Resonance of internal waves in fjords: A finite-difference model

After the time periodicity is removed from the problem, the spatial distribution of internal waves in a stratified fluid is governed by a hyperbolic equation. With boundary conditions specified all along the perimeter of the domain, information is transmitted in both directions (forward and backward) along every characteristic, and, unlike the typical temporal hyperbolic equation, the internal-wave equation is not amenable to a simple forward integration. The problem is tackled here with a finite-difference, relaxation technique by constructing a time-dependent, dissipative problem, the final steady state of which yields the solution of the original problem. Attempts at solving the problem for arbitrary topography then reveal multiple resonances, each resonance being caused by a ray path closing onto itself after multiple reflections. The finite-difference formulation is found to be a convenient vehicle to discuss resonances and to conclude that their existence renders the problem not only singular but also extremely sensitive to the details of the topography. The problem is easily overcome by the introduction of friction. The finite-difference representation of the problem is instrumental in serving as a guide for the investigation of the resonance problem. Indeed, it keeps the essence of the continuous problem and yet simplifies the analysis enormously. Although straightforward, robust and successful at providing a numerical solution to a first few examples, the relaxation component of the integration technique suffers from lack of efficiency. This is due to the particular nature of the hyperbolic problem, but it remains that numerical analysts could improve or replace the present scheme with a faster algorithm

Mid-summer vertical behavior of a high-latitude oceanic zooplankton community

Vertical behavior, such as diel vertical migration (DVM) and swarming are widespread among zooplankton. At higher latitudes, synchronized DVM is mostly absent during summer and predominantly herbivorous copepods tend to form large near-surface swarms. This behavior is risky because it can make them vulnerable to visual predators. Here, we used ca. 12 days of mid-summer (28 June to 10 July 2018) high-frequency acoustic data collected on board of an autonomous surface vehicle (Sailbuoy) to study the vertical behavioral patterns of a zooplankton community in the Norwegian Sea (69◦–71◦ N). Comparing acoustic data with zooplankton net samples, we could distinguish the sound scatters into (1). lipid-rich older developmental stages of Calanus spp., (2). younger developmental stages of Calanus spp., smaller copepods and krill and (3). unknown group of strong sound scatters that may have been younger stages of planktivorous fish. We observed shorter-range classic DVM during much of the study period, where in two days, the migration appeared to be pronounced (> 50 m in amplitude), largely synchronous and occurred in the presence of sound scatterer group 3. The observed zooplankton community was concentrated in the upper 20 m in cloudy and calm days but retreated to greater depths at increased near-surface turbulence. This turbulence-driven vertical retreat appeared to be synchronized across the zooplankton community, potentially indicating a schooling behavior

Timing of Calanus finmarchicus diapause in stochastic environments

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Water mass modification in an Arctic fjord through cross-shelf exchange: The seasonal hydrography of Kongsfjorden, Svalbard

Kongsfjorden and the West Spitsbergen Shelf is a region whose seasonal hydrography is dominated by the balance of Atlantic Water, Arctic waters, and glacial melt. Regional seasonality and the cross-shelf exchange processes have been investigated using conductivity-temperature-depth (CTD) observations from 2000–2003 and a 5-month mooring deployment through the spring and summer of 2002. Modeling of shelf-fjord dynamics was performed with the Bergen Ocean Model. Observations show a rapid and overwhelming intrusion of Atlantic Water across the shelf and into the fjord during midsummer giving rise to intense seasonality. Pockets of Atlantic Water, from the West Spitsbergen Current, form through barotropic instabilities at the shelf front. These leak onto the shelf and propagate as topographically steered features toward the fjord. Model results indicate that such cross-front exchange is enhanced by north winds. Normally, Atlantic Water penetration into the fjord is inhibited by a density front at the fjord mouth. This geostrophic control mechanism is found to be more important than the hydraulic control common to many fjords. Slow modification of the fjord water during spring reduces the effectiveness of geostrophic control, and by midsummer, Atlantic Water intrudes into the fjord, switching from being Arctic dominant to Atlantic dominant. Atlantic Water continues to intrude throughout the summer and by September reaches some quasi steady state condition. The fjord adopts a ‘‘cold’’ or ‘‘warm’’ mode according to the degree of Atlantic Water occupation. Horizontal exchange across the shelf may be an important process causing seasonal variability in the northward heat transport to the Arctic

A major Calanus finmarchicus overwintering population inside a deep fjord in northern Norway: implications for cod larvae recruitment success

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Warming of Atlantic Water in two west Spitsbergen fjords over the last century (1912-2009)

The recently observed warming of west Spitsbergen fjords has led to anomalous sea-ice conditions and has implications for the marine ecosystem. We investigated long-term trends of maximum temperature of Atlantic Water (AW) in two west Spitsbergen fjords. The data set is composed of more than 400 oceanographic stations for Isfjorden and Grønfjorden (78.1°N), spanning from 1876 to 2009. Trends throughout the last century (1912–2009) indicate an increase of 1.9°C and 2.1°C in the maximum temperature during autumn for Isfjorden and Grønfjorden, respectively. A recent warming event in the beginning of the 21st century is found to be more than 1°C higher than the second warmest period in the time series. Mean sea-level pressure (MSLP) data from ERA-40 and ERA-Interim data sets produced by the European Centre for Medium-Range Weather Forecasts and mean temperature in the core of the West Spitsbergen Current (WSC) at the Sørkapp Section along 76.3°N were used to explain the variability of the maximum temperature. A correlation analysis confirmed previous findings, showing that variability in the oceanography of the fjords can be explained mainly by two external factors: AW temperature variability in the WSC and regional patterns of the wind stress field. To take both processes into consideration, a multiple regression model accounting for temperature in the WSC core and MSLP over the area was developed. The predicted time series shows a reasonable agreement with observed maxima temperature in Isfjorden for the period 1977–2009 (N=24), with a statistically significant multiple correlation coefficient of 0.60 (R2=0.36) at P<0.05.publishedVersio

Annual changes in Arctic fjord environment and modern benthic foraminiferal fauna:Evidence from Kongsfjorden, Svalbard

The relationships between modern Arctic benthic foraminifera and their ecological controls, along with their sensitivity to rapid environmental changes, is still poorly understood. This study examines how modern benthic foraminifera respond to annual environmental changes in the glaciated Arctic fjord Kongsfjorden, western Svalbard. Large environmental gradients due to the inflow of warm and saline Atlantic Water and the influence of tidewater glaciers characterise the fjord hydrography. A transect of six multi-corer stations, from the inner to the outer fjord, was sampled in the late summers of 2005 to 2008 to study the distribution of living (rose Bengal stained) benthic foraminifera. Physical properties of the water masses were measured concurrently. In general, nearly the entire Kongsfjorden region was dominated by ubiquitous N. labradorica foraminiferal assemblage that successfully exploited the local food resources and thrived particularly well in the presence of Atlantic-derived Transformed Atlantic Water (TAW). Further, the annual investigation revealed that Kongsfjorden underwent large interannual hydrological changes during the studied years related to variable inflow of warm and saline Atlantic Water. This led to a strong fauna variability particularly at the two marginal sites: the glacially influenced inner fjord and marine influenced shelf region. We also observed significant species shift from the ‘cold’ to ‘warm’ years and an expansion of widespread and sub-arctic to boreal species into the fjord

Variability and decadal trends in the Isfjorden (Svalbard) ocean climate and circulation – An indicator for climate change in the European Arctic

Isfjorden, a broad Arctic fjord in western Spitsbergen, has shown significant changes in hydrography and inflow of Atlantic Water (AW) the last decades that only recently have been observed in the Arctic Ocean north of Svalbard. Variability and trends in this fjord’s climate and circulation are therefore analysed from observational and reanalysis data during 1987 to 2017. Isfjorden experienced a shift in summer ocean structure in 2006, from AW generally in the bottom layer to AW (with increasing thickness) higher up in the water column. This shift, and a concomitant shift to less fast ice in Isfjorden are linked to positive trends in the mean sea surface temperature (SST) and volume weighted mean temperature (VT) in winter (SSTw/VTw: 0.7 ± 0.1/0.9 ± 0.3 °C 10 yr−1) and summer (SSTS/VTS: 0.7 ± 0.1/0.6 ± 0.1 °C 10 yr−1). Hence, the local mean air temperature shows similar trends in winter (1.9 ± 0.4 °C 10 yr−1) and summer (0.7 ± 0.1 °C 10 yr−1). Positive trends in volume weighted mean salinity in winter (0.21 ± 0.06 10 yr−1) and summer (0.07 ± 0.05 10 yr−1) suggest increased AW advection as a main reason for Isfjorden’s climate change. Local mean air temperature correlates significantly with sea ice cover, SST, and VT, revealing the fjord’s impact on the local terrestrial climate. In line with the shift in summer ocean structure, Isfjorden has changed from an Arctic type fjord dominated by Winter Deep and Winter Intermediate thermal and haline convection, to a fjord dominated by deep thermal convection of Atlantic type water (Winter Open). AW indexes for the mouth and Isfjorden proper show that AW influence has been common in winter over the last decade. Alternating occurrence of Arctic and Atlantic type water at the mouth mirrors the geostrophic control imposed by the Spitsbergen Polar Current (carrying Arctic Water) relative to the strength of the Spitsbergen Trough Current (carrying AW). During high AW impact events, Atlantic type water propagates into the fjord according to the cyclonic circulation along isobaths corresponding to the winter convection. Tides play a minor role in the variance in the currents, but are important in the side fjords where exchange with the warmer Isfjorden proper occurs in winter. This study demonstrates that Isfjorden and its ocean climate can be used as an indicator for climate change in the Arctic Ocean. The used methods may constitute a set of helpful tools for future studies also outside the Svalbard Archipelago.publishedVersio