El Niño and Probability
Large datesets key to understanding how ENSO affects global weather and risk of climate extremes
Susan Bacon, Summer 2003
The laws of probability are useful for day-to-day activities, such as beating a friend at a game of poker, trying to win the lottery, and even guessing the chances that a favorite sports team will win a big game. As researchers from ESRL's Physical Sciences Division (PSD) recently reported, these rules of chance are also essential for climate forecasting. In a paper published in the American Meteorological Society's Journal of Climate, Prashant Sardeshmukh, Gilbert Compo, and Cécile Penland used probability calculations to show that a large set of data is the key for learning how the El Niño Southern Oscillation (ENSO) affects weather outside of the tropics. Furthermore, a dataset this large can be used to detect the impact of ENSO related shifts in the mean state and changes in variability on the risk of seasonal climate extremes.
The results from this study indicate that it's not enough to use instrumental data collected from thermometers, barometers and other weather-measuring devices over the past 100 years, since this instrumental record contains no more than a few dozen ENSO events. Rather, the PSD research team found that using climate models to create larger data sets can give additional useful information about how ENSO affects weather around the world.
Benefits of Large Datasets
The importance of using a large data set can be illustrated in a simple study of coin tosses. Any honest coin has a 50-50 chance of landing on heads or tails. This means that if a coin is flipped 100 times, it's likely that about 50 of the flips will show heads, and about 50 will show tails. With 100 flips, the coin-tosser will have a good chance of predicting a 50 percent chance (probability) of getting a heads or a tails on any one of the flips. But if the coin is flipped only a few times, this 50-50 split may be harder to achieve. It's easy to imagine that 5 tosses could produce 1 heads and 4 tails, or 3 heads and 2 tails, or all heads and no tails. Drawing a conclusion about the probability of getting a heads or tails on any toss based on only 5 flips - a small sample size - can lead to inaccurate results.
Similar challenges exist when using a small data set to understand the effects of ENSO [a phenomenon in which sea surface temperatures in the Eastern Pacific Ocean become higher or lower than the long-term average and cause changes that reverberate through the atmosphere perturbing weather patterns] on a system as complex as climate. But now, with increasingly sophisticated models and computer power, researchers don't have to rely only on the instrumental record. The climate science community can also create large data sets (ensembles) to more accurately identify the links between ENSO and resulting weather patterns.
ENSO causes some of the most significant, yet predictable, shifts in global weather. Its impacts are easiest to see in tropical areas, but it can also affect weather outside of the tropics. For instance, an El Niño winter, in which sea surface temperatures are higher than normal in the east central Pacific, brings cooler and wetter weather to the Southern United States, and warmer and drier weather to the Pacific Northwest. A La Niña winter in the east central Pacific, on the other hand, has lower than normal sea surface temperatures and causes nearly opposite weather patterns.
Describing ENSO impacts on weather not only provides better information on how the climate system works in general, but it also gives researchers clues about the likelihood that extreme weather events - from floods and droughts, to blizzards and heat waves - will take place in areas outside of the tropics. This advance warning helps people prepare, and so reduces the loss of lives and other damage extreme weather can cause.
Researchers know there is a connection between ENSO and weather outside of the tropics, but remain challenged when forecasting how a specific ENSO event will affect weather in thes