“Colder North Eurasia, warmer North America” pattern in December 2023 and its blocking precursor

Abstract

A seesawing temperature pattern occurred over northern mid-latitudes in December 2023, with cold anomalies over northern parts of Eurasia and warm conditions over North America. The recurrent dipole temperature pattern in boreal winter has been revealed to be associated with concurrent changes in atmospheric variability (e.g., Siberian high and Alaska ridge) and boundary forcing (e.g., Arctic sea ice and North Atlantic sea surface temperature), whereas its precursor features remain less clear. Based on reanalysis data and two sets of coupled simulations, this study revealed that strong blocking activity over the Greenland-Eurasia sector in November was an important precursor to the remarkable “colder North Eurasia, warmer North America” pattern in December 2023. Increased tropospheric blocking frequency in November favored vertically propagating planetary waves to weaken the stratospheric polar vortex. The stratospheric response that persisted into December was characterized as the shift of polar vortex towards Eurasia and away from North America, linked to the enhanced zonal wavenumber-1 planetary waves. Downward propagation of the amplified wavenumber-1 component into the troposphere in December contributed to stronger continental high pressure over Eurasia and weaker ridge over Alaska, causing the “colder North Eurasia, warmer North America” pattern. High consistency between the results of observations and two sets of numerical simulations confirms the critical role of preceding tropospheric blocking, which is valuable for sub-seasonal prediction of the dipole temperature pattern across northern mid-latitudes.