|Abstract or Summary
- A time-dependent, spectral, barotropic model and a similar
two-layer primitive equation model are developed to investigate the
planetary-scale wave responses to various types of large-scale
forcing: vorticity and heat sources, and sea surface temperature (SST)
anomalies. Both models are linearized about the zonal mean states of
The characteristics of forced Rossby waves are extensively studied
based on both the barotropic model experiments and the theory of Rossby
wave propagation on the sphere (Hoskins and Karoly, 1981). In particular,
both model and theoretical results show that the responses are
dominated by ultra-long wave components (zonal wavenumber m = 1, 2, and
3), and that the large responses occur for vorticity sources located at
the subtropics and at the high latitudes near 45°N. The model experiments
for the wavenumber-dependent sources located at various latitudes
show that the ultra-long waves behave like a north-south seesaw between
the high and middle latitudes (m = 1) or between the high latitudes and
subtropics (m = 2). The north-south seesaw of zonal wavenumber 1
component is in good agreement with that observed by Gambo and Kudo
(1983). The responses of long waves (m > 3) are, however, localized in
the source regions with relatively small amplitudes.
The characteristics of baroclinic responses to prescribed heat
sources located at various latitudes are also examined. Over the
source latitudes baroclinic responses are dominant; however, the remote
responses have a barotropic structure. The north-south seesaws appearing
in the barotropic model are also observed in the baroclinic model.
A series of baroclinic model experiments, in which surface heat
fluxes and internal heating are computed in terms of the model variables,
are also conducted to investigate the linear effect of
sea-surface temperature (SST) anomalies on the atmospheric circulation.
The experiments for prescribed SST anomalies, taken equal to twice
those of Rasmusson and Carpenter (1982), simulate many aspects of the
associated observed atmospheric anomalies, and suggest, therefore, that
a large part of the atmosphere's responses occur via linear dynamics.
It is also suggested that the rather weak responses in the North
Pacific are due to the lack of a zonally varying basic state. In the
case where the SST anomalies are located in the middle latitudes, the
responses are about five times smaller than for the tropical SST
anomalies. This result is also fairly consistent with the GCM experimental
results by Chervin et al. (1976).
Subsequent experiments, using climatological January SSTs in the
tropics, suggest that the tropical Pacific SST can be an important
factor in maintaining the climatological standing waves, at least over
the western half of the Northern Hemisphere.