Source: Meehl, G. A., Arblaster, J. M., Bitz, C. M., Chung, C. T., & Teng, H. (2016). Antarctic sea-ice expansion between 2000 and 2014 driven by tropical Pacific decadal climate variability. Nature Geoscience. doi:10.1038/ngeo2751
The expanding ice puzzle
Sea ice in the Arctic is melting rapidly due to Earth’s increasing temperatures driven by rising greenhouse gases, but the opposite is happening in Antarctica, where sea ice has expanded slightly in the past few decades. In the winter of 2014, Antarctic sea ice reached a new record maximum. This unexpected observation has perplexed scientists in recent years, because current climate model predictions on average show a decrease in Antarctic sea ice. Unfortunately this has caused confusion amongst the public and is frequently used by climate deniers as evidence to argue that climate change isn’t happening.
An important thing to know about sea ice expanding in Antarctica is that this isn’t the whole story. While the ice averaged over the entire continent has increased very gradually since the satellite record began in 1979, this big picture look hides the fact that sea ice is actually melting in some regions and expanding in others. These strong regional changes in sea ice provide a clue to the cause of these trends, indicating that changes in regional atmospheric systems might be responsible. The rate of sea ice expansion has also sped up in the last decade. Fluctuations in the circulation of the atmosphere happen on timescales of decades and could be driving the acceleration in sea-ice expansion. A group of scientists from the US and Australia looked closely at changes in the atmosphere and were able to link the observed change in Antarctic sea-ice to atmospheric changes and explain and why the real world doesn’t match up with most climate models.
Strong winds blowing away from the Antarctic coast are strongly correlated to increasing sea ice extent, because when the winds blow ice away from the coast this allows new ice to be formed, essentially forming a sea ice factory. A low pressure system off the coast of the western side of Antarctica, the Amundsen Sea Low, is known to drive these strong winds, causing sea ice to expand. The Amundsen Sea Low has intensified since the year 2000, but why has this low pressure increased, and what changes in the atmosphere has caused this?
Much like El Nino effects atmosphere and rainfall patterns around the globe, there are other natural climate cycles in the tropical ocean and atmosphere that have global effects. The researchers chose to investigate a particular natural pattern in the atmosphere called the Interdecadal Pacific Oscillation (IPO), because in the time that the IPO has been negative since the year 2000, the trend in Antarctic sea ice was nearly 5 times as fast as from 1979-1999. Only a small number of current climate models have a negative IPO since 2000, and these models have a significant increase in Antarctic sea ice, while all the rest of the models with positive IPO have sea ice melting.
So what does a negative IPO mean? The most noticeable change in the atmosphere then the IPO is negative is that there is less rainfall than normal in the eastern tropical Pacific Ocean. To test whether less rainfall in the eastern tropical Pacific could be driving the stronger Amundsen Sea Low and hence the increased sea ice extent, the authors picked a model that had the characteristic low rainfall in the eastern tropical Pacific, and found the intensification of the Amundsen Sea Low was very similar to what has been observed in the real world.
What’s next for Antarctic ice?
The scientists concluded that rainfall changes in the eastern tropical Pacific are actually the biggest cause of the stronger Amundsen Sea Low that is driving the growth of sea ice in Antarctica. The surprising result shows how complex interactions between the atmosphere, oceans and ice can lead to unexpected changes in our climate and cause confusion among scientists and the wider community. It also means that climate models, which all have different variations in the IPO at different times, aren’t always going to match up with the real world on timescales of a few decades. Over longer time periods, and with many different models, we hope to get closer to the truth. Predicting how the IPO and other natural climate patterns will change in the future is very challenging, so it is still uncertain how Antarctic sea ice will change in the future.