temperature (SST) along the Equator occurs because this area is exposed to more sun
rays than the regions lying along the Tropic of Cancer and the Tropic of Capricorn.
According to NOAA, the Oceanic Nino Index (ONI) is a key element in monitoring and classifying the relative intensities of El Nino and La Nina. The normal
value of the ONI is 0.5: if the value exceeds (+0.5) higher it shows that the central
and eastern tropical Pacific Ocean is warmer than normal, indicating El Nino
conditions. If the value lies (À0.5) lower it shows that the region is cooler than
normal, indicating La Nina conditions. The ONI values are measured for a consecutive 3 months average of SST in the central and eastern tropical Pacific Ocean,
which lies between 120
and 170
west. Scientists have named the region as the
Niño 3.4 region. The average SST of each month is measured in the Nino 3.4 region
and is averaged with previous and succeeding months; then the consecutive 3 months
average is compared with a 30-year average. The obtained difference from the
average temperature is the ONI value for those 3 months.
ENSO intensity is classified based on SST (https://ggweather.com):
• Weak (with a 0.5 to 0.9 SST anomaly)
• Moderate (1.0 to 1.4)
• Strong (1.5 to 1.9)
• Very strong (!2.0)
Walker Circulation
Walker circulation is the ocean-based air circulation system that affects weather on
the Earth. The change in surface pressure and temperature over the eastern and
western tropical Pacific Ocean results in Walker circulation. In recent studies from
NASA, Walker circulation is the result predominantly of the subsurface current,
which originates from the deep ocean, leading to the accumulation of warm water in
the Western Pacific Ocean.
Neutral Phase
There are certain times where El Nino and La Nina fail to exist and the temperature
of ocean surface water remains close to average SST (Fig. 10.1). Sometimes the
ocean may appear as a prevailing El Nino or La Nina but the atmosphere does not
accompany the changes, and vice versa. Neutral conditions refer to the winds,
temperature, rainfall, and convection (rising air) across the tropical Pacific regions
being closer to their long-term averages. Generally, the trade winds blow from the
eastern to western Pacific along the surface while in the atmosphere (troposphere)
the winds blow in the opposite direction, from western to eastern Pacific, forming a
looping pattern over the Pacific Ocean called the Walker circulation. These trade
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A. Stanley Raj and B. Chendhoor
rays than the regions lying along the Tropic of Cancer and the Tropic of Capricorn.
According to NOAA, the Oceanic Nino Index (ONI) is a key element in monitoring and classifying the relative intensities of El Nino and La Nina. The normal
value of the ONI is 0.5: if the value exceeds (+0.5) higher it shows that the central
and eastern tropical Pacific Ocean is warmer than normal, indicating El Nino
conditions. If the value lies (À0.5) lower it shows that the region is cooler than
normal, indicating La Nina conditions. The ONI values are measured for a consecutive 3 months average of SST in the central and eastern tropical Pacific Ocean,
which lies between 120
and 170
west. Scientists have named the region as the
Niño 3.4 region. The average SST of each month is measured in the Nino 3.4 region
and is averaged with previous and succeeding months; then the consecutive 3 months
average is compared with a 30-year average. The obtained difference from the
average temperature is the ONI value for those 3 months.
ENSO intensity is classified based on SST (https://ggweather.com):
• Weak (with a 0.5 to 0.9 SST anomaly)
• Moderate (1.0 to 1.4)
• Strong (1.5 to 1.9)
• Very strong (!2.0)
Walker Circulation
Walker circulation is the ocean-based air circulation system that affects weather on
the Earth. The change in surface pressure and temperature over the eastern and
western tropical Pacific Ocean results in Walker circulation. In recent studies from
NASA, Walker circulation is the result predominantly of the subsurface current,
which originates from the deep ocean, leading to the accumulation of warm water in
the Western Pacific Ocean.
Neutral Phase
There are certain times where El Nino and La Nina fail to exist and the temperature
of ocean surface water remains close to average SST (Fig. 10.1). Sometimes the
ocean may appear as a prevailing El Nino or La Nina but the atmosphere does not
accompany the changes, and vice versa. Neutral conditions refer to the winds,
temperature, rainfall, and convection (rising air) across the tropical Pacific regions
being closer to their long-term averages. Generally, the trade winds blow from the
eastern to western Pacific along the surface while in the atmosphere (troposphere)
the winds blow in the opposite direction, from western to eastern Pacific, forming a
looping pattern over the Pacific Ocean called the Walker circulation. These trade
230
A. Stanley Raj and B. Chendhoor