Forecast El Nino has arrived; what it means for the Maritime provinces

People rest on chairs on the Halifax Waterfront in Halifax on Thursday, July 6, 2023. THE CANADIAN PRESS/Darren Calabrese

A long predicted El Nino phase in the Pacific Ocean has emerged. On Thursday, June 11, the Climate Prediction Center branch of the National Oceanic and Atmospheric Administration (NOAA) released that “El Nino conditions are present and expected to strengthen into the Northern Hemisphere winter 2026-27.”

El Nino is the presence of a vast stretch of warmer than normal ocean waters in the central-tropical Pacific to the coastline of South America. The presence of an El Nino is known to heavily influence weather conditions in North America over the length of seasons.

You may have heard this El Nino referred to as a “super” El Nino. There is no meteorological definition of a “super” El Nino. There is a definition for a strong El Nino and very strong El Nino though. A strong El Nino occurs when a measure called the Oceanic Nino Index, or ONI, reaches 1.5 to 2 degrees. A very strong El Nino when the ONI reaches more than 2 degrees. Current forecasts have a strong or very strong El Nino developing by or through the fall of 2026.

The ONI is defined as “the running three-month average sea surface temperatures in the east-central tropical Pacific between 120°-170°W, near the International Dateline, and whether they are warmer or cooler than average.”

The Big Picture

A study of past El Nino years reveals a general pattern across the continent. That pattern is most prominent in the winter but can be reflected in other seasons, such as the summer, as well.

It consists of persistent ridges (rises to the north) of the jet stream in western parts of the continent. Persistent periods of hotter and drier weather often develop for areas under the ridge.

The ridges then descend into troughs (dips to the south) in the vicinity of the Great Lakes. The troughs often contain cooler air and associated areas of low pressure. Areas in the trough can experience periods of unseasonably cool and wet weather conditions.

The Maritime provinces sit right on the eastern edge of the trough. Sometimes falling under the influence of the cooler, wetter pattern in the trough and at other times firmly under a ridge (hotter, drier) rising to peak over the North Atlantic.

Past strong El Nino Summers

For the Maritimes this means that there is not as clear a “signal” or “trend” for an El Nino season compared to western and central areas of Canada. Past summers during or moving into strong El Nino events have displayed a diverse range of temperature and precipitation patterns. Here are some examples of observed conditions during some of those summers:

• Summer of 1982 (wet and cool): Weather conditions in the Maritimes for June through August in 1982 averaged cooler and wetter. Rain totals near-to-above normal for the season. Reanalysis of atmospheric patterns for that season show that the Maritimes likely was placed almost directly under the jet stream as it flowed out of a trough centred near the Great Lakes. Frequent low-pressure systems moving through the jet stream contributing to the higher-than-normal amount of rain.

• Summer of 1997 (dry and cool): Much like the summer of 1982 temperatures were cooler than average for the period of June through August. Unlike 1982 though rain finished mostly below average for the summer season.

• Summer of 2023 (warmer): In contrast to 1982 and 1997 temperatures finished near-to-above average for summer. Reanalysis of the atmosphere for that year show a positive height anomaly above the Maritimes. That means that the jet stream was likely positioned to the north in a ridge for prolonged periods of time. The more northerly position of the jet stream allowing for warmer, humid air to flow more frequently into the Maritimes from the sub-tropical Atlantic. That sub-tropical air contributing to the catastrophic flash flooding in Nova Scotia in July as it fed into thunderstorms.

The hurricane season

A benefit of El Nino is that it can contribute to a below average hurricane season as far as the number of tropical storms and hurricanes that develop.

During El Nino years more wind shear is often observed over the Atlantic basin. Wind shear is a change in direction and speed with height in the atmosphere. As powerful as a tropical storm or hurricane is that wind shear can serve to disrupt or sometimes even prevent the development of those types of weather systems. El Nino and the associated wind shear are a major factor in the prediction of a below-average Atlantic hurricane season this year.

That doesn’t mean the Maritimes couldn’t still be impacted by a tropical storm or hurricane. That has happened during past El Nino years. As an example, Hurricane Lee in 2023 transitioned to powerful post-tropical storm before impacting the Maritimes on Sept. 16. The storm felling trees and creating widespread power outages across the region.