Numerical investigation of the Arctic ice-ocean boundary layer; implications for air-sea gas fluxes

Abstract. In ice-covered regions it can be challenging to determine air-sea exchange – for heat and momentum, but also for gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we seek a mechanistic interpretation for the rate of air-sea gas exchange (k) derived from radon-deficits. These require an estimate of the water column history extending 30 days prior to sampling. We used coarse resolution (36 km) regional configuration of the MITgcm with fine near surface vertical spacing (2 m) to evaluate the capability of the model to reproduce conditions prior to sampling. The model is used to estimate sea-ice velocity, concentration and mixed-layer depth experienced by the water column. We then compared the model results to existing field data including satellite, moorings and Ice-tethered profilers. We found that model-derived sea-ice coverage is 88 to 98 % accurate averaged over Beaufort Gyre, sea-ice velocities have 78 % correlation which resulted in 2 km/day error in 30 day trajectory of sea-ice. The model demonstrated the capacity to capture the broad trends in the mixed layer although with a bias and model water velocities showed only 29 % correlation with actual data. Overall, we find the course resolution model to be an inadequate surrogate for sparse data, however the simulation results are a slight improvement over several of the simplifying assumptions that are often made when surface ocean geochemistry, including the use of a constant mixed layer depth and a velocity profile that is purely wind-driven.

Download Full-text

Numerical investigation of the Arctic ice–ocean boundary layer and implications for air–sea gas fluxes

Ocean Science ◽

10.5194/os-13-61-2017 ◽

2017 ◽

Vol 13 (1) ◽

pp. 61-75 ◽

Cited By ~ 4

Author(s):

Arash Bigdeli ◽

Brice Loose ◽

An T. Nguyen ◽

Sylvia T. Cole

Keyword(s):

Gas Exchange ◽

Sea Ice ◽

Mixed Layer ◽

General Circulation ◽

Mixed Layer Depth ◽

Circulation Model ◽

Horizontal Resolution ◽

The Arctic ◽

Massachusetts Institute Of Technology ◽

Institute Of Technology

Abstract. In ice-covered regions it is challenging to determine constituent budgets – for heat and momentum, but also for biologically and climatically active gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we sought to evaluate if numerical model output helps us to better estimate the physical forcing that drives the air–sea gas exchange rate (k) in sea ice zones. We used the budget of radioactive 222Rn in the mixed layer to illustrate the effect that sea ice forcing has on gas budgets and air–sea gas exchange. Appropriate constraint of the 222Rn budget requires estimates of sea ice velocity, concentration, mixed-layer depth, and water velocities, as well as their evolution in time and space along the Lagrangian drift track of a mixed-layer water parcel. We used 36, 9 and 2 km horizontal resolution of regional Massachusetts Institute of Technology general circulation model (MITgcm) configuration with fine vertical spacing to evaluate the capability of the model to reproduce these parameters. We then compared the model results to existing field data including satellite, moorings and ice-tethered profilers. We found that mode sea ice coverage agrees with satellite-derived observation 88 to 98 % of the time when averaged over the Beaufort Gyre, and model sea ice speeds have 82 % correlation with observations. The model demonstrated the capacity to capture the broad trends in the mixed layer, although with a significant bias. Model water velocities showed only 29 % correlation with point-wise in situ data. This correlation remained low in all three model resolution simulations and we argued that is largely due to the quality of the input atmospheric forcing. Overall, we found that even the coarse-resolution model can make a modest contribution to gas exchange parameterization, by resolving the time variation of parameters that drive the 222Rn budget, including rate of mixed-layer change and sea ice forcings.

Download Full-text

Phytoplankton bloom phenomena in the North Atlantic Ocean and Arabian Sea

ICES Journal of Marine Science ◽

10.1093/icesjms/fsu241 ◽

2015 ◽

Vol 72 (6) ◽

pp. 2021-2028 ◽

Cited By ~ 5

Author(s):

John F. Marra ◽

Tommy D. Dickey ◽

Albert J. Plueddemann ◽

Robert A. Weller ◽

Christopher S. Kinkade ◽

...

Keyword(s):

Water Column ◽

Mixed Layer ◽

Arabian Sea ◽

Phytoplankton Bloom ◽

Mixed Layer Depth ◽

Ocean Dynamics ◽

Layer Depth ◽

Diurnal Variability ◽

Water Column Stratification ◽

Iceland Basin

Abstract We review bio-optical and physical data from three mooring experiments, the Marine Light–Mixed Layers programme in spring 1989 and 1991 in the Iceland Basin (59°N/21°W), and the Forced Upper Ocean Dynamics Experiment in the central Arabian Sea from October 1994 to 1995 (15.5°N/61.5°E). In the Iceland Basin, from mid-April to mid-June in 1989, chlorophyll-a concentrations are sensitive to small changes in stratification, with intermittent increases early in the record. The spring increase occurs after 20 May, coincident with persistent water column stratification. In 1991, the bloom occurs 2 weeks earlier than in 1989, with a background of strong short-term and diurnal variability in mixed layer depth and minimal horizontal advection. In the Arabian Sea, the mixing response to the northeast and southwest monsoons, plus the response to mesoscale eddies, produces four blooms over the annual cycle. The mixed layer depth in the Arabian Sea never exceeds the euphotic zone, allowing interactions between phytoplankton and grazer populations to become important. For all three mooring experiments, change in water column stratification is key in producing phytoplankton blooms.

Download Full-text

Wintertime Methane Emission From the Barents and Kara Seas and Sea of Okhotsk: Satellite Evidence.

10.5194/egusphere-egu21-5628 ◽

2021 ◽

Author(s):

Leonid Yurganov ◽

Dustin Carroll ◽

Andrey Pnyushkov ◽

Igor Polyakov ◽

Hong Zhang

Keyword(s):

Remote Sensing ◽

Sea Ice ◽

Water Column ◽

Mixed Layer Depth ◽

Deep Ocean ◽

Ice Cover ◽

The Arctic ◽

Atmospheric Methane ◽

Arctic Seas ◽

Ir Radiation

Existence of strong seabed sources of methane, including gas hydrates, in the Arctic and sub-Arctic seas with proven oil/gas deposits is well documented. Enhanced concentrations of dissolved methane in deep layers are widely observed. Many of marine sources are highly sensitive to climate change; however, the Arctic methane sea-to-air flux remains poorly understood: harsh natural conditions prevent in-situ measurements during winter. Satellite remote sensing, based on terrestrial outgoing Thermal IR radiation measurements, provides a novel alternative to those efforts. We present year-round methane data from 3 orbital sounders since 2002. Those data confirm that negligible amounts of methane are fluxed from the seabed to the atmosphere during summer. In summer, the water column is strongly stratified from sea-ice melt and solar warming. As a result, ~90% of dissolved methane is oxidized by bacteria. Conversely, some marine areas are characterized by positive atmospheric methane anomalies that begin in November. During winter, ocean stratification weakens, convection and winter storms mix the water column efficiently. We also find that the amplitudes of the seasonal cycles over Kara and Okhotsk Seas have increased during last 18 years due to winter concentration growth. There may be several factors responsible for sea-air flux: growing emission from clathrates due to warming, changes in methane transport from the seabed to the surface, changes in microbial oxidation, ice cover, etc. Finally, methane remote sensing results are compared to available observations of temperature in deep ocean layers, estimates of Mixed Layer Depth, and satellite microwave sea-ice cover measurements.&#160;

Download Full-text

Modeling submerged biofouled microplastics and their vertical trajectories

10.5194/bg-2021-236 ◽

2021 ◽

Author(s):

Reint Fischer ◽

Delphine Lobelle ◽

Merel Kooi ◽

Albert Koelmans ◽

Victor Onink ◽

...

Keyword(s):

Southern Ocean ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Euphotic Zone ◽

Equatorial Pacific ◽

Open Ocean ◽

Subtropical Gyre ◽

Layer Depth ◽

High Biological Activity ◽

Near Surface

Abstract. The fate of (micro)plastic particles in the open ocean is controlled by physical and biological processes. Here, we model the effects of biofouling on the subsurface vertical distribution of spherical, virtual plastic particles with radii of 0.01–1 mm. For the physics, four vertical velocity terms are included: advection, wind-driven mixing, tidally induced mixing, and the sinking velocity of the biofouled particle. For the biology, we simulate the attachment, growth and loss of algae on particles. We track 10,000 particles for one year in three different regions with distinct biological and physical properties: the low productivity region of the North Pacific Subtropical Gyre, the high productivity region of the Equatorial Pacific and the high mixing region of the Southern Ocean. The growth of biofilm mass in the euphotic zone and loss of mass below the euphotic zone result in the oscillatory behaviour of particles, where the larger (0.1–1.0 mm) particles have much shorter average oscillation lengths (< 10 days; 90th percentile) than the smaller (0.01–0.1 mm) particles (up to 130 days; 90th percentile). A subsurface maximum concentration occurs just below the mixed layer depth (around 30 m) in the Equatorial Pacific, which is most pronounced for larger particles (0.1–1.0 mm). This occurs since particles become neutrally buoyant when the processes affecting the settling velocity of the particle and the motion of the ocean are in equilibrium. Seasonal effects in the subtropical gyre result in particles sinking below the mixed layer depth only during spring blooms, but otherwise remaining within the mixed layer. The strong winds and deepest average mixed layer depth in the Southern Ocean (400 m) result in the deepest redistribution of particles (> 5000 m). Our results show that the vertical movement of particles is mainly affected by physical (wind-induced mixing) processes within the mixed layer and biological (biofilm) dynamics below the mixed layer. Furthermore, positively buoyant particles with radii of 0.01–1.0 mm can sink far below the euphotic zone and mixed layer in regions with high near-surface mixing or high biological activity. This work can easily be coupled to other models to simulate open-ocean biofouling dynamics, in order to reach a better understanding of where ocean (micro)plastic ends up.

Download Full-text

An Enhanced PIRATA Dataset for Tropical Atlantic Ocean–Atmosphere Research

Journal of Climate ◽

10.1175/jcli-d-16-0816.1 ◽

2018 ◽

Vol 31 (4) ◽

pp. 1499-1524 ◽

Cited By ~ 13

Author(s):

Gregory R. Foltz ◽

Claudia Schmid ◽

Rick Lumpkin

Keyword(s):

Time Series ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Tropical Atlantic ◽

Layer Depth ◽

Near Surface ◽

Vertical Grid ◽

Storage Rate ◽

The Difference ◽

Data Dropouts

The Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) provides measurements of the upper ocean and near-surface atmosphere at 18 locations. Time series from many moorings are nearly 20 years in length. However, instrumental biases, data dropouts, and the coarse vertical resolutions of the oceanic measurements complicate their use for research. Here an enhanced PIRATA dataset (ePIRATA) is presented for the 17 PIRATA moorings with record lengths of at least seven years. Data in ePIRATA are corrected for instrumental biases, temporal gaps are filled using supplementary datasets, and the subsurface temperature and salinity time series are mapped to a uniform 5-m vertical grid. All original PIRATA data that pass quality control and that do not require bias correction are retained without modification, and detailed error estimates are provided. The terms in the mixed-layer heat and temperature budgets are calculated and included, with error bars. As an example of ePIRATA’s application, the vertical exchange of heat at the base of the mixed layer ( Q− h) is calculated at each PIRATA location as the difference between the heat storage rate and the sum of the net surface heat flux and horizontal advection. Off-equatorial locations are found to have annual mean cooling rates of 20–60 W m−2, while cooling at equatorial locations reaches 85–110 W m−2 between 10° and 35°W and decreases to 40 W m−2 at 0°. At most off-equatorial locations, the strongest seasonal cooling from Q− h occurs when winds are weak. Possible explanations are discussed, including the importance of seasonal modulations of mixed-layer depth and the diurnal cycle.

Download Full-text

Autonomous profiling float observations of the high biomass plume downstream of the Kerguelen plateau in the Southern Ocean

Biogeosciences Discussions ◽

10.5194/bgd-11-17413-2014 ◽

2014 ◽

Vol 11 (12) ◽

pp. 17413-17462 ◽

Cited By ~ 4

Author(s):

M. Grenier ◽

A. Della Penna ◽

T. W. Trull

Keyword(s):

Southern Ocean ◽

Water Column ◽

Mixed Layer ◽

Dissolved Inorganic Carbon ◽

Mixed Layer Depth ◽

Light Limitation ◽

Clear Correlation ◽

Kerguelen Plateau ◽

Layer Depth ◽

Chl A

Abstract. Natural iron fertilisation from Southern Ocean islands results in high primary production and phytoplankton biomass accumulations readily visible in satellite ocean colour observations. These images reveal great spatial complexity with highly varying concentrations of chlorophyll, presumably reflecting both variations in iron supply and conditions favouring phytoplankton accumulation. To examine the second aspect, in particular the influences of variations in temperature and stratification, we deployed four autonomous profiling floats in the Antarctic Circumpolar Current near the Kerguelen plateau in the Indian sector of the Southern Ocean. Each "bio-profiler" measured more than 250 profiles of temperature (T), salinity (S), dissolved oxygen, chlorophyll fluorescence (Chl a), and particle backscatter in the top 300 m of the water column, sampling up to 5 profiles per day along meandering trajectories extending up to 1000 km. Comparison of surface Chl a estimates (top 50 m depth; analogous to values from satellite images) with total water column inventories revealed largely linear relationships, suggesting that dilution of chlorophyll by mixed layer depth variations plays only a minor role in the spatial distributions observed by satellite, and correspondingly that these images provide credible information on total and not just surface biomass accumulations. Regions of very high Chl a accumulation (1.5–10 μg L-1) were associated predominantly with a narrow T–S class of surface waters, which appears to derive from the northern Kerguelen plateau. In contrast, waters with only moderate Chl a enrichments (0.5–1.5 μg L-1) displayed no clear correlation with water properties, including no dependence on mixed layer depth, suggesting a diversity of sources of iron and/or its efficient dispersion across filaments of the plume. The lack of dependence on mixed layer depth also indicates a limited influence on production by light limitation. One float became trapped in a cyclonic eddy, allowing temporal evaluation of the water column in early autumn. During this period, decreasing surface Chl a inventories corresponded with decreases in oxygen inventories on sub-mixed layer density surfaces, consistent with significant export of organic matter and its respiration and storage as dissolved inorganic carbon in the ocean interior. These results are encouraging for the expanded use of autonomous observing platforms to study biogeochemical, carbon cycle, and ecological problems, although the complex blend of Lagrangian and Eulerian sampling achieved by the floats suggests that arrays rather than single floats will often be required.

Download Full-text

A Modeling Study of Irrigation Effects on Surface Fluxes and Land–Air–Cloud Interactions in the Southern Great Plains

Journal of Hydrometeorology ◽

10.1175/jhm-d-12-0134.1 ◽

2013 ◽

Vol 14 (3) ◽

pp. 700-721 ◽

Cited By ~ 90

Author(s):

Yun Qian ◽

Maoyi Huang ◽

Ben Yang ◽

Larry K. Berg

Keyword(s):

Soil Moisture ◽

Great Plains ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Surface Fluxes ◽

Specific Humidity ◽

Surface Model ◽

Layer Depth ◽

Near Surface ◽

Southern Great Plains

Abstract In this study, the authors incorporate an operational-like irrigation scheme into the Noah land surface model as part of the Weather Research and Forecasting Model (WRF). A series of simulations, with and without irrigation, is conducted over the Southern Great Plains (SGP) for an extremely dry (2006) and wet (2007) year. The results show that including irrigation reduces model bias in soil moisture and surface latent heat (LH) and sensible heat (SH) fluxes, especially during a dry year. Irrigation adds additional water to the surface, leading to changes in the planetary boundary layer. The increase in soil moisture leads to increases in the surface evapotranspiration and near-surface specific humidity but decreases in the SH and surface temperature. Those changes are local and occur during daytime. There is an irrigation-induced decrease in both the lifting condensation level (ZLCL) and mixed-layer depth. The decrease in ZLCL is larger than the decrease in mixed-layer depth, suggesting an increasing probability of shallow clouds. The simulated changes in precipitation induced by irrigation are highly variable in space, and the average precipitation over the SGP region only slightly increases. A high correlation is found among soil moisture, SH, and ZLCL. Larger values of soil moisture in the irrigated simulation due to irrigation in late spring and summer persist into the early fall, suggesting that irrigation-induced soil memory could last a few weeks to months. The results demonstrate the importance of irrigation parameterization for climate studies and improve the process-level understanding on the role of human activity in modulating land–air–cloud interactions.

Download Full-text

Mixed layers along the Atlantic water path in the Nordic seas in HighResMIP models

10.5194/egusphere-egu21-1983 ◽

2021 ◽

Author(s):

Anne Marie Tréguier ◽

Torben Koenigk ◽

Iovino Doroteaciro ◽

Lique Camille ◽

David Docquier

Keyword(s):

High Resolution ◽

Mixed Layer ◽

Mixed Layer Depth ◽

Atlantic Water ◽

The Arctic ◽

Greenland Sea ◽

Layer Depth ◽

Nordic Seas ◽

Heat Exchanges ◽

Mixed Layers

Atlantic water flows over the Greenland-Iceland-Scotland Ridge into the Norwegian Sea. Along its path towards the Arctic, the Atlantic water is cooled by strong air-sea fluxes. Deep winter mixed layers modify the stratification and properties of the Atlantic water and precondition its flow into the Arctic, thus influencing Arctic sea ice and climate. Atlantic water also recirculates in the Greenland sea where deep water formation contributes to the dense limb of the Atlantic Meridional Overturning Circulation. It is thus of paramount importance to represent mixed layer deepening and lateral heat exchanges processes in the Nordic Seas in climate models.Heat exchanges in the Nordic Seas are influenced by narrow current branches, instabilities and eddies, which are not accurately represented in low resolution climate model (with grid ~ 50-100km). &#160;Here we examine the mixed layer dynamics and heat exchanges using the latest generation of European high resolution global coupled models in the framework of HighResMip (5-15km grids in the Nordic Seas). We investigate in detail the effect of model resolution on the mixed layer depth and water mass formation in relation with the Atlantic water circulation and modification between the Norwegian and the Greenland Sea. First results show an increased northward ocean heat transport, a more realistic representation of the ocean current system in the Nordic Seas, and consequently an improved spatial distribution of the turbulent surface heat flux compared to standard resolution CMIP6 models. The mixed layer depth itself however varies strongly between different HighResMIP models. Summarizing, our assessment of the high resolution coupled simulations of the historical period demonstrates that future climate projections at high resolution have a huge potential, but also limitations.

Download Full-text

A model for the Artic mixed layer circulation under a melted lead: Implications on the near-surface temperature maximum formation

10.5194/tc-2020-322 ◽

2020 ◽

Author(s):

Alberto Alvarez

Keyword(s):

Sea Ice ◽

Surface Temperature ◽

Mixed Layer ◽

Circulation Pattern ◽

Acoustic Properties ◽

The Arctic ◽

Near Surface ◽

Ice Surface ◽

Storage Rate ◽

Surface Convection

Abstract. Leads in the sea ice pack have been extensively studied due to their climate relevance. An intense heat exchange between the ocean and the atmosphere occurs at leads in winter. As a result, a major salt input to the Arctic mixed layer is generated at these locations by brine rejection. Leads also constitute preferential melting locations in the early melting season, but their oceanography and climate relevance, if any, still remain unexplored during this period of the year. This study investigates the oceanographic circulation under a melted lead, resulting from the combined effect of the lead geometry, solar radiation and sea ice melting. Results derived from an idealized framework, suggest the daily generation of near surface convection cells that extend from the lead sides to the lead center. Convection cells disappear when melting is diminished during the period of minimum solar insolation. The cyclical generation and evolution of convection cells with the solar cycle, impacts the heat storage rate in the mixed layer below the lead. The contribution of this circulation pattern to the generation of the Near Surface Temperature Maximum (NSTM), is discussed in terms of its capability to inject warm surface waters below the open and sea ice surface. It has been suggested that the NSTM probably affects the oceanographic structure and acoustic properties of the upper ocean and the overlying ice cover.

Download Full-text

On the Variability of Stratification in the Freshwater-Influenced Laptev Sea Region

Frontiers in Marine Science ◽

10.3389/fmars.2020.543489 ◽

2020 ◽

Vol 7 ◽

Cited By ~ 1

Author(s):

Markus A. Janout ◽

Jens Hölemann ◽

Georgi Laukert ◽

Alexander Smirnov ◽

Thomas Krumpen ◽

...

Keyword(s):

Sea Ice ◽

Interannual Variability ◽

Water Column ◽

River Water ◽

Heat Storage ◽

Mixed Layer Depth ◽

Laptev Sea ◽

Layer Depth ◽

Freshwater Influence ◽

The Laptev Sea

In this paper, we investigate the seasonal and spatial variability of stratification on the Siberian shelves with a case study from the Laptev Sea based on shipboard hydrographic measurements, year-round oceanographic mooring records from 2013 to 2014 and chemical tracer-based water mass analyses. In summer 2013, weak onshore-directed winds caused spreading of riverine waters throughout much of the eastern and central shelf. In contrast, strong southerly winds in summer 2014 diverted much of the freshwater to the northeast, which resulted in 50% less river water and significantly weaker stratification on the central shelf compared with the previous year. Our year-long records additionally emphasize the regional differences in water column structure and stratification, where the northwest location was well-mixed for 6 months and the central and northeast locations remained stratified into spring due to the lower initial surface salinities of the river-influenced water. A 26 year record of ocean reanalysis highlights the region’s interannual variability of stratification and its dependence on winds and sea ice. Prior the mid-2000s, river runoff to the perennially ice-covered central Laptev Sea shelf experienced little surface forcing and river water was maintained on the shelf. The transition toward less summer sea ice after the mid-2000s increased the ROFI’s (region of freshwater influence) exposure to summer winds. This greatly enhanced the variability in mixed layer depth, resulting in several years with well-mixed water columns as opposed to the often year-round shallow mixed layers before. The extent of the Lena River plume is critical for the region since it modulates nutrient fluxes and primary production, and further controls intermediate heat storage induced by lateral density gradients, which has implications for autumnal freeze-up and the eastern Arctic sea ice volume.MAIN POINTS1.CTD surveys and moorings highlight the regional and temporal variations in water column stratification on the Laptev Sea shelf.2.Summer winds increasingly control the extent of the region of freshwater influence under decreasing sea ice.3.Further reductions in sea ice increases surface warming, heat storage, and the interannual variability in mixed layer depth.

Download Full-text