The Central American Cold Surge

We frequently talk about how the tropics and subtropics visit Utah in the form of cool-season atmospheric rivers, but there are situations in which high- or mid-latitude airmasses plunge southward into the tropics.  This is especially common to the east of the Rocky Mountains and Sierra Madre, which frequently act to channel relatively cool airmasses to Central America or, in extreme events, northern South America.

A great example of these Central American Cold Surges is affecting Mexico and Central America today.  Note in the loop below (especially the lower panel), the surge of relatively cool high pressure from the central United States into Central America.   To save bandwidth, I haven't plotted a longer loop to fully show the history of this event, but the airmass currently plunging into Central America originates in northwest Canada.  Brrr...

As these cold surges move southward, they are modified and warmed, but they are still relatively cold when they get to Mexico and Central America.  Check out the stiff north winds observed along the east coast of Mexico this morning.  The 59ºF with a wind of 23 mph at Tampico can't feel too comfortable for sunbathing.

Source: MesoWest

That 59 also represents the lowest temperature observed in Tampico since mid March, which was probably when they had their last major cold surge [note that wind gusts (green dashed lines) are also the strongest since that mid-March event].

Source: MesoWest

The cold surge passed Palenque, MX this morning and is about to reach Guatemala.

Source: MesoWest

Amongst the more remarkable mountain weather phenomenon that are produced by these Central American Cold Surges are strong gap winds that push through Chivella Pass in southern Mexico and extend over the Gulf of Tehuantepec.

Source: The COMET Program

This results in some very unusual phenomena.  First, one often sees a cold front pushing into the tropical eastern Pacific, which can sometimes be accompanied by a narrow rope cloud.

Source: Steenburgh et al. (1998)

Then there are very strong gap outflow winds over the Gulf of Tehuantepec.  These winds are very unusual because along the the center of the outflow jet they are inertially balanced, which causes them to curve rightward (relative to the flow) at a rate precisely predicted by the rotation rate of the Earth.  This is perhaps the cleanest example of atmospheric inertial flow anywhere in the world.   

Source: Steenburgh et al. (1998)

Finally, interactions between the strong winds and the ocean lead to an upwelling of nutrient-rich water to near the surface, which is an important aspect of the marine ecology of the Gulf of Tehuantepec.  

In 1998, I wrote a paper on these gap outflow winds with David Schultz and Brian Colle.  It was an entirely curiosity driven research project for which we had little-to-no financial support.  This paper has become the most popular of my research career, with citations in everything from oceanographic to renewable energy journals.  It is a prime example of why I often tell my students to never let the best laid plans get in the way of good serendipitous research.