Impact of the stratosphere on El Niño-Southern Oscillation

The European Consortium EC-EARTH climate model version 3.1 is used to assess the effects of a well-resolved stratosphere on the representation of El Niño-Southern Oscillation (ENSO). Three 100-year  long experiments with fixed radiative forcing representative of the present climate are compared: one with the top at 0.01hPa and 91 vertical levels (HIGH-TOP), another with the top at 5hPa and 62 vertical levels (LOW-TOP), and another high-top experiment with the stratosphere nudged to the climatology of HIGH-TOP from 10hPa upwards (NUDG). The differences in vertical resolution between HIGH-TOP and LOW-TOP start at around 100hPa. This study focuses on the canonical ENSO phenomenon, which is the most important source of variability and predictability on seasonal-to-interannual timescales.

 

Preliminary results indicate that EC-EARTH realistically simulates the ENSO SST pattern in the tropical Pacific regardless of vertical resolution, although HIGH-TOP (LOW-TOP) overestimates (underestimates) the SST variability during boreal summer (winter). In both configurations, the SST tongue is narrower meridionally and slightly shifted towards the central-western Pacific compared to observations, a common bias of climate models. Resolving the stratosphere has a clear effect on the power spectrum of the Niño3.4 index: as compared to observations where there is a well-known frequency range of 2-7 years, HIGH-TOP and LOW-TOP have a prominent peak centered at 4-5 years but additionally both simulations display another peak, towards higher (~ 2yrs) and lower (~ 7yrs) frequencies, respectively. Another impact of including a well-resolved stratosphere is to systematically enhance the amplitude of the SST, wind and convective anomalies in the tropical Pacific throughout the entire ENSO cycle. Finally, similar differences are obtained when comparing HIGH-TOP and NUDG, suggesting an active role of the tropical stratospheric variability on ENSO.