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Svein Østerhus and Tor Gammelsrød

Abstract

Over the past decade, the multiyear oceanographic time series from ocean weather station Mike at 66°N, 2°E indicate a warming by about 0.01°C yr−1 in the deep water of the Norwegian Sea. The time of onset of this warming is depth dependent, starting at 2000-m depth in 1987 but not at the 1200-m level until 1990. The warming abruptly halts around 1993 for a couple of years before it culminates in the absolute maximum temperatures in the end of the 50-yr-long record. This warming is attributed to variations in the amount and direction of interchanges between the three deep basins of the Nordic seas: the Arctic Ocean, the Greenland Sea, and the Norwegian Sea. The reduction of deep convection in the Greenland Sea from the early 1980s and the increased horizontal exchange with the relatively warm deep waters of the Arctic Ocean are the proximate cause of the warming, leading to a constant rise in the temperature of the “parent” Greenland Sea deep water (GSDW) from the early 1980s. These changing GSDW characteristics were passed on to the deep Norwegian Sea via the Jan Mayen Channel through Mohn Ridge, entering at a depth determined by the sill depth of this passage (2200 m) and propagating to shallower depths thereafter. The cessation of deep warming in the Norwegian Sea from 1993 to 1995—not shown by the GSDW—is attributed to the reversal of flow in the Jan Mayen Channel as GSDW production was greatly reduced, as confirmed by direct current measurements. The importance of the Arctic Ocean–Nordic seas system to global climate emphasizes the importance of identifying and understanding the mechanisms that control the interbasin dynamics of the Nordic seas and simulating them realistically in models.

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Len Zedel, Lee Gordon, and Svein Osterhus

Abstract

The Ocean Ambient Sound Instrument System (OASIS) consists of a conventional RD Instruments Acoustic Doppler Current Profilers (ADCPs), modified to allow the recording of high-quality ambient sound in the frequency range from 1 to 75 kHz. In addition to the usual capabilities of an ADCP, this combination of acoustic instrumentation allows wind direction and speed to be inferred from a subsurface platform. The method of wind speed determination from ambient sound levels is explained identifying some of the techniques and limitations. Wind direction is inferred from surface drift velocities using the ADCP data. The design of both the hardware and software required to make combined ADCP and ambient sound recordings is discussed. The capabilities of the system are demonstrated using observations made in the Norwegian Sea at Ocean Weather Station Mike. Using previously published algorithms (and calibration constants) ambient sound–based wind speeds are found that closely match direct wind observations. The typical standard deviations for hourly wind speed estimates is 1.5 m s−1 when using acoustic frequencies less than 10 kHz. Higher acoustic frequencies show greater variance in wind speed estimates. OASIS estimates of the 12-h average wind directions have an error standard deviation of 25° with no mean bias.

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Carina Bringedal, Tor Eldevik, Øystein Skagseth, Michael A. Spall, and Svein Østerhus

Abstract

The Atlantic meridional overturning circulation and associated poleward heat transport are balanced by northern heat loss to the atmosphere and corresponding water-mass transformation. The circulation of northward-flowing Atlantic Water at the surface and returning overflow water at depth is particularly manifested—and observed—at the Greenland–Scotland Ridge where the water masses are guided through narrow straits. There is, however, a rich variability in the exchange of water masses across the ridge on all time scales. Focusing on seasonal and interannual time scales, and particularly the gateways of the Denmark Strait and between the Faroe Islands and Shetland, we specifically assess to what extent the exchanges of water masses across the Greenland–Scotland Ridge relate to wind forcing. On seasonal time scales, the variance explained of the observed exchanges can largely be related to large-scale wind patterns, and a conceptual model shows how this wind forcing can manifest via a barotropic, cyclonic circulation. On interannual time scales, the wind stress impact is less direct as baroclinic mechanisms gain importance and observations indicate a shift in the overflows from being more barotropically to more baroclinically forced during the observation period. Overall, the observed Greenland–Scotland Ridge exchanges reflect a horizontal (cyclonic) circulation on seasonal time scales, while the interannual variability more represents an overturning circulation.

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