The Climate of PeruAlejandro Soto12 August 2010

Peru can be divided into three basic climate regions:

1. The desert coast

2. The mountains/highlands

3. The eastern lowlands

The desert coast is subtropical (just like Los Angeles) and generally sees very little rainfall.

The mountains are cool-to-cold with rainy summers and dry winters.

The eastern lowlands have a tropical rainforest climate, thus it is hot and wet throughout the year with no real seasonal changes in weather.

What climate regions will we see?

Desert coasts

Mountains/highlands

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The National Atlas of the United States of AmericaU.S. Department of the InteriorU.S. Geological Survey

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pageprecip_ca3.pdf INTERIOR-GEOLOGICAL SURVEY, RESTON, VIRGINIA-2005

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PRECIPITATIONPrecipitation varies widely across the United States, from a low of 2.3 inches per year in California's Death Valley to a high of 460 inches on Hawaii's Mount Waialeale. Nevada ranks as the driest state, with an average annual precipitation of 9.5 inches, and Hawaii is the wettest, at 70.3 inches. The average annual precipitation for California is 21.44 inches.

Average Annual Precipitation (in inches)1961-1990

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15 in/yr = 380 mm/yr

5 in/yr = 127 mm/yr10 in/yr = 254 mm/yr

Peruvian coastal rainfall vs SoCal rainfall

How dry is the desert coast?

What causes the extreme dry conditions of the Peruvian coast?

1. The Humboldt current in the ocean

2. The prevailing surface winds

3. The Walker circulation in the atmosphere

• The Humboldt current is a eastern boundary current that brings cold water from the southern polar region into the mid-latitudes, including northern Chile and Peru.

The Humboldt Current

• The Humboldt current is part of a larger ocean current system.

• You can think of the Humboldt current as an “anti-Gulf-Stream”. The Gulf Stream brings warm water to eastern North America while the Humboldt current brings cold water to western South America.

Global Prevailing Surface Winds

Blue: WesterliesYellow: Northeasterly Trade WindsBrown: Southeasterly Trade Winds

The surface winds of the southeastern Pacific follow a similar path to the ocean currents.

The surface winds and Humboldt current work together to generate upwelling of deep cold ocean water.

The Walker circulation is an atmospheric circulation that crosses the Pacific ocean basin. Convective storm systems on the western side of the Pacific drive a circulation that delivers cold, dry air to the eastern Pacific.

Incidentally, this is a mirror image to the ocean overturning driven by the Humboldt current and meridional surface winds.

The Walker Circulation

What’s the net result?

• The Humboldt current, the prevailing surface winds, and the Walker circulation all conspire to deliver cold, dry air to the western shores of the mid and southern coasts of South America. The Peruvian coast is the northern most region that desert coastal land due to this delivery of cold, dry air.

• During the southern summer, when the inter-tropical convergence zone moves southward, the Peruvian coast does get some rainfall, which is why the annual mean precipitation is higher on the Peruvian coast than on the Atacama coast in Chile. But for 9 months or more the coast of Peru is dry and sees almost no rainfall.

Just a snapshot. Go to video!

Let’s see it in action.

What’s the exception to all of this? (Hint: El Niño.)

• In an average year, the climate on the coast of Peru is cold and dry.

• But every now and then (i.e. a frequency of 2 to 5 years) the entire system is reversed and extensive rainfall is seen on the Peruvian coast.

• This is the El Niño Southern Oscillation phenomena.

What happens during El Niño?

• The normal circulation.

What happens during El Niño?

• The El Niño circulation.

• When El Niño occurs, there is extensive rainfall on the Peruvian coast. Convective storms generate so much rain that flash flooding often occurs in the region.

Important note that will be returned to later:

The thermocline changes significantly in slope and height between El Niño and non-El Niño conditions. When the thermocline reduces slope during El Niño, less deep cold water is delivered to the surface.

Now, let’s talk about the highland climate.

The mountains and highlands receive greater amounts of rainfall for two main reasons:

1. orographic effects

2. the inter-tropical convergence zone

These two processes work together to give almost six months of precipitation in the mountains followed by a six month dry season.

The best way to see this? Again, let’s go to the video!

Alright, that’s the current Peruvian climate, which is cool, but what happened in the past?

There is evidence that during the mid-holocene (~6,000 years ago), the ENSO was reduced in strength. Thus, the western shores of South America would have been much drier for longer periods of time than the current climate.

This reduction may have been driven by the overall increase in insolation due to orbital forcing. The insolation increase would have reinforced the convective activity in the western Pacific, thus overwhelming any activity in the eastern Pacific.

Mid-Holocene (~6 kya)

Early- to Mid-Pliocene (3-5 Mya)

The early- to mid-pliocene climate was ~3 K warmer than the current climate. This would have increased ocean temperatures in the high latitudes leading to warmer deep ocean temperatures. Thus, the temperature contrast between surface and deep ocean water would have been less, which would have created a deeper mean thermocline. An annually deeper mean thermocline creates the conditions for a “permanent” El Niño.

• So, with increased regional insolation the El Niño phenomenon is suppressed and the Peruvian coast becomes extremely dry.

• With increased global mean temperatures, the El Niño phenomenon becomes amplified and essentially permanent leading to a warm and we Peruvian coast.

• The western slopes of the Peruvian Andes follow the patterns of the coast while the eastern slopes are more sensitive to the location and movement of the inter-tropical convergent zone.

Published References:

Clarke, A.J. (2008), An Introduction to the Dynamics of El Niño and the Southern Oscillation, Academic Press, New York. [Chapters 1 and 2 have good descriptions without too much fluid mechanics mathematics.]

Chiang, J.C.H. (2009), “The Tropics in Paleoclimate,” Annual Review of Earth and Planetary Sciences, 37, 263-297.

Crowley, T.J. and North, G.R. (1991), Paleoclimatology, Oxford University Press, New York. [I mainly looked at Chapter 3, although I skimmed all of Part II.]

Unpublished References, but still Useful:

Madl, P., “The El Niño (ENSO) Phenomenon,” http://www.sbg.ac.at/ipk/avstudio/pierofun/atmo/elnino.htm

Wikipedia: “Climate of Peru”, http://en.wikipedia.org/wiki/Climate_of_Peru

Wikipedia: “Humboldt Current”, http://en.wikipedia.org/wiki/Humboldt_Current

Back-up

ANRV374-EA37-12 ARI 23 March 2009 18:57

the maritime continent, and also tropical Africa and South America (in seasons other than borealsummer).

The tropical Pacific region, in particular, features prominently in the tropical hypothesis. Thetropical Pacific climate today possesses an interesting zonal asymmetry; it is warmer to the westthan in the east (Figure 2a, left panel ), despite the fact that they both occupy the same equatoriallatitudes. The tropical western Pacific possesses a warm pool of water exceeding 28.5!C, andmoist convection is located here as well as over the maritime continent. In the eastern Pacific, the

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