3:00 PM - 3:15 PM
[ACG33-06] Projected changes of the Northern Annular Mode linked to SST variability
Keywords:Northern Annular Mode, El Niño-Southern Oscillation, Global warming
The Northern Annular Mode (NAM), also known as the Arctic Oscillation (AO), is the most dominant pattern of atmospheric circulation variability in the wintertime Northern Hemisphere extratropics. The NAM is characterized by the same-signed sea-level pressure (SLP) anomalies around the climatological Aleutian Low (AL) and Azores High, and the opposite-signed SLP anomaly around the Icelandic Low. The latter two lobes constitute the North Atlantic Oscillation (NAO). While future projection studies suggest the NAM phase shift towards a more positive phase on average with global warming, it is unclear how the statistical properties of the NAM as a mode of interannual variability will modulate. In this study, we investigate modulations of the NAM under global warming. In particular, we examine seasonality of the NAM and its change. We use the large ensemble global atmospheric simulation dataset d4PDF, which consists of a 100-member ensemble of historical experiment driven by observed sea surface temperature (SST), sea ice, and historical radiative forcing for 60 years, and a 90-member ensemble of global warming experiment with a mean SST increase to the global warming level of 4K.
In the warmer climate, the relative magnitude of the AL anomaly becomes larger throughout winter. This change is partly attributed to the stronger influence from El Niño-Southern Oscillation (ENSO). Indeed, the fraction of NAM variance explained by the ensemble mean variability, which mostly arises from SST and sea ice variability, greatly increases with global warming. In the warmer climate, the ENSO teleconnection reaches further east to the North Atlantic, leading to a stronger association of the AL anomaly with NAO. Climatologically, this linkage strengthens from early to late winter. In the warmer climate, a strong linkage already emerges in early winter. We further compare the six background SST warming patterns prescribed to the warming experiments. The result suggests that an El Niño-like SST warming contributes to the strengthening of the ENSO-NAM correlation. This modulation is contributed to by an eastward shift of tropical convective activity anomalies associated with ENSO and changes in the waveguide structure of the background circulation.
In the warmer climate, the relative magnitude of the AL anomaly becomes larger throughout winter. This change is partly attributed to the stronger influence from El Niño-Southern Oscillation (ENSO). Indeed, the fraction of NAM variance explained by the ensemble mean variability, which mostly arises from SST and sea ice variability, greatly increases with global warming. In the warmer climate, the ENSO teleconnection reaches further east to the North Atlantic, leading to a stronger association of the AL anomaly with NAO. Climatologically, this linkage strengthens from early to late winter. In the warmer climate, a strong linkage already emerges in early winter. We further compare the six background SST warming patterns prescribed to the warming experiments. The result suggests that an El Niño-like SST warming contributes to the strengthening of the ENSO-NAM correlation. This modulation is contributed to by an eastward shift of tropical convective activity anomalies associated with ENSO and changes in the waveguide structure of the background circulation.