Influence of sea ice on the Meridional Overturning Cell in the North Atlantic

Cecilie Mauritzen
Service Hydrographique et Océanographique de la Marine
Brest, France

Sirpa Häkkinen
Laboratory for Hydrospheric Processes
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771

1. Introduction

We investigate the influence of sea ice exported from the Arctic on the "Meridional Overturning Cell" (MOC) in the North Atlantic. We hypothesize that variations in fresh water input in the form of ice from higher latitudes influences convective overturn ing as well as the MOC. Results from a series of observational and modeling studies suggest that there might be a connection between these phenomena. The period in the late 1960s when fresh water capped the Labrador Sea, the vertical extent of convection, and thus the production of Labrador Sea Water, was limited (Lazier 1980). Observations suggest that there are large interannual variations in the sea ice cover in the Polar and Subpolar Seas (Parkinson 1991). Deser and Blackmon (1993) found that decadal fluctuations in SST east of Newfoundland (which may be taken as an indicator of convection depth) are traced to lag the Labrador Sea ice cover by 2 years. Modeling studies (Walsh et al 1985; Häkkinen, 1995) suggest that there are long-term trends of lower and higher ice export at Fram Strait lasting for 5-10 years. In this paper we vary the amount of ice exported from the Arctic and study the effect on the MOC with the aid of a numerical model.

2. Model

The model is a fully prognostic coupled ocean-ice model with topography-following depth coordinates (previously described by Häkkinen and Mellor, 1992, and Häkkinen, 1993). In the present setup the model domain covers 15S to the Bering Strait (Figure 1). The open ocean wind forcing is from monthly ECMWF wind stress climatology. In the Arctic, geostrophic wind stress over sea ice is used. Heat exchange is derived from ECMWF monthly climatologies of wind, temperature and humidity, and model generated surfa ce temperature and derived specific humidity. Cloudiness is taken from ISCCP measurements, the precipitation-evaporation field is obtained from NMC operational analysis, and river runoff is from Russell and Miller (1990).

Figure 1: Model grid and topography (1000m isobath).

3. Meridional Overturning Cell

It has proved quite useful, when analyzing numerical models of the North Atlantic, to distinguish between a primarily horizontal gyre circulation and a meridional overturning circulation (the "Meridional Overturning Cell", or MOC), since it is the latter that is primarily responsible for carrying heat to northern latitudes. There are several ways to derive the MOC streamfunction: we zonally integrate the velocity field in density coordinates (Figure 2) since the northward flow of warm water is light, and the returnflow is dense (an integration in depth coordinates produces spurious overturning (Mauritzen and Häkkinen, 1997, see also Döös and Webb, 1994). A 6 Sv overturning cell extends as far north as 80N, and another 12 Sv downwells between 45N and 55N, which is quite realistic. Of course, by zonally integrating one loses information about east-west variations. The three dimensional picture is more complex, but also more informative. We analyze the 3-D field by separating the water into density classes and geographical regions, i.e., a box decomposition. We define the cross-isopycnal transports as the residual of isopycnal transports and we correct for model drift. The result is a complex box diagram (see eg. Mauritzen and Häkkinen, 1996). For present p urposes the circulation is shown schematically (Figure 3). As the shallow limb of the MOC (shown in black) flows northeastward through the subpolar gyre it becomes gradually denser. It remains thermocline water (lighter than

) unti l it reaches three specific locations: near the Greenland-Scotland Ridge, in the Nordic Seas/Barents Sea, and in the Labrador Sea. In these three regions truly "dense water" is formed. Thus the downwelling of the MOC consists of three different "branches" . Only the latter two branches are forced directly by the atmosphere. Near the Greenland-Scotland Ridge thermocline waters mix with the dense overflows from the Nordic Seas so that entrainment occurs at depth without direct atmospheric buoyancy forcing. S uch a partition of the MOC agrees well with observations (see eg. Schmitz and McCartney, 1993, Dickson and Brown, 1994).

Figure 2: Annually averaged meridional overturning cell, calculated as a zonally averaged streamfunction and plotted as a function of latitude and potential density (contour interval 3 Sv).

Figure 3: Schematic of the 3-D MOC in the subpolar gyre. Dense waters are indicated in gray. The arrows indicate both diapycnal transfers and flow direction. "A" represents dense water formation in the Labrador Sea; "B" represents entrainment near the Gre enland-Scotland Ridge; "C" represents dense water formation in the Nordic Seas.

4. Ice export

How does the model respond to variations in the ice export? We ran two experiments, one with an ice export from the Arctic approximately 2800km³/yr (consistent with Aagaard and Carmack's (1989) observations), the other with an ice export of roughly 2 000km³/yr (consistent with Vinje et. al.'s 1996 observations) (Figure 4). In the subpolar gyre atmospherically forced dense water formation increases by 4-5 Sv and the MOC increases by nearly that amount (Figure 5). Ice thus strongly influences the La brador Sea branch of the MOC. In contrast, there are only weak changes in the Nordic Seas: the two experiments yield dense water exports across the Greenland-Scotland Ridge of 6.5 Sv and 6.7 Sv. The increase in dense water formation in the subpolar gyre o ccurs as a consequence of the increase in surface salinities which is caused by the reduced import of sea ice into the gyre. The subpolar gyre seems particularly sensitive to variations in the ice export because the ice melts in the region of dense water formation, ie. the Labrador Sea. In addition, the effect of variations in ice export amplifies in the subpolar gyre: the reduction in ice export through the Fram Strait is ~30%, whereas the resulting reduction of sea ice import to the subpolar gyre is dec reased by 60% (Figure 4).

Figure 4: Annual average ice export through the Fram Strait and the Denmark Strait. Dashed lines represent the reduced ice-export experiment.

Figure 5: Upper two thick curves: annual average strength of MOC at ~40N. Lower two thick curves: annual average diapycnal transport from thermocline to dense waters (across ) in the subpolar gyre. Dashed lines represent the re duced ice-export experiment. As a backdrop is shown the seasonal variability of the MOC for the two experiments (dotted lines).

5. Conclusions

These modeling studies suggest that sea ice export from the Arctic plays a significant role in determining the strength of the MOC in the North Atlantic, in particular for the branch of the MOC that downwells in the Labrador Sea. Implicit is a correspond ing sensitivity in the northward heat transport. Previous modeling studies suggest that there are periods of higher and lower ice export through the Fram Strait lasting 5-10 years. The present results suggest corresponding changes in the oceanic heat tran sport. Such changes are likely to have an impact on the climate system as a whole.

References

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