Tropical Pacific - North Pacific teleconnection in a coupled GCM

A state of the art coupled ocean-atmosphere model is used to study the connection between Tropical Pacific and North Pacific variability. The performance of the model is tested in terms of its sensitivity to the horizontal resolution of the atmospheric component, comparing two twentieth century simulations at T30 and T106 horizontal resolution. Despite a better simulation of the frequency and the spatial distribution of the Tropical Pacific anomalies associated with the El Nino Southern Oscillation (ENSO) in the high resolution experiment, the response of simulated atmospheric and oceanic anomalies in the North Pacific are scarcely different than in the T30 experiment, where the ENSO variability is weaker and more frequent than observed. The poor performance of the tropical-extratropical connection in the Pacific Ocean is studied in terms of the coupling occurring between atmospheric and oceanic fields in the North Pacific sector. Surface zonal wind stress and upper ocean heat content variability is significantly linked with the Tropical Pacific SST, but the connection with the SST of the North Pacific itself is stronger. In the model, the portions of North Pacific wind stress and upper ocean heat content variances linked with both North and Tropical Pacific SST are realistic, but their spatial distributions differ depending on the horizontal atmospheric resolution.

A significant portion of North Pacific SST variability is influenced by the tropical Pacific SST, both in the observed datasets and in the model. By means of the coupled manifold technique the North Pacific SST has been decomposed into two parts, one forced from the Tropical Pacific variability and the other free from it. In the observations, the Empirical Orthogonal Function (EOF) analysis applied to the North Pacific SST anomalies suggests that the dominant mode of variability of the total field is a combination of the first modes of variability of both forced and free components. In the high resolution experiment the dominant mode of variability of the North Pacific SST appears to be almost totally forced by the Tropical Pacific SST, while in the low resolution case the performance is more realistic, even in terms of the connection between SST in the North Pacific and in the global ocean. In both observations and model results the North Pacific variability forced from the Tropical Pacific SST seems to be mostly related to the high-frequency timescale variability.