Syllabus: Mains – GS I (Geography)

Why in the News?

• In recent decades, tropical cyclones starting near the Equator have been less destructive.
• The most recent big cyclone like this in the Indian area was Cyclone Okchi in 2017. It caused a lot of damage in Kerala, Tamil Nadu, and Sri Lanka.
• But, in the future, these cyclones might happen more often. This is because of two things: global warming and a repeating event called the Pacific Decadal Oscillation (PDO). The PDO comes back every 20-30 years.

What are Equatorial-Origin or Low Latitude Cyclones?

• Equatorial-Origin or Low Latitude Cyclones (LLCs) are cyclones that start between 5°N and 11°N.
• They are smaller in size than cyclones in higher areas but can get much stronger very quickly.
• Normally, cyclones don't form near the equator (low latitude), but when the water is warm, they can gather more moisture and become more powerful.
• Most of these cyclones begin in the Western Pacific Ocean.
• The last big cyclone like this in the Indian area was Cyclone Ockhi in 2017.
• It traveled more than 2000 km and caused a lot of damage in Kerala, Tamil Nadu, and Sri Lanka.
• In the post-monsoon season (Oct-Nov-Dec), the north Indian Ocean (NIO) is a hotspot for LLCs.
• They make up about 60% of all Tropical Cyclones in the NIO since 1951. However, they haven't received as much attention as they should have.

What is the Pacific Decadal Oscillation (PDO)?

• The Pacific Decadal Oscillation (PDO) is a long-term change in the Pacific Ocean.
• It's like a cycle that happens every 20-30 years, and it has both "cool" and "warm" phases.
• Positive (Warm) PDO: During this phase, the western part of the Pacific Ocean becomes cooler, while the eastern part gets warmer. It's the opposite during a negative PDO.
• The term Pacific Decadal Oscillation (PDO) was first used around 1996 by a scientist named Steven Hare.
• Impact of PDO:
• On Global Climate: The PDO's phase can have a big impact on the world's weather. It affects things like hurricanes in the Pacific and Atlantic, droughts and floods around the Pacific, how well marine life does, and patterns in global land temperatures.
• On Cyclones: When the PDO is warmer (positive phase), there tend to be fewer cyclones that start near the equator.
• In 2019, the PDO started getting cooler, going into a negative phase.
• If this continues, it could mean more of those equatorial-origin cyclones in the months after the monsoon.

ENSO and PDO:

• Positive PDO with ENSO is generally unfavorable, but a negative PDO brings more rain to India.
• When the Pacific Decadal Oscillation (PDO) is in a positive phase along with the El Niño-Southern Oscillation (ENSO), it's often not good.
• However, when the PDO is negative, it brings more rain to India.
• When Both ENSO and PDO Match:
• If both ENSO and PDO are in the same phase, it's believed that the impacts of El Niño or La Niña events may be stronger.

PDO vs. ENSO:

• Difference in Repeating:
• El Niño or La Niña events happen in the Pacific Ocean every 2-7 years, but the PDO has a pattern that lasts much longer, often over decades.
• Identifying Positive PDO Takes Time:
• You can figure out if the PDO is in a "positive" or "warmer phase," but it takes several years of measuring ocean temperatures and their interaction with the atmosphere.
• In contrast, you can determine the stage of an ENSO event in any given year.

What is El Niño-Southern Oscillation (ENSO)?

ENSO is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe.  We also focus on ENSO because we can often predict its arrival many seasons in advance of its strongest impacts on weather and climate.

• The El Niño-Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean.
• On periods ranging from about three to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal.
• This oscillating warming and cooling pattern, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the United States and other parts of the world.
• El Niño and La Niña are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.
• El Niño:  A warming of the ocean surface, or above-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean.
• Over Indonesia, rainfall tends to become reduced while rainfall increases over the central and eastern tropical Pacific Ocean.
• The low-level surface winds, which normally blow from east to west along the equator (“easterly winds”), instead weaken or, in some cases, start blowing the other direction (from west to east or “westerly winds”).
• In general, the warmer the ocean temperature anomalies, the stronger the El Niño (and vice-versa).
• La Niña:  A cooling of the ocean surface, or below-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean.
• Over Indonesia, rainfall tends to increase while rainfall decreases over the central and eastern tropical Pacific Ocean.
• The normal easterly winds along the equator become even stronger.
• In general, the cooler the ocean temperature anomalies, the stronger the La Niña (and vice-versa).
• Neutral: Neither El Niño or La Niña. Often tropical Pacific SSTs are generally close to average.
• However, there are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa).

Mains Question:

• What are the key environmental and geographical factors that influence the formation and distribution of tropical cyclones in various ocean basins around the world, and how do they differ? (15M)