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Why does sea level change?

Seasonal changes

An important cause of intra-annual sea level change is the annual warming/cooling cycle in both hemispheres - in each hemisphere the oceans warm and expand in summer and cool and contract in the winter. Thus the sea levels in each hemisphere are higher in summer and early autumn, and lower in winter and early spring.

In addition there is an increase of water stored on land in the Northern Hemisphere winter and thus less in the ocean, leading to a lower global average sea level at this time of year.

Other changes are related to the latitudinal movement of the weather patterns in the ocean-atmosphere system, which produce changes in ocean currents and thus changes in sea level.

Plot of global- and hemsipheric-mean sea level

Global- and hemispheric-mean sea level from TOPEX/Poseidon and Jason-1. The two hemispheric signals partially cancel to produce a global signal with smaller amplitude.

Plot of the amplitude of the annual signal

Amplitude of the annual signal in millimetres.

Inter-annual to decadal

Inter-annual to decadal variability in the ocean-atmosphere system can cause large-scale changes in sea level. For example, El Niño events cause coherent changes of up to 20-30cm in sea level from the eastern Pacific Ocean to the eastern Indian Ocean, as shown by the plots of sea level from tide gauges below.

Plot of some tide gauge records

Tide gauge records from Fremantle (SW Australia), Kwajalein Atoll (Western Tropical Pacific) and San Francisco (Eastern Pacific). Monthly data is shown in light blue, and the data with the annual signal removed is shown in dark blue. Note the correlated/anti-correlated signals at time of El Niño events (e.g. 1983 and 1997).

Other decadal changes (e.g. the Pacific Decadal Oscillation and the Indian Ocean Dipole) also cause significant large-scale changes in sea level as does exchange of water with the atmosphere and storage on and in the land.


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