Sea Ice Nonlinearities Act to Rectify and Filter Oceanic and Atmospheric Forcing

The nonlinearities controlling sea ice thermodynamics integrate forcing from the ocean and atmosphere in surprising ways, rendering it difficult to understand the processes affecting sea ice response to climate change. In this study, a simple ice thickness model is forced by realistic stochastic atmospheric and oceanic heat fluxes. Ensemble experiments show that the nonlinearities in the system rectify the added zero-mean noise on weather time scales leading to a change in the mean sea ice state. Most notably, there is a thinning in summer when sea ice is already at its minimum. The sea ice system integrates high-frequency forcing to influence longer time scales, thus changing not only the mean state but also the interannual-to-decadal variability of sea ice. Adding a trend to the forcing variables yields estimates of the dominant drivers of the current and future ice loss in the Arctic, with a prevalent role of ice–ocean heat flux over surface heat fluxes. This study reveals sea ice as a fundamental climate component, absorbing the energy into its mean state and transforming weather fluctuations with time scales of days to weeks into internal variability on time scales of months to decades. Significance Statement Understanding how sea ice responds to changes in the Arctic climate is crucial to predict its future. Using a simple model, ice thickness is shown to react in unexpected ways to small changes in atmospheric and oceanic conditions. Sea ice absorbs parts of those changes to modify its average thickness and transforms short-term weather fluctuations (lasting days to weeks) into longer-term changes in ice thickness (lasting months to decades). When it comes to Arctic warming, trends in the atmosphere and ocean have different impacts on the ice melt. The ocean plays a bigger role in determining when a seasonally ice-free Arctic will occur. This study emphasizes that sea ice is a key part of the climate system.