Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Ocean Observation and Research Department
western tropical Pacific during the La Niña years after
El Niño. The first major westerly wind burst
occurred in June causing the equatorial jet of .
m/s and warm water convergence. This propagated as
equatorial downwelling Kelvin waves and switched
from cold phase to warm phase in the central Pacific.
Similar major westerly wind bursts occurred in
December and May , the last one induced the
strong ocean and atmosphere coupling and the coupled
system propagated slowly into the eastern pacific. In
December , the El Niño matured as the subsurface
temperature anomaly at m-m depth indicated
C warmer than normal, and the sea surface temperature
was also C warmer than normal. However the
C isotherm depth shoaled rapidly with propagation
of upwelling equatorial Kelvin waves, and the sea surface
temperature returned to normal. Thus this -
El Niño evolution indicated quite different features
from those of the - El Niño, i.e. the former one
developed slowly and moderately, but the later one
quickly developed as the largest El Niño on record.
The TRITON data will be utilized to determine what
controls such El Niño characteristics, focusing on the
surface mixing layer including salinity effect and water
mass exchange between the tropics and subtropics in
the thermocline layer.
() Numerical simulation by a high resolution model
The Recharge-Discharge Oscillator has been recognized
as one of the ENSO cycle mechanisms, and the
meridional transport of the warm water volume
(WWV) in the equatorial ocean has been investigated
as a key process. Though the interior ocean transport
has been analyzed, the transport in the western boundary
region is not fully understood because of the variable
and complicated structure of the boundary currents.
In this study, the interannual variation of the
WWV transport in the equatorial Pacific Ocean is
investigated to elucidate the role of the western boundary
currents by diagnosing a high resolution model.
The WWV is defined as the thickness from sea
surface to the depth of C isotherm and the monthly
anomaly, which is the deviation from the climatological
monthly mean, is investigated. Time series observations
of Nino sea surface temperature (SST) anomaly and
the WWV anomaly in the equatorial region (S-N)
are shown in Fig. . The model simulates the realistic
interannual variation inferred from Nino SST anomaly,
the WWV reduction in El Niño period, and the phase
lag between Nino SST anomaly and the WWV variation
as predicted in the Recharge-Discharge Oscillator
theory. To investigate the role of the boundary currents,
the WWV and the volume transport are computed in
three regions (west of E, E-W, east
of E) in the equatorial zonal band (S-N).
The schematic diagram for - El Niño is shown in
Figure .
WWV (1e20 cm3)
4
3
2
1
0
-1
-2
-3
-4
-5
-6
-7
1982 1984 1986 1988 1990 1992 1994 1996 1998 2000
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
Fig. 4 Time series observation of Nino3 SST anomaly (red line)
and the WWV anomaly (black line) in the equatorial region.
The WWV is determined from the depth of 20C isotherm
anomaly integrated over the basin (8S-8N, 156E-95W).
20N
15N
10N
5N
EQ
5S
10S
15S
2
1.5
6
9
1.2
0.4
28 14
-7 -2.4
14
3
15 16
20S
120E 130E 140E 150E 160E 180 160W 140W 120W 100W 80W
20N
15N
10N
5N
EQ
5S
10S
15S
1 5.5
+7.5 -13
17
3
16 15
20S
120E 130E 140E 150E 160E 180 160W 140W 120W 100W 80W
Fig. 5 Schematic diagram of the change of the WWV anomaly
(italic) in the western and interior regions and the transport
anomaly across the boundaries (black) during the 1997/98
El Niño developing period (upper panel) and during mature
to termination period (lower panel) in Sverdrup.
Nino3 SSTA
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