Warming water and melting land ice have raised global mean sea level 4.5
centimeters (1.7 inches) from 1993 to 2008. But the rise is by no means
uniform. This image, created with sea surface height data from the
Topex/Poseidon and Jason-1 satellites, shows exactly where sea level has
changed during this time and how quickly these changes have occurred.
It’s also a road map showing where the ocean currently stores the growing
amount of heat it is absorbing from Earth’s atmosphere and the heat it
receives directly from the Sun. The warmer the water, the higher the sea
surface rises. The location of heat in the ocean and its movement around
the globe play a pivotal role in Earth’s climate.
Light blue indicates areas in which sea level has remained relatively
constant since 1993. White, red, and yellow are regions where sea levels
have risen the most rapidly—up to 10 millimeters per year—and
which contain the most heat. Green areas have also risen, but more
moderately. Purple and dark blue show where sea levels have dropped, due
to cooler water.
The dramatic variation in sea surface heights and heat content across the
ocean are due to winds, currents and long-term changes in patterns of
circulation. From 1993 to 2008, the largest area of rapidly rising sea
levels and the greatest concentration of heat has been in the Pacific,
which now shows the characteristics of the Pacific Decadal Oscillation
(PDO), a feature that can last 10 to 20 years or even longer.
In this “cool” phase, the PDO appears as a horseshoe-shaped pattern of
warm water in the Western Pacific reaching from the far north to the
Southern Ocean enclosing a large wedge of cool water with low sea surface
heights in the eastern Pacific. This ocean/climate phenomenon may be
caused by wind-driven Rossby waves. Thousands of kilometers long, these
waves move from east to west on either side of the equator changing the
distribution of water mass and heat.
This image of sea level trend also reveals a significant area of rising
sea levels in the North Atlantic where sea levels are usually low. This
large pool of rapidly rising warm water is evidence of a major change in
ocean circulation. It signals a slow down in the sub-polar gyre, a
counter-clockwise system of currents that loop between Ireland, Greenland
and Newfoundland.
Such a change could have an impact on climate since the sub-polar gyre may
be connected in some way to the nearby global thermohaline circulation,
commonly known as the global conveyor belt. This is the slow-moving
circulation in which water sinks in the North Atlantic at different
locations around the sub-polar gyre, spreads south, travels around the
globe, and slowly up-wells to the surface before returning around the
southern tip of Africa. Then it winds its way through the surface currents
in the Atlantic and eventually comes back to the North Atlantic.
It is unclear if the weakening of the North Atlantic sub-polar gyre is
part of a natural cycle or related to global warming.
This image was made possible by the detailed record of sea surface height
measurements begun by Topex/Poseidon and continued by Jason-1. The
recently launched Ocean Surface Topography Mission on the Jason-2
satellite (OSTM/Jason-2) will soon take over this responsibility from
Jason-1. The older satellite will move alongside OSTM/Jason-2 and continue
to measure sea surface height on an adjacent ground track for as long as
it is in good health.
Topex/Poseidon and Jason-1 are joint missions of NASA and the French space
agency, CNES. OSTM/Jason-2 is collaboration between NASA; the National
Oceanic and Atmospheric Administration; CNES; and the European
Organisation for the Exploitation of Meteorological Satellites. JPL
manages the U.S. portion of the missions for NASA's Science Mission
Directorate, Washington, D.C.