Originally I had hoped to complete this series in two posts. That proved to be impractical due to the length of An Introduction To ENSO, AMO, and PDO — Part 2, which discusses the Atlantic Multidecadal Oscillation (AMO). The Pacific Decadal Oscillation (PDO) is discussed in An Introduction To ENSO, AMO, and PDO — Part 3.
This three-part series of posts is an introduction to three sea surface temperature-based indices that are commonly referred to during discussions of global weather and climate. The three are (1) El Niño-Southern Oscillation (ENSO), which is typically expressed as El Niño and La Niña events, (2) Atlantic Multidecadal Oscillation (AMO), and (3) Pacific Decadal Oscillation (PDO). The intent of these posts is to provide the reader with a basic understanding of the phenomena. They are not intended to be single-source all-inclusive references.
Part 1 discusses El Niño and La Niña events.
EL NIÑO AND LA NIÑA EVENTS
An El Niño event is a natural warming of the central and eastern tropical Pacific that is caused by an occasional change in coupled ocean-atmosphere processes. The phrase “coupled ocean-atmosphere processes” refers to the many climate factors that all interact. The interacting components include sea surface temperature, surface winds, surface and subsurface ocean currents, atmospheric circulation, precipitation, cloud cover, sea level pressure, sea surface height, etc. How a few of those factors interact will be covered in this post.
Figure 1 includes three cross-sectional views of equatorial Pacific Ocean temperatures from 300 meters in depth to the surface and from Indonesia in the west to the South American Coast in the east. Illustrated are a “Normal” December (Cell a) in 1996, an El Niño December (Cell b) in 1997, and a La Niña December (Cell c) in 1998. “Normal” is listed as “ENSO Neutral” in the illustration, which means the conditions do not reflect an El Niño or a La Niña event. The month of December is shown because El Niño and La Niña events normally peak during the months of November, December and January.
Note how in the “Normal” state (Cell a), from the surface to 200 meters in depth, the warmer subsurface waters are in the west and the cooler waters are in the east. This is caused by trade winds pushing warm surface waters from east to west. The westward moving warm surface waters run into Indonesia so they accumulate in the western Pacific in an area called the Pacific Warm Pool. Just as important: the trade winds blowing across the surface from east to west also cause the waters to be about ½ meter higher in the west than in the east. I tried to show that (with limited success) by adding slopes to the ocean surfaces of the Figure 1. During the El Niño phase, the trade winds first slow, then reverse. Since the trade winds are no longer “holding” the water in place in the western Pacific, gravity causes the warm water to slosh to the east. Referring again to Figure 1, note how during the El Niño (Cell b) the warm subsurface waters have extended east to the South American coast. The shift in the location of that warm water causes changes in precipitation and atmospheric circulation patterns, which raise surface temperatures globally.
Cell c in Figure 1 shows the equatorial Pacific temperature profile during a La Niña event. Close inspection will allow you to note that the sea surface temperatures in the east are cooler than during “Normal” ENSO-neutral conditions shown in Cell a. This is caused by a strengthening of the trade winds as the coupled ocean-atmosphere processes attempt to return to “Normal” conditions. The processes overshoot in their attempt. The higher-than-normal trade winds return the warm surface waters to the western Pacific and also expose more of the cool waters in the east than “normal”.
El Niño events are an important part of climate on Earth. They discharge heat from the tropical Pacific. Atmospheric circulation and ocean circulation then carry that heat poleward, where it can be more easily radiated into space. In doing so, El Niño events help to reduce the temperature difference between the tropics and the poles that would exist without them.
La Niña events are just as important because they recharge the heat that had been discharged during the El Niño before it. They accomplish this by reducing cloud cover over the tropical Pacific. During a La Niña, the strength of the Pacific trade winds rise above “Normal” conditions, which reduce cloud cover over the central and eastern tropical Pacific. The reduced cloud cover allows more downward shortwave radiation (visible light) to warm the tropical Pacific. The stronger-than-normal trade winds then push this water that has been warmed by the sun back to the Pacific Warm Pool and the Western Pacific.
Note again that El Niño events discharge heat from the tropical Pacific and La Niña events recharge it. The above paragraph briefly explained how a La Niña recharges the heat that had been released during an El Niño. Now for a quick discussion of how an El Niño releases more heat than normal from the tropical Pacific. Figure 2 illustrates the Sea Surface Temperatures of the Pacific Ocean during the three ENSO phases. Cell a shows the ENSO Neutral or “Normal” phase. The sea surface temperatures greater than 28 deg C are shown in red. Note how the warmer surface waters are located in the western Pacific, in an area called the Pacific Warm Pool. During the El Niño (Cell b) more of the tropical Pacific sea surface is covered by the warm water. The increase in surface area covered by warmer water allows more heat to be released from the tropical Pacific to the atmosphere.
Of the three phases of El Niño-Southern Oscillation, the El Niño is the truly unusual phase. It involves the transport of warm water from the western tropical Pacific to the east. These changes are the opposite of the “Normal” and La Niña conditions. A La Niña, on the other hand, is simply an exaggeration of “Normal” conditions. As noted above, trade winds during the La Niña rise above normal conditions. These stronger trade winds increase the strength of the ocean currents moving from east to west.
Some of the interaction/interdependence of the coupled ocean-atmosphere processes can be seen in Figure 3. During Normal conditions, vast amounts of heat and moisture are pumped, rising, from the West Pacific Warm Pool into the atmosphere. This convection is driven by the surface heat. As the warm, moist air rises, it draws dry air in behind it, creating a “loop” of atmospheric circulation called Walker Circulation. The warm, moist air cools as it rises and can hold less water. Rain is the result.
The warm waters slosh east during the El Niño. The convection, clouds, and precipitation accompany the warm water east. Thus, the change in location of the warm water alters the normal ocean-atmospheric patterns and the locations of atmospheric circulation cells.
NOAA’s Bill Kessler provides a great description (in layman terms) of the interactions between the ocean and atmosphere in his El Niño FAQ webpage, starting with 1. What is El Niño and how does it relate to the usual situation in the tropical Pacific? So there is no reason to repeat it here.
The differences between “Normal”, El Niño, and La Niña conditions can also be seen in maps of Sea Surface Temperature (SST) anomalies for the Pacific, Figure 4. During an El Niño (Cell b), the Sea Surface Temperature (SST) anomalies of the central and eastern tropical Pacific can be more than 3 degrees C warmer than “normal”. Again, this is caused when the warmer waters from the western tropical Pacific slosh to the east. A La Niña is shown in Cell C. Note that the Sea Surface Temperature (SST) anomalies for the central and eastern tropical Pacific are lower than “normal”. This is caused by increase in trade winds, which exposes the cooler subsurface waters in that location. It also sends warm water to the west at a higher rate than normal.
Keep in mind that the Pacific Ocean covers a major portion of the globe. The maps in Figures 2 and 4 include longitudes that stretch from 120E to 70W, or 170 degrees, or nearly halfway around the globe. The massive amounts of moisture being pumped into the atmosphere in the tropical Pacific are, therefore, a major part of world climate. So a change in its location and strength during an ENSO event can be felt in temperatures and precipitation worldwide.
Note: The 1997/98 El Niño is used as an example in this post because it was an extremely strong El Niño event and caused exaggerated changes in the coupled ocean-atmosphere processes. Also, there were no strong volcanic eruptions during or following that El Niño. Strong volcanic eruptions also impact global temperatures and can also skew the impacts of ENSO events. This will be illustrated later in this post.
A NOTE ON THE USE OF THE WORD ANOMALY WHEN DISCUSSING VARIABLES SUCH AS TEMPERATURE
An example will help illustrate why anomalies and not raw data are used during discussions of variables such as temperature. Figure 5 is a time-series graph of monthly Global Sea Surface Temperatures (not anomalies) from November 1981 to June 2010. As you will note there are wide seasonal swings in temperatures over a one-year period. These swings make it difficult to determine how the Sea Surface Temperature (SST) for a given month compares to earlier or later readings for the same month or determine how the temperatures compare to normal conditions from one month to the next.
A data supplier such as the Hadley Centre selects a base period, normally 30 years, then averages the January temperatures over that base period, and repeats this process for all months. These monthly average base period temperatures are then subtracted from an actual reading to show how much that reading differs from the mean for that month. If the reading for a month is higher than the average, the difference is positive. And the reverse holds true if the reading is below the average. The difference is referred to as an anomaly. Figure 6 illustrates Global Sea Surface Temperature (SST) anomalies for the same period as Figure 5. Year-to-year and month-to-month variations are much easier to see when using anomalies.
Also for example, by converting the data to anomalies, it is much easier to then compare Sea Surface Temperatures (SST) from a warmer part of the globe, the tropics for example, to a cooler area like the Southern Ocean surrounding Antarctica, or to compare different variables such as Sea Surface Temperatures, Sea Level Pressure, Cloud Cover, etc.
There are other reasons for using anomalies, and there are other methods used to calculate them, but those discussions would add little to this post.
THE SOUTHERN OSCILLATION PORTION OF ENSO
As noted above, ENSO stands for El Niño-Southern Oscillation. El Niño is the expression of the Sea Surface Temperature portion of this natural phenomenon. Sea Level Pressure (SLP) is also impacted by this complex coupled ocean-atmosphere process. During El Niño and La Niña events, Sea Level Pressures (SLP) in the eastern and western Pacific vary. The Southern Oscillation Index is a measure of those changes in Sea Level Pressures (SLP) and is calculated as the difference in Sea Level Pressures (SLP) between Tahiti and Darwin, Australia.
SEA SURFACE TEMPERATURE-BASED ENSO INDICES
The sea surface temperature anomalies of four areas of the equatorial Pacific are used by researchers to express the frequency and strength of El Niño and La Niña events. They are known as the NINO regions, and their locations are shown in Figure 7.
NINO3.4 SST Anomalies are referred to most commonly, because, of the four, they correlate well (not perfectly) with the resulting changes in global temperature. Figure 8 is a long-term time-series graph of NINO3.4 SST anomalies, extending from the start of the dataset in January 1870 to present, June 2010.
NOAA uses a running 3-month average of NINO3.4 SST anomalies in its Oceanic NINO Index (ONI), because, as shown in Figure 8, the NINO3.4 SST anomalies can be noisy. The averaging smoothes or minimizes some of the noise. A rise in the NINO.4 SST anomalies becomes an official El Niño event according to NOAA when the three-month average is greater than or equal to +0.5 deg C and stays equal to or above for 5 successive “quarters”, such as (1) Oct-Nov-Dec, (2) Nov-Dec-Jan, (3) Dec-Jan-Feb, (4) Jan-Feb-Mar, and (5) Feb-Mar-April. Likewise, a drop in NINO3.4 SST anomalies becomes an official La Niña when the three-month average is less than or equal to -0.5 deg C and stays below that value for 5 consecutive quarters.
Table 1 is a copy of the NOAA Historic ONI Data with the official El Niño periods highlighted in red and La Niña in blue. Since 1950, El Niño events occurred every two to five years, with a few multiyear or back-to-back El Niño events. Note also that every El Niño event is not followed by a La Niña event.
Link to the source of Table 1:
The threshold of +/- 0.5 deg C is used because the global impacts of El Niño and La Niña events (temperature, precipitation, etc.) are noticeable above (El Niño) or below (La Niña) those values.
Also keep in mind that it is not the fact that the NINO3.4 SST anomalies are above a certain threshold that causes the changes in global weather and temperature during an El Niño. It is the change in location of the warm water to the central and eastern tropical Pacific and the changes in the locations and strengths of all of the coupled ocean-atmosphere processes (ocean surface wind, convection, precipitation, atmospheric circulation, etc.) that cause the changes in global precipitation and temperature. That is, the changes in NINO3.4 SST anomalies are only one of the many factors that cause changes in global temperature. The changes in NINO3.4 SST anomalies simply correlate well (but not perfectly) with global temperature changes.
Note: The Sea Surface Temperature (SST) data used in this post is the Optimum Interpolation Sea Surface Temperature data (OI.v2) from the National Climate Data Center (NCDC). It is also referred to as Reynolds SST.
On a year-to-year basis, El Niño events are clearly visible in graphs of global and regional temperature anomalies. The strong El Niño events of 1982/83, 1986/87/88, 1997/98, and 2009/10 clearly stand out in the graph of the global Sea Surface Temperature (SST) anomalies since 1979, Figure 9. So do the La Niña events of 1988/89, 1998/99/00/01, and 2007/08.
The same is true when looking at time-series graphs of global land surface temperature (LST) anomalies and Global Lower Troposphere Temperature (TLT) anomalies. A time-series graph of Global Lower Troposphere Temperature (TLT) anomalies is shown in Figure 10. El Niño and La Niña events are dominant. The Lower Troposphere Temperature (TLT) anomaly data used in this post are from Remote Sensing Systems (RSS).
For those new to discussions of global temperatures, Lower Troposphere Temperature is one of the metrics used to measure global temperature. The troposphere is the lowest level of the atmosphere, and the lower troposphere is approximately the bottom 3,000 meters. Since 1979, satellites have been used to measure atmospheric temperatures at various heights.
Notice how the Sea Surface Temperature (SST) anomalies, Figure 9, and Lower Troposphere Temperature (TLT) anomalies, Figure 10, drop in 1991. They then rebound about 1994/95. As noted in both illustrations, that dip and rebound was NOT caused by a La Niña.
ONLY VOLCANIC ERUPTIONS CAN HAVE A GREATER IMPACT ON GLOBAL TEMPERATURES
The only other natural variable that has a greater effect on the year-to-year variations in global temperature is an explosive volcanic eruption, one that releases sun-blocking aerosols into the stratosphere. These aerosols can overwhelm the strongest of El Niño events. For example, the strength of the El Niño event in 1982/83 rivaled that of the 1997/98 “El Niño of the Century”. This can be seen in a graph of Sea Surface Temperature anomalies for the NINO3.4 region, Figure 11.
Back to volcanic eruptions: While the 1982/82 El Niño was almost as strong as the 1997/98 event, note how in Figure 12 that there was only a minor spike in the global sea surface temperature anomalies in 1982/83 and in Figure 13 there was a dip in Lower Troposphere Temperature anomalies at that time. There should have been spikes in both datasets that were almost as large as the ones in 1997/98. But the effect of the 1982/83 El Niño was counteracted by the eruption of El Chichon, an active volcano in northwestern Chiapas, Mexico. Note also that the temperatures in Figures 12 and 13 dip in 1991, then rebound a few years later. That dip and rebound was caused by the explosive volcanic eruption of Mount Pinatubo, which is located on the Philippines island of Luzon. The aerosols discharged by Mount Pinatubo overwhelmed the moderate 1991/92 El Niño and the El Niño conditions in 1993.
A NOTE ABOUT THE RANGE OF TEMPERATURE VARIATIONS
The ENSO-induced variations in the sea surface of the central and eastern tropical Pacific are much greater than the variations in global Sea Surface Temperature, global Land Surface Temperature, and global Lower Troposphere Temperature anomalies. Figures 8 and 11 illustrated Sea Surface Temperature anomalies for the NINO3.4 region (central equatorial Pacific). The temperature scales on those graphs were shown in deg C. On the other hand, the temperature scales on the Global Sea Surface Temperature anomalies and the Lower Troposphere Temperature anomalies shown in many of the other graphs were considerably smaller, in tenths of a deg C or less. Many times those scales are overlooked by those new to the discussion of ENSO. Figure 14 compares Global Sea Surface Temperature anomalies to Sea Surface Temperature anomalies for the NINO3.4 region. The variations in NINO3.4 Sea Surface Temperature anomalies dwarf the responses in global Sea Surface Temperature anomalies.
In fact, in order to bring variations of the NINO3.4 Sea Surface Temperature anomalies more into line with the changes in Global Sea Surface Temperature anomalies, the NINO3.4 data need to be multiplied by a significant scaling factor. A scaling factor of 0.1 was used in Figure 15.
Figures 16 and 17 are .gif animations of maps of the Goddard Institute for Space Studies (GISS) surface temperature anomaly dataset GISTEMP that were created through the KNMI Climate Explorer. The dataset is their standard product, LOTI, which is the merged land plus sea surface data with 1200km radius smoothing. To minimize seasonal and weather noise, each map shows 12-month average temperature anomalies. The years 1982 to 2009 were used as base years for anomalies because that is the length of the satellite-based Reynolds (OI.v2) Sea Surface Temperature data used in GISTEMP.
Figure 16 captures the evolution and decay of the 1997/98 El Niño and the lagged responses in land and sea surface temperatures around the globe. It starts a few months before the sea surface temperature anomalies in the equatorial Pacific began to rise and ends after a few months of negative SST anomalies in the central equatorial Pacific.
Note the lags in the European and Asian responses at mid latitudes and high latitudes, and also those over North America at the same latitudes. Also note how the sea surface temperature response travels eastward. The El Niño event occurs in the tropical Pacific; then the Atlantic Ocean responds as the changes in atmospheric circulation travel eastward; next is the response in the Indian Ocean, and last, the western Pacific.
Figure 17 presents a portion of the 1998/99/00/01 La Niña, and it captures the lagged responses of global sea and land surface temperature anomalies to a La Niña event. The animation starts while equatorial Pacific sea surface temperature anomalies are still elevated. In other words, its start point overlaps with the end of the El Niño animation in Figure 16.
As noted earlier, El Niño events are an important part of climate on Earth. The temperature difference between the poles and the equator would be much greater without the occasional release of additional heat from the tropical Pacific. Their impacts are easily seen in global and regional temperature and precipitation records. ENSO events cause temperatures to change globally through the transport and transfer of heat, and outside of the tropical Pacific, they cause temperatures to vary without the transfer of heat.
Many climate scientists treat the ENSO phenomenon as noise and assume that its signal can be easily removed from the global temperature record. I have written numerous posts about how and why this is incorrect. The latest includes a two-part video with numerous animations. Refer to La Niña Is Not The Opposite Of El Niño – The Videos. There are links to more than a dozen posts that discuss ENSO events in detail, including:
More Detail On The Multiyear Aftereffects Of ENSO – Part 1 – El Nino Events Warm The Oceans
More Detail On The Multiyear Aftereffects Of ENSO – Part 2 – La Nina Events Recharge The Heat Released By El Nino Events AND…During Major Traditional ENSO Events, Warm Water Is Redistributed Via Ocean Currents.
More Detail On The Multiyear Aftereffects Of ENSO – Part 3 – East Indian & West Pacific Oceans Can Warm In Response To Both El Nino & La Nina Events
Sea surface temperature data and maps are available through the NOAA NOMADS webpage:
GISTEMP land and sea surface temperature data and maps and the RSS lower troposphere temperature data are available through the KNMI Climate Explorer:
The equatorial Pacific Ocean temperature cross sections in Figure 1 were cropped from those available through the ECMWF website. “Full field” were used:
The three illustrations used in Figure 3 are available from NOAA:
The map of the NINO regions is also available through NOAA: