Characteristics of extratropical cyclone variability in the Northern Hemisphere and their response to rapid changes in Arctic sea ice

Extratropical cyclones are critical weather systems that affect large-scale weather and climate changes at mid-high latitudes. However, prior research shows that there are still great difficulties in predicting extratropical cyclones for occurrence, frequency, and position. In this study, mean sea level pressure (MSLP) data from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) are used to calculate the variance statistics of the MSLP to reveal extratropical cyclone activity (ECA). Based on the analysis of the change characteristics of ECA in the Northern Hemisphere, the intrinsic link between ECA in the Northern Hemisphere and Arctic sea ice is explored. The results show that the maximum ECA mainly occurs in winter over the mid-high latitudes in the Northern Hemisphere. The maximum ECA changes in the North Pacific and the North Atlantic, which are the largest variations in the Northern Hemisphere, are independent of each other, and their mechanisms may be different. Furthermore, MSLP is a significant physical variable that affects ECA. The North Atlantic Oscillation (NAO) and North Pacific Index (NPI) are significant indices that impact ECA in the North Atlantic and North Pacific, respectively. The innovation of this paper is to explore the relationship between the activity of extratropical cyclones in the Northern Hemisphere and the abnormal changes in Arctic sea ice for the first time. The mechanism is that the abnormal changes in summer-autumn and winter Arctic sea ice lead to the phase transition of the NPI and NAO, respectively, and then cause the occurrence of ECA in the North Pacific and North Atlantic, respectively. Arctic sea ice plays a crucial role in the ECA in the Northern Hemisphere by influencing the polar vortex and westerly jets. This is the first exploration of ECAs in the Northern Hemisphere using Arctic sea ice, which can provide some references for the in-depth study and prediction of ECAs in the Northern Hemisphere.