The seasonality of Antarctic sea ice trends

Antarctic sea ice is experiencing a weak overall increase in area that is the residual of opposing regional trends. This study considers their seasonal pattern. In addition to traditional ice concentration and total ice area, temporal derivatives of these quantities are investigated (‘intensification’ and ‘expansion’ respectively). This is crucial to the attribution of trends, since changes in forcing directly affect ice areal change (rather than ice area). Diverse regional trends all contribute significantly to the overall increase. Trends in the Weddell and Amundsen—Bellingshausen regions compensate in magnitude and seasonality. The largest concentration trends, in autumn, are actually caused by intensification trends during spring. Autumn intensification trends directly oppose autumn concentration trends in most places, seemingly as a result of ice and ocean feedbacks. Springtime trends are reconcilable with wind trends, but further study of changes during the spring melting season is required to unravel the Antarctic sea ice increase

Numerical modelling of the riverine thermal bar

A Finite-Volume discretisation of the Navier–Stokes equations is used to study various aspects of the physics and ecology of the riverine thermal bar. The classical thermal bar is a down-welling plume which is formed twice a year in temperate lakes when the shallows warm or cool through the temperature of maximum density (Tmd). The riverine thermal bar is a similar sinking plume arising at the confluence of river and lake waters which are on either side of the Tmd. The dynamics of this poorly understood riverine case may be considerably more complex due to the additional effects of river salinity and velocity on the down-welling plume. A series of deep-lake simulations forms the initial study of the riverine thermal bar in the Selenga River delta in Lake Baikal, Siberia. While the decrease in the Tmd with depth (pressure) prevents the classical thermal bar from sinking far, this study shows that a saline riverine thermal bar may be able to sink to greater depths and thus take part in Baikal's vigorous deep-water renewal. Attention then focusses on a model of the smaller Kamloops Lake in British Columbia, which is used to reproduce the only field observations of a riverine thermal bar and test the effects of coriolis forces, bathymetry, and surface heating on the resulting flow field. Plankton ecosystem models are then coupled to these validated dynamics, and results are presented which extend and test the findings of a previous modelling study on the effects of the classical thermal bar on plankton populations

A review of the physics and ecological implications of the thermal bar circulation

AbstractFollowing recent applications of numerical modelling and remote sensing to the thermal bar phenomenon, this paper seeks to review the current state of knowledge on the effect of its circulation on lacustrine plankton ecosystems. After summarising the literature on thermal bar hydrodynamics, a thorough review is made of all plankton observations taken in the presence of a thermal bar. Two distinct plankton growth regimes are found, one with production favoured throughout the inshore region and another with a maximum in plankton biomass near the position of the thermal bar. Possible explanations for the observed distributions are then discussed, with reference to numerical modelling studies, and the scope for future study of this interdisciplinary topic is outlined

Influence of tides on melting and freezing beneath Filchner-Ronne Ice Shelf, Antarctica

An isopycnic coordinate ocean circulation model is applied to the ocean cavity beneath Filchner-Ronne Ice Shelf, investigating the role of tides on sub-ice shelf circulation and ice shelf basal mass balance. Including tidal forcing causes a significant intensification in the sub-ice shelf circulation, with an increase in melting (3-fold) and refreezing (6-fold); the net melt rate and seawater flux through the cavity approximately doubles. With tidal forcing, the spatial pattern and magnitude of basal melting and freezing generally match observations. The 0.22 m a(-1) net melt rate is close to satellite-derived estimates and at the lower end of oceanographic values. The Ice Shelf Water outflow mixes with shelf waters, forming a cold (<-1.9 degrees C), dense overflow (0.83 Sv) that spills down the continental slope. These results demonstrate that tidal forcing is fundamental to both ice shelf-ocean interactions and deep-water formation in the southern Weddell Sea. Citation: Makinson, K., P. R. Holland, A. Jenkins, K. W. Nicholls, and D. M. Holland (2011), Influence of tides on melting and freezing beneath Filchner-Ronne Ice Shelf, Antarctica, Geophys. Res. Lett., 38, L06601, doi: 10.1029/2010GL046462

The effect of a new drag-law parameterization on ice shelf water plume dynamics

A drag law accounting for Ekman rotation adjacent to a flat, horizontal bou ndary is proposed for use in a plume model that is written in terms of the depth-mean velocity. The drag l aw contains a variable turning angle between the mean velocity and the drag imposed by the turbulent bound ary layer. The effect of the variable turning angle in the drag law is studied for a plume of ice shelf wat er (ISW) ascending and turning beneath an Antarctic ice shelf with draft decreasing away from the groundi ng line. As the ISW plume ascends the sloping ice shelf–ocean boundary, it can melt the ice shelf, wh ich alters the buoyancy forcing driving the plume motion. Under these conditions, the typical turning ang le is of order 10° over most of the plume area for a range of drag coefficients (the minus sign arises for th e Southern Hemisphere). The rotation of the drag with respect to the mean velocity is found to be signifi cant if the drag coefficient exceeds 0.003; in this case the plume body propagates farther along and across the b ase of the ice shelf than a plume with the standard quadratic drag law with no turning angle

The transient response of ice-shelf melting to ocean change

Idealised modelling studies have shown that the melting of ice shelves varies as a quadratic function of ocean temperature. However, this result is the equilibrium response, derived from steady ice— ocean simulations subjected to a fixed ocean forcing. This study considers instead the transient response of melting, using unsteady simulations subjected to forcing conditions that are oscillated with a range of periods. The results show that the residence time of water in the sub-ice cavity offers a critical timescale. When the forcing varies slowly (period of oscillation ≫ residence time), the cavity is fully-flushed with forcing anomalies at all stages of the cycle and melting follows the equilibrium response. When the forcing varies rapidly (period ≤ residence time), multiple cold and warm anomalies coexist in the cavity, cancelling each other in the spatial mean and thus inducing a relatively steady melt rate. This implies that all ice shelves have a maximum frequency of ocean variability that can be manifested in melting. Between these two extremes, an intermediate regime occurs in which melting follows the equilibrium response during the cooling phase of the forcing cycle, but deviates during warming. The results show that ice shelves forced by warm water have high melt rates, high equilibrium sensitivity, and short residence times, hence a short timescale over which the equilibrium sensitivity is manifest. The most rapid melting adjustment is induced by warm anomalies that are also saline. Thus, ice shelves in the Amundsen and Bellingshausen seas, Antarctica, are highly sensitive to ocean chang

The effect of meltwater plumes on the melting of a vertical glacier face

Freshwater produced by the surface melting of ice sheets is commonly discharged into ocean fjords from the bottom of deep fjord-terminating glaciers. The discharge of the freshwater forms upwelling plumes in front of the glacier calving face. We simulate the meltwater plumes emanated into an unstratified environment using a non-hydrostatic ocean model with an unstructured mesh and subgrid-scale mixing calibrated by comparison to established plume theory. The presence of an ice face reduces the entrainment of seawater into the meltwater plumes, so the plumes remain attached to the ice front, in contrast to previous simple models. Ice melting increases with height above the discharge, also in contrast to some simple models, and we speculate that this ‘overcutting’ may contribute to a tendency of icebergs to topple inwards toward the ice face upon calving. The overall melt rate is found to increase with discharge flux only up to a critical value, which depends on the channel size. The melt rate is not a simple function of the subglacial discharge flux, as assumed by many previous studies. For a given discharge flux, the geometry of the plume source also significantly affects the melting, with higher melt rates obtained for a thinner, wider source. In a wider channel, two plumes are emanated near the source and these plumes eventually coalesce. Such merged meltwater plumes ascend faster and increase the maximum melt rate near the center of the channel. The melt rate per unit discharge decreases as the subglacial system becomes more channelised

On the conditional frazil ice instability in seawater

It has been suggested that the presence of frazil ice can lead to a conditional instability in seawater. Any frazil forming in the water column reduces the bulk density of a parcel of frazil-seawater mixture, causing it to rise. Due to the pressure-decrease in the freezing point, this causes more frazil to form, causing the parcel to accelerate, and so on. We use linear stability analysis and a non-hydrostatic ocean model to study this instability. We find that frazil ice growth caused by the rising of supercooled water is indeed able to generate a buoyancy-driven instability. Even in a gravitationally stable water column, the frazil ice mechanism can still generate convection. The instability does not operate in the presence of strong density stratification, high thermal driving (warm water), a small initial perturbation, high background mixing or the prevalence of large frazil ice crystals. In an unstable water column the instability is not necessarily expressed in frazil ice at all times; an initial frazil perturbation may melt and refreeze. Given a large enough initial perturbation this instability can allow significant ice growth. A model shows frazil ice growth in an Ice Shelf Water plume several kilometres from an ice shelf, under similar conditions to observations of frazil ice growth under sea ice. The presence of this instability could be a factor affecting the growth of sea ice near ice shelves, with implications for Antarctic bottom water formation