Model Bias Reduction and the Limits of Oceanic Decadal Predictability: Importance of the Deep Ocean

Sévellec, Florian and Fedorov, Alexey V. (2013) Model Bias Reduction and the Limits of Oceanic Decadal Predictability: Importance of the Deep Ocean. Journal of Climate, 26 (11), 3688-3707. (doi:10.1175/JCLI-D-12-00199.1).

Abstract

Ocean general circulation models (GCMs), as part of comprehensive climate models, are extensively used
for experimental decadal climate prediction. Understanding the limits of decadal ocean predictability is
critical for making progress in these efforts. However, when forced with observed fields at the surface, ocean
models develop biases in temperature and salinity. Here, the authors ask two complementary questions related
to both decadal prediction and model bias: 1) Can the bias be temporarily reduced and the prediction
improved by perturbing the initial conditions? 2) How fast will such initial perturbations grow? To answer
these questions, the authors use a realistic ocean GCM and compute temperature and salinity perturbations
that reduce the model bias most efficiently during a given time interval. The authors find that to reduce this
bias, especially pronounced in the upper ocean above 1000 m, initial perturbations should be imposed in the
deep ocean (specifically, in the Southern Ocean). Over 14 yr, a 0.1-K perturbation in the deep ocean can
induce a temperature anomaly of several kelvins in the upper ocean, partially reducing the bias.Acorollary of
these results is that small initialization errors in the deep ocean can produce large errors in the upper-ocean
temperature on decadal time scales, which can be interpreted as a decadal predictability barrier associated
with ocean dynamics. To study the mechanisms of the perturbation growth, the authors formulate an idealized
model describing temperature anomalies in the Southern Ocean. The results indicate that the strong mean
meridional temperature gradient in this region enhances the sensitivity of the upper ocean to deep-ocean
perturbations through nonnormal dynamics generating pronounced stationary-wave patterns.

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