Global Implications of Arctic Sea Ice Retreat

The 21-st century hiatus in surface global warming has been linked to equatorial Pacific cooling caused by a prevailing La Nina state. Multiple studies have suggested that the observed equatorial Pacific anomalies can be explained solely by natural climate variability. However, climate models that were able to capture natural variability failed to predict the hiatus. We argue that the changes in high latitude atmospheric circulation induced by the anomalously rapid arctic summer ice retreat during the hiatus influenced tropical atmospheric circulation in a way that supports prolonged La Nina conditions.

We used NCAR and ECMWF reanalysis, and SSM/I ice concentration datasets to investigate the polar influence on tropical dynamics. Static stability calculations over the Arctic region indicate that from mid-September to mid-October convective plumes can rise above newly opened ocean areas. These disturbances generate barotropic waves in the upper troposphere that then propagate towards the equator. Due to the asymmetry in ice-ocean-land distribution, these waves create three telecommunication patterns with alternating senses of east-west trade wind circulation in the equatorial Indo-Pacific region. Thus, the persistent sea ice retreat in 1999-2017 caused anomalous westerly-easterly-westerly (W-E-W) pattern of trade wind anomalies across the Indo-Pacific, similar to the wind anomalies during canonical La Nina phases. We will discuss the non-linear interactions between these disturbances and the mean circulation, including a reflected wave train that propagates northward to mid-latitudes to modify the Aleutian Low circulation. To filter out the natural variability footprint, we will compare the recent hiatus period with the previous periods when natural variability indices had the same polarity. Finally, we will discuss the implication of our studies for numerical modeling.