Intense surface currents in the Tropical Pacific during 1996-1998.
Understanding the role
of ocean in the El Niño-Southern Oscillations (ENSO) requires detailed
knowledge of the currents variability, which control the ocean-atmosphere
coupling through the sea surface temperature variation. The tropical Pacific
Ocean near surface currents and their force balance are investigated based
on the drifting buoys data and the TOPEX/POSEIDON altimetry. Both data
sets are combined using the multivariate optimal interpolation to present
the currents on a regular space-time grid.
The currents fields for the time period December 1996 - August 1998 show
dramatic changes of the equatorial Pacific circulation forced by the recent
ENSO events. Eastward current anomalies of ~1 m/s were recorded in December
1996 in the western Pacific in response to sporadic westerly wind bursts.
These anomalies reached the eastern boundary by April 1997, and in the
summer of 1997 a band of strong eastward flow formed across the basin along
the equator. This circulation pattern existed until the very beginning
of 1998 when it rapidly switched to a cold La Nina phase. The westward
equatorial jet appeared in January-April 1998 prior to the restoration
of the trade winds in the tropical Pacific (see
Fig.1, model currents and drifter observations are colored in black and
red, respectively).
Monthly mean fields of surface velocity, satellite altimetry and wind stress
were used to evaluate the applicability of linear zonal momentum balance
near the equator. Anomaly local acceleration at the equator (
)
is found to be balanced by the difference between the vertical gradient
of zonal wind stress anomaly (
,
where H is the vertical length scale) and zonal pressure gradient
anomaly (presented in terms of the sea level gradient 
).
This three-term balance displays significant basin-wide variations and
implies that the equatorial Pacific is not in equilibrium with local wind
forcing due to the presence of propagating waves (see Fig.2).
)
is found to be balanced by the difference between the vertical gradient
of zonal wind stress anomaly (,
where H is the vertical length scale) and zonal pressure gradient
anomaly (presented in terms of the sea level gradient ).
This three-term balance displays significant basin-wide variations and
implies that the equatorial Pacific is not in equilibrium with local wind
forcing due to the presence of propagating waves (see Fig.2).
Drifter currents and Reynolds and Smith [1994] sea surface temperature
fields were used to assess the role of heat advection in the equatorial
Pacific. We selected two areas bounded by -4S and 4N, and choose
longitudinal bands from 180E to 220E, and from 240E to 280E to represent
central and eastern parts of the ocean, respectively. Based on different
measurement sets Picaut et al. [1996] have demonstrated the dominance
of zonal advection in the migration of the eastern edge of the western
Pacific warm pool. This result strongly suggests that heat storage rate
in the central Pacific in mainly balanced by the advection of the temperature
front separating western and central Pacific waters.
Figure
3 supports this hypothesis demonstrating close agreement between 
(bold line) and minus advection in the central tropical Pacific.
Besides of close correspondence between 
and -
, a time delay between these two series likely exists reflecting probably
the effect of secondary vertical motions accompanying horizontal movement
of an ocean front. For example, negative value of 
+
demands additional sink of heat to complete the balance. The strongest
negative deviation was observed around the beginning of 1998 when
La Nina currents contributed to thermocline shoaling that additionally
emphasized the effect of upwelling in the cooling of the upper ocean. This
conclusion differs from that drawn by Delcroix and Picaut [1998] who found
that although the equatorial upwelling extends toward the western Pacific
during La Nina, its effectiveness in changing the near-surface salinity
(and temperature) is questionable.
(bold line) and minus advection in the central tropical Pacific.
Besides of close correspondence between
and -
, a time delay between these two series likely exists reflecting probably
the effect of secondary vertical motions accompanying horizontal movement
of an ocean front. For example, negative value of +
demands additional sink of heat to complete the balance. The strongest
negative deviation was observed around the beginning of 1998 when
La Nina currents contributed to thermocline shoaling that additionally
emphasized the effect of upwelling in the cooling of the upper ocean. This
conclusion differs from that drawn by Delcroix and Picaut [1998] who found
that although the equatorial upwelling extends toward the western Pacific
during La Nina, its effectiveness in changing the near-surface salinity
(and temperature) is questionable.
The advection driven balance less probably exists in the eastern Pacific,
as mean thermocline shoaling has to result in a stronger effect of vertical
motions. Indirect evidence comes from time series of 
and advection term (see Figure
3), which seems to vary almost independently. Obvious exceed of the
SST rate of change over the advection recorded during 1994, 1997 El Ninos
demonstrates the necessity of extra source of heat probably associated
with currents convergence that cancels upwelling, and contrary an additional
sink of heat (supported by currents divergence) had to exist during the
final stage of 1997-98 El Nino and subsequent La Nina.
and advection term (see Figure
3), which seems to vary almost independently. Obvious exceed of the
SST rate of change over the advection recorded during 1994, 1997 El Ninos
demonstrates the necessity of extra source of heat probably associated
with currents convergence that cancels upwelling, and contrary an additional
sink of heat (supported by currents divergence) had to exist during the
final stage of 1997-98 El Nino and subsequent La Nina.
Preliminary examination of the anomaly temperature balance shows that at
the beginning of El Nino a development of positive SST anomaly in the central
Pacific is mainly supported by horizontal advection (advection feedback),
while the upwelling (thermocline feedback) dominates rapid decrease of
the SST in the eastern Pacific at final stage of El Nino