Laura de Steur

Characteristics of colored dissolved organic matter (CDOM) in the Arctic outflow in the Fram Strait: Assessing the changes and fate of terrigenous CDOM in the Arctic Ocean

Absorption coefficients of colored dissolved organic matter (CDOM) were measured together with salinity, delta O-18, and inorganic nutrients across the Fram Strait. A pronounced CDOM absorption maximum between 30 and 120 m depth was associated with river and sea ice brine enriched water, characteristic of the Arctic mixed layer and upper halocline waters in the East Greenland Current (EGC). The lowest CDOM concentrations were found in the Atlantic inflow. We show that the salinity-CDOM relationship is not suitable for evaluating conservative mixing of CDOM. The strong correlation between meteoric water and CDOM is indicative of the riverine/terrigenous origin of CDOM in the EGC. Based on CDOM absorption in Polar Water and comparison with an Arctic river discharge weighted mean, we estimate that a 49-59% integrated loss of CDOM absorption across 250-600 nm has occurred. A preferential removal of absorption at longer wavelengths reflects the loss of high molecular weight material. In contrast, CDOM fluxes through the Fram Strait using September velocity fields from a high-resolution ocean-sea ice model indicate that the net southward transport of terrigenous CDOM through the Fram Strait equals up to 50% of the total riverine CDOM input; this suggests that the Fram Strait export is a major sink of CDOM. These contrasting results indicate that we have to constrain the (C)DOM budgets for the Arctic Ocean much better and examine uncertainties related to using tracers to assess conservative mixing in polar waters. Citation: Granskog, M. A., C. A. Stedmon, P. A. Dodd, R. M. W. Amon, A. K. Pavlov, L. de Steur, and E. Hansen (2012), Characteristics of colored dissolved organic matter (CDOM) in the Arctic outflow in the Fram Strait: Assessing the changes and fate of terrigenous CDOM in the Arctic Ocean, J. Geophys. Res., 117, C12021, doi:10.1029/2012JC008075.

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

A new ocean observing system has been launched in the North Atlantic in order to understand the linkage between the meridional overturning circulation and deep water formation. For decades oceanographers have understood the Atlantic Meridional Overturning Circulation (AMOC) to be primarily driven by changes in the production of deep water formation in the subpolar and subarctic North Atlantic. Indeed, current IPCC projections of an AMOC slowdown in the 21st century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep water formation. The motivation for understanding this linkage is compelling since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic (OSNAP), to provide a continuous record of the trans-basin fluxes of heat, mass and freshwater and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the RAPID/MOCHA array at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014 and the first OSNAP data products are expected in the fall of 2017.

Strong winter cooling over the Irminger Sea in winter 2014–2015, exceptional deep convection, and the emergence of anomalously low SST

Deep convection is presumed to be vital for the North Atlantic Meridional Overturning Circulation,even though observational evidence for the link remains inconclusive. Modeling studies have suggested thatconvection will weaken as a result of enhanced freshwater input. The emergence of anomalously low seasurface temperature in the subpolar North Atlantic has led to speculation that this process is already at work.Hereweshow that strong atmospheric forcing in the winter of 2014–2015, associated with a high North AtlanticOscillation (NAO) index, produced record mixed layer depths in the Irminger Sea. Local mixing removed thestratification of the upper 1400mand ventilated the basin to middepths resembling a state similar to themid-1990s when a positive NAO also prevailed. We show that the strong local atmospheric forcing ispredominantly responsible for the negative sea surface temperature anomalies observed in the subpolar NorthAtlantic in 2015 and that there is no evidence of permanently weakened deep convection.

Warm water pathways toward Nioghalvfjerdsfjorden Glacier, Northeast Greenland

Nioghalvfjerdsfjorden Glacier (79NG) is the largest of three marine-terminating outlet glaciers draining the Northeast Greenland Ice Stream. To understand how Atlantic waters supply waters in the cavity beneath the floating 79NG, we analyze historic and recent bathymetric, hydrographic, and velocity observations obtained on the Northeast Greenland continental shelf. The bathymetry is characterized by a trough system, consisting of the Westwind Trough and the Norske Trough in the northern and southern part of the continental shelf, respectively. Atlantic waters recirculating in Fram Strait cross the shelf break and enter the trough system at its southeastern inlet toward the inner shelf. Warm Atlantic Intermediate Water (AIW) present below 200 m in the Norske Trough shows large contributions of the recirculating Atlantic water. We found that the bathymetry is sufficiently deep to provide a direct subsurface pathway for warm AIW between the shelf break and the 79NG cavity via the Norske Trough. Likewise, based on the hydrographic data, we show that the Norske Trough supplies AIW warmer than 1°C to the 79NG, which is not present in the Westwind Trough. Our moored and lowered velocity measurements indicate that a boundary current carries warm AIW along the northeastern slope of Norske Trough toward the 79NG. We suggest that anomalies in Atlantic water temperatures in Fram Strait could reach 79NG within less than 1.5 years, thereby modifying the glaciers basal melt rates.

The underwater soundscape in western Fram Strait: Breeding ground of Spitsbergen's endangered bowhead whales

In the Arctic, warming and concomitant reductions in sea ice will affect the underwater soundscape, with the greatest changes likely being linked to anthropogenic activities. In this study, an acoustic recorder deployed on an oceanographic mooring in western Fram Strait documented the soundscape of this area, which is important habitat for the Critically Endangered Spitsbergen bowhead whale population. The soundscape was quasi-pristine much of the year, with low numbers of ships traversing the area. However, during summer/autumn, signals from airgun surveys were detected >12h/day. Mean received peak-to-peak SPLs for loud airgun pulses reached 160.46±0.48dB 1μPa when seismic-survey ships were close (at ~57km). Bowhead whales were present almost daily October-April in all years, with singing occurring in almost every hour November-March. Currently, loud anthropogenic sound sources do not temporally overlap the peak period of bowhead singing. This study provides important baseline data for future monitoring.

The Arctic Ocean Seasonal Cycles of Heat and Freshwater Fluxes: Observation-Based Inverse Estimates

This paper presents the first estimate of the seasonal cycle of ocean and sea ice net heat and freshwater (FW) fluxes around the boundary of the Arctic Ocean. The ocean transports are estimated primarily using 138 moored instruments deployed in September 2005 to August 2006 across the four main Arctic gateways: Davis, Fram and Bering Straits, and the Barents Sea Opening (BSO). Sea ice transports are estimated from a sea ice assimilation product. Monthly velocity fields are calculated with a box inverse model that enforces volume and salinity conservation. The resulting net ocean and sea ice heat and FW fluxes (annual mean

Arctic freshwater export: Status, mechanisms, and prospects

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Upstream Sources of the Denmark Strait Overflow: Observations from a High-Resolution Mooring Array

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Characteristics of colored dissolved organic matter (CDOM) in the Arctic outflow in the Fram Strait: Assessing the changes and fate of terrigenous CDOM in the Arctic Ocean: CDOM DYNAMICS IN ARCTIC OCEAN

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Pan-Arctic Oceanic volume, heat and freshwater transport time series during 2005 to 2006, link to model results in NetCDF Format

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