Variability and changes of tropical-extratropical cloudband events over the Southern Hemisphere using a synoptic climatology approach

The aridity of the Southern Hemisphere subtropical latitudes is frequently punctuated by near-planetary scale bands of cloud that diagonally link the tropics to the mid-latitudes. These cloud bands preferentially occur in the subtropical convergence zones situated in each of the three ocean basins. However, much of the research focused on these regions has considered seasonal averages, which are not suitable for fully capturing the synoptic-scale variability of these events. The MetBot, first developed to identify cloud bands over southern Africa and now successfully adapted to the South American area, overcomes this problem by identifying these tropical-extratropical cloud bands in daily data. In this study, we use the near-real-time version of the MetBot, currently implemented to monitor the progress of cloudband seasons over the Southern Hemisphere. Cloud bands are identified in the daily outgoing longwave radiation data from the High-Resolution Infrared Radiation Sounder (HIRS), developed by the National Oceanic and Atmospheric Administration (NOAA), with a spatial resolution of 1° and latency time of a couple of days (Lee, 2011). The results of the near-real-time monitoring area are available at <https://hart-ncg.github.io/real-time/about.html>. Here, we considered the cloud bands dataset with events identified across the Southern Hemisphere between 1979 and 2020 to evaluate their variability and recent changes. The climatological analysis highlights four hotspot regions: South Pacific and South America, which are the strongest; southern Africa, with events only forming through austral summer; and Australia, with summer (winter) events over the eastern (western) coast. The hemispheric scale of the results provides a unified view of the interannual and intraseasonal variability of the cloud bands, including the preferential location of the convective activity during different ENSO phases. By isolating the systems responsible for most of the rainy season precipitation over the subtropics, we also identify changes in the cloud band activity in recent years and their impact on the total precipitation. Future applications include the use in tandem with S2S forecast data, as currently being implemented over South America, as well as a tool to evaluate historical and future climate model simulations of various resolutions.  

Lee, Hai-Tien and NOAA CDR Program (2011): NOAA Climate Data Record (CDR) of Daily Outgoing Longwave Radiation (OLR), Version 1.2. NOAA National Climatic Data Center. doi:10.7289/V5SJ1HH2