Seasonal Variation of the Sea Surface Temperature Growth Rate of ENSO

El Niño-Southern Oscillation(ENSO)exhibits a distinctive phase-locking characteristic,first expressed during its onset in boreal spring,developing during summer and autumn,reaching its peak towards winter,and decaying over the next spring.Several studies have demonstrated that this feature arises as a result of seasonal variation in the growth rate of ENSO as expressed by the sea surface temperature(SST).The bias towards simulating the phase locking of ENSO by many state-of-the-art climate models is also attributed to the unrealistic depiction of the growth rate.In this study,the seasonal variation of SST growth rate in the Niño-3.4 region(5°S-5°N,120°-170°W)is estimated in detail based on the mixed layer heat budget equation and recharge oscillator model during 1981-2020.It is suggested that the consideration of a variable mixed layer depth is essential to its diagnostic process.The estimated growth rate has a remarkable seasonal cycle with minimum rates occurring in spring and maximum rates evident in autumn.More specifically,the growth rate derived from the meridional advection(surface heat flux)is positive(negative)throughout the year.Vertical diffusion generally makes a negative contribution to the evolution of growth rate and the magnitude of vertical entrainment represents the smallest contributor.Analysis indicates that the zonal advective feedback is regulated by the meridional immigration of the intertropical convergence zone,which approaches its southernmost extent in February and progresses to its northernmost location in September,and dominates the seasonal variation of the SST growth rate.