What would happen if the difference in pressure in Tahiti was negative?

The correct option is Below average and late monsoon.

Step-by-Step Explanation:

1. Understanding the Southern Oscillation Index (SOI)
The Southern Oscillation Index (SOI) is a key indicator used to monitor the El Niño-Southern Oscillation (ENSO) cycle. It is calculated based on the differences in monthly sea-level pressure anomalies between Tahiti (representing the central/eastern Pacific) and Darwin, Australia (representing the western Pacific/eastern Indian Ocean).

2. Meaning of a Negative Pressure Difference
When the pressure anomaly in Tahiti is negative compared to Darwin, the resulting SOI is negative. A sustained negative SOI value indicates the presence of El Niño conditions, where warm waters shift eastward across the equatorial Pacific, and the atmospheric pressure over Tahiti is lower than normal.

3. Impact on the Monsoon
Under El Niño conditions, the normal tropical atmospheric circulation (Walker Circulation) is weakened. This disruption has a direct impact on monsoon systems, such as the Indian Summer Monsoon:
- The trade winds that carry moisture-laden air from the oceans toward the subcontinent weaken.
- The onset of the monsoon winds is delayed, leading to a late monsoon.
- The overall moisture transport is reduced, causing below-average seasonal rainfall.

4. Conclusion
Therefore, a negative pressure difference in Tahiti leads to a below average and late monsoon.