*National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan
**Department of Earth and Planetary Physics, Graduate School of Science, The University of Tokyo, Japan; Frontier Research System for Global Change, Research, Tokyo, Japan
It has been pointed out that the Indian Ocean gives birth to a unique coupled ocean-atmosphere mode which may induce unusual rainfall in the surrounding area including the tropical East Africa (Saji et al., 1999). They have called the new climate signal the "Dipole Mode (DM)" based on the sea surface temperature (SST) and corresponding wind anomalies over the tropical Indian Ocean. According to Saji et al.(1999), the DM event actually occurred in 1961, 1967, 1972, 1982, 1994, and 1997.
The DM structure is characterized by the anomalous cold SST in the southeastern tropical Indian Ocean and the anomalous warm SST in the western tropical Indian Ocean. In response to the SST anomalies, atmospheric convection over the eastern (western) tropical Indian Ocean is suppressed (enhanced), and easterly wind anomaly over the central Indian is intensified. The anomalous wind related to the DM event causes a shoaling (deepening) of the thermocline in the eastern (western) equatorial Indian Ocean. This results in the cooling (warming) of SSTs in the east (west) by increasing (decreasing) entrainment from below the thermocline. Thus, the positive feedback due to the air-sea interaction is crucial in the variability related to the DM event. This situation is quite similar to the ENSO in the tropical Pacific.
We are successful in simulating the DM events for the first time, using a coupled general circulation model (CGCM) without flux correction. Also, we have reproduced the DM event in 1982, 1994, and 1997, using an ocean general circulation model (OGCM) which is the oceanic part of CGCM. In the meeting, we first present the DM event simulated using a high resolution CGCM (cf. Matsuura et al., 1999) and an OGCM. Then we analyze the heat budget to clarify the manner in which the SST anomalies are formed.
The temporal evolution and spatial patterns of the simulated DM events in CGCM are mostly in good agreement with observational evidence and the OGCM result in 1994. The simulated DM events are clearly independent of the model El Nino events in the present coupled model (Iizuka et al., 2000). The observational analysis also suggests that the DM event may occur independently of the ENSO as a unique air-sea coupled phenomenon over the tropical Indian Ocean (Saji et al., 1999; Webster et al., 1999). The heat budget analysis demonstrates that an air-sea interaction, strongly influenced by the ocean dynamics, is essential in the evolution of the model DM event.
