Optimal Growth of Central and East Pacific ENSO Events

Central Pacific (CP) and Eastern Pacific (EP) type ENSO events are inves-2 tigated using linear inverse modeling (LIM). Optimal initial conditions and growth 3 rates for CP or EP ENSO events are identified explicitly using a CP or EP ENSO 4 norm. The dominant difference in initial conditions that lead to CP and EP ENSO 5 events is the role of the second empirical orthogonal function of tropical sea sur-6 face temperature, which represents the Pacific Meridional Mode (PMM). Op-7 timal initial conditions for CP-type ENSO events include warm SST anomalies 8 in the central subtropical Pacific (a characteristic of the PMM) while optimal 9 initial conditions for EP-type ENSO events are focused in the eastern equato-10 rial Pacific and southern hemisphere subtropics along about 25 • S. Thermocline 11 anomalies differ in initial structures, and in their influence on SST for CP and 12 EP events. Results point to different roles of the PMM and thermocline vari-13 ations in the evolution of CP and EP ENSO events.