Polynyas gained their name from a Russian term meaning ‘ice hole’ (Williams et al 2007). They are regions of persistent or re-occurring open water, and/or thin ice up to 0.3m in thickness, that exist within the thick sea ice (Massom et al 1998). Polynyas are non-linear, and vary spatially and temporally depending on the physical environment (Henshaw 2003). They are sensitive to changes in local and regional ice conditions, and mechanical and thermodynamic forcing by the ocean and atmosphere (Barber and Massom 2007). Polynyas can range in size from 10 to 105 km2 (Morales-Maqueda et al 2004).
The unique characteristics of polynyas render them important on a local, regional and global scale (Williams et al 2007). They alter water masses, gas and heat fluxes across the ocean-atmosphere interface, sea-ice production, biogeochemistry, nutrient dynamics, primary productivity and regional ecology. Polynyas are the result of two complementary mechanisms and are either mechanically or thermodynamically driven Massom et al 1998).
Figure 1. The Terra Nova Bay polynya as captured by the Moderate
Resolution Imaging Spectroradiometer on NASA’s Aqua satellite on
October 16, 2007.
Sourced from http://earthobservatory.nasa.gov/.
Polynya classification varies depending on whether classification is based on their physical position in the environment, or by the processes that form and maintain them. Some authors classify polynyas as either ‘coastal’ or ‘open water’, while others prefer to name polynyas as either ‘shelf’ or ‘deep water’ (Williams et al 2007). Polynyas limited by land are referred to as ‘shore' or 'coastal' polynyas, or if limited by fast ice are known as ‘flaw' polynyas. Polynyas that appear several times during one winter, or annually in the same position are referred to as ‘re-occurring' polynyas (Morales-Maqueda et al 2004).
Polynyas categorised by the mechanisms that form and maintain them, and are traditionally classed as either ‘latent heat’ or ‘sensible heat’ polynyas. Latent heat polynyas tend to be coastal, where as sensible heat polynyas tend to be occur over open water. Additional names used to describe latent heat polynyas include ‘mechanically forced’ and ‘ice-divergent’ polynyas; and additional names used to describe sensible heat polynyas include ‘convective’ and ‘thermal’ polynyas (Williams et al 2007). Often polynyas result from a combination of both of these mechanisms, and are then referred to as ‘mixed-mode’ polynyas.
Sensible Heat Polynyas
Sensible heat polynyas are thermally driven and usually form above deep waters in sea ice concentrated regions (Morales-Maqueda et al 2004). They form from an oceanic sensible heat flux; where the heat flux is large enough to melt existing sea ice, inhibit the formation of new ice, or decrease the rate of ice accumulation; allowing wind-driven advection or increased solar radiation to decrease the local concentration of sea ice (Reddy et al 2007). Thus sensible heat polynyas often exhibit low ice production, and their size is governed by the extent of the warm water anomaly that creates them (Morales-Maqueda et al 2004, Martin 2001).
Due to the mechanisms that create sensible heat polynyas, they are to an extent, self maintaining. The positive sensible heat flux means the surface water cools and becomes more dense than the surrounding water. Sinking of this dense surface water then drives further convection. The loop tends to cease in spring when the atmosphere warms, or if a fresh water flux places a low salinity cap on the convection (Martin 2001).
Sensible heat polynyas tend to occur in regions of upwelling. These regions are often associated with tidal activity in channels, bays or straits; or interaction between topographic features with currents such as seamounts (Morales-Maqueda 2004). In the Antarctic only a small number of re-occurring sensible heat polynyas have been observed; for example in the Ross Passage and over the Pennell Bank.
Latent Heat Polynyas
Latent heat polynyas are mechanically driven and usually form in coastal regions adjacent to a lee shore (Martin 2001). Mechanical processes such as katabatic winds or currents result in ice divergence where adjacent sea ice is transported offshore (Martin 2001, Morales-Maqueda et al 2004). This allows the ocean surface to be in contact with the cooler atmosphere, creating a large positive sensible heat flux. As a result new ice is rapidly formed, releasing latent heat as it freezes (Massom et al 1998).
It should be noted that latent heat polynyas are not maintained through the release of latent heat when sea water freezes as the name suggests. The latent heat released by ice formation is “used up” to maintain the surface waters at freezing point, and exerts no control over freezing rates (Morales-Maqueda et al 2004). Rather, mechanical forcing drives newly formed frazil ice offshore maintaining the polynya. Therefore the maximum size of latent heat polynyas is a balance between ice export from the polynya and ice production within the polynya (Morales-Maqueda et al 2004).
The majority of polynyas in the Antarctic are latent heat polynyas, which frequent the Antarctic coast through winter (Kern 2009). They tend to form in lee of ice shelves, grounded icebergs, headlands, islands and downwind of ice tongues in shallow water which is near freezing point and can not be supplied with warm water from below (Martin 2001, Markus 1998). Unlike sensible heat polynyas, latent heat polynyas are important "ice factories’" and thus play a huge role in the water mass modification.
Figure 2. Schematic representation of the physical processes taking place in deep water and
shelf water polynyas. Sourced from Morales-Maqueda et al (2004).
Sea Ice Production
Latent heat polynyas are regions of exceptionally high sea ice production. During winter, water within latent heat polynyas is usually held at freezing point (Morales-Maqueda et al 2004). This is maintained by heat loss to the atmosphere being balanced by the latent heat of fusion as ice forms (Thorsten 1998). In some cases, the difference in ocean-atmosphere temperature can be up to 200C, and the resulting heat flux over polynyas can be one to two orders of magnitude larger than estimations over thicker surrounding ice (Tamura et al 2008, Massom et al 1998). With new ice being constantly transported out of the polynya, these conditions facilitate rapid ice formation; therefore polynyas are often referred to as “ice factories”. Polynyas have been estimated to produce on average 10m.y-1 of ice, which is exceptionally high compared with other regions. Figure 3 illustrates the exceptionally high sea ice production in coastal polynyas relative to other coastal regions in the Antarctic. The Ross Sea Polynya, Cape Darnley Polynya, Mackenzie Bay Polynya and the Mertz Glacier Polynya are on average among the most productive in the Antarctic (Figure 3).
Figure 3. Spatial distribution of annual cumulative sea-ice production averaged over 1992–2001 calculated using ERA-40
data with enlargements along the coasts of theWeddell Sea, Ross Sea and East Antarctica. The 200- and 1000-m isobaths are
indicated by thin lines. Sourced from Tamura et al (2008).
Antarctic Bottom Water Formation
High sea ice production in polynyas has implications for underlaying water mass properties. As a result, latent heat polynyas in the Antarctic are major contributors to the formation of Antarctic Bottom Water (ABW). ABW is the densest water mass in the ocean, and is vital in driving abyssal and consequently global thermohaline circulation (Tamura et al 2008). Sea ice has a salinity of approximately 5%o compared with sea water which is approximately 34.5%o. This is because salt is expelled during sea ice production, which creates a dense brine layer at the surface. The density difference between the brine layer and surrounding water means it sinks, forming a water mass termed ‘shelf water’. Shelf water sinks down over the continental shelf and slope, into the deep ocean to eventually form ABW (Gordon 2001). This water mass is characterised by high salinity, low temperature, and high dissolved oxygen and carbon dioxide concentrations due to the increased solubility of gases in the poles. ABW is therefore vital in ventilating the deep ocean, as well as driving global circulation. This highlights the importance of polynyas on a global scale.
Polynyas play an important role in regional meteorology, and have the potential to dominate local climate. They interact with the overlying atmosphere through a series of feedback mechanisms which modify meso-scale atmospheric motions (Minnett 2007). Formation of the internal boundary layer (ITBL), cloud modification through plume formation and convection associated with polynyas are able to alter the heat and moisture content of the atmospheric boundary layer, the boundary layer stability, and the surface heat budget above and downwind of the polynya. During winter especially, enhanced turbulent fluxes across the ocean-atmosphere interface introduces heat and moisture into the normally dry, cold polar atmosphere.
When cool air is transported over the relatively warm open water of polynyas, large latent and sensible heat fluxes result from temperature and moisture differences between the ocean and atmosphere. As heat and moisture from the ocean is transported vertically into the atmosphere, they modify vertical profiles of temperature, humidity and wind speed, creating an internal boundary layer (ITBL) (Andreas and Murphy 1986). Convergence of sensible and latent heat fluxes then cause near surface air to warm and become more moist as it crosses the polynya, leading to a reduction in the ocean-atmosphere heat and moisture flux with fetch. This reduction can be up to 20% over fetches of tens of kilometres and up to 50% over hundreds of kilometres (Fiedler et al 2010). Not only boundary layer processes, but also microphysical and radiative processes are important above polynyas due to the complication of the boundary layer by moisture and cloud formation (Renfrew and King 1999).
During winter, plumes and clouds are formed above and downwind of polynyas. Water vapour lost to the atmosphere over a polynya forms convective cloud plumes, creating a positive sea ice formation-albedo feedback. Clouds that stem from polynyas result in an atmosphere to surface radiative flux that melts sea ice. This in turn stimulates a greater water vapour flux to the atmosphere leading to cloud formation which acts to enhance melting (Barber and Massom 2007). These convective cloud plumes have been observed to reach heights of 4km (Fiedler et al 2010). However, there is limited fetch over polynyas, and as a result the downwind boundary layer returns to being stable, although sometimes cloud plumes have been observed for a 100km downwind of large polynyas (Khvorostyanov et al 2003).
Polynyas also play a role on local climate during summer. Polynyas allow a large amount of short-wave radiation to penetrate into the oceanic mixed layer, impacting on the heat and mass balance of the ice pack and the ocean (Morales-Maqueda et al 2004). In spring increased short-wave radiation penetrates into the water column and facilitates melting, this also enhances the albedo feedback facilitating further melt. The early spring melt exhibited by polynyas has ecological implications.
Polynyas tend to harbour a high concentration of marine life. This can be largely attributed to the high chlorophyll-a concentrations observed in polynyas, with phytoplankton abundance recordings being some of the highest on earth (Arrigo et al 2012). However the extent of enhanced productivity between polynyas varies according to polynya duration, ice and snow cover and physical circulation (Tremblay and Smith 2007).
High primary productivity in polynyas is primarily due to the higher penetration of solar radiation into the water column of polynyas in spring, compared with surrounding ice concentrated regions. Irradiance in polar regions is a major control over primary productivity, and as a result polynyas are characterized by enhanced phytoplankton growth relative to surrounding regions (Tremblay and Smith 2007). Whether polynyas exhibit a greater annual productivity, or just experience a pulse in productivity earlier in the season is still largely unknown (Tremblay and Smith 2007).
Polynyas tend to be concentrated over continental shelves. Continental shelves are already highly productive due to their nutrient rich waters, relative to surrounding offshore waters (Tremblay and Smith 2007). As well as this, the Antarctic is rarely deficit of macronutrients, and phytoplankton growth is more likely to be limited by trace metals, particularly iron (Arrigo et al 2012). With the onset of spring when nutrient availability is high coming out of winter, and the water column is stratified from a fresh water melt layer and continental freshwater fluxes, combined with increased irradiance and low grazing pressure; polynyas facilitate large phytoplankton blooms (Montes-Hugo and Yuan 2012).
Phytoplankton form the base of the food-web in the Antarctic, and increased primary production in polynyas result in an oasis of life relative to other regions. Phytoplankton blooms in Antarctic polynyas are typically dominated by diatoms, due to their ability to rapidly respond to changes in environmental conditions (Deibel and Daly 2007). Diatom production in polynyas has also shown to benefit recruitment and life cycles of some zooplankton species, through higher egg production rates and shorter generation times (Deibel and Daly 2007).
Enhanced primary production supports secondary producers. The major zooplankton grazers found within Antarctic polynyas include krill, copepods and salps (Arrigo and van Dijken 2003). Krill especially, are a keystone species in the Antarctic ecosystem, and form a major part of the diet of many Antarctic species. As a result, polynyas tend to also support an abundance of high trophic level species and top predators including seals, penguins, sea birds and whales throughout the spring and summer (Karnovsky et al 2007).
Enhanced productivity is a major driver of marine bird and mammal congregations around polynyas. However polynyas also serve a range of other functions, which is evident over winter months when productivity is minimal due to the lack of solar radiation. For many species polynyas serve as a breathing hole, access to nesting and breeding sites, provide refuge from predation and access to open water (Ainley et al 2003, Joiris 1991). Consequently, food-web enhancement in polynyas can not be solely attributed to increased productivity (Tremblay and Smith 2007).
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