Located roughly midway down the Sea of Cortez on the western coast of Sonora, Mexico, the town of San Carlos is a top weekend destination for scuba divers from the Western U.S. A rich and varied ecosystem, the area is subject to tremendous swings in temperature, nutrient levels, and fauna between the winter and summer months.
Divers accustomed to the bright colors and crystal clear waters of the Caribbean may at first be put off by the more subdued bottom topography and variable visibility, but will soon appreciate the diversity of this region, and the sheer density of life that inhabit these waters.
When planning for a trip during the fall or spring, it is advisable to get recent information on water temperatures, as they may change substantially over a matter of a week or two. Changes in water temperatures typically lag behind changes in air temperatures by several months.
Spring, for instance will generally have settled over San Carlos quite solidly by March, with air temperatures reaching 85-95 degrees Farenheit/29.5-35C, but water temperatures will often be 62-68F/16.7-20C. During March and April, as the days grow longer and the water temperature creeps up, visibility can be highly variable, from 40-50 feet some days, to a near “pea soup” the next, as phytoplankton blooms move swell and ebb. April diving is substantially warmer than March, with waters ranging from 72-77F/22-25C, but a good 5-7mil wetsuit is still a must for most people at these temperatures
The end of May is the traditional start to the summer dive season in San Carlos.
Summer diving is outstanding. The warmer water and increased sunlight have consumed much of the dissolved nutrients, leaving crystal clear waters. A substantial base of the food chain is converted during the spring into algal biomass to feed other consumers. By June, temperatures range from 82-89F/27-32C, and will remain warm through September.
The months of October and November are transition months in the Sea of Cortez, Typically they are still relatively mild but as the circulation patterns along the Midriff Islands (near Bahia Kino) change, the arrival of sudden upwelling can bring with it a 10 degree drop in water temperatures and an associated drop in visibility over a 1-2 week period.
The arrival of these winter waters brings with them an explosion in marine life. Many of the larger billfish and tuna will depart, but the plankton draws myriad smaller creatures from the deep such as the brilliant Dall’s goby, nudibranchs, and many other species. For those equipped with a heavy wetsuit or drysuit, winter diving in San Carlos can allow scuba divers to see an entirely new mix of species that are absent from the summer dives.