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The Indian Ocean Dipole

Small and inconspicuous to the eye, a sprawling number of wingless locusts have begun their lives hopping across the land in Somalia’s arid interior. As they grow and gain flight in the next few weeks, these very insects could form swarms spanning the size of cities like Bangalore—the latest in a series of locust invasions that have affected Eastern Africa since the beginning of the year.  These swarms have been decimating crops across the region, putting food security and the livelihood of thousands of people at risk in what has been dubbed by many as the worst locust plague in decades.

Surprisingly, however, Eastern Africa isn’t the only region being visited by gargantuan locust swarms. Thousands of miles away, Pakistan and North-Western India were in the midst of a similar plight, with the latter alone having seen nearly 4,20,000 acres of land affected by the second week of February. In both Eastern Africa and South Asia, some of the heaviest rains in decades had preceded the vermin influx. Prolonged moisture content in otherwise arid lands had created the perfect breeding grounds for the locust population. Elsewhere, in lands even farther separated, enormous wildfires threatened entire populations of humans and wildlife. Curiously, all of these events could be traced to a common root phenomenon linking them all.

As the climate changes, so do the processes that maintain an equilibrium across the Earth’s surface. Eastern Africa and South Asia are linked by the commonality of the Indian Ocean and the patterns of climate that it governs. Being largely enclosed in nature, the Indian Ocean undergoes an annual cycle during which temperatures alternate between warm and cold along its two longitudinal coasts, a phenomenon called the Indian Ocean Dipole, that has gotten increasingly skewed in recent years.

During Spring, the Sun’s north-bearing movement above the Equator causes temperatures in the tropical Indian Ocean to rise. The Easterly winds, which blow westwards over the tropics, push warm waters towards the west of the Indian Ocean, where Eastern Africa forms a barrier. The warming of water along this coast and its subsequent higher rates of evaporation give rise to more moisture in the skies above it, which eventually translates to high rainfall in the region.

Image credits: BOM, Australia

As the Sun moves north onto the Tropic of Cancer during the peak of Summer, the heat over Southern Asia creates a low-pressure zone that draws out moisture from the Western Indian Ocean, forming the southwest monsoons of India. Meanwhile, complementing the warm west is the Eastern side of the Indian Ocean, where the gathering of displaced cool water occurs, inhibiting the formation of moisture in the atmosphere above it. This is known as a Positive Indian Ocean Dipole, of which a particularly strong episode may result in the prevalence of higher rainfall and weather extremities such as cyclonic storms across the Western Indian Ocean, as seen towards the end of last year.

After the northern hemisphere’s summer, the Sun begins its journey south. During a regular season, as the Sun drops below the Equator and moves towards the Tropic of Capricorn, the Easterlies weaken whereas the Westerlies strengthen, moving the mass of warm water towards the East of the Indian Ocean, while cooling the western side, thereby creating the Negative Indian Ocean Dipole. The timing behind these shifts contributes to a sensitive balance between the wet and dry seasons across the countries adjoining the Indian Ocean. Yet, it is precisely this balance that has been disturbed through various irregularities in recent years.

Image Credits: BOM, Australia

The shift in balance resulted in dire situations in Africa as well as India. Over 2.8 million people had been displaced across Eastern Africa due to heavy flooding and multiple cyclonic storms, with the subsequent locusts simply aggravating an already displeasing situation.  Meanwhile, India had seen a death toll of over 2100 and heavy flooding across several states. The 2019 Indian Monsoons had not only been the strongest since 1994 but had also withdrawn the latest in the history of the country’s monsoons, receding nearly a month after its norm.

However, while the strong positive dipole meant wetness across these lands, something worse had transpired elsewhere. In lands further East across the Indian Ocean from Africa, the waters had been cool for considerably too long. The return of warmth to the coasts of Western Indonesia and Australia was long delayed. Thus, while the heavy rainfall in Africa and ensuing monsoons in India caused widespread flooding and ushered in a locust blitz, large swathes of land across Indonesia and Australia had become dry, with rain forests and bushland becoming vulnerable to combustion. Nearly 50 million acres of land had fallen to wildfires across the two countries, in particular affecting wildlife to an extent where recovery is unlikely. Dozens of human casualties had been recorded, along with an innumerable number of animal species being lost or further endangered. Following the enormous fires that had seen smoke circumnavigate the entire globe back to its origin, Australia soon found itself at the risk of a deluge, from what is, in some places, the heaviest rainfall in thirty years. The pattern had been the same in India. Prior to its strong monsoon in 2019, it witnessed a dry spell that induced its own set of forest fires, as seen in Bandipur National Park, drought across several states and a scarcity of water that brought its fourth-largest city to its knees.

Image credits: BOM, Australia

The climate has been fluctuating in wildly different ways across the globe. Be it crop-devastating locust invasions, flooding, or widespread forest fires, irregularities can manifest themselves in seemingly unrelated ways, despite having a common root. As extreme events grow in frequency and magnitude, as they have in the last few decades, the choice to actively deal with these issues may just decide the fate of mankind.

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