Atmosphere-ocean feedback during El Nino, composite image. El Nino is a natural climate pattern that occurs when the surface waters in the eastern Pacific Ocean warm up, leading to a coupled climate pattern where changes in ocean temperatures are linked to changes in the atmosphere. The Walker circulation, which is a large-scale atmospheric circulation pattern over the tropical Pacific, is affected by El Nino. During El Nino (at left), there is an increase in the amount of rising air, clouds, and rain over the warm waters in the central Pacific, which changes the Walker circulation. As a result, the trade winds and upper-level winds slow down, and the overall circulation weakens. This weakening of the trade winds allows the surface to continue to warm, creating a feedback mechanism. On the other hand, when there is no El Nino occurring (at right), the Walker circulation is driven by the warm waters in the far western Pacific. It consists of rising air and storms over this region, west-to-east winds aloft, descending air, and dry conditions over the eastern Pacific.