Manfred Bersch

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.

Transformation of the Labrador Sea Water in the subpolar North Atlantic

Development, spreading and decay of the thermohaline properties of two Labrador Sea Water (LSW) classes are described. During the development phase, a specific LSW class repeatedly mixed by winter convection in the Labrador Sea becomes colder, denser, thicker and deeper. Once convection weakens, the LSW class becomes isolated from the upper layer and starts to decay, rapidly losing its volume while retaining the same density due to isopycnal mixing with the neighbouring warm saline intermediate waters. A similar pattern in temperature, salinity and density is seen in the other basins with different time lags from about two years in the Irminger Sea to ten years in the northern Iceland Basin and Rockall Trough regions. The influence of LSW on the thermohaline properties of other North Atlantic water masses is also discussed.

On the circulation of the northeastern North Atlantic

Abstract Along a section from Cape Farvel at the southern tip of Greenland to the Porcupine Bank off the Irish coast (WOCE section A1E/AR7E), obtained in September 1991, geostrophic velocities were calculated from CTD measurements and referenced to velocities recorded by an Acoustic Doppler Current Profiler (ADCP) in the upper 500 m. The mean accuracy of the absolute geostrophic velocities is estimated at±3.6 cm/s, which is comparable to that resulting from the assumption of a layer of no motion. The derived flow field is essentially columnar without a pronounced layer of no motion. Maximum velocities of 20–30 cm/s occur in the upper 1000 m of the East Greenland Current and the Porcupine slope current and at 2000 m depth in the Denmark Strait overflow. The meandering of the Irminger Current crossing the Reykjanes Ridge and an enhanced mesoscale variability along the Rockall Plateau and in the Rockall Trough are indicated. Compared to Schmitz and McCartney (1993) the estimated volume transports suggest an intensified meridional circulation with an increased supply from the subtropics to the Subpolar Mode Water (SPMW) and an increased entrainment of SPMW into the deeper layers. In the bottom layer with densities σ 0 ⩾ 27.80 kg/m 3 a reduced circulation of Iceland-Scotland Overflow Water (ISOW) in the Irminger Basin and a recirculation of ISOW in the Iceland and West European basins are found. About 12 Sv of bottom water are transported southwards and join the Deep Western Boundary Current.

Topographic effects of the Maud Rise on the stratification and circulation of the Weddell Gyre

Abstract Hydrographic data indicate that due to the Maud Rise (65°S, 3°E) the upwelling of relatively warm and more saline Lower Circumpolar Deep Water (LCDW) in the Weddell Gyre and the entrainment of LCDW into the upper layer are enhanced, which increase the ocean heat loss and enrich the salinity of the upper layer. By these processes the water column is preconditioned for deep convection. An anomalous water column with a diameter of about 150–200 km, notable for its relatively cold deep water and its relatively saline upper layer, is situated above the crest of the Maud Rise. Water from its main pycnocline spreads laterally within the upper 500 m and compensates for the entrainment of LCDW into the upper layer. Current meter data from the Winter Weddell Sea Project 1986 show a marked temporal and spatial variability of the southward transport of LCDW in the lee of the Maud Rise. An anticyclone of LCDW, which had cut off from the main body of LCDW east of the Maud Rise, migrated southward along the western slope with a velocity of about 3 cm s−1. It was followed by a cyclone, which had probably formed at the front of the anomaly due to a strengthened flow, undergoing an enhanced meandering. The wavelength of the entire disturbance is estimated to be about 380 km, one-third of the circumference of the Maud Rise anomaly. If the disturbance repeats continuously, about two to three anticyclones detach per year.

A twentieth-century reanalysis forced ocean model to reconstruct the North Atlantic climate variation during the 1920s

The observed North Atlantic multi-decadal variability for the period 1872–2009 is reconstructed with the Max Planck Institute ocean model, which is forced with an ensemble of the atmospheric twentieth century reanalysis. Special emphasis is put on the early part of the experiments, which includes a prominent climate variation during the 1920s. The experiments are in agreement with selected hydrographic records, indicating a transition from cold and fresh North Atlantic water properties, prior to the 1920 climate variation, towards warm and saline waters afterwards. Examining the variation reveals that sea level pressure (SLP) anomalies prior to the 1900s resemble a negative phase of North Atlantic Oscillation and associated weak winds result in a weak North Atlantic Current (NAC) and sub-polar gyre (SPG). This leads to a reduced transport of warm and saline waters into the higher latitudes. Simultaneously, Arctic freshwater release results in the accumulation of cold and fresh water properties, which cover the upper layers in the Labrador Sea and subsequently suppress convection. From the 1910s, the Arctic freshwater export is reduced, and, NAC and SPG are strengthened as a result of an increased SLP gradient over the North Atlantic. Concurrently, Labrador Sea convection and Atlantic meridional overturning circulation (AMOC) increase. The intensified NAC, SPG, and AMOC redistribute sub-tropical water into the North Atlantic and Nordic Seas, thereby increasing observed and modelled temperature and salinity during the 1920s.

Upper layer thermal structure in the Atlantic sector of the Southern Ocean

By means of 7 XBT* sections between South America resp. South Africa and Antarctica the variability of the Subtropical Convergence (STC), the Polar Frontal Zone (PFZ) and the Weddell-Scotia Confluence (WSC) are particularly discussed. Between 45°W and 56°W, the STC is shifted to the South by 2.3 degrees of latitude confining a tongue of subtropical surface water near the continental slope of the Patagonian shelf. Further South, the PFZ is displaced to the North by 7 degrees of latitude between 58°W and 39°W. An intense Subantarctic Front has been observed northwestwards of South Georgia. Even East of the South Sandwich Islands the WSC can be detected by a positive temperature anomaly.