Understanding Pressure Differences and Monsoon Patterns

The question asks what happens if there is a negative difference in pressure in Tahiti and its effect on monsoon. This relates to a significant climate phenomenon known as the Southern Oscillation, which involves fluctuations in the air pressure difference between the tropical eastern Pacific (like Tahiti) and the western Pacific (like Darwin, Australia).

The Southern Oscillation Index (SOI)

The Southern Oscillation Index (SOI) is a measure used to track the strength and phase of the Southern Oscillation. It is calculated based on the difference in sea-level pressure between Tahiti and Darwin. Specifically, it's usually the standardized difference between the monthly mean sea-level pressure anomaly at Tahiti and Darwin.

• Positive SOI: Generally indicates higher pressure in Tahiti and lower pressure in Darwin. This is associated with La Niña conditions.
• Negative SOI: Generally indicates lower pressure in Tahiti and higher pressure in Darwin. This is associated with El Niño conditions.

A "negative difference in pressure in Tahiti" in the context of the Southern Oscillation often implies that the pressure in Tahiti is lower than average, especially compared to Darwin. This situation contributes to a negative SOI, which is characteristic of El Niño.

El Niño and its Impact on Monsoons

El Niño is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean. It is linked to the negative phase of the Southern Oscillation (negative SOI). El Niño has widespread impacts on weather patterns across the globe, including affecting monsoon systems in various regions, particularly in parts of Asia.

During an El Niño event (associated with a negative pressure difference pattern involving low pressure in Tahiti relative to Darwin):

• The typical atmospheric circulation patterns are disrupted.
• Rainfall shifts away from the western Pacific towards the central and eastern Pacific.
• This often leads to droughts or significantly reduced rainfall in regions that typically receive rain from monsoon systems, such as parts of Southeast Asia, Australia, and notably, the Indian subcontinent.
• The monsoon season in these affected areas tends to be weaker (below average rainfall) and sometimes starts later than usual (late monsoon).

Conversely, during La Niña (associated with a positive SOI), the pattern is often reversed, leading to stronger monsoons and above-average rainfall in many of these regions.

Analysing the Options

Given that a negative pressure difference in Tahiti (contributing to a negative SOI and El Niño) is associated with disruptions to typical monsoon patterns, let's look at the options:

• Above average late monsoon: El Niño usually means below-average rainfall, so "above average" is unlikely.
• Drought: El Niño often causes drought conditions in monsoon-dependent regions due to reduced rainfall. This is a possible outcome.
• Below average and late monsoon: This precisely matches the typical impact of El Niño on many monsoon systems – less rain than usual and a delayed start.
• Above average and early monsoon: El Niño is associated with weaker and later monsoons, not stronger and earlier ones. This option describes conditions more typical of La Niña.

Based on the well-documented effects of El Niño on monsoons, a negative pressure difference pattern in Tahiti is strongly linked to a below-average and late monsoon.

Conclusion

A scenario involving a negative difference in pressure in Tahiti, particularly when considered in the context of the pressure difference with Darwin (contributing to a negative SOI), points towards El Niño conditions. El Niño is known to negatively impact many monsoon systems, typically resulting in less rainfall (below average) and a delayed start to the monsoon season (late monsoon).

Therefore, if the difference in pressure in Tahiti is negative (implying conditions leading to a negative SOI/El Niño), the likely consequence for a monsoon system sensitive to these patterns is a below average and late monsoon.

Revision Table: Southern Oscillation and Monsoon

Additional Information: ENSO Cycle

The Southern Oscillation is part of a larger climate phenomenon called the El Niño-Southern Oscillation (ENSO) cycle. ENSO has three phases:

• El Niño: Characterized by warmer-than-average sea surface temperatures in the eastern and central tropical Pacific Ocean and a negative SOI.
• La Niña: Characterized by cooler-than-average sea surface temperatures in the eastern and central tropical Pacific Ocean and a positive SOI.
• Neutral: Conditions are near average.

The ENSO cycle is a major driver of global climate variability and significantly influences temperature and precipitation patterns worldwide, including the strength and timing of monsoon seasons in many parts of the world.