Nmj. Hall et al., The extratropical signal generated by a midlatitude SST anomaly. Part II: Influence on seasonal forecasts, J CLIMATE, 14(12), 2001, pp. 2696-2709
A simple GCM based on a primitive equation model with empirically derived t
ime-independent forcing is used to make forecasts in the extended to season
al range. The results are analyzed in terms of the response to a midlatitud
e Pacific sea surface temperature anomaly (SSTA), represented here by a hea
ting perturbation. A set of 90-day, 30-member ensemble forecasts is made wi
th 54 widely differing initial conditions, both with and without the SSTA.
The development of the response, defined as the difference between ensemble
means, is split into three 30-day averages: month 1, month 2, and month 3.
During month 1, ensemble members separate, and the local response and remot
e teleconnections are established. The local response is not very sensitive
to the initial condition.
In month 2, the extended range, the responses are relatively strong and var
y greatly from one initial condition to another. However, a linear analysis
reveals that large variations in the response do not correlate strongly wi
th large variations in the initial condition. The initial perturbations req
uired to generate the observed variations in the response are relatively sm
all, and may be difficult to isolate in a real forecasting situation.
In month 3, the seasonal range, variations between responses are much small
er. The initial condition loses its influence and the responses all start t
o resemble the equilibrium response discussed in Part I.