Abstract
The 2011/12 La Niña, referred to as a double-dip La Niña event with following a previous La Niña event, was not well predicted by most climate models when starting from early-mid 2011. Based on a developed El Niño-Southern Oscillation (ENSO) ensemble prediction system, this paper investigates the key predictors for the 2011/12 La Niña, to determine which conditions favor a double-dip La Niña event up to one year in advance. The key predictors were isolated in a 100-member ensemble hindcast experiment. Results show that continuous easterly surface winds and persistent subsurface cold conditions preceded the second-year cooling in mid-2011. And a significant difference can be viewed between the best and the worst ensemble forecasts arose from the stochastic model-error perturbations. The detailed comparisons between the best and the worst ensemble forecasts further illustrate that the stochastic model-error perturbations play a significant role in improving the prediction skills of the best ensemble members during the 2011/12 12-month forecast process, through capturing the transition of sea surface temperature (SST) over the tropical Pacific (i.e., from a warm condition to a cold condition) in boreal spring.
