Any kind of winter precipitation can play havoc with a wood deck, especially if you have any steps or a ramp as part of the structure. Dull, gray winter skies often don’t allow the moisture to dry properly; and that leads to algae forming and that’s the cause of most of the slippery conditions. A good quality sealer helps prevent algae build-up, but that’s a chore for warmer weather.
Until spring can warm the air, pick a sunny day and use a pump-up sprayer to apply a coat of warm water mixed with trisodium phosphate or an oxygen-based bleach. Use a stiff brush to aggressively scrub the wood surface, followed by rinsing with a garden hose.
Finally, lightly scatter some sand over the surface, and this should help keep you slip-free until the spring.