Coastal Upwelling Season in San Diego: Cold Water and Plankton Blooms
Every spring, San Diego’s coastal waters transform. Prevailing northwesterly winds push surface water offshore, allowing deep, cold, nutrient-rich water to surge upward from the continental shelf. This phenomenon—coastal upwelling—creates the conditions that define freediving in San Diego from April through September, and understanding it is essential for planning safe, productive dives.
What Is Coastal Upwelling?
Coastal upwelling occurs when wind-driven surface currents move water away from the shore. To fill the void, water from depths of 200–600 feet rises to the surface. This deep water is cold—typically 48–55°F—and saturated with nutrients like nitrogen and phosphorus that have accumulated in the oxygen-poor depths.
In San Diego, upwelling is most intense from April through June, when the North Pacific High-pressure system strengthens and sends consistent northwesterly winds down the coast. The Scripps Institution of Oceanography, located right here in La Jolla, has monitored these patterns for decades and confirmed that our region experiences some of the strongest upwelling on the California coast.
The Plankton Bloom Effect
When nutrient-rich water reaches sunlit surface layers, phytoplankton—microscopic algae—explode in population. This bloom can turn the water a murky green or brown, sometimes so thick it reduces visibility to just 10–15 feet. While visually frustrating, plankton blooms are a sign of a healthy, productive ecosystem.
Zooplankton follow, feeding on phytoplankton. Fish populations surge in response. Kelp forests thicken. The entire food web intensifies. For freedivers, this means abundant marine life around La Jolla Cove, the Coronado Islands, and Sunset Cliffs—but it also means adapting your technique and expectations.
Visibility and Conditions by Location
La Jolla Cove sees the worst visibility during peak upwelling (May–June), often dropping to 15–20 feet. The cove’s protected position means water mixes slowly; plankton lingers. Plan dives for early morning before wind stirs up additional particulates, and expect reduced light penetration at depth.
Bird Rock and the Coronado Islands typically maintain slightly better visibility—25–35 feet—because stronger surge and current help flush the water column. However, currents can intensify during peak upwelling. Check Element for real-time conditions before heading out.
Sunset Cliffs and the deeper reefs south of Ocean Beach often escape the worst blooms because offshore winds push plankton-laden surface water away from the southern coast. This area frequently offers 40–50 feet of visibility when La Jolla is socked in.
Temperature and Thermal Protection
Upwelled water chills rapidly. By May, surface temperatures at La Jolla Cove drop to 54–58°F. At 60 feet, expect 50–52°F. A 5mm or 7mm wetsuit becomes mandatory; many serious freedivers switch to dry suits or thick 9mm neoprene during peak upwelling months.
Cold water accelerates heat loss and triggers the mammalian dive reflex more aggressively. Breath-holding capacity may increase slightly, but nitrogen narcosis and CO2 buildup remain serious concerns. Respect your limits, dive conservatively, and always use a buddy line.
Nutrient Enrichment: The Upside
Don’t view upwelling as purely an obstacle. The nutrients that cloud visibility also fuel kelp forests and attract pelagic species. Giant kelp grows explosively in upwelled water. Garibaldi, sheephead, and calico bass concentrate in kelp. Leopard sharks cruise sandy bottoms. Spiny lobster emerge from deeper refuges.
Plankton blooms also reduce UV penetration, lowering surface glare and making underwater visibility feel clearer despite the green tint. Many experienced freedivers actually prefer upwelling season for the life abundance and reduced sun exposure.
Seasonal Timing and Planning
April–May: Upwelling begins. Visibility drops sharply. Water temperature slides below 60°F. Expect variable conditions; some days are clearer than others.
June–July: Peak upwelling. Coldest water. Thickest blooms at protected sites like La Jolla Cove. Southern breaks (Ocean Beach, Sunset Cliffs) may offer relative clarity.
August–September: Upwelling weakens. Water gradually warms. Visibility begins to improve. Blooms dissipate. This is the sweet spot—cold enough for good visibility, warm enough to be comfortable.
October onward: Upwelling ceases. Water temperatures climb to 65–70°F. Visibility exceeds 60 feet. Plankton crashes. Warmer water means fewer nutrients and less dramatic marine activity, but conditions are optimal for recreational diving.
Adapting Your Approach
During upwelling season, prioritize southern and offshore sites over La Jolla during peak bloom months. Dive early in the day before wind mixes the water column further. Invest in a quality thermal suit—the cold is non-negotiable. Reduce target depth if visibility is severely limited; a safe, shallow dive beats a risky deep one in murky water.
Use Element to track water temperature, visibility forecasts, and upwelling intensity. Our app pulls real-time data from NOAA buoys and the Scripps Institution, giving you the edge you need to time your dives perfectly.
Coastal upwelling is San Diego’s seasonal gift and challenge. Respect it, adapt to it, and you’ll discover why our waters are among the most dynamic on the California coast.