El Niño, Harmful Algal Blooms, and Crab Season

Contributed by UCSC Scientist and CeNCOOS Principal Investigator Dr. Clarissa Anderson

California is now in the throngs of the continual storm systems that are the hallmark of El Niño in California. Already, this El Niño is shaping up to be one of the top three on record with some similarities to the 1997/98 El Niño-Southern Oscillation (ENSO) event, but with some distinguishing differences as well.

This El Niño is different most notably in the atmospheric dynamics (the Southern Oscillation part of the name) and in the anomalous temperature patterns in the central Equatorial Pacific (as discussed in Francisco Chavez’s post).

While the warmer than average sea surface temperatures off the coast are likely now attributable to El Niño patterns, it is still somewhat difficult to differentiate anomalous warming from the warm “blob” (or Pacific Warm Anomaly) that produced similar patterns throughout the California Current for the last two or more years.  What is clear is that the atmospheric changes brought about by El Niño’s arrival over the North Pacific have been strong enough to diminish the influence of the “ridiculously resilient ridge” of high pressure that has been sitting over California and preventing storm systems from alleviating the long drought (see posts on the California Weather Blog).

Changes in Algal Bloom Likelihood

In this respect, El Niño is winning out against the blob and shifting to weather patterns that are not necessarily as conducive to phytoplankton growth and algal bloom development. Pseudo-nitzschia (the algae responsible for shellfish and other fisheries closures in 2015) is not entirely gone though, with cryptic populations resurging periodically, particularly during calm periods following storms when light levels allow for growth. It has not appeared to be prolonged enough or vigorous enough growth to lead to accompanying high rates of domoic acid production. For this reason, there is hope that the residual pool of domoic acid in deeper waters and sediments that has been intoxicating crabs and devastating fishermen will soon be depleted and/or mixed offshore.


A bigger question remains as to what the spring will bring now that there is an established “seeding” population of Pseudo-nitzschia that seems to be persisting off the coast. Warm sea water anomalies are typically expected during the spring upwelling season following an El Niño winter, and these are similar to conditions that were present during the massive bloom of 2015. Will the nutrient conditions be such that DA production is favored? Will it reach the historic levels we saw in 2015 and be as pervasive or long-lived? Time will tell, and we are mobilizing resources to study this in greater detail as the dynamics evolve. Already, our HAB model suggests a south to north progression of elevated DA probabilities (seen in the .gif above), as we might expect if El Nino is influencing the pattern.

You can learn more about Clarissa’s work on her blog, and more about this event on the CeNCOOS El Niño page