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

-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

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

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

?

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

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

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

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

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）

(℃/d

ay)

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

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

END

-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 pool Positive zonal gradient of SST Negative zonal gradient of low-level pressure

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

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