Stephen Davenport, DTN Weather Risk Communicator, Long Range

The oceans and their slowly changing temperature patterns have a profound impact on the configuration of the world's atmospheric circulations. Foremost among these climate drivers is the gradual oscillation between cool and warm waters in the equatorial Pacific, known as the El Nino Southern Oscillation, or ENSO. This cycles between La Nina (cool phase) and El Nino (warm phase), typically every one to three years.

Currently, ENSO is in a La Nina state, but the forecast through summer 2026 is indicative of the opposite, El Nino state. Impacts to North American weather patterns will be noticeable.

This has been a rather weak La Nina episode, which despite its limited power has helped to carve out the shape of North America's pressure patterns through the winter and set the stage for occasional arctic outbreaks, especially across the North-Central U.S. The atmosphere is complex and there are other factors involved at shorter "sub-seasonal" time ranges -- like the state of the stratospheric polar vortex -- but what we have seen so far this season has approximated to idealized wintertime La Nina patterns.

THE EFFECTS OF LA NINA AND ITS DISAPPEARANCE

During La Nina, the jet stream pushes northwards over the eastern Pacific, with high pressure tending to occur over the Gulf of Alaska. This causes a large-scale trough to form downstream, with the jet stream dipping southwards across the Central and Eastern U.S.

As this has been a weak La Nina, however, its impacts have been less consistent. Interruptions to the broad setup have come from the high-pressure center occasionally retrograding westwards towards the Bering Sea, allowing Pacific-sourced air to wash across the U.S. This has brought a couple of periods of unusually warm weather, with some record-high temperatures here and there.

But what lies ahead? La Nina is due to weaken imminently, and there are already signs that cold weather anomalies in the tropical Pacific are being eroded from the west as trade winds shift. A steady disappearance of colder water will occur through late winter and spring, leading to a period of neutral ENSO conditions. Eventually the ocean should flip the script and warm towards El Nino by the fall.

CHANGES IN THE TRADE WINDS

Trade winds blow east-to-west along the equator, but when they go through periods of strengthening and easing, they alter ocean surface currents. A stronger flow across the tropics pushes relatively warm water westwards and forces an upwelling of colder water from the deep to replace it. As this process continues, the cooling spreads across the tropical Pacific basin and La Nina develops.

However, when the trade winds slacken -- or even reverse -- warm water sloshes back eastwards, starting a change towards the El Nino phase. This is what we are starting to see, with "westerly wind bursts" developing near and west of the International Dateline. Those westerlies, according to global weather models, are due to strengthen considerably, which could precipitate a steady or even rapid warming of the Pacific Ocean, and thus the demise of La Nina.

THE EMERGENCE OF EL NINO

The western Pacific is not only warming at the surface. There is a growing pool of warmer-than-average water at depth as well, waiting to unleash itself. This growing subsurface warmth is usually a precursor to a change to El Nino.

For a while, the effects of this La Nina will linger, as there is a lagged response in the atmosphere to such slowly evolving climate drivers. But through the spring, the neutral ENSO will mean weaker impacts on global circulations, so it will be less of a long-range forecasting tool.

However, if El Nino develops as expected, it will probably strongly influence conditions later in 2026, with very different patterns developing for next winter.

HURRICANE SEASON

El Nino could start to make its effect felt through the summer and into the fall. Not least is its probable impact on the hurricane season. Although it is far too soon to start bandying around any numbers, we can at least note the canonical impacts of El Nino on hurricanes. Most importantly it tends to increase wind shear over the Atlantic Ocean -- the change in wind speed and direction with height. As hurricanes need low wind shear to develop, then there could be somewhat subdued activity in the Atlantic this year.

WINTER OF 2026-27

On the reasonable assumption that El Nino will persist through next winter, we can take a very early look at how things might shape up. Although it is far too early to make a forecast, we can take a look at typical patterns, which are very different from those during La Nina winters. Instead of high pressure in the East Pacific, there tends to be an area of low pressure, with the polar jet stream diverting farther north. All other things being equal, this tends to reduce the chances of arctic outbreaks, with the northern U.S. and at least Western Canada usually having milder winters than usual. However, the Pacific jet is usually stronger, subduing temperatures across the southern U.S. due to more frequent passage of low-pressure systems.

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