I originally wrote this in response to JeepmanSV talking about enjoying the ocean water temp in a thread asking about Mazatlan weather in October. However after reading it through I thought it might be better to put it in a stand-alone thread.
Water temperature universally depends on four major factors:
1. Air temperature
2. Length of daylight hours
3. Currents (including upwelling)
It's also important we add two sub-headings: The strength and particularly the originating direction of numbers 3 and 4. We need also to recognize that water temperatures will be slower to change than air temperature. It can easily take a month of longer warmer daylight hours in the Spring before ocean water temperatures start to reciprocate. Likewise well into the Fall the water may still be as warm as it was in the summer, since the shorter and cooler days haven’t had enough time to really take effect.
It should be said ocean water temperatures can vary dramatically in some areas of the globe. Rhode Island summers with the Gulf Stream means you can go out with just your bathing suit on in 80 degree waters. Six months later you’ll be lucky if that same water reaches the 40’s. In addition we should add that some areas, such as Southern California, can see wild swings in water temperature in a very short time due to upwelling. As much as several degrees in just a single day!
That being said, Mazatlan virtually never has big swings in water temperature from day to day, and the summer to winter change is mild compared to many areas. However due to factors we’ll talk about in a minute, the ocean highs and lows are somewhat greater in Mazatlan than other comparable areas along the same meridian.
For example Mazatlan is almost exactly the same distance from the equator as Hawaii. However summer water temps are a little warmer than Honolulu, and winter water temperatures in Mazatlan are surprisingly cooler than the Islands. The summer ocean temperatures in Maz run about 82 degrees F. (27.8 C.), whereas in Honolulu it’s more like 79 F. (26 C.) Late January to February is typically the coolest time for water temps. A mild 72 F. (22.2 C.) in Hawaii, but only about 65 F. (18.3 C.) in the Golden Zone. That’s because of our items #3 and #4….current and wind.
While there isn’t anything like a Gulf Stream bringing very warm waters all the way up to New England, there are summer hurricanes and tropical storms that push warmer waters up from Southern Mexico. In addition they bring in a lot of still moist air which keeps the water surface generally more calm and baking in the summer sun. In the winter those currents stop, and strong cool winds start being pushed down from the Sonora Desert. This churns up the water (called upwelling), and blows the warmer surface water south. Of course as the surface water moves away it gets replaced with colder water coming up from the bottom. Think of upwelling as nature’s version of taking your straw and stirring up your ice filled drink.
The key to understanding ocean water temps in Mazatlan is to be aware of when air temps are most likely to occur, and factoring in the approximate month lag time from climate changes to corresponding water temperature changes. For example summer is generally considered to run from June through September. Yet with the delay for warming the water over the air, you’ll likely have much warmer water in October then you did in May. That same principle applies to snow bird visitors coming for the winter months. Those arriving in November and December will generally see water temperatures not having reached their winter low. However visitors coming in March may have longer warmer days, but are unlikely to see much change yet in the ocean water.