Weather System Affects Summer
By SHILLINGTON, David
Surface ocean temperatures influence weather. ——————- – The recent hot dry summer has caused a number of people to think about climate change and to ask what the El Nino event is and how it affects weather in the Pacific Ocean and New Zealand.
El Nino is an unusual warming of surface ocean waters of the eastern tropical Pacific Ocean and is part of what is termed the Southern Oscillation (SO).
The Southern Oscillation is the normal see-saw pattern of reversing surface air pressure between the eastern and western tropical Pacific. When the surface pressure is high in the eastern tropical Pacific, it is low in the western tropical Pacific, and vice-versa. Because the ocean warming and pressure reversals are, for the most part, simultaneous, scientists call this phenomenon the El Nino/Southern Oscillation or ENSO. El Nino is a natural feature of the global climate system and is not caused by global warming.
Scientists have been measuring the amount of heat that is stored in the upper ocean and how this has changed as the seasons and years progress and have noticed a cyclic event occurring amongst apparently very chaotic temperature data.
About half of the solar radiation (incoming heat energy from the sun) is absorbed by the Earth’s surface and a large proportion of it is in the upper ocean layer. Some is absorbed by the air and the rest is reflected back into space (the Earths albeido).
In general, the water on the surface of the ocean is warmer than at the bottom because it is heated by the sun. In the tropical Pacific, winds generally blow in a westerly direction from Peru towards Indonesia. As the water moves west it heats up even more because it’s exposed to the sun for longer periods.
In the western Pacific the air is heated by the water below it, increasing the buoyancy of the lower atmosphere thus increasing the likelihood of rain. This is why heavy rain storms are typical near Indonesia while Peru is relatively dry.
In the east, the water cools the air above it, and the air becomes too dense to rise to produce clouds and rain. At the same time an upwelling occurs in the eastern Pacific along the west coast of South America, replacing the water moving towards Indonesia.
Upwelling describes the movement of deeper colder water from the bottom of the ocean upwards towards the surface away from the shore. Nutrient-rich water is responsible for supporting the large fish population commonly found in this area. Peruvian fishing grounds are one of the five richest in the world, and the term El Nino is Spanish for “The Christ Child” because it came at about the time of the celebration Christmas. The changes in the fishing conditions as described by remote fisherman relate well to the scientists-defined El Nino events.
Because the trade winds push surface water westward toward Indonesia, the sea level is roughly half a meter higher in the western Pacific than in the east. The sea is not exactly level and ships have to travel slightly uphill westwards along the Pacific Ocean. When an El Nino event takes place, the trade winds relax and the water that once “piled up” in the western Pacific, sloshes back towards the east, ending the nutrient-rich upwelled coastal water (with a downturn in the fishing industry as well). These events are characterised by warmer than average surface water temperatures being recorded for more than five successive months. When the water temperatures drop to below average (these changes can be up to 1.5degrees) for defined periods of time, then an opposite La Nina effect is noted. The effects of the current La Nina in our own backyard, seem to have established more beach visitor days for holiday makers but very difficult times for farmers and others whose livelihood is affected by the vagaries of weather patterns.
* David Shillington is Head of the School of Applied Health Sciences at UCOL.
(c) 2008 Evening Standard; Palmerston North, New Zealand. Provided by ProQuest Information and Learning. All rights Reserved.