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Abstract

A 150 year-long numerical simulation of present-day climate using the Max-Planck Institute's
coupled ocean-atmosphere model ECHAM4-T42-OPYC3 is analysed with regard to the
interannual variability of the strength of the Indian summer monsoon and its relation to land-
surface and ocean interactions. Individual years are categorised into three classes of monsoons:
normal, strong and weak (greater or less than one standard deviation of precipitation over
India). The ensembles of anomalous monsoons are then sub-divided into a composite of cases
coinciding with sea surface temperature (SST) anomalies in the Pacific related to the El Nifio-
Southern Oscillation (ENSO) phenomenon and a second composite of anomalous monsoons
occurring, when no SST anomalies are found in the Pacific. The coupled model shows
variations of the SST in the Pacific which are as large as and occur at a similar frequency as in
observations. Thus it provides the basis for a realistic simulation of the interannual monsoon
variability in ENSO-related conditions, but it overemphasizes the biennial component of
occurrence. As in observations, about a third of all cases of weak monsoons occur in the
summer when an El Nico begins to develop in the Pacific, while strong monsoons are often
associated with La NiNa events. This is an improvement from earlier coupled model
simulations. In the model simulation, a modulation of the strength of the monsoon is due to a
change of the large-scale land/ocean temperature gradient in the Indian Ocean sector in the mid-
troposphere. The two composites show different developments during the annual cycle. In
ENSO-related strong monsoon cases the atmosphere over land warms up during the spring in
association with generally warmer tropics as a remnant from a warm event in the previous
winter. During the summer months the warming in the Indian Ocean region is replaced by a
cooling in association with the developing La NiNa, while the land remains significantly warmer
than normal. Therefore, in the coupled simulation the Indian Ocean only shows very small SST
anomalies, while observed SSTs may vary in connection with ENSO events at a time lag of four
months. Also for non-ENSO related monsoons a warming occurs over land in the summer, but
then neither the Indian Ocean nor the tropical west Pacific exhibit any significant anomalies
during the spring. Simulated temperature anomalies responsible for these modulations are
relatively small. Independent of the origin of the monsoon anomaly, strong monsoons differ
from weak monsoons by a significant precipitation pattern over India and a modification of the
850 hPa zonal wind field over the maritime continent and the West Pacific. Similar to
observations, the anomalous monsoons in the coupled model are related to precursors in the
200 hPa zonal wind field in the spring, but no evidence could be found for a significant
influence from the Eurasian snow pack in the spring on the subsequent Indian summer
monsoon. While the model shows many realistic features, the variation of the monsoon occurs
against a background of a deficient regional rainfall pattern in India and too small a range of
Indian Ocean SST variations in conjunction with ENSO events.