The evolution of light and vertical mixing across a phytoplankton ice-edge bloom

Elem Sci Anth ◽

10.1525/elementa.357 ◽

2019 ◽

Vol 7 ◽

Cited By ~ 11

Author(s):

Achim Randelhoff ◽

Laurent Oziel ◽

Philippe Massicotte ◽

Guislain Bécu ◽

Martí Galí ◽

...

Keyword(s):

Chlorophyll A ◽

Vertical Mixing ◽

World Ocean ◽

Ocean Dynamics ◽

The Arctic ◽

Upper Ocean ◽

Melt Mixing ◽

Atlantic Sector ◽

West Greenland ◽

Ice Edge

During summer, phytoplankton can bloom in the Arctic Ocean, both in open water and under ice, often strongly linked to the retreating ice edge. There, the surface ocean responds to steep lateral gradients in ice melt, mixing, and light input, shaping the Arctic ecosystem in unique ways not found in other regions of the world ocean. In 2016, we sampled a high-resolution grid of 135 hydrographic stations in Baffin Bay as part of the Green Edge project to study the ice-edge bloom, including turbulent vertical mixing, the under-ice light field, concentrations of inorganic nutrients, and phytoplankton biomass. We found pronounced differences between an Atlantic sector dominated by the warm West Greenland Current and an Arctic sector with surface waters originating from the Canadian archipelago. Winter overturning and thus nutrient replenishment was hampered by strong haline stratification in the Arctic domain, whereas close to the West Greenland shelf, weak stratification permitted winter mixing with high-nitrate Atlantic-derived waters. Using a space-for-time approach, we linked upper ocean dynamics to the phytoplankton bloom trailing the retreating ice edge. In a band of 60 km (or 15 days) around the ice edge, the upper ocean was especially affected by a freshened surface layer. Light climate, as evidenced by deep 0.415 mol m–2 d–1 isolumes, and vertical mixing, as quantified by shallow mixing layer depths, should have permitted significant net phytoplankton growth more than 100 km into the pack ice at ice concentrations close to 100%. Yet, under-ice biomass was relatively low at 20 mg chlorophyll-a m–2 and depth-integrated total chlorophyll-a (0–80 m) peaked at an average value of 75 mg chlorophyll-a m–2 only around 10 days after ice retreat. This phenological peak may hence have been the delayed result of much earlier bloom initiation and demonstrates the importance of temporal dynamics for constraints of Arctic marine primary production.

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Adjusting Neural Network to a Particular Problem: Neural Network-Based Empirical Biological Model for Chlorophyll Concentration in the Upper Ocean

Applied Computational Intelligence and Soft Computing ◽

10.1155/2018/7057363 ◽

2018 ◽

Vol 2018 ◽

pp. 1-10 ◽

Cited By ~ 1

Author(s):

Vladimir Krasnopolsky ◽

Sudhir Nadiga ◽

Avichal Mehra ◽

Eric Bayler

Keyword(s):

Neural Network ◽

Chlorophyll A ◽

Ocean Dynamics ◽

Sea Surface ◽

Biological Model ◽

Upper Ocean ◽

Wide Field ◽

Biological Processes ◽

Chl A ◽

The Impact

The versatility of the neural network (NN) technique allows it to be successfully applied in many fields of science and to a great variety of problems. For each problem or class of problems, a generic NN technique (e.g., multilayer perceptron (MLP)) usually requires some adjustments, which often are crucial for the development of a successful application. In this paper, we introduce a NN application that demonstrates the importance of such adjustments; moreover, in this case, the adjustments applied to a generic NN technique may be successfully used in many other NN applications. We introduce a NN technique, linking chlorophyll “a” (chl-a) variability—primarily driven by biological processes—with the physical processes of the upper ocean using a NN-based empirical biological model for chl-a. In this study, satellite-derived surface parameter fields, sea-surface temperature (SST) and sea-surface height (SSH), as well as gridded salinity and temperature profiles from 0 to 75m depth are employed as signatures of upper-ocean dynamics. Chlorophyll-a fields from NOAA’s operational Visible Imaging Infrared Radiometer Suite (VIIRS) are used, as well as Moderate Resolution Imaging Spectroradiometer (MODIS) and Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) chl-a concentrations. Different methods of optimizing the NN technique are investigated. Results are assessed using the root-mean-square error (RMSE) metric and cross-correlations between observed ocean color (OC) fields and NN output. To reduce the impact of noise in the data and to obtain a stable computation of the NN Jacobian, an ensemble of NN with different weights is constructed. This study demonstrates that the NN technique provides an accurate, computationally cheap method to generate long (up to 10 years) time series of consistent chl-a concentration that are in good agreement with chl-a data observed by different satellite sensors during the relevant period. The presented NN demonstrates a very good ability to generalize in terms of both space and time. Consequently, the NN-based empirical biological model for chl-a can be used in oceanic models, coupled climate prediction systems, and data assimilation systems to dynamically consider biological processes in the upper ocean.

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Short Commentary on Marine Productivity at Arctic Shelf Breaks: Upwelling, Advection and Vertical Mixing

10.5194/os-2017-68 ◽

2017 ◽

Author(s):

Achim Randelhoff ◽

Arild Sundfjord

Keyword(s):

Barents Sea ◽

Dominant Role ◽

Vertical Mixing ◽

Ice Cover ◽

Shelf Break ◽

The Arctic ◽

Ample Evidence ◽

Atlantic Sector ◽

Spatially Resolved ◽

Short Commentary

Abstract. The future of Arctic marine ecosystems has received increasing attention in recent years as the extent of the sea ice cover is dwindling. Although the Pacific and Atlantic inflows both import huge quantities of nutrients and plankton, they feed into the Arctic Ocean in quite diverse regions. The strongly stratified Pacific sector has a historically heavy ice cover, a shallow shelf and dominant upwelling-favourable winds, while the Atlantic sector is weakly stratified, with a dynamic ice edge and a complex bathymetry. We argue that shelf break upwelling is likely not a universal but rather a regional, albeit recurring feature of the new Arctic. Instead, it is the regional oceanography that decides its importance through a range of diverse factors such as stratification, bathymetry and wind forcing. Teasing apart their individual contributions in different regions can only be achieved by spatially resolved timeseries and dedicated modelling efforts. The Northern Barents Sea shelf is an example of a region where shelf break upwelling likely does not play a dominant role, in contrast to the shallower shelves north of Alaska, where ample evidence for its importance has already accumulated.

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Subsurface Tropical Instability Waves in the Atlantic Ocean in Model and Observations

10.5194/egusphere-egu21-53 ◽

2021 ◽

Author(s):

Mia Sophie Specht ◽

Johann Jungclaus ◽

Jürgen Bader

Keyword(s):

Atlantic Ocean ◽

Vertical Mixing ◽

Ocean Surface ◽

Ocean Dynamics ◽

Upper Ocean ◽

Tropical Atlantic ◽

Mean Flow ◽

Instability Waves ◽

Tropical Atlantic Ocean ◽

Tropical Instability Waves

<div> Tropical instability waves (TIWs) near the ocean surface are present in all tropical oceans and are known to be important for air-sea interactions and regional climate variability. Recent studies based on observations in the Pacific Ocean found that apart from TIWs at the surface, there also exist subsurface TIWs (subTIWs) which can alter vertical mixing. To date, most studies have focused on TIW related dynamics near the ocean surface. However, to properly assess vertical mixing in the upper ocean, improved understanding of the vertical structure of TIWs and the influence of subTIWs is needed. In this study, we show subTIW presence in the Atlantic Ocean for the first time using mooring observations.Further, we characterize subTIWs in the tropical Atlantic Ocean with a special focus on subTIW spatial and temporal variability and their effect on mixing. For this, data covering almost two decades are used that were generated from a comprehensive, global, high-resolution ocean model forced by the reanalysis ERA5. We find subTIWs between 40 m depth and the thermocline in both model and observations and unlike TIWs, subTIWs are frequently active both north and south of the Equator. The results of our study suggest that subTIWs induce a multi-layer shear structure which has the potential to destabilize the mean flow and thereby cause mixing. These effects are strongest north of the Equator where TIWs and subTIWs act simultaneously, implying possible TIW/subTIW interactions. We conclude that subTIWs are a feature of the tropical Atlantic Ocean with regionally varying implications for vertical mixing and heat fluxes. In addition, subTIWs differ from TIWs in their temporal and regional occurrences Therefore, subTIWs should be considered in future assessments of upper ocean dynamics, particularly in subTIW dominated regions. </div>

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Ocean role in the winter sea ice openings north of Svalbard

10.5194/egusphere-egu2020-20578 ◽

2020 ◽

Author(s):

Christophe Herbaut ◽

Marie-Noelle Houssais ◽

Anne-Cecile Blaizot

Keyword(s):

Sea Ice ◽

Atlantic Water ◽

Ocean Model ◽

Warm Water ◽

The Arctic ◽

Boundary Current ◽

Atlantic Sector ◽

Ice Melt ◽

Ice Edge ◽

Strong Winds

The winter trend in the sea ice coverage in the Atlantic sector of the Arctic Ocean has been linked to the Atlantic Water heat transport, providing significant skill to decadal prediction (Yeager et al, 2015, &#197;rthun et al., 2019). The Atlantic Water meets the sea ice north of Svalbard where it has the potential to melt significant amounts of ice and contribute to the formation of a cool, fresh surface layer (Rudels et al., 2004). In this study we investigate the origin of the intra-seasonal variability of winter sea ice melt north of Svalbard and evaluate its contribution to recurrent sea ice openings in this region.&#160;Based on outputs of a simulation with a high resolution regional ice-ocean model over the period 1995-2017, a number of large, short-term ice melt events could be identified in winter which can contribute up to 40% of the total winter ice melt. Most of these events show enhanced signature along the Atlantic Water path. However different types of events have been established depending on the scenario responsible for enhanced sea ice melt. Enhanced melt can happen concomitantly to large ice edge convergence over preexisting warm surface waters, a scenario which predominates during close-up of large ice openings. Large melt rates can also be driven by entrainment of warm water into the mixed layer in response to strong winds or to enhanced advection of warm water during episodes of increased transport in the boundary current. The latter process is however less efficient than entrainment. We conclude that increased southerly winds, which can sustain altogether ice edge retreat and efficient ice melt through entrainment and advection of heat into the region, create optimal conditions for major ice openings such as those observed north of Svalbard.

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The evolution of a shallow front in the Arctic marginal ice zone

Elem Sci Anth ◽

10.1525/elementa.413 ◽

2020 ◽

Vol 8 ◽

Author(s):

Samuel Brenner ◽

Luc Rainville ◽

Jim Thomson ◽

Craig Lee

Keyword(s):

Wind Stress ◽

The Arctic ◽

Upper Ocean ◽

Survey Period ◽

Radar Images ◽

Marginal Ice Zone ◽

Ice Conditions ◽

Frontal Dynamics ◽

Ice Edge ◽

High Degree

The high degree of heterogeneity in the ice-ocean-atmosphere system in marginal ice zones leads to a complex set of dynamics which control fluxes of heat and buoyancy in the upper ocean. Strong fronts may occur near the ice edge between the warmer waters of the ice-free regions and the cold, fresh waters near and under the ice. This study presents observations of a well-defined density front located along the ice edge in the Beaufort Sea. The evolution of the front over a ~3-day survey period is captured by multiple cross-front sections measured using an underway conductivity-temperature-depth system, with simultaneous measurements of atmospheric forcing. Synthetic aperture radar images bookending this period show that the ice edge itself underwent concurrent evolution. Prior to the survey, the ice edge was compact and well defined while after the survey it was diffuse and filamented with coherent vortical structures. This transformation might be indicative of the development an active ocean eddy field in the upper ocean mixed layer. Over the course of hours, increasing wind stress is correlated with changes to the lateral buoyancy gradient and frontogenesis. Frontal dynamics appear to vary from typical open-ocean fronts (e.g., here the frontogenesis is linked to an “up-front” wind stress). Convective and shear-driven mixing appear to be unable to describe deepening at the heel of the front. While there was no measurable spatial variation in wind speed, we hypothesize that spatial heterogeneity in the total surface stress input, resulting from varying ice conditions across the marginal ice zone, may be a driver of the observed behaviour.

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Spectral Characterization of Dissolved Organic Matter in Seawater and Sediment Pore Water from the Arctic Fjords (West Svalbard) in Summer

Water ◽

10.3390/w13020202 ◽

2021 ◽

Vol 13 (2) ◽

pp. 202

Author(s):

Meilian Chen ◽

Ji-Hoon Kim ◽

Sungwook Hong ◽

Yun Kyung Lee ◽

Moo Hee Kang ◽

...

Keyword(s):

Organic Matter ◽

Dissolved Organic Matter ◽

Pore Water ◽

Phytoplankton Bloom ◽

Ocean Surface ◽

The Arctic ◽

Sediment Pore Water ◽

Ice Edge ◽

Accumulation Trend ◽

Arctic Fjords

Fjords in the high Arctic, as aquatic critical zones at the interface of land-ocean continuum, are undergoing rapid changes due to glacier retreat and climate warming. Yet, little is known about the biogeochemical processes in the Arctic fjords. We measured the nutrients and the optical properties of dissolved organic matter (DOM) in both seawater and sediment pore water, along with the remote sensing data of the ocean surface, from three West Svalbard fjords. A cross-fjord comparison of fluorescence fingerprints together with downcore trends of salinity, Cl−, and PO43− revealed higher impact of terrestrial inputs (fluorescence index: ~1.2–1.5 in seawaters) and glaciofluvial runoffs (salinity: ~31.4 ± 2.4 psu in pore waters) to the southern fjord of Hornsund as compared to the northern fjords of Isfjorden and Van Mijenfjorden, tallying with heavier annual runoff to the southern fjord of Hornsund. Extremely high levels of protein-like fluorescence (up to ~4.5 RU) were observed at the partially sea ice-covered fjords in summer, in line with near-ubiquity ice-edge blooms observed in the Arctic. The results reflect an ongoing or post-phytoplankton bloom, which is also supported by the higher levels of chlorophyll a fluorescence at the ocean surface, the very high apparent oxygen utilization through the water column, and the nutrient drawdown at the ocean surface. Meanwhile, a characteristic elongated fluorescence fingerprint was observed in the fjords, presumably produced by ice-edge blooms in the Arctic ecosystems. Furthermore, alkalinity and the humic-like peaks showed a general downcore accumulation trend, which implies the production of humic-like DOM via a biological pathway also in the glaciomarine sediments from the Arctic fjords.

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Upper Ocean Dynamics Select for Synechococcus Light Color Generalists

10.1002/essoar.10506787.1 ◽

2021 ◽

Author(s):

Raisha Lovindeer ◽

Lucas Ustick ◽

Francois W. Primeau ◽

Adam Camillo Martiny ◽

Katherine Mackey

Keyword(s):

Ocean Dynamics ◽

Upper Ocean ◽

Light Color

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Attribution of the Recent Winter Sea Ice Decline over the Atlantic Sector of the Arctic Ocean*

Journal of Climate ◽

10.1175/jcli-d-15-0042.1 ◽

2015 ◽

Vol 28 (10) ◽

pp. 4027-4033 ◽

Cited By ~ 97

Author(s):

Doo-Sun R. Park ◽

Sukyoung Lee ◽

Steven B. Feldstein

Keyword(s):

Sea Ice ◽

Arctic Ocean ◽

Environmental Changes ◽

Arctic Sea Ice ◽

The Arctic ◽

Positive Trend ◽

Ir Radiation ◽

Atlantic Sector ◽

Sea Ice Decline ◽

The Arctic Ocean

Abstract Wintertime Arctic sea ice extent has been declining since the late twentieth century, particularly over the Atlantic sector that encompasses the Barents–Kara Seas and Baffin Bay. This sea ice decline is attributable to various Arctic environmental changes, such as enhanced downward infrared (IR) radiation, preseason sea ice reduction, enhanced inflow of warm Atlantic water into the Arctic Ocean, and sea ice export. However, their relative contributions are uncertain. Utilizing ERA-Interim and satellite-based data, it is shown here that a positive trend of downward IR radiation accounts for nearly half of the sea ice concentration (SIC) decline during the 1979–2011 winter over the Atlantic sector. Furthermore, the study shows that the Arctic downward IR radiation increase is driven by horizontal atmospheric water flux and warm air advection into the Arctic, not by evaporation from the Arctic Ocean. These findings suggest that most of the winter SIC trends can be attributed to changes in the large-scale atmospheric circulations.

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Is the World Ocean Warming? Upper-Ocean Temperature Trends: 1950–2000

Journal of Physical Oceanography ◽

10.1175/jpo3005.1 ◽

2007 ◽

Vol 37 (2) ◽

pp. 174-187 ◽

Cited By ~ 46

Author(s):

D. E. Harrison ◽

Mark Carson

Keyword(s):

Vertical Structure ◽

World Ocean ◽

Time Variability ◽

Upper Ocean ◽

Ocean Temperature ◽

Similar Magnitude ◽

Subsurface Temperature ◽

Temperature Trends ◽

Uncertainty Estimates ◽

The World

Abstract Subsurface temperature trends in the better-sampled parts of the World Ocean are reported. Where there are sufficient observations for this analysis, there is large spatial variability of 51-yr trends in the upper ocean, with some regions showing cooling in excess of 3°C, and others warming of similar magnitude. Some 95% of the ocean area analyzed has both cooled and warmed over 20-yr subsets of this period. There is much space and time variability of 20-yr running trend estimates, indicating that trends over a decade or two may not be representative of longer-term trends. Results are based on sorting individual observations in World Ocean Database 2001 into 1° × 1° and 2° × 2° bins. Only bins with at least five observations per decade for four of the five decades since 1950 are used. Much of the World Ocean cannot be examined from this perspective. The 51-yr trends significant at the 90% level are given particular attention. Results are presented for depths of 100, 300, and 500 m. The patterns of the 90% significant trends are spatially coherent on scales resolved by the bin size. The vertical structure of the trends is coherent in some regions, but changes sign between the analysis depths in a number of others. It is suggested that additional attention should be given to uncertainty estimates for basin average and World Ocean average thermal trends.

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