Seasonal and mesoscale variability of phytoplankton in the Arabian Sea. from satellite observations to models

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1 Seasonal and mesoscale variability of phytoplankton in the Arabian Sea from satellite observations to models Marina Lévy NIO Winter school, Feb 2015 LOCEAN-IPSL, France 1

2 Introduction

3 Arabian Sea Chlorophyll Modis Oman India

4 Questions ü How can the seasonal and mesoscale variabilities be described? ü What are the physical factors that control these Chl variabilities? ü What can we learn from models? ü How is the seasonal variability modulated by the mesoscale variability?

5 Bibliography Lévy et al (2007) JGR Basin-wide seasonal evolution of the Indian Ocean's NAS phytoplankton blooms Koné et al (2009) Geophys Monograph Series Physical and biogeochemical SCTR controls of the phytoplankton seasonal cycle in the Indian Ocean : a modeling study Resplandy et al (2011) JGR Contribution of mesoscale processes to nutrient budgets in the Arabian Sea

6 Method Observations Ocean Color (SeaWiFs) for Chla at the ocean surface NAS Model Physics: Global SCTR OGCM NEMO at 0.5 and 1/12 resolution, 30 vertical levels Interannual simulation ERA Biogeochemistry: PISCES : 2P 2Z, Fe, Si, N limitations

7 Seasonal variability SeaWiFS Chla (Climatology) Lévy et al., 2007 Semi-annual Chla cycles Strong regional variations in intensity Variations in timing (cycles not in phase)

8 Seasonal variability August January SeaWiFS Chla (Climatology) The bloom phenology is controlled by Physical processes Biological processes

9 Seasonal variability Winter (NEM) Summer (SWM) CIC Each bloom is described by two parameters: - Time of bloom onset - Cumulated increase in Chl : CIC

10 Boreal WINTER Cumulated Increase in Chla Chla in Jan Month of bloom peak Lévy et al., 2007

11 Boreal WINTER Cumulated Increase in Chla Month of bloom peak Chla in Jan Month of bloom onset

12 Boreal WINTER Cumulated Increase in Chla Month of bloom peak Bloom regions Month of bloom onset

13 ½ NEMO-PISCES CIC Month of onset Koné et al. 2009

14 N Limitations Nitrogen is the main limiting nutrient at bloom onset Silicate limits diatom - production at the end of the bloom Koné et al. 2009

15 NEMO-PISCES

16 Boreal SUMMER Summer cumulated Increase in Chla Month of bloom peak August Chla

17 Boreal SUMMER Summer cumulated Increase in Chla Month of bloom peak August Chla Month of bloom onset

18 Boreal SUMMER Cumulated Increase in Chla Month of bloom peak Bloom regions Month of bloom onset

19 ½ NEMO-PISCES Summer Koné et al. 2009

20 N fluxes Boreal Summer - > Biogeochemical provinces coincide with regions of seasonal N delivery Koné et al. 2009

21 NEMO-PISCES Seasonal Entrainement of nitrate driven by the deepening of the surface mixedlayer during the NEM Koné et al, 2009

22 NEMO-PISCES Seasonal Entrainement of nitrate driven by the deepening of the surface mixedlayer during the NEM Koné et al, 2009

23 NEMO-PISCES Seasonal Entrainement of nitrate driven by the deepening of the surface mixedlayer during the NEM Koné et al, 2009

24 Summer Winter Unlike most other regions in the ocean, it is not possible to classify the IO into unique biogeochemical provinces Need to separate the two seasons.

25 Summary of part 1 Models and ocean color obs give complementary informa5ons Ver5cal dynamics drives most of the Chl variability in the IO at seasonal 5me scales Chl is a proxy for ver5cal exchanges

26 How do eddies modulate the phytoplankton response in the Arabian Sea? Laure Resplandy

27 Oma n Arabian Sea Chlorophyll Modis Indi a

28 Modulation by sub-mesoscale in the Arabian Sea Winter Chl SeaWiF s Summer Chl SeaWiF s Chl in-situ (JGOFS 1995) Chl Modis

29 Strategy: high resolution bio-physical regional model EKE ocean physics NEMO 1/12 (~9 km) boundary & initial conditions from 1/2 model (Kone et al. 2009)

30 I. Impact of mesoscale on seasonal blooms Winter mean Observations Model

31 I. Impact of mesoscale on seasonal blooms Summer mean Observations Model

32 I. Impact of mesoscale on seasonal blooms

33 I. Impact of mesoscale on seasonal blooms Winter snapshot Summer snapshot Resplandy et al., JGR 2011

34 I. Impact of mesoscale on seasonal blooms Winter snapshot Surface Chl Chl Integrated MLD vertical velocities Promote restratification & intense vertical velocities Sustain phytoplankton (F1) Decoupling between surface and integrated phytoplankton (F2)

35 I. Impact of mesoscale on seasonal blooms Limiting nutrient budget N/ t = lateral advection vertical advection vertical mixing Bio sources/sinks advection = Mean + Eddy Budget over blooming regions: 0-80 meters Summer Winter

36 I. Impact of mesoscale on seasonal blooms lateral advection vertical advection vertical mixing Total Mean Eddy Total advection = Mean + Eddy

37 I. Impact of mesoscale on seasonal blooms Winter vertical mixing Major input by vertical mixing advection Mean Eddy lateral vertical Crucial role of eddy vertical advection in Central Arabian Sea (40-50%) Resplandy et al., JGR 2011

38 I. Impact of mesoscale on seasonal blooms Surface Chl Chl Integrated Summer snapshot MLD vertical velocities Great Whirl + Socotra eddy Similarity between surface and integrated Chl weak vertical velocities except Great Whirl & Socotra Eddy Is the eddy contribution only lateral

39 I. Impact of mesoscale on seasonal blooms Summer Mean Advection Eddy lateral vertical Central AS: eddy advection (50-70%) Resplandy et al., JGR 2011

40 Nitrate flux mmol/ m2/d I. Impact of mesoscale on seasonal blooms Total = Mean + Eddy Oman upwelling region M A M J J A S O N D J F M early upwelling Eddies boost vertical input (60-90%) late upwelling Mean advection Eddy: offshore transport Resplandy et al., JGR 2011

41 Conclusions Mesoscale strongly modulate the seasonal cycle of phytoplankton - lateral exchanges between coastal and central AS - vertical exchanges between surface and subsurface - the onset of coastal upwelling - the re-stratification in winter