Why does sea level change?
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
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.
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
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.