By Dr Kyaw (Meteorology School)

 

Difficult to Define?

A monsoon is not merely a period of heavy rain. It is a large-scale weather system driven by a seasonally reversed circulation, a massive shift in wind patterns caused by temperature differenc­es between land and ocean. This system is a magnificent natural mechanism for redistributing en­ergy across the globe.

 

Consequently, pinpointing the monsoon's onset and withdrawal requires more than just measur­ing rainfall. It demands an under­standing of the complex interplay between wind direction, atmos­pheric humidity, air pressure, sea surface temperature, and kinetic energy. Due to this complexity, meteorologists have developed increasingly sophisticated indices to define these transitions with ever greater precision.

 

The Evolution of Indices: From Traditional to Modern Science

The history of monsoon indi­ces reflects our growing under­standing of climate. Early agrari­an communities relied on natural signs — like trees shedding leaves, birds nesting, or wind shifts to pre­dict the monsoon. With the estab­lishment of meteorological depart­ments, indices based on precise rainfall measurements, such as India's Kerala-based index, were introduced.

 

Following the 1960s, the ad­vent of computer models and sat­ellite data allowed scientists to incorporate global variables like wind flow, moisture transport, and atmospheric energy. Modern indi­ces, such as the Webster & Yang Index, became more complex but also more accurate.

 

Accurately determining the onset and withdrawal of the mon­soon is crucial for regional eco­nomic planning, agriculture, and water management. However, de­claring a specific "monsoon onset" or "withdrawal" day is inherently complex, as it involves analyz­ing a multi-layered physical pro­cess. Therefore, meteorologists increasingly rely on indices that encapsulate these various physi­cal perspectives, a culmination of decades of scientific debate and refinement.

 

This article will summarize the development, methodology, strengths, and weaknesses of key indices, tracing their evolution from early rainfall-based meas­ures to modern indices incorpo­rating complex physics.

 

Definition of Key Monsoon Indices

(1) The Kerala-Based Index

• Historical Development: Since the late 1960s, the India Meteorological De­partment (IMD) has used the "Monsoon Onset over Kerala" (MOK) definition. This index represents an early approach that for­malized rainfall observa­tions with mathematical criteria.

• Methodology: The onset is declared when prede­fined stations in Kerala report rainfall exceed­ing a specific threshold (e.g., 2.5 millimetres) for two consecutive days, alongside a sustained shift in wind direction to south-westerlies.

• Strengths: It is simple, intuitive, and directly relevant for agricultural planning.

• Weaknesses: It can be triggered by pre-mon­soon thunderstorms, leading to false starts. As a regional measure, it poorly represents the dynamics of the broader monsoon system (e.g., it cannot predict the onset over Myanmar).

 

(2) The Webster & Yang Index (W&Y Index)

• To overcome the limita­tions of rainfall-only indi­ces, scientists developed indices like the W&Y In­dex, which uses wind and energy variables to better capture the global nature of the monsoon.

• Historical Develop­ment: Introduced in 1992 by meteorologists Peter Webster (UK) and Yang Song (China), this index defines monsoon transitions by analyzing the pronounced shift in wind speed and direction between upper and lower atmospheric levels over a broad Asian region (Web­ster & Yang, 1992).

• Methodology: The index is calculated as the dif­ference in zonal wind be­tween the lower (850 hPa) and upper (200 hPa) trop­osphere: W&Y Index = U (850 hPa) - U (200 hPa). A significant change in this value marks the onset or withdrawal.

• Significance: It is a dy­namic precursor that measures the core char­acteristic of the monsoon: the seasonal wind rever­sal.

 

(3) The Masumoto Index

• Historical Develop­ment: Japanese me­teorologist Masumoto proposed this index in 1997 for the Southeast Asian region, utilizing 5-day averaged (pentad) rainfall data. Notably, his­torical records indicate a Myanmar meteorolo­gist contributed to this project (Masumoto et al., 1997).

• Methodology: The onset is defined when the area's average pentad rainfall exceeds the annual mean (e.g., 11 mm) and persists for a specified number of consecutive pentads. The middle day of the first qualifying pentad is des­ignated the onset date.

• Significance: While rainfall-based, it intro­duces the concept of persistence, helping to distinguish the genuine monsoon season from short-lived storm events. Myanmar's Department of Meteorology and Hy­drology currently uses this index as a primary tool.

 

(4) The Myanmar-Indochina Southwest Monsoon (MSWM) Index

Post-2000, with rapid ad­vancements in climate science, researchers began focusing on physically coherent variables, such as moisture transport, tailored to specific regions.

• Historical Develop­ment: Around 2020, our research team proposed the MSWM Index for the Indochina Peninsula. It combines data on mois­ture, rainfall, pressure gradients, and cloudi­ness, using a Change Point Index (CPI) based on Vertically Integrat­ed Moisture Transport (VIMT) (Kyaw et al., 2020).

• Methodology: It identi­fies the date of a sudden change (a "change point") in the VIMT over the re­gion. A shift from neg­ative to positive values indicates onset, while the reverse marks withdraw­al.

• Significance: Though computationally com­plex, VIMT is a physical­ly robust variable. The MSWM Index is well-suit­ed for modern climate modelling and comput­er-based calculations.

 

(5) The East Asian Summer Mon­soon (EASM) Index

The East Asian monsoon, vital for China, Japan, and Korea, is an­other complex system discussed extensively by experts.

• Historical Develop­ment: Among several indices, one proposed by Chinese meteorologist William K.-M. Lau in 2000, which uses Zonal Wind Shear, is widely used for its accuracy (Lau, 2000).

• Methodology: Similar to the W&Y Index, it cal­culates the wind shear (difference) between U (850 hPa) and U (200 hPa) over a defined East Asian region.

• Significance: It directly correlates with region­al weather systems like China's Meiyu, Korea's Changma, and Japan's Baiu frontal systems, al­lowing for precise track­ing of the East Asian monsoon.