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The role of atmospheric rivers (ARs) for extreme ablation and snowfall is examined at Brewster Glacier in the Southern Alps, the site of the longest glacier mass balance record in New Zealand. By global standards, New Zealand is strongly impacted by ARs. Here it is shown for the first time (in New Zealand) that ARs strongly contribute to extreme snowfall and ablation and thus mass balance overall. Vertically integrated water vapor transport (IVT) exceeds 1,600 (800) kg.m(-1).s(-1) for the largest ablation (snowfall) events, marking these as very strong ARs. The proximity to the freezing threshold during extreme snowfall events indicates the sensitivity of mass balance to temperature variation. Importantly, similarly high rates of IVT during some extreme ablation and snowfall events also occur outside of conventional AR spatial structures. This finding indicates that AR detection algorithms may substantially underestimate the importance of extreme IVT for New Zealand and elsewhere.

Plain Language Summary New Zealand glaciers are very sensitive to climate variation and change-yet relatively little is known about the individual weather patterns that cause extreme ablation and accumulation events. Similarly, while the importance of atmospheric rivers (ARs) for extreme weather events in New Zealand is generally recognized, their importance for glacier mass balance has not been studied before. ARs are long, narrow corridors of very high atmospheric water vapor transport, with volumetric flow rates equivalent to the world's largest rivers. Here we show (for the first time) that ARs do play a key role for glacier mass balance in New Zealand. Importantly, we also show that some common methods for identifying ARs do not capture all of the most extreme ablation and melt events-even though they are driven by water vapor transport rates equivalent to conventionally defined ARs. As such, past studies have likely underestimated the importance of AR-type events, both in New Zealand and elsewhere.