6 CLIMATELocal hydrometeorological variables (temperature, humidity, sunshine, atmospheric pressure, wind speed, precipitation, etc.), and therefore water availability, are influenced by climate. Climate (or climatological normal) is a measure of the average configuration of weather variables over a particular region and over a given period, generally 30 years. Discussions of climate, in the context of water availability, include climate variability and climate change (or global warming). Climate variability concerns how the climate fluctuates around the long-term average, while climate change refers to the long-term continuous change towards a statistically significant long-term variation in either the average state of the climate or its variability.6.1 CLIMATE VARIABILITYCommon drivers of climate variability are large-scale circulation patterns, sunspots, and volcanic eruptions. The first, however, is the most important and can significantly alter climatic, meteorological and hydrometeorological variables around the world. The period of resulting change can be expressed in months, years, or even decades. 6.1.1 El Niño – Southern Oscillation There are a number of climate variability patterns that influence local hydrometeorological conditions. The most important is the El Niño Southern Oscillation (ENSO). ENSO is a coupled ocean-atmosphere phenomenon linked to sea surface temperature (SST) anomalies in the central and eastern equatorial Pacific and the associated sea level pressure difference, the Southern Oscillation (SO) (Rasmusson and Carpenter, 1982). The Southern Oscillation is the fluctuation in atmospheric pressure between Tahiti and Darwin, Australia. Negative (positive) pressure is accompanied...... middle of article ...... among the most common are: Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO) , the Atlantic Multidecadal Oscillation, the Quasi-Biennial Oscillation, El Niño Modoki, etc. In the United States, for example, it was found that PDO could modulate the effect of ENSO; El Niño (La Niña) during the positive (negative) phase of the PDO, and lead to stronger climate responses than when they evolve in opposite phases (Gershunov and Barnett, 1998b). There is, however, a possibility that stronger El Niño and La Niña events result from random decadal variation in ENSO and may even be responsible for the PDO (McPhaden et al., 2006; Rodgers et al., 2004). . Regardless, knowledge of all patterns of climate variability that influence local climate conditions is important and can help predict precipitation and other climate variables that affect local climate conditions..