Picture an orchestra playing without a conductor. First the brass comes in, then the strings, and sometimes the percussion takes a solo. That is how weather over North America works: the atmosphere, ocean, and land take turns being the lead when forecasting weather from a week to two months out.
Weather regimes are persistent patterns of air motion that determine whether winter will be mild or summer rainy. There are four of them, plus days with no clear regime. Scientists trained computer models, each receiving data from only one source: for example, ocean temperature or soil moisture. This way, they could see which "musician" matters most at different lead times.
The season also shifts the balance of power. In winter and summer, weather is easier to predict than in spring and autumn, because the atmosphere tends to get stuck in one regime more often. For some regimes, the ocean matters more; for others, sea ice or soil moisture does. If meteorologists know which information source is most important in a given situation, month-long forecasts could become more accurate.
The frequency of the 'No Regime' class varies from 18% in winter to 44% in summer, making summer forecasts particularly difficult: the atmosphere is more often in a transitional state without a pronounced large-scale structure.
In Kim Stanley Robinson's novel 'Forty Signs of Rain', scientists attempt to predict abrupt changes in weather regimes due to a shutdown of the Gulf Stream. This study resonates with that idea: it shows which slow components of the Earth system might serve as early indicators of circulation regime shifts.