RAPID: What Caused the Record Warmth and Loss of Antarctic Sea ice in the Austral Summer of 2022, and will Sea Ice Remain Low Over 2022-2024?
In the austral winter of 2021/2022 a drastic decline in Antarctic sea ice extent (SIE) has taken place, and February 2022 marked the lowest SIE on record since consistent satellite sea ice observations began in 1979. Combined with the loss of SIE, the most extreme heat wave ever observed globally (as estimated by temperature anomalies from climatology) took place over East Antarctica in March 2022 as temperatures climbed over +40°C from climatology, an event that climate models struggle to replicate. Extreme events have an oversized footprint in socioeconomic impacts, but also serve as litmus tests for climate models and their predictive capabilities, and thus our understanding of extreme events and the ability of climate models in simulating such events is of key interest both in a scientific and social context. This project will use novel tools to diagnose the factors that led to the record low Antarctic SIE and heat wave focusing on the impact of winds and ocean temperatures. Currently (June 2022) Antarctic SIE remains at record low levels for the time of year, raising the prospect of a long-lasting period of low SIE, yet annual forecasts of Antarctic sea ice do not yet exist. To address this issue, this project will also create exploratory annual sea ice forecasts for the 2022-2024 period. These extreme events have questioned our current understanding of Antarctic climate variability. Motivated by the timing of these events and our recent development of novel analysis tools, this project will address the following research questions: (R1) Can local winds account for the observed 2021/2022 sea ice loss, or are remote sea surface temperature (SST) anomalies a necessary ingredient? (R2) Are sea ice conditions over 2022-2024 likely to remain anomalously low? (R3) Can a state-of-the-art climate model simulate a heat wave of comparable magnitude to that observed if it follows the observed circulation that led to the heat wave? The main approach will be to use a nudging technique with a climate model, in which one or several variables in a climate model are nudged toward observed values. The project authors used this tool to attribute Antarctic sea ice variability and trends over 1979-2018 to winds and SST anomalies. This project will apply this tool to the period 2019-2022 to address R1 and R3 by running two different model experiments over this time period in which the winds over Antarctica and SSTs in the Southern Ocean are nudged toward observed values. In addition, we will diagnose the relevant modes of atmospheric variability over 2019-2022 that are known to influence Antarctic sea ice to gain further insight into the 2022 loss of SIE. To address R2, we plan to extend the model simulations but without nudging, using the model as a forecast model (as its 2022 initial conditions will be taken from the end of the nudged simulations and capture important aspects of the observed state). We expect that if current upper ocean heat content is anomalously high, low SIE conditions may continue over 2022-2024, as happened over 2017-2019 following the previous record low of SIE in 2016/2017. To further address R3, we will compare observations and model simulations using novel atmospheric heat transport calculations developed by the project team.
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