Climate State Dependence of Arctic Precipitation Variability

29 april 2020

Arctic precipitation is projected to increase more rapidly than the global mean in warming climates. However, warming-induced changes in the variability of Arctic precipitation are largely unknown. Scientists from KNMI, in collaboration with the Institute for Marine and Atmospheric research Utrecht (Utrecht University), Energy and Sustainability Research Institute Groningen (University of Groningen), and the Water Systems and Global Change Group (Wageningen University & Research), have shown that the increase in precipitation variability towards warmer climates does not scale with the increase in mean precipitation. This is mainly attributed to the origin: while the increase in mean precipitation is largely attributed to the increase in surface evaporation, the increase in precipitation variability is attributed to variability in poleward moisture transport.

Changes in the hydrological cycle due to global warming

Global warming will affect the Earth's hydrological cycle mainly because of the increasing moisture-holding capacity of the atmosphere. While most research on the changing hydrological cycle is focused on assessing trends in mean quantities, currently little is known about the variability in the hydrological cycle of the Arctic region, and especially about how the variability will change towards other climates. Changes in variability are however crucial to study in order to interpret climate trends, as strong temporal variations on decadal timescales can temporarily obscure or enhance long-term trends. Additionally, interannual variability of the hydrological cycle is one of the governing aspects of precipitation extremes. 

To study this climate variability, long timeseries are the most appropriate. As observations and reananalyses are not available over such time periods, this research used five quasi-equilibrium climates simulated by a global climate model (EC-Earth) to study the changes in precipitation variability and the underlying mechanisms in different climates. These quasi-equilibrium simulations have a length of 400 years length and are forced with a broad range of CO2 concentrations (0.25, 0.5, 1, 2, and 4 times the current global mean).

Change in variability does not scale with mean

It was hypothesized that the change in precipitation variability would scale with the change in mean. However, it was found that the changes in mean precipitation were dominant in winter, while precipitation variability showed a stronger increase in summer towards warmer climates. 

The increase in mean precipitation towards warmer climates is driven by the increase in surface evaporation in winter, which is attributed to retreating sea ice and also the intrusion of relatively warm ocean water into the Arctic, resulting in enhanced energy exchange between the ocean and atmosphere. In contrast, wintertime precipitation variability is governed by both poleward moisture transport and surface evaporation, which oppose each other as they both influence the vertical and meridional moisture gradients.

In summer, poleward moisture transport and surface evaporation also exhibit a negative correlation, but the regressions are much weaker. Those weaker regressions are caused by the higher air temperatures due to incoming solar radiation, resulting in smaller vertical temperature gradients between the ocean and atmosphere (and therefore increased stability over the oceans). During the summer season, the increasingly stronger relation between Arctic sea level pressure variability and precipitation variability towards warmer climates enhances variability.

Figure showing Arctic mean and variability of precipitation, moisture and evaporation.
Arctic means (a–c) and variability (d–f) of total precipitation, PMT, and evaporation in winter (blue) and summer (red) for the five climate states (means over 70–90°N). Error bars for variability indicate the 5th and 95th percentiles uncertainty.

Read more here: Climate State Dependence of Arctic Precipitation Variability