The Atmospheric In uence of Tropical Diabatic Heating Associated with Developing ENSO on Indian Monsoon

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.