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ICES M arine Science Symposia, 219: 375-377. 2003

Formation o f intermediate water in the Greenland Sea during the 1990s

Jo h a n n e s K arstensen, Peter Schlosser, Jo h a n n Blindheim, J o h n Bullister, a n d D ouglas W allace

Karstensen, J., Schlosser, P., Blindheim, J., Bullister, J., and Wallace, D. 2003.

F orm ation o f intermediate w ater in the Greenland Sea during the 1990s. - ICES M arine Science Symposia, 219: 375-377.

The transform ation rates o f upper water into intermediate water (500 to 1600 m) of the central Greenland Sea are deduced from annual changes in C FC tracer inventories between 1991 and 2000. Transform ation was found to be interm ittent in time, mainly taking place in the winters of 1994/1995 and 1999/2000. F orm ation rates are o f the order o f 0.2 to 0.9 Sv (1 Sv = 106 m 3 s_1), equivalent to a 10-year average o f up to 0.2 Sv. Associated changes in heat content o f the intermediate layer are consistent with a winter-time heat loss o f 20 W m-2 over 1 m onth (75 W n r 2 over 1 week) at the sea surface.

Keywords: Greenland Sea, heat flux, variability, water mass formation.

J. Karstensen and P. Schlosser: Lam ont-D oherty Earth Observatory o f Columbia University, 61 R t 91V, Palisades, N Y , 10964, USA [tel: +1 845 3658707: fa x : +1 845 3658176: e-mail: jkarsten@ ldeo.columbia.edu: peters@ldeo.columbia.edu]; J. Blind- heim: IM R , PO B ox 1870, NO-5024 Bergen, Norway [tel: +47 5 238569: fa x : +47 5 238531; e-mail: johan.blindheim@imr.no]; J. Bullister: P M E L IN O A A , 7600 Sand Point Way, N.E. Seattle, WA. 98115, USA [tel: +1 206 5266741; fa x : +1 206 5266744; e-mail: johnb@ pmel.noaa.gov]; D. Wallace: IfM , Diisternbrooker Weg 20, D-24105 Kiel, Germany, [tel: +49 431 6004200; fa x : +49 431 6004202; e-mail:

dwallace@ifm.uni-kiel.de]. Correspondence to J. Karstensen.

Introduction

The Greenland Sea is a prominent site for water mass transformation. Its wind-driven cyclonic circu­

lation causes a doming of the isopycnals towards the gyre center that promotes convection due to winter-time buoyancy loss. Convection to interme­

diate depth (of the order of 1500 m) was observed frequently from the late 1980s, whereas deeper con­

vection was never directly observed. The convection to intermediate depth forms Greenland Sea Arctic Intermediate Water (GSAIW) (Swift and Aagaard, 1981; Blindheim, 1990). Here we explore formation rates of GSAIW for the time interval 1991 to 2000 combining hydrographic and transient tracer data along a 75°N section crossing the Greenland Sea gyre.

Characteristics and formation of GSAIW

The convection process is the first step in the process of ventilating the ocean's interior. The intensity of

atmospheric heat loss is the major driver in the pro­

cess, whereas salt fluxes play a role at least in trig­

gering convection of surface water (Rudels, 1990).

Variable surface forcing leads to variable transfor­

mation of upper waters in terms of transformed volume and tracer characteristics.

The temporal evolution of temperature and CFC 11 (Figure 1) show a deep penetration of trac­

ers during the winters of 1994/95 and 1999/2000 as a consequence of convection and subsequent transport into the interior. In particular, these convection events changed the CFC inventories substantially (Figure 2).

Formation rates were calculated as the amount of mixed-layer water needed to produce the observed change in CFC inventory. As the ocean does not lose CFCs through its surface (in contrast to for example heat) and atmospheric time histories are quasi-constant over the period considered in this study (Walker et ai, 2000), inventories can be ass­

umed either to increase or remain quasi-constant, as upper waters have only weak lateral gradients.

Thus, our strategy was to calculate the height of a

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376 J. Karstensen et al.

11 (pmol/kg)

1991 1992 1993 1994 1995 199E 1997 199S 1999 2000 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Figure 1. Tem poral evolution o f potential tem perature (°C; left) and C F C 11 (pmol k g '1; right) between N ovem ber 1991 and May 2001. N ote the convective events in winter 1994/1995 and 1999/2000 bringing cold water with high C F C 11 signals to intermediate depths. C ontours are produced assuming th at the tracer signal is im printed in M arch o f each year o f observation.

220

=•2000

1 180

I"160

® 140

c120 100

110 - 1 0 00

1 90

o 80

§ 70

>

50

Figure 2. Change in C F C 11 and C F C 12 inventories (1 x 103mol) considering an area o f 4.5 x 10® m 2 and a depth range from 500 to 1600 m. N o te the inventory change between 1994/95 and 1999/2000.

column of upper waters required to yield the obse­

rved increases in the CFC inventory over time. We assumed that the Atlantic Water layer plays a role in the convection process and analyzed the changes in inventories in the water column that reaches from below the core of this layer (500 m depth) to a depth of 1600 m, where no increase in CFC was observed during the 1991 to 2000 time interval (Figure 1).

For CFC 11 we found an increase in the inventory of about 30 to 40 kmol in both winters with corresponding values for CFC 12 between 20 and

CFC 11

CFC 12

_ru y u m u u u u i_

11/9111/9211/9311/94 11/95 11 /96 4/97 8/98 6/99 5/2000

30 kmol (Figure 2). This translates into replacement of a water layer of 100 to 200 m depth, correspond­

ing to volumes between 4.5 and 9 x 1012 m3, respec­

tively (radius of gyre 1.2 x H)5 m). These numbers assume that the CFCs are completely confined in the gyre after convection and only “older waters”

are removed from the gyre by the addition of rec­

ently ventilated, high CFC waters. However, con­

sidering a 50% entrainment of “older water” with a pre-convective CFC inventory, water equivalent to a column of about 300 m thickness has to be exchanged to yield the observed increase in CFC inventories.

Export of newly formed water

The related export of water from the convective site (subduction) is most likely caused by isopycnal eddy transfer, as shown by Khatiwala and Visbeck (2000) for the Labrador Sea. They found a relaxation time of the density field in the order of 7 months. Using this value we obtain a transport between 0.3 and 0.6 Sv for both winters, which increases to 0.9 Sv using a 50% entrainment. The annual average trans­

port is in the order of 0.1 to 0.2 Sv, because only two major convection events occurred during the 10 years for which data were analyzed. The average numbers compare well with the findings of Rhein (1996) averaging over the 1980s and early 1990s.

Although a salinification of the intermediate layer is observed over the 10 years, interannual salinity changes do not appear to be significant. In contrast, temperature is affected in a sense that the convec­

tion interrupts the general warming trend in the

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Formation o f intermediate water in the Greenland Sea 377

layer (0.02 K y r 1) by holding the average tempera­

ture at a constant level between successive winters.

Surface heat fluxes calculated from this value yield reasonable values of about 20 W n r 2 if averaged over 1 month or 75 W n r 2 if averaged over 1 week.

Conclusions

The transformation of surface water into intermedi­

ate water in the Greenland Sea during the 1990s was found to be intermittent in time (two main events) and associated with an average transport of about 0.2 Sv. Frequently, the North Atlantic Oscillation (NAO) index is used as indicator for the atmo­

spheric forcing and thus convection activity. A posi­

tive (negative) index is related to weak (strong) convection activity in the Greenland Sea. Since the mid-1970s, a generally positive NAO index (with a few exceptions) prevailed and the highest values were observed in the 1990s. Thus, if this correlation holds, low GSAIW formation rates are not sur­

prising. As the GSAIW spreads into the Nordic Seas, lower formation rates may have consequences for the ventilation of the deeper waters in this region. However, the present data set is not suf­

ficiently dense to provide hard evidence for such a conclusion.

Acknowledgements

We thank the crew of the RV “Johann Hjort”

as well as all the cruise participants for helping to collect the data over the years. The study was supported by the National Oceanographic and Atmospheric Administration through grant NA86GP0375.

References

Blindheim, J. 1990. Arctic Intermediate W ater in the Norwegian Sea. Deep-Sea Research, 37: 1475-1489.

Khatiw ala, S., and Visbeck, M. 2000. A n estimate o f the eddy-induced circulation in the L abrador Sea. Geophysical Research Letters, 27: 2277-2280.

Rhein, M. 1996. Convection in the Greenland Sea, 1982-1993.

Journal o f Geophysical Research, 101: 18183-18192.

Rudels, B. 1990. Haline convection in the Greenland Sea.

Deep-Sea Research, 37: 1491-1511.

Swift, J. H., and Aagaard, K. 1981. Seasonal transitions and water mass form ation in the Iceland and Greenland Sea.

Deep-Sea Research, 28: 1107-1129.

W alker, S. J., Weiss, R. F., and Salameh, P. K. 2000. Recon­

structed histories o f the annual mean atmospheric mole fractions for the halocarbons CFC-11, C F C -12, CFC-113 and carbon tetrachloride. Journal o f Geophysical Research, 105: 14285-14296.

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