The Impact of Sea Ice Concentration and Sea Surface Temperature Boundary Forcing in different Experimental Setups with ECHAM6 on the changes in Northern hemisphere atmospheric circulation regimes

We performed three sets of experiments with the atmospheric model ECHAM6 where sea ice concentration (SIC) and sea surface temperature (SST) boundary forcing has been prescribed differently. The first set compares present-day boundary forcing from an early phase in the 1980s and a late phase from the 2000s. The second set of experiments follow the PAMIP protocol. Additionally, the second set has been rerun with ECHAM6-SWIFT, which implements a fast interactive ozone chemistry module for the stratospheric polar vortex. We show the impact of separated and combined SIC and SST changes on atmospheric circulation regimes in the troposphere and stratosphere over the Northern hemisphere. In particular, changes in the occurrence frequency of circulation regimes depend on the boundary forcing in a nonlinear way. Furthermore, we discuss how the fast interactive ozone chemistry impacts the response of atmospheric circulation regimes to SIC and SST changes. The results are further compared to reanalysis data separated in an early period from 1979 to 2000 and late period from 2000 to 2021.