12:00 PM - 12:15 PM
[ACG34-12] Northern midlatitude summer warming pattern tied to Pacific and Atlantic multidecadal variability
★Invited Papers
Keywords:multidecadal variability, Pacific decadal oscillation, atlantic multidecadal variability
Significant surface air temperature warming during summer 1979-2020 is not uniformly distributed in the northern midlatitude over land, but rather confined to several longitudinal sectors including Europe, Central Siberia-Mongolia and both coasts of North America. These hot spots are accompanied by a chain of high-pressure ridges from an anomalous, circumglobal, Rossby wave train in the upper troposphere. From reanalysis data and several Coupled Model Intercomparison Project Phase 6 (CMIP6) baseline experiments, we find that the circulation trend pattern is associated with fluctuations of the Atlantic Multidecadal Variability (AMV) and the Interdecadal Pacific Oscillation (IPO). The phase shift of AMV in the 1990s is particularly noteworthy for accelerating warming averaged over the northern midlatitude land.
The amplitude of the observed trend in both surface air temperature and the upper-level geopotential height generally falls beyond the range of multidecadal trends simulated by the CMIP6 preindustrial control runs, supporting the likelihood that anthropogenic forcing played a critical role in the observed trend. On the other hand, the fidelity of the simulated low-frequency modes of variability and their teleconnections, especially on multidecadal timescales, is difficult to assess because of the relatively short observational records. Our mechanistic modeling results indicate that synoptic eddy-mean flow interaction is a key to the formation of the anomalous wavetrain but how the multidecadal modes can modulate the synoptic eddies through atmosphere-ocean and atmosphere-land interactions remains poorly understood. This gap in our knowledge makes it challenging to quantify the roles of the low-frequency modes and external forcings in causing the observed multidecadal trends.
The amplitude of the observed trend in both surface air temperature and the upper-level geopotential height generally falls beyond the range of multidecadal trends simulated by the CMIP6 preindustrial control runs, supporting the likelihood that anthropogenic forcing played a critical role in the observed trend. On the other hand, the fidelity of the simulated low-frequency modes of variability and their teleconnections, especially on multidecadal timescales, is difficult to assess because of the relatively short observational records. Our mechanistic modeling results indicate that synoptic eddy-mean flow interaction is a key to the formation of the anomalous wavetrain but how the multidecadal modes can modulate the synoptic eddies through atmosphere-ocean and atmosphere-land interactions remains poorly understood. This gap in our knowledge makes it challenging to quantify the roles of the low-frequency modes and external forcings in causing the observed multidecadal trends.