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

1. 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

2. questions • 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).

3. OPA Dynamics 1992-2000 • Sponge layer 30S and 30N • 7-day ERS-TAO stress/1-day NCEP fluxes • Coupled on-line to: • PISCES (Aumont et al., 2002) • Retroaction light-chlorophyll Model OPA vs TAO currents. Correct structure but bias at 110°W (Lengaigne et al., 2003, Vialard et al., 2002) Coupled dynamical-biogeochemical Model

4. Green Algae Cyanobacteria Coccolith. bloom Prochlorococcus Diatoms • 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: Ex: 10 days synthesis in Feb 1998

5. Seawifs Model Chl SST AVHRR SST-1C DATA(1998-2000): MODEL (1998-2000): The patterns of SST and Chl are reasonable but upwelling too weak

6. DATA TIW TIW=BREAK LA NINA AUG-SEP JUL MAY EL NINO TIW TIW MODEL LA NINA AUG-SEP JUN-JUL MAY EL NINO EQ. SST CHLORO SEA LEV. ANO. 1999 1998 1999 1998

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

8. b a b a b 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 • a )bloom is modulated by an irruption of TIW signal that propagates to the west • 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. • The blooms of dominant diatoms correspond to strong outbursts of Iron in the surface layers

9. BLOOM 1: end of June • Model resembles • the data with TIW • undulations • Strong upwelling • at 130°W: Iron • input and bloom • EUC surfaces. • Strong shear of • zonal currents  • generation of very • strong TIWs east • of 130°W, • near 110°W SST TMI CHLORO MODEL SEAWIFS 0m 50m 100m 200m U at EQ Fe at EQ CHL at EQ 130°W 130°W 23°C

10. BLOOM 2: end of August • Model chl is too • Strong west of 130°W • Model resembles • the data with TIW • undulations and • Separation near • 130°W • This separation • corresponds to • the westward • moving TIW, now • near 130°W that advects chl-poor and warm waters from the north. 130°W 130°W SST TMI CHLORO MODEL SEAWIFS U at EQ Fe at EQ CHL at EQ 23°C

11. CONCLUSIONS • 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.