63
boundary layer is not sufficiently buoyant (i.e., warm and moist) to penetrate through the overlying layer (i.e., the free troposphere), whose vertical
temperature profile tends to be in moist convective equilibrium with the
warmest regions of the tropical oceans, where SST is near or slightly above
28 C (Sarachik, 1978). One of the most prominent features of the tropical
precipitation climatology is the "equatorial dry zone" , a triangular shaped
region whose northern part corresponds closely to the cold tongue in the
SST field. The weakening of the equatorial cold tongue during the warm
phase of the ENSO cycle allows the bands of deep convection that surround
the dry zone to encroach upon it: the rain area over the warm waters to
the west of the dateline shifts eastward, and the intertropical convergence
zone (ITCZ) along'" 7 ON expands southward, as illustrated in Fig. 21.
The encroachment tends to be most pronounced when the warm phase of
the ENSO cycle coincides with the season of the year (January through
May) when the cold tongue tends to be relatively weak to begin with. At
these times intermittent heavy rainfall is observed along the equator across
the entire width of the Pacific and even in the coastal deserts of Ecuador
and northern Peru. The fluctuations in surface wind and precipitation
associated with the ENSO cycle are dynamically consistent: enhanced precipitation is associated with enhanced convergence of the surface wind field.
Note that much of the convergence in the surface wind field is associated
with the meridional wind component.
The sea-level pressure changes in Fig. 19 are part of a larger pattern
that affects the Australia and much of the tropical Indian Ocean as well.
The warm (weak tradewind) phase of the ENSO cycle is characterized
by above normal pressure to the west, over the tropical Indian Ocean,
Indonesia, and northern Australia and below normal pressure to the east,
over Pacific Islands such as Tahiti (17 S, 149 W) and Easter Is. (27 S,
109 W) and along the South American coast, and a weakened east-to-west
gradient across the central Pacific. The normalized difference between the
normalized sea-level pressure anomalies at Tahiti and Darwin (12 S, 131
E) is used as an index of the Southern Oscillation (the SO in ENSO)4. This
4The term Southern Oscillation was coined by Sir Gilbert Walker approximately 75 years ago to
describe this recurrent pattern of sea-level pressure fluctuations, which he believed to be related to
climate fluctuations over many parts of the globe. However, Walker was not aware of the dramatic
contrasts in SST, surface wind, and precipitation over the equatorial Pacific that occur in association
with it. The work of Jacob Bjerknes (1966, 1969), half a century later, provided a basis for linking the
Southern Oscillation with episodes of abnormally warm SST along the coast of South America, which had
hitherto been regarded as a local, primarily oceanographic phenomenon, known as EI Nino (Eguiguren
1894, Rasmusson and Carpenter 1982, Deser and Wallace 1987).
boundary layer is not sufficiently buoyant (i.e., warm and moist) to penetrate through the overlying layer (i.e., the free troposphere), whose vertical
temperature profile tends to be in moist convective equilibrium with the
warmest regions of the tropical oceans, where SST is near or slightly above
28 C (Sarachik, 1978). One of the most prominent features of the tropical
precipitation climatology is the "equatorial dry zone" , a triangular shaped
region whose northern part corresponds closely to the cold tongue in the
SST field. The weakening of the equatorial cold tongue during the warm
phase of the ENSO cycle allows the bands of deep convection that surround
the dry zone to encroach upon it: the rain area over the warm waters to
the west of the dateline shifts eastward, and the intertropical convergence
zone (ITCZ) along'" 7 ON expands southward, as illustrated in Fig. 21.
The encroachment tends to be most pronounced when the warm phase of
the ENSO cycle coincides with the season of the year (January through
May) when the cold tongue tends to be relatively weak to begin with. At
these times intermittent heavy rainfall is observed along the equator across
the entire width of the Pacific and even in the coastal deserts of Ecuador
and northern Peru. The fluctuations in surface wind and precipitation
associated with the ENSO cycle are dynamically consistent: enhanced precipitation is associated with enhanced convergence of the surface wind field.
Note that much of the convergence in the surface wind field is associated
with the meridional wind component.
The sea-level pressure changes in Fig. 19 are part of a larger pattern
that affects the Australia and much of the tropical Indian Ocean as well.
The warm (weak tradewind) phase of the ENSO cycle is characterized
by above normal pressure to the west, over the tropical Indian Ocean,
Indonesia, and northern Australia and below normal pressure to the east,
over Pacific Islands such as Tahiti (17 S, 149 W) and Easter Is. (27 S,
109 W) and along the South American coast, and a weakened east-to-west
gradient across the central Pacific. The normalized difference between the
normalized sea-level pressure anomalies at Tahiti and Darwin (12 S, 131
E) is used as an index of the Southern Oscillation (the SO in ENSO)4. This
4The term Southern Oscillation was coined by Sir Gilbert Walker approximately 75 years ago to
describe this recurrent pattern of sea-level pressure fluctuations, which he believed to be related to
climate fluctuations over many parts of the globe. However, Walker was not aware of the dramatic
contrasts in SST, surface wind, and precipitation over the equatorial Pacific that occur in association
with it. The work of Jacob Bjerknes (1966, 1969), half a century later, provided a basis for linking the
Southern Oscillation with episodes of abnormally warm SST along the coast of South America, which had
hitherto been regarded as a local, primarily oceanographic phenomenon, known as EI Nino (Eguiguren
1894, Rasmusson and Carpenter 1982, Deser and Wallace 1987).