Major surface melting over the Ross Ice Shelf part I: Foehn effect

Abstract

West Antarctica (WA), especially the Ross Ice Shelf (RIS), has experienced more frequent surface melting during the austral summer recently. The future is likely to see enhanced surface melting that will jeopardize the stability of ice shelves and cause ice loss. We investigate four major melt cases over the RIS via Polar Weather Research and Forecasting (WRF) simulations (4 km resolution) driven by European Centre for Medium‐Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5) reanalysis data and Moderate Resolution Imaging Spectroradiometer (MODIS) observed albedo. Direct warm air advection, recurring foehn effect, and cloud/upper warm air introduced radiative warming are the three major regional causes of surface melting over WA. In this paper, Part I, the first two factors are identified and quantified. The second paper, Part II, discusses the impact of clouds and summarizes all three factors from a surface energy balance perspective. With a high‐pressure ridge located westward towards the Sulzberger Ice Shelf (77° S, 148° W) and a low‐pressure center located between 165° and 180° W, warm marine air from the Ross Sea is advected towards the coastal RIS and leads to surface melting. When the high‐pressure ridge is located farther east towards Marie Byrd Land (120–150° W), the foehn effect can cause a 2–4°C increase in surface temperature on the leeside of the mountains. For three of four melt cases, more than 40% of the melting period experiences foehn warming. Isentropic drawdown is usually the dominant foehn mechanism and contributes up to a 14°C temperature increase, especially when strong low‐level blocking occurs on the upwind side. The thermodynamic mechanism can be important depending on the strength of moisture uptake and condensation on the windward side. Meanwhile, sensible heat flux contributes less to foehn warming, but still plays an important role in the melting. The prediction of future stability of the RIS should include foehn warming as a major driver.

Major surface melting over the Ross Ice Shelf part II: Surface energy balance

“…Mountain Range X (MRX) and Mountain Range Y (MRY) are represented by the blue lines. Domain 1 covers a larger area than Figure 1 and is shown in Zou et al (2021) [Colour figure can be viewed at wileyonlinelibrary.com] warm advection, and cloud impacts, have been confirmed to contribute to the surface temperature increase (Nicolas et al, 2017;Scott et al, 2019;Zou et al, 2019). In Part I (Zou et al, 2021), four historical extensive melt cases over the RIS are analyzed based on Polar WRF (PWRF) simulations.…”

“…Domain 1 covers a larger area than Figure 1 and is shown in Zou et al (2021) [Colour figure can be viewed at wileyonlinelibrary.com] warm advection, and cloud impacts, have been confirmed to contribute to the surface temperature increase (Nicolas et al, 2017;Scott et al, 2019;Zou et al, 2019). In Part I (Zou et al, 2021), four historical extensive melt cases over the RIS are analyzed based on Polar WRF (PWRF) simulations. Recurring foehn effect, with isentropic drawdown as the dominant factor, occurs during more than 40% of the melting period and contributes an up to 4 • C temperature increase, which significantly favors surface melting (Zou et al, 2021).…”

“…In Part I (Zou et al, 2021), four historical extensive melt cases over the RIS are analyzed based on Polar WRF (PWRF) simulations. Recurring foehn effect, with isentropic drawdown as the dominant factor, occurs during more than 40% of the melting period and contributes an up to 4 • C temperature increase, which significantly favors surface melting (Zou et al, 2021). Direct marine air advection from the Ross Sea region mainly affects the coastal RIS adjacent to the Ross Sea and rarely causes impacts farther inland (Zou et al, 2021).…”

“…Over the RIS, with strong marine air advection from the Amundsen/Ross Sea, the enhanced longwave radiation induced by low-level liquid clouds can prolong the melting (Nicolas et al, 2017). However, under strong foehn cases, the eastern RIS (near Siple Coast) can experience an increase in SWD and SHF, due to foehn clearance and turbulent mixing, respectively (Zou et al, 2019;2021).…”

“…This article applies PWRF v4.1.1 to simulate the state of atmosphere over Antarctica with a 20-km resolution domain (domain 1) and a 4-km high-resolution nest (domain 2; Figure 1), with 71 vertical levels, the lowest being 4 m above the surface (Zou et al, 2021). As a regional climate model developed by the Polar Meteorology Group of the Byrd Polar and Climate Research Center at The Ohio State University, PWRF can provide reliable near-surface simulations in the polar regions, especially during the austral summer melting (Hines and Bromwich, 2008;Bromwich et al, 2013;Zou et al, 2019).…”