Marine Pollution Bulletin Vol. 43, Nos. 1±6, pp. 61±70, 2001 Ó 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S0025-326X400)00234-4 0025-326X/01 $ - see front matter Two Regimes of the Arctic's Circulation from Ocean Models with Ice and Contaminants A. Y. PROSHUTINSKY* and MARK JOHNSON 1 Institute of Marine Science, University of Alaska, Fairbanks, AK 99775-7220, USA A two-dimensional barotropic, coupled, ocean±ice model Ocean. Shokal'skii /1940), Gordienko and Karelin with a space resolution of 55.5 km and driven by atmo- /1945), and Buinitskii /1951, 1958) assumed that the spheric forces, river run-o, and sea-level slope between the surface circulation in the Arctic Ocean was mainly Paci®c and the Arctic Oceans, has been used to simulate driven by the in¯ow of Atlantic water through Fram the vertically averaged currents and ice drift in the Arctic Strait, producingan excess of water in the basin and Ocean.Results from 43 years of numerical simulations of causingan out¯ow of surface water and ice alongthe water and ice motions demonstrate that two wind-driven shorter routes to the Greenland Sea. Data from vessel circulation regimes are possible in the Arctic, a cyclonic drifts in the Eurasian subbasin indicated a movement and an anti-cyclonic circulation.These two regimes appear of surface water from the Eurasin subbasin to Fram to alternate at 5±7 year intervals with the 10±15 year pe- Strait. Nansen /1902), who discovered waters of At- riod.It is important to pollution studies to understand lantic origin in the Arctic Ocean, mentioned that water which circulation regime prevails at any time.It is antici- exchange between the Arctic Ocean and the Greenland pated that 1995 is a year with a cyclonic regime, and during and Norwegian Seas was caused by dierences of water this cyclonic phase and possibly during past cyclonic re- temperature and salinity, e.g., he assumed a thermoh- gimes as well, pollutants may reach the Alaskan shelf.The aline circulation. These scientists believed the Arctic regime shifts demonstrated in this paper are fundamentally Ocean water and ice circulation was mainly thermoh- important to understanding the Arctic's general circulation aline in nature and closely related to the world's ocean and particularly important for estimating pollution trans- circulation. port. Ó 2001 Elsevier Science Ltd.All rights reserved. Subsequently, some authorities /Zubov and Somov, 1940; Shuleikin, 1941; Shirshov, 1944; Treshnikov, 1959) adopted a dierent view on the nature of the water circulation, namely, that the in¯ow of Atlantic waters Introduction into the Arctic Basin was caused principally by the The variability of the general circulation of the Arctic is out¯ow of surface water to the Greenland Sea under the not as well known as it is in many other oceans. in¯uence of the existingwind system. This point of view Knowledge of the circulation is important to under- was con®rmed by experiments undertaken by Gudko- standingthe potential for pollutants and radionuclides vich and Nikiforov /1965) with a hydraulic model sim- to be carried to locations distant from a source. The ulatingthe system of wind-driven currents in the Arctic thrust of the present work is to examine the major Ocean. The experiments showed winds to be a principal forcingmechanisms that drive the Arctic's circulation, factor in formingthe system of permanent currents in and with the use of a basin scale model, identify the the Arctic Basin. The winds of the Arctic anti-cyclone relative strengths of each forcing mechanism, and the generate anti-cyclonic currents in the Amerasian sub- Arctic's response. The potential for pollutants to be basin. The wind system in the Eurasian subbasin and at dispersed depends on the total circulation comprised of the periphery of the Arctic anti-cyclone induces a wide the ¯ow resultingfrom each forcingmechanism. Trans-Arctic current originating in the Chukchi Sea and Two dierent historical views describe factors de- emerging through Fram Strait. Simulating the in¯ow of terminingthe movement of water and ice in the Arctic water as the only factor drivingthe circulation did not produce a circulation pattern similar to that actually *Correspondingauthor /now at the Woods Hole Oceanographic observed. In accordance with this concept, Gudkovich Institute, USA). Tel.: +1-508-289-2796; fax: +1-508-457-2181. /1961a,b) explained the seasonal and year-to-year vari- E-mail addresses: [email protected] /A.Y. Proshutinsky), [email protected] /M. Johnson). ation in the circulation of surface water and ice by a 1 Tel.: +1-907-474-6933. suitable rearrangement of the baric ®elds. 61 Marine Pollution Bulletin To con®rm Gudkovich's conclusions, a series of two- Arctic is at least the same order of magnitude or larger, dimensional numerical models were run /Fel'zenbaum, in terms of maximum observed current speeds, as the 1958; Campbell, 1965; Galt, 1973; Ponomarev and thermohaline forcing. Thus, the circulation regimes Fel'zenbaum, 1975; Proshutinsky, 1988, 1993), where presented in this study may be observable in the ocean, wind ®elds were the major factor drivingcirculation in dependingon the Arctic's linearity. the Arctic Ocean. It is interestingthat Galt /1973) found For this paper we have examined the way the circu- that in¯ow/out¯ow forcingand bottom topography lation in the Arctic responds due to the sea-level slope, both play a more important role in the circulation than the winds /derived from pressure), and the input from does wind forcing. However, in the research of Tres- rivers. We identify two wind-driven regimes, a cyclonic hnikov and Baranov /1972), it was demonstrated that and an anti-cyclonic regime, which alternate over a ten the major features of the water circulation may be ob- to ®fteen year period. When estimatingpathways that tained by means of diagnostic calculations according to pollutants may follow, it is important to distinguish density ®elds. These authors stated that `we could not between these two regimes, and identify which phase is evaluate the role of individual factors in the formation present. of currents because the observed ®eld of masses already That the position of the sea surface may serve as an contains the combined /and still indistinguishable) ef- integral index of processes of interaction between the fects of wind, baroclinicity, and bottom relief'. A similar atmosphere and the ocean was an idea ®rst stated by conclusion could be made accordingto the results of the Duvanin /1951). This result was con®rmed later by dif- diagnostic model by Ponomarev /1977). Prognostic ferent authors dependingon the problem they solved, models /Semtner, 1976) testify to the signi®cant role of e.g., `the position of the sea surface is an integral indi- the irregularity of the ®eld of mass in the formation of cator of most processes and phenomena related to the the three-dimensional water circulation. formation of the hydrological regime' /Galerkin, 1961), A series of dierent model runs was presented by or `the only indicator of synoptic variability of hydro- Semtner /1987), Flemingand Semtner /1991), H akkinen logical conditions in arctic seas, for which correspond- and Mellor /1992), Hibler and Bryan /1986), Riedlinger ing®eld data exist, consists of the sea-level heights' and Preller /1991), Piacsek et al. /1991). All of these /Krutskikh, 1978) Year-to-year and seasonal ¯uctua- models and numerical experiments were directed to de- tions of the ocean level in this sense may be viewed as an velopingnew models and to describingbetter observed indicator of large-scale interaction between the atmo- features of water and ice dynamics without special at- sphere and ocean. In particular, the sea surface can ev- tention to the variability of the oceanographic regime of idently be used as an indicator of large-scale circulation the Arctic Ocean. Recent papers by Hakkinen /1993, of water and ice of the Arctic Ocean and as an indicator 1995) were dedicated to modelingand explain such of the regime driving pollution transport. We use sea phenomena as deep convection in the Greenland Sea level in our paper as an integral index to describe vari- and the Great Salinity anomaly. Her investigations ability of the Arctic Ocean circulation. demonstrated again the signi®cance of atmospheric cir- This paper is organized in the following manner. culation in the coupled ice±ocean system in the Arctic. Basic equations and boundary conditions are described In this paper we assume that the general circulation of in Model Equations and Boundary Conditions. The the Arctic can be characterized by four broad responses data base is discussed in Data Base. The model results to dierent forcings. The response from the observed are given in Results and the ®nal section contains a sea-level slope between the North Paci®c ocean and the summary and discussion. Arctic is northward ¯ow through Bering Strait and out through Fram Strait, with the interior response yet to be clearly described. A second response is due to forcingby Model Equations and Boundary Conditions the atmospheric winds and pressure ®elds that drive We shall use a system of equations of motion and both a baroclinic and a barotropic response. In this continuity written in the vector notation and in the paper we treat the Arctic as a barotropic ¯uid. A third stereographic polar coordinate system response is due to the in¯ow from rivers. Associated dU with river in¯ow is a fourth response, the total ther- f k  U ÀgDmrf N m2r2U mohaline ¯ow, due to the introduction of ¯uid at a dt h boundary. cT 1 À cT À T i s b ; 1 We have modeled the response due to sea-level slope, q winds and river input.