Stratification in the North Sea: response to different atmospheric forcing

2020 ◽  
Author(s):  
Anıl Akpınar ◽  
Matthew Palmer
Keyword(s):  
North Sea ◽  
Heat Fluxes ◽  
Physical Factors ◽  
Similar Response ◽  
Atmospheric Conditions ◽  
Atmospheric Variability ◽  
The North ◽  
Shelf Seas ◽  
Shelf Sea ◽  
Nutrient Injection

<p>On-set of spring stratification is one of the physical factors that influence the productivity of the continental shelves. Atmospheric convective mixing determines the on-set of spring stratification. This is particularly important in seasonally stratified shelf seas, where stratification constrain nutrient injection to the water column. Higher productivity in stratified period relies on intermittent diapycnal mixing events. Thus, the on-set and intensity of stratification is important for the functioning of the shelf-sea ecosystem. In this study, we investigate on-set of stratification, and its relation with the atmospheric conditions as well as imprints of sub-mesoscale features. We use high resolution in-situ measurements from 10 glider deployments, spanning over 18 months in the central North Sea. Focusing on two consecutive winters, we present year to year variability in the timing and intensity of stratification. An early initiation of stratification is observed in 2018/2019, which is also intense compared to the previous year of 2017/2018. We find that reduced wind stress and net air-sea heat fluxes result in an early on-set of stratification in 2018/2019. In February 2019, intermittent increases in chlorophyll are observed, corresponding to a minimum in sea-to-air heat loss. Similarly, in 2019 an earlier spring bloom is observed. We investigate this period with NEMO model outputs at 7km resolution (AMM7) and show a similar response, emphasizing the influence of atmospheric variability on dynamics of the shelf-sea.   </p>

Physical preconditioning of oxygen depletion in shelf seas

2020 ◽  
Author(s):  
Charlotte Williams ◽  
Claire Mahaffey ◽  
Matthew Palmer ◽  
Naomi Greenwood
Keyword(s):  
North Sea ◽  
Late Summer ◽  
Global Ocean ◽  
Physical Conditions ◽  
Annual Variability ◽  
The North ◽  
Biogeochemical Models ◽  
Shelf Seas ◽  
Shelf Sea ◽  
The North Sea

<p>The global ocean dissolved oxygen (DO) inventory is decreasing and the areal extent of DO deficiency is increasing. In the shelf sea BML, net DO removal can occur as a result of restricted ventilation due to seasonal thermal stratification, oxygen consumption via pelagic and benthic respiration of organic matter, and nitrification. DO decline is becoming evident in several shelf seas, with recent model studies estimating that large regions of the Northwest European continental shelf seas (325,000 to 400,000 km<sup>2</sup>) have the potential to become seasonally deficient in DO in late summer. It is therefore of vital importance that DO is monitored accurately and effectively in shelf seas.</p><p>Here we present results from AlterECO project, which aimed to provide an alternative, novel framework for the monitoring of shelf sea ecosystem health indicators, including DO, via the deployment of 20 gliders in the North Sea (NW European shelf). Between November 2017 and May 2019 the gliders provided 18 month continuous measurements of T, S, chlorophyll fluorescence, and DO in the seasonally stratified study area, capturing the onset and breakdown of two spring blooms. In both years the gliders captured a weakly stratified, deep (>60m) thermocline in late autumn which was responsible for oxygen deplete (75%)  ‘pools’ in the North Sea. Our results show that preconditioning of pre-bloom transitional periods as well as episodic mixing events drive inter-annual differences in BML DO concentrations. Large inter-annual variability between pre-bloom physical conditions was observed, with the occurrence of anticyclone Hartmut in February 2018 resulting in a much colder water column (and therefore higher solubility of DO) in spring 2018 than 2019. Additionally we will demonstrate that the erosion of mini-blooms during the onset of stratification results in mixing of supersaturated DO surface water into the BML, helping to prevent DO deficiency in the BML in late summer. Comparisons of our high resolution glider data with the latest state of the art biogeochemical models (AMM15-ERSEM) will also be presented. We postulate that understanding the drivers of inter-annual variability in pre-bloom physical conditions is crucial in terms of understanding and predicting DO depletion in shelf seas.</p><p> </p>

A satellite-based Lagrangian view on the origin of water-masses in the northern European shelf seas

2021 ◽  
Author(s):  
Ezra Eisbrenner ◽  
Léon Chafik
Keyword(s):  
North Sea ◽  
Water Mass ◽  
Sea Water ◽  
Water Masses ◽  
Subpolar Gyre ◽  
The North ◽  
Shelf Seas ◽  
Shelf Sea ◽  
The North Sea ◽  
Northern North Sea

<p>Knowledge about water-mass properties is critical to understanding how ocean climate variability impacts the shelf seas. Disentangling the origin of shelf sea water-masses and associated driving mechanisms is, therefore, a significant step towards improving the predictive skill related to water-mass evolution. Especially more conservative water-mass properties, even of surface waters, have the potential to reveal links between the shelf seas and large-scale ocean circulation regimes when traced back to their origin. The northern North Sea for example as the main gateway for water-masses to one of Europe's largest shelf sea areas is largely supplied by water-masses from the open North Atlantic, a connection which can be seen from, e.g., sea surface salinity.</p><p>The aim of this study is to identify the origin of northern North Sea water-masses and distinguish pathway variability relative to the subpolar gyre regimes. This is done using Lagrangian trajectories, calculated using satellite-derived velocity fields. The results of the Lagrangian statistics mainly indicate that on inter-annual time-scales the North Atlantic subpolar gyre strength largely influences the water-masses found in the North Sea. The relation is found to originate from varying pathways and therefore origin. We conclude that on inter-annual time scales the subpolar gyre strength is a good proxy and skillful predictor of water-mass variability in the North Sea.</p>

scholarly journals Shelf sea tidal currents and mixing fronts determined from ocean glider observations

Ocean Science ◽  
2018 ◽  
Vol 14 (2) ◽  
pp. 225-236 ◽  
Author(s):  
Peter M. F. Sheehan ◽  
Barbara Berx ◽  
Alejandro Gallego ◽  
Rob A. Hall ◽  
Karen J. Heywood ◽  
...  
Keyword(s):  
North Sea ◽  
Thermohaline Circulation ◽  
Heat Fluxes ◽  
Water Masses ◽  
Tidal Mixing ◽  
Primary Control ◽  
Surface Heat Fluxes ◽  
The North ◽  
Shelf Sea ◽  
North Western

Abstract. Tides and tidal mixing fronts are of fundamental importance to understanding shelf sea dynamics and ecosystems. Ocean gliders enable the observation of fronts and tide-dominated flows at high resolution. We use dive-average currents from a 2-month (12 October–2 December 2013) glider deployment along a zonal hydrographic section in the north-western North Sea to accurately determine M2 and S2 tidal velocities. The results of the glider-based method agree well with tidal velocities measured by current meters and with velocities extracted from the TPXO tide model. The method enhances the utility of gliders as an ocean-observing platform, particularly in regions where tide models are known to be limited. We then use the glider-derived tidal velocities to investigate tidal controls on the location of a front repeatedly observed by the glider. The front moves offshore at a rate of 0.51 km day−1. During the first part of the deployment (from mid-October until mid-November), results of a one-dimensional model suggest that the balance between surface heat fluxes and tidal stirring is the primary control on frontal location: as heat is lost to the atmosphere, full-depth mixing is able to occur in progressively deeper water. In the latter half of the deployment (mid-November to early December), a front controlled solely by heat fluxes and tidal stirring is not predicted to exist, yet a front persists in the observations. We analyse hydrographic observations collected by the glider to attribute the persistence of the front to the boundary between different water masses, in particular to the presence of cold, saline, Atlantic-origin water in the deeper portion of the section. We combine these results to propose that the front is a hybrid front: one controlled in summer by the local balance between heat fluxes and mixing and which in winter exists as the boundary between water masses advected to the north-western North Sea from diverse source regions. The glider observations capture the period when the front makes the transition from its summertime to wintertime state. Fronts in other shelf sea regions with oceanic influence may exhibit similar behaviour, with controlling processes and locations changing over an annual cycle. These results have implications for the thermohaline circulation of shelf seas.

Introduction to the North Sea Project

1993 ◽  
Vol 343 (1669) ◽  
pp. 379-382 ◽  
Keyword(s):  
North Sea ◽  
Royal Society ◽  
Economic Importance ◽  
The North ◽  
The Future ◽  
Shelf Seas ◽  
Shelf Sea ◽  
The North Sea ◽  
Heavy Burden ◽  
Intellectual Challenge

It is more than ten years since the Royal Society last hosted a discussion to focus on the Shelf Seas. The intervening period has been a time of both exciting scientific developments and an emerging awareness of the practical and economic importance of our shallow coastal seas. Concern about the welfare of the environment in these waters, on which we place an increasingly heavy burden, has added to the intellectual challenge of understanding the processes that control that environment and lent impetus to the development of improved models of the shelf sea system which will be needed for informed management in the future as pressures from our agriculture and industry increase.

scholarly journals Metabarcoding reveals different zooplankton communities in northern and southern areas of the North Sea

2020 ◽  
Author(s):  
Jan Niklas Macher ◽  
Berry B. van der Hoorn ◽  
Katja T. C. A. Peijnenburg ◽  
Lodewijk van Walraven ◽  
Willem Renema
Keyword(s):  
North Sea ◽  
Zooplankton Community ◽  
Taxonomic Diversity ◽  
Molecular Techniques ◽  
Trophic Levels ◽  
List Type ◽  
The North ◽  
Shelf Sea ◽  
The North Sea ◽  
Zooplankton Communities

AbstractZooplankton are key players in marine ecosystems, linking primary production to higher trophic levels. The high abundance and high taxonomic diversity renders zooplankton ideal for biodiversity monitoring. However, taxonomic identification of the zooplankton assemblage is challenging due to its high diversity, subtle morphological differences and the presence of many meroplanktonic species, especially in coastal seas. Molecular techniques such as metabarcoding can help with rapid processing and identification of taxa in complex samples, and are therefore promising tools for identifying zooplankton communities. In this study, we applied metabarcoding of the mitochondrial cytochrome c oxidase I gene to zooplankton samples collected along a latitudinal transect in the North Sea, a shelf sea of the Atlantic Ocean. Northern regions of the North Sea are influenced by inflow of oceanic Atlantic waters, whereas the southern parts are characterised by more coastal waters. Our metabarcoding results indicated strong differences in zooplankton community composition between northern and southern areas of the North Sea, particularly in the classes Copepoda, Actinopterygii (ray-finned fishes) and Polychaeta. We compared these results to the known distributions of species reported in previous studies, and by comparing the abundance of copepods to data obtained from the Continuous Plankton Recorder (CPR). We found that our metabarcoding results are mostly congruent with the reported distribution and abundance patterns of zooplankton species in the North Sea. Our results highlight the power of metabarcoding to rapidly assess complex zooplankton samples, and we suggest that the technique could be used in future monitoring campaigns and biodiversity assessments.HighlightsZooplankton communities are different in northern and southern areas of the North SeaMetabarcoding results are consistent with known species distributions and abundanceMetabarcoding allows for fast identification of meroplanktonic species

scholarly journals Depth-averaged instantaneous currents in a tidally dominated shelf sea from glider observations

2016 ◽  
Author(s):  
Lucas Merckelbach
Keyword(s):  
Real Time ◽  
North Sea ◽  
Current Data ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Real Time Mode ◽  
The North ◽  
Shelf Sea ◽  
The North Sea ◽  
Better Than

Abstract. Ocean gliders have become ubiquitous observation platforms in the ocean in recent years. They are also increasingly used in coastal environments. The coastal observatory system COSYNA has pioneered the use of gliders in the North Sea, a shallow tidally energetic shelf sea. For operational reasons, the gliders operated in the North Sea are programmed to resurface every 3–5 hours. The glider's deadreckoning algorithm yields depth averaged currents, averaged in time over each subsurface interval. Under operational conditions these averaged currents are a poor approximation of the instantaneous tidal current. In this work an algorithm is developed that estimates the instantaneous current (tidal and residual) from glider observations only. The algorithm uses a second-order Butterworth low-pass filter to estimate the residual current component, and a Kalman filter based on the linear shallow water equations for the tidal component. A comparison of data from a glider experiment with current data from an ADCP deployed nearby shows that the standard deviations for the east and north current components are better than 7 cm s−1 in near-real time mode, and improve to better than 5 cm s−1 in delayed mode, where the filters can be run forward and backward. In the near-real time mode the algorithm provides estimates of the currents that the glider is expected to encounter during its next few dives. Combined with a behavioural and dynamic model of the glider, this yields predicted trajectories, the information of which is incorporated in warning messages issued to ships by the (German) authorities. In delayed mode the algorithm produces useful estimates of the depth averaged currents, which can be used in (process-based) analyses in case no other source of measured current information is available.

scholarly journals Depth-averaged instantaneous currents in a tidally dominated shelf sea from glider observations

2016 ◽  
Vol 13 (24) ◽  
pp. 6637-6649 ◽  
Author(s):  
Lucas Merckelbach
Keyword(s):  
Real Time ◽  
North Sea ◽  
Current Data ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Real Time Mode ◽  
The North ◽  
Shelf Sea ◽  
The North Sea ◽  
Better Than

Abstract. Ocean gliders have become ubiquitous observation platforms in the ocean in recent years. They are also increasingly used in coastal environments. The coastal observatory system COSYNA has pioneered the use of gliders in the North Sea, a shallow tidally energetic shelf sea. For operational reasons, the gliders operated in the North Sea are programmed to resurface every 3–5 h. The glider's dead-reckoning algorithm yields depth-averaged currents, averaged in time over each subsurface interval. Under operational conditions these averaged currents are a poor approximation of the instantaneous tidal current. In this work an algorithm is developed that estimates the instantaneous current (tidal and residual) from glider observations only. The algorithm uses a first-order Butterworth low pass filter to estimate the residual current component, and a Kalman filter based on the linear shallow water equations for the tidal component. A comparison of data from a glider experiment with current data from an acoustic Doppler current profilers deployed nearby shows that the standard deviations for the east and north current components are better than 7 cm s−1 in near-real-time mode and improve to better than 6 cm s−1 in delayed mode, where the filters can be run forward and backward. In the near-real-time mode the algorithm provides estimates of the currents that the glider is expected to encounter during its next few dives. Combined with a behavioural and dynamic model of the glider, this yields predicted trajectories, the information of which is incorporated in warning messages issued to ships by the (German) authorities. In delayed mode the algorithm produces useful estimates of the depth-averaged currents, which can be used in (process-based) analyses in case no other source of measured current information is available.

The contribution of the deep chlorophyll maximum to primary production in a seasonally stratified shelf sea, the North Sea

2013 ◽  
Vol 113 (1-3) ◽  
pp. 153-166 ◽  
Author(s):  
Liam Fernand ◽  
Keith Weston ◽  
Tom Morris ◽  
Naomi Greenwood ◽  
Juan Brown ◽  
...  
Keyword(s):  
Primary Production ◽  
North Sea ◽  
Deep Chlorophyll Maximum ◽  
Chlorophyll Maximum ◽  
The North ◽  
Shelf Sea ◽  
The North Sea
Sign in / Sign up

Export Citation Format

Share Document