Low-Frequency Signals in Midtropospheric Submonthly Temperature Variance

Abstract

Low-frequency signals in the daily variability of temperature in the midtroposphere are investigated, thereby complementing published studies of changes in day-to-day temperature variability and in extreme weather events at the surface. The results are based upon approximately four decades of upper-air data from radiosondes and the National Centers for Environmental Prediction?National Center for Atmospheric Research (NCEP?NCAR) reanalyses. The annual mean field of 500-hPa submonthly temperature variance, var(T), is oriented zonally across most of the globe, with maxima in the midlatitudes over the major landmasses of North America and Asia and over the oceans of the Southern Hemisphere. Seasonally, var(T) shifts equatorward from the warm to cool season in both hemispheres. Therefore, var(T) reflects day-to-day changes in temperature about the jet stream associated with baroclinic synoptic-scale systems. Year-to-year changes in var(T) over the Northern Hemisphere are greatest over the major landmasses of North America, northern Europe, and Asia. There is also evidence of an influence of ENSO upon the interannual variability of var(T) over the northern portion of North America during winter, where there is a westward displaced maximum in cold events relative to warm events. Trend analysis over the Northern Hemisphere shows that there has been a significant increase in submonthly temperature variance over the northeastern portion of North America, the North Atlantic, and Scandinavia, representing as much as 30% of the climatological values of var(T) in these regions. These regional trends are most apparent during the Northern Hemisphere winter and spring seasons. The zonally averaged var(T) has generally decreased over polar latitudes and increased over the midlatitudes of the Northern Hemisphere, although there are considerable differences from season to season. Averaged over the entire Northern Hemisphere, var(T) exhibits a slight upward trend since the late 1950s in the NCEP?NCAR reanalysis, although this trend is significant in the spring season only. The robustness of this springtime trend, however, is in doubt, because the trend found from a radiosonde-only dataset is negative. For the conterminous United States, the two datasets do agree by showing mostly small positive trends in most seasons. These positive trends, however, are not statistically significant, and therefore the authors cannot state with confidence that there has been a change in synoptic-scale temperature variance in the midtroposphere over the United States since 1958.