Atmospheric blocking over the high-latitude North Atlantic blocks the westerly jet stream, causing the eastward propagation of weather systems to stall for 7 days or so. An earlier study by Häkkinen et al. (2011) used the 20th century atmospheric reanalysis product of NOAA (20CR) to show that the wintertime blocking frequency over high latitude North Atlantic covaries with the Atlantic Multidecadal Oscillation (AMO, or AMV). For instance, during a negative AMO phase in 1970s – 1980s, the wintertime blocking days decreased, while during a positive AMO phase in 1930s – 1960s, the blocking days increased significantly. Häkkinen et al. (2011) concluded that the space-time structure of the wind stress and wind stress curl associated with a blocked regime could lead to weaker ocean gyres and weaker air-sea heat exchange (i.e., less cooling of the ocean), both of which contribute to a positive AMO phase. A new study published in Journal of Climate (Kwon et al., 2019) proposed a very different hypothesis. They argues that the blocking frequency is rather modulated by the AMO. The new study used the same reanalysis product (20CR) to perform lead-lag correlation analysis to find that the AMO leads the multidecadal variability of blocking frequency by several years. Further composite analysis showed that warm SST anomalies over the western subpolar region during a positive AMO phase maintain the atmosphere aloft warmer. This in turn reduces the meridional gradient of the air temperature across the westerlies and thus weaken the meridional eddy heat flux in the lower troposphere. Due to the weakened meridional eddy heat flux, the westerly jet shifts southward, increasing the frequency of Rossby wave breaking.
Figure 5 from Kwon et al. (2019): Lag-correlations between (a) the AMV index and PC2 of the winter number of blocking days, and (b) the AMV and NAO indices. The blue (red) curves are for the correlations using the yearly values before (after) applying a 10-year low-pass filter to both time series. The dashed curves indicate the corresponding statistical significance at the 5% level. The lag is positive (negative) when the AMV index leads (lags).
Impact of Multidecadal Variability in Atlantic SST on Winter Atmospheric Blocking. J. Climate, https://doi.org/10.1175/JCLI-D-19-0324.1 https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-19-0324.1, T. M. (2019).
Häkkinen, S., Rhines, P. B., & Worthen, D. L. (2011). Atmospheric blocking and Atlantic multidecadal ocean variability. Science, 334(6056), 655-659. https://science.sciencemag.org/content/334/6056/655