Detlef Quadfasel

The Arctic Circumpolar Boundary Current

Abstract The main water transformations in the Arctic Mediterranean take place in the boundary current of Atlantic Water, which crosses the Greenland–Scotland ridge from the North Atlantic into the eastern Norwegian Sea. It enters and flows around the Arctic Ocean before it exits the Arctic Mediterranean as the East Greenland Current, primarily through Denmark Strait. On route, it experiences numerous branchings and mergings. By examining how the properties of this “circumpolar” boundary current evolve, it is possible to identify and describe the processes causing the water mass transformations in the Arctic Mediterranean. It is also possible to follow the Arctic Ocean deep waters as they spread into the Nordic Seas and eventually provide 40% of the overflow water supplying the North Atlantic Deep Water.

Outflow of dense water from a Svalbard Fjord into the Fram Strait

Abstract A plume of dense bottom water from Storfjorden in Svalbard spreads northward along the eastern side of Fram Strait. During its descent to depths of more than 2000 m the plume entrains some 500% of surface and intermediate waters. Based on observations from 1986 it appears to contribute about 5–10% to the annual deep water production of the polar basins.

Is the Arctic Ocean warming

Recent high-quality hydrographic measurements in the Arctic Ocean have revealed a warming of up to 1 K of the Atlantic Layer when compared with Russian climatologies of the 1940s to 1970s. About half of this warming can be attributed to the different methods by which the two data sets were obtained: the climatologies are based on discrete bottle data in the vertical and necessarily involve smoothing in space and time, whereas the modern, quasi-synoptic data are from continuously recording conductivity-temperature-depth sondes and give much better resolution in the vertical. Modern surveys also focused more on boundary current regimes where narrow warm bands of Atlantic Water are present. The remainder of the warming detected can be explained with the following physical arguments: (1) an increased inflow of Atlantic Water in the early 1990s and (2) a higher temperature of this inflowing water. Temperature time series in the Barents Sea since the beginning of this century suggest that the warming of the early 1990s is not a long-term climate signal but, rather, is related to the inherent natural variability of the system with timescales of decades.

Circulation in the Timor Sea

In October 1987 and March 1988, measurements were taken across the Sahul shelf and the southwestern end of the Timor Strait to the edge of Indonesian waters. The shipboard instrumentation comprised a conductivity-temperature-depth (CTD) rosette, an acoustic Doppler current profiler (ADCP) and a Pegasus dropsonde. Complementary data came from satellite-tracked drifters, continental shelf moorings, and a Nansen bottle survey in 1976. Pegasus and ADCP measurements in the strait suggested a total transport of about 7 Sv toward the Indian Ocean, with about half of this in the upper 350 m. However, transports may at times be higher, because a drifter in July 1983 revealed speeds of l m s−1 in the strait, twice those measured on the surveys. Data from moored current meters implied a transport on the shelf of roughly 1 Sv, except in the autumn transition of the monsoon in 1985 when it exceeded 3 Sv. Water properties measured on the 1987 and 1988 surveys suggested components from the Flores and Banda seas, Indian Ocean Central Water and a high-salinity subsurface plume from evaporation on the inner Sahul shelf.

Currents and transports of the Monsoon Current south of Sri Lanka

The zonal monsoon circulation south of India/Sri Lanka is a crucial link for the exchange between the northeastern and the northwestern Indian Ocean. The first direct measurements from moored stations and shipboard profiling on the seasonal and shorter-period variability of this flow are presented here. Of the three moorings deployed from January 1991 to February 1992 along 80°30′E between 4°11′N and 5°39′N, the outer two were equipped with upward looking acoustic Doppler current profilers (ADCPs) at 260-m depth. The moored and shipboard ADCP measurements revealed a very shallow structure of the near-surface flow, which was mostly confined to the top 100 m and required extrapolation of moored current shears toward the surface for transport calculations. During the winter monsoon, the westward flowing Northeast Monsoon Current (NMC) carried a mean transport of about 12 Sv in early 1991 and 10 Sv in early 1992. During the summer monsoon, transports in the eastward Southwest Monsoon Current (SMC) were about 8 Sv for the region north of 3°45′N, but the current might have extended further south, to 2°N, which would increase the total SMC transport to about 15 Sv. The circulation during the summer was sometimes found to be more complicated, with the SMC occasionally being separated from the Sri Lankan coast by a band of westward flowing low-salinity water originating in the Bay of Bengal. The annual-mean flow past Sri Lanka was weakly westward with a transport of only 2–3 Sv. Using seasonal-mean ship drift currents for surface values in the transport calculations yielded rather similar results to upward extrapolation of the moored profiles. The observations are compared with output of recent numerical models of the Indian Ocean circulation, which generally show the origin of the zonal flow past India/Sri Lanka to be at low latitudes and driven by the large-scale tropical wind field. Superimposed on this zonal circulation is local communication along the coast between the Bay of Bengal and the Arabian Sea.

Observed and modelled stability of overflow across the Greenland–Scotland ridge

Across the Greenland–Scotland ridge there is a continuous flow of cold dense water, termed ‘overflow’, from the Nordic seas to the Atlantic Ocean. This is a main contributor to the production of North Atlantic Deep Water that feeds the lower limb of the Atlantic meridional overturning circulation, which has been predicted to weaken as a consequence of climate change. The two main overflow branches pass the Denmark Strait and the Faroe Bank channel. Here we combine results from direct current measurements in the Faroe Bank channel for 1995–2005 with an ensemble hindcast experiment for 1948–2005 using an ocean general circulation model. For the overlapping period we find a convincing agreement between model simulations and observations on monthly to interannual timescales. Both observations and model data show no significant trend in volume transport. In addition, for the whole 1948–2005 period, the model indicates no persistent trend in the Faroe Bank channel overflow or in the total overflow transport, in agreement with the few available historical observations. Deepening isopycnals in the Norwegian Sea have tended to decrease the pressure difference across the Greenland–Scotland ridge, but this has been compensated for by the effect of changes in sea level. In contrast with earlier studies, we therefore conclude that the Faroe Bank channel overflow, and also the total overflow, did not decrease consistently from 1950 to 2005, although the model does show a weakening total Atlantic meridional overturning circulation as a result of changes south of the Greenland–Scotland ridge.

A note on the seasonal variability of the South Java Current

The seasonal cycle of the South Java Current is explored using current measurements with satellite-tracked drifters and climatological compilations of ship drifts. It is shown that the changing monsoon winds and the variations of the freshwater flux from the Indonesian archipelago are responsible for the annual cycle of the flow, while the actual reversals between the seasons are strongly influenced by remote forcing through equatorial and coastal long waves from the central Indian Ocean.

Formation of Southern Hemisphere Thermocline Waters: Water Mass Conversion and Subduction*

Abstract The ventilation of the permanent thermocline of the Southern Hemisphere gyres is quantified using climatological and synoptic observational data. Ventilation is estimated with three independent methods: the kinematic method provides subduction rates from the vertical and horizontal fluxes through the base of the mixed layer, the water age uses in situ age distribution of thermocline waters, and the annual-mean water mass formation through air–sea interaction is calculated. All three independent estimates agree within their error bars, which are admittedly large. The subduction rates are mainly controlled through their vertical and lateral components with only minor transient eddy contributions. The vertical transfer, derived from Ekman pumping, ventilates over most of the areas of the subtropical gyres, while lateral transfer occurs mainly along the Subtropical and Subantarctic Fronts, where it injects mode and intermediate waters. For the permanent thermocline the overall ventilation of the Sout...

Convection and deep water formation in the Arctic Ocean-Greenland Sea System

Abstract The processes of convection and deep water formation in the Nordic Seas are reviewed. In the Arctic Ocean the formation of deep water results from brine release due to freezing, which increases the density of the shelf waters. The dense shelf waters sink on the continental slopes into the deep basins entraining ambient waters from the strongly stratified Arctic Ocean proper. In the European Polar Seas—the Nordic Seas—deep water is only formed in the Greenland Sea through haline convection, resulting in a weakly stratified water column. The exchanges through Fram Strait, the deep connection between the Arctic Ocean and the Nordic Seas are used to quantify the average rate of deep water formation. The net deep outflow from the Arctic Ocean corresponds to the production of Arctic Ocean Deep Water. By considering the total deep outflow and mixing ratios derived from θ-S characteristics of the different basins the formation rate of Greenland Sea Deep Water may by inferred. Both sources provide about 0.5 Sv making the arctic contribution to the World Ocean deep waters at least 1 Sv.

Renewal of deep water in the Red Sea during 1982–1987

Between October 1982 and May 1983 the deep water in the northern Red Sea was ventilated through slope convection. The deep water became cooler, fresher, and more oxygenated. Dense bottom water formed in the Gulf of Suez during winter cooling sank down the continental slope, thereby entraining near-surface and intermediate depth waters. From the application of a one-dimensional plume model, a vertical transport of 0.58 × 106 m3 s−1 over a period of 7 months was estimated. Long-term time series of atmospheric heat fluxes show that such a major convection event can occur every 4–7 years. The corresponding renewal time of the Red Sea deep water is thus of the order of 40–90 years, in agreement with previous estimates.