Ocean-Sea Ice Dynamics as Simulated by a Circulation Model
3. Model design and experimental set-up
4.1 Upper ocean circulation and sea ice Cover in the Arctic Mediterranean
4.1.1 P r e s e n t - d a y ( c o n t r o l r u n )
The control run is aimed at simulating the present-day circulat,ion. In the following, we show annual mean fields which apply t,o the last year of the integration period.' The ocean circulation averaged over the top 80 m in the polar and subpolar seas3 is presented in Fig. 4a. The model captures the characteristic features of the ob- served flow pattern markedly well. A strong cyclonic gyre dominates the Nordic Seas, consisting of the EGC (East Greenland Current) in the west, and the NAC (Norwegian Atla,ntic Current) in the east. The latter transports wa,rm &nd salty water frorn the Athntic to the North, while the EGC carries cold, relatively fresh polar water t o the South. where it leaves the Nordic Seas through Denmark Strait:.
Atlantic water enters the Ba,rents Sea. bringing some heat into the Arctic Ocean.
This current constitutes the southern branch of an overall cyclonic flow pattern in the eastern Arctic Ocean. The Canadian Basin in the western Arctic is dominated by the anticyclonic Beaufort Gyre. The western anticyclonic gyre meets the eastern cyclonic circulation in the central Arctic, thereby forining the current system of the Transpolar Drift (TPD). The T P D carries polar waters towards the outlets of the Arctic Ocean, namely Fram Stra,it and Nares Strait (Canadian Archipelago).
Nea,r-surface salinities are shown in Fig. 4b. High salinities (> 35 psu) in the Nor- wegian and the western Barents Seas indicate the inflow of Atlantic waters from the South. In the Arctic Ocean proper, salinities are much lower with minima in the Siberian shelf seas due t o inflowing river water. Low-saline shelf waters are advected into the central Arctic Ocean, eventually leaving the Arctic Ocean through Frain Strait or the Canadian Archipelago. The s o u t l ~ w ~ r d flow of polar water in the EGC causes low salinit.ies in the western Nordic Seas.
The distribution of sea ice is presented in Fig. 5. We recognize a typical pattern that is well-known from other inodel studies (e.g., Hibler 1979; Harder 1996) and which is consistent with sonar data (e.g., Hibler 1979; Bourke & McLaren 1992). This pa.ttern is cl~ar~cterized by maxiinum ice thickness north of Ca,nada, a n ice thickness of 3--- 4 m near the pole, z~nd relatively thin ice t o the north of Siberia. The mean ice drift is indicated by arrows in Fig. 5. It resembles the upper ocean circulation, with an anticyclonic gyre over t,he Canadian Basin, a T P D , outflow through Fram Strait, a,nd an EGC.
2h,lulti-year a.veraging is not necessary, since internal interannual variability proves to be negli- gible in thc modelled Arctic Ocean.
3T11e top 80 m are represented by the three topn~ost levels of the model grid and comprise the surface mixed layer with the upper part of the cold halocline in the Arctic Ocean.
size of the Beaufort Gyre.
Variations in the Transpolar Drift during the Holocene were hypothesized by Dyke et al. (1997) from radiometric analyses of driftwood collected in the Canadian Ar- chipelago. For the mid-Holocene, coinrident with the large freshwater input t>o the Laptev Sea (see Section 2.3), the driftwood record suggests an eastward shift of the T P D and increased outflow through Fram Strait. T h e model results (Fig. 8b) reveal a strong connection between Arctic river runoff and the ocean circulation which inay help to explain the TPD's variability during the Holocene.
6. Conclusions
Coinpiling data and information from the available literature, we tried t o paint a consistent picture of freshwater influx into the Arctic Ocean for the early and middle Holocene. A quantifica.tion, however, is difficult and subject t o considerable uncer- tainty. In the future, we expect t o gain more insight into the past Arctic freshwater budget by utilizing coupled climate models. Recent efforts in paleoclimate modeling intercoinparison, however, revealed considerable discrepancies among the various models in use concerning mid-Holocene P -
Ern
high latitudes (cf. de Noblet et al.2000).
Even though soine speculative assumptions were necessary in order t o construct the freshwater forcing used in our experiments, we believe t h a t the rough magnitudes a.re realistic. Our results show t h a t important effects on the polar oceanic circulation are associa,ted with these magnitudes. The Arctic Ocean surface circulation is not siinply driven by winds, as it is often claimed. Freshwater is dispersed by the oceanic flow field in upper layers. while the freshwater distribution is vitally important for driving the circulation. The results suggest that long-terin Holocene variability in Arct,ic freshwater forcing caused considerable va,riability in Arctic Ocean dynamics on a century-to-millennium time scale.
A gradual deepening of the Bering Strait until t h e mid-Holocene was probably as- sociated with an increasing heat flux into the Arctic Ocean. The model shows that the inflowing heat exerts a strong influence on polar sea ice. An intensified warm Bering Strait inflow causes a decline in sea ice coverage in the Chukchi and East Siberian Seas. Where sea ice is replaced by Open water, the surface albedo decrea- ses. It is therefore likely, that the gradually increasing influx of Pacific water during the early Holocene slowly affected the polar climate. Regarding the global iinpact of variable Arctic freshwater forcing on the oceanic circulation, the model results suggest only a small effect. Even though the freshwater influx applied in our ex- periments can be considered as extreme (massive freshwater input from the North American/Greenland coasts, doubled runoff into the Laptev Sea), the influence On the strengt11 of the THC is negligible.
In the present work, we studied the influence of Arctic freshwater forcing oll the coupled ocean-sea ice system. Examining the dynamical impact of varying atmos- pheric forcing a,nd ocean bottorn topography acting both sepa.rately and in concert,
would b e t h e logical next s t e p towards understanding t h e role of t h e Arctic Ocean in Holocene cliinate variability.
Acknowledgments. We thank R . Gerdes and K. Herterich f01- their support. The careful reviews of J.-H. Kim, D. Handorf and H. von Storch helped to improve the manuscript. We kindly acknowlcdge financial support from the German Federal Ministry for Education, Science and Research (BMBF) through the KIHZ and DEKLIM projects.
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Region Present 7 ka 10 ka Norwegian coast
Barents Sea Kara Sea Laptev Sea East Siberian Sea North American/North Greenland coast Total
-P
-Table 1: Fkeshwater input from continents to the Arctic Ocean for the present-day control run, Experiment 7 ka, and Experiment 10 ka. Units are km3 y r l .
Input
Output
Fig. 1: Present-day freshwater budget of the Arctic Ocean (only the major contributors are shown). Values (in km3 y r l ) are based on Aagaard & Carmack (1989), Steele et al.
(1996), and Prange (2003). Sea ice is mainly exported through R a m Strait. P - E denotes net precipitation.
Fig. 2: Arctic rivers implemented in the model and their mean discharge (in km3 y r l ) . Values for Taimyra and Pyasina are taken froin Treshnikov (1985). For all other rivers the flow into the ocean was calculated based 011 gauged discharge data, provided by the Global Runoff Data Centre (GRDC) a t the Federal Institute of Hydrology, Koblenz, Germany.
Fig. 3: Domain of the model. The inodel equations are defined on a rotated grid. Both the geographical and the inodel grid coordinates are displayed. The frame marks t h e area that is shown in Figs. 4-8.
Fig. 4: Annual mean fields of the upper Arctic Ocean (averaged over 0-80 m) in t h e present-day control run: (a) Velocity, (b) salinity (contour interval is 0.5 psu). Labels refer t o the rotated model grid.
Fig. 5: Annual mean sea ice thickness in the present-day control run. The contour interval is 1 m. Thc mean ice drift pattern is indicated by arrows. Labels refer t o the rotated nlodel grid.
Fig. 6: Differences in mean upper ocean fields (averaged over 0-80 m ) between Experiment 10 ka and the present-day control run (i.e., 10 ka - present-day): (a) Salinity (contour interval is 0.5 psu), (b) velocity.
Fig. 7: Difference in mean sea ice thickness between Experiment 10 ka and the present-day control run. The contour interval is 0.1 m.
Fig. 8: Differences in mean upper ocean fields (averaged over 0-80 m) between Experiment 7 ka and the present-day control run (i.e., 7 ka - present-day): (a) Salinity (contour interval is 0.5 psu), (b) velocity.
Fig. 9: Meridional overturning streamfunction in the North Atlantic averaged over the last 5 years of the integration period: (a) Present-day control run, (b) Experiment 10 ka,
(C) Experiment 7 ka. The contour intervals are 1 Sv. Positive va.lues represent clockwise rotation in the plane of the figure. Labels refer to the geographical grid.
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