The state climatologist discusses the current drought and the signs of climate change. Is the current drought a harbinger of the future?
With the winter season once again seemingly passing California by, officials are bracing for a fourth year of drought. Earlier this month, the Public Policy Institute of California held a half-day conference in Sacramento focusing on how the state can manage through another dry year and become more drought resilient.
Is the current drought a sign of things to come? Michael Anderson, state climatologist with the Department of Water Resources, kicked off the PPIC conference, Managing Drought, with a presentation addressing that question.
He began with the basics. “California’s climate and weather really depends heavily on its topography,” he said. “The shape of California and it’s location on the planet are fundamental pieces to how and why we get what we get.”
He presented a slide with two graphs on it, noting that the top graph is the monthly distribution of the 8-station index, an index of eight rain gauges that provides a measure of precipitation in the Sacramento River Basin, and the bottom graph is the runoff of the Sacramento River. He said that the average for the index is 50 inches, pointing out that the top graph shows that half of that annual precipitation falls in December, January, and February. “So you have a 90-day window where you get half of the annual precipitation,” he said. “In some ways, that’s nice and tidy – you get everything and make use of it, but it also makes it all or nothing to some degree.”
He also pointed out that the peak in runoff on the lower graph doesn’t coincide with the peak precipitation in the top graph. “Part of that is because some of that precipitation falls as snow and then melts,” he said. “It’s about 18 million acre-feet on average. … We talk about averages but before long I’ll have you convinced that averages don’t matter anymore.”
He then presented a slide depicting the variation in precipitation for the United States, explaining that this graph was produced by Mike Dettinger at Scripps/USGS who looked at the standard deviation of yearly precipitation across the country. “Everywhere you see a pink dot, one year looks an awful lot like the next, and notice where Washington DC is as that’s really important. That’s where a lot of the federal programs are. Compare that to what’s going on in California. We win. We have more variability from year to year than anywhere else in the country. That makes things both good and bad.”
He presented a slide showing the average number of days to receive half of that precipitation, and noted that it’s just a few storms each year are the core of California’s water supplies. “I told you that it was 90 days to get half your annual precipitation,” he said. “The chart shows the number of days of precipitation it takes to actually get that 50%, and you’ll notice those numbers are down around fifteen. So we have 90 days to get 15 days of precipitation, so that means when we get rain, it’s going to come not just evenly a little bit day to day, but it comes in punches.”
He then presented a slide showing how atmospheric rivers are formed and said, “This is how to make rain happen in California. Just a few moving pieces and parts of the Pacific Ocean – and this doesn’t even describe what’s going on in the Atlantic that may have a role in the process as well.”
He noted that the bright colors indicate water vapor, and the bright colors indicate there’s a lot in the intertropical convergence zone. The filaments coming out of the tropics are called atmospheric rivers, and 90% of the equator to pole transport of water vapor happens in those filaments, he explained. “In the wintertime, the storm tracks head south and comes back up in the spring, which kind of fits our nice 90 day window, so we have to have that coming south,” he said. “That’s influenced by things going on in the pole and in the tropics, such as ENSO – the El Nino Southern Oscillation, and MJO tropical convection, or the Madden Julian Oscillation; these are different processes in the tropics that can really kick the atmosphere.”
But these are not the only things going on. There are also polar processes, but the poles are changing quickly and we don’t know a lot about them. “Now that’s kind of fun,” he said. “You don’t know a whole lot about something that is changing rather rapidly and could have significant impacts on what’s going to happen to you.”
It all has to line up and when it lines up right, we get a flood, and when it doesn’t line up, we don’t, he said. “The size of the atmospheric river results from the alignment of key processes, and the absence of these means we’re dry,” he said.
Mr. Anderson said last year Michael Dettinger, Dan Cayan, and himself were kicking around the notion that there ought to be a way figure out what’s happening, but nothing really stands out in the physical world that appears to be driving things. “It has an interesting correlation, so they were looking at precip and looking at thresholds,” he said. “The red one is the top 5% of the amount of rain in a day, so the rainiest days, the top 5%. Well that pattern is starting to come out, versus everything else. So Michael Dettinger did a subset of atmospheric rivers, ones that originate around Hawaii and have distinct characteristics in terms of the alignment of the atmospheric river hitting California. Now you look at that, and when we go dry, we’re not getting atmospheric rivers; when we get a wet period, we’re getting more of them. We don’t know what drives this, but we know it’s important.”
He then presented a slide with two graphs on it with water vapor observations. He noted that the one on the left was just after Christmas in 2013, and the numbers indicate that it was very dry. Over Yosemite there was just a half a millimeter of water vapor. “It’s not going to rain when that is what we have over the top of us,” he said. “At the end of January and beginning of February, things finally did change, the ridge broke down, and now over 3 centimeters of water vapor. The magic threshold is 2.5; once we get above 2.5, we can actually have a decent rainstorm. We now have data, so we’re just starting now to compile a record that actually gets to the process, rather than just waiting to see what the outcome is, and that’s an important piece, too.”
The current drought: What is happening now?
He then presented a slide with a time series of the 8-station index, reminding that the average is 50 inches, but in the wettest year in 1983, it was almost 90 inches. “Back in 1924, only 17” fell all season long,” he said. “Water year 1976-77 was on pace for that until September came along, it got a little wet and kicked it up above it, but it’s right in that same ballpark. Also notice here that 9 of 14 years of the 21st century are below average.”
He then presented a different chart of the 8-station index, noting that the pink line indicates precipitation for 2013-2014, the eighth driest water year on record. He reminded how the year started out pretty good but then went flat. “It was New Years and they are talking about ice skating on Tenaya Lake and I got my first question, ‘Is this what climate change is? and I said, ‘No this is a dry year; hang on, we’ll see what happens.’ Sure enough, we got bailed out in March. Big March – almost average snowpack in the north, but that’s very important – it was only in the north. South folks didn’t get bailed out. That kind of erased worries. It came in March, a little late, but it works.”
The next year starts and we were doing great, he said. “This was about the time I announced that network for observing extreme precipitation at AGU,” he said. “It was so nice of Mother Nature to give us three atmospheric rivers right then, so it rained on 25,000 scientists, just to help drive the point home. At that point, we’re wetter than the wettest year on record. What could possibly go wrong?”
There was another stormy week, and still we were wetter than the wettest year, and then it went dry, he said. “It didn’t just go dry; it went drier than it had been in over 100 years of observations. Well, that’s a little disconcerting because first you’re thinking okay, we’re good, we can ride this out, we’ll get through that year and we’ll get into the next year. Trick is you had all your expectations set, and then Mother Nature changes. Mother Nature is no longer contributing base flow or anything else, and you’re going to try and operate water systems in the entire absence of Mother Nature supporting you through that period. … So in a 14 month period, we spanned the entire distribution of our observations.”
It was the eighth driest on record, but if you only look at certain perspectives, you’re going to miss a lot that’s going on, Mr. Anderson pointed out.
He then presented a slide with a three year time series of precipitation, noting that the three year period begins July 1st and ends June 30th, which is the weather service’s version of a water year. “For our wettest year, it is 1981-1984. 1974-77 had been the driest, and darker green area was the past three years. Notice how many times and how far that is under that 74-77 line. … we have a three year period that is record dry for the whole state.”
He then presented a slide of snow water equivalent for 13 year model mean for May 4th, noting this is from NASA MODUS satellite data. “This is about a month after peak, and we’re starting to get peak runoff season. We see lots of blues, and that’s about a meter of snow water equivalent, so 39 inches of snow water equivalent everywhere you see that nice dark pretty blues, and on the right is the elevation so you can see that it corresponds to those high level elevations.”
“Here’s what happened this year,” he said, presenting a satellite shot of snow water equivalent for May 4. He pointed out there isn’t really any snow in the Feather River watershed, so there’s no peak runoff. There are remnants here and there, but not a lot, he said.
Mr. Anderson said that a USDA colleague from Washington DC went through the data for reservoirs and compiled an average of reservoir levels to determine how the drought reservoir levels compared to 1977. “In 2011, we were fantastic and things were looking awesome,” he said. “In 2012, we started off great, but then things really fell apart, then 2013 and 2014, and he said, look, you’re doing great, you’re better than 1977. I talked to water managers who said, yes, there’s more water there, but there’s also more demands on it, and so the amount of water actually available is less. So that water, even though it’s there, is already spoken for. It has its jobs that it has to do.”
He then presented a graphic of major reservoir levels and noted that the red lines are where reservoir levels should be, and the blue bars are where we are. “We had a fantastic December, and things were really good, but here’s where we are and still the red lines are way above. If you look at things down here in the San Joaquin, things didn’t get better and then the case on the Merced River, it actually got worse.”
Looking to the future: Signs of change
“What are we looking at now doesn’t look like how it did and by the way, it’s not going to keep looking this; it’s going to continue to change,” he said. “This is a small list of things we think are going to happen: Warmer temperatures, smaller snowpack, more rain and less snow, earlier snowmelt, more variability, more extremes. We might be seeing some of these already.”
He then presented another graph of the 8-station index, noting that the blue bars are the period of record, the red bars are from 1971-2000, and the green bars are the 21st century. “You’ll see some change,” he said. “The period of record average is 50; but that last 30 years of the 20th century, we were at 53”, so we had some bonus water being delivered. Now if you look at the 21st century, we’re down to 49.”
He pointed out that the big green bar in December makes you think that December is super wet, but he reminded that 2012 was the third driest in almost 100 years of records with a third of an inch, and 2014 with 8/10ths of an inch was fourth driest. “Those two years are going into that average and that still gets to 12. So when the switch is on, it’s on and when it’s off, it’s off. Averaging it together doesn’t mean a whole lot.”
The variability is evident in the Sacramento River runoff, he said. “Winter runoff isn’t there, and it’s not there to the tune of almost 2 million acre-feet,” he said. “That makes things kind of entertaining to try and manage water though that.”
He then presented a chart of precipitation and temperature from 1895 to 2013, noting that this data is based on a calendar year. He noted that the yellow triangles are the period of record going back to 2895, the diamonds are the 20th century values, and the black squares are 21st century values.
“Now something ought to jump off the page at you really quickly,” he said. “How many black squares are below the period record average? The answer is none of them. We’re not in this part of the distribution anymore. We’re already on the move, and this weird year out here in 1934 that is just so far off the charts, now it has a lot of friends. We say 2013 was drier than 76 and yes it was, but it was also two degrees warmer. It’s not the same drought anymore, and it’s not going to be because we’re not going to keep sampling here; expectations are this is going to get another 3 to 4 degrees warmer.”
He then presented the next slide, noting that April through July runoff is on the bottom and the water year runoff for the Sacramento River is on the y axis;the period of record is the triangle, diamonds are 20th century values, green squares are 21st century. “If you count them, 9 out of the 14 are down at the bottom here. Five of the lowest 20 April through July flows have been since 2001.”
Things are getting interesting and we had better find a way to keep track of it, Mr. Anderson said, presenting a slide of the California Climate Tracker, which compiles website takes the weather service cooperative observer data, rainfall and temperature. “They came up with their own regions, and with each of those, you can break down the record from 1895 to present … for the south coast where we have quite a few people, and the region where we do quite a bit of ag, it was the driest water year they had ever seen.
“Temperature wise, everywhere you see red was a record, and that’s going to keep happening,” he said. “That’s not just an aberration anymore.”
In conclusion …
Mr. Anderson then wrapped it up with his summarizing thoughts:
Current drought and other 21st Century droughts have shown record-setting characteristics and are warmer than 20th Century counterparts.
Atmospheric river events provide significant inputs into annual precipitation totals. There are fewer such events in drought years, and characteristics of atmospheric events will change with climate change. “When they are there, we have water; when they are not, we don’t. What that means then instead of thinking about how much rain falls in a year, you’ve got to start thinking about opportunities to have these storms come and get us. Our best opportunities are in those three months where we on average get half of our annual precipitation.”
Planning for future droughts can take advantage of information in the historical record, including paleo reconstructions. The trick will be to increase our understanding of causal mechanisms and watershed condition/response over different time scales. “Now, just because things are changing doesn’t mean you can abandon the past. There’s information in there and you really need to pay attention to it. Pay attention to what’s in the historical record including paleo reconstructions. The trick is increasing your understanding of causal mechanisms and process and watershed condition response under differing conditions. It’s there. We just have to figure it out.”
Averages are not so useful anymore. We need to understand variability and process: “You really need to start thinking about variability and thinking about process to really understand how things are going to play out from here on out.”