Surfing - The regularity of swell varies across the globe and throughout the year. During winter, heavy swells are generated in the mid-latitudes, when the north and south polar fronts shift toward the Equator. The predominantly westerly winds generate swells that advance eastward. So, waves tend to be largest on west coasts during the winter months. However, an endless train of mid-latitude cyclones causes the isobars to become undulated, redirecting swells at regular intervals toward the tropics.