One of El Niño's key ingredients is missing

El Niño, the climate cycle in the tropical Pacific Ocean that can reconfigure weather patterns, is slowly but steadily building for 2019 — but there is a component of it that is still missing in action.

Why it matters: El Niño events are one of the most consequential forces that can tip the odds in favor of particular weather patterns. But they require both unusually warm ocean temperatures in the tropical Pacific and changes in the atmosphere — and so far, the atmosphere hasn't changed.

The big picture: El Niño events can make or break a ski season in the Western U.S. by directing storms across the southern tier of the country, and they tend to favor milder-than-average conditions in the Midwest and Pacific Northwest. They can also boost global average temperatures, adding to the influence of human-caused global warming, which acts on far longer timescales.

During the past two decades, scientists have become adept at forecasting the onset and intensity of El Niño events. A telltale sign of El Niño's arrival is a sharp increase in equatorial tropical Pacific Ocean temperatures.

• Part of its definition is now being met, as sea surface temperatures in part of the Pacific have reached 1°C, or 1.8°F, above average for this time of year.

Between the lines: Yet, the ocean temperatures are only part of the equation, as indicated by forecasters' reluctance to announce El Niño's arrival. Instead, to get an El Niño requires a series of interactions between the ocean and the atmosphere. This is why meteorologists refer to El Niño as a "coupled system."

• Right now, the ocean is asking the atmosphere to play along, and the air is pretending not to hear anything. That is expected to change soon, however.

"While warmer-than-average surface waters in the equatorial Pacific are an essential element of El Niño, the atmospheric response is just as critical," says Mike Halpert, deputy director of NOAA's Climate Prediction Center, via email.

Signs that experts like Halpert are looking for before declaring an El Niño advisory include:

• A slackening of the easterly trade winds along and north of the equator.
• Increased rainfall near the International Dateline.
• Clear and persistent sea level pressure changes across the world's largest ocean.

The bottom line: Halpert says that temporarily, the Pacific appears to be dominated by other tropical weather cycles. But he and other forecasters expect the ocean to eventually win over the weather patterns, with at least a 90% chance that El Niño will develop this winter.