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 weaken, however. "El Niño existed long before humans began directly influencing the climate," Tõnisson said.

Human-made global warming nevertheless does add an extra energy to this natural system. Rising average temperatures in the atmosphere and the oceans serve to make extreme weather events more intense than they had been before. Tõnisson observed: "The human impact lies in the fact that, since average temperatures have risen, there is more energy throughout the atmosphere and the ocean. This means that El Niño phenomena get amplified: While heavy rainfall used to be the case, now the rainfall is getting exceptionally heavy."

Global climate variability also affects Europe's atmospheric circulation via the jet streams, which among other things has been bringing wetter-than-usual winters to the Mediterranean region. Again, this influence does not extend as far up as Estonia's latitudes. "Here, local factors often have a greater impact than El Niño itself," the senior researcher added.

What kind of weather can we expect for Midsummer in Estonia?

Summer weather in the Nordic region is determined primarily by local conditions and nearby pressure systems. As for the current start of summer, the just-ended month of May saw temperatures about half a degree warmer than the long-term average in Estonia, though also displayed large regional temperature differences.
Weather stations recorded temperatures of around 15 degrees in some areas along the North coast, even as temperatures inland approached 30 degrees at exactly the same time.

According to the forecasts, similarly variable weather will go on heading to Midsummer and through the rest of June. This means over the coming weeks, mini-heatwaves alternating with thunderstorms. "When the hot weather does arrive, it may be very short-lived—just a day, a couple of days, or even half a day, when temperatures will briefly jump to 31 degrees, after which the thunderstorms will return. [...] It seems that many such contrasts can be expected, and they will often end with thunderstorms," Tõnisson said.

Long-range forecasts are not very reliable under Estonia's highly variable conditions. As a result, it is currently impossible to predict the weather for the entire summer with precision. However, the scientist offered a hint regarding the upcoming holidays – albeit not a hint which might be welcome. "The reliability of long-term forecasts is highly limited, but at the moment it seems quite feasible that it will rain around Midsummer's Day, although we should be spared any snow at least."

As for the rest of summer, longer dry periods and more consistent beach weather are more likely during the second half of the summer. Tõnisson added. "If a somewhat lengthier heatwave is coming, it will likely take place sometime in mid-July or during the latter half of that month."

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