Sextant to Satellite
157
I became the theoretician in a group of oceanographers, led by Walter Duing,
who participated in a huge field program, involving an armada of ships, to study
mainly atmospheric convection over the tropical Atlantic during the summer of 1975.
At the time, so little was known about tropical oceanic variability that some of our
colleagues questioned whether measurements over the relatively short period of a
few weeks or months would reveal anything interesting. We consulted with Dennis
Moore, but his ideas concerning strange waves that use the equator as a guide to
race across ocean basins seemed so “dream-like” that we doubted their relevance to
actual oceanic phenomena. We nonetheless decided to make most measurements very
close to the equator where a curious undercurrent lurks. This led to a serendipitous
discovery: energetic meanders of the equatorial currents with a period of a few weeks.
7
At about the same time, Richard Legeckis
8 discerned similar meanders in satellite
photographs of sea surface temperature patterns in the tropical Pacific. A stability
analysis of the time-averaged currents yielded results, concerning unstable waves, in
accord with measurements of the spatial and temporal structure of the meanders.
12
The relevance of theories concerning stable equatorial waves (developed by
Taroh Matsuno
9 and Dennis Moore
10 ) to actual oceanic phenomena remained in
question until Carl Wunsch and Adrian Gill
11 used the available theories to explain
curious oscillations, at periods of a few days, in sea level as measured at a number of
islands in the tropical Pacific.
The oscillations in sea level at a few islands, and the meanders of the equatorial
currents mentioned earlier, are phenomena of minor importance. Their discovery, and
the success of theories
12 to explain their main features, nonetheless galvanized the
community of tropical oceanographers because seemingly “dream-like” theories suddenly were of direct relevance to observations. Attention now turned to the response
of the oceans to time-dependent winds.
How long, after an abrupt cessation of the winds, before a current such as
the Gulf Stream disappears? George Veronis and Henry Stommel
13 first addressed
this question and found that the answer depends on the time it takes Rossby waves
to propagate across an ocean basin. In midlatitudes this is on the order of decades,
but the speed of the waves increases with decreasing latitude and is infinite at the
equator. Lighthill’s seminal paper on the generation of the Somali Current,
14 which
demonstrated that the removal of that singularity requires the introduction of equatorially trapped waves, motivated numerous theoretical and observational studies of
a variety of phenomena. (Several people earned Ph.D. degrees correcting errors in
Lighthill’s paper. He neglected equatorial Kelvin waves, for example.) Henry Stommel persuaded younger colleagues to test these ideas concerning the generation of
western boundary currents after the abrupt onset of the southwest monsoons. Bob
Knox struck gold—time series of currents and winds—by asking officers at a British
military base on the desolate island Gan within sight of the equator to make regular
measurements from a small vessel. Ants Leetmaa spent much time in the Seychelles
and east Africa, chartering vessels for additional oceanographic measurements.
157
I became the theoretician in a group of oceanographers, led by Walter Duing,
who participated in a huge field program, involving an armada of ships, to study
mainly atmospheric convection over the tropical Atlantic during the summer of 1975.
At the time, so little was known about tropical oceanic variability that some of our
colleagues questioned whether measurements over the relatively short period of a
few weeks or months would reveal anything interesting. We consulted with Dennis
Moore, but his ideas concerning strange waves that use the equator as a guide to
race across ocean basins seemed so “dream-like” that we doubted their relevance to
actual oceanic phenomena. We nonetheless decided to make most measurements very
close to the equator where a curious undercurrent lurks. This led to a serendipitous
discovery: energetic meanders of the equatorial currents with a period of a few weeks.
7
At about the same time, Richard Legeckis
8 discerned similar meanders in satellite
photographs of sea surface temperature patterns in the tropical Pacific. A stability
analysis of the time-averaged currents yielded results, concerning unstable waves, in
accord with measurements of the spatial and temporal structure of the meanders.
12
The relevance of theories concerning stable equatorial waves (developed by
Taroh Matsuno
9 and Dennis Moore
10 ) to actual oceanic phenomena remained in
question until Carl Wunsch and Adrian Gill
11 used the available theories to explain
curious oscillations, at periods of a few days, in sea level as measured at a number of
islands in the tropical Pacific.
The oscillations in sea level at a few islands, and the meanders of the equatorial
currents mentioned earlier, are phenomena of minor importance. Their discovery, and
the success of theories
12 to explain their main features, nonetheless galvanized the
community of tropical oceanographers because seemingly “dream-like” theories suddenly were of direct relevance to observations. Attention now turned to the response
of the oceans to time-dependent winds.
How long, after an abrupt cessation of the winds, before a current such as
the Gulf Stream disappears? George Veronis and Henry Stommel
13 first addressed
this question and found that the answer depends on the time it takes Rossby waves
to propagate across an ocean basin. In midlatitudes this is on the order of decades,
but the speed of the waves increases with decreasing latitude and is infinite at the
equator. Lighthill’s seminal paper on the generation of the Somali Current,
14 which
demonstrated that the removal of that singularity requires the introduction of equatorially trapped waves, motivated numerous theoretical and observational studies of
a variety of phenomena. (Several people earned Ph.D. degrees correcting errors in
Lighthill’s paper. He neglected equatorial Kelvin waves, for example.) Henry Stommel persuaded younger colleagues to test these ideas concerning the generation of
western boundary currents after the abrupt onset of the southwest monsoons. Bob
Knox struck gold—time series of currents and winds—by asking officers at a British
military base on the desolate island Gan within sight of the equator to make regular
measurements from a small vessel. Ants Leetmaa spent much time in the Seychelles
and east Africa, chartering vessels for additional oceanographic measurements.