Abstract
AbstractThe extratropical teleconnection from the tropical Pacific in boreal summer exhibits a significant shift over the past 70 years. Cyclonic circulation anomalies over the North Atlantic and Eurasia associated with El Niño in the later period (1978–2014) are absent in the earlier period (1948–1977). An initialized atmospheric model ensemble, performed with prescribed sea surface temperature (SST) boundary conditions, replicates some key features of the shift in the teleconnection, providing clear evidence that this shift is not simply due to internal atmospheric variability or random sampling. Additional ensemble simulations, one with detrended tropical SSTs and another with constant external forcing are analyzed. In the model, the teleconnection shift is associated with climatological atmospheric circulation changes, which are substantially reduced in the simulation with detrended tropical SSTs. These results demonstrate that the climatological atmospheric circulation and associated teleconnection changes are largely forced by tropical SST trends.
Highlights
El Niño–Southern Oscillation (ENSO) is the dominant mode of interannual climate variability in the tropics
Despite considerable sampling uncertainty, there seems to be consistent observational evidence indicating that there has been a shift in the boreal summer teleconnection from the tropical Pacific over the past 70 years
In this study we have investigated an interdecadal shift in the boreal summer extratropical teleconnection from the tropical Pacific over the last 70 years
Summary
El Niño–Southern Oscillation (ENSO) is the dominant mode of interannual climate variability in the tropics. The teleconnection to the extratropics during boreal summer is largely consistent with barotropic Rossby wave propagation driven by precipitation anomalies in the tropical Pacific (O'Reilly et al, 2018; Shaman, 2014b; Shaman & Tziperman, 2011). O'Reilly et al (2018) found that the observed summer teleconnection from the tropical Pacific to the North Atlantic sector is seemingly absent before 1978 or so. If model simulations are able to replicate the observed changes in the summer teleconnection this would provide evidence that the changes in the teleconnection represent real changes in the climate system, rather than sampling of random internal atmospheric variability. We investigate the shifting summer teleconnection from the tropical Pacific to the extratropics from 1948 onward, using reanalysis data sets and a series of atmospheric general circulation model (AGCM) simulations. Further AGCM experiments reveal that the forced and/or natural tropical SST trends likely contribute to the changes in the observed teleconnection
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