How Upwelling Seasons Affect San Diego Water Temperature and Visibility

San Diego’s reputation as a year-round freediving destination masks a critical reality: upwelling cycles dramatically reshape water conditions every few months. Understanding when and why upwelling occurs isn’t just useful—it’s essential for planning safe, productive sessions.

What Is Coastal Upwelling?

Upwelling happens when northwesterly winds push surface water away from the California coast, forcing nutrient-rich, cold deep water to rise and replace it. This process is strongest in late spring and summer, typically May through September, when the Pacific High pressure system dominates and winds blow consistently from the northwest.

For San Diego freedivers, this means predictable seasonal patterns. Winter water (December–March) sits around 57–60°F. By late spring, upwelling can drop temperatures to 52–55°F at depth—a 5–10 degree swing that fundamentally changes exposure protection needs and bottom time.

The Spring Transition (March–May)

March typically delivers the warmest water of the year, often reaching 62–64°F. This is the sweet spot for many San Diego freedivers. Visibility at Sunset Cliffs and Coronado Islands can exceed 40 feet on calm days. Nitrogen narcosis feels gentler, and thermal comfort extends dive windows.

By late April, upwelling begins. You’ll notice the change first at deeper sites like the Coronado Islands’ outer reefs and the kelp forests off Mission Beach. Surface water remains warm, but descend beyond 30 feet and the thermocline becomes obvious—sometimes a 10-degree drop in just a few feet.

Peak Upwelling (June–August)

Summer is paradoxical in San Diego: beautiful weather coincides with some of the year’s coldest water and worst visibility.

Water Temperature: Upwelling reaches peak intensity in July and August. Surface temps may stay 60–62°F, but at 40–60 feet you’re looking at 52–54°F. The thermocline becomes razor-sharp, sometimes occurring at 20 feet.

Visibility: Upwelling brings nutrient-rich water that fuels phytoplankton blooms. Visibility at La Jolla Shores and Pacific Beach often drops to 15–25 feet during peak upwelling. The Coronado Islands and outer kelp forests can be even worse—10–15 feet is common. Blooms occasionally create murky, greenish water that feels disorienting at depth.

This is when proper thermal protection becomes non-negotiable. A 5mm or 7mm wetsuit isn’t optional; it’s your baseline. Many San Diego freedivers switch to hooded suits or even drysuits during this window.

The Fall Reversal (September–November)

Upwelling weakens dramatically in September as the Pacific High pressure system retreats. Winds shift, surface water warms, and the water column begins to restratify.

By October, water temps climb back to 59–61°F. Visibility improves noticeably—60+ feet is achievable at Sunset Cliffs on good days. The thermocline softens and sits deeper (40+ feet). This is the second-best visibility window of the year, after spring.

November is often exceptional. Water temps hover around 58–60°F, visibility hits 50+ feet, and upwelling has largely ceased. If you can only freedive one month, October or November rivals April for overall conditions.

Winter Stability (December–February)

Winter water is cold but consistent. Upwelling is minimal because northwesterly winds are weaker and more variable. Water temps stabilize around 57–59°F. Visibility depends more on storm runoff and swell than upwelling.

Rainy periods bring murky water (Tecolote Canyon near Mission Bay is especially affected), but clear days can produce 40+ feet of visibility. Winter also brings occasional warm-water events—the 2023–24 marine heatwave pushed temps above 65°F into January, a stark reminder that climate variability is overlaying traditional patterns.

Site-Specific Upwelling Effects

Not all San Diego sites respond equally to upwelling. Exposed, deep reefs like the Coronado Islands and outer Mission Beach feel upwelling first and most intensely. Protected bays like La Jolla Shores, Torrey Pines, and Sunset Cliffs warm more slowly and cool less dramatically—though they still experience 5–8 degree swings.

For summer freediving, La Jolla Shores is your best bet during peak upwelling. The shallow, protected bay buffers temperature extremes. Visibility still drops, but it rarely falls below 20 feet.

Practical Takeaways

Plan your exposure protection around the upwelling calendar, not the calendar date. A 3mm suit works in April; it won’t cut it in July. Track water temps obsessively from June through August. Download the Element app to monitor real-time conditions and upwelling forecasts—knowing whether upwelling is intensifying or weakening this week changes everything about site selection and depth targets.

Upwelling is part of San Diego’s marine character. Respect it, and you’ll dive smarter, stay warm, and find the clearest water possible when it’s available.