Nonlinear structure and evolution of African easterly waves

HARDIN, NATHAN R. Nonlinear Structure and Evolution of African Easterly Waves. (Under the direction of Assistant Professor Anantha R. Aiyyer). We use the years of 1980-2001 to examine the climatology, structure, and evolution of African Easterly Waves (AEWs) during the June-October(JJASO) period characterized by tropical cyclone development. A variety of datasets including the ECMWF ERA40 reanalysis, CLAUS brightness temperature, National Center for Atmospheric Research (NCAR) outgoing longwave radiation (OLR), and sea surface temperature (SST) from the National Oceanic and Atmospheric Administration’s (NOAA) Climate Data Center (CDC) were used to examine AEW behavior. A feature based detection approach using potential vorticity (PV) is employed to create a census of AEWs. The density plots show preferred AEW tracks. A northern track contribution, poleward of 15N, is characterized by drier waves, which take longer to form into tropical cyclones. The southern track, located equatorward of 15N, is characterized by more moist waves. Both AEW tracks are responsible for tropical cyclone precursors. AEW wave structures were composited based on a nondimensional parameter referred to as the β-Rossby number. Linear wave composites reveal a classical Rossby wave structure, whereas nonlinear AEWs exhibited a more robust wave signature. This nonlinear AEW structure is supported by more intense convection, deeper moisture, and more intense upward motion. These differences suggest nonlinear waves may be more likely to undergo tropical cyclogenesis due to better preconditioning. Wave evolution was studied by compositing genesis and null AEW cases. Genesis cases reveal a more steady state structure, with characteristics found in the nonlinear wave composites. Conversely, null AEWs are characterized by significant weakening, with decaying moisture, upward motion, and convective signals. Examination of β-Rossby number behavior reveals a propensity for genesis AEWs to maintain nonlinearity as they propagate across the Atlantic, whereas null case AEWs continually become more linear. Examination of β-Rossby number frequency within the genesis and null cases reveal a similar β-Rossby number mode. The genesis cases are characterized by a skew towards higher β-Rossby number values, suggesting more genesis waves exhibit nonlinear structure. Individual case studies were undertaken in order to better understand factors controlling which AEWs undergo genesis versus those that do not. Analysis reveals multiple unfavorable factors facing an AEW which did not undergo tropical cyclogenesis. These factors include dry air intrusion, increased vertical wind shear, and anomalously cool SSTs. All of these factors contributed in preventing the AEW from becoming a tropical cyclone. The AEW that did undergo tropical cyclogenesis received significant contributions from a midlatitude trough (e.g., enhanced synoptic lift). Without the superposition of the trough, the AEW likely would never have experienced genesis. Findings from the case studies suggest significant variability with regards to the behavior and evolution of individual AEW cases. Nonlinear Structure and Evolution of African Easterly Waves