Atlantic Nino

Why in news?

• IITM Pune researchers have shown that the IMD prediction system is deficient in predicting the Atlantic Niño and, therefore, its effect on the Indian monsoon. Monsoon 2021 is a clear example of this missed link.

What is Atlantic Nino?

• The Atlantic Equatorial Mode or Atlantic Nino or Atlantic Zonal Mode is a quasiperiodic interannual climate pattern of the equatorial Atlantic Ocean.
• It is the dominant mode of year-to-year variability that results in alternating warming and cooling episodes of sea surface temperatures accompanied by changes in atmospheric circulation.
• The term Atlantic Niño comes from its close similarity with the El Niño-Southern Oscillation (ENSO) that dominates the tropical Pacific basin.
• For this reason, the Atlantic Niño is often called the little brother of El Niño.
• The Atlantic Niño is characterized by a sea surface temperature anomaly centered on the equator between 0° and 30°W.
• This sea surface temperature anomaly is closely related to a change in the climatological trade winds. A warm anomaly is associated with relaxed trade winds across a large swath of the equatorial Atlantic basin, while a cool anomaly is associated with enhanced easterly wind stress in the same region.
• These trade wind fluctuations can be understood as the weakening and strengthening of the Atlantic Walker circulation.
• The equatorial warming and cooling events associated with the Atlantic Niño are known to be strongly related to rainfall variability over the surrounding continents, especially in West African countries.

El Nino versus Atlantic Nino:

• Unlike El Niño, the Atlantic Niño does not have sea surface temperature anomalies that switch sign from east to west, but rather a single basin-wide anomaly. Additionally, the amplitude of the Atlantic Niño tends to be about half that of El Niño.
• El Niño usually builds up slowly during northern summer before reaching maximum strength in late fall or winter, bringing a wide range of climate impacts throughout much of the planet.
• Atlantic Niño, in contrast, tends to peak in summer when ENSO is usually inactive, is usually shorter in duration, is overall much weaker than ENSO, and has more modest and local climate impacts.
  • For example, Atlantic Niño often disrupts the West African summer monsoon, leading to reduced rainfall in the Sahel region, and is linked to increased frequency of flooding in northeastern South America and the West African sub-Sahel countries bordering the Gulf of Guinea.

How Atlantic Nino affect monsoon in India?

• Atlantic Niño’s impact on the monsoon has been known since 2014 when a study led by INCOIS showed that the number of low-pressure systems is greatly reduced by the Atlantic Niño, leading to deficit monsoons.
• In 2021, Atlantic Niño has made an appearance. Sea surface temperatures in the eastern Atlantic have remained more than a degree higher than normal this summer >> This year has seen a sharply lower number of low-pressure systems in Indian ocean, which contribute up to 60 per cent of the seasonal total rainfall over the core monsoon zone.
• The Atlantic and Indian Oceans are not directly connected in the tropics via the ocean.
• The Atlantic Niño affects the monsoon by producing atmospheric waves, which propagate into the Indian Ocean.
• These waves affect air temperatures over the Indian Ocean and influence the land-ocean thermal contrast as well as Low Pressure Systems (LPSs). The biggest rainfall deficits from the Atlantic Niño tend to occur over the Western Ghats and the core monsoon zone.
• The deficit patterns are a sign of the Atlantic Niño’s influence.

Add ons:

• Monsoon forecast models in India rely heavily on El Niño. But only about 50 per cent of the dry years are explained by El Niño. Therefore monsoon prediction during non-El Niño years will be a challenge. Clearly, Atlantic Niño is a significant player in monsoon evolution and models and forecasters must pay attention to this.

PRACTICE QUESTION:

Consider the following statements:

1. The Atlantic Nino affects the monsoon by producing atmospheric waves, which propagate into the Indian Ocean

2. Unlike El Nino, the Atlantic Nino does not have sea surface temperature anomalies that switch sign from east to west, but rather a single basin-wide anomaly.

Which of the statements given above is/are correct?

(a) 1 0nly

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2

Answer