800 OCEANOGRAPHY
additional influences of the sun and planets, sometimes
enhancing and sometimes reducing the moon-induced tide.
Perhaps more importantly, variations in ocean water depth
greatly influence the movement of the tidal “wave”, this
influence becoming very apparent in the continental shelf-
slope region. In spite of the these complexities, we have at the
present time an excellent understanding of the physics of tide
generation and tidal wave propagation. Tide prediction algo-
rithms have been developed and proven surprisingly accurate
among much of the world’s coastlines. Of course, accuracy is
a relative word and should be measured against the require-
ments of ocean scientists. As these requirements dictate still
more accurate tidal elevation predictions, new algorithms
must be developed. An example of such a development is
the recent work in spectral modelling of shallow water tidal
wave propagation, with accuracy in sea surface elevation of
the order 2 centimeters (Vincent and Le Provost, 1988).Recent Advances in Oceanographic Research The global
impacts of El Nino events have become more clearly defined
in the interval since this article first appeared. The need to
develop models for prediction of El Nino occurrences became
obvious following the severe event in 1982/83 and the even
stronger one in 1996/97. There has been a significant effort
to integrate advances in observations from satellites (with
coverage of the global ocean) with in situ measurements
from ships and buoys and to incorporate these into coupled
numerical models of the climatology of the ocean and atmo-
sphere. The evidence seems compelling that the occurrence
of El Nino is closely associated with the oscillation in sea
level pressure between the western and central regions of
the tropical Pacific Ocean. A non-dimensional index of this
oscillation is found by subtracting the measured pressure
in Darwin, Australia from that measured in Tahiti and then
dividing by the standard deviation of this pressure difference
obtained from the long term records at these locations.
Under conditions without an El Nino this index is gener-
ally slightly positive but can undergo reversals of sign. During
significant El Nino events this index becomes strongly nega-
tive, indicating that the typical low pressure on the western
side of the tropical Pacific has changed dramatically.
Conceptually, during normal years the strong easterly trade
winds move warm surface water into the western portion of the
Pacific with much cooler water occurring along the west coast
of South America and extending into the Eastern Equatorial
Pacific. The elevation of the sea surface is about 0.5m higher
near Indonesia than off Ecuador. Prior to the onset of El Nino
the east to west pressure gradient along the equator dimin-
ishes as the Southern Oscillation Index becomes negative. As
a result the trade winds diminish (even reversing direction) and
the warmer surface water in the western tropical Pacific moves
eastward eventually warming the near surface waters along the
equator and the west coast of South America. The effects of
the occurrence of an El Nino on weather are significant and
on a global scale. Following an El Nino there can be an over
recovery with stronger trade winds and more intense upwell-
ing of colder deep water along the Peruvian Coast. This La
Nina is associated with a positive Southern Oscillation indexand has important global effects as well. The connection of El
Nino and the Southern Oscillation is sufficiently strong to give
rise to the acronym ENSO to denote these two phenomena and
their interconnections.
Another large scale oscillation has been detected for the
North Pacific Region. Termed the Pacific Decadal Oscillation,
PDO, it is chiefly characterized by an alternation of a warm
phase with a cool phase, with time scales of 2–3 decades.
During the warm phase, sea surface temperatures in the cen-
tral North Pacific are lower than average with warmer than
average temperatures occurring along the west coasts of
North and South America. For the cool phase these anomalies
reverse with cooler water occurring along the west coast and
the central region sea surface temperatures being warmer than
average. Although there is no persistent connection between
the PDO and ENSO the most severe El Nino events have all
coincided with the warm phase of the PDO.
Observations of the oceanographic and meteorological con-
ditions at the sea surface on a global scale from satellites have
become increasingly useful. An example of this is provided by
the 12 year success of the measurement of the dynamic topog-
raphy of the sea surface using a high frequency radar altim-
eter and sea surface temperature using a microwave radiometer
onboard the Topex Poseidon satellite. The technical difficulties
in the accurate determination of sea surface topography are for-
midable. The position of the satellite relative to the Earth must
be accurately known, the earth’s geoid (the level surface most
closely aligned with mean sea level) must be well defined and
the effect of open ocean tides must be removed from the obser-
vations. The Topex Poseidon satellite repeats observations over
the same oceanic region once every 10 days and during that
period it collects data from 95% of the ice free global ocean.
The reported accuracy of the departures of the mean elevation
of the sea surface from the geoid is about 0.04m. Thus, the
change in elevation of the sea surface across the Gulf Stream of
the order of 1.0 meters is easily discerned. Given the spatial dis-
tribution of sea surface elevation, (x, y) then the surface geo-
strophic current components, u and v, in the x and y directions
respectively are found from the following equations expressing
the near balance between the Coriolis force and the component
of gravity along the sloping isobaric sea surface:fu g (x, y)/ y , fv g (x, y)/ y ,where g is the acceleration of gravity and f is, as before,
the Coriolis parameter. The observations from the Topex
Poseidon satellite provide estimates of the geostrophic cur-
rents at very high spatial resolution and with a temporal
resolution of 10 days. These data are useful since surface
currents measured directly are in substantial agreement with
the calculated geostrophic currents. The 12 year mission of
this satellite (1992–2005) allowed for detailed observations
of development and time history of the very intense ENSO
event in 1997/98 and the subsequent La Nina. In addition to
radar altimeter observations other satellite borne instruments
have been used to map the distribution and evolution of sea
surface temperatures using infrared radiometers, to estimate
surface wind stress over the sea and ocean surface wave char-
acteristics from scattering of microwave radars.C015_001_r03.indd 800C015_001_r03.indd 800 11/18/2005 11:10:05 AM11/18/2005 11:10:05 AM