The seasonal predictability induced by the transient eddies
In the northern mid-latitude especially North Pacific and North Atlantic where the transient eddies are active, the eddy vorticity flux contributes to formation and maintenance of stationary circulations. The convergence and divergence of the eddy vorticity flux induce negative and positive geopotential height tendency, respectively. Therefore vorticity flux by transient eddies plays an important role for the interannual variability of stationary circulations. In this study, the potential seasonal predictability and actual prediction skill associated with tendency of the geopotential height at 500 hPa (GPH500) induced by the transient eddy vorticity flux is investigated using the hindcast dataset of JMA/MRI-CGCM from 1981 to 2010. The prediction skill (anomaly correlation coefficient: ACC) for GPH500 in the central North Pacific is higher than that in other mid-latitude regions in boreal winter (December-February: DJF), which is consistent with a spatial pattern of the potential predictability with a perfect model assumption. Relatively high ACC as well as potential predictability of the eddy vorticity forcing seen in the central North Pacific and North Atlantic of DJF indicate its contribution to the predictability of GPH500, although the scores for the eddy forcing are lower compared to GPH500. The spatial pattern of the variability in the eddy forcing is similar to that in GPH500. The potential predictability, the actual prediction skill (ACC) and the variability for GPH500 can be explained through those for the eddy vorticity forcing to some extent. The results suggest that better simulation of the transient eddy activity and eddy vorticity flux leads to more accurate prediction of stationary circulations in seasonal prediction models.
In addition, the influence on the eddy vorticity forcing associated with El Niņo-Southern Oscillation (ENSO) is clearly depicted with composite analysis for both the Japanese 55-year Reanalysis (JRA-55) and the JMA/MRI-CGCM hindcast data. The El Niņo and La Niņa events modulate the spatial distribution of the tropical diabatic heating through a change of precipitation pattern. This modulation causes not only tropical but also mid-latitude circulations such as the pattern of zonal mean wind, eddy vorticity forcing and GPH500. El Niņo (La Niņa) events tend to shift the westerly jet stream, that is, the storm track southward (northward) in the northern hemisphere. The shift of the storm track changes the distribution of the eddy forcing by transient eddies, consequently mid-latitude circulation patterns. Corresponding to GPH500 negative (positive) anomaly in the central North Pacific in DJF El Niņo (La Niņa) events, the eddy vorticity flux induces negative (positive) GPH500 tendency. The ENSO composite analysis illustrates these modulations in the JMA operational seasonal forecast model as well as JRA-55 reanalysis, proving that the coupled model is capable of successively representing the modulation of mid-latitude stationary patterns seen in GPH500 through modulation of the eddy forcing during DJF El Niņo and La Niņa events.