Nick Christen, Craig Arsedo
Things are starting to get interesting out in the Pacific! There’s been quite a bit of media hype over the past few months, so I’ve decided, in conjunction with a classmate of mine at San Francisco State University’s Department of Earth and Climate Sciences, to share what we actually know (and don’t know) about the developing situation.
As we further progress into 2014, the climate circulation phenomenon known as El Niño looks increasingly imminent. For those who aren’t aware of the pattern, it’s a component of the greater El Niño Southern Oscillation (ENSO) – a global coupled oceanic and atmospheric phenomenon that involves fluctuations in sea surface temperatures and mean sea level pressure. When sudden changes in these properties occur, short-term climate change effects on the order of nine to 12 months are felt around the globe. El Niño is officially declared when the three-month running mean of East Pacific sea surface temperature (SST) is warmer than normal by at least 0.5 degrees Celsius, according to the Climate Prediction Center, a subdivision of the National Oceanic and Atmospheric Administration.
Introduction and Background
So-called “normal” or ENSO-neutral conditions are marked by consistent easterly trade winds in the tropical Pacific, displacing surface waters toward the west and allowing cool, nutrient-rich water to surface via upwelling along the west coast of South America. For this reason, the western Pacific region is very warm and experiences some of the most enhanced convection and precipitation in the world, contrasted with Peru and Ecuador’s coastal deserts at the same latitude. Trade winds pile up the warm water in the West Pacific, making sea levels about 0.5 meters higher than in the East. When an El Niño occurs the easterly trade winds relax, and in some cases reverse direction, allowing for the deep warm waters in the western Pacific to “slosh” back toward South America. This deep moving pool of warm water is called a Kelvin Wave. The maximum atmospheric convection, and therefore cloudiness and precipitation, follows this warm pool which rearranges the overall atmospheric circulation across the entire tropical Pacific. Warmth that is usually (under ENSO-Neutral conditions) confined to the waters of the West Pacific is allowed to spread out and make much more contat with the surface. La Niña, the second component of ENSO, is just the opposite—it’s marked by a strengthening of the easterly trade winds and therefore increased eastern Pacific upwelling and cooler waters.
Figure 1. ENSO index: red values greater than +0.5 represent El Niño, blue values less than -0.5 show La Niña. Values reaching ± 1 are moderate events, while ±1.5 or more are considered strong events. Graph by NOAA.
The Pacific cycles from El Niño to La Niña, with the ENSO-Neutral times in between. Typically El Niño occurs anywhere from two to seven years apart with very strong events occurring about once every 15 to 20 years as seen in Figure 1. El Niño can be seen as a natural means by which the ocean releases its heat into the atmosphere. To get an idea of the heat exchange, we look at the upper-ocean heat content in the equatorial Pacific — that’s the volume of water from 10S to 10N latitude, 150E to 75W longitude, and down to a depth of 200 meters. In the strong warming event associated with the 1997 El Niño, that heat content increased by some 3.5e22 Joules (Kessler 50). To put this in perspective, the US Energy Infrmation Administrtion’s total count for the annual global energy consumed is in the range of 5.0e20 Joules (LESS energy than in the natural processes of a strong El Niño by a factor of 100!) . . . Take a moment to let that sink in. That’s why El Niño can have a massive effect on global wind, temperature, pressure and precipitation patterns.
*The unit “Joule” is a measure of energy or heat. For example, one Joule is the heat required to raise the temperature of one gram of water by 0.24 degrees Celsisus. A Joule is also the same as a Watt of power per second.
When so much energy is given off, the Polar Jet Stream deviates from its normal path in response to meridional temperature gradient changes. And the degree to which this deviation occurs is crucial when we talk about weather impacts over North America, for example. The driver of mid-latitude storms, the Polar Jet, tends to be strengthened and displaced southward during a srtong El Niño and can be positioned right over California. A Polar Jet that sets up farther south can tap into subtropical moisture, which allows for the intensification of mid-latitude storms.
The impacts on weather from El Niño vary greatly depending on its intensity, which is determined by magnitude of sea surface temperature anomalies along with ocean-atmosphere dynamics. Many Californians have a fixed idea that El Niño means wet and La Niña means dry, but it’s important to realize that no two El Niños are created equal. In California, El Niño’s effects are especially variable – and even more so in the northern part of the state. As a general rule, the weaker the Niño, the less of an impact it will have on California’s precipitation. The most recent event in 2009-10 was weak-to-moderate, and state-wide precipitation wasn’t much greater than normal. And let’s not forget the notably dry years of 1976, 1989, 1992-93, and 2006-07, all of which coincided with weak Niños.
Relationship between San Franciso rainfall and El Nino is also a good representation for all of Caifornia
The precipitation relationship doesn’t get reliable until we get into the range of moderate to strong Niños, such as 1986-87, 1982-83, 1971-72, and 1957-58 . . . and of course “super” Niños like 1997-98. All of these events resulted in much wetter-than-normal conditions across California.
Dry Times Persist
We would like to emphasize just how much California could benefit from the enhanced precipitation induced by a strong El Niño event. The state is currently in a multi-year drought that began in about mid-2011 and is meteorologically the most severe to strike the area since record keeping began in the late 19th century. Climate scientists have gone back further in time (by studying tree rings, for example), and have determined that California is likely in its worst drought since the 1500s. Since late 2012, especially, we have seen incredible ridging over the Northeast Pacific, so persistent that it has earned the non-meteorological name “Ridiculously Resilient Ridge” among scientists.
Figure 2. Ridiculously Resilient Ridge of 2013 – non-meteorological term used to describe a persistent ridge that never seems to break down. This is the number one cause of the severe drought. Image by Accuweather.
This large area of high atmospheric pressure anomalies has blocked storms from entering the American West, making 2013 California’s driest calendar year on record. The past three water years have all been sub-par, forcing fields to lay fallow, lakes to dry up, and the Sierra snow pack to be practically nonexistent.
Figure 3. Satellite image by NASA of snow pack in Sierra taken January 2013 (left) and January 2014 (right).
Figure 4. Current reservoir levels throughout California. Information provided by the California Department of Water Resources.
NASA photographs of Lake Mead reflect drought conditions thoughout the Colorado River Basin in the 21st century.
The western states as a whole—especially the Intermountain West—have been experiencing drier-than-average conditions for much of the 21st century. The reduced flows of the Colorado River and consistently low water levels in Lake Mead and Lake Powell since about 2000 are classic signs of the prevailing dry pattern, which several scientists (including us here at SFSU) link to the multi-decadal state of the Pacific Decadal Oscillation (PDO).
Figure 5: Image of PDO phases, University of Washington
The North Pacific’s surface heat distribution oscillates between warm and cool phases approximately every 15 to 30 years, with warm phases tending to support more frequent and stronger El Niños, and cool regimes favoring La Niña. “Warm” and “cool” do not describe the overall surface temperatures of the North Pacific, but rather the distribution of the anomalies. In a “warm” phase, the positive temperature anomalies line up along the eastern edge of the basin, forming a horseshoe shape of warm water in the east surrounding a cool core of negative anomalies out in the Northwest Pacific. A “cool” phase is just the opposite. With the PDO in a cool phase since roughly 1998, sea levels have risen in the West Pacific, drought has engulfed much of the western United States, and the La Nina of 2010-11 was one of the strongest in recorded history. The index used to measure the PDO has abruptly turned positive (warm) in the beginning of 2014, indicating that a shift to a warm phase could be underway.
Figure 6. PDO Index, where the y-axis is temperature anomaly and x-axis is time in years. Blue regions correspond to negative (cool) phases, while orange denotes a positive (warm) PDO. The black is a binomial filter that works to remove noise in the data and create a smooth graph. The shaded color regions are unmodified raw data. Graph provided by NOAA.
El Niño in 2014?
Figure 8. Equatorial Pacific cross-section showing the large Kelvin Wave (yellow, orange and red colors) beginning to surface at 90 degrees West. Chart provided by NOAA.
Beginning in January and continuing through April, there have been a series of distinct westerly wind bursts (WWB) over the West Pacific, which have led to the formation of an incredibly large subsurface Kelvin Wave. A westerly wind burst is exactly what the name says – a relatively sort-lived burst of westerly winds in tropical regions where the easterly trades usually dominate. Not surprisingly, the surface waters respond to the impulse of a WWB by accelerating eastward in the form of a Kelvin Wave. It is believed that these events are crucial in the development of an El Niño. The current Kelvin Wave has now propagated all the way across the Pacific basin and reached the coast of South America, with warm waters already beginning to reach the surface. Both sea level height and SST anomalies have rapidly turned positive in the Eastern Pacific, especially in the region of the Humboldt Current directly off the coast of Peru and Ecuador. Due to the elevated upper-ocean heat content in the tropical Pacific, sea levels have gone up as much as 10 to 20 centimeters. While that may not sound much, it reflects a tremendous amount of thermal expansion due to warmer waters, and it’s a huge indicator of just how much heat lurks below the surface. Here’s what that looks like when measured by satellites from NASA. Not only are the height anomalies similar to those that preceded the 1997 El Niño, but they are spread out over a larger north-south area. WOW!
NASA satellite measurements of sea-surface height anomalies
SST anomalies in the far eastern Pacific have already reached El Niño thresholds, but no El Niño will be declared until the Oceanic Niño Index (ONI) reflects that the conditions persist for at least three months. This is simply because SST’s are variable and can occasionally enter El Niño territory, only to fade back to neutral by the following month. The last time we saw such a “false start” was in the fall of 2012, when SST’s warmed but atmospheric conditions never allowed for a full El Niño event to form. Given the amount of SST warming and westerly wind events along the equatorial Pacific thus far in 2014, it seems reasonable to expect that El Niño should be declared by July. Most computer models agree on this.
There are stories popping up all over the media, making bold claims that we’re in for a Super-Niño, and that “2014 is the new 1997.” We can say with a fair level of certainty that the upper-oceanic heat content in the first part of 2014 has been at least on par with 1997 levels, possibly greater. But the uncertainty is just how much of that heat will make it to the surface and interact with the atmosphere. The further development of both atmospheric and oceanic circulation pattens needs to be watched closely throughout the upcoming summer. We need to look at atmospheric pressure patterns like the Madden-Julian Oscillation (MJO), SST oscillations like the Indian Ocean Dipole, tropical cyclone activity in the Pacific, WWB’s, and of course further development of Kelvin Waves. Continued warming in the eastern Pacific will lead to further relaxation of the trade winds, allowing more warm water to move eastward. It looks as though a lot of this “positive feedback” is already getting underway, but we won’t know about a possible El Niño’s strength until late summer.
Another note: adding to the difficulty in this year’s ENSO forecast is the fact that a significant portion of Tropical Atmospheric Ocean (TAO Array) instruments used for moorings in the tropical Pacific have been taken offline due to budget constraints. When computer models deal with holes in data, calculations are a lot more messy making for less reliable forecasts. So they kind of say: “Uhh, it’s hot down here but we don’t exactly know HOW hot.” Of course we still have very reliable satellites, but they can’t see what’s underneath the surface to give us the same information we get from moorings. The TAO Array is truly an integral part of the ENSO forecast and warning system.
Later in the summer we will have a better idea on the possible strength and effects of this El Niño. So stay tuned, Californians, and remember never to celebrate the end of a drought too soon!
Things to keep in mind . . .
Perhaps this year more than ever, many Americans welcome the threat of a strong El Niño – whether it’s drought relief in the West, fewer Atlantic Basin Hurricanes, or a mild winter to contrast the relentless Polar Vortex of 2013 across the northern and eastern states. While America could especially benefit, there are also some serious things to prepare for and be aware of. After an incredible 2013 fire season and what promises to be another epic one this summer, heavy El Niño-induced rains in California could do a lot more than just happily fill reservoirs. Going into the next rainy season, soil will be parched and in many cases unstable where large areas have been burned. So even without a huge 1997-type of event, there’s an elevated chance of mudslides nd flooding across the state. Furthermore, El Niño typically brings milder winters in California which translate to significantly higher snow levels – making it tough for water officials to decide how much water to release between storms.
Global impacts of El Niño vary widely. Australia, Indonesia, India and parts of China are prone to drought, while Peru and Ecuador – normally coastal deserts – see their rainfall increase by up to ten times. As agriculture and fisheries are affected, the global market takes a hit. It suddenly becomes more than just an issue of predicting the weather.
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