Dr. Nathan Mantua discusses how climate change and seasonal weather patterns influence coastal and marine salmon habitats
Earlier this fall, the UC Davis Coastal and Marine Sciences Institute Center for Coastal Ocean Issues and the Delta Science Program convened the California Salmon and Climate Variability Symposium to explore how variable and changing ocean and hydrologic conditions affect Central Valley salmon and their management. Dr. Nathan Mantualeads the Landscape Ecology Team at NOAA’s Southwest Fisheries Science Center in Santa Cruz, California. His research interests include climate variability and predictability, climate impacts on natural resources, and the use of climate information in resource management. In this presentation, he discusses the connection between atmospheric conditions and the coastal and marine habitats of salmon.
Dr. Nathan Mantua began by reviewing the salmon life cycle and how climate influences Pacific salmon. Salmon have a complex life cycle where they are spawning and rearing for some time in freshwater; they may or may not use the estuary for extended rearing periods, but they will transition through. “On the freshwater side, flow and temperature are critical pressure points that the climate system can exert its influence on,” he said.
On the marine side, ocean conditions play a big part. The ocean environment involves a whole suite of habitat changes; it’s an extremely dynamic environment off the Pacific Coast off of North America. “When the physical environment changes, the food web changes, and we see lots of evidence for changing growth rates and early marine survival rates that end up having impacts on total adult abundance and stock productivity,” he said.
Precipitation for the last four water years has been well below average. While none of the individual years were the driest year in this climate record, Dr. Mantua noted that they approach that, and the consecutive years have contributed successively to this extended drought period. He also noted that in terms of rankings, the 2014-15 water year wasn’t the worst but it’s on the very dry side; it also coincided with very wet years in the middle part of the country.
“If we look at similar record of temperature for the last 120 water years, you’ll see lots of variability – mostly cold years in the early part of the record; some very warm years in the 30s and early 40s, then a mix of cold and warm years up until the 80s where virtually every year now has been warm,” he said. “What is really remarkable to me is the fact that 2014 and 2015 are so far on that outer edge – in fact 2015 shattered the water year temperature record. If we look at just winter, 2014 set a new record for the state of California for the warmest in that 120 year period, and 2015 is another half degree Centigrade above that, so extraordinarily warm temperatures the last two winters.”
The loss of temperature control in Shasta Reservoir was related to the fact that not only was the cold water pool small, it just wasn’t very cold because of the incredible warmth of consecutive years of climate feeding into that system, he said. That extraordinarily warmth was confined to the Pacific Coast and extended all the way into Northern Alaska, all the way up to the Arctic, so it was widespread, he noted.
The reason the West Coast and California the bull’s eye for the wrath of the climate system for the last two years is a persistent circulation pattern known in the media known as the Ridiculously Resilient Ridge. “It’s actually been in slightly different forms the last four winter seasons, but the past two, it’s had pretty similar character with high pressure along the Pacific Coast going from the sub-Arctic all the way down into California, causing very warm dry condition especially on the Pacific Coast,” he explained. “It comes with this cool wet trough, and last winter and the winter before, the eastern seaboard was experiencing lots of blizzards, cold weather, and pretty miserable conditions of a different flavor. This system has been linked back to conditions in the tropical Pacific, the warm temperatures not in the El Nino side, not towards South American coasts, but actually towards the western tropical Pacific that were persistent during these two winter periods appear to favor that condition. They don’t really explain why it was so strong and persistent, but there’s good evidence that the tropical, extra tropical teleconnection was at work, and it’s at least part of the story.”
California’s precipitation is highly variable. “We know from stream gauge records and from our own experiences looking at streams in California that there’s a whole lot of variability in seasonal flows and in interannual variability in flows,” he said, presenting a graph showing streamflow from the Carmel River. “You can see the order of magnitude changes seasonally which are sort of a regular feature, but then there are years where there is no peak at all, at least at this particular gauge. 1976-77 stand out as very poor water years with no flow at that gauge in the Carmel. If I had 2014 here, you would see a similar story. The Carmel River did not flow to sea in water year 2014.”
“The dynamic variability in California watersheds and these small coastal watersheds especially have this extreme from virtually no flow in dry summer periods to high and persistent flows in wet years, is a characteristic that the native fish have had to evolve with,” Mr. Mantua said.
He then showed some slides of temperatures in the Sacramento River downstream from Keswick Dam in recent years, starting with 2006. “The black horizontal lines are estimates of the incubation period based on degree-day models or when those fish were in spawning or when those eggs were in the gravel and the temperatures they experienced, and you can see that at least in 2006, the 56 degree water extended down to about 60 miles from Keswick,” he said. The last really good water year with exceptional snowpack in the Sierras was 2011 about 40 to 60 miles of river experiencing cooler temperatures; compare that with 2014 when they lost temperature control.
“When you get late in the summer and in the fall, the variability that we see even in very highly managed systems with the big reservoir like Shasta is still large,” Mr. Mantua said. “There is a climate sensitivity there in spite of a lot of engineering and a lot of activity to try and reduce the sensitivity and increase the resiliency of the system.”
He then presented a graphic of the year to year variation in the mean distance over the whole incubation period in the summer from 1990-2014, noting that this comes from a historical simulation of stream temperatures for the Sacramento River below Shasta and Keswick. “They have an energy balance model that is assimilating observations of stream temperature, and using the release temperature and volumes along with weather information to predict the longitudinal profile of temperatures down the mainstem Sacramento. I just want to point out that the variability you see early in the record was before the temperature control device was being operated to where we start to manage that with the temperature control device, but even in that case, this last year really stands out as very little habitat on average and a loss of temperature control by the end of the season.”
He then displayed a graphic depicting how the ocean is being driven by changes in the atmospheric forcing. “During the summer half of the year or the warm season, the sea level pressure field over the north Pacific is dominated by high pressure in the atmosphere,” he explained. “This blue arrow is meant to indicate that the winds nearshore blowing from north to south. They tend to be intense, and they are most intense around the Oregon border to Cape Mendocino down to about San Francisco. This is the core of the coastal upwelling, the seasonal upwelling area in the northeast Pacific which is a process where winds from the north blowing towards the equator are sweeping surface water are offshore; that water is replaced by deeper, usually colder and nutrient rich water that fuels exceptional productivity. It makes the California current system one of the most productive marine ecosystems on earth.”
“The winter half of the year is characterized by having a much smaller north Pacific high; its center is located at a lower latitude, sort of between Hawaii and Baja and Southern California,” he said. “The intensity of the winds are smaller, so there is still upwelling off the California coast, but it’s intermittent weak during the winter. As you go further north, into northern California especially, Oregon and Washington, you have a lot more winds from the south with storms that cause coastal downwelling. This seasonal interplay has a huge impact on the structure of the ocean and on its productivity.”
He then displayed two charts of ocean temperatures from 2010, the one on the left from January and the one on the right from June. “The feature that I wanted to draw your attention to here is that the isotherms (the lines of constant temperature) more or less parallel the lines of latitude so offshore is about 11 degrees in the winter,” he said. “During the summer, it’s a very different picture as summer insolation is increasing, the days are getting longer, and generally speaking the oceans warm up at high latitudes, but not along the California coast. We actually see this cold spot that forms between central Oregon down to about Central California with the coldest water between Cape Mendocino and the Gulf of the Farallones. The coldest water of the whole year at the Bodega Marine Lab on average is the first week of June. That’s the cold season because of the intense upwelling.”
It’s also the fog season, Mr. Mantua pointed out, presenting a map showing the contrast in daily maximum temperatures between the coast and the interior. “Fog is really important for supporting cooler habitats in coastal valleys and coastal areas,” he said. “The contrast in daily maximum temperatures can be something like 20 – 3 degrees centigrade just by going 50 miles from a coastal valley into the interior. The inversion that is capping the fog in the coastal marine layer has a height of about 400 meters on average so if you get above that altitude, you pop out into that hot dry part of California. If you go below that, it’s much cooler.”
Mr. Mantua presented a chart showing interannual fog variability along the coast. “Fog variability is also pretty large between years and even between decades,” he said. “Researchers used airport data for coastal cities, in this case Arcata and Monterey as they are highly correlated with each other and with other coastal sites in California. You see a lot of variability between years. The last really good fog year was 2010.”
”They were able to extend this record back by showing that fog on the coast is well correlated with the temperature contrasts between the coast and the interior, so they used surface air temperature records that go back to 1900 as a proxy and those surface-air temperature records suggest it used to be a lot foggier in California in the early 20th century,” he said. “It’s about a 30% decline in fog frequency along the coast in the 20th century, and this loss of fog is well correlated with coastal temperatures in the Northeast Pacific.”
“If we then flip that fog index around and overlay it with the sea surface temperature along the Pacific Coast, there’s a very strong correspondence,” Mr. Mantua said. “In fact, coastal ocean temperatures in the California current system all the way up into the Gulf of Alaska tended to be much cooler in the early 20th century. A lot of warming into the 40s and you see there isn’t much of an indication of a long-term trend since the 1940s. There’s a lot of variability.”
The last two years in the northeast Pacific have gone outside the envelope again, setting records for high temperatures, especially in the Gulf of Alaska in that warm blob spot, he said. “Jim Johnston and I published a paper last year where we looked at that history of coastal temperature variability both on land and in the Northeast Pacific ocean and showed that a time series for the atmospheric forcing that we related to sea level pressure in this broad area between Hawaii and the Pacific Coast, parallels and actually leads the ocean temperature variations by a few months, indicating that changes in the wind field and the circulation are the dominant control over this history of ocean temperature along the Pacific Coast.”
This is different than the ridge pattern, he said. “It really doesn’t project that strongly onto the persistent ridge that we’ve seen, but the warming of the last two years, especially off of California has been amplified by the fact that we’ve had low pressure just to the north and east of Hawaii at the same time of the high pressure closer to the Pacific Coast.”
He then presented a recent graphic showing sea surface temperature anomalies in the eastern part of the Pacific. He noted warm ocean temperatures extending from the Gulf of Alaska down to southern California, as well as the signature of the tropical El Nino with the warming right on the equator that goes from the coast of South America out into the mid Pacific. The California current system and the El Nino phenomena of the tropics are not always in sync, he said. “In fact the warming of the last two years has been stronger off of our coast than in the tropics until just the last few months,” he said.
“During the development of that extraordinary El Nino, the two areas warmed at about the same time, during the summer of 97 through the fall and peaking in the winter, and then getting really cold in the spring and summer of 1998,” he explained. “Beyond spring, it gets a little tricky. In 1982-83, the tropics warmed a lot in the 1982 to develop this extraordinary El Nino event. The coast of California didn’t really experience that warming until that winter, and it persisted all the way through the summer. The really weird ecosystem observations associated with that event off of our coast happened in the summer of 1983, so it was the next summer, not the summer before, so they don’t always go together at the same time, but we do know there are some important mechanisms that tie the two together in some robust fashion and it’s primarily through that teleconnection to the Aleutian Low.”
The resilient ridge created a pattern that diverted the storm track, keeping it dry and warm along the Pacific Coast, and creating this big blob of warm water in the Gulf of Alaska, basically by keeping storms from extracting heat from the ocean, he said. The typical El Nino winter situation has just the opposite pattern; intense low pressure in the Gulf of Alaska and a storm track that’s pointed mostly towards Northern Mexico and the Southern U.S. with mild winters for the Pacific Northwest. Northern California is on a dividing line that at least during the 1998 and 1983 El Nino years.
“This pattern of low pressure was very intense and close enough to shore that the stormtracks spent a lot of time hitting us and precipitation even in Oregon and Washington was about average that year,” he said. “We had good snowpacks, exceptional precipitation for the state, and lots of flooding and mudslides and storm damage on the coast, so the intensity of the El Nino really matters for Central and Northern California. And it will have a bearing on what happens this winter.”
Mr. Mantua then summed it up. “Our Mediterranean climate promotes extreme changes between seasons and years for salmon habitats, but habitats in our freshwater and estuaries have experienced increased exposure to climate variations because of the cumulative impacts that have basically simplified those systems, so now when the climate changes, they respond a lot faster than they might have historically. Salmon sensitivity to these variations is especially high for populations that using suboptimal habitats.”