final_program_abstracts[1]

11 IMSC Session Program
Decadal predictability of the Atlantic: estimation of optimal
perturbations
Thursday - Poster Session 1
Ed Hawkins and Rowan Sutton
NCAS - Climate, University of Reading, Reading, UK
Future decadal climate forecasts are likely to rely on ensembles initialised using small
perturbations to ocean and atmosphere conditions. In order to design efficient
ensembles there is a need to identify those perturbations that grow most rapidly. Such
perturbations may also be useful to identify where new ocean observations could
improve forecast skill. We have employed two different methods to estimate such
optimal perturbations, or ‘singular vectors’, for decadal predictions of the Atlantic
Ocean.
Firstly, we use linear inverse modelling (LIM) to find the initial condition anomalies
which grow most rapidly under a particular norm of interest. We have utilized a wide
range of different GCMs to explore the sensitivity of the results. The regions
consistently identified as most sensitive to small perturbations are located in the far
North Atlantic. Significant non-normal amplification is found, and the mechanisms of
amplification generally involve a basin-wide overturning circulation response to the
small perturbations. We also demonstrate multi-decadal predictability of the
overturning strength, and of basin-wide temperature and salinity fields.
Secondly, we are using an ensemble based technique – Climatic Singular Vectors
(CSVs) - which, unlike the LIM approach, enables optimal perturbations to be
estimated for specific initial conditions, e.g. a high or low overturning strength.
Reliable CSVs can be obtained in the HadCM3 GCM, although the methodology is
computationally very expensive. Amplification again occurs in the far North Atlantic,
indicating that these regions are optimal for additional ocean observations to improve
decadal climate predictions.
For more details:
Hawkins & Sutton, 2009, Journal of Climate, 22, p3960
Hawkins & Sutton, 2010, Journal of Climate, submitted
Abstracts 271