Recent Satellite Studies
15
eastern Pacific the low cloudiness of the southern subtropical anticyclones tends to extend
northwards to the Equator; in some months the minimum is found at the Equator itself.
Over the eastern Pacific a secondary zone of maximum cloudiness is sometimes found
several degrees south of the Equator but this forms in situ and does not arise through the
southward translation of the North Pacific ITCZ. Over the Indian Ocean in January a
cloudy zone extends northeastwards from Malagasy to Sumatra. In this case the average
is often unrepresentative of the distribution on individual days and in individual years.
There are occasions when the Indian Ocean cloud systems take up broadly the orientation
shown in Fig. 8 but the Indian Ocean zones are more variable in their character and
location than are those of the East Pacific. The zone established at the Equator on
23 January 1967 (Fig. 6) was earlier to be found further to the north as shown in Fig. 9,
which is an ESSA 3 mosaic for 12 January 1967. An older zone, seen south of the Equator
in Fig. 9, was in process of disruption by the tropical cyclone and it did not subsequently
reform; instead, the newer zone to the north moved southwards to take its place. In
Fig. 10 is plotted the daily latitude at longitude 60 ° E of the convergence zones which
10 0
N
15 0
' "\0' 00\
0\
. \
\
..
. "
'. /
'0 •
.. \ .,. \
• .r \
••
•
·\i'· \ . .
/\
. \ \ I \ j.\..
\ \j \lyl
.'
S ~ ____ ~~ ____ ~~ ____ ~~ ____ ~~ ____ ~~ __ ~
10
20
III
10
20
28
Jonuary 1967
Februory 1967
Fig. 10. The movement over the Indian Ocean of the centre of the Intertropical Convergence Zone
at 60° E during January and February 1967. (JOHNSON, 1970)
moved over the Indian Ocean during January and February 1967. Zones repeatedly
formed over the north of the ocean and shifted southwards irregularly over periods of
weeks to replace or merge with older, more southerly systems.
Whatever their form, wave, vortex or amorphous, the cloud clusters of the trades
are typically westward-moving with a speed of translation of order 5° of longitude per
day. They are long-lived and can usually be tracked over periods of 10 days or more.
Over the Indian Ocean, the disturbances are not predominantly westward-moving but
such systems do occur in due season over those regions subject to tropical cyclones. The
cloud clusters which form in the monsoon Westerlies, however, are quasi-stationary and
15
eastern Pacific the low cloudiness of the southern subtropical anticyclones tends to extend
northwards to the Equator; in some months the minimum is found at the Equator itself.
Over the eastern Pacific a secondary zone of maximum cloudiness is sometimes found
several degrees south of the Equator but this forms in situ and does not arise through the
southward translation of the North Pacific ITCZ. Over the Indian Ocean in January a
cloudy zone extends northeastwards from Malagasy to Sumatra. In this case the average
is often unrepresentative of the distribution on individual days and in individual years.
There are occasions when the Indian Ocean cloud systems take up broadly the orientation
shown in Fig. 8 but the Indian Ocean zones are more variable in their character and
location than are those of the East Pacific. The zone established at the Equator on
23 January 1967 (Fig. 6) was earlier to be found further to the north as shown in Fig. 9,
which is an ESSA 3 mosaic for 12 January 1967. An older zone, seen south of the Equator
in Fig. 9, was in process of disruption by the tropical cyclone and it did not subsequently
reform; instead, the newer zone to the north moved southwards to take its place. In
Fig. 10 is plotted the daily latitude at longitude 60 ° E of the convergence zones which
10 0
N
15 0
' "\0' 00\
0\
. \
\
..
. "
'. /
'0 •
.. \ .,. \
• .r \
••
•
·\i'· \ . .
/\
. \ \ I \ j.\..
\ \j \lyl
.'
S ~ ____ ~~ ____ ~~ ____ ~~ ____ ~~ ____ ~~ __ ~
10
20
III
10
20
28
Jonuary 1967
Februory 1967
Fig. 10. The movement over the Indian Ocean of the centre of the Intertropical Convergence Zone
at 60° E during January and February 1967. (JOHNSON, 1970)
moved over the Indian Ocean during January and February 1967. Zones repeatedly
formed over the north of the ocean and shifted southwards irregularly over periods of
weeks to replace or merge with older, more southerly systems.
Whatever their form, wave, vortex or amorphous, the cloud clusters of the trades
are typically westward-moving with a speed of translation of order 5° of longitude per
day. They are long-lived and can usually be tracked over periods of 10 days or more.
Over the Indian Ocean, the disturbances are not predominantly westward-moving but
such systems do occur in due season over those regions subject to tropical cyclones. The
cloud clusters which form in the monsoon Westerlies, however, are quasi-stationary and