ENSO (El Nino) and Indian Ocean Dipole

Subject: Geography

Context:

The India Meteorological Department has forecast rainfall during August-September to be 104% of long-period average with a model error of ± 8%. The prediction assumes “neutral” ENSO (El Nino) and Indian Ocean Dipole conditions continuing during the remaining part of the monsoon season

Concept:

ENSO (El Nino)

• 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.
• Though ENSO is a single climate phenomenon, it has three states, or phases, it can be in.  The two opposite phases, “El Niño” and “La Niña,” require certain changes in both the ocean and the atmosphere because ENSO is a coupled climate phenomenon.
• 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 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”).
• 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 tropical Pacific Ocean.  The normal easterly winds along the equator become even stronger.
• Neutral:  Neither El Niño or La Niña. Often tropical Pacific SSTs are generally close to average.

Indian Ocean Dipole

• The Indian Ocean Dipole was so strong that it overrode concerns of a drought in India last June and brought torrential rainfall — the most India has seen in decades.
• It also lasted nearly a month more than what is normal. This extended rainfall continued in several parts of West Asia, Oman, Yemen and in the Horn of Africa — Ethiopia, Somalia, Kenya — so much so that that the dry sand became heavily moisture laden, facilitating the formation of several locust swarms.
• While this dipole was beginning to take shape by late 2018 — and locust outbreaks were growing in Africa — it increased last year.
• Due to favourable winds, it helped swarms to fly and breed in traditional grounds in Iran, Afghanistan, Pakistan and India.

Indian Ocean Dipole

• Sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as the Indian Ocean Dipole or IOD.
• The IOD is one of the drivers of Indian Monsoon. The IOD has three phases: neutral, positive and negative
• Neutral IOD phase: Water from the Pacific flows over to East Indian Ocean ( between the islands of Indonesia). Air rises above this area and falls over the western half of the Indian Ocean basin, blowing westerly winds along the equator. Temperatures are close to normal across the tropical Indian Ocean, and hence the neutral IOD does not affect the Indian Southwest Monsoon.
• Positive IOD phase: Westerly winds weaken along the equator allowing warm water to shift towards Africa. Changes in the winds also allow cool water to rise up from the deep ocean in the east. This sets up a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west. Generally this means there is more moisture than normal in the atmosphere over West Indian Ocean & Arabian Sea. This changes the path of weather systems coming towards India, often resulting in more rainfall during Southwest Monsoon.

• Negative IOD phase: Westerly winds intensify along the equator, allowing warmer waters to concentrate near Equatorial East Indian Ocean. This sets up a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west. A negative IOD typically adversely affects the Indian Southwest Monsoon rainfall resulting in below-average rainfall over India.