Abstract:
In order to understand the relationship between the El Ni~no-Sourthern Oscillation
(ENSO) and the Indian summer monsoon, the atmospheric response to the tropical
diabatic heating is studied in a general circulation model (GCM). We modify an existing
GCM by adding a relatively small diabatic heating with an idealized structure.
In this method, the atmospheric circulation forced by the inserted forcing interacts
with convection and existing atmospheric circulation to produce a coupled dynamical
convective response (GCM e ect).
In experiments using climatological sea surface temperature (SST), idealized anomalous
cooling over the Maritime continent induces anomalous downward motion (similar
to one branch of Walker circulation during El Ni~no) and asymmetric anti-cyclonic
circulation extending toward India, which opposes the climatological monsoon
ow.
In turn, this anomalous circulation weakens the Indian monsoon precipitation and it
seems to explain the relationship between ENSO and the Indian monsoon. Another
coupled dynamical convective response is the response of the diabatic heating related
to surface convergence, when the idealized heating is over elevated SST. Depending
on the longitudinal and latitudinal location of the forcing, the pattern of atmospheric
response in
uencing the Indian region changes and the sensitivity of the diabatic
heating response in terms of magnitude is also di erent.
In further experiments, a slab ocean model is applied over the western Paci c and
Indian Ocean. The air-sea interaction reduces mean precipitation in the region of
a large rainfall such as the equatorial Indian Ocean. When the idealized heating is
inserted over the warm SST, the response of the full diabatic heating shows a negative
feedback. On the other hand, the idealized cooling shows a positive feedback
of its response. This is because strengthened atmospheric circulation induced by the
idealized forcing (both heating and cooling) releases latent heat
ux from the ocean
to the atmosphere and cools down the ocean temperature. The anomalous cold SST
gives the positive feedback to the heating experiments and the negative feedback to
the cooling experiments.
For the more realistic experiments the idealized heating over the central/ eastern
Paci c is also inserted in the model in addition to the western Paci c cooling.
The diabatic heating response of the wide idealized heating over the central/ eastern
Paci c enhances the western Paci c cooling by anomalous downward motion as compensating
vertical motion over the western Paci c. The Kelvin-wave like response
also appears to propagate eastward all the way to the Indian region, thus interacting
with the monsoon
ow. The observed diabatic heating (diagnosed) of El Ni~no
events is used to simulate the anomalous atmospheric circulation related to the Indian
monsoon. For the normal monsoon of the 1997/98 event the Indian Ocean warming
is added to compensate the e ect of the western Paci c cooling on India. For the
dry monsoon of 1982 event negative precipitation is simulated but the atmospheric
circulation anomaly is too large compared with observation.