Why Does ENSO State Tend to Oscillate?
• The short answer is “equatorial ocean dynamics” (Fig. 23 in powerpoint; something that was poorly understood in the late 60s when Bjerknes did his ENSO work.) • The key observations were made in the 1970s by Klaus Wyrtki, an oceanographer at the University of Hawaii. Wyrtki had a network of tide gauges in the tropical Pacific which gave records of sea level. In the tropics, monthly average sea level is an excellent substitute for the monthly average depth of the thermocline — that is, for the thickness of the upper ocean warm layer. Wyrtki showed that an El Nino event is associated (preceded, in fact) by a transfer of warm water from west to east. The top panel of Fig. 24 is a reworking of a figure showing increased sea level between Oct. 1975 and Oct. 1976 from a 1979 paper of Wyrtki’s. The bottom panel shows that west to east movement of warm water also occurred between 1981 and 1982. The next figure (Fig. 25) shows a sequence of sea level (thermocline depth) maps for 1975-6. Init
