Roaring and Screaming: The Science of South Swells

Inside Aliso Marble
Solid south swell at a Southern California shorebreak.

The Roaring Forties, the Furious Fifties and the Screaming Sixties. As wave-riders, these enigmatic markers play a vital role in our summertime enjoyment.  Summer is a time of long days and warm, trunkable water. When massive storms rumble through the Southern Ocean, swell can reach everywhere from Tahiti to Chile to Hawaii to California to Alaska. In this article, we’ll analyze how our summertime south swells are generated and how they impact waveriders in the Pacific.

Image: Rick McClain
Image: Rick McClain

The Roaring Forties refers to the area between 40 and 50 degrees latitude south with the Fifties and Sixties below. Famous for relentless winds, unruly seas and treacherous navigation, this area is unique on our planet because there is very little land. The 60th parallel south passes through nothing but Ocean. Because there is little or no land to slow the flow of air, winds here whip around the globe unimpeded. These winds are aided in their westerly flow by the rotation of the Earth, known as the coriolis effect.

NOAA South Swell Alert 6:13
NOAA advisory June 4, 2013

During the Southern Hemisphere winter, there is a great imbalance between the steady solar energy reaching the equator and the absence of energy hitting the Antarctic. Warm tropical air rises, moving south while the cold polar air sinks and moves north to take its place. This is convection: the constant effort of the atmosphere to equalize temperature and pressure. Cyclogenesis is the development or strengthening of low pressure systems in the atmosphere. This happens when a warm air mass collides with a cold air mass. The atmospheric pressure drops as the warm air rushes upward. Air flows into the low pressure area from surrounding areas of high pressure. The lower the pressure the faster the wind. The faster the wind, the longer it blows and the larger the area, the bigger the swell.

Large low pressure system in the SPAC. Image: Stormsurf

In the Southern Ocean, large lows form below Australia and are driven to the east by the prevailing westerly flow. As these storms move into the South Pacific Ocean (SPAC), they enter the swell window for Hawaii and California. Even though the storms can be enormous, our south swells are generally smaller and less consistent than our winter time west and northwest swells. Because south swells travel between 4,000 to 7,000 miles to reach our shores, their energy decays and becomes less consistent. While our winter swells only travel 500 to 3,000 miles with much less deterioration. Because of their long range travel, significant south swells approach our coast with very long periods in excess of 20 seconds.

Great Circles directed at Southern California Image: Stormsurf
Great Circles directed at Southern California Image: Stormsurf

The trajectory of south swell storms is very important in determining the quality of our surf. Storms that stay near the 60th latitude decay more as the swell moves through the Ocean. Storms that track to the north and strengthen near New Zealand in the Roaring Forties produce stronger, more consistent surf for California. These swells also have more west in their direction meaning that they can impact more surf spots. Straight south swells from around 180 degrees move past or are shadowed from large swathes of the California coast. Because of how steep these south swells approach our coast, they are often walled and closed out. The addition of local NW windswell is important to break up the walls and provide better shaped waves.

Powerful storm in the Furious Fifties Image: Stormsurf
Powerful storm in the Furious Fifties Image: Stormsurf

The amount of ice around the Antarctic continent influences the size and strength of Southern Ocean storms. The area of fetch is larger when there is less ice. There is more water for the wind to blow across and transfer energy. During May and June, in the austral fall, ice is minimal and the storm track is coming alive.  Many of our largest south swells materialize in the early summer.

In the 1950’s and 60’s, Dr. Walter Munk of the Scripps Institute of Oceanography studied the propagation of swell energy originating in the South Pacific Ocean across the whole of the Pacific Basin. His team discovered that a large storm near Antarctica will create swell energy that eventually reaches the shores of Alaska. This research was vital to our understanding of how swells travel. Now we hope for strong activity in the Roaring Forties. We anxiously watch swell models, hoping for purple blobs to grow and spread across the Southern Ocean.


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Swell Lines Magazine

Bodysurfing yarns woven 'tween crest & trough