Sea Surface Temperature Indexes
Because ENSO is largely dependent on the ocean, sea surface temperature (SST) is
the important factor in determining ENSO (Bjerknes 1969; Rasmussen and Carpenter 1982; Wyrtki 1985). Nino 1 and Nino 2 are the regions chosen for measurements,
later combined into Nino 1 + 2. Nino 3 and Nino 4 are the regions where data are
acquired consistently by passing ships. Later the region 3.4 was recognised as the
representative for ENSO (Barnston et al. 1997), which covers both the Nino 3 and
Nino 4 regions. The temperature changes of the Nino 3.4 region are shown in ONI.
Outgoing Longwave Radiation Indexes
In this satellite-based data collection, the outgoing longwave radiation (OLR)
emitted from the cloud tops is observed with the advance very high-resolution
radiometer (AVHRR) instrument. With this we could detect the areas that are rainier
or drier than normal over the tropical Pacific Ocean. Negative OLR indicates the
greater convection that leads to greater cloud coverage, usually of El Nino conditions. Greater convective activity over the central and eastern tropical Pacific shows
that the cloud tops are higher and colder thus emitting less infrared (IR) radiation into
space. The positive OLR is exactly the opposite of the negative index. The OLR are
not greatly related to ENSO but act as a key element in linking the remote climate
teleconnection outside the tropical Pacific (Chiodi and Harrison 2013).
Wind Indexes
A wind index is the measure of change in air flow in the upper and lower part of the
Walker circulation.
Hadley Circulation
The Equator is that part of Earth that receives more sun radiation than either the
Northern or Southern Hemisphere. When overheating occurs at the Equator the heat
needs to be evenly distributed towards nearby locations, which is possible through
the convection process. Because the Equator is hot, the hot air molecules rise,
forming a low-pressure zone; when the air is further away from the ground, the
heat source begins to cool down. As the heat source begins to cool the air wants to
fall down, but it cannot fall directly down as the warm air is rising, so it is pushed to
the sides; then the warm air actually cools even further so that it begins to fall back
234
A. Stanley Raj and B. Chendhoor
Because ENSO is largely dependent on the ocean, sea surface temperature (SST) is
the important factor in determining ENSO (Bjerknes 1969; Rasmussen and Carpenter 1982; Wyrtki 1985). Nino 1 and Nino 2 are the regions chosen for measurements,
later combined into Nino 1 + 2. Nino 3 and Nino 4 are the regions where data are
acquired consistently by passing ships. Later the region 3.4 was recognised as the
representative for ENSO (Barnston et al. 1997), which covers both the Nino 3 and
Nino 4 regions. The temperature changes of the Nino 3.4 region are shown in ONI.
Outgoing Longwave Radiation Indexes
In this satellite-based data collection, the outgoing longwave radiation (OLR)
emitted from the cloud tops is observed with the advance very high-resolution
radiometer (AVHRR) instrument. With this we could detect the areas that are rainier
or drier than normal over the tropical Pacific Ocean. Negative OLR indicates the
greater convection that leads to greater cloud coverage, usually of El Nino conditions. Greater convective activity over the central and eastern tropical Pacific shows
that the cloud tops are higher and colder thus emitting less infrared (IR) radiation into
space. The positive OLR is exactly the opposite of the negative index. The OLR are
not greatly related to ENSO but act as a key element in linking the remote climate
teleconnection outside the tropical Pacific (Chiodi and Harrison 2013).
Wind Indexes
A wind index is the measure of change in air flow in the upper and lower part of the
Walker circulation.
Hadley Circulation
The Equator is that part of Earth that receives more sun radiation than either the
Northern or Southern Hemisphere. When overheating occurs at the Equator the heat
needs to be evenly distributed towards nearby locations, which is possible through
the convection process. Because the Equator is hot, the hot air molecules rise,
forming a low-pressure zone; when the air is further away from the ground, the
heat source begins to cool down. As the heat source begins to cool the air wants to
fall down, but it cannot fall directly down as the warm air is rising, so it is pushed to
the sides; then the warm air actually cools even further so that it begins to fall back
234
A. Stanley Raj and B. Chendhoor