Date of Degree

9-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Marco Tedesco

Committee Members

Cecile Agosta

Xavier Fettweis

James Booth

Z. Johnny Luo

Subject Categories

Atmospheric Sciences | Climate | Glaciology

Keywords

Antarctic Peninsula, Larsen C ice shelf, climate model, foehn wind, surface melt

Abstract

Surface melting over the Antarctic Peninsula (AP) plays a crucial role for the stability of ice shelves and dynamics of grounded ice, hence modulating the mass balance in a region of the world which is particularly sensitive to increasing surface temperatures. Understanding the processes that drive melting using surface energy and mass balance models is fundamental to improving estimates of current and future surface melting and associated sea level rise through ice-shelf collapse. This is even more important in view of the specific challenges presented by how circulation patterns over the topographically-complex Antarctic Peninsula, especially foehn winds, impact surface melt. In this dissertation, I evaluate the regional climate model Modèle Atmosphérique Régionale (MAR) over the Antarctic Peninsula (AP) at a 10 km horizontal resolution. An initial run is assessed with both in situ data from AWS stations and melt occurrence estimates from passive and active microwave data. A subsequent run over the full 1982-2017 period is used to extract the primary patterns of surface melt variability and determine how interannual variability of these patterns impacts meltwater infiltration into the snowpack.

MAR version 3.5.2 is used for a first run over the 2000-2014 period. This is the first time that this model, which has been validated extensively over Greenland, has been applied to the Antarctic Peninsula at a high resolution. Near-surface atmosphere model outputs are first compared to data retrieved from 10 automatic weather stations. We find that the atmosphere is adequately represented by MAR for the summer season, but note regionally-specific temperature biases which may impact surface melt. Using a non-parametric Mann-Kendall test, we find a slight summer and spring cooling trend in the northeast Antarctic Peninsula (including the Larsen C ice shelf) which confirms trends reported in previous literature. Trends in other regions of the AP or in the winter season are inconclusive.

Model outputs uses outputs between 1999 and 2009 are analyzed to understand the impacts of mesocscale wind circulation patterns. This period which coincides with the availability of active microwave data from the QuikSCAT mission which is used for comparison with the model. The primary regional focus is the northern East Antarctic Peninsula (East AP), where we define smaller sub-regions according to divergent melt occurrence biases. Melting in the East AP can be initiated both by sporadic westerly foehn flow over the AP and by northerly winds advecting warm air from lower latitudes. To assess MAR’s ability to simulate these physical processes, this study takes a unique approach, examining model biases for melt occurrence on the Larsen Ice Shelf, as evaluated by satellite estimates from passive and active microwave data, with concurrent temperature biases associated with wind direction biases as evaluated by three automatic weather stations (AWS). Our results indicate that satellite estimates show greater melt frequency, a larger melt extent, and a quicker expansion to peak melt extent than MAR in the center and east of the Larsen C ice shelf. The difference between the remote sensing and modeled estimates reduces in the north and west of the East AP. Our results indicate that although MAR shows an overall warm bias, it also shows fewer warm, strong westerly winds than reported by AWS stations, which may lead to an underestimation of melt. The underestimation of foehn flow in the east of the Larsen C may potentially be resolved by removing the hydrostatic assumption in MAR or increasing spatial resolution. The underestimation of southwesterly flow in particular may be reduced by using higher-resolution topography.

The interannual behavior of foehn-induced surface melt is explored using two novel methods applied to regional climate model outputs. These methods estimate changes in the interannual frequency of foehn flow and the associated impact on snow melt, snowpack density and meltwater vertical percolation depth. The first method extracts spatial patterns of melt occurrence using empirical orthogonal function (EOF) analysis. The second method introduces the terms “foehn index” and “foehn melt index”, expanding a detection method previously employed at AWS stations to now capture the total surface area where foehn flow is detected and when foehn-induced meltwater is produced. The analysis of the interannual strength of the main EOF modes and foehn melt index show a change in the behavior of foehn winds in recent years (since 2013), such that foehn-induced melt has declined in December and January but strengthened in March. Due to enhanced densification estimated during 2015-2017, we examine the effects of extreme foehn-induced melting events on the modeled snowpack during this period and find that consistent late-season melt resulted in increased densification of the upper snowpack, with potential implications for future ice shelf stability.

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