NUMERICAL SIMULATIONS OF BERING SHELF CIRCULATION 



BRING 

 3 I — -y S IMIT ALASI<A y=100 km 



TOPOGRAPHY 



NO IMPOSED 

 CURRENT 



Fig. 9. Numerical simulations of a rectangular Bering Sea Shelf, showing the 

 Anadyr Current to be a boundry current of concentrated flow resulting 

 from northward flow across the shoaling shelf; the current is 

 concentrated on the left looking up-slopc. From Kinder, Chapman & 

 Whitehead, 1986. 



is produced by wind effects driving water against the shoreline 

 boundaries adjacent to the straits and thereby altering the 

 pressure head through them (Aagaard et ai. 1985; Coachman 

 & Aagaard, 1988). The Anadyr Current is much less constrained 

 by boundaries (also true of the downstream Chukchi Sea 

 portion of the ecosystem). It also seems unlikely that the large- 

 scale driving force due to the difference in sea levels changes 

 sufficiently in magnitude or rapidly enough to cause variations 

 like those observed in the Chirikov basin; in particular, it is 

 unlikely the Anadyr Current ever ceases or reverses. This is of 

 crucial importance to the ecosystem — it guarantees a relatively 

 steady, continuous, and large supply of nutrients. Furthermore, 

 the residence times of water parcels within the gulf, which is the 

 site of the first production center (cf. Fig. 3), are likely to be 

 much less variable than they are in the Chirikov basin; at 1 5 to 

 20 cm s ', water parcels will transit the gulf in about 1 month. 

 In contrast with the Gulf of Anadyr and Chukchi Sea, the 

 middle active production center of the ecosystem — the Chirikov 

 basin — has restrictive physical boundary conditions on its flow 

 field. Two straits restrict inflow of water from the south, 

 Shpanberg to the east of St. Lawrence and Anadyr to the west, 

 and there is only one outlet, Bering Strait. From the Gulf of 

 Anadyr, most of the Anadyr Current most of the time flows 

 northward through Anadyr Strait into the Chirikov basin. It is 

 joined by a transport of the less saline Bering Shelf water from 

 south of St. Lawrence Island. Bering Shelf water enters the 

 basin also around the eastern end of St. Lawrence through 

 Shpanberg Strait, while the eastern part of this strait usually 



carries Alaskan Coastal water from the south; the latter water 

 mass forms a bouyancy boundary current along the entire 

 Alaskan coast. All these waters pass northward out of the 

 Chirikov basin through Bering Strait. Crossing Chirikov basin, 

 the three water masses tend to remain discretely side-by-side; 

 there is little lateral mixing between them (Coachman et ah 

 1975). The production center is in the center and western part 

 of the basin, associated with Anadyr and Bering Shelf water 

 masses, not Alaskan Coastal. 



The northward flow is constricted passing through the 

 straits, which has important physical consequences for 

 ecosystem production. In transiting the straits, the currents 

 increase speed and greater amounts of bottom-friction-generated 

 turbulent energy becomes available for mixing the water 

 columns. Any stratification that has developed upstream from 

 the straits is largely broken down, and materials (in particular 

 nutrients) that have collected in the lower layer are redistributed 

 into the upper layer. 



A south-to-north section through Anadyr Strait from a 

 3-dimensional nonlinear dynamical model (Deleersnijder & 

 Nihoul, 1988) shows these effects graphically (Fig. 10). 

 Immediately north of Anadyr Strait, there are high amounts of 

 turbulent kinetic energy in the upper 20 m (Fig. IOC) and 

 upwelling velocities as great as 5 X 10'cms'(Fig. lOA); these 

 together have nearly broken down strong thermal stratification 

 existing south of the strait (Fig. lOB). The result is a large 

 plume of colder, nutrient-rich water spreading from Anadyr 

 Strait northward along the Siberian shore and eastward across 

 the Chirikov basin, frequently visible in satellite imagery 

 (Nihoul et ai, 1990). The effect is registered in hydrographic 

 dataas much reduced vertical stratification; compare the plume 

 of small lower-upper layer salinity differences in Fig. 7. 

 Notice also a similar plume extending northward from Bering 

 Strait. 



The ecological importance of the enhanced vertical mixing 

 in Anadyr Strait and. subsequently, in Bering Strait is enormous; 

 it effectively "resets" the system downstream from areas of 

 large production so that another production cycle can occur. 

 Production takes place in the waters transiting the Gulf of 

 Anadyr and some stratification develops from Anadyr runoff. 

 Nutrients become depleted in the upper layer. The "resetting" 

 feature mixes nutrients back into the upper layer where they 

 can fuel another round of high production. The Chirikov basin 

 production, and layering in its northern part due to Alaskan 

 Coastal water in the tunneling flow field (cf. Fig. 7), create a 

 similar situation just south of Bering Strait. These conditions 

 are "reset" by the Bering Strait flow leading to large production 

 in the southeastern Chukchi (Fig. 3). 



In the tightly bounded Chirikov basin another aspect of the 

 flow field is important. The in- and outflows through the 

 bounding straits are not one hundred percent correlated; there 

 are periods ranging from days to weeks when the in-flow via 

 Anadyr and Shpanberg Straits does not equal the outflow 

 through Bering Strait. During these times water volume in the 

 basin is not conserved, which is compensated by either a rise or 

 fall in sea level (cf. Aagaard et ai, 1985); changes in sea level 

 at Nome as great as 1 m in a couple of days have been observed. 



22 



