In FY2019, NCAR scientists further investigated the mechanisms by which short-timescale perturbations to atmospheric processes can affect interannual variability such as the El Niño-Southern Oscillation (ENSO). To this end a coupled simulation of NCAR's Community Climate System Model was compared to a simulation in which the model's atmospheric diabatic tendencies are perturbed at each time step using a Stochastically Perturbed Parameterized Tendencies scheme.
The simulation with stochastic parameterization compares better with 20th-century reanalysis in having lower inter-annual sea surface temperature (SST) variability and more irregular transitions between El Niño and La Niña states (Fig). The broadening of the SST spectrum is consistent with a reduced decorrelation time scale of the ENSO eigenmode from 17 to 11 months, which is closer to 8 months as obtained in 20th-century reanalysis. It was shown that for the simple model of a linear damped oscillator, such a noise-induced stabilization and broadening of the spectrum is obtained when its frequency parameter is perturbed (i.e., a randomization of phases).
These results can be used to inform next generation physical parameterization development. NCAR scientists will continue study of other areas where fast-slow interactions play a fundamental role and stochastic parameterization can add value to weather predictions and climate projections.
These findings were published in the Journal of Climate.