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

Abstract

The West Antarctic climate is under the combined impact of synoptic and regional drivers. Regional factors have contributed to more frequent surface melting with a similar pattern recently, which accelerates ice loss and favors global sea‐level rise. Part I of this research identified and quantified the two leading drivers of Ross Ice Shelf (RIS) melting, viz. foehn effect and direct marine air advection, based on Polar WRF (PWRF) simulations. In this article (Part II), the impact of clouds and the pattern of surface energy balance (SEB) during melting are analyzed, as well as the relationship among these three factors. In general, net shortwave radiation dominates the surface melting with a daily mean value above 100 W·m−2. Foehn clearance and decreasing surface albedo respectively increase the downward shortwave radiation and increase the absorbed shortwave radiation, significantly contributing to surface melting in areas such as western Marie Byrd Land. Also, extensive downward longwave radiation caused by low‐level liquid cloud favors the melting expansion over the middle and coastal RIS. With significant moisture transport occurring over more than 40% of the time during the melting period, the impact from net radiation can be amplified. Moreover, frequent foehn cases can enhance the turbulent mixing on the leeside. With a Froude number (Fr) around 1 or slightly larger, fast downdrafts or reversed wind flows can let the warm foehn air penetrate down to the surface with up to 20 W·m−2 in sensible heat flux transfer to the ground. However, when the Froude number is close to infinity with breaking waves on the leeside, the contribution of turbulence to the surface warming is reduced. With better understanding of the regional factors for the surface melting, prediction of the future stability of West Antarctic ice shelves can be improved.

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

“…The bias of surface temperature simulation from PWRF in WA is less than ∼1 • C, which is reduced by 2 • C compared with AMPS. Also, the surface wind and radiation from PWRF show more reliable results (Zou et al, 2021). Thus, the transport of heat and moisture from the ocean to the RIS can be described more precisely.…”

“…By contrast, the mountain waves over MRY are more turbulent with larger Fr numbers (Figure 6b heat transfer between foehn flow and the surface on the leeside. Also, the surface temperature on the leeside of MRY increased to ∼2 • C. However, when Fr is much larger than 1 (e.g., Fr = 2,000) or closer to infinity, the diminished turbulent mountain waves will not enhance the heat transfer and increase the surface temperature effectively (Zou et al, 2021). In general, the 2005 and 2016 cases experienced stronger surface warming on the leeside of mountains, and MRY had stronger mountain waves compared with MRX, including breaking waves and hydraulic jumps.…”

“…Direct marine air intrusion plays a more significant role in the two prolonged and less widespread melt events (1982/1983 and 1991/1992 cases). With longer duration, the 1982/1983 and 1991/1992 cases affect smaller areas that are closer to the coast (Figure 2; Zou et al, 2021). In other words, the expansion of the melting is slower.…”

“…And a Fr number close to infinity will lead to turbulence on the leeside. The relationship between Fr number and sensible heat transfer is thoroughly discussed in Part II (Zou et al, 2021). Daily Fr numbers are calculated for both MRX and MRY (red boxes in Figure 1b).…”

“…However, direct marine intrusion can promote the formation of low-level liquid cloud by advecting moisture inland, and thus affects the surface energy balance (SEB). This linkage between marine advection and cloud impacts is analyzed in detail in the second part of the investigation of this major melt event over the RIS (Zou et al, 2021). With a high-pressure ridge located over coastal MBL (90-120 • W) and a low-pressure center located over the Nickerson Ice Shelf (150-180 • W), warm marine air needs to cross the mountain ranges over MBL to reach the eastern RIS (Figure 1b).…”