Strengthening El Niño unlikely to affect Estonia's summer weather

The El Niño–Southern Oscillation (ENSO) won't directly alter Estonia's summer, though the weather we can expect for Midsummer is still changeable.

ENSO is a global climate phenomenon driven by irregular variations in tropical Pacific winds and sea surface temperatures, involving alternating El Niño (warming) and La Niña (cooling) phases coupled with an atmospheric oscillation, that influences weather patterns across much of the tropics, subtropics, and higher latitudes.

While it does not directly affect Estonia much, travelers heading farther abroad should take note.

Hannes Tõnisson, senior researcher at Tallinn University, helped viewers of "Terevisioon" understand ENSO, ENSO-like phenomenon which do impact Estonian weather, and also dropped some hints on the perennial summer question here: What kind of weather can we expect for Jaanipäev (Midsummer).

In order to help explain the mechanism occurring in the tropical Pacific Ocean, you might draw a parallel with temperature changes in Estonia's coastal waters. Winds blowing from the land and out to sea push warmer surface water offshore, and bring cold water from deeper layers up to the surface. "It is similar to the situation on our northern coast in summer, when temperatures in Estonia can reach 30 degrees [Celsius], but a southerly wind is blowing and the water at Tallinn's beaches is only 4 degrees," Hannes Tõnisson, senior researcher at Tallinn University, told "Terevisioon".

A similar upswelling of cold water happens on a much larger, global scale, along the west coast of South America. Under normal conditions, these "trade winds" blow westward towards Australia, on the other side of the Pacific. This pattern of air circulation tends to keep water temperatures in the eastern Pacific relatively low.

From time to time, however, these winds weaken. As the airflow slows, the colder water no longer rises from the depths, and the ocean's surface layer begins to warm. A self-reinforcing cycle develops whereby the winds eventually reverse direction, transforming a vast area of previously cool ocean into a warmer one.

As water temperatures change, rainfall patterns shift too. For example, many areas of South America, and also for instance the African nation of Kenya, can expect heavy downpours, while Indonesia and Australia are more typically affected by prolonged drought.

While weather anomalies are often associated with climate change today, El Niño is a natural cycle which has existed for thousands of years. Scientists have not yet identified the original trigger that causes the trade winds to wea