Figure 5. Linear regression between the principal component of the leading EOF mode and the anomalies in (a) sea level pressure, (b) surface heat flux, and (c) SST. Vectors represent regression coefficients between the principal component and zonal and meridional wind stress anomalies and are in units of dyn/cm2. Contours intervals are 0.1 mb for (a), 0.2 W/m2 for (b), and 0.05° K for (c). Positive values are shaded. Positive (negative) values in (b) indicate less (more) surface heat flux out of the ocean. (From Yu et al., 2000.)

fluxes associated with the altered atmospheric circulation during ENSO events.

5. Impacts of Indian Ocean on the

ENSO Cycle

As mentioned earlier, variations in the Indian Ocean may be capable of influencing ENSO activity through either the atmospheric circulation or the oceanic throughflow. To examine this possibility, ENSO simulations were contrasted between the Indo-Pacific Run and the Pacific Run. The former run includes the effects of Indian Ocean SST variability on ENSO, while the latter excludes that effect. Findings obtained from this comparative study were reported by Yu et al. (2002) and Yu (2005). It was found that the magnitude and frequency of the ENSO cycle are affected by the addition of an interactive Indian Ocean in the CGCM. Figure 6 compares the time series of the NINO3 index (SST anomalies in 90°-150°W; 5°S-5°N) calculated from the two runs. The first noticeable difference in this figure is that the Indo-Pacific Run produces stronger ENSO amplitudes than the Pacific Run. The standard deviation of the NINO3 SST anomalies is increased from 0.50°C for the Pacific Run to 0.78°C for the Indo-Pacific Run. The latter is closer to the observed value. The second noticeable difference is that the Indo-Pacific Run has a greater decadal variation in ENSO intensity than the Pacific Run. The former run can be broadly separated into two "strong variability decades" (years 10-21 and 38-52) with large warm and cold events and a "weak variability decade" (years 22-37) with weak warm and cold events. No such clear decadal differences are present in the Pacific Run. It appears that by including the Indian Ocean, the CGCM produces more realistic ENSO amplitude and stronger variability on decadal time scales.

The changes in the ENSO cycle when the Indian Ocean coupling is included may be expected, as the CGCM can now resolve

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