De. Waliser et al., SHORTWAVE FEEDBACKS AND EL-NINO-SOUTHERN-OSCILLATION - FORCED OCEAN AND COUPLED OCEAN-ATMOSPHERE EXPERIMENTS, J GEO RES-O, 99(C12), 1994, pp. 25109-25125
Changes in tropical sea surface temperature (SST) can produce changes
in cloudiness that modify incoming solar shortwave (SW) radiation, whi
ch in turn affects SST. The effects of this negative feedback on Pacif
ic interannual variability are examined in forced ocean model and hybr
id coupled ocean-atmosphere model simulations. Two empirical schemes a
re used to model the large-scale, low-frequency response of surface SW
to SST anomalies. The first scheme attempts to account for the nonloc
al nature of the atmospheric response to SST based on patterns of cova
riability analyzed through singular value decomposition. In the observ
ations the primary coupled mode of variability is composed of a SW ano
maly in the central Pacific that covaries with anomalous SST in the ea
stern Pacific. This is applied in the model as a nonlocal SW feedback.
The second scheme examines the effects of a purely local feedback wit
h a spatially varying coefficient of magnitude chosen similar to the f
irst scheme. In almost all cases the second scheme behaved similarly t
o the first, presumably because the correlation scale of SST is large
enough for El Nino-Southern Oscillation (ENSO) dynamics that there is
little sensitivity to the local approximation in the SW feedback. In s
imulations forced by time series of observed wind stress the SW feedba
ck induced very minor SST damping. Results from a simplified heat budg
et analysis showed that while the SW feedback increased the local heat
flux damping on SST, it also induced a mean shallowing of the mixed l
ayer. The resulting changes in both the local mean vertical temperatur
e gradient and the zonal velocity response to the wind stress acted to
oppose the local heat flux damping effects. When observed SW anomalie
s were applied to forced simulations, the simulated SST anomalies were
modified as expected, and agreement with observed SST improved. In co
upled simulations the SW feedbacks had greater impact than in the case
of specified stress. The main effects were to decrease the magnitude
of the warm and cold SST anomalies in the central Pacific, while leavi
ng the pattern and evolution of ENSO anomalies essentially unchanged e
lsewhere. The SW feedbacks thus produce a modest improvement of the mo
del ENSO SST pattern compared with observations, although they tended
to shorten the period of the model ENSO cycle. Overall the results sug
gest that large-scale SW feedbacks are of quantitative importance to s
imulating some aspects of the ENSO cycle but are not critical to the o
verall occurrence of the phenomenon.