Abstract
A unique analysis is applied to the normal modes of the U.S. Navy's global operational numerical weather prediction model to investigate time-varying responses in the tropics and extratropics to tropical Pacific sea surface temperature anomalies. With this new analysis, the modes are partitioned according to their latitudinal variances. This allows the response energy to be separated into tropical and extratropical contributions. The partitioned responses are derived by grouping those modes whose fractional variance within a prescribed latitudinal band δμ exceeds some threshold value β. Since the parameters δμ and β may be chosen arbitrarily, this technique greatly increases the flexibility of the normal-mode diagnostic approach.
The partitioned responses reveal distinct differences between the evolution and vertical scales of the dominant modes in the tropics and extratropics. In the tropics, the structure is dominated by the external mode and a medium-depth internal mode. The internal mode is determined by the profile of the large-scale divergence and subsequent rotational wind (Walker circulation) response driven by enhanced convection. In the extratropics, the dominant structure is equivalent barotropic. The external rotational modes grow rapidly within the extratropics in a manner that suggests that meridional propagation alone does not fully explain the growth of the extratropical response.
