Easterly wave diagnostics

We present an objective method for identifying synoptic scale easterly waves and the position of easterly jet cores in gridded datasets. These diagnostics were designed to examine African easterly wave (AEW) activity over the African continent and Atlantic Ocean using the 700hPa streamfunction field. However, the method is fully customizable to apply to other phenomena in any gridded dataset. The method has been developed to: (i) reduce the amount of manual data interpretation (ii) reduce the likelihood of unrelated phenomena being identified as AEWs and (iii) facilitate completely objective comparisons between AEWs with different structures on multiple scales, in order to describe their variability.

These diagnostics have been used in the analysis of AEW activity over the African continent and Atlantic Ocean during summer 2004 and 2005. The results show that the ‘average' AEW during this period bears a close resemblance to composite structures from previous research, although there is marked variability in the characteristics and ultimate fate of AEWs. It will be demonstrated that this method performs reliably when compared to methods of AEW identification used in previous studies. It will also be illustrated how these diagnostics, in combination with potential vorticity thinking, promote a new way of discussing synoptic scale weather systems in this region.