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We have argued that the outflow of relatively dense water from the Arctic Ocean to the Greenland Sea occurs over a range of depths and that its subsequent interactions with Greenland Sea waters result in several distinct intermediate and deep salinity maxima within the Greenland Sea. The principal saline core overlying the Greenland slope represents withdrawal of water from about 1700 m in the Arctic Ocean, but although it appears through mixing with GSDW to account for much of the salinity structure in the Greenland Sea, such mixing cannot by itself form NSDW, since the latter lies slightly off the mixing line on the cold and saline side. Instead, NSDW must at least in part involve a deeper source within the Arctic Ocean, apparently lying below 2000 m. A noteworthy aspect of the mixing, regardless of the Arctic Ocean source level, is that it appears to have a diapycnal component. We do not know the energy source for such mixing. For example, we find no evidence in the smaller-scale CTD profiles to suggest that double diffusive processes are important. An alternative energy source might be dynamic instabilities in the boundary current, although observations farther north suggest that there the flow over the slope is baroclinically stable [Foldvik et al., 1988]. Whether this is also true over less steep topography elsewhere remains to be explored.
Regardless of the details of the various sources and ensuing mixing, there is now clear evidence that the temperature-salinity properties of the various deep basins south of Fram Strait are controlled by the interplay between two different hydrographic regimes. One is the relatively warm and saline deep water sphere of the Arctic Ocean, the properties of which in part are accounted for by shelf processes, and the other is the much colder and fresher regime of the Greenland Sea, which is driven by open-ocean convection. A curious aspect of this situation is that the freshness of the Greenland Sea also is the result of water export from the Arctic Ocean, apparently primarily in the form of low-salinity sea ice [Aagaard and Carmack, 1989], so that in a sense both regimes are of Arctic Ocean origin, with the deep Greenland Sea attempting to mix again descendants of the salt and fresh water which were separated by freezing over the shelves of the Arctic Ocean.
Given this interplay within the Greenland Sea between source waters of very different characteristics, we might expect variability in the deep hydrographic structure if one or both of the sources also varied. Figure 8 shows two stations at essentially identical locations within the convective region of the central Greenland Sea; they represent conditions near the end of the convective season, but are seven years apart. Note that below 1500 m the deep water warmed markedly during the intervening years, generally, by 0.05°-0.08°C, depending on depth. The salinity increase during the same period appears to have been confined to below 2000 m and did not exceed about 0.002, which is close to the accuracy of these measurements. (In preparing Figure 8, we have increased the published [Clarke et al., 1984] 1982 Hudson salinities by 0.001, based on a careful comparison of the various standards used in the 2 years.)
Figure 8. Potential temperature-salinity characteristics at a site in the central Greenland Sea during 1982 and 1989, together with the depth of each observation. The inset shows the change in properties from 1982 to 1989, with positive values representing warming and increasing salinity.
Tritium/
He and CFC (chlorofluorocarbon)
measurements in the Greenland Sea over the same period suggest little deep convective
renewal during these 7 years [Schlosser
et al., 1991]. The observed thermohaline changes in the deep water therefore
likely resulted from a shutdown of the deep surface-driven convective regime
in the central Greenland Sea, which normally supplies cold low-salinity water,
while the outpouring of relatively warm and saline water from the Arctic Ocean
continued. In this connection, note in Figure 8
that the warming and slight increase in salinity reach a maximum in the depth
interval 2000-2500 m. This is the interval in which the type 3 salinity intrusion
appears to be concentrated (compare station 204, Figure
4). The absence of apparent warming during 1989 above 1500 m is consistent
with the observation that during the late winter of 1989 convection reached
to at least 1600 m depth after a period of several years of very limited convection
[Meincke
et al., 1990]. That is, the recent strengthening of the convection partially
reversed the warming trend of the 1980's, cooling (and presumably freshening;
see Figure 8) the water column above about 1500
m.
From this perspective, the importance of the warm Arctic Ocean outflow lies in its contribution of sensible heat to the Greenland Sea, which more than compensates for the effect on density of the small addition of salt by the same outflow. The net effect of this outflow is therefore to destabilize the middle and lower water column in the convective region of the central Greenland Sea. This preconditions the deep ocean to overturning after significant surface-driven convection develops in the upper water column. In this manner, the warm Arctic Ocean outflow serves as a regulatory mechanism which tends to maintain the long-term continuity of the deep ventilation in the Greenland Sea.
A crude transport estimate for the warm and saline outflow from the Arctic
Ocean can be made as follows. In both of the sections across the Greenland slope
between 75°-76°N portrayed in our Figure 2 and
in Figure 10 of Aagaard
et al. [1985], the cross-sectional area of water more saline than 34.910
is about 25 km
. The mean
salinity of this water is close to 34.913, and it principally represents the
type 1 salinity maximum. Direct current measurements in this section during
1987-1988 from two instruments moored 5 m above the bottom (locations given
in Figure 1) show a year-long mean flow of 13.2
cm s
toward SSW along
the slope in water 1250-1300 m deep. Measurements of shorter duration (106-224
days) the following year, made 5 m and 600 m above the 1800 m isobath, i.e.,
slightly farther seaward, gave record-length means from 8.0-9.7 cm s
. A time-weighted mean value suggests that the flow through this section at
depths of 1200-1300 m is about 12 cm s.
For a cross section of 25 km,
this yields a transport of 3 Sv with a mean salinity of 34.913. From the regression
line of Figure 4, water of this salinity represents
a mixture containing two-thirds Arctic Ocean water, suggesting that the outflow
rate of the intermediate salinity maximum from the Arctic Ocean (type 1) is
about 2 Sv. A small amount of the type 2 water has also been included in this
calculation, while the type 3 water, which is important to the formation of
NSDW, has largely been omitted. A similar calculation can be made for a section
farther north, namely, that of Smethie et al. [1988; their Figure 5a]
near 78°N. The cross-sectional area of water in that section more saline than
34.910 is about 70 km,
and the mean salinity of this water is close to 34.917. If we assign the year-long
mean speed of 2.3 cm s
at 1378 m measured about 150 km farther north by Foldvik
et al. [1988; their Table 1] during 1984-1985, the transport of the
saline water through this section is 1.6 Sv. From the regression line of Figure
4, water of salinity 34.917 has a type 1 content of about 80%, suggesting
an outflow rate from the Arctic Ocean of 1.3 Sv. Our range of 1.3-2.0 Sv contrasts
with the smaller estimates of Smethie
et al. [1988] (0.8-0.9 Sv) and Heinze
et al. [1990] (0.7-1.0 Sv), both based on box models. Apart from issues
of long-term variability, our larger outflow values are reasonable considering
the various model assumptions, particularly (1) the capping in the models of
the deep water sphere at 1500-1700 m (thereby missing the large amount of saline
Arctic Ocean outflow above those depths) and (2) the assumption that all the
outflow mixes with GSDW (which is clearly not the case, since a significant
fraction continues southward into the Iceland Sea without mixing).
Converting our estimated outflow rate into a production rate for NSDW is problematic,
principally because of uncertainties about the production process and the fraction
of saline water which actually participates in NSDW production. Nonetheless,
a set of fairly reasonable assumptions suggest a production rate quite consistent
with calculations using the growing body of transient tracer data. For example,
if we assume for the moment that (1) the total outflow of saline waters of types
1 and 3 from the Arctic Ocean is 2 Sv, (2) only one half of this outflow mixes
with GSDW (the remainder entering the Iceland Sea; see Figure
5), and (3) the portion which is mixed does so in a 2:1 ratio (see Figure
4), then a production rate of new NSDW of 1.5 Sv is implied. Since the volume
of the Norwegian Sea below 1700 m is 8.6 × 10
km [Smethie
et al., 1988], a deep water formation rate of 1.5 Sv would give a replacement
time of 18 years. This is comfortably in the range of time scales estimated
from transient tracer box models [e.g.,
Schlosser, 1985; Smethie
et al., 1988; Heinze
et al., 1990], although certain details of these models may, as we've
suggested, not be realistic. In the present case, these model assumptions apparently
do not lead to unrealistic replacement times, possibly because in the real ocean
only a portion of the initially large outflow from the Arctic Ocean mixes with
GSDW to produce NSDW.
On the other hand, it is now clear that the steady state assumptions implicit in these kinds of calculations have limited applicability to the Greenland Sea, and in particular that the production of GSDW is highly variable, so that both the properties and the effective renewal rates of the NSDW can be expected to vary on time scales of a few years. It is also clear that the outflow of deep waters from the Arctic Ocean represents a complex pattern in potential temperature-salinity space. In particular, what we have here classified as type 1 water cannot by itself account for the various salinity maxima south of Fram Strait, but instead additional sources from deeper levels in the Arctic Ocean must also be invoked. Furthermore, the mixing regime and the circulation pattern are more complex than has been generally appreciated. For example, diapycnal mixing is important, there is significant NSDW production outside the western Greenland Sea (e.g., in northernmost Fram Strait), deep waters from the Arctic Ocean also feed the Iceland Sea, and large amounts of NSDW probably never enter the Norwegian Sea but instead recirculate to the north and/or are discharged into the Arctic Ocean [cf. Smethie et al., 1988; Swift and Koltermann, 1988]. We suggest that as attempts continue to construct plumbing diagrams of the thermohaline circulation of the various arctic seas and their interconnections, these diagrams will grow increasingly more complex and more interconnected and will increasingly indicate significant variability. This undoubtedly has important implications for climate modeling.
Acknowledgments. We are grateful to Gerd Rohardt, who took care of the CTD instrumentation and processing and assured the quality of this data set. The three reviewers of the manuscript provided exceptionally helpful suggestions. Partial support came from the Arctic Program, Office of Naval Research. NOAA Pacific Marine Environmental Laboratory contribution 1235; Alfred Wegener Institute for Polar and Marine Research contribution 290.
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