scholarly journals Hydrography, transport and mixing of the West Spitsbergen Current: the Svalbard Branch in summer 2015

2018 ◽  
Author(s):  
Eivind Kolås ◽  
Ilker Fer
Keyword(s):  
Heat Flux ◽  
Heat Loss ◽  
Large Fraction ◽  
Turbulent Heat Flux ◽  
The Arctic ◽  
Turbulent Heat ◽  
Fram Strait ◽  
The West ◽  
Transport And Mixing ◽  
West Spitsbergen

Abstract. Measurements of ocean currents, stratification and microstructure were made in August 2015, northwest of Svalbard, downstream of the Atlantic inflow in Fram Strait in the Arctic Ocean. Observations in three sections are used to characterize the evolution of the West Spitsbergen Current (WSC) along a 170-km downstream distance. Two alternative calculations imply 1.5 to 2 Sv (1 Sv = 106 m3 s−1) is routed to recirculation and Yermak branch in Fram Strait, whereas 0.6 to 1.3 Sv is carried by the Svalbard branch. The WSC cools at a rate of 0.20 °C per 100 km, with associated bulk heat loss per along-path meter of (1.1–1.4) x 107 W m−1, corresponding to a surface heat loss of 380–550 W m−2. The measured turbulent heat flux is too small to account for this cooling rate. Estimates using a plausible range of parameters suggest that the contribution of diffusion by eddies could be limited to one half of the observed heat loss. In addition to shear-driven mixing beneath the WSC core, we observe energetic convective mixing of an unstable bottom boundary layer on the slope, driven by Ekman advection of buoyant water across the slope. The estimated lateral buoyancy flux is O(10−8) W kg−1, sufficient to maintain a large fraction of the observed dissipation rates, and corresponds to a heat flux of approximately 400 W m−2. Convectively-driven bottom mixing followed by the detachment of the mixed fluid, and its transfer into the ocean interior can lead to substantial cooling of the WSC, at a rate comparable to that expected from diffusion by eddies.

scholarly journals Hydrography, transport and mixing of the West Spitsbergen Current: the Svalbard Branch in summer 2015

Ocean Science ◽  
2018 ◽  
Vol 14 (6) ◽  
pp. 1603-1618 ◽  
Author(s):  
Eivind Kolås ◽  
Ilker Fer
Keyword(s):  
Heat Flux ◽  
Heat Loss ◽  
Large Fraction ◽  
Turbulent Heat Flux ◽  
The Arctic ◽  
Turbulent Heat ◽  
Fram Strait ◽  
The West ◽  
Transport And Mixing ◽  
West Spitsbergen

Abstract. Measurements of ocean currents, stratification and microstructure were made in August 2015, northwest of Svalbard, downstream of the Atlantic inflow in Fram Strait in the Arctic Ocean. Observations in three sections are used to characterize the evolution of the West Spitsbergen Current (WSC) along a 170 km downstream distance. Two alternative calculations imply 1.5 to 2 Sv (1 Sv = 106 m3 s−1) is routed to recirculation and Yermak branch in Fram Strait, whereas 0.6 to 1.3 Sv is carried by the Svalbard branch. The WSC cools at a rate of 0.20 ∘C per 100 km, with associated bulk heat loss per along-path meter of (1.1-1.4)×107 W m−1, corresponding to a surface heat loss of 380–550 W m−2. The measured turbulent heat flux is too small to account for this cooling rate. Estimates using a plausible range of parameters suggest that the contribution of diffusion by eddies could be limited to one half of the observed heat loss. In addition to shear-driven mixing beneath the WSC core, we observe energetic convective mixing of an unstable bottom boundary layer on the slope, driven by Ekman advection of buoyant water across the slope. The estimated lateral buoyancy flux is O(10−8) W kg−1, sufficient to maintain a large fraction of the observed dissipation rates, and corresponds to a heat flux of approximately 40 W m−2. We conclude that – at least in summer – convectively driven bottom mixing followed by the detachment of the mixed fluid and its transfer into the ocean interior can lead to substantial cooling and freshening of the WSC.

scholarly journals Estimating subpixel turbulent heat flux over leads from MODIS thermal infrared imagery with deep learning

2021 ◽  
Vol 15 (6) ◽  
pp. 2835-2856
Author(s):  
Zhixiang Yin ◽  
Xiaodong Li ◽  
Yong Ge ◽  
Cheng Shang ◽  
Xinyan Li ◽  
...  
Keyword(s):  
Heat Flux ◽  
Spatial Resolution ◽  
Arctic Region ◽  
Open Water ◽  
Turbulent Heat Flux ◽  
The Arctic ◽  
Turbulent Heat ◽  
Moderate Resolution Imaging Spectroradiometer ◽  
Thermal Infrared Imagery ◽  
Ice Surface Temperature

Abstract. The turbulent heat flux (THF) over leads is an important parameter for climate change monitoring in the Arctic region. THF over leads is often calculated from satellite-derived ice surface temperature (IST) products, in which mixed pixels containing both ice and open water along lead boundaries reduce the accuracy of calculated THF. To address this problem, this paper proposes a deep residual convolutional neural network (CNN)-based framework to estimate THF over leads at the subpixel scale (DeepSTHF) based on remotely sensed images. The proposed DeepSTHF provides an IST image and the corresponding lead map with a finer spatial resolution than the input IST image so that the subpixel-scale THF can be estimated from them. The proposed approach is verified using simulated and real Moderate Resolution Imaging Spectroradiometer images and compared with the conventional cubic interpolation and pixel-based methods. The results demonstrate that the proposed CNN-based method can effectively estimate subpixel-scale information from the coarse data and performs well in producing fine-spatial-resolution IST images and lead maps, thereby providing more accurate and reliable THF over leads.

scholarly journals Estimating subpixel turbulent heat flux over leads from MODIS thermal infrared imagery with deep learning

2021 ◽  
Author(s):  
Zhixiang Yin ◽  
Xiaodong Li ◽  
Yong Ge ◽  
Cheng Shang ◽  
Xinyan Li ◽  
...  
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Spatial Resolution ◽  
Important Variable ◽  
Turbulent Heat Flux ◽  
The Arctic ◽  
Turbulent Heat ◽  
Modis Imagery ◽  
Mixed Pixels ◽  
Thermal Infrared Imagery

Abstract. Turbulent heat flux (THF) over leads is an important variable used for monitoring climate change in the Arctic. Presently, THF over leads is often calculated from satellite imagery. The accuracy of the estimated THF is low for mixed pixels that consist of ice and leads, because the mixed pixels along lead boundaries will lower the accuracy of the surface temperature measured over leads and the corresponding lead map. To address this problem, a deep residual convolutional neural network (CNN)-based framework is proposed to estimate THF over leads at the subpixel scale (DeepSTHF) with remotely sensed imagery. The DeepSTHF allows the production of a sea surface temperature (SST) image and a corresponding lead map with a finer spatial resolution than the input SST image using two CNNs, so that the subpixel scale THF can be estimated from them. The proposed approach is assessed using simulated and real MODIS imagery and compared against the conventional bicubic interpolation and pixel-based methods. The results demonstrate that the proposed CNN-based method can effectively estimate subpixel-scale information from the coarse data and performs well in producing fine spatial resolution SST images and lead maps, thereby allowing researchers to obtain more accurate and reliable THF over leads.

scholarly journals Sensitivity of Northern Hemisphere climate to ice-ocean interface heat flux parameterizations

2020 ◽  
Author(s):  
Xiaoxu Shi ◽  
Dirk Notz ◽  
Jiping Liu ◽  
Hu Yang ◽  
Gerrit Lohmann
Keyword(s):  
Heat Flux ◽  
Heat Exchange ◽  
Sea Ice ◽  
Mixed Layer ◽  
Climate Model ◽  
Turbulent Heat Flux ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Turbulent Heat ◽  
Flux Parameterization

Abstract. We investigate the impact of three different parameterizations of ice-ocean heat exchange on modeled ice thickness, ice concentration, and water masses. These three parameterizations are (1) an ice-bath assumption with the ocean temperature fixed at the freezing temperature, (2) a turbulent heat-flux parameterization with ice-ocean heat exchange depending linearly on the temperature difference between the mixed layer and the ice-ocean interface, and (3) a similar turbulent heat-flux parameterization as (2) but with the temperature at the ice-ocean interface depending on ice-ablation rate. Based on model simulations with the standalone sea-ice model CICE, the ice-ocean model MPIOM and the climate model COSMOS, we find that (3) leads (in comparison to the other two parameterizations) to a thicker modeled sea ice, warmer water beneath high-concentration ice and cooler water towards the ice edge, and higher salinity in the Arctic Ocean mixed layer. Finally, in the fully coupled climate model COSMOS, the most realistic parameterization leads to an enhanced Atlantic meridional overturning circulation (AMOC), a more positive North Atlantic Oscillation (NAO) mode and a weakened Aleutian Low.

scholarly journals Sensitivity of Northern Hemisphere climate to ice–ocean interface heat flux parameterizations

2021 ◽  
Vol 14 (8) ◽  
pp. 4891-4908
Author(s):  
Xiaoxu Shi ◽  
Dirk Notz ◽  
Jiping Liu ◽  
Hu Yang ◽  
Gerrit Lohmann
Keyword(s):  
Heat Flux ◽  
Heat Exchange ◽  
Sea Ice ◽  
Temperature Difference ◽  
Climate Model ◽  
Turbulent Heat Flux ◽  
The Arctic ◽  
Ice Thickness ◽  
Turbulent Heat ◽  
Sea Ice Thickness

Abstract. We investigate the impact of three different parameterizations of ice–ocean heat exchange on modeled sea ice thickness, sea ice concentration, and water masses. These three parameterizations are (1) an ice bath assumption with the ocean temperature fixed at the freezing temperature; (2) a two-equation turbulent heat flux parameterization with ice–ocean heat exchange depending linearly on the temperature difference between the underlying ocean and the ice–ocean interface, whose temperature is kept at the freezing point of the seawater; and (3) a three-equation turbulent heat flux approach in which the ice–ocean heat flux depends on the temperature difference between the underlying ocean and the ice–ocean interface, whose temperature is calculated based on the local salinity set by the ice ablation rate. Based on model simulations with the stand-alone sea ice model CICE, the ice–ocean model MPIOM, and the climate model COSMOS, we find that compared to the most complex parameterization (3), the approaches (1) and (2) result in thinner Arctic sea ice, cooler water beneath high-concentration ice and warmer water towards the ice edge, and a lower salinity in the Arctic Ocean mixed layer. In particular, parameterization (1) results in the smallest sea ice thickness among the three parameterizations, as in this parameterization all potential heat in the underlying ocean is used for the melting of the sea ice above. For the same reason, the upper ocean layer of the central Arctic is cooler when using parameterization (1) compared to (2) and (3). Finally, in the fully coupled climate model COSMOS, parameterizations (1) and (2) result in a fairly similar oceanic or atmospheric circulation. In contrast, the most realistic parameterization (3) leads to an enhanced Atlantic meridional overturning circulation (AMOC), a more positive North Atlantic Oscillation (NAO) mode and a weakened Aleutian Low.

scholarly journals Deep Flow Variability Offshore South-West Svalbard (Fram Strait)

Water ◽  
2019 ◽  
Vol 11 (4) ◽  
pp. 683
Author(s):  
Manuel Bensi ◽  
Vedrana Kovačević ◽  
Leonardo Langone ◽  
Stefano Aliani ◽  
Laura Ursella ◽  
...  
Keyword(s):  
Thermohaline Circulation ◽  
Deep Layer ◽  
The Arctic ◽  
Series Data ◽  
Fram Strait ◽  
Atmospheric Forcing ◽  
The West ◽  
Flow Variability ◽  
Dense Water ◽  
West Spitsbergen

Water mass generation and mixing in the eastern Fram Strait are strongly influenced by the interaction between Atlantic and Arctic waters and by the local atmospheric forcing, which produce dense water that substantially contributes to maintaining the global thermohaline circulation. The West Spitsbergen margin is an ideal area to study such processes. Hence, in order to investigate the deep flow variability on short-term, seasonal, and multiannual timescales, two moorings were deployed at ~1040 m depth on the southwest Spitsbergen continental slope. We present and discuss time series data collected between June 2014 and June 2016. They reveal thermohaline and current fluctuations that were largest from October to April, when the deep layer, typically occupied by Norwegian Sea Deep Water, was perturbed by sporadic intrusions of warmer, saltier, and less dense water. Surprisingly, the observed anomalies occurred quasi-simultaneously at both sites, despite their distance (~170 km). We argue that these anomalies may arise mainly by the effect of topographically trapped waves excited and modulated by atmospheric forcing. Propagation of internal waves causes a change in the vertical distribution of the Atlantic water, which can reach deep layers. During such events, strong currents typically precede thermohaline variations without significant changes in turbidity. However, turbidity increases during April–June in concomitance with enhanced downslope currents. Since prolonged injections of warm water within the deep layer could lead to a progressive reduction of the density of the abyssal water moving toward the Arctic Ocean, understanding the interplay between shelf, slope, and deep waters along the west Spitsbergen margin could be crucial for making projections on future changes in the global thermohaline circulation.

scholarly journals Water mass distribution in Fram Strait and over the Yermak Plateau in summer 1997

2000 ◽  
Vol 18 (6) ◽  
pp. 687-705 ◽  
Author(s):  
B. Rudels ◽  
R. Meyer ◽  
E. Fahrbach ◽  
V. V. Ivanov ◽  
S. Østerhus ◽  
...  
Keyword(s):  
Arctic Ocean ◽  
Water Mass ◽  
Atlantic Water ◽  
The Arctic ◽  
Fram Strait ◽  
The West ◽  
The Arctic Ocean ◽  
Yermak Plateau ◽  
Eurasian Basin ◽  
West Spitsbergen

Abstract. The water mass distribution in northern Fram Strait and over the Yermak Plateau in summer 1997 is described using CTD data from two cruises in the area. The West Spitsbergen Current was found to split, one part recirculated towards the west, while the other part, on entering the Arctic Ocean separated into two branches. The main inflow of Atlantic Water followed the Svalbard continental slope eastward, while a second, narrower, branch stayed west and north of the Yermak Plateau. The water column above the southeastern flank of the Yermak Plateau was distinctly colder and less saline than the two inflow branches. Immediately west of the outer inflow branch comparatively high temperatures in the Atlantic Layer suggested that a part of the extraordinarily warm Atlantic Water, observed in the boundary current in the Eurasian Basin in the early 1990s, was now returning, within the Eurasian Basin, toward Fram Strait. The upper layer west of the Yermak Plateau was cold, deep and comparably saline, similar to what has recently been observed in the interior Eurasian Basin. Closer to the Greenland continental slope the salinity of the upper layer became much lower, and the temperature maximum of the Atlantic Layer was occasionally below  0.5 °C, indicating water masses mainly derived from the Canadian Basin. This implies that the warm pulse of Atlantic Water had not yet made a complete circuit around the Arctic Ocean. The Atlantic Water of the West Spitsbergen Current recirculating within the strait did not extend as far towards Greenland as in the 1980s, leaving a broader passage for waters from the Atlantic and intermediate layers, exiting the Arctic Ocean. A possible interpretation is that the circulation pattern alternates between a strong recirculation of the West Spitsbergen Current in the strait, and a larger exchange of Atlantic Water between the Nordic Seas and the inner parts of the Arctic Ocean.Key words: Oceanography: general (Arctic and Antarctic oceanography; water masses) - Oceanography: physical (general circulation)

scholarly journals Seasonal and Mesoscale Variability of the Two Atlantic Water Recirculation Pathways in Fram Strait

2021 ◽  
Author(s):  
Zerlina Hofmann ◽  
Wilken-Jon von Appen ◽  
Claudia Wekerle
Keyword(s):  
Sea Ice ◽  
Atlantic Water ◽  
The Arctic ◽  
Fram Strait ◽  
Mesoscale Variability ◽  
The West ◽  
East Greenland Current ◽  
Prime Meridian ◽  
Set Up ◽  
West Spitsbergen

<p>Atlantic Water, which is transported northward by the West Spitsbergen Current, partly recirculates (i.e. turns westward) in Fram Strait. This determines how much heat and salt reaches the Arctic Ocean, and how much joins the East Greenland Current on its southward path. We describe the Atlantic Water recirculation's location, seasonality, and mesoscale variability by analyzing the first observations from moored instruments at five latitudes in central Fram Strait, spanning a period from August 2016 to July 2018. We observe recirculation on the prime meridian at 78°50'N and 80°10'N, respectively south and north of the Molly Hole, and no recirculation further south at 78°10'N and further north at 80°50'N. At a fifth mooring location at 79°30'N, we observe some influence of the two recirculation branches. The southern recirculation is observed as a continuous westward flow that carries Atlantic Water throughout the year, though it may be subject to broadening and narrowing. It is affected by eddies in spring, likely due to the seasonality of mesoscale instability in the West Spitsbergen Current. The northern recirculation is observed solely as passing eddies on the prime meridian, which are strongest during late autumn and winter, and absent during summer. This seasonality is likely affected both by the conditions set by the West Spitsbergen Current and by the sea ice. Open ocean eddies originating from the West Spitsbergen Current interact with the sea ice edge when they subduct below the fresher, colder water. Additionally the stratification set up by sea ice presence may inhibit recirculation.</p>

scholarly journals Estimation of turbulent heat flux over leads using satellite thermal images

2019 ◽  
Vol 13 (6) ◽  
pp. 1565-1582 ◽  
Author(s):  
Meng Qu ◽  
Xiaoping Pang ◽  
Xi Zhao ◽  
Jinlun Zhang ◽  
Qing Ji ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Heat Flux ◽  
Surface Temperature ◽  
Turbulent Heat Flux ◽  
The Arctic ◽  
Landsat 8 ◽  
Turbulent Heat ◽  
Thermal Images ◽  
Modis Data ◽  
Ice Leads

Abstract. Sea ice leads are an important feature in pack ice in the Arctic. Even covered by thin ice, leads can still serve as prime windows for heat exchange between the atmosphere and the ocean, especially in the winter. Lead geometry and distribution in the Arctic have been studied using optical and microwave remote sensing data, but turbulent heat flux over leads has only been measured on-site during a few special expeditions. In this study, we derive turbulent heat flux through leads at different scales using a combination of surface temperature and lead distribution from remote sensing images and meteorological parameters from a reanalysis dataset. First, ice surface temperature (IST) was calculated from Landsat-8 Thermal Infrared Sensor (TIRS) and Moderate Resolution Imaging Spectroradiometer (MODIS) thermal images using a split-window algorithm; then, lead pixels were segmented from colder ice. Heat flux over leads was estimated using two empirical models: bulk aerodynamic formulae and a fetch-limited model with lead width from Landsat-8. Results show that even though the lead area from MODIS is a little larger, the length of leads is underestimated by 72.9 % in MODIS data compared to TIRS data due to the inability to resolve small leads. Heat flux estimated from Landsat-8 TIRS data using bulk formulae is 56.70 % larger than that from MODIS data. When the fetch-limited model was applied, turbulent heat flux calculated from TIRS data is 32.34 % higher than that from bulk formulae. In both cases, small leads accounted for more than a quarter of total heat flux over leads, mainly due to the large area, though the heat flux estimated using the fetch-limited model is 41.39 % larger. A greater contribution from small leads can be expected with larger air–ocean temperature differences and stronger winds.

scholarly journals Mechanism of seasonal Arctic sea ice evolution and Arctic amplification

2016 ◽  
Vol 10 (5) ◽  
pp. 2191-2202 ◽  
Author(s):  
Kwang-Yul Kim ◽  
Benjamin D. Hamlington ◽  
Hanna Na ◽  
Jinju Kim
Keyword(s):  
Heat Flux ◽  
Sea Ice ◽  
Arctic Ocean ◽  
Turbulent Heat Flux ◽  
Sea Surface ◽  
The Arctic ◽  
Turbulent Heat ◽  
Arctic Amplification ◽  
Atmospheric Column ◽  
The Arctic Ocean

Abstract. Sea ice loss is proposed as a primary reason for the Arctic amplification, although the physical mechanism of the Arctic amplification and its connection with sea ice melting is still in debate. In the present study, monthly ERA-Interim reanalysis data are analyzed via cyclostationary empirical orthogonal function analysis to understand the seasonal mechanism of sea ice loss in the Arctic Ocean and the Arctic amplification. While sea ice loss is widespread over much of the perimeter of the Arctic Ocean in summer, sea ice remains thin in winter only in the Barents–Kara seas. Excessive turbulent heat flux through the sea surface exposed to air due to sea ice reduction warms the atmospheric column. Warmer air increases the downward longwave radiation and subsequently surface air temperature, which facilitates sea surface remains to be free of ice. This positive feedback mechanism is not clearly observed in the Laptev, East Siberian, Chukchi, and Beaufort seas, since sea ice refreezes in late fall (November) before excessive turbulent heat flux is available for warming the atmospheric column in winter. A detailed seasonal heat budget is presented in order to understand specific differences between the Barents–Kara seas and Laptev, East Siberian, Chukchi, and Beaufort seas.

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