Jens Meincke

The great salinity anomaly in the northern North Atlantic 1968-1982

The widespread freshening of the upper 500–800m layer of the northern North Atlantic, which this paper describes, represents one of the most persistent and extreme variations in global ocean climate yet observed in this century. Though a range of explanations have been advanced to explain this event, including in situ changes in the surface moisture flux, this paper describes the Great Salinity Anomaly as largely (though not entirely) an advective event, traceable around the Atlantic subpolar gyre for over 14 years from its origins north of Iceland in the mid-to-late 1960s until its return to the Greenland Sea in 1981–1982. The overall propagation speed around this subpolar gyre is estimated at about 3cm s−1. Of the total salt deficit associated with the anomaly as it passed south along the Labrador Coast in 1971–1973 (about 72 × 109 tonnes), a deficit equivalent to about two thirds of this figure (47 × 109 tonnes) ultimately passed through the Faroe-Shetland Channel to the Barents Sea, Arctic Ocean and Greenland Sea during the mid-1970s. Possible effects on deep water formation and on the representativeness of historical section data are discussed.

Long-Term Coordinated Changes in the Convective Activity of the North Atlantic

The North Atlantic is a peculiarly convective ocean. The convective renewal of intermediate and deep waters in the Labrador Sea and Greenland/Iceland Sea both contribute significantly to the production and export of North Atlantic Deep Water, thus helping to drive the global thermohaline circulation, while the formation and spreading of 18-degree water at shallow-to-intermediate depths off the US eastern seaboard is a major element in the circulation and hydrographic character of the west Atlantic. For as long as time-series of adequate precision have been available to us, it has been apparent that the intensity of convection at each of these sites, and the hydrographic character of their products have been subject to major interannual change, as shown by Aagaard (1968), Clarke et al (1990), and Meincke et al (1992) for the Greenland Sea, in the OWS BRAVO record from the Labrador Sea, (eg Lazier,1980 et seq.), and at the PANULIRUS / Hydrostation “S” site in the Northern Sargasso off Bermuda (eg Jenkins, 1982, Talley and Raymer, 1982). This paper reviews the recent history of these changes showing that the major convective centres of the Greenland- and Labrador Seas are currently at opposite convective extrema in our postwar record, with vertical exchange at the former site limited to 1000 m or so, but with Labrador Sea convection reaching deeper than previously observed, to over 2300 m. As a result, Greenland Sea Deep Water has become progressively warmer and more saline since the early ‘70’s due to increased horizontal exchange with the Arctic Ocean through Fram Strait, while the Labrador Sea Water has become progressively colder and fresher over the same period through increased vertical exchange; most recently, convection has become deep enough there to reach into the more saline NADW which underlies it, so that cooler, but now saltier and denser LSW has resulted.

Rapid freshening of the deep North Atlantic Ocean over the past four decades

The overflow and descent of cold, dense water from the sills of the Denmark Strait and the Faroe–Shetland channel into the North Atlantic Ocean is the principal means of ventilating the deep oceans, and is therefore a key element of the global thermohaline circulation. Most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse-gas concentrations predict a weakening thermohaline circulation in the North Atlantic as the subpolar seas become fresher and warmer, and it is assumed that this signal will be transferred to the deep ocean by the two overflows. From observations it has not been possible to detect whether the oceans overturning circulation is changing, but recent evidence suggests that the transport over the sills may be slackening. Here we show, through the analysis of long hydrographic records, that the system of overflow and entrainment that ventilates the deep Atlantic has steadily changed over the past four decades. We find that these changes have already led to sustained and widespread freshening of the deep ocean.

Direct measurements of volume transports through Fram Strait

Heat and freshwater transports through Fram Strait are understood to have a significant influence on the hydrographic conditions in the Arctic Ocean and on water mass modifications in the Nordic seas. To determine these transports and their variability reliable estimates of the volume transport through the strait are required. Current meter moorings were deployed in Fram Strait from September 1997 to September 1999 in the framework of the EU MAST III Variability of Exchanges in the Northern Seas programme. The monthly mean velocity fields reveal marked velocity variations over seasonal and annual time scales, and the spatial structure of the northward flowing West Spitsbergen Current and the southward East Greenland Current with a maximum in spring and a minimum in summer. The volume transport obtained by averaging the monthly means over two years amounts to 9.5 ± 1.4 Sv to the north and 11.1 ± 1.7 Sv to the south (1 Sv = 106 m3s-1). The West Spitsbergen Current has a strong barotropic and a weaker baroclinic component; in the East Greenland Current barotropic and baroclinic components are of similar magnitude. The net transport through the strait is 4.2 ± 2.3 Sv to the south. The obtained northward and southward transports are significantly larger than earlier estimates in the literature; however, within its range of uncertainty the balance obtained from a two year average is consistent with earlier estimates.

Saline outflow from the Arctic Ocean: Its contribution to the deep waters of the Greenland, Norwegian, and Iceland seas

AbstractSince 1985 various investigators have proposed that Norwegian Sea deep water (NSDW) is formed by mixing of warm and saline deep water from the Arctic Ocean with the much colder and fresher deep water formed by convection in the Greenland Sea (GSDW). We here report on new observations which suggest significant modification and expansion of this conceptual model. We find that saline outflows from the Arctic Ocean result in several distinct intermediate and deep salinity maxima within the Greenland Sea; the southward transport of the two most saline modes is probably near 2 Sv. Mixing of GSDW and the main outflow core found over the Greenland slope, derived from about 1700 m in the Arctic Ocean, cannot by itself account for the properties of NSDW. Instead, the formation of NSDW must at least in part involve a source which in the Arctic Ocean is found below 2000 m. The mixing of various saline outflows is diapycnal. While significant NSDW production appears to occur in northern Fram Strait, large amounts of saline Arctic Ocean outflow also traverse the western Greenland Sea without mixing and enter the Iceland Sea. During the past decade, deep convection in the Greenland Sea has been greatly reduced, while deep outflow from the Arctic Ocean appears to have continued, resulting in a markedly warmer, slightly more saline, and less dense deep regime in the Greenland sea.

Interannual thermohaline changes in the northern North Atlantic 1991–1996

Abstract In the period 1991–1996 the WOCE hydrographic section A1E/AR7E between Greenland and Ireland was repeated five times. The observed thermohaline changes altered the baroclinic structure along the eastern margin of the subpolar gyre significantly. Between June 1995 and August 1996 an overall increase of the temperature and thickness and a decrease of the density of the Subpolar Mode Water (SPMW) layer were observed, accompanied by an increase of its salinity east of the Reykjanes Ridge and a decrease of its salinity in the Irminger Sea. The changes were most pronounced in the Iceland Basin, where the Subarctic Front retreated westwards, coinciding with a strong weakening of the Westerlies as determined by the North Atlantic Oscillation. They are related to a local reduction of the Ekman upwelling and the ocean-to-atmosphere heat flux on the one hand and to the advection of anomalies from the subtropics on the other hand. The eastward spreading of the different Labrador Sea Water (LSW) vintages led to a corresponding cooling of the LSW in the Irminger Sea and in the Iceland Basin in the period 1991–1996. The renewal of the LSW in the Rockall Trough occurred more sporadically, indicating that the North Atlantic Current (NAC) impedes the southward spreading of LSW in the eastern Atlantic. The changes in 1996 seem to have also counteracted this spreading.

The North Atlantic current and its associated hydrographic structure above and eastwards of the mid-atlantic ridge

Abstract Based on CTD data sets obtained in 1981–1984, XBT profiles, and long-term current meter moorings, the large-scale circulation field of the northeastern Atlantic north of the Azores was investigated. The mean volume transport through a standard meridional CTD section between 40°N and 52°N along the eastern flank of the Mid-Atlantic Ridge (MAR) was estimated to be 30 ± 9 Sv, with the North Atlantic Current (NAC) transporting 26 Sv. The NAC was found to be composed of clearly defined current branches (jets), that appear in temperature-salinity diagrams as a modal structure of the Central Water. Whereas the northernmost current branch (subarctic front) was found to be topographically fixed at the Gibbs Fracture Zone, the number, intensity and T - S structure of the remaining current branches, as well as their path over the MAR, are subject to intense variability. From 2 years of observations the branches were found to continue into the basins east of the MAR. They appeared as mesoscale features in a region of increased eddy kinetic energy and are interpreted to result from baroclinic instability. No indications of a branch of the NAC moving south, i.e. a recirculation as part of the North Atlantic subtropical gyre, were found.

Possible predictability in overflow from the Denmark Strait

The overflow and descent of cold dense water from the Denmark Strait sill — a submarine passage between Greenland and Iceland — is a principal means by which the deep ocean is ventilated, and is an important element in the global thermohaline circulation. Previous investigations of its variability — in particular, direct current measurements, in the overflow core since 1986 — have shown surprisingly little evidence of long-term changes in flow speed. Here we report significant changes in the overflow characteristics during the winter of 1996–97, measured using two current-meter moorings and an inverted echo sounder located at different depths in the fastest part of the flow. The overflow warmed to the highest monthly value yet recorded (2.4 °C), and showed a pronounced slowing and thinning at its lower margin. We believe that the extreme warmth of the overflow caused it to run higher on the continental slope off east Greenland, so that the lower current meters and the echo sounder were temporarily outside and deeper than the fast-flowing core; model simulations appear to confirm this interpretation. We suggest that the extreme warmth of the overflow is a lagged response to a warming upstream in the Fram Strait three years earlier (caused by an exceptional amplification of the winter North Atlantic Oscillation). If this is so, overflow characteristics may be predictable.

Note on the thermal structure of the Greenland Sea gyres

Abstract Closely spaced temperature profile measurements in the Greenland Sea during August 1986 reveal the existence of two cyclonic gyres, one in the northern Greenland Basin and one in the Boreas Basin. Their western boundaries and the shear zone in between them are expected to be areas of winterly deep water formation.

Time series of freshwater-transport on the East Greenland Shelf at 74N

The major flux of freshwater from the Arctic Ocean to the convective regions in the Nordic Seas and the Northern North Atlantic is linked to the shelf branches of the East Greenland Current. This flux is partitioned into a solid (ice) and a liquid phase. Time series measurements of the liquid component are hard to obtain since the seasonally varying ice cover prevents the use of standard moored instrumentation in the near surface gradient layers where most of the freshwater transport takes place. Using 40m-polyethylene tubes to protect sensors and buoyancy in the upper portion of the moorings and employing a bottom-mounted acoustic Doppler current profiler have yielded the first time series of upper layer temperatures, salinities and currents for a location at the outer East Greenland shelf at 74 ◦ N. The observed seasonal variability suggests the dominance of local warming/cooling and melting/freezing rather than advective processes. Estimates of liquid freshwater transport yield a mean of 869 km 3 /a, whereas the total freshwater transport amounts to a value between 1250 to 1750 km 3 /a. Zusammenfassung Der Transport von Suswasser aus dem Arktischen Ozean in die konvektiven Wirbel der Gronland-, Islandund Labradorsee erfolgt im Ostgronlandstrom. Er besteht aus zeitlich veranderlichen Anteilen in fester (Eis) und flussiger Form. Zeitreihenmessungen des flussigen Suswasseranteiles sind schwierig zu erhalten, da die saisonal veranderliche Eisbedeckung des Ostgronlandstromes den Einsatz verankerter Sensoren in Oberflachennahe, dort wo sich der starkste vertikale Gradient des Salzgehaltes befindet, sehr risikoreich macht. Durch die Verwendung von 40 m langen Polyathylenrohren, in denen die Temperatur- und Salzgehaltssensoren sowie die Auftriebselemente des oberen Teiles der Verankerungskette untergebracht wurden, konnten erstmals 3-jahrige Zeitreihen (2000–2003) der Schichtung des Ostgronlandstromes auf dem ostgronlandischen Schelf bei 74 ◦ N erhalten werden. Die fur Transportabschatzungen notwendigen Stromungsmessungen wurden durch den Einsatz eines am Grund verankerten, akustischen Doppler-Profilstrommessers gewonnen (2001–2002). Die beobachteten Jahresgange von Temperatur und Salzgehalt werden durch lokale Erwarmung/Abkuhlung bzw. Schmelzen/Gefrieren dominiert. Der flussige Suswassertransport fur den Zeitraum 2001–2002 ergab sich zu 869 km 3 /a, die Werte fur den gesamten Suswassertransport lagen im Bereich 1250 bis 1750 km 3 /a.