Inside Katabatic Winds Over the Terra Nova Bay Polynya: 1. Atmospheric Jet and Surface Conditions

Abstract

Katabatic winds are a common feature of the Antarctic coastline (King & Turner, 1997;Parish & Bromwich, 1987). Longwave radiation from the Antarctic ice sheet cools the snow surface and removes heat from the near-surface air, creating ubiquitous, strong, low-level temperature inversions (Connolley, 1996). The shallow cold air flows downhill and is funneled into coastal valleys, where the sloping terrain allows the air parcels to gain momentum from gravity. In many situations, particularly on the East Antarctic Coast, the katabatic winds peter out quickly beyond the generating slopes due to surface friction, gravity wave dispersion (Renfrew, 2004;Vignon et al., 2020), opposing thermal winds from the build-up of cold air at the slope bottom (Vihma et al., 2011), opposing synoptic forcing (Seefeldt et al., 2007), or entrainment from above the jet (Manins & Sawford, 1979). In some situations, the synoptic pressure gradient forcing is favorable (downstream) or weak (Bromwich et al., 1990;Seefeldt & Cassano, 2012;Seefeldt et al., 2007) or the winds are decoupled from the surface (Andreas et al., 2000) and katabatic winds are able to propagate hundreds of kilometers across flat ice shelves and ocean surfaces.The Antarctic katabatic winds have been the source of considerable attention from the time of the first explorers due to their major impacts on navigation, visibility and human survival. The katabatic winds often form strong jets that are characterized by low-level, high-wind-speed cores that are surrounded by regions of strong vertical and horizontal wind shears. Katabatic jumps, or rapid increases in wind speed associated with the onset of a katabatic wind event (Ball, 1956), are common phenomena in many Antarctic coastal regions (e.g., Vignon et al., 2020). Katabatic winds can extend over the ocean and trigger mesoscale cyclones and other atmospheric features that affect regions far from the initial katabatic wind source (Bromwich,

Fetch‐Limited, Strongly Forced Wind Waves in Waters With Frazil and Grease Ice – Spectral Modeling and Satellite Observations in an Antarctic Coastal Polynya

“…The assumption made in this study that β in can be computed in the same way over water and ice has no theoretical background and is justified only by the lack of data to verify it. • Whitecapping is strongly reduced in regions where frazil streaks are present -not only within streaks themselves, but also in open-water areas between them -confirming existing qualitative observations from TNBP (Ackley et al, 2022;Guest, 2021aGuest, , 2021b. Crucially, in the model this effect is obtained without any modifications to the formulation of the S ds term.…”

“…Whitecapping is strongly reduced in regions where frazil streaks are present – not only within streaks themselves, but also in open‐water areas between them – confirming existing qualitative observations from TNBP (Ackley et al., 2022; Guest, 2021a, 2021b). Crucially, in the model this effect is obtained without any modifications to the formulation of the S ds term.…”

“…As a step toward broadening the picture and extending wave–ice interactions analyses to a wider range of conditions, we turn our attention toward a setting with features that in many ways are the opposite of the MIZ‐typical conditions described above: coastal (or latent heat) polynyas during katabatic wind events (Morales Maqueda et al., 2004). Polynya openings are associated with very high wind speeds, often exceeding 30 m⋅s −1 , and advection of very cold and dry continental air masses, resulting in offshore drift of the ice pack and extremely high ocean–atmosphere turbulent heat and moisture fluxes (up to 2,000 W⋅m −2 ; Guest, 2021a, 2021b). All these factors combined lead to strong turbulence and convective, wind‐ and wave‐induced mixing in the ocean mixed layer (OML; Herman et al., 2020), and to intense frazil ice formation (Nakata et al., 2021; Thompson et al., 2020).…”

Inside Katabatic Winds Over the Terra Nova Bay Polynya: 2. Dynamic and Thermodynamic Analyses

“…On May 8, 2017, as part of the PIPERS program, the R/V Nathaniel B. Palmer steamed on a downwind transect across the Terra Nova Bay polynya (TNBP) during a strong katabatic wind event, while the author and colleagues performed surface layer (ship platform) and upper‐air (rawinsonde) measurements. The physical setting, measurements, surface characteristics and details regarding the vertical structure and horizontal evolution of the state variables wind vector U , the potential temperature θ , and the specific humidity q , for this case are presented in Guest (2021), hereafter referred to as “Part 1.” The figures and results in Part 1 will be referred to frequently throughout this study.…”

Processes of stratification breakdown and restratification in Antarctic coastal polynyas

“…The katabatic winds blow northward off the ice shelf front. To mimic the katabatic wind patterns in the Antarctic Mesoscale Prediction System as analyzed by Guest (2021a) and Wenta and Cassano (2020), the wind field specified in this study has a half-elliptic shape with the highest speed in the central region between 10.3° and 12.3°E and between 76° and 75°S, and it linearly decays outward to 0 in a region of ∼55 km wide (Figure 2-4b). Based on the winds measured near coastal polynyas around Antarctica (e.g., Adolphs and Wendler 1995;Knuth and Cassano 2011;Wendler et al 1997) and the wind data presented in Figure 2-2, the katabatic wind speed at 10-m height, Vwind, is assumed to oscillate with time t following the form,…”