San Juan, Puerto Rico

The Cuba soil warming and vegetation change sent me searching for evidence of temperature changes in the region and perhaps closer to Luquillo. I found Claude E. Duchon's 1986 paper Temperature Trends at San Juan Puerto Rico [Bulletin Am. Meteor. Soc. 67(11):1370-1377.] San Juan has apparently warmed up "monotonically" at every month and hour of the day Duchon checked for the time span 1956-1982. On a mean annual basis it turned out to be 2.1 C rise for the 27 years of record. Daily minimum temperatures have risen the most, 3.0 C per 27 years. The first guess was that as San Juan got bigger, more metropolitan and sophisticated an urban heat island developed and became more pronounced. Unfortunately, the rate of warming was off the charts for trends due to urban warming relative to population growth. Duchon's theory is that the transition from palm tree agriculture to concrete modernity accounts for the trend and he offers the notion that the airport local heat island is especially important. The airport has grown over time.

Duchon presents other data that permit the alternative hypothesis that may be of some interest to our friends at Luquillo. They show that wind speeds have declined and the direction of the winds has also changed. In the 1950s, the morning winds came from the ESE (about 15 degrees south of east) and are now about 45 degrees south of east. Afternoon winds had come from 70 degrees east of north (from the offshore) now (1982) the come from 5 progressively had to cross more land, i.e. more of the island, before it could reach San Juan. Since the islands are very much heated relative to the surrounding oceans [the island is a heat island], the air that comes over more land gets heated more.

The biggest difference is in the morning and that is when the temperature trend is greatest, 3 degrees in 27 years. Afternoon wind speeds have not changed much but morning winds have slowed down from 3.5 meters per second to about 2.5 meters per second. So in the morning it takes the air longer to get across the land area before it gets to San Juan. It crosses more land and crosses it slower. It can heat up more.

At Luquillo there should have been similar wind direction and perhaps speed changes. The winds have, in recent years, come more out of the southeast and less out of the east. Why? We are not sure but I have research in of the subtropical North Atlantic has changed (1899-1990) and this would change wind direction (see Davis et al., 1997. J. of Climate).


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