6 CLIMATELocal hydrometeorological variables (temperature, humidity, sunshine, atmospheric pressure, wind speed, precipitation, etc.), and therefore the availability of water, are influenced by the climate. Climate (or climatological normal) is a measure of the average trend of meteorological variables in a particular region and over a period of time, usually 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 change in the average state of the climate or its variability.6.1 VARIABILITY CLIMATE Common drivers of climate variability are large-scale circulation patterns, sunspots, and volcanic eruptions. The former, however, is the most important and can dramatically alter climate, weather and hydrometeorological variables around the world. The time period of the resulting change can be in terms of months, years or even decades. 6.1.1 El Niño-Southern Oscillation There are numerous patterns of climate variability that influence local hydro-meteorological 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 air pressure between Tahiti and Darwin, Australia. The negative (positive) pressure is accompanied... in the middle of the map... the most common ones include: the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation, the Quasi Biennial Oscillation, El Niño Modoki, etc. In the United States, for example, it has been found that PDO can modulate the effect of ENSO; El Niño (La Niña) during the positive (negative) phase of the PDO, and lead to stronger climate responses, compared to when they evolve in opposite phases (Gershunov and Barnett, 1998b). There is, however, the possibility that stronger El Niño and La Niña events result from a 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 influence