An Analysis of the Spring-to-Summer Transition in the West Central Plains for Application to Long Range Forecasting

Abstract

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.

Changes in Blocking Characteristics during the First Part of the 21st Century

“…There was little change in the Southern Hemisphere BI. In the Northern Hemisphere, the increses found here were consistent with the results of others [6], [7], [17], [25] who examined 'partial' climatologies examining only certain reigions during certain seasons. The work of [6] implied that the period 1970 -1999 showed a relative minimum in Atlatnic Region winter season blocking.…”

“…The Atlantic region showed the weakest increases which is consistent with the results of [7]. The large increase in the occurrence and days within Pacific Region Blocking is consistent with the results of [17] who examined spring and summer blocking in this region. The work of [25] also showed a large increase in blocking for the eastern Europe and western Russia region for the spring and summer seasons as well.…”

“…The JMA classifies ENSO phases using SST within the bounded region of 4˚S to 4˚N, 150˚W to 90˚W, and defines the inception of an ENSO year as 1 October, and its conclusion on 30 September of the next year. This index is widely used in other published works (see [17] and references therein) and a list of years is given in (Table 1). Also, [19] found that while the JMA index is more sensitive to La Niña events than other definitions, it is less sensitive than other indices to El Niño events.…”

“…This decrease is not significant at any standard level of statistical significance. [17] and [25] showed increases within the warm season in recent decades, on the other hand, [6] showed that over the Atlantic for the winter season that blocking days over the same period covered by [1] was associated with a significant minimum. Additionally, [7] showed an increase in blocking since 2000 as shown with three blocking indexes including the index used here.…”

“…The definition for ENSO used here is described in [17] and references therein and a short description is given here. The Japanese Meteorological Agency (JMA) El Nino and Souther Oscillation (ENSO) index is available through the Center for Ocean and Atmospheric Prediction Studies (COAPS) from 1868 to present [18].…”