El Niño affects the
weather in large parts of the world. The effects depend
strongly on the location and the season. The strongest
effects on precipitation are in South-East Asia and the
western Pacifc Ocean, especially in the dry season
(August-November). There are temperature effects throughout
most of the tropics. The number of tropical cyclones also
depends on El Niño in most basins. In boreal winter
the effects are most wide-spread: from southern Africa to
eastern Russia and most of the Americas.
For the four meteorological seasons we computed how El Niño
and La Niña perturbed the average weather of the last century.
We used the GPCC V7 analysis of monthly mean precipitation and the
CRU TS 3.22 analysis of temperature.
Blue colours indicate that during El Niño there was, on
average, more rain than normal, red colours indicate drought during El
Niño. La Niña has the opposite effect in almost all
locations. As a measure of the strength of the relationship we used the
correlation coefficient with the Niño3.4 index. The square of this
number gives the fraction of the variance that is explained by this aspect
of El Niño.
March-May In boreal spring the
strongest effects are in the western Pacific Ocean: along the equator
rainfall increases during El Niñ and at 10°-15° North
and South rainfall decreases. The north of Mexico and the desert
states of the U.S. usually get more rain. The North-East of Brasil
often stays drier than usual during El Niño. Even in our part
of Europe it rains more on average during El Niño.
In these months eastern
Indonesia often suffers droughts during El Niño. The rain zone
has moved east to the islands along the equator in the Pacific Ocean.
The Indian Monsoon is often weaker during El Niño, although by
no means always.
September-November This season the
effects of El Niño are strongest. Almost all of Indonesia, the
Philippines and eastern Australia are drier than usual during most El
Niño events. Large parts of India are often drier than usual,
but the Sri Lanka and some southern states get more rain. East
Africa, parts of Central Asia and Spain are also on average wetter
than normal during El Niño in this season, as are Chili and
In boreal winter the
Philippines and East Indonesia stay drier, whereas the Pacific islands
along the equator remain wetter. Florida also gets more rain than
normal during El Niño, this effect extends to other southern
states of the U.S. and into Mexico. South Africa is more frequently
dry, as is the northern coast of South America and some of the leeward
Antilles. In Uruguay en South Brasil rainfall increases on average.
Along the coasts of Ecuador and Peru rainfall increases when the
coastal waters heat up, an effect also named El Niño but not
always coincident with the warming along the equator that affects the
rest of the world.
In the temperature maps, red colours denote locations that on
average are warmer during El Niño and cooler during La
Niña. Blue colours are colder during El Niño and/or
warmer during La Niña. Some North America effects are
non-linear: the effect of La Niña is not the opposite of the
effect of El Niño.
In this season El Niño
causes warmer weather in most of the tropics. The north-western coast
of North America is also warmer than usual. In constrast, the
south-east of the U.S. and north-eastern Mexico are often warmer
during La Niña.
The heat signal is very
clear in India, West Africa and eastern South America. Summer in
East-Asia and eastern Canada is often somewhat cooler than normal.
The east coast of
Central and South America, India and southern Australia are often
warmer during El Niño.
The effects of El
Niño on temperature are clearest in boreal winter, when El
Niño normally is strongest. Northern North and South
America, Australia and also southern Africa usually have warmer
weather than normal during El Niño.
During El Niño there are on average fewer hurricanes over
the Atlantic Ocean, the Caribian Sea and the Gulf of Mexico. La
Niña often brings more. The west coast of Mexico and the
United States see more landfalling hurricanes during El Niño.
In the central Pacific Ocean El Niño brings more typhoons,
both north and south of the equator. Their more easterly genesis
makes that fewer of these tropical cyclones reach Australia. In the
northern Pacific Ocean the area with typhoons also shifts east. Ther
are no effects on the number of cyclones over the Indian Ocean.