Measuring the impact of observations on the predictability of the Kuroshio Extension in a shallow-water model
Kramer, W., Dijkstra, H. A., Pierini, S. and van Leeuwen, P. J. (2012) Measuring the impact of observations on the predictability of the Kuroshio Extension in a shallow-water model. Journal of Physical Oceanography, 42 (1). pp. 3-17. ISSN 0022-3670
To link to this article DOI: 10.1175/JPO-D-11-014.1
In this paper sequential importance sampling is used to assess the impact of observations on a ensemble prediction for the decadal path transitions of the Kuroshio Extension (KE). This particle ﬁltering approach gives access to the probability density of the state vector, which allows us to determine the predictive power — an entropy based measure — of the ensemble prediction. The proposed set-up makes use of an ensemble that, at each time, samples the climatological probability distribution. Then, in a post-processing step, the impact of diﬀerent sets of observations is measured by the increase in predictive power of the ensemble over the climatological signal during one-year. The method is applied in an identical-twin experiment for the Kuroshio Extension using a reduced-gravity shallow water model. We investigate the impact of assimilating velocity observations from diﬀerent locations during the elongated and the contracted meandering state of the KE. Optimal observations location correspond to regions with strong potential vorticity gradients. For the elongated state the optimal location is in the ﬁrst meander of the KE. During the contracted state of the KE it is located south of Japan, where the Kuroshio separates from the coast.