Earth Science and Climate Change

El Nino and La Nina, and Climate Variability & Predictability

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El Niño – Southern Oscillation (ENSO) is one of the most important modes of variability of year-to-year climate in the Earth System. A distinct feature of ENSO is large positive and negative swings in the sea surface temperatures (SSTs) in the equatorial tropical Pacific that are referred to as El Niño (warmer SSTs) and La Niña (colder SSTs). SST variations have distinct fingerprints on various climate features over remote regions over the globe – droughts and floods over Indonesia; frequency of hurricanes in the Atlantic Ocean basin; variations in surface temperature over the United States during northern summer etc. Because of larger thermal inertia of oceans, slower variations in ENSO SSTs can be predicted during next few seasons, and connections between variations in SSTs and global climate, therefore, impart useful predictability to the near-term evolution of climate. Indeed, seasonal predictions based on the ENSO are now made operationally and are provided to the user community. Such long-range forecasts are an important aspect of developing climate services and managing risks and advantages associated with climate variability. Another important aspect of ENSO is understanding its variations on slower time scales and what physical reasons may be responsible for it. Similar to ENSO that is one specific mode of climate variability in the Earth System; other modes of climate variability also exist. These modes span a multitude of time scales varying from weather, monthly, seasonal and decadal. Examples include Blocking, North Atlantic Oscillation (NAO), Pacific North-American (PNA) Oscillation, Madden-Julian Oscillation (MJO), Pacific Decadal Oscillation (PDO) and Atlantic Multi-Decadal Oscillation (AMO) etc. These modes of climate variability also have regional fingerprints in surface temperature and precipitation variability, and therefore, can have significant influence on different aspects of societal activities. Understanding causes of their onset, persistence and decay, together with interactions among different modes of variability, are important for understanding and quantifying their predictability. It is also argued that regional aspects of anthropogenic climate change will also be manifested via its influence on models of climate variability.