Equatorial Waves and Peter Webster

In this celebration of Peter Webster’s career, I will reflect on Peter’s scientific contribution to our knowledge of equatorial waves, as well as Peter’s contribution to my own career. Starting with Peter’s Ph.D. thesis paper on the response of the tropical atmosphere to local, steady forcing (Webster 1972) we can see that the knowledge of the shallow-water equatorially-trapped wave solutions was being applied to understand the steady large-scale tropical circulation; that is, the Kelvin wave to the east of the latent-heat forcing and the Rossby wave to the west. Peter’s work with the equatorial wave modes continued in the late 1980s with the papers Webster and Chang (1988) and Zhang and Webster (1989). In these the equatorial wave solutions were used to explain aspects of the transient behaviour of the equatorial region, using a shallow-water equivalent depth applicable to the dry (i.e. not convectively-coupled) waves. In the early 1990s, Peter’s main attention turned to TOGA COARE, which is where I began my own interest in equatorial waves as a student helper in the field project offices in Australia and the Solomon Islands. During the COARE IOP the Webster and Lukas (1992) paper on the scientific basis and experimental design became my bible of what the massive undertaking of COARE was all about. Thanks to COARE, for which Peter was the scientific lead, I got to see the observed equatorial waves as they appear in tropical convection, and I also got my foot in the door to my period of work and study in Boulder at NCAR, the University of Colorado, and NOAA. Importantly for my development, my first class at the University of Colorado was taught by Peter, and he was also my Ph.D. academic adviser. Interestingly, as shown in Webster and Lukas (1992), the role of the shallow-water equatorial waves for the propagation and organization of tropical moist convection was not fully appreciated before and during COARE. Indeed, many of my meetings with my University professor (i.e. Peter) involved intense discussions of the shallow-water equivalent depth and its relation to vertical structure. Peter would ask: How can the equatorial waves, as derived using the dry primitive equations, appear for such a shallow equivalent depth, implying a short vertical wavelength, in observations of convection? My Ph.D. thesis papers on the convectively-coupled equatorial waves (Wheeler and Kiladis 1999; Wheeler, Kiladis and Webster 2000) were made significantly better based on these discussions. Peter’s late-1980s theoretical work on the role of zonal basic-state flows on equatorial waves was also an important contribution to the development of my ideas. We now know that many of the zonally-propagating convective features observed during COARE, on time scales of 2 days to 30 days, are well described as moist equatorial waves with a much shallower equivalent depth than the dry waves that Peter earlier studied. In summary, the theory and application of equatorial waves for our understanding of the steady and transient behaviour of the tropics owes a lot to the work of Peter, and is just one of many areas in which he has made a mark.