[A-CG30] Extratropical oceans and atmosphere

[ACG30-09] Modulations of Northern Annular Mode due to global warming

*Kento Usui¹, Takashi Kawamura¹, Satoru Okajima², Shion Sekizawa², Ayumu Miyamoto², Masaki Toda², Yu Kosaka², Hisashi Nakamura² (1.Department of Earth and Planetary Science, The University of Tokyo, 2.Research Center for Advanced Science and Technology, The University of Tokyo)

Keywords:Northern Annular Mode (NAM), Global warming, ENSO

Modulations of the winter Northern Annular Mode (NAM) under global warming are investigated using ERA5 reanalysis data and the outputs from historical and +4K global warming simulations of d4PDF. Thanks to the large ensemble size of d4PDF, the modulations of the NAM can be detected with great significance, and it is possible to separate atmospheric variability into internal variability and variability due to external forcing (mainly SST variability). It is found that the structure and amplitude of sea-level pressure anomalies associated with NAM in d4PDF historical simulations are mostly consistent with those of ERA5, albeit with excessively strong anomalies at the Pacific center of action (near the Aleutian low). Comparison between historical simulation and +4K simulations shows that the sea-level pressure anomalies around the Pacific center of the action strengthens with global warming. Besides the fraction of variance due to external (SST) forcing greatly rises from 9.4% to 32.1%. These suggest that global warming enhances the influence from ENSO on NAM. There are two possible explanations for this change. First, the pattern of equatorial Pacific precipitation anomalies associated with ENSO shifts eastward with global warming, which displaces the teleconnection pattern eastward with stronger influence on the North Atlantic sector of NAM. Second, the background westerlies get stronger and the wavelength of the stationary Rossby waves increases. We also examined the transient eddy activity changes associated with ENSO, and found that the anomalies extend more downstream toward the North Atlantic in the warmer climate. This suggests that the transient eddy activity anomalies associated with ENSO have stronger feedback to North Atlantic circulation anomalies associated with NAM in the warmer climate.