Gereon Budéus

On the hydrography of the Northeast Water Polynya

The water masses and circulation in the area of the Northeast Water Polynya, located on the East Greenland Shelf north of 79°N, are described on the basis of an R/V Polarstern cruise during spring and summer 1993. The baroclinic flow shows northward components close to the East Greenland coast and eastward components at the northern limit of the polynya. An anticyclonic half circle is formed by this and the southward flowing East Greenland Current. In the south the circle is not closed. The upper water column, occupied by Polar Water, is affected by this circulation pattern, while deeper waters in the trough system of the area seem to spread independently. Different types of deep waters are found in different troughs, all being of Atlantic origin, though they seem not to be directly connected to Return Atlantic Water. It is shown that what is called Polar Water must be formed, at least partly, on the Greenland Shelf and that deepwater formation does not occur in the investigated area.

Fate of vent-derived methane in seawater above the Håkon Mosby mud volcano (Norwegian Sea)

Abstract The Hakon Mosby mud volcano (HMMV) is a cold methane-venting seep situated at the Norwegian–Barents–Spitsbergen continental margin. Methane discharged by the vent creates a plume in the ambient seawater at 1200 m water depth. Here, we study the hydrographic regime to evaluate its influence on the distribution pattern of methane as it is shown by the concentration gradients. The stable carbon isotopic signature of methane is used to trace the vent methane in the lateral and vertical direction and to trace its fate in the hydrosphere. Direct methane release into the bottom water occurs in the central zone of the HMMV. Methane included in the subseafloor reservoir and in the plume above the vent has the same carbon isotopic ratios, which means that the released methane is not oxidized. The shape of the methane plume is determined by spreading predominantly along its original isopycnal. However, vent methane is traceable in seawater up to 800 m above the HMMV by methane values, which exceed the background and high carbon isotopic signature heterogeneity. The fate of vent methane in the hydrosphere is dominated by dilution and mixing with background methane within the bottom water rather than by oxidation as it is shown by the carbon isotopic ratios. Methane released by the HMMV moves northward with deep intermediate waters, which are detached from the ocean surface and may enter the polar Arctic Ocean. Consequently, methane discharged at the HMMV results in a direct input of fossil methane into the Recent methane reservoir of the deeper ocean and reduces the capacity of the deep ocean as a sink for atmospheric methane.

On the generation of the Northeast Water Polynya

In spring and summer of 1993 a multidisciplinary survey of the Northeast Water (NEW) Polynya, located on the continental shelf northeast of Greenland, was undertaken by R/V Polarstern. Hydrographic and remotely sensed data from this expedition are analyzed with respect to the generation and the seasonal development of the NEW. Its formation is concluded to result from the combined effect of a fast ice barrier, extending perpendicular from the coast and bridging a trough system, and a northward flowing coastal current. The fast ice barrier protects the region downstream of it from ice import while current driven ice export continues. The temporal development and spatial distribution of hydrographie parameters in the NEW is primarily controlled by its generation mechanism. Continental and sea ice melt induce vertical stability in certain parts of the polynya, giving rise to enhanced primary production there.

Winter convective events and bottom water warming in the Greenland Sea

From observations on yearly cruises to the central Greenland Sea between 1993 and 1996, conclusions are drawn with respect to winter convection and bottom water renewal. The data indicate that winter convection was extremely weak after 1993, not even ventilating the intermediate waters. This is remarkable, since the salinities in the upper layers increased considerably between 1993 and 1995, thus providing presumably favorable conditions for winter convection. With the absence of deep reaching winter convective events, the temperatures in the deeper waters of the Greenland Gyre increased steadily by about 0.03 K between 1993 and 1996. We conclude from the development of mainly the thermal structure on a zonal transect that an explanation for the temperature increase can be given by a large-scale downward water movement of about 150 m/yr in the central Greenland Sea. The data indicate that this process is independent of changes in the dynamically induced density distribution. It is therefore possible that a downward movement, perhaps masked by other processes, may continue for many years. If this is the case, resulting flushing times would be of the order of 20–30 years only. The presence of a large-scale circulation cell with downward movement in the central Greenland Gyre would explain the observed warming of the bottom waters without the demand for an actually active heat source. It is also in accordance with the observed increase of chemical tracer concentrations in the deep waters.

Nutrient status of the Northeast Water Polynya

The nutrient distribution in the Northeast Water Polynya (NEW) was investigated intensively between the end of May and the beginning of August 1993 during the R/V Polarstern cruise ARK IX. The major characteristics were low initial nitrate concentrations (ca. 4 μM) in the surface mixed layer of the East Greenland Shelf Water, accompanied by high silicate values (ca. 10–14 μM). These concentrations were not reduced by phytoplankton growth. Silicate was rather homogeneously distributed in the entire water column, whereas nitrate increased continuously with depth to about 13 μM. Phosphate concentrations were about 1.1 μM and had a similar distribution to that of silicate. During the course of the summer, nutrients became depleted, and nitrate was exhausted in large parts of the NEW. Silicate was reduced to values of less than 2 μM at some stations which implies that diatom growth continued despite nitrate depletion, ammonium serving as a nitrogen source. The polynya is fertilised by water with the initial nutrient concentrations downstream of the Norske Oer Ice Shelf. This process continuously supplies nutrients to the surface throughout the year and these are transported northward by the anticyclonic surface circulation following the topography of the trough system. The northern boundary of this tongue of relatively nutrient-rich water is controlled by the uptake of nutrients by phytoplankton in summer. Its extemsion is variable due to interactions between biological processes, circulation and ice cover. In the Ob Bank region the nutrient distribution can be altered by the inflow of Polar Water from the north when strong northerly winds prevail as happened during the first part of the study.

Distribution and exchange of water masses in the Northeast Water Polynya (Greenland Sea)

On the basis of classical hydrographic and nutrient analysis, water masses and their spreading in the Northeast Water (NEW) Polynya were investigated from RV Polarstern ARK IX (1993) data. It is shown that a local water body, East Greenland Shelf Water, occupies the top layer in the NEW and that this water is different from Polar Water exported from the Arctic Polar Ocean. Polar Water, as well as the underlying and also imported Knee Water, follows a path crossing the broad East Greenland Shelf diagonally from northeast to southwest but both waters do not enter the NEW Polynya. Intermediate waters in the NEW are also modified locally. A local source of silicate, contributing to an intermediate silicate maximum in the trough system, is identified in the centre of the anticyclonic movement over Belgica Bank. Furthermore, it is confirmed that there is no one-directional through-flow of deeper waters in the trough system. Belgica Trough and Westwind Trough contain two different water types of Atlantic origin, which are not directly related to Return Atlantic Waters. The deeper waters in Norske Trough are supplied from Belgica Trough over a sill of about 250 m depth.

Summary of the Northeast Water Polynya formation and development (Greenland Sea)

Abstract The mechanisms forming the Northeast Water Polynya (NEWP), located on the shelf northeast of Greenland, are elaborated for different sub-domains of the polynya for the years 1990–1993. Sea-ice formation, sea-ice melt and the horizontal divergence of ice-floe velocity owing to winds and currents are investigated for the sub-domains. Several shelf-ice barriers of different size extend perpendicular to prevailing currents and winds causing a divergence of the ice-field flow in their lee. In summer, winds are weak and the northwand flowing coastal current in combination with Norske Oer Shelf Ice (NOSI) constitutes the dominant forcing of the Northeast Water summer polynya forming in the southern part of the NEW area. During this season the polynya gradually increases its size towards the north since the air-sea heat budget is positive then and now ice-formation takes place. In winter, strong northerly wind outbreaks push newly formed sea-ice out of Ob Bank region in the northern NEW area which is sheltered by Ob Bank Shelf Ice (OBSI) and the eastern coast of Kronprins Christians Land. Since during the winter season the air-sea temperature difference can amount to up to 30°C new ice forms rapidly balancing the ice export.

Wind-triggered events of phytoplankton downward flux in the Northeast Water Polynya

Phytoplankton carbon fluxes were studied in the Northeast Water (NEW) Polynya, off the eastern coast of Greenland (79� to 81� N, 6� to 17� W), during summer 1993. The downward flux of organic particles was determined during 54 days using a sediment trap moored at a fixed location, below the pycnocline (130 m). The hypothesis of the present study is that wind events were ultimately responsible for the events of diatoms downward flux recorded in the trap. Wind conditions can influence the vertical transport of phytoplankton by affecting (1) the environmental conditions (e.g. hydrostatic pressure, nutrient concentrations, and irradiance) encountered by phytoplankton during their vertical excursion, and (2) the aggregation and disaggregation of phytoplankton flocs. The first mechanism affects the physiological regulation of buoyancy, whereas the second one affects the size and shape of settling particles. Using field data (wind velocity, density profiles and phytoplankton abundance), we assessed the potential aggregation and the vertical excursion of phytoplankton in surface waters. The results show that, upstream from the trap, wind and hydrodynamic conditions were sometimes favourable to the downward export of phytoplankton. Lag-correlation between time series of wind and phytoplankton downward flux shows that flux events lagged wind events by ca. 16 days. Given that the average current velocity in the top 100 m was ca. 10 cm s � 1 , a lag of 16 days corresponded to a lateral transport of ca. 130 km, upstream from the sediment trap, where phytoplankton production was lower than at the location of the trap. According to that scenario, 21% to 60% of primary production was exported to depth during wind events. If we had assumed instead a tight spatial coupling between the material collected in the trap and the relatively high phytoplankton production at the location of the trap, we would have concluded that <7% of primary production was exported to depth. The difference between the two scenarios has great implications for the fate of phytoplankton. Our results stress the importance of investigating the spatial coupling between surface and trap data before assessing the pathways of phytoplankton carbon cycling. D 2002 Elsevier Science B.V. All rights reserved.

The anatomy of the Arctic Frontal Zone in the Greenland Sea

From a series of two-dimensional expendable bathythermograph (XBT) surveys, conductivity-temperature-depth (CTD) sections, surface drifters, and acoustic Doppler current profiler (ADCP) observations, the properties and structure of the Arctic Frontal Zone in the Greenland Sea have been determined. The Arctic Frontal Zone appears to be a large-scale, climatic “multifrontal” frontal zone. The structure of the frontal zone can be discerned from subsurface as well as from surface hydrographic parameters, even in summer, when a seasonal thermocline covers the subsurface hydrographie structure. The Arctic Frontal Zone consists of two semipermanent frontal interfaces with warm, saline Norwegian Atlantic Water to the east and Arctic Water from the Greenland Sea gyre to the west. The two frontal interfaces are bounding a band of shallow cyclonic cold eddies and anticyclonic warm eddies with horizontal scales of the order of 40–50 km. The typical diameter of the eddies can be scaled with the local internal Rossby radius of deformation. The eddy kinetic energy of the surface flow in the frontal zone is of the order of 60 to 85 cm2 s−2. The zonal density gradient in the Arctic Frontal Zone maintains a mean northward geostrophic transport of 3.8 Sv, averaged over a number of cruises. This transport is mainly connected with the frontal interface on the western side of the warm and saline Norwegian Atlantic Water. The estimated cross-frontal eddy transports of heat and salt appear to be of considerable importance for the conditioning of the Greenland Sea gyre.

The North East Water polynya (Greenland Sea)

Physical observations of the North East Water (NEW) polynya, located near the north-eastern corner of Greenland, are presented. Data were collected in June 1991 by RV Polarstern. An idea is put forward to explain how the NEW is generated. A northward coastal current interacts with a persistent shelf ice barrier under which water can flow but that retains ice floes and therefore protects the NEW area from ice advection. Since in summer, the combination of currents, barrier and air-sea heat balance gives rise to a polynya. The distribution of upper water column vertical stability in the NEW is also influenced by its generation process. Surface melt water is retained by the shelf ice barrier, causing neutral vertical stability in its lee. Sea ice melting and land runoff then act as two distinct sources of vertical stability enabling the development of plankton blooms, especially in the northern part of the NEW.