Look at the SST (Sea Surface Temperatures) off the coast of Peru and in Eastern Indonesia. The reason they are looked at in the Eastern and
Western Pacific is because it can also indicate the low and high
pressure systems that set up in those areas. Off the coast of peru, if
the sea surface temps are colder than normal, that means that deeper,
colder water has surfaced due to prevailing strong winds pushing the
existing surface water away and allowing the deeper water to rise.
These types of high pressure systems set up along the coast of Peru and
can be watched by looking at sea surface temp maps. When high pressure
sets up along Peru's coast, low pressure tends to set up in the western
pacific and the surface water temps are warmer than usual in SE Asia
near Indonesia. Storms spin off that large low pressure system and the
high pressure systems off the coast of peru block them so they are
pushed to the north and ultimately will slide across the Pacific
Northwest US. This is called a La Nina.
The opposite scenario has high pressure in the Western Pacific and Low
Pressure off the coast of Peru. That low pressure system will spin
storms off that coast and they tend to hit the southwestern US. This
is called El Nino.
Out of every 5 years, 2 of them tend to be El Nino and 2 tend to be La Nina (some more severe than others).
