Reply on RC1

Nicolaj Hansen

doi:10.5194/tc-2021-69-ac1

Abstract

Abstract. Antarctic surface mass balance (SMB) is largely determined by precipitation over the continent and subject to regional climate variability related to the Southern Annular Mode (SAM) and other climatic drivers at the large scale. Locally however, firn and snowpack processes are important in determining SMB and the total mass balance of Antarctica and global sea level. Here, we examine factors that influence Antarctic SMB and attempt to reconcile the outcome with estimates for total mass balance determined from the GRACE satellites. This is done by having the regional climate model HIRHAM5 forcing two versions of an offline subsurface model, to estimate Antarctic ice sheet (AIS) SMB from 1980 to 2017. The Lagrangian subsurface model estimates Antarctic SMB of 2473.5Â±114.4âGtâyrâ1, while the Eulerian subsurface model variant results in slightly higher modelled SMB of 2564.8Â±113.7âGtâyrâ1. The majority of this difference in modelled SMB is due to melt and refreezing over ice shelves and demonstrates the importance of firn modelling in areas with substantial melt. Both the Eulerian and the Lagrangian SMB estimates are within uncertainty ranges of each other and within the range of other SMB studies. However, the Lagrangian version has better statistics when modelling the densities. Further, analysis of the relationship between SMB in individual drainage basins and the SAM is carried out using a bootstrapping approach. This shows a robust relationship between SAM and SMB in half of the basins (13 out of 27). In general, when SAM is positive there is a lower SMB over the plateau and a higher SMB on the westerly side of the Antarctic Peninsula, and vice versa when the SAM is negative. Finally, we compare the modelled SMB to GRACE data by subtracting the solid ice discharge, and we find that there is a good agreement in East Antarctica but large disagreements over the Antarctic Peninsula. There is a large difference between published estimates of discharge that make it challenging to use mass reconciliation in evaluating SMB models on the basin scale.

Highlights

The Antarctic Ice Sheet (AIS) has the potential to raise global sea level by 58 m (Fretwell et al, 2013) and it is of utmost importance to understand its role in present sea level change in order to project it into the future
In the model mean (1980–2017) of the three surface mass balance (SMB) simulations (Fig. 1a), we see that the majority of the total AIS (ToAIS) has a positive SMB; only a few regions show a negative SMB: Larsen ice shelf, George IV ice shelf, coastal regions of Queen Maud Land, the Transantarctic Mountains, near Amery ice shelf, and some coastal areas in East Antarctica
We estimate the Antarctic SMB to range from 2583.4 ± 121.6 to 2473.5 ± 114.4 Gt yr−1 over the total area of the ice sheet including shelves and between 1995.4 ± 99.3 and 1963.3 ± 96.2 over the grounded part, for the period from 1980 to 2017

Summary

Introduction

The Antarctic Ice Sheet (AIS) has the potential to raise global sea level by 58 m (Fretwell et al, 2013) and it is of utmost importance to understand its role in present sea level change in order to project it into the future. The ice sheet mass balance (MB) can be split into atmospheric and ice dynamic components: MB = SMB − D, (1). Blowing snow is not taken into consideration in this study, so the SMB is defined here as SMB = P −S −RO. Of these components, precipitation is by far the largest contributor (Krinner et al, 2007) and consists primarily of snow at higher altitudes. If SMB < D, the total mass balance is negative and the ice sheet loses mass and thereby contributes to global sea level rise. We focus on the SMB component of the mass balance, to pinpoint the immediate forcing to ice sheet dynamic instability. To estimate the SMB, we use an atmospheric regional climate model (RCM) to force a subsurface model, which outputs the SMB

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