Role of Indian Ocean Dipole (IOD) in Indian Monsoon

Influence of Indian Ocean Dipole (IOD) on Climate

The Indian Ocean Dipole (IOD) is characterized by the variance in sea surface temperature between two distinct regions – a western region situated in the Arabian Sea (western Indian Ocean) and an eastern counterpart located in the eastern Indian Ocean to the south of Indonesia. This arrangement resembles a dipole due to the presence of two distinct poles, leading to its nomenclature. The IOD is referred to as the 'Indian Nino' as it arises from alterations in the thermal and atmospheric conditions of the Tropical Indian Ocean.

The IOD can manifest in two forms: positive IOD and negative IOD, each exerting an impact on Indian summer monsoon rainfall. The occurrence of a positive IOD arises when the sea surface temperatures register higher than usual levels in the Arabian Sea, while simultaneously showing lower temperatures than normal in the tropical eastern Indian Ocean. Conversely, a negative IOD occurs when this temperature pattern is reversed.

A positive IOD engenders heightened monsoon rainfall and an increased frequency of active monsoon days (characterized by above-normal rainfall) across the Indian subcontinent. In contrast, a negative IOD leads to diminished rainfall levels and an augmentation in monsoon break days (characterized by no rainfall).

The Indian Ocean Dipole (IOD)

The Indian Ocean Dipole (IOD), which has a significant impact on the climate of Australia and neighboring countries within the Indian Ocean Basin, is characterized by the disparity in sea surface temperature between two distinct regions. One region is situated in the western portion of the Arabian Sea (western Indian Ocean), while the other is positioned south of Indonesia in the eastern Indian Ocean. The IOD exerts a notable influence on rainfall patterns in the surrounding Indian Oceanic region.

Exploring the Indian Ocean Dipole

The term "Indian Ocean dipole" signifies a phenomenon where two poles of the same substance exhibit contrasting qualities. This phenomenon is observed in the Indian Ocean and pertains to the sea surface temperatures on opposing sides of the ocean, each characterized by a distinct temperature profile.

The western segment of the Indian Ocean experiences warmer temperatures (positive phase) followed by a cooling trend (negative phase) relative to the eastern Indian Ocean during the cyclic fluctuation of sea surface temperatures known as the Indian Ocean Dipole (IOD), also referred to as the Indian Nino. This phenomenon is a feature of the tropical region encompassing the Indian Ocean.

When the western side of the ocean becomes warmer, the eastern side turns cooler, and vice versa. This oscillating temperature behavior in the Indian Ocean is labeled as the Indian Ocean Dipole. This pattern of temperature variability bears similarity to the El Nino and La Nina events in the Pacific Ocean.

Mechanism of the Indian Ocean Dipole

The Indian Ocean, renowned as the warmest among the planet's five oceans, is encircled by land on three sides. The movement of tropical easterlies impacts the oceanic waters, with warm waters from the Pacific Ocean entering the Indian Ocean through the channels between the Australian and Indonesian islands on the eastern side.

The tropical easterlies travel from the northeastern regions of the northern hemisphere to the southeastern parts of the southern hemisphere. These winds, found within the latitude range of 0 to 30 degrees, play a crucial role in affecting the surface temperature of the ocean. Importantly, the Indian Ocean remains isolated from the Arctic seas due to its confinement by the Eurasian Landmass from the north.

The interaction between the waters of the Pacific Ocean and those of the Indian Ocean causes shifts in the temperature of the latter. These temperature variations determine the distinct phases of the IOD:

Neutral Indian Ocean Dipole Positive Indian Ocean Dipole Negative Indian Ocean Dipole

During the Neutral Indian Ocean Dipole phase, which coincides with spring when the sun's energy is evenly distributed between the northern and southern hemispheres, easterly winds flow between 0 and 30 degrees North and South of the equator. These easterlies or trade winds impact the temperature distribution across the eastern and western Indian Oceans, leading to uniform temperature levels. This phase is known as the Neutral Period of the Indian Ocean Dipole (IOD).

The Positive Indian Ocean Dipole manifests as a result of reduced westerly winds near the equator, facilitating the flow of warm water towards Africa. Concurrently, the deep ocean releases cold water, contributing to a temperature disparity in the tropical Indian Ocean. During this phase, the eastern region experiences colder water while the western region witnesses warmer waters. This positive dipole occurrence in the Indian Ocean is believed to have a favorable influence on the monsoon.

In contrast, the Negative Indian Ocean Dipole involves the intensification of westerly winds along the equatorial line, leading to the accumulation of warmer waters near Australia. This temperature variation impacts the tropical Indian Ocean, resulting in warmer-than-average waters in the eastern sector and cooler-than-average waters in the western region. This phenomenon hampers the progress of the monsoon across India.

Impact of Indian Ocean Dipole on the Indian Monsoon

Studies indicate that central India often experiences above-average rainfall during IOD years. Notably, a high IOD index tends to counteract the influence of ENSO (El Nino-Southern Oscillation) during various ENSO years, such as 1983, 1994, and 1997, thereby enhancing monsoon rainfall. Furthermore, both the eastern and western poles of the IOD, located near Indonesia and off the coast of Africa respectively, independently and cumulatively impact the quantum of rainfall during the Monsoon period in the Indian subcontinent.

A linkage exists between below-normal sea surface temperatures (SST) in the eastern Indian Ocean and higher-than-normal rainfall in central India. Conversely, the interaction between a negative IOD and El Nino can lead to severe drought conditions. Positive IOD events result in an increased frequency of cyclones in the Arabian Sea, while negative IOD events contribute to enhanced cyclogenesis (formation of tropical storms) in the Bay of Bengal. The Indian Ocean Dipole and the Monsoon phenomenon exhibit a unique interplay.

Impact of Indian Ocean Dipole on India

Notable IOD effects, such as those observed in 1997 and 1998, have triggered exceptionally heavy monsoons across the Indian Subcontinent. Such effects are often attributed to a large positive IOD. Similar outcomes were witnessed in 2006. Since 1980, there have been approximately 12 instances of positive IODs, with only one significant negative IOD occurring in 2010. The recurrent occurrence of positive IOD events is noteworthy and raises concerns, as it has contributed to events like the Black Saturday bushfires.

An exceptional instance occurred in 2007 when a positive IOD coincided with La Nina. This event was highly uncommon, having occurred only once previously, in 1967. The combination of a potent negative IOD and a robust La Nina led to catastrophic flooding in Queensland during 2010–2011 and in Victoria in 2011.

Predictive models suggest that clusters of consecutive positive IOD events are expected to arise approximately twice every 1000 years. The 20th century has also witnessed an upsurge in the frequency, strength, and prevalence of positive IOD occurrences.

 

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