ENSO
What it is. The coupled ocean–atmosphere oscillation of the tropical Pacific, and by a distance the largest single source of seasonal predictability on the planet. Warm phase El Niño, cool phase La Niña, with an episode declared when the ONI holds beyond ±0.5 °C for five consecutive overlapping seasons.
What it does. It shifts the Walker circulation, and with it the position of the Pacific jet. In an El Niño winter the southern tier of the United States tends wet and the Ohio Valley and Pacific Northwest dry; La Niña reverses it. The tropical response is strong and reliable; the mid-latitude response is a shift in the odds, never a guarantee. Skill collapses through the northern spring — the predictability barrier — which is why an outlook made in April for the following winter deserves less weight than the same outlook made in August.
Indian Ocean Dipole
What it is. The Indian Ocean's own east–west seesaw, peaking in the northern autumn and usually decaying by December. Frequently, but not always, co-varying with ENSO.
What it does. A positive IOD loads rainfall onto East Africa and takes it away from Indonesia and southern Australia; a negative IOD does the reverse. It is the single most useful driver for the Australian and East African seasons, and it modulates the Indian summer monsoon alongside ENSO.
NAO & the Arctic Oscillation
What it is. The dominant mode of North Atlantic winter variability, and the AO is essentially its hemispheric expression. It is a fast mode — it reorganises in days — which is precisely what makes forecasting its seasonal mean so hard.
What it does. Positive NAO means a strong, northward storm track: mild and wet in northwest Europe, dry in the Mediterranean, mild in the eastern United States. Negative NAO blocks, drives cold outbreaks into Europe and the eastern US, and wets the Mediterranean. Models predict the winter-mean NAO with real but modest skill, and — this is the famous part — with a signal that is too weak relative to its own noise, so the ensemble mean has to be scaled up to be believed.
The stratospheric polar vortex
What it is. The winter circumpolar westerly jet in the stratosphere. Around six times a decade in the north it collapses in a sudden stratospheric warming, and the anomaly propagates downward into the troposphere.
What it does. A vortex disruption is followed, within roughly two weeks, by a negative Arctic Oscillation at the surface that can persist four to eight weeks — a genuine source of week-3-to-8 predictability, and one of the few mechanisms that lets a
subseasonal forecast beat climatology in mid-latitude winter. Our sibling site
polarvortex.earth watches it directly.
QBO
What it is. The quasi-biennial oscillation: a downward-propagating reversal of the equatorial stratospheric winds on a roughly 28-month cycle. The most regular thing in the atmosphere, and still not perfectly predictable.
What it does. Through the Holton–Tan relationship, an easterly QBO is associated with a weaker, more disturbed polar vortex and so a greater chance of a negative-NAO winter. It also modulates the MJO. The effect is real in the composite and modest in any one year.
MJO
What it is. An eastward-propagating envelope of tropical convection circling the globe every 30 to 60 days. Subseasonal rather than seasonal, but it is how the tropics talk to the mid-latitudes on the way to a season.
What it does. The MJO is the main lever for weeks 2–4 forecasts: particular phases load the odds for atmospheric rivers into the US West Coast, for cold-air outbreaks, and for tropical cyclogenesis. Over a whole season it mostly averages out — but the number of strong MJO events in a season does not, and that is where it re-enters.
The slow ocean — PDO, AMV & friends
What it is. The Pacific Decadal Oscillation and the Atlantic Multidecadal Variability are patterns of ocean temperature that persist for years to decades. They are less predictors than context: the background state an ENSO event happens on top of.
What it does. AMV is a strong control on Atlantic hurricane activity and Sahel rainfall. PDO modulates how strongly a given ENSO event projects onto North American weather. Because they are slow, they are also the part of a seasonal forecast that persistence alone gets mostly right.
Land memory — snow, soil and ice
What it is. The surface remembers. Soil moisture anomalies persist for months, snow cover changes the albedo and the surface energy balance, and sea ice does both while also moving the baroclinic zone.
What it does. Dry soil in spring amplifies summer heat — one of the clearer seasonal mechanisms there is, and central to European and Great Plains heatwave outlooks. Snow-cover and sea-ice pathways into the winter circulation are actively argued over in the literature and should be treated as open questions, not settled ones.
The trend
What it is. Not a mode of variability at all, but the largest single term in most temperature outlooks. A model's own drift is removed by taking anomalies against its hindcast climatology; the real-world trend is not removed, and should not be.
What it does. Over much of the globe the honest baseline for a seasonal temperature outlook is 'warmer than the 1991–2020 normal', before a single dynamical model is consulted. A forecast that beats climatology only because it captured the trend has still beaten climatology — but it is worth knowing which part of the skill came from where.