Geoscience Reference
In-Depth Information
about 1870. While the NAO has no preferred
timescale of variability, a number of epochs can be
defined. From about 1890-1900 it was in a most
negative mode, followed by a mostly positive
period from about 1900-1950. This was followed
by a negative period from about 1960-1980,
followed by a general rise into the mid-1990s.
This recent rise fostered winters that, compared
to normal, were warmer over much of northern
Eurasia, and wetter (drier) conditions over
Northern Europe-Scandinavia (southern Europe-
Mediterranean), in association with a northward
shift of storm tracks. Since the late 1990s, the NAO
has regressed back to a generally more neutral
phase.
The PDO, NPO and PNA
While ENSO has been previously discussed, is
should be pointed out that ENSO can be linked in
a variety of ways to 'ENSO-like' patterns for which
multi-decadal signals are prominent. Climate
signals are especially well expressed in the
northwest Pacific, including Alaska. Of particular
note is the Pacific Decadal Oscillation, or PDO,
which has an index based on North Pacific sea
surface temperatures. Its time series parallels the
7.3 Oceanographic observations
Meteorological and oceanographic measurements in the oceans are made on about 7000 ships of
the Voluntary Observing Fleet and by moored or floating buoys. 'Selected ships' observe air and
sea surface temperature, pressure and its tendency, wind, present and past weather, humidity,
clouds and waves. Supplementary (and Auxillary) ships make the same observations omitting sea
surface temperature, pressure tendency, waves (and clouds). The UK Meteorological Office operates
seven moored deep-water buoys on the edge of the continental shelf west of the British Isles and
two in the North Sea. There are similar buoys in the Pacific and Atlantic oceans off Canada and the
USA, with about 65 such buoys operated by the USA. They measure pressure, air temperature,
humidity, wind velocity, sea surface temperature, and wave height and period. Drifting buoys are
now used worldwide. They measure barometric pressure and its tendency, and sea surface
temperature while some also measure air temperature and wind velocity. The data are telemetered
to the Argos satellites, which fix the buoys' position, and transmit them to Oslo, Toulouse and
Søndre Strømfiord in West Greenland, as well as to Argos ground stations in the USA and France.
Ocean currents are determined from the drift of ships where the difference between a ship's dead-
reckoned position - determined from its previous position based on a navigational fix - and its actual
location is ascribed solely to the effect of surface currents. They are also measured by current meters
on surface and bottom moorings, that are installed by oceanographic research vessels and may
operate for up to two years before being recovered. The temperature and salinity properties of the
oceans are determined from conductivity, temperature and depth (CTD) sensors developed in the
1970s. These measure resistances of the sensors to variations in conductivity, temperature and
pressure. Conductivity depends on both temperature and salinity, so with these measurements a
temperature salinity profile is obtained and relayed to the research vessel. A global array of free-
drifting profiling floats (Argo) that measure the temperature and salinity of the upper 2000m of the
ocean began operations in 2000 and currently has 3000 floats in the world's oceans. The data are
retrieved when the float periodically surfaces and are transmitted to receiving stations by satellite
link.
