Abstract

In which direction is the influence larger: from the Arctic to the mid-latitudes or vice versa? To answer this question, CO2 concentrations have been regionally increased in different latitudinal belts, namely in the Arctic, in the northern mid-latitudes, everywhere outside of the Arctic and globally, in a series of 150 year coupled model experiments with the AWI Climate Model. This method is applied to allow a decomposition of the response to increasing CO2 concentrations in different regions. It turns out that CO2 increase applied in the Arctic only is very efficient in heating the Arctic and that the energy largely remains in the Arctic. In the first 30 years after switching on the CO2 forcing some robust atmospheric circulation changes, which are associated with the surface temperature anomalies including local cooling of up to 1 °C in parts of North America, are simulated. The synoptic activity is decreased in the mid-latitudes. Further into the simulation, surface temperature and atmospheric circulation anomalies become less robust. When quadrupling the CO2 concentration south of 60° N, the March Arctic sea ice volume is reduced by about two thirds in the 150 years of simulation time. When quadrupling the CO2 concentration between 30 and 60° N, the March Arctic sea ice volume is reduced by around one third, the same amount as if quadrupling CO2 north of 60° N. Both atmospheric and oceanic northward energy transport across 60° N are enhanced by up to 0.1 PW and 0.03 PW, respectively, and winter synoptic activity is increased over the Greenland, Norwegian, Iceland (GIN) seas. To a lesser extent the same happens when the CO2 concentration between 30 and 60° N is only increased to 1.65 times the reference value in order to consider the different size of the forcing areas. The increased northward energy transport, leads to Arctic sea ice reduction, and consequently Arctic amplification is present without Arctic CO2 forcing in all seasons but summer, independent of where the forcing is applied south of 60° N. South of the forcing area, both in the Arctic and northern mid-latitude forcing simulations, the warming is generally limited to less than 0.5 °C. In contrast, north of the forcing area in the northern mid-latitude forcing experiments, the warming amounts to generally more than 1 °C close to the surface, except for summer. This is a strong indication that the influence of warming outside of the Arctic on the Arctic is substantial, while forcing applied only in the Arctic mainly materializes in a warming Arctic, with relatively small implications for non-Arctic regions.

Highlights

  • Over the last 30 years, Arctic amplification, an increase of Arctic surface temperatures twice the amount of the Northern hemisphere mean temperature increase, has been observed in the field and in climate projections (Cohen et al 2014)

  • The response to sea ice loss has been isolated from the complete greenhouse gas impact in the coordinated multi-model ensemble of CMIP5 models (Zappa et al 2018)—the robust results being a southward shift of the jet stream and a strengthening of the Siberian High in late winter consistent with many studies prescribing idealized sea ice conditions

  • Even without any Arctic forcing in the 60 Ns experiment, the amount of Arctic sea ice declines markedly due to energy transport from the low to the high latitudes

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Summary

Introduction

Over the last 30 years, Arctic amplification, an increase of Arctic surface temperatures twice the amount of the Northern hemisphere mean temperature increase, has been observed in the field and in climate projections (Cohen et al 2014). The impact of shrinking sea ice in the Arctic on mid-latitudes remain subject to a large uncertainty due to a notoriously small signal-to-noise ratio in the northern mid-latitudes, a limited time period of observations, and different designs of modelling studies (Cohen et al 2018). Screen et al (2018) give a synthesis of long coupled model experiments which reveal some consistent features as response to sea ice loss despite differences in the model set-up: hemisphere-wide atmospheric warming, intensification of Aleutian low and Siberian high, weakening of the Icelandic low, weakening and southward shift of the midlatitude westerly winds in winter. Experiments within PAMIP include applying Arctic sea ice anomalies with and without SST anomalies in uncoupled and coupled mode in short 1-year and long 100-year simulations.

Experiment setup
Atmospheric temperature response
Near‐surface temperature response in the Arctic forcing experiments
Meridional energy transport
Synoptic activity
Discussion and conclusions
Full Text
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