The Indian monsoon system
Note
A seasonal monsoon forecast is a probability, not a date, because the system is a planetary land-sea thermal contrast modulated by remote ocean states such as ENSO and the Indian Ocean Dipole. Intense summer heating over northwest India draws the inter-tropical convergence zone north, reversing the pressure gradient and pulling moist southwesterlies across the Arabian Sea and Bay of Bengal. Elevated heating of the Tibetan Plateau and the resulting tropical easterly jet sustain the circulation; the cross-equatorial Somali jet feeds it. Retreat brings the northeast monsoon, the main rainy season for southeastern India.
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Questions on this topic
Consider the following statements regarding the northeast (retreating) monsoon over peninsular India and the factors that modulate its rainfall activity:
- The northeast monsoon season over peninsular India spans October to December, with October typically contributing the major share of seasonal rainfall in states like Kerala.
- The Madden-Julian Oscillation (MJO) in a suppressed phase over the Bay of Bengal region tends to inhibit cyclogenesis and reduce rainfall activity during the northeast monsoon season.
- The northeast monsoon draws its primary moisture from the Arabian Sea, and weakening of Arabian Sea branch winds is the dominant factor in reducing rainfall over Kerala during this season.
Which of the statements given above is/are correct?
Explanation
Statement 1 is correct: the northeast monsoon season over peninsular India runs from October to December, and October historically accounts for the largest share of total seasonal rainfall in Kerala — a well-established climatological pattern. Statement 2 is correct: the Madden-Julian Oscillation operates in active and suppressed phases that alternate roughly every 30–60 days. When the MJO moves into a suppressed phase over the Indian Ocean–Bay of Bengal region, it weakens convective activity, suppresses cyclogenesis in the Bay of Bengal, and reduces precipitation — thereby diminishing one of the northeast monsoon's key moisture-delivery mechanisms for peninsular India. Statement 3 is incorrect on a fundamental point of monsoon geography: the northeast monsoon over Kerala and the rest of peninsular India draws its primary moisture from the Bay of Bengal, not the Arabian Sea. Easterly winds carry this Bay of Bengal moisture onshore. The Arabian Sea is a key source for the southwest monsoon, not the northeast monsoon. It is the weakening of easterly winds and the absence of Bay of Bengal weather systems — not Arabian Sea branch winds — that suppresses northeast monsoon rainfall over Kerala.
Consider the following statements regarding the Indian monsoon system — its onset, progression, and withdrawal:
- The large-scale Indian monsoon system is conventionally considered to first become established over the Kerala coast, as the Bay of Bengal branch drives the initial planetary-scale organisation of rainfall.
- The Indian monsoon's withdrawal, which begins over Northwest India, proceeds more rapidly across the interior of the country than it does over the southeastern tip of the peninsula.
- A local rainfall event at a given location can precede the large-scale monsoon onset at that location, because the large-scale onset is defined by the integration of local rainfall into a planetary-scale interconnected system rather than by the mere occurrence of first rainfall.
Which of the statements given above is/are correct?
Explanation
Statement 1 is incorrect. The conventional IMD operational definition does treat Kerala as the point of monsoon onset over the Indian peninsula, but climatological and network-based research now indicates that the first large-scale organisation of the Indian monsoon system actually begins over Northeast India (around mid-May), driven partly by thunderstorm clusters and a northward-moving Bay of Bengal system — not over Kerala. Kerala marks peninsular onset, not the first planetary-scale onset. Statement 2 is correct. The monsoon's large-scale withdrawal begins in Northwest India (around late September) and retreats rapidly across much of the country within about a month, but its retreat over the extreme southeastern tip of the peninsula continues into December — meaning the southeastern tip lags far behind the interior, confirming that withdrawal over the interior is comparatively rapid. Statement 3 is correct. This reflects the fundamental conceptual distinction between local rainfall onset and large-scale monsoon onset. Local or transient weather events can produce rainfall at a location before that location's rainfall becomes embedded within the broader, planetary-scale monsoon system. Large-scale onset is defined as the moment when local rainfall onset becomes part of the largest interconnected rainfall cluster — a 'phase transition' in the climate network — not simply the first occurrence of rain. This is why Northeast India and the southwestern peninsula may have locally defined rainy seasons that are 20–30 days longer than their large-scale monsoon seasons.
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