*Mark Sponsler is the creator of StormSurf.com, “a website dedicated to delivering the highest quality marine weather data to those who ride waves.” Sponsler’s weekly forecast videos focus on El Nino indicators year round, regardless of media hype.
El Nino can have profound effects on global weather and ocean conditions. Under normal conditions, trade winds over the equatorial Pacific blow from east to west (Peru towards Asia). This causes warm water to sequester near Asia, and cool water to upwell off Peru. This in turn results in high pressure over the cool water off Peru (producing stable atmospheric conditions) while low pressure and tropical precipitation locks down off Asia.
But about once every 7 years, the trade winds over the equator relax if not reverse direction, with the effect being a flow that travels reverse of the normal direction, or from west to east (Asia towards Peru). When the trades relax or weaken, this situation is know as westerly anomalies. That is, compared to normal for the time of year, the winds have a westerly component to them And when trades fully reverse (they start blowing from the west to the east), this is known as a Westerly Wind Burst (WWB).
WWBs can last for 10-20 days and can blow as hard as 20+ kts. This situation typically occur between New Guinea and the International dateline. When a WWB occurs, it sets the oceans surface in motion moving to the east. The result is warm water off Asia starts migrating east across the tropical Pacific towards Peru. WWBs also typically spawn tropical cyclones, sometimes straddling both sides of the equator simultaneously. Typically the westerly anomaly/WWB cycle starts in Spring or early Summer, with a single WWB, followed by 2-4 more through early Fall, with the warmest waters reaching Ecuador around Christmas.
The warm water doesn’t flow from west to east on the surface. It falls to depth, down about 200 meters, forming a pocket or bubble of anomalously warm water (+7 degs C above normal). That pocket is called a Kelvin Wave, and it travels west to east under the equator for 2.5 months before being forced to the surface first as it encounters the Galapagos Islands, then eventually impacts Ecuador. For each WWB that occurs during the EL Nino cycle, a corresponding Kelvin Wave develops. The more WWBs, the more Kelvin Waves, and the greater the warming in the east.
As a Kelvin Wave erupts in the East Pacific, it warms surface water there, and typically pretty dramatically. This in turn has effects on fisheries and wildlife, especially in the Galapagos, Peru, Central American corridor. Where normally cold upwelled nutrient rich waters are present, now a warmer and far less bountiful food chain is present for fish. Fish stocks leave the area. Local economies that survive off fishing turn non-productive.
Eventually these warm waters, as they erupt near the Galapagos start migrating west by prevailing trades over the East Pacific, moving towards the dateline. Note – the trades don’t completely dissipate in the east. As this cycle progresses, a defined warm water ‘tongue’ develops extending from the Galapagos east and within 5 degrees north and south of the equator reaching west to the the dateline. Depending how warm the surface pool gets and how much area it covers compared to normal determines whether the situation will qualify to be labeled as El Nino. The area of concern is from 5N to 5S and from 120W to 170W, the NINO3.4 area. If temps in this region hold at +0.5 deg C above normal for 3 months, it considered a minimal El Nino. At the top end temps of +1.5 result in a classification of a strong El Nino. As of the Sept monthly reading, this years event was at +1.67 degs C. In comparison, the two strongest El Ninos at their peak in Dec/Jan were at +2.32 (’97/98) and +2.21 (’82/83). Theses were considered Super El Ninos. This years event is already the 6th strongest El Nino ever (as of Sept), and building. And it is tracking mid-way on it’s development path between the ’82/83 and the ’97/98 events, making it possible it too could reach Super El Nino status.
As the surface warm pool builds, it starts interacting with the atmosphere above it, enabling greater evaporation and increasing humidity levels, reducing surface air pressure and increasing the odds for rain in locations of the planet typically bone dry. Likewise as water cools over Asia, surface air pressure builds and drought sets in. Precipitation follows the warmer water across the Pacific, resulting in a complete reversal of the Pacific Basin weather patten.
From a surf perspective, the weather changes associated with El Nino are most pronounced in Fall and Winter months in the Northern Hemisphere. The warm pool feeds more energy into the jetstream, which in turns causes the jet to track further south forming a strong semi-permanent low pressure system just east of the dateline and south of the Aleutians easing into the Gulf of Alaska. The upper level energy feeds development of larger, stronger and more consistent storms tracking across the Norther Pacific, which in turn generates larger, stronger and more consistent surf aimed at breaks in which the jetstream is flowing towards, like Hawaii, California, the Pacific Northwest, Mexico. But because the jetstream is further south, it also offers the specter of much rain and stormy local conditions, making surf conditions less than ideal. During pronounced episodes during strong El Nino years, the jetstream can drive storm energy straight from Siberia clear across the Pacific and directly into the US, often tracking right into normally drier regions of Central and Southern California. This can bring significant rain and snowfall to regions that are typically desert like, having severe economic and ecologic affects. In the Atlantic, the El Nino enhanced jetstream creates upper level shear that suppresses hurricane production.
So El Nino can be a mixed blessing, depending on where you live.
After El Nino has run it’s course, typically in the early summer following it’s peak, a new pattern emerges: La Nina that has almost the opposite effect. Colder than normal water starts to develop in the eastern equatorial Pacific in the mid summer as the trades start raging from east to west (Peru to Asia). Strong high pressure rebuilds over the eastern equatorial Pacific while low pressure follows the warm waters being blown back towards Asia. By Christmas time the year following El Nino, the North Pacific jetstream is displaced well north, driving up towards the Aleutians into Alaska and northern Canada, and high pressure dominates the NE Pacific pattern. As a result storm and swell production starts to decrease. And the whole cycle is then set to start again.