Persistent radiative states in the Arctic: Drivers, trends, and model representation

Abstract

The Arctic climate is rapidly changing. The ability for the high latitudes to effectively emit longwave radiation to space is vital for shedding excess energy received in the tropics, thus regulating the global energy balance. Several Arctic field campaigns have found the presence of two preferred radiative states, each of which heavily modulate the surface energy balance, affecting whether processes such as the melting or freezing of ice and snow can occur. These states are characterized by persistent optically thick clouds (‘opaque’) rapidly transitioning to persistent optically thin clouds or cloud-free conditions (‘transmissive’), and vice versa. Using satellite observations, models, and in-situ measurements, we investigate the spatial and temporal occurrence of these states, their underlying mechanisms, models’ ability to represent them, and their long-term trends. Our findings demonstrate that CloudSat and CALIPSO can effectively detect bimodal longwave flux distributions across the polar regions. We show that these two radiative states are ubiquitous throughout the Arctic and Antarctic and are connected to both liquid-containing and ice-only clouds. The frequency of these states varies by region and season. Models, including reanalyses, struggle to represent the correct frequency of the two states. However, cloud-resolving, polar-specific products, such as the Arctic System Reanalysis,generally perform better than coarser global models. Due to the non-Gaussian nature of polar longwave flux distributions, some traditional model evaluation methods, especially those relying on monthly means, likely miss errors in the representation of these states entirely. Thus, we urge model evaluators to consider the existence of distinct states in their assessments and, more generally, to adopt evaluation methods that can assess non-Gaussian distributions. Long-term observations from the North Slope of Alaska reveal that pronounced changes in downwelling longwave radiation are linked to changing frequencies of these states,specifically, a disappearance of transmissive conditions. These changes are linked to both local and non-local moisture processes, which contribute to the formation of optically thick clouds. This shift highlights a strong positive feedback mechanism, where decreasing the transmissive state amplifies the warming and moistening of the Arctic atmosphere, further promoting the opaque state. Any region prone to future sea ice loss may experience a similar feedback, and thus, a disappearance of the transmissive state.