. They are also evaluated independently for NWP and NP from the meteorological variables averaged within (135-142◦E, 55-59◦N) and (147-153◦E, 59-60◦N), respectively (see Fig. 9 of the original). In the present study, eastern part of the northern shelf is taken as NP.
Fsis determined by the ice production rateVi and can be written by
Fs=ρiVi(So−Si)10−3, (3) whereSo andSi are the ocean surface and the ice inside salinity. Then,So−Si gives the total amount of rejected salt and it is set to be 0.69So here (c.f., Martin et al., 1998). Vi is based on the surface heat
Ch =0.002,Tw =-1.8◦C. From eq. 4, the surface salinity flux depends on both the air temperatureTa
and the magnitude of wind speed|Ua|.
b is determined by a competition between ice formation within open water area and offshore ice advection by cross-shore windVa, and presented by the following differential equation:
db dt +Vi
hi
b= 0.03Va, (5)
wherehi is the collection depth of newly formed frazil ice, and is defined to be 0.1 m here.