Over shorter time periods, there are important natural cycles in the sea surface temperatures (SSTs) that result in major atmospheric changes over multi-year and multi-decadal time scales. Examples of this are the well-known El Niño (the warm phase of the El Niño-Southern Oscillation (ENSO)) and its alter ego La Niña (ENSO cool phase), that occur every 3-5 years on average. The Pacific Decadal Oscillation (PDO) and Atlantic Multi-decadal Oscillation (AMO) occur in 10-50 year cycles. Most of these phenomena have only been discovered within the last 30 years, so there is much we have to learn about them. Their effects are complex, and can affect many areas of the globe. Although the effects on weather and temperatures are mostly found in certain preferred regions, they may also create an imbalance in the global climate system. Thus, a strong, long-lasting El Niño or La Niña for example, can affect the mean global temperatures.
- El Niño and La Niña
El Niño occurs when there is a reversal in the low level winds in the Tropical Pacific Ocean, resulting in westerly (west to east) flow instead of the normal easterly trade winds. The root cause for this phenomenon is not well understood. The abnormal westerlies cause the warmer waters of the central and western Pacific to gradually move toward South America, leading to higher than normal SST (Figure 1). This results in more moist, unstable conditions along the west coast of South America (Peru, Ecuador, and Chile), leading to greater precipitation than normal and even severe flooding. It affects the local economy also because the fishing industry suffers due to a lack of nutrients in the ocean, causing the fish to go elsewhere. Due to feedback from the lower levels into the upper atmosphere caused by deep convective clouds (thunderstorms), El Niño produces other effects on a global scale that include: (1) a higher frequency of tropical storms and hurricanes in the Eastern North Pacific between Mexico and Hawaii, (2) lower hurricane frequency in the Atlantic, (3) wet winters in the southern United States, and (4) increased drought in Australia. El Niño usually results in warmer than normal average global temperatures, which has certainly been the case in 1998-99 and 2015-16.
Figure 1. Surface wind differences from normal (arrows) and sea surface temperatures (SST) associated with an El Nino (left) and La Nina (right) pattern. Arrow lengths are proportional to wind speed. (Source: NASA Jet Propulsion Laboratory)
La Niña is just the opposite situation. Stronger than normal easterly trade winds (right panel of Figure 1) during a La Niña result in observations of much cooler than normal (3-5°C) SSTs in the tropical eastern and central Pacific Ocean. This leads to: (1) drought along the coasts of Peru and Chile, (2) more precipitation in the northern U. S. and Canada, (3) drier in the southern U.S., and (4) more frequent hurricanes in the North Atlantic Ocean basin. La Nina usually results in cooler than normal average temperatures for the U.S. See Figure 2 for a summary of the effects of both El Nino and La Nina.
Figure 2. A summary of conditions that accompany El Nino and La Nina in North America and the North Pacific Ocean. (NOAA/NCEP/NWS)
The frequency of both El Niño and La Niña is irregular, but there are usually two or three of each within the course of a decade. Duration is short, lasting no more than 18 months. There is some evidence that El Niños have become more frequent within the last 50 years, while La Niñas have become less frequent. It is not known if this is a random occurrence or is somehow related to climate change. Climate models predict an increase in the frequency of extreme El Niños by a factor of two.
2. Pacific Decadal Oscillation (PDO)
A longer period but equally important phenomenon discovered in 1997 is the Pacific Decadal Oscillation or PDO, marked by alternating colder and warmer than normal SSTs in the Northeast Pacific north of 20°N. During a warm (positive) phase, the northeast Pacific warms while the western Pacific cools (left panel of Figure 2). The opposite occurs during a cool (negative) phase (right panel of Figure 2). As with the ENSO, the cause of PDO is not known. The effects of the PDO on temperature and precipitation are pronounced in the northeast Pacific, northwest U. S., Canada, and Alaska. Analysis of tree ring data back to the year 993 shows that the PDO has significant year-to-year variability but a major period interval of 50-70 years. Since we have been in a cool phase since the mid-1990’s, there is evidence that this has contributed to the “pause” in global warming observed since that time. The theory is that if we return to a warm phase, the warming trend in global temperature may resume.
Figure 2. Sea surface temperatures (SST) and surface winds associated with the warm phase (left) and cool phase (right) of the Pacific Decadal Oscillation (PDO). (Source: NASA JPL)
1. Cai, W., et al., 2014: Increasing frequency of extreme El Niño events due to greenhouse warming. Nature Climate Change, Vol. 4, 111-116.
2. MacDonald G. M. and R.Case, 2005: Variations in the Pacific Decadal Oscillation over the past millennium. Geophys. Research Letters, Vol. 32, L08703, 4 pages.
3. Kosaka, Y. and S-P. Xie, 2013: recent global-warming hiatus ties to equatorial Pacific surface cooling. Nature, vol. 501, pages 403-407.