Initialization shock in decadal hindcasts due to errors in wind stress over the tropical Pacific

In a recently published paper, H. Pohlmann and colleagues from the MiKlip projects FLEXFORDEC and ATMOS/MODINI investigate the problem of low decadal prediction skill in the tropical Pacific using the MiKlip decadal prediction system.

Low prediction skill in the tropical Pacific is a common problem in decadal climate prediction systems especially for lead years (LY) 2-5. Multi-model studies, which are analyzing decadal prediction systems, reveal in general high prediction skill over the oceans where the low-frequency climate variability is dominant. However, low prediction skill is often present over the equatorial and North Pacific for LY 2-5, affecting also the prediction skill of the Pacific Decadal Oscillation (PDO). In the tropical East Pacific, the prediction skill is in many systems even lower than in uninitialized experiments. While the low prediction skill in the North Pacific is assumed to result from biases in the representation of ocean mixing processes in the climate models the reason for the low prediction skill in the tropical Pacific is unknown.

Graphic from paper
Figure 1: SST anomalies (a,c: average over 1961-1970; b,d: average over 1995-2004) in the MPI-ESM CMIP5 system (a,b) and the sensitivity experiment (c,d).

H. Pohlmann and colleagues look into the problem of reduced prediction skill by analyzing the Max Planck Institute Earth System Model (MPI-ESM) decadal hindcasts for the fifth phase of the Climate Model Intercomparison Project (CMIP5) and performing a sensitivity experiment in which hindcasts are initialized from a model run forced only by surface wind stress. In both systems, sea surface temperature (SST) variability in the tropical Pacific is successfully initialized, but most skill is lost at lead years 2-5. Utilizing the sensitivity experiment enables them to pin down the reason for the reduced prediction skill in MPI-ESM to errors in wind stress used for the initialization. A spurious trend in the wind stress forcing displaces the equatorial thermocline in MPI-ESM unrealistically. When the climate model is then switched into its forecast mode, the recovery process triggers artificial El Niño and La Niña events at the surface (see the figure 1). Their results demonstrate the importance of realistic wind stress products for the initialization of decadal predictions.

Reference:

Pohlmann, H., J. Kröger, R. J. Greatbatch, W. Müller, 2016: Initialization shock in decadal hindcasts due to errors in wind stress over the tropical Pacific. Clim. Dyn., doi:10.1007/s00382-016-3486-8.

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