5. variability of SST and surface chlorophyll

Seasonal variability of current anomaly, temperature, and chlorophyllinferred from space north of New Guinea Island

OS41B-1718

1. introduction

The New Guinea Coastal Current (NGCC) flowing along the northern coast of New Guinea is part of the low latitude western boundary current system of the South Pacific connecting subtropical to equatorial water masses. The mean current is northwestward (Fig. 1) and is strongly influenced by the monsoonal wind regime. The NGCC flows northwestward between June and September (trade winds) and reverses between December and March (northwest monsoon) [1].

2. data and methodologyData sources

� Monthly along track Topex/Poseidon, Jason 1, and 2 sea level anomaly (SLA) at 1 Hz rate (6.2 km, 10 days, Jan. 1994-Dec. 2010) reprocessed at CTOH [3]

� Monthly SeaWiFS chlorophyll [4] � Monthly Pathfinder AVHRR SST [5]

Fig. 1. Bathymetry (m) and main geographical features in the New Guinea regionNB = New Britain I.NI = New Ireland I.

Vectors are the mean drifter-estimated surface currents [2]

Calculation of the cross track geostrophic current

� The cross track geostrophic current anomaly (Ua) is calculated from the along track SLA gradient filtered with a σ = 0.3° (or 1.2° near the equator) Gaussian filter. It is the weighted sum of Uf calculated in the f-plane, away from the equator, and Uβcalculated in the β-plane near the equator [6] :

Ua = Wf Uf + Wβ Uβ

with β = 2.3 10-11 m-1 s-1

� Weight functions ensure a smooth transition between the equator and higher latitudes:

Wβ = exp[-(θ / θs)²] and Wf = 1 - Wβ with θs = 2.2°

2

2

l

gU

∂∂= η

ββlf

gU f ∂

∂= η

references1. Ueki et al., Journal of Geophysical Research, 20032. Lumpkin and Garrafo, Journal of Atmospheric and Oceanic Technology, 20053. Roblou et al., in Coastal Altimetry, chapter 9, Springer Verlag Berlin Heidelberg, 20114. McClain et al., Deep Sea Research II, 2004

5. Vázquez-Cuervo et al., Journal of Climate, 20106. Lagerloef et al., Journal of Geophysical Research, 19997. Qu et al., Geophysical Research Letters, 20088. Colin et al., in Proceeding of the third C.S.K. Symposium (1973), Bangkok, Thailand, 1974

3. variability of SLA

Fig. 2. (a) Power spectra of SLA along the northern coast of New Guinea (b) Phase of the annual harmonic of SLA (month of max.) along the tracks

track numbers:049, 125, 201, 023,099

4. variability of the cross track current anomaly

Fig. 3. Cross track current anomaly (Ua)

(a) Power spectra along the northern coast of New Guinea

(b) Explained variance of annual + semi annual harmonics along the tracks

(c) Seasonal cycle (annual + semi annual harmonics) along the northern coast of New Guinea

� Spectral analysis (Fig. 2a): main peaks at seasonal (6-month and 1-year periods) and interannual (20-month, 2.5-, and 4.5-year periods) time scales

� The semi annual component varies nearly in phase in the western equatorial Pacific (maximum in April-May; [7])

� The annual component near the coast leads the off shore SLA signal by 2 months (Fig. 2b)

���� It drives an along-track SLA gradient

and the associated geostrophic cross-track current

� Spectral analysis (Fig. 3a): the annual time scale dominates along the coast; the semi annual harmonic contributes 2 to 4% (Tab. 1). Annual and semi annual harmonics are out of phase at the northernmost track (Fig. 3c)

� The seasonal cross track current anomaly forms a coherent vein from VitiazStrait to the northern tip of New Guinea (Fig. 3b)���� Its width is 80-150 km; its core is 100-150 km off shore

� The current anomaly is northwestward in austral winter (trade winds) and southeastward in austral summer (northwest monsoon) (Fig. 3c)

� Variations north of New Guinea lead variations east of Vitiaz Strait (track 099) by one month

Numbers indicate ascending tracks used to calculate the cross track current

0.300.160.150.090.10annual

0.040.030.020.040.02semi

annual

992320112549tracknb.

Tab. 1. explained variance of Ua

Fig. 4. Seasonal (annual + semi annual harmonics) component of (a, c) SST (°C) and (b, d) surface chlorophyll (mg m-3) in February (a, b) and August (c, d). The seasonal component of the cross track current anomaly is superimposed.

� Bismarck Sea: semi annual SST and annual surface chlorophyll� Vitiaz Strait: annual SST and nearly constant chlorophyll

Austral winter (trade winds)� NGCC is northwestward, SST is ≈ 27°C and chlorophyll is ≈ 0.15 mg m-3 in the western Salomon Sea

� Cool water is advected westward through Vitiaz Strait into Bismarck Sea and narrow vein of cool water hugging the coast from Vitiaz Strait to 142°E (Fig. 4c)

� Narrow vein of low chlorophyll through Vitiaz Strait into Bismarck Sea below the small upwelling south of New Britain (Fig. 4d)

Austral summer (northwest monsoon)� NGCC weakens or may reverse into a southeastward flow

� Coastal upwelling [8] from December to April (SST < 28.6°C and chlorophyll > 0.2 mg m-3) between 138°E and 146°E south of the equator (Fig. 4a, b)

� No clear exit of cool and chlorophyll-rich surface water through Vitiaz Strait; instead, hints of a recirculation into the eastern Bismarck Sea

���� the NGCC does not seem to reverse in Vitiaz Straitin austral summer at the seasonal timescale

6. conclusion� The altimetric cross track current anomaly represents the seasonal NGCC variations along the northern

coast of New Guinea

� Surface chlorophyll is annual: influence of coastal upwelling� SST is semi annual: influence of upwelling and advection of cool water from the Solomon Sea

Trade winds (Jun.-Sep.)� Northwestward NGCC� No upwelling

� Advection of cool water from the western Solomon Sea into Bismarck Sea

Northwest monsoon (Dec.-Mar.)� Weak or southeastward NGCC� Upwelling

� No advection of cool water from the western Solomon Sea

track numbers:049, 125, 201, 023,099

Five altimetric ascending tracks orthogonal to the New Guinea (NG) coast allow investigating the variability of this narrow along-coast surface current. The associated seasonal patterns of SST and surface chlorophyll are also examined.

Marie-Hélène Radenac1, Fabien Léger1, Pierre Dutrieux2, Christophe Menkes3, Gérard Eldin1

1LEGOS (CNRS-IRD-CNES-UPS), Toulouse, France, marie-helene.radenac@legos.obs-mip.fr2BAS, Cambridge, United Kingdom

3LOCEAN-IRD, Nouméa, New Caledonia