Intraseasonal Oscillation in the Indian Summer Monsoon Simulated by Global and Nested Regional Climate Models

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Abstract
Simulations of the intraseasonal oscillation (ISO) in the Indian summer monsoon by a general circulation model (GCM) and a nested regional climate model (RCM) are described. The ISO is the leading mode of subseasonal variability in both models. It is shown to be associated with circulation and precipitation anomalies that propagate northward from the equatorial Indian Ocean to the foothills of the Himalayas on the 30–50-day timescale. The spatial structure, timescale, and propagation characteristics of the simulated ISO are found to be similar to those of the leading observed intraseasonal mode. In particular, both of the simulated versions and the observed version all involve periodic deepening and filling of the monsoon trough resulting from northward propagation of troughs and ridges from the equatorial region. Some differences do occur, however: the GCM version of the ISO is too zonally symmetric and the ISO is too strong in both models. During the positive phase of the ISO (i.e., when the ISO acts to enhance the monsoon trough), composite low-level circulation anomalies in the monsoon trough region are found to be somewhat weaker in the RCM than in the GCM because the RCM signal is obscured to a greater degree by noise associated with other modes of variability. In the GCM, large precipitation anomalies are found to be associated with the positive and negative phases of the ISO in many areas, particularly at the latitudes of the monsoon trough. However, the use of a fine-resolution nested RCM leads to the identification of important spatial detail not present in the GCM distributions. This is particularly true in mountainous regions, most notably in the foothills of the Himalayas: here the RCM simulates a strong precipitation signal, which appears to represent an orographic component of the response to circulation anomalies associated with the ISO, whereas this precipitation signal is absent in the GCM. The use of a nested RCM also allows the phase relationship between the oscillations in the two models to be studied. The relationship is found to be close in most years, suggesting that the regional ISO in the RCM is modulated by the driving GCM circulation via the lateral boundary forcing on the 30–50-day timescale. Several examples are also found, however, where the GCM and RCM diverge, showing that the northward-propagating mode can occur independently of any global forcing on the same timescale, in agreement with observational evidence.

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