ENSO-neutral conditions continue and are expected to remain through the fall. Let’s hit the road (virtually) and take a trip around El Niño/Southern Oscillation land! Who needs Carhenge when you have the Walker circulation?
Buckle your seatbelts
Perhaps you recently looked at the table of historical ENSO episodes, where the Oceanic Niño Index is recorded. This index, the three-month-average sea surface temperature anomaly in the Niño3.4 region (anomaly = departure from long-term average), is our primary metric for ENSO. (In climate prediction, we refer to any three-month-average period, e.g. February–April, as a season, so I’ll use that going forward here.)
Perhaps you looked at the table, and perhaps you noticed that the last five seasons, starting with October–December 2019 and going through February–April 2020, are all at or (very slightly) above 0.5°C, and colored red in the table. What’s that about? Was El Niño going on all winter and we were asleep at the wheel? Not so fast.
There are a few things going on here. First, as we are always talking about here on the Blog, ENSO is a coupled system, involving both the tropical Pacific Ocean and the atmosphere. In the case of El Niño, warmer-than-average ocean surfaces in the central and eastern equatorial Pacific lead to more rising air, clouds, and rain, changing the Walker circulation. The atmosphere in turn affects the ocean through changes in the near-surface winds. When the surface temperature anomaly is expected to persist, and the ocean and the atmosphere are both showing characteristic changes, we call this El Niño conditions—and coupled conditions did not persist this past winter.
There’s no clear threshold where ocean-atmosphere coupling switches on. We have examples in recent years (2014-15 and 2018–19) where the ocean surface warmed up and it took a few months for an atmospheric response to kick in. (Check out Nat’s post and paper investigating the delay.) Those years, the ocean surface was predicted to continue warming, and El Niño was expected. This time around, the Niño3.4 surface temperature anomaly was predicted to hover right around 0.5°C for a few months, but forecasters did not expect the surface to continue warming, nor for the atmosphere to exhibit the characteristic El Niño response.
Stakeholders request a simple, official metric for ENSO and the 0.5°C anomaly in the Niño3.4 region was chosen because it is most representative of ENSO. (Tony covers the history of this choice here.) Usually, the atmosphere responds to a consistent anomaly of 0.5°C or more, but not always. So, occasionally we’ll end up with red on the ENSO table—meaning ocean criteria were met—without evidence of a full-blown El Niño. Such is the agony of borderline conditions!
The world’s tallest haystack
Finally, there’s another issue that is likely coming into play here. The tropical Pacific is experiencing climate change along with the rest of the world, and it’s affecting the “average.” What counts as “average” is getting warmer over time. NOAA uses a 30-year period to define average, and right now that is 1986–2015 for the Oceanic Niño Index. In January of 2021, we’ll update the average period to 1991–2020.
This update will result in changes to the ONI values dating back to 2006. Some seasons that were warm enough to qualify as El Niño when compared to the older (cooler) average won’t be when compared to the more recent (warmer) average. Since winter 2019–2020 was right on the edge, it’s likely that at least one season will drop to 0.4°C, and we’ll no longer have five consecutive seasons at or above the El Niño threshold.
The short version of all of the above is “no, El Niño was not present this past winter.” It’s like meeting the age requirement to get a driver’s license, but not passing the written test.
On the road again
Where are we headed next? There’s a 65% chance that ENSO-neutral conditions will last through the summer. As predicted, ocean surface temperature anomalies decreased through April and into May. Winds over the surface of the tropical Pacific, the trade winds, have been stronger over the past few weeks, helping to cool the surface. Also, an area of cooler water beneath the surface has expanded over the past weeks.
Some of the computer models are hinting at increasing chances for La Niña next winter, although there is a wide range of potential outcomes. Forecasters estimate the odds of La Niña developing in early winter are about equal to the odds that neutral will continue, with lower chances for El Niño.
June will find us emerging from the spring predictability barrier, a time where it’s more difficult to accurately predict ENSO development. Nature’s in the driver’s seat, so it’s guaranteed to be an interesting trip.
The Northern Water Board of Directors allocated 15,000 acre-feet of Regional Pool Program (RPP) water during its May 14, 2020, Board meeting. RPP water is available for lease by eligible Northern Colorado water users, with sealed bids due May 28, 2020. Bid prices per-acre-foot must be greater than or equal to $27.40, a floor price the Board selected based on the 2020 agricultural assessment rate.
Due to the COVID-19 pandemic, interim procedures have been instituted for the May 2020 RPP allocation. The interim procedure and additional Regional Pool information are available at http://northernwater.org/regionalpool.
The following forms are required to submit a bid:
Pre-Approval Form – To confirm eligibility, interested bidders must email or mail the Pre-Approval Form to Northern Water. In person delivery will not be accepted in 2020. A new Pre-Approval Form is required each year.
Carrier Consent Form – If the RPP water will be delivered by a carrier, such as a ditch or reservoir company, bidders and their carriers must complete the Carrier Consent Form or provide a signed agreement stating that the carrier will deliver the RPP water to the bidder. This form must also be emailed or mailed to Northern Water; in person delivery will not be accepted.
Bid Form – Sealed bids will be accepted at Northern Water’s headquarters through a “self-serve” process. Bidders will sign in at a kiosk in the lobby and print a bid label for their sealed bid envelope. The label will identify the bidder name, date and time stamp, and bid number. Secure the label to the bid envelope and place in the drop box. Sealed bids may also be mailed to Northern Water, but must be received before the deadline.
Sealed bids are due by 2 p.m. May 28 at Northern Water’s headquarters, 220 Water Ave., Berthoud, CO 80513. As described above, sealed bids can be mailed or hand delivered; email and fax bid forms will not be accepted. RPP leases will be awarded based on highest bids per acre-foot. Sealed bids will be opened during a 9 a.m., June 1 Zoom video conference. The link to the Zoom video conference will be available at http://northernwater.org/RegionalPool.
Many staff are working remotely due to the COVID-19 pandemic and are not available to answer questions in person. Questions regarding the Regional Pool Program and bid submittal can be emailed to email@example.com or by calling Sarah Smith at 970-622-2295 or Water Scheduling at 970-292-2500.
FromThe Washington Post (Brady Dennis and Juliet Eilperin):
The Environmental Protection Agency has decided not to limit perchlorate, a chemical that has long been detected in the drinking water of many Americans and linked to potential brain damage in fetuses and newborns and thyroid problems in adults, according to two agency officials briefed on the matter.
They spoke on the condition of anonymity because the decision hasn’t been announced.
The move, which comes despite the fact that the EPA faces a court order to establish a national standard for the chemical compound by the end of June, marks the latest shift in a long-running fight over whether to curb the chemical used in rocket fuel.
Under President Barack Obama, the EPA had announced in 2011 that it planned to set the first enforceable limits on perchlorate because of its potential health impacts. Both the Defense Department and military manufacturers have long resisted any restrictions on the chemical, which is also used in fireworks, munitions and other ignition devices. It naturally occurs in some areas, such as parts of the Southwest.
In an email Thursday, EPA spokeswoman Corry Schiermeyer said the agency “has not yet made a final decision” on whether to limit perchlorate in drinking water. “The next step in the process is to send the final action to the Office of Management and Budget for interagency review,” she said. “The agency expects to complete this step shortly.”
The EPA also issued a news release Thursday in which Administrator Andrew Wheeler hailed the fact that levels of perchlorate exposure have declined since 2011. Though no federal standards regulating perchlorate levels in drinking water exist, some states have already acted to reduce the amounts in their drinking water systems. California and Massachusetts, for example, have set limits for perchlorate at levels far lower than what the EPA had previously proposed.
Here’s the release from the University of California Riverside:
Grasslands across the globe, which support the majority of the world’s grazing animals, have been transitioning to shrublands in a process that scientists call “woody plant encroachment.”
Managed grazing of drylands is the most extensive form of land use on the planet, which has led to widespread efforts to reverse this trend and restore grass cover due to the belief that it results in less water entering streams and groundwater aquifers.
A new study led by Adam Schreiner-McGraw, a postdoctoral hydrology researcher at the University of California, Riverside, modeled shrub encroachment on a sloping landscape and reached a startling conclusion: Shrub encroachment on slopes can increase the amount of water that goes into groundwater storage. The effect of shrubs is so powerful that it even counterbalances the lower annual rainfall amounts expected during climate change.
Until now, researchers have thought that because woody plants like trees and shrubs have deeper roots than grass, woody plant encroachment resulted in less water entering streams and groundwater aquifers. This belief stemmed from scientists performing their related studies on flat ground.
“It is striking that ecosystem composition is what controls projected future changes to groundwater recharge,” Schreiner-McGraw said. “This does not mean that climate change is not important, but that vegetation change is potentially more important and something that scientists and land managers should focus more effort on understanding.”
Co-author Hoori Ajami, an assistant professor of groundwater hydrology at UC Riverside, said the paper looks at the combined effects of climate and vegetation change on groundwater-recharge processes in arid environments.
“Most studies to date have looked at these changes in isolation,” Ajami said. “Here we illustrate that the combined effects of vegetation change and climate change could be greater or less than the sum of its parts.”
The intrusion of shrubs into grasslands is often considered a problem because it reduces the amount of forage available for livestock grazing and can lead to more bare ground patches and subsequent increase in soil erosion. This process of creating more bare ground is called “xerification.” Climate change contributes to xerification, but fire suppression and overgrazing play the biggest roles.
It makes sense that shrubs, which have deep root systems along with thick stems and many leaves, capture more water than grass does as it percolates down through the soil, leaving less available water to replenish the underground aquifers. Research on “diffuse recharge,” the process by which water replenishes groundwater supplies over a large area, seems to bear this out for flat landscapes. Xerification of grasslands has thus been viewed as bad for both livestock and the water cycle.
“We approached this research with a simple premise that topography plays a role in redistributing available water, and this should affect the outcomes of xerification,” said co-author Enrique R. Vivoni, a professor at Arizona State University.
The group looked at focused recharge, which occurs when hillslopes funnel water into concentrated areas, such as streambeds. Streambeds often have sandy bottoms, which allow water to quickly infiltrate and prevent the deep-rooted shrubs from sucking it up.
Data from a highly monitored desert mountain slope in New Mexico was used to simulate the effects of woody plant encroachment and climate change on water resources. The team discovered that not only did the shrubs increase focused groundwater recharge, but that they did so even under conditions where climate change reduced the amount of rainfall.
They also modeled a more extensive form of shrub encroachment called thicketization, in which plants grow in dense stands with no bare patches, and found, as in prior flat landscape research, the shrubs reduced the amount of groundwater recharge on slopes as well.
On hillslopes, bare soil in between patches of shrubs is necessary to drive water into streambeds. Increased runoff increases focused groundwater recharge.
“We were surprised to find that a transition from grassland to shrubland can increase sustainability of groundwater aquifers,” said Schreiner-McGraw. “The best way to increase focused recharge in this system is to increase the amount of runoff from hillslopes that gets concentrated in the streambeds.”
Climate change will most likely increase groundwater recharge by making rainstorms larger, but less frequent. Larger storms increase the amount of runoff that reaches sandy-bottom channels and increases groundwater recharge. Findings from this study suggest vegetation will also play an important part in groundwater recharge in the future.
Though the study took place in New Mexico, Schreiner-McGraw said it applies to similar environments. Large parts of California are also desert savannahs. Southern California and the Central Valley have landforms similar to those found in the New Mexico study site. These areas could experience similar hydrological processes, though atmospheric rivers create storms very different from monsoon storms, so more research is required.
“The study highlights the role of long-term monitoring in understanding water balance dynamics of watersheds, and the role that process-based modeling plays in understanding system dynamics,” Ajami said.
Here’s the release from the Cary Institute of Ecosystem Studies:
In the Grand Canyon reach of the Colorado River, two species play an outsized role in the fate of mercury in the aquatic ecosystem, and their numbers are altered by flood events. So reports new research, published in Science Advances, that is among the first to meld ecotoxicology and ecosystem ecology to trace how mercury flows through aquatic food webs and then spreads to land.
Mercury is an environmental contaminant that occurs in ecosystems globally. In its organic form, it is a potent neurotoxin that can harm people and wildlife. Mercury accumulation in animals and how it magnifies along food chains is well studied. Less well understood are the pathways mercury takes through food webs to reach top predators, such as fish and birds, and how those pathways might change after large ecosystem disturbances, such as floods.
Emma Rosi is an aquatic ecologist at Cary Institute of Ecosystem Studies and co-lead author on the paper. She explains, “By combining data on mercury concentrations in aquatic life with well-studied food webs, we were able to reveal how mercury moves through an ecosystem. We found that flooding and an invasive species both influenced the flow of this contaminant of global concern.”
The traits of organisms living in an ecosystem – their physiology, what they eat, and what eats them – determine contaminant movement and exposure. These factors have rarely been included in models of contaminant flux and fate. “Pairing contaminant concentrations and highly detailed food webs has the potential to improve the management of contaminants in ecosystems,” Rosi notes.
To study these pathways, the research team developed mercury-based food webs for six sites spanning 225 miles of the Colorado River, extending downstream from the Glen Canyon Dam in Grand Canyon National Park. Food web sampling took place seasonally over two years. At each site, they measured algae, invertebrates, and fish to determine who was eating what – and what that meant for mercury exposure at each level of the food web.
Insects (blackflies and midges) and invasive New Zealand mudsnails were the dominant invertebrates in the river. These animals play a vital role in moving energy and contaminants from the bottom of the food web to fish predators at the top. Fish included native Bluehead Sucker, Flannelmouth Sucker, Speckled Dace, and Humpback Chub, as well as non-native species such as Common Carp, Fathead Minnow, and Rainbow Trout.
The stomach contents of invertebrates and fish were assessed to identify what they ate and in what amounts. Algae, detritus, and animals were analyzed for mercury concentrations and, combined with the diet data, the team estimated the amount of mercury that animals were consuming throughout the year.
Food web complexity varied across the study sites. Just below the Glen Canyon Dam, food webs were simple with few species and food web connections. Further downstream, food webs had higher species diversity and more connections. Across the study sites, regardless of food web complexity, relatively few species were key players in the movement of mercury.
Algae and tiny particles of detritus were the source of 80% of mercury flowing to invertebrates.
In sites closest to the dam, invasive mudsnails dominated the food webs. Trout were the only fish in this part of the river, and they are unable to digest mudsnails. Mercury accumulated by the snails did not move up the food chain. Because the snails are fully aquatic, mercury cycled back into the river’s detrital food web when they died.
Blackfly larvae were the source of 56-80% of the mercury flowing to fish. Blackflies are preferred prey for fish, such as Rainbow Trout, and blackflies had higher mercury contaminations compared to other invertebrates. Blackflies that escape predation and emerge from the river as flying adults move mercury from the river to land. This can expose terrestrial predators, such as birds and bats, to mercury that started out in the river.
The amount of mercury that blackflies moved to land was dependent on the number of hungry fish in any part of the river. At some sites, fish ate nearly 100% of the blackfly larvae, leaving few left to emerge. At other sites, there were a lot more blackflies than the fish could eat. When these blackflies emerged as adults, the mercury inside them hitched a ride to terrestrial food webs along the river.
One year into sampling, the study sites were flooded as part of a planned dam release. The team was able to explore the effects of the flood on mercury movement in the food webs. At sites near the dam, the flood washed away large numbers of New Zealand mudsnails and led to a boom in blackfly populations. With the rise in blackflies, more mercury flowed to trout. Because trout gobbled up nearly all the blackflies in their larval form, very little of the mercury accumulated in these abundant insects was transported to land by the flying adults.
Rosi explains, “Changes to the animal populations in an ecosystem will impact how mercury moves through a food web. This was especially apparent at sites where flooding changed the proportion of blackflies relative to fish. Flooding dramatically altered mercury pathways in the simple tailwater food web near the dam, but not in the more complex food webs downstream.”
“Invasive species and dams are common in rivers globally, and both factors were at play in the Grand Canyon reach of the Colorado River.” Rosi says. “We found that flooding changed the species present at our study sites, and mercury flow changed with those shifts.”
“Understanding the factors that control the movement of mercury through food webs can help resource managers protect ecosystems that are susceptible to mercury pollution,” says David Walters, USGS scientist and co-lead author of the study.
Rosi concludes, “This study is exciting because it sheds light on the depth of understanding we can achieve when we merge ecological and ecotoxicological thinking. Species traits, animal populations, predator-prey interactions, and disturbance can all influence the movement of contaminants in the environment. Understanding the complex interplay of these factors can improve risk management of animal exposures in the environment.”