The first part of the ENSO mechanism is usually described as the piling up of war water in the western Pacific due to westerly equatiorial (trade) winds into a warm water pool to produce La Nina conditions.

The second mechanism is a weakening of these westerly atmospheric flows leading to a change in the thermocline profile to produce El Nino conditions.

It seems obvious to ask where this weakening comes from: at latitudes to the west of the warm water pool, to the east or both. Fortunately the laws of causality exclude the possibility that changes come from somewhere else.

Naively this seems to boil down to asking if easterlies are generated in the Indian Ocean pushing the Walker circulation eastward or whether some drop in central Pacific pressure pulls the Walker circulation eastwards?

Are significant amounts of water evaporated from the Pacific warm water pool and then dumped on Australia as is common in strong La Ninas? This would need to be distinguished from the contribution to Australian precipitation from evaporation in the Indian Ocean. This seems to me to be needed for any simple volumetric mass balance description of ENSO.

What is the effect of the Indian Ocean dipole on ENSO?

## Abstract

> The influence of the Indian Ocean Dipole (IOD) on the interannual atmospheric pressure variability of the Indo-Pacific sector is investigated. Statistical correlation between the IOD index and the global sea level pressure anomalies demonstrates that loadings of opposite polarity occupy the western and the eastern parts of the Indian Ocean. The area of positive correlation coefficient in the eastern part even extends to the Australian region, and the IOD index has a peak correlation coefficient of about 0.4 with the Darwin pressure index, i.e. the western pole of the Southern Oscillation, when the former leads the latter by one month. The correlation analysis with seasonally stratified data further confirms the lead role of the IOD. The IOD-Darwin relation has undergone interdecadal changes; in the last 50 years the correlation is highest during the most recent decade of 1990–99, and weakest during 1980–89.

> Saji et al. (1999) found that the coupled ocean-atmosphere phenomenon evolves with an east-west dipole in the SST anomaly, and named it the Indian Ocean Dipole. The Dipole Mode Index (DMI ) is thus defined as the SST anomaly difference between the eastern and the western tropical Indian Ocean (see insets in Fig. 3a for the regions used to compute the DMI ). The changes in the SST during the IOD
events are found to be associated with the changes in the surface wind of the central equatorial Indian Ocean. In fact, winds reverse direction from westerlies to easterlies during the peak phase of the positive IOD events when SST is cool in the east and warm in the west. The effect of the wind is even more significant at the thermocline depths through the oceanic adjustment process (Rao et al. 2002); the ther

The correlation coeffient between the pressure index and the SST index time series is 0.65 (0.74 for June-November) when the latter leads the former by one month.

Fig.3 shows cross correlation coefficients above 2.5 with 99% CL for:

* WP-DMI with a -4.5 month WP lag : 0.3
* Darwin-DMI with a -1 month Darwin lag : 0.4

and coefficients below -0.25 with 99% CL for:

* IOSPL-POSLP with a -2 month IOSPL lag : -0.275
* SOI-DMI with a -2.5 month SOI lag. : -0.4


WP : western Pacific
DMI : dipole mode index
IOSPL : Indian ocean sea level pressure
POSPL : Pacific ocean sea level pressure
SOI : southern oscillation index

During positive IOD events sea level pressure anomalies in the Indonesia-Australia region are positive and those in the western Indian Ocean region are negative.

The Nino3 index has a broad 3-6 year spectral peak whereas the spectral peak of the IOD index is around 2 years.

> Thus the inherent periodicity associated with the IOD events, which is different from that of the Pacific ENSO events, may provide covariability between the IOD and the pressure fluctuation at Darwin, i.e., one pole for the Southern Oscillation. The correlation analysis in the present study clearly supports this hypothesis; the IOD index shows a significant correlation with the sea level pressure anomaly at Darwin. The correlation analysis further shows the lead role of IOD in determining the evolution of such a correlation with the Darwin and central-west Pacific indices.

> Though the IOD correlation with the pressure variations in the eastern Pacific is insignificant, the former has a significant correlation with the SOI, and the pressure difference index of the tropical Pacific. It can be explained by
the fact that pressure variations in the western Pacific will set up anomalous winds that force the oceanic Kelvin waves initiating changes in
the eastern Pacific. The above novel relationship, however, undergoes decadal modulation.

> A significant reduction of the IOD impact on the Darwin pressure is observed for the recent decade; 1980 through 1989. Understanding the physical mechanism that determines such decadal modulations of the ocean-atmosphere coupled system in the Indo-Pacific sector is underway using sophisticated coupled ocean atmosphere models.