Geoscience Reference
In-Depth Information
were merchant ships whose crews voluntarily made observations of the upper
oceans and reported them to a central location. Besides using nonstandardized
recording methods, these ships provided information only from shipping lanes,
leaving large portions of the ocean surface unobserved.
Oceanographers have been creative in developing various ways of retrieving
information about the oceans. In May 1990, for example, they took advantage
of a spill of 60,000 sports shoes from a cargo ship in the North Atlantic. In
another incident 29,000 bathtub toys were spilled in a storm near the date line
in 1992. When the spilled items started turning up along the North American
coast several months later, oceanographers used their locations to infer speeds
and directions of ocean currents.
The satellite era for earth observations, which began with the launch of
TIROS- 1
in April 1961, brought global observing coverage of the ocean's sur-
face. Sea surface temperature, surface winds, phytoplankton distributions, sea
surface elevations, and other variables describing the state of the ocean's sur-
face are extracted from satellite observations.
Observing the ocean away from the surface presents a formidable challenge
because the world's oceans are enormous and, for the most part, remote. Ocean
circulation patterns and thermal structure beneath the surface are primarily
inferred from indirect measurements or from drifters. Direct measurements
of ocean temperature, pressure, salinity, and density are either the result of
extrapolating in space from local measurements scattered around the globe,
or extrapolating in time from intensive observing periods. Thus, we have less
confidence in our climatological picture of the ocean than the atmosphere, and
we have only a rudimentary idea of how the ocean's climate varies in time.
The heat capacity of the ocean dwarfs that of the atmosphere. There is more
heat energy in the top 3 m of the ocean than in the entire atmospheric column
above it. The specific heat of water, defined as the amount of heat needed to
increase the temperature of 1 kg of water by 1 K, is about 4218 J/(kg K), and
varies slightly with temperature and pressure. This is four times the specific
heat of air, which is about 1007 J/(kg K), again depending on temperature and
pressure. Furthermore, the mass of the atmosphere is minuscule compared with
that of the oceans (5.3 10
18
kg versus 1.4 10
21
kg, respectively). Conse-
quently, the heat content of the world's oceans is immense compared with that
of the atmosphere, and sea surface temperature distributions are very powerful
forcing functions for the atmosphere.
Figure 2.15
shows the sea surface temperature climatology, annually aver-
aged for the period 1900-1997. (Note that the zonal boundaries of the plot
are shifted eastward by 20° of longitude compared with the figures of the at-
mosphere in the previous section so the Atlantic Ocean basin is not disrupted.)
Regions over the oceans with no data are covered by sea ice for at least some
portion of the year. Note the following features:
• Meridional temperature gradients are larger in middle and high latitudes
than in the tropics.
• Sea surface temperatures in the eastern sides of the Atlantic and Paciic
Ocean basins are cooler than in the western sides of the basins in both
hemispheres, similar to the structure seen in the low-level atmospheric
temperature (
Fig. 2.6)
. The western tropical Pacific, known as the
western
