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Air-Sea-Ice Interactions at the Ronne Polynya, Southern Weddell Sea, Antarctica

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Air-Sea-Ice Interactions at the Ronne Polynya, Southern Weddell Sea, Antarctica Air-sea-ice interactions at the Ronne Polynya, southern Weddell Sea, Antarctica Emma Kathleen Fiedler A thesis submitted for the degree of Doctor of Philosophy University of East Anglia School of Environmental Sciences March 2009 c This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with the author and that no quotation from the thesis, nor any information derived therefrom, may be published without the author’s prior, written consent. Abstract Polynyas are regions of open water or thin ice in sea ice. They play an important role in the regional meteorology and oceanography of the high latitudes and in the global ocean circulation. The surface heat budget of the Ronne Polynya, Antarctica, was investigated, using a combination of field observations and modelling. Three flights were conducted over the polynya in February 2007 using a British Antarctic Survey instrumented aircraft. The polynya was observed to be mostly covered with thin ice perforated with holes and was comprised of two distinct regimes, an inner region of newly-formed and thin ice and an outer region of thicker, more consolidated ice. The sensible heat flux over the polynya was of the order of 100 W m−2 and decreased with fetch, primarily as a result of the thickening ice cover. The Bowen ratio was 3.8 ± 0.3. The sensible heat transfer and drag coefficients were calculated, −3 −3 at CHN10 = (0.7 ± 0.1) × 10 and CDN10 = (1.1 ± 0.2) × 10 . The heat transfer coefficient is similar to that found over heterogeneous sea ice and is significantly lower than has been used in previous studies of heat fluxes over polynyas, which are often assumed to be open water. The transfer coefficients were not found to be a function of fetch or ice conditions as represented by the surface temperature and albedo. Heat budget calculations indicated little new ice was being formed at the time of the case studies but that both regimes could potentially be important for wintertime ice production and dense water formation. The data were used to validate the output of sensible heat flux, potential temperature and boundary layer depth from a simple fetch-dependent model, showing that for these case studies, surface temperatures and transfer coefficients appropriate to an ice-covered surface were required for accurate modelling. 2 Contents Abstract 2 Acknowledgements 7 1 Introduction 8 1.1 Air-sea-ice interactions in the Antarctic . 8 1.1.1 Polynyas . 10 1.1.2 Atmosphere - polynya interactions . 13 1.1.3 Ocean - polynya interactions . 18 1.2 The Ronne Polynya . 20 1.3 Project aims . 23 2 Aircraft Observations of the Ronne Polynya 26 2.1 Flight tracks . 26 2.2 Synoptic conditions . 27 2.3 Instrumentation and data processing . 32 2.3.1 The BAT probe . 37 3 2.3.2 Total temperature probe . 41 2.3.3 Humidity sensors . 42 2.3.4 Radiation sensors . 43 2.3.5 Video . 48 3 Observations of the Convective Internal Boundary Layer 49 3.1 Vertical profiles . 49 3.2 Ice shelf height . 54 3.3 Entrainment . 55 3.3.1 The entrainment parameter . 58 3.4 Summary . 63 4 Turbulent Fluxes over the Ronne Polynya 65 4.1 Methods . 65 4.1.1 Eddy covariance method . 65 4.1.2 Obtaining timeseries of fluctuating quantities from high fre- quency measurements . 67 4.1.3 Data quality control . 69 4.1.4 Bulk method . 75 4.1.5 Transfer coefficients . 76 4.2 Observations . 79 4.2.1 Sensible heat flux . 79 4 4.2.2 Flux sampling error . 81 4.2.3 Observed transfer coefficients . 83 4.2.4 Surface ice conditions . 85 4.2.5 Relationships between transfer coefficients and ice conditions . 90 4.2.6 Latent heat flux . 92 4.2.7 Momentum flux . 92 4.3 Summary . 97 5 Modelling of the Ronne Polynya Case Studies 100 5.1 Introduction to the model . 100 5.2 The model construction . 101 5.3 Modelling of the case studies . 103 5.3.1 Sensible heat flux . 104 5.3.2 Potential temperature . 108 5.3.3 Convective internal boundary layer depth . 112 5.4 Summary . 117 6 Surface Heat Budgets and Buoyancy Flux 119 6.1 Heat budget . 120 6.1.1 Background . 120 6.1.2 The Ronne Polynya surface heat budget . 129 6.2 Ice production volume . 132 5 6.3 Surface buoyancy flux . 137 7 Conclusions 141 7.1 Summary . 141 7.2 Discussion and conclusions . 145 7.3 Further work . 147 References 150 6 Acknowledgements I would firstly like to thank my supervisor at UEA, Ian Renfrew, for all his help- ful advice and guidance along the way. I would also very much like to thank my supervisors at BAS, John King, Tom Lachlan-Cope and Keith Nicholls for all their input to this project. Thanks also to Alex Weiss and Russ Ladkin of BAS and the support team at Rothera for their help with this research, as well as all those at BAS who made my participation in the fieldwork possible. Thanks also to Dave Sproson for programming help and Nina Petersen for friendly chats about spectral analysis. Finally, I’d also like to say thanks to the ENV PhDers, particularly the climbers, quizzers and tea-break takers, as well as those at Rothera who made my time there so fantastic. This PhD was funded by a NERC studentship with a CASE award at the British Antarctic Survey. 7 Chapter 1 Introduction 1.1 Air-sea-ice interactions in the Antarctic Fluxes of heat and moisture between the upper surface of the ocean and the lowest layers of the atmosphere play an important role in determining the meteorological and climatological conditions of the high latitudes (King and Turner, 1997). As the specific heat capacity of the ocean is high, the heat stored has a moderating effect on the climate (King and Turner, 1997). The largest heat fluxes occur in wintertime when the ocean is significantly warmer than the atmosphere. However, the presence of sea ice significantly reduces the fluxes. In winter, the flux of heat from the ocean to the atmosphere over snow-covered pack ice can be two orders of magnitude smaller than for the open ocean (Maykut, 1978). Therefore the role of sea ice is important in many meteorological processes and has a major impact on the climatology of the polar regions. The production of dense, saline ocean bottom water in the seas surrounding the Antarctic continent has an important influence on the global ocean circulation (King and Turner, 1997). Cooling of the ocean surface and the rejection of dense, brine- rich water during sea ice formation promotes downwelling and contributes to the transformation of intermediate and deep waters, affecting the oceanic overturning 8 Figure 1.1: Map of Antarctica, showing locations referred to in the text. Image from http://blackmaps.files.wordpress.com/2009/03/antarctica-map.jpg. 9 circulation (Morales Maqueda et al., 2004). Therefore, air-sea-ice interactions in the Antarctic play an important role in both the regional meteorology and oceanography, as well as globally through their impact on the ocean circulation. Figure 1.1 is a map of Antarctica, showing the locations of areas mentioned in the following sections. 1.1.1 Polynyas The presence of even small fractions of open water and thin ice in sea ice can significantly alter the surface balances of heat and moisture. These open water areas can be in the form of leads, which are linear narrow fractures in the pack ice caused by divergent motion of the ice, see figure 1.2(a), or polynyas, see figure 1.2(b). Polynyas are larger, non-linear areas of open water or thin ice in the sea ice pack ranging in area from 10 to 105 km2 (Barber et al., 2001). They are formed and maintained either through the divergent motion of sea ice due to wind stress or the action of ocean currents, or through melting due to an influx of oceanic sensible heat which also prevents the formation of new ice (Morales Maqueda et al., 2004). Polynyas have been known to exist since the early days of Antarctic exploration, because of the access they provided to the coastal areas early on in the Antarctic summer (King and Turner, 1997). In addition to the surface heat and moisture budgets, polynyas can also modify the surface momentum balance since they allow the oceanic mixed layer direct con- tact with the surface winds (Morales Maqueda et al., 2004). They also have an effect on biogeochemical air-sea fluxes and on tracer transport by the ocean as a result of convection and vertical mixing, and, as a consequence of turbulence and freezing, can also provide conditions for the entrainment of pollutants and sedi- ments (Morales Maqueda et al., 2004). As they often recur regularly, polynyas are also important as biological habitats and, in the Arctic, have been used as hunting 10 open lead sea ice newly-frozen sea ice (a) An open lead in pack ice. From http://a76.dk/grafik/lorita 2104 open lead.jpg Ronne Ice Shelf (Antarctic Peninsula) wind direction (offshore) Ronne Polynya (b) The Ronne Polynya, southern Weddell Sea, Antarctica, looking upwind towards the Ronne Ice Shelf. Figure 1.2: Photos showing examples of a lead (a) and a polynya (b). 11 grounds by Inuit for the past 3000 years (Smith et al., 1990). A wind-driven, or coastal, polynya is caused by the offshore advection of sea ice by strong, cold, continental winds.
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