Changes in climate are not caused only by conditions in the atmosphere. The top few metres of the ocean can store as much heat as the entire atmosphere, and relatively small changes in ocean circulation can move vast amounts of heat around the planet.
The biological pump
The ocean acts as a long-term reservoir for carbon dioxide, which is a primary greenhouse gas (a gas that absorbs radiation in the atmosphere). Microscopic marine plants (phytoplankton) in the surface waters take up dissolved carbon dioxide during photosynthesis, just as land plants do, and convert it to organic carbon. About 10–30% of the carbon taken up by phytoplankton sinks out of surface waters, eventually settling in the abyssal depths, where it is effectively removed from the ocean–atmosphere system. This process is known as the biological pump, and is thought to have a major role in mitigating climate change.
The great ocean conveyor belt
Cooling of surface waters near Antarctica, and in the Greenland and Labrador seas, creates cold, heavy water that sinks, setting up what is known as the great ocean conveyor belt. This brings warm surface water from the equator to the poles, and returns cool waters at depth. The cooler deep water mixes so that it warms and rises as it travels towards the equator. The conveyor belt slowly moves water from one ocean basin to another, redistributing heat, salt and nutrients. This recirculation smooths out the earth’s temperature – without it, the poles would be considerably colder and the tropics significantly hotter.
Water masses are oceanic waters that have different temperatures or salinity levels from each other. The main water masses around New Zealand are subtropical and subantarctic waters. Subtropical water has arrived from near the equator. Heated and evaporated by the sun, it can be as warm as 22°C and as salty as 35.6. Subantarctic water is relatively cool and fresh – rarely exceeding 10°C, with a salinity of 34.4.
The journey of water is estimated to take 1,200 years to come full circle. The conveyor belt moves past New Zealand at a depth of about 2–5 kilometres along the east of the Campbell Plateau, and around Chatham Rise and the Hikurangi Plateau.
El Niño/Southern Oscillation (ENSO)
ENSO is a fluctuation of the ocean–atmosphere system in the tropical Pacific Ocean that occurs every 3–7 years, and has worldwide effects.
During normal years, trade winds on either side of the equator blow water westward so that it piles up in the western Pacific, near New Guinea – like a fan blowing water across a bath. As it crosses the Pacific, the water is warmed by the sun. Waters near New Guinea are as hot as any on the planet, and are called the warm pool.
During El Niño years the trade winds weaken and the warm pool flows back to the east, leading to a warming of the sea in the eastern equatorial Pacific Ocean. The increase in temperature (and corresponding cooling in the western Pacific Ocean) changes the heat that moves from the ocean to the atmosphere, which subsequently alters the winds over the Pacific Ocean. Around New Zealand at these times, the winds tend to prevail more from the south-west than usual. In El Niño years the weather is wetter in the west and south, and drier in the east and north.
This simplest of ocean properties turns out to be very difficult to consistently define. Originally, ocean salinity was defined as the amount of all salts in the water measured in parts per thousand by weight. However, in modern practice salinity is measured electronically and is defined as a ratio that has no units. Thus, the old measure of ocean salinity of about 35‰ (parts per thousand) becomes 35.
These large-scale processes can have an impact on particular parts of the ocean. For example, during the 1992–93 El Niño weather pattern, the north-east coast of New Zealand experienced dense and extensive plankton blooms. These were so severe and long-lasting that nearly the entire kelp population from Whangārei to Great Barrier Island was killed.
La Niña is a weather pattern consisting of stronger trade winds and a larger warm pool. Changes in the atmosphere and ocean tend to be the opposite of those during El Niño. In La Niña years, New Zealand’s winds tend to be more from the north-east than normal. The weather is drier in the west and south, and wetter in the east and north.