SAN DIEGO – As national forecasters predict El Niño is likely to take over soon, a new study from the UC San Diego’s Scripps Institution of Oceanography could give researchers a better idea of how these conditions could impact local waters.

The study, which looked at over a century of records from the Scripps Pier in La Jolla, focused on seawater salinity to detect El Niño and La Niña years of the past, providing a basis of understanding how the coastal ocean connects to wider climate patterns.

“The study is great, because it’s one of the first we’ve seen that really connects the coastal ocean as a part of the atmospheric variability system,” lead researcher and oceanography PhD student, Sierra Byrne, told 

“It’s already been established that we know how the rain is going to change when we’re in an El Niño versus a La Niña,” she continued, “but we haven’t seen anyone really fitting that into what does that mean for a coastal ocean.”

Generally, climate patterns like El Niño and La Niña are marked by water temperatures: El Niño years, which are characteristic of increased rainfall, bring warmer-than-average waters to the central-east tropical Pacific. La Niña, on the other hand, brings cooler-than-average temperatures and less rainfall.

Longer climate variability also plays a role in these patterns, specifically Pacific Decadal Oscillation (PDO). As Byrne explained, PDO ocean temperature fluctuations take place over the span of a decade – or longer – and are generally considered the “long-lived El Niño.” 

The relative freshness of the water, analyzed in the new UCSD study on salinity, is just another manifestation of this large-scale atmospheric variability.

It should be expected during years of heavy precipitation – the El Niño pattern – more freshwater drains down through local waterways and eventually ends up flowing into the ocean, thus creating changes to the saltiness of ocean water.

“It’s a pretty visible signature when you’re recording salinity data to see these huge drops in salinity that come after rain events,” Byrne said.

Using 105 years of facility measurements recorded off the Scripps pier, the study found that historically both Niño patterns and PDOs were associated with variability in the salinity in the waters near San Diego.

Daily sea surface salinity measurements made at Scripps Pier from 1916 to 2021. The x-axis shows the year and the y-axis displays the recorded salinity, or saltiness, in the water. (Screenshot by KSWB/FOX 5 from "Southern California winter precipitation variability reflected to 100-year ocean salinity record" study)
Daily sea surface salinity measurements made at Scripps Pier from 1916 to 2021. The x-axis shows the year and the y-axis displays the recorded salinity, or saltiness, in the water. (Screenshot by KSWB/FOX 5 from “Southern California winter precipitation variability reflected to 100-year ocean salinity record” study)

For instance, the signature of a 30-year-long negative phase of the PDO – when the water was cooler, meaning less precipitation and drought conditions – from the 1940s to 1970s revealed a pattern of minimal salinity variation in pier samples.

“It is the reaction to how the climate responds to this climate forcing in the new pattern, and that changes the rainfall we get, which then changes streamflow, which then changes the salinity,” Byrne said.

While there are other ways to observe these kinds of trends, the approach of capturing spikes in salinity in a location like the Scripps Pier is particularly novel for this purpose. 

That’s because it’s away from major river sources. The nearest freshwater source comes from the Los Peñasquitos Lagoon seven kilometers, or about 4.2 miles, to the north and the San Dieguito River 12 kilometers, or eight miles, to the south.

Despite that, it can still pick up this signal of heavy freshwater flow that’s indicative of climate patterns that facilitate more precipitation.

“We don’t have a major river source, so frequently, these types of studies that can show a long distance freshening from a rain event are regions where there’s a consistent freshwater output from a big river that might have a lot more range,” Byrne said. 

In the findings of the study, “(we) really see that these must be some pretty strong rates for the freshwater that’s coming out of something like a local stream.”

This connection between freshwater in the ocean and climate variability, Byrne said, will be important to improving coastal monitoring.

Heavy precipitation events, like the winter storms seen earlier this year, can have a devastating impact on seaside communities – from landslides and sinkholes to car accidents caused by dangerous conditions.

Even after the rain clears, the freshwater could have picked up possible pollutants, like fertilizers and trash, as it flows into the ocean as run-off, making the coastal ocean potentially hazardous to beachgoers.

Current guidance for San Diego residents is to wait about 72 hours after a rain event before getting into the water, Bryne said. 

But this record of salinity could allow researchers in future studies to understand where the water is going and how residents could best protect themselves and their health from hazardous materials.

The study, according to Byrne, serves as a great jumping off point for further studies on the connection between climate variability and coastal dynamics.

“This is a really zoomed out view,” Byrne said. “That’s really an interest for us was getting this background variability of what we have seen in the past, given 105 years to look at … maybe we can have a better idea going forward (of) what we can expect.”

This extends to possible scenarios for how climate change will impact patterns like PDO, El Niño and La Niña.

“In terms of projecting as things change with future climate, we look more at past variability in an effort to understand what we might expect in natural variability,” Byrne said, “things that the climate is doing naturally on its own.”