Southern Mexico and Central America
Written by Keith Jaszka – Weather Routing Inc.
Each year, localized windstorms known as Tehuano winds blast the Gulf of Tehuantepec with dust and can wreak havoc on unprepared mariners. The topography of the Isthmus of Tehuantepec, the narrow strip of land separating the Gulf of Mexico from the Pacific, plays an important role in these events. Directly north of the Gulf of Tehuantepec is an approximately 40 km-wide gap in Mexico’s Sierra Madre Mountains called Chivela Pass. When a high pressure ridge of Arctic origin moves southward across the Central United States and Gulf of Mexico, the resulting N’ly flow of cold air becomes blocked by the Sierra Madre Mountains. As described by the U.S. National Hurricane Center (NHC), this accumulation of cold, dense air combined with warm, humid air over the tropical waters of the Pacific create a strong north-south pressure gradient across the isthmus. The enhanced pressure gradient forces N’ly winds to be funneled through the narrow Chivela Pass and persist up to several hundred kilometers offshore, giving rise to Tehuano winds. Bands of patchy/puffy (i.e. cumuliform) clouds streaming southward across the W’rn Gulf of Mexico on satellite imagery are often a precursor to Tehuano winds. In addition, a “rope cloud” (i.e. arced line of cumuliform clouds) often marks the leading edge of Tehuano winds once an event is underway (Figure 1.).
These windstorms can occur over the Gulf of Tehuantepec during any time of the year, but are most common during October through March, when Arctic high pressure ridges are stronger (i.e. composed of colder air) and contribute to a stronger pressure gradient over the Isthmus of Tehuantepec. According to the NHC, gale-force (> 34 knots) Tehuano winds typically occur in November through March and are most common in December. However, these winds may reach gale-force as early as September or as late as May. Storm-force Tehuano winds of 48 knots or greater may occur as early as October and as late as April and are most frequent in January. Lastly, the average duration of a Tehuano wind event is 48 hours and these winds occasionally reach hurricane-force (> 64 knots) during the winter months.
Farther south, cold air can also infiltrate through valleys and passes over Central America and produce enhanced northeast winds through bays and gulfs along the west coast (Figure 2). This frequently occurs in the Gulfs of Fonseca, Papagayo, Dulce and Panama during October through April. Typically, the winds will increase in these regions approximately 24-48 hours after the northerly winds increase across the Gulf of Tehuantepec. These wind surges can last anywhere from 2-4 days in these regions.
Before crossing these gulfs, always check the latest weather forecasts for the area. For the Gulf of Tehuantepec, you can check the airport observation in Veracruz, which is located north of Chivela Pass. If there are high northerly winds reported here, then these winds are funneling into the Gulf of Tehuantepec. It is best to wait approximately 12-24 hours after the winds have eased in Veracruz to allow the higher winds and swells in the Gulf of Tehuantepec to abate.
Keith Jaszka is a meteorologist at Weather Routing Inc. (WRI). WRI has been providing forecasts and route recommendations to ships worldwide for over 50 years. To find out more about WRI, please go to the following links: www.wriwx.com and www.seaweather.net.