Northern Hemisphere Flow Regime Transitions, Blocking, and the Onset of Spring in the Central USA during Late Winter 2019 and 2021

Abstract

Studies have shown that maxima in the time series of Northern Hemisphere (NH) integrated enstrophy (IE) can be associated with large-scale flow regime transitions and, often, the onset and decay of blocking events. During February and March 2019, and then February 2021, strong IE maxima were associated with changes in the NH flow regimes that brought very cold conditions to the central United States. The colder conditions in the central USA during late winter 2019 and 2021 were also associated with very strong Pacific or Atlantic Region blocking events. Using the NCEP re-analyses, three different teleconnection indexes, and surface weather data from nine different cities in the central USA, IE maxima, flow regime transitions, and surface weather regimes are identified. The mean temperature and precipitation characteristics for the cities named here during the different large-scale flow regime characteristics are compared. The results have demonstrated that relatively warm conditions occurred through the first part of February 2019 before a period of anomalously colder (as much as 12 °C below normal) and drier weather, with more snow, persisted into early March. This period was bookended by maxima in the NH IE time series, changes in the character of the main NH teleconnection indexes, and a strong simultaneous NH blocking episode. Following the cold period, the temperature regime returned to values that were closer to seasonal normal values, which were then discussed as a possible indicator of a transition from a winter to a spring regime.