Food & Climate 

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Drought driving factors in the world and the Mediterranean

There are three main factors that we know affect the appearance and severity of droughts in different regions of the world.

These factors are explained in more detail in other units and here we show how they are interlinked and how they influence the world in general and the Mediterranean in particular. These factors are:

· El Niño episodes (ENSO)
· The North Atlantic Oscillation (NAO)
· Other possible influences. The Mediterranean Oscillation 

El Niño Southern Oscillation (ENSO)

The Southern Ocean Oscillation is made up of two phases, El Niño and La Niña, and has affected climate for many centuries, primarily in the Southern Hemisphere.  Recent research shows that it may also have some impact on the climate of the Northern Hemisphere.

Different temperature and rainfall conditions are seen during El Niño and La Niña events, not only in the Tropical Pacific but also in distant regions of the world including the Northern latitudes. These variations can sometimes bring about extraordinary episodes of dangerous floods and droughts.

During El Niño events,  temperatures in the East Pacific increase along with evaporation rates and rainfall amounts, while lower than average rainfall amounts are seen on the West side of the Ocean.  The opposite conditions are seen during the La Niña phase.

In order to know if El Niño, La Niña or a Neutral stage are occurring and what their magnitude is, different indices are used. One measures the differences in sea surface temperatures across the Tropical Pacific Ocean (SST), another measures the differences in pressure between the east and west coasts of the Southern continents (SOI). These indices are used to make climate forecasts to enable us to predict the possible impacts of extreme events on our Environment and Society.

This complex system is explained fully in the Weather Unit, take a look if you're interested!

North Atlantic Oscillation (NAO)

Just like the Southern Oscillation, there is also a similar weather cycle in the Northern Hemisphere.  The North Atlantic Oscillation is one of the main natural factors which influences climate in the North Atlantic and Mediterranean regions, particularly in the winter.  You can find out more about the NAO in both the Oceans Unit and in the Weather Unit.

The NAO index varies from year to year, but over the past few decades has remained in one phase for intervals lasting several years. In the picture you can see a representation of what a positive NAO Index means.

A high-pressure cell is established over the North Atlantic Ocean creating long periods of very stable and quite dry conditions during the winter for the Mediterranean basin.  During negative NAO conditions, Mediterranean winters are much wetter.  

Other influences over the Mediterranean basin.

Scientists have shown that the NAO is the main factor which influences winter rainfall patterns and temperature over the Mediterranean region.  But not all the variations we see in the Mediterraean climate can be explained by NAO and ENSO.  Other global climate phenomena may also be responsible.

It's also possible that the extent of the monsoon in Asia and the amount of rainfall in the African Sahel region may also influence the Mediterranean climate. 

Differences in atmospheric pressure between the Eastern and Western Mediterranean basins sets up the Mediterranean Oscillation.  In a similar way to the NAO, this may affect wind and rainfall patterns in the region. 

About this page:

Author:  Marta Moneo and Dr. Ana Iglesias - Universidad Politécnica de Madrid, España
1. scientific reviewer: Alex de Sherbinin - CIESIN, Columbia University, USA
2. scientific reviewer: Lily Parshall - Goddard Institute for Space Studies, Columbia University, USA
educational reviewer: Emilio Sternfeld - Colegio Virgen de Mirasierra - España
last published: 2004-05-12