A global blocking climatology published by this group for events that occurred during the late 20th century examined a comprehensive list of characteristics that included block intensity (BI). In addition to confirming the results of other published climatologies, they found that Northern Hemisphere (NH) blocking events (1968–1998) were stronger than Southern Hemisphere (SH) blocks and winter events are stronger than summer events in both hemispheres. This work also examined the interannual variability of blocking as related to El Niño and Southern Oscillation (ENSO). Since the late 20th century, there is evidence that the occurrence of blocking has increased globally. A comparison of blocking characteristics since 1998 (1998–2018 NH; 2000–2018 SH) shows that the number of blocking events and their duration have increased significantly in both hemispheres. The blocking BI has decreased by about six percent in the NH, but there was little change in the BI for the SH events. Additionally, there is little or no change in the primary genesis regions of blocking. An examination of variability related to ENSO reveals that the NH interannual-scale variations found in the earlier work has reversed in the early 21st century. This could either be the result of interdecadal variability or a change in the climate. Interdecadal variations are examined as well.

The spring-to-summer transition is of special importance in long range forecasting, as the general circulation transitions to a less energetic regime. This affects the Midwestern United States in a profound way, since agriculture is very sensitive to the variability of weather and climate. Beginning at the local scale, surface temperature observations are used from a representative station in the West Central Missouri Plains region in order to identify the shift from late spring to early summer. Using upper-air re-analyses as a supplement, the 500-mb height observations are examined to find a spring-to-summer transition date by tracking the location of a representative contour. Each of these is used to identify spring-to-summer transition date and then statistical analysis is performed on this long-term data set. Finally, teleconnections, specifically the influence of El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), and blocking are examined in order to quantify interannual variability. It was found that examining these criteria, developed in an earlier study that covered a much shorter time period, produced similar statistics to this 68-year study of spring-to-summer transitions. It was also found that the onset of La Niña was associated with hotter summers in the region, a result first found in the earlier study, but this association was much stronger here.

Enstrophy in a fluid relates to the dissipation tendency in a fluid that has use in studying turbulent flows. It also corresponds to vorticity as kinetic energy does to velocity. Earlier work showed that the integrated regional enstrophy (IRE) was related to the sum of the positive Lyapunov exponents. Lyapunov exponents are the characteristic exponent(s) of a dynamic system or a measure of the divergence or convergence of system trajectories that are initially close together. Relatively high values of IRE derived from an atmospheric flow field in the study of atmospheric blocking was identified with the onset or demise of blocking events, but also transitions of the large-scale flow in general. Kolmogorv-Sinai Entropy (KSE), also known as metric entropy, is related to the sum of the positive Lyapunov exponents as well. This quantity can be thought of as a measure of predictability (higher values, less predictability) and will be non-zero for a chaotic system. Thus, the measure of IRE is related to KSE as well. This study will show that relatively low (high) values of IRE derived from atmospheric flows correspond to a more stable (transitioning) large-scale flow with a greater (lesser) degree of predictability and KSE. The transition is least predictable and should be associated with higher IRE and KSE.