WPGM2008講演スライド パプアニューギニア沖沿岸湧昇とエルニーニョ発生

1. Takuya Hasegawa, Kentaro Ando, Keisuke Mizuno
(JAMSTEC/IORGC)
&
Roger Lukas
(University of Hawaii)
Relationship between SST Cooling over the Western Part
of the Pacific Warm Pool and Coastal Upwelling
along the North Coast of Papua New Guinea
Prior to the 2002/03 El Nino
2008 WPGM Cairns

2. -Background-
Western equatorial Pacific
Coastal upwelling along PNG can contribute to
generate positive zonal gradient of SST
Warm SST related to
Pacific warm pool Onset of El Nino
Coastal upwelling
（Rotschi et al. 1973
Lukas1988;
Tsuchiya et al. 1989;
Kuroda 2000;
Webster & Lukas 1992;
Ueki et al. 2003)
WWB
Papua New Guinea
(PNG)
Eastward movement of
Pacific warm pool
Positive zonal gradient of SST
negative zonal gradient of SLP
High SLP Low SLP
Enhancement of westerly wind

3. ＳＳＴ horizontal map （NOAA OI-SST）
Dec
1981
Mar
1997
Dec
2001
Cold SST along the north coast of
PNG before onset of El Nino
Coastal cooling before onset of El Nino events
Papua New
Guinea
El Nino events after 1980s:
1982/83
1986/87
1991/92
1997/98
2002/03
2006/07
El Nino events after 1980s:
1982/83
(1986/87)
(1991/92)
1997/98
2002/03
2006/07 (but weak signal)
Half of El Nino events after 1980s:
Cold SST along the north coast of
Papua New Guinea
However, role of coastal oceanic variation
in SST cooling over the western part of
the Pacific warm pool, is not clear at present

4. western part of the Pacific warm pool (WPWP)
-Target-
SST cooling in WPWP
Papua New Guinea (PNG)
This study:
# Exploring mechanism of SST cooling in WPWP
# With special reference to coastal upwelling along
the north coast of PNG
# Discuss an onset mechanism of El Nino
?

5. Satellite data
-Data-
# Sea surface temperature (SST): TMI satellite data
# Sea surface wind (SSW): QuikSCAT satellite data
# T, S, current velocity, air-temp., etc.: TRITON moored buoy data
# CTD & ADCP data: KY0111 cruse of R/V Kaiyo
# Data period: 1999~present
Focus the oceanic variation in WPWP
prior to onset of 2002/03 El Nino event
WPWP
PNG
CTD data from R/V Kaiyo
TRITON mooring buoy data

6. Upper ocean temperature structure observed by R/V Kaiyo
2.7˚S EQ
CTD stations
28˚C
Coastal upwelling in 2001 boreal winter with cold SST along PNG coast
Shallower near the coast
Latitude-depth diagram of CTD-observed temperature
80m
20m
CTD-observation of the R/V Kaiyo
for 21-23 December 2001
(Prior to onset of 2002/03 El Nino)
Snapshot of daily SST on 22 December 2001
SSTs in the upwelling region:
Lower than those around it
Shows coastal upwelling
-Results-
Cold SST area along coast

7. 22 Dec 2001
Snapshot of SSW
Cold SST
North coast of PNG：
・Northwesterly SSW → Coastal upwelling
(Lukas1988; Webster & Lukas 1992)
Snapshot of TMI SST (color) & QuikSCAT SSW （arrow）
Northwesterly wind
parallel to the coast

8. TMI-SST
20Dec2001
10Jan2002
30Dec2001
Cold SST area:
Extension from the coastal area toward the equator
[℃]
Extension of the cold SST area from the coast

9. Cooling mechanism in WPWP
Using four TRITON buoys in BOX (daily data):
# Horizontal heat advection; from box-averaged zonal current
velocity at 10m depth (U) & horizontal gradient of
mixed layer temperature (MLT)
# Net surface heat flux: from buoy meteorological data
# Vertical heat advection, entrainment cooling, diffusion; Residual
# Daily data; tide-killer & 7-day running mean filtered
Cooling period of MLT
Dec 2001
MLT
Cooling: -0.7 (℃)
（Explaining 50% of MLT cooling）
(℃/day)
Large contribution of zonal heat advection to MLT cooling,
as compared to the net surface heat flux
Temporal change rate of observed MLT
Due to zonal heat advection
Due to net surface heat flux
(℃) Heat budget analysis
TRITON
buoys
Due to meridional heat advection
1 JAN
2002

10. onset of 2002/03
El Nino
-Summary and discussion-
eastward shift of
warm pool
zonal heat advection
Schematic diagram explaining
the proposed mechanism for
onset of 2002/03 El Nino
SST cooling
●About half of El Nino events after 1980s: Coastal SST cooling before onset
●Case study of the 2002/03 El Nino using buoy data etc.:
[Western edge of Pacific warm pool]
PNG
positive zonal gradient of SST
& strong westerly SSW
Newly shows a role of coastal upwelling in SST cooling over the
western edge of the Pacific warm pool. It may also contribute to
onset of El Nino. To explore a relationship between SST cooling
and enhancement of westerly wind is next target.
Northwesterly wind

11. END

12. -Background-
Onset of El Nino：
# Various mechanisms/hypotheses
・Eastward movement of Pacific warm pool (Wyrtki 1975, etc.）
Western equatorial Pacific
Eastward movement of Pacific warm pool
→ Air-sea coupled instability mode/slow mode
・ Positive zonal gradient of SST & westerly wind over the western side of Pacific warm pool
(Yamagata 1985; Philander et al. 1984)
・Westerly Wind Burst (WWB) related to MJO→SST cooling→Positive SST gradient
→low-level zonal pressure gradient (negative pressure gradient)
→enhancement of westerly wind→eastward movement of Pacific warm pool
(Kessler et al. 1995; Kessler and Kleeman 2000; Yu et al. 2003; Tziperman et al. 2007 )
Eastern equatorial Pacific
Warm SST related to
Pacific warm pool Onset of El Nino
WWB SST cooling in the
western edge of
Pacific warm pool
High pressure Low pressure
Eastward movement of
Pacific warm poolPositive zonal gradient of SST
Negative zonal gradient of low-level pressure

13. Time series of SSW component parallel to north coast of PNG
Uwin
(raw)
Vwin
(raw)
Parallel component (Positive: Northwesterly wind)
(raw & 7-day running mean filtered)
CTD Observation by R/V Kaiyo
Coastal upwelling period:
Strong northwesterly wind

14. TRITON-buoys
ADCP current velocities at depth of 40m during 13-23 December 2001
Cooling Mechanism?
Eastward &
Northeastward
ADCP-measured 40m-depth current velocities
Cold temperature; transported to the equatorial region,
by eastward/northeastward advection