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Weather or Whether.

...El Niño and La Niña, hurricanes, saints, and the rainy season

By Bill Wilson 

Well, Alberto is our baptism to the first day of summer and the first good soak (almost an inch--.86 inches) of the rainy season. In Mexico, the season is often variable depending on region. Usually, the rainy season lasts from May to October, and the dry season is between the middle of November and April. During the wet season, showers usually only last a brief time although some gullywashers and electrical storms can be spectacular. 

There is a correlation between our rainy season and the hurricane season. But the 2024 Atlantic hurricane season officially began on June 1, it got off to the slowest start since 2014. This was due to a large stationary heat dome over Central America and Mexico, as tropical cyclogenesis in June often occurs over the Gulf of Mexico and northern Caribbean Sea. 

We had rain in May and early June but not in the amounts to replenish the presas (our reservoirs).

My stations (2) have different algorithms that make predictions based on various factors like elevation, a date of expected rainy season, latitude and longitude.  My one station is set to June and since then we have had 1.24 inches (31.5 mm) most of it today. After 20 years of visiting and living in San Miguel de Allende I've seen rainy seasons that were very wet and some I asked, "This is it?" 

Back in 2005, our first 3-week scouting trip to San Miguel, Jackie and I were staying in Casa la Cuesta and went to sleep during a rocking thunderstorm in September. We woke up early around 4 am and the sound and light show was still going on. We had a good laugh. Since then I've talked to professional farmers, guides, teachers and locals about the start of the perennial rainy season. Some mention San Isidro, the feast day of farmers or Santa Cruz day and others cite the hurricane season. 

And climatologists will mention cycles and definite changing patterns. 

Weather or Whether (a column I write for Insiders newspaper) it's May or June or earlier or later matters not. You'll hear in español,"por la milpas." Or, “for the fields,” especially corn. Corn and water are essential. And as we enter summer we welcome the rains as this spring it's been unusually hot with temps over 100ºF for many days. We also learned about heat domes. 

And you thought summer started with Memorial Day sales in US. Actually, it’s the summer solstice about to happen this afternoon. This year's summer solstice arrives (today) June 20 for the Northern Hemisphere, according to NASA. Because the Earth rotates on a tilt, the summer solstice takes place when the Northern Hemisphere is tilted toward the sun which will peak at 4:51 pm EST. 

Remember here in Mexico we are in Mexican time (Central zone and no DST) so adjust your welcoming ritual accordingly. As to another weather factor, NOAA officially declared the powerful 2023-2024 El Niño over recently and a shift to ENSO-neutral conditions, which could have several important ramifications for the Atlantic hurricane season, Jun 13, 2024.

About El Niño and La Niña: During normal conditions in the Pacific ocean, trade winds blow west along the equator, taking warm water from South America towards Asia. To replace that warm water, cold water rises from the depths — a process called upwelling. El Niño and La Niña are two opposing climate patterns that break these normal conditions. Scientists call these phenomena the El Niño-Southern Oscillation (ENSO) cycle. El Niño and La Niña can both have global impacts on weather, wildfires, ecosystems, and economies. 

Episodes of El Niño and La Niña typically last nine to 12 months, but can sometimes last for years. El Niño and La Niña events occur every two to seven years, on average, but they, like the rainy season don’t occur on a regular schedule.  Generally, El Niño occurs more frequently than La Niña. During El Niño, trade winds weaken. Warm water is pushed back east, toward the west coast of the Americas. El Niño means Little Boy in Spanish. 

South American fishermen first noticed periods of unusually warm water in the Pacific Ocean in the 1600s. The full name they used was El Niño de Navidad, because El Niño typically peaks around December. El Niño can affect our weather significantly. The warmer waters cause the Pacific jet stream to move south of its neutral position. With this shift, areas in the northern U.S. and Canada are dryer and warmer than usual. But in the U.S. Gulf Coast and Southeast, these periods are wetter than usual and have increased flooding. El Niño causes the Pacific jet stream to move south and spread further east. During winter, this leads to wetter conditions than usual in the Southern U.S. and warmer and drier conditions in the North. 

El Niño also has a strong effect on marine life off the Pacific coast. During normal conditions, upwelling brings water from the depths to the surface; this water is cold and nutrient rich. 

During El Niño, upwelling weakens or stops altogether. Without the nutrients from the deep, there are fewer phytoplankton off the coast. This affects fish that eat phytoplankton and, in turn, affects everything that eats fish. The warmer waters can also bring tropical species, like yellowtail and albacore tuna, into areas that are normally too cold.

La Niña La Niña means Little Girl in Spanish. La Niña is also sometimes called El Viejo, anti-El Niño, or simply "a cold event." La Niña has the opposite effect of El Niño. During La Niña events, trade winds are even stronger than usual, pushing more warm water toward Asia. Off the west coast of the Americas, upwelling increases, bringing cold, nutrient-rich water to the surface. These cold waters in the Pacific push the jet stream northward. This tends to lead to drought in the southern U.S. and heavy rains and flooding in the Pacific Northwest and Canada. During a La Niña year, winter temperatures are warmer than normal in the South and cooler than normal in the North. La Niña can also lead to a more severe hurricane season. 

La Niña causes the jet stream to move northward and to weaken over the eastern Pacific. During La Niña winters, the South sees warmer and drier conditions than usual. The North and Canada tend to be wetter and colder. And during La Niña, waters off the Pacific coast are colder and contain more nutrients than usual. This environment supports more marine life and attracts more cold-water species, like squid and salmon, to places like the California coast.

Living here in the mountains of central Mexico (almost like high desert) at 6,300 feet our weather is usually mild and we notice big changes. A lot of whining about heat recently. Today, instead pour a glass of nice Mexican of Chilean red wine and toast Tlaloc, god of rain here in Mexico.

Bill Wilson has lived in San Miguel for 15 years and has been a journalist, photographer and editor of daily newspapers and weekly magazines.  He’s covered everything from Cub Scout news to the United Nations.

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