[A-CG32] Climate Variability and Predictability on Subseasonal to Centennial Timescales

[ACG32-02] Subseasonal modulations of the Northern Annular Mode and ENSO remote influence

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

Keywords:Northern Annular Mode, El Niño-Southern Oscillation, d4PDF

The Northern Annular Mode (NAM) is the most prominent pattern of atmospheric circulation variability in the wintertime Northern Hemisphere. Its positive (negative) phase is associated with negative (positive) sea level pressure (SLP) anomalies over the Arctic region and positive (negative) anomalies in midlatitudes over the North Atlantic and Pacific. Whether the NAM is a single, physically organized mode of variability like its Southern Hemisphere counterpart, or a statistical artifact of an empirical orthogonal function analysis remains an open question. One proposed mechanism argues that the NAM is associated with the North Atlantic Oscillation (NAO) and the Aleutian-Icelandic low seesaw (AIS). Considering the NAM influence on surface air temperature and precipitation, understanding its formation mechanism is important for seasonal prediction. However, the small sample size and pronounced multidecadal modulations in the AIS have limited clear identification of the mechanism only from observational data. In this study, we examine subseasonal modulations of the NAM using “Database for Policy Decision-Making for Future Climate Change” (d4PDF), which is a 100-member ensemble atmospheric general circulation model simulations driven by historical sea surface temperature (SST), sea ice variability and radiative forcing. Our analysis shows that the NAM-associated Aleutian low anomalies become enhanced from early to late winter with the pronounced AIS emerging in late winter. Our further analysis of the SST-driven component (evaluated as the ensemble average) reveals a drastic subseasonal change of ENSO influence on the NAM such that the positive NAM is associated with El Niño in early winter but with La Niña in late winter. The ENSO teleconnection thus leads to in-phase covariations of the Aleutian and Icelandic lows in early winter but their seesaw in late winter, contributing to the subseasonal modulation of the NAM. These results support the hypothesis that, while the NAO dominates throughout the season, only in late winter the teleconnection from the Pacific can drive the NAO via AIS, forming the NAM.