Short commentary on marine productivity at Arctic shelf breaks: upwelling, advection and vertical mixing

Randelhoff, Achim; Sundfjord, Arild

doi:10.5194/os-14-293-2018

Cited by 35 publications

(21 citation statements)

References 51 publications

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“…We have not found any indications to that effect during our sampling campaigns. For a more detailed description of the issue of wind-driven upwelling in Arctic shelfbreak areas, see Randelhoff and Sundfjord (2018). All else aside, even actual upwelling in winter would not enhance productivity in our study area since, as we have shown, the surface mixed layer is already replete by that time and well before the onset of the next spring bloom.…”

Section: Thermal Convection In Wintermentioning

confidence: 81%

Seasonality of the Physical and Biogeochemical Hydrography in the Inflow to the Arctic Ocean Through Fram Strait

et al. 2018

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Eastern Fram Strait and the shelf slope region north of Svalbard is dominated by the advection of warm, salty and nutrient-rich Atlantic Water (AW). This oceanic heat contributes to keeping the area relatively free of ice. The last years have seen a dramatic decrease in regional sea ice extent, which is expected to drive large increases in pelagic primary production and thereby changes in marine ecology and nutrient cycling. In a concerted effort, we conducted five cruises to the area in winter, spring, summer and fall of 2014, in order to understand the physical and biogeochemical controls of carbon cycling, for the first time from a year-round point of view. We document (1) the offshore location of the wintertime front between salty AW and fresher Surface Water in the ocean surface, (2) thermal convection of Atlantic Water over the shelf slope, likely enhancing vertical nutrient fluxes, and (3) the importance of ice melt derived upper ocean stratification for the spring bloom timing. Our findings strongly confirm the hypothesis that this "Atlantification," as it has been called, of the shelf slope area north of Svalbard resulting from the advection of AW alleviates both nutrient and light limitations at the same time, leading to increased pelagic primary productivity in this region.

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Section: Thermal Convection In Wintermentioning

confidence: 81%

Seasonality of the Physical and Biogeochemical Hydrography in the Inflow to the Arctic Ocean Through Fram Strait

et al. 2018

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“…The shelf north of Svalbard is 150–200 m deep versus 50–60 m in the Beaufort Sea where upwelling is particularly common. The much greater depth north of Svalbard implies that shelf break upwelling is not likely to be important here; the outer shelf is too deep for the surface and Ekman layers to overlap and interact (see Randelhoff & Sundfjord, ). In addition, the wind field is highly variable both in strength and direction with only few short periods of easterly (i.e., upwelling favorable) winds lasting several consecutive days (Figure a).…”

Section: Resultsmentioning

confidence: 99%

“…After an initial look into heat exchange at the ocean‐atmosphere interface, we investigate the role of vertical fluxes in the water column as deduced from current shear variance as well as the influence of tides. While wind‐driven upwelling is a well‐documented process in parts of the Canadian Arctic (e.g., Pickart et al, ), the shelf geometry north of Svalbard is not favorable for shelf break upwelling driven by along‐slope winds (Randelhoff & Sundfjord, ). We show, however, that Ekman pumping can lead to instances with considerable isopycnal uplift.…”

Section: Resultsmentioning

confidence: 99%

Variability and Redistribution of Heat in the Atlantic Water Boundary Current North of Svalbard

et al. 2018

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We quantify Atlantic Water heat loss north of Svalbard using year-long hydrographic and current records from three moorings deployed across the Svalbard Branch of the Atlantic Water boundary current in 2012-2013. The boundary current loses annually on average 16 W m −2 during the eastward propagation along the upper continental slope. The largest vertical fluxes of >100 W m −2 occur episodically in autumn and early winter. Episodes of sea ice imported from the north in November 2012 and February 2013 coincided with large ocean-to-ice heat fluxes, which effectively melted the ice and sustained open water conditions in the middle of the Arctic winter. Between March and early July 2013, a persistent ice cover-modulated air-sea fluxes. Melting sea ice at the start of the winter initiates a cold, up to 100-m-deep halocline separating the ice cover from the warm Atlantic Water. Semidiurnal tides dominate the energy over the upper part of the slope. The vertical tidal structure depends on stratification and varies seasonally, with the potential to contribute to vertical fluxes with shear-driven mixing. Further processes impacting the heat budget include lateral heat loss due to mesoscale eddies, and modest and negligible contributions of Ekman pumping and shelf break upwelling, respectively. The continental slope north of Svalbard is a key example regarding the role of ocean heat for the sea ice cover. Our study underlines the complexity of the ocean's heat budget that is sensitive to the balance between oceanic heat advection, vertical fluxes, air-sea interaction, and the sea ice cover. Plain Language SummaryThe Atlantic Water boundary current carries heat into the Arctic Ocean as it flows through Fram Strait and along the continental slope north of Svalbard. Using observations from bottom-mounted instruments, we investigated different processes leading to heat loss from the Atlantic Water layer in the region north of Svalbard. Most of the changes recorded over the course of 1 year from September 2012 to September 2013 at 81.5 ∘ N, 31 ∘ E are driven by changes further upstream and by air-sea heat exchange. However, significant local heat loss can be caused by mixing due to wind or tides. Seasonal differences are large and predominantly caused by absence or presence of sea ice (autumn/early winter versus spring/early summer), influence of melt water and wind on the stability of the water column, and a seasonally changing light regime.

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“…Nutrient supply from AW and BSW to the overlying PSW occurs by autumn and winter mixing, upwelling along the shelf break and Polar Front, tidal forcing and episodic windmixing in spring/summer [31,37,[49][50][51]. Recent evidence from the shelf-slope north and west of Svalbard shows that thermally induced convective mixing [52] is more important than wintertime shelf-break upwelling in the AW inflow region [53]. Atlantification-the increasing advection of warm (greater than 0°C), saline, nutrient-rich AW at intermediate depth (150-900 m)-is occurring along the continental slope north of Svalbard and over the Barents shelf from the south [24,[54][55][56][57].…”

Section: (B) the Barents Seamentioning

confidence: 99%

Nitrate supply and uptake in the Atlantic Arctic sea ice zone: seasonal cycle, mechanisms and drivers

Henley

Porter

Hobbs

et al. 2020

Phil. Trans. R. Soc. A.

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Nutrient supply to the surface ocean is a key factor regulating primary production in the Arctic Ocean under current conditions and with ongoing warming and sea ice losses. Here we present seasonal nitrate concentration and hydrographic data from two oceanographic moorings on the northern Barents shelf between autumn 2017 and summer 2018. The eastern mooring was sea ice-covered to varying degrees during autumn, winter and spring, and was characterized by more Arctic-like oceanographic conditions, while the western mooring was ice-free year-round and showed a greater influence of Atlantic water masses. The seasonal cycle in nitrate dynamics was similar under ice-influenced and ice-free conditions, with biological nitrate uptake beginning near-synchronously in early May, but important differences between the moorings were observed. Nitrate supply to the surface ocean preceding and during the period of rapid drawdown was greater at the ice-free more Atlantic-like western mooring, and nitrate drawdown occurred more slowly over a longer period of time. This suggests that with ongoing sea ice losses and Atlantification, the expected shift from more Arctic-like ice-influenced conditions to more Atlantic-like ice-free conditions is likely to increase nutrient availability and the duration of seasonal drawdown in this Arctic shelf region. The extent to which this increased nutrient availability and longer drawdown periods will lead to increases in total nitrate uptake, and support the projected increases in primary production, will depend on changes in upper ocean stratification and their effect on light availability to phytoplankton as changes in climate and the physical environment proceed. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

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Short commentary on marine productivity at Arctic shelf breaks: upwelling, advection and vertical mixing

Cited by 35 publications

References 51 publications

Seasonality of the Physical and Biogeochemical Hydrography in the Inflow to the Arctic Ocean Through Fram Strait

Seasonality of the Physical and Biogeochemical Hydrography in the Inflow to the Arctic Ocean Through Fram Strait

Variability and Redistribution of Heat in the Atlantic Water Boundary Current North of Svalbard

Nitrate supply and uptake in the Atlantic Arctic sea ice zone: seasonal cycle, mechanisms and drivers

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