Figure 1. Summary of observed annual mean beach width change since 1998. Lines are linear best fit beach width trends between El Nino years for individual beaches. Beaches with negative trends between the 2016 and 2024 El Ninos (upper panel) are grouped as currently eroding on a multi-year time scale. Beaches with 2016-2023 positive trends are grouped as currently accreting (lower panel).

San Diego County beach change over the last several decades can be conceptualized as the interplay between two primary variables:

• Available nearshore sand supply.


• Wave-driven beach erosion and accretion in response to the El Nino-Southern Oscillation (ENSO) climate pattern, with each multi-year period between El Nino winters being a distinct "beach recovery phase" (trend lines, Figure 1).

Unusually high winter waves and severe beach erosion are often associated with episodic El Nino winters, which typically occur every 2-7 years. Natural beach widening, or recovery from El Ninos, happens more gradually over the course of the milder winter years between El Nino events.

During the recovery phase, the beach does not necessarily increase monotonically in annual width, because there is still a modest range of winter and summer wave variability from year to year. However, the net change, or trend, is positive over the recovery years assuming there is a sufficient supply of nearshore sand to migrate shoreward onto the subaerial beach during the summer high tides.

South Torrey Pines State Beach (yellow squares, upper panel) is considered the closest thing to a well-monitored "control beach" in the County. It is long (3.5km), wide (70m), and relatively stable. It hasn't been nourished directly, but is located at the southern boundary of the Oceanside Littoral Cell. As a result, the beach's sand supply is likely enhanced by the overall southward littoral drift in the Cell, and the beach remains mostly sandy year-round.

Prior to 2021, the last three ENSO beach recovery phases (post-1998, 2003, 2010, 2016 El Ninos) at South Torrey Pines show a consistent pattern that begins with an El Nino winter erosion year of varying severity (2-10+ m), followed by multiple years of beach recovery with an average rate of approximately +1m/year (yellow trend line slopes between 2003 and 2015, upper panel, Fig. 1). It is a useful reference beach when interpreting the erosion-recovery phases of the other beaches in the County.

• 2024 was an El Nino beach year, marking the end of the latest ENSO recovery phase (2016-2023). Resetting the 2016-2023 beach width trend lines accordingly, most North County beaches experienced a net loss across this 7-yr period owing to two atmospheric river winters (2017,2023) and the extreme wave event in January, 2021 (upper panel, Fig 1.). This poor recovery phase outcome is more reminiscent of the 1978-1998 time period with more frequent above average wave years (Bromirski et al, 2013). Fortunately, the 2024 El Nino was mild compared to the 2010 and 2016 El Ninos, with most beaches showing slight gains or losses in annual width since last year.


• Nearly all the San Diego County State Beaches have negative trends in annual beach width (-1m to -3m/year) over the past 3 years (Eroding Beaches, upper panel, Fig. 1).


• Moonlight SB (blue diamonds, upper panel) has a large sudden gain in width (+20m) in 2024 owing to a beach nourishment.


• Silver Strand in South County had a significant positive trend (+1m/yr) in the 2016-2023 recovery phase (Accreting Beaches, lower panel, Fig. 1), but also experienced the largest 1 year loss (-10m) during this 2024 El Nino year.


• The 2024-2024 Winter Waves and Beaches Outlook predicts above average waves and beach loss conditions with the forecasted El Nino, and 2024 will likely be another year of net beach loss.

References

Bromirski, P. D., D. R. Cayan, J. Helly, and P. Wittmann (2013), Wave power variability and trends across the North Pacific, J. Geophys. Res. Oceans, 118, 6329-6348, doi:10.1002/2013JC009189.