Unusual blooms in the Equatorial Pacific in 1998 : A model point of view

Menkes C. E., Gorgues T., Alvain S., Aumont O., Bopp L., Moulin C., Dandonneau Y., Cravatte S., Vialard J.

(LODYC, LSCE, Paris)

SeaWifs, Ryan et al., 2002

• The Strong 1998 La Nina return produced very strong and unusual blooms (Chavez et al., 1999, Ryan et al., 2002).

• What type of species were dominant in bloom 2 ? Why ?• What was the exact role of Tropical Instability Waves • Did the major bloom really propagate eastward ?

•To address these question we use a coupled dynamical-biogeochemical model and a new method for deriving phytoplankton species from SeaWifs (Alvain et al., 2004).

Coupled dynamical-biogeochemical Model

• OPA Dynamics 1992-2000• Sponge layer 30S and 30N• 7-day ERS-TAO stress/1-day NCEP fluxes

Model OPA vs TAO currents. Correct structure but bias at 110°W (Lengaigne et al., 2003, Vialard et al., 2002)

Coupled on-line to:

•PISCES (Aumont et al., 2002)•Retroaction light-chlorophyll

•The model allows to distinguish between diatoms or pico-nano planktons. To do so in reality, we use the algorithms of Alvain et al. (2004) that allows to extract from SeaWifs reflectances at different wavelengths, five dominant groups of species:

Green AlgaeProchlorococcus

CyanobacteriaDiatoms

Coccolith. bloom

Ex: 10 days synthesis in Feb 1998

DATA(1998-2000): MODEL (1998-2000):

SST AVHRR SST-1C

Seawifs Model Chl

The patterns of SST and Chl are reasonable but upwelling too weak

EQ. SST CHLORO SEA LEV. ANO.

DATA

MODEL

AUG-SEP

AUG-SEP

TIW=BREAK

TIWTIW

TIW

JUL

JUN-JUL

LA NINA

LA NINA

MAY

MAY

EL NINO

EL NINO 1998

1999

1998

1999

•The dominant ( more than 70%) group during the 1998 blooms are diatoms in both model and SeaWifs at the equator.• Only in 1998 are diatoms dominant at the equator

DATA MODEL

• Again, the eastward propagation in 1998 is not obvious and the diatom blooms (july and sep-aug) seem separated .

•The blooms of dominant diatoms correspond to strong outbursts of Iron in the surface layers

•In the model, the b) Aug-Sep bloom is not associated to EUC surfacing but to enhanced vertical diffusion and upwelling of Fe due to the trades.

b

b

b

•a )bloom is modulated by an irruption of TIW signal that propagates to the west

a

a

a

•The first a) Jun-July bloom corresponds to a patch of eastward current where the eastward EUC surfaces near 120°W (Picaut et al., 2002, Izumo et al., 2002)and strong westward current is to the east

U at EQ Fe at EQ CHL at EQ

23°C

0m50m100m

200m

•Model resembles the data with TIWundulations•Strong upwellingat 130°W: Iron input and bloom•EUC surfaces.

SST TMI

CHLORO MODEL SEAWIFS

BLOOM 1: end of June

130°W 130°W

•Strong shear of zonal currents generation of verystrong TIWs eastof 130°W, near 110°W

U at EQ Fe at EQ CHL at EQ

23°C

SST TMI

CHLORO MODEL SEAWIFS

BLOOM 2: end of August •Model chl is too Strong west of 130°W•Model resembles the data with TIWundulations andSeparation near 130°W

130°W 130°W

• This separationcorresponds to the westward moving TIW, nownear 130°W that advects chl-poor and warm waters from the north.

• The coupled OPA-PISCES model reproduces some of the major features of the 1998 unusual blooms.• These blooms are dominated by diatoms and 1998 is the only period when this is true in both model and observations.• The first bloom in June-July is associated with strong upwelling near 130°W linked to EUC surfacing with north-south undulations due to instability waves generated by strong current shear.• This shear generates near 110°W an extremely strong TIW signal that advects nutrient-poor and warm waters from the north and that reduces the bloom to the west as it propagates.• Then a second bloom appears to the east of 120°W, confined to the eastern Pacific and is not obviously propagating eastward either in the data or in the model.