An overview of the satellite chlorophyll patterns in the North Atlantic. André Valente CCMMG, Azores University Eumetrain - Ocean and sea week - Lisbon, Portugal 2011/11/02

My introduction Hello! My name is André! I am PhD student in the University of Azores.

Contents 1 – What is chlorophyll and why is it important? 2 – How can we measure chlorophyll from space? 3 – What can we do with the images? 4 - Important concepts. 5 – Chlorophyll patterns in the North Atlantic. - Large-scale. - Seasonal time-scale. - Interannual time-scales. - Shorter time-scales.

What is phytoplankton and chlorophyll? Phytoplankton are microscopic sea plants. They are the base of the marine food chain. Chlorophyll a is a pigment of phytoplankton. Is responsible for absorbing sunlight during photosynthesis. Phytoplankton Zooplankton Chlorophyll a ~ Phytoplankton biomass ~ Primary productivity

Phytoplankton Zooplankton Nutrients+Light Phytoplankton growth More food available! Why is phytoplankton important? 1 - Supports almost all marine life. 2 - Half of the biological production on the planet. 3 - Key role in the global carbon cycle.

 By measuring the “color” of the ocean! chlorophyll satellite images = ocean color images  Hypothesis: chlorophyll is the only coloring agent in the water.  Chlorophyll absorbs the “blue” radiation, so the higher the chlorophyll concentration, the lower is the “blue” water-leaving radiance Open ocean: Blue water – Low chlorophyll Green water – High chlorophyll The hypothesis fails in coastal waters (other coloring material) ‏ How to measure chlorophyll from space?

WATER AIR L wat L sup Eletromagnetic Radiation L traj sensor The sun emits electromagnetic radiation in the visible part of the spectrum. “ocean color” How to measure chlorophyll from space?

The higher the chlorophyll concentration, the lower is the “blue” radiance emerging from the water. C chl L w (443nm)‏ L w (551nm)‏ ~ Water leaving radiance for different chl concentrations Water leaving radiance (or ocean color)‏ (Yoder and Kennelly, 2006)‏

. Sensors: MODIS, SeaWiFS, MERIS, etc Spatial resolution: 1km2 Temporal resolution: Daily Since 1998-present Freely available. It takes 100min to make one orbit. About 15 orbits per day. The result is one image per day at the same time every day. The satellite passes every day around 14pm above Lisbon. What is like a chl satellite image?

. 1 day composite (2009/05/29)‏ 7 day composite (all images 2009/05/ /06/01)‏ 1 month composite (all images 2009/05/ /05/31)‏ What does a chl satellite image look like?

We have: Daily, high resolution, global images of chlorophyll patterns. Huge amount of data (since that gives around 5000 daily images). We can study plankton distribution in time and space: Identify large-scale and regional patterns. Determine seasonal and interannual cycles. Delineate ecological provinces. Determine trends (climate change). Using other environmental variables (currents, water masses, winds, tides, bathymetry, etc,) we can identify the forcing mechanisms. So what can we do now?

Important concepts: nutrients, light and mixing Nutrients + Light Phytoplankton growth But nutrients and light are the inverse of one another: What brings nutrients to the euphotic zone? The biological pump (Levy et al, 2008)‏ Nutrient increases with depth. Light decreases with depth

Ocean transport and mixing processes maintains the supply of nutrients from deep waters to surface waters. Other nutrient inputs: atmospheric deposition, river runoff in the coastal zone and nitrogen fixation. Important concepts: supply of nutrients

The spatial changes in phytoplankton abundance is the result of regional differences in the amount of nutrient fluxed into the euphotic zone. (Yentsch, 1989) ‏ Important concepts: supply of nutrients

Important concepts: mixed layer depth (MLD)‏ AugNov May Feb Mixed Layer Depth Climatology (de Boyer Montégut et al 2004)‏

Important concepts: mixed layer depth (MLD)‏ MLD influences the rate of primary production by regulating the basic substrates: light and nutrients. Winter storms deepen the mixed layer Warmer temperatures, weak winds and insulation shallows mixed layer Set Nov Jan Feb Mar Apr May Jun Jul Aug 50m 20m 50m 20m 10m 30m 50m 100m 200m 300m

1 - Large scale and time scale>1year: Controlled by the thermohaline and the wind-driven circulations. These circulations regulate the subsurface nutrient distribution. 2 - Seasonal time-scale: Modulated by winter mixing and stratification. 3 – Interannual time-scale: Variations in winter mixing and stratification. 4 - Shorter time scales: Controled by mesoscale eddies (10-100km) and submesoscale features such as fronts and filaments (~1-10km). The physical supply of nutrients

Large scale and time scale>1year Annual chl mean for 1998 Different surface chlorophyll patterns and therefore productivity. Where?

Large scale and time scale>1year Annual chl mean for 1998 Coastal upwelling Subpolar Gyre Coastal waters Different surface chlorophyll patterns and therefore productivity. Why? Subtropical Gyre

Large scale and time scale>1year Annual chl mean for 1998 Coastal waters: More productive than open ocean. Nutrient supply from rivers and anthropogenic nutrient inputs. Upwelling of nutrients from tidal mixing in the shelf But be careful... the chl algorihm was not designed for coastal waters. Mineral sediments tipically induce erroneously high satellite-derived chl.

Large scale and time scale>1year Annual chl mean for 1998 Coastal upwelling: Eastern boundaries Winds induce the upwelling of deeper cold, nutrient-rich waters. Very productive regions.

Large scale and time scale>1year Annual chl mean for 1998 Subpolar gyre: High productivity. Strong seasonal blooms of phytoplankton. Subtropical gyre: Low productivity. Weak seasonal blooms of phytoplankton.

Why the difference in productivity between gyres? Surface winds drive double-gyre systems and thermocline differences. Subpolar gyre : cyclonic circulation, upwelling and a raised thermocline. Subtropical gyre : anticyclonic circulation, downwelling and a depressed thermocline. (Williams and Follows, 2003)‏ 1 – Different nutricline depths (Znitrate>1mMol/m3)

In the subpolar gyre the thermocline and the nutricline is closer to the surface. There are more nutrients available and the potential for higher productivity is greater. WOA nitrate August climatology (mMOL/m3). light Why the difference in productivity between gyres? 1 – Different nutricline depths (Znitrate>1mMol/m3)

Subpolar gyre: Strong winter mixing (mixed layer depths >200m)‏ Subtropical gyre: Weak winter mixing (mixed layer depths ~ 100m)‏ WOA nitrate August climatology (mMOL/m3) and WOA mixed layer depth March climatology. Why the difference in productivity between gyres? 2 – Different winter mixing light mld

Why the difference in productivity between gyres? Subpolar gyre: Shallow nutricline Strong winter mixing Nutrient abundant Light can be limiting Subtropical gyre: Deeper nutricline Weak winter mixing Nutrient limited Light abundant.

Seasonal time-scale AugNov May Feb The vernal, or spring, bloom. A feature of many seasonal seas in the global ocean. The most famous is the spring bloom of the North Atlantic, clearly detectable from space. Satellite Chlorophyll Climatology

Maximum MLDs ~ 100m Weak bloom in winter Nutrient limited Maximum MLDs > 200m Strong bloom in spring Seasonal time-scale Levy et al, 2005 The seasonality is mainly driven by variations in the mixed layer depth.

Seasonal time-scale MLD's shallow to 100m (due to surface warming), and the spring bloom begins. Why? Wherever MLD's are greater than 200m, chlorophyll is low. Why? Circles are ARGO floaters. The color is the mixed layer depth (0-400m)‏ Chl April 2007Chl March 2007

Seasonal time-scale Critical Depth Hypothesis formalized by Sverdrup in 1953 (and almost always used) Henson et al, 2006 For a spring bloom to occur the MLD must be shallower than a certain critical depth (Zc). For MLD>Zc production is inhibited, the cells are being continuously mixed below the euphotic layer for periods greater than their doubling time.

MLD shallowing and Chl increase Seasonal time-scale Why does MLD shallows? Qnet>0, ocean gains heat - warmer air temperatures - weaks winds - solar heating

Seasonal time-scale Follows and Dutkiewicz, 2002 Bloom timing: A northward propagating front of chlorophyll.

Seasonal time-scale Subpolar region: the bloom is more intense where greatest heat input favours restratification. Subtropical region: the bloom is intensified where there is greater surface heat loss and wind mixing, consistent with nutrient limitation. Follows and Dutkiewicz, 2002

Interannual time-scale Interannual variability in winter-time convection and the corresponding influence on the supply of nitrate to the euphotic zone and the response in primary production (BATS). (Williams and Follows, 2003)‏

Interannual time-scale Henson et al, 2009 Follows and Dutkiewicz, 2002 NAO positive NAO negative NAO positive – NAO negative Interannual variability in the timing of the spring bloom due to the variation in wind mixing, linked to NAO.

Shorter time-scales Controled by mesoscale eddies (10-100km) and submesoscale features such as fronts and filaments (~1-10km). Lehahn et al, 2007 (Williams and Follows, 2003)‏

The physical regime of the oceans dictates the phytoplankton distributions and hence primary production in the oceans; the forces involved are those associated with the sun's heating and cooling, which drives the ocean's circulation. (Yentsch, 1989)‏

Thank You!!

References : de Boyer Montégut, C., G. Madec, A. S. Fischer, A. Lazar, and D. Iudicone (2004), Mixed layer depth over the global ocean: an examination of profile data and a profile-based climatology, J. Geophys. Res., 109, C12003, doi: /2004JC Follows, M., Dutkiewicz, S., Meterological modulation of the North Atlantic spring bloom. Deep-Sea Research II 49, 321–344. Henson, S. A., I. Robinson, J. T. Allen, and J. J. Waniek (2006), Effect of meteorological conditions on interannual variability in timing and magnitude of the spring bloom in the Irminger Basin, North Atlantic, DeepSea Res., Part I, 53, 1601– 1615, doi: /j.dsr Henson, S. A., J. P. Dunne, and J. L. Sarmiento (2009), Decadal variability in North Atlantic phytoplankton blooms, J. Geophys. Res., 114, C04013, doi: /2008JC Lehahn, Y., F. d'Ovidio, M. Lévy and E. Heitzel (2007). Stirring of the Northeast Atlantic spring bloom: a lagrangian analysis based on multi-satellite data, J. Geophys. Res., 112, C08005, doi: /2006JC Lévy, M., Y. Lehahn, J.-M. André, L. Mémery, H. Loisel, and E. Heifetz (2005). Production regimes in the Northeast Atlantic : a study based on SeaWiFS chlorophyll and OGCM mixed-layer depth, J. Geophys. Res., Vol.110,No.C7,C07S10, doi: /2004JC00277 Lévy, M. (2008). The modulation of biological production by oceanic mesoscal turbulence, Lect. Notes Phys., 744, , DOI / _9, Transport in Geophysical flow: Ten years after, J. B. Weiss and A. Provenzale (Eds), Springler Williams R. G. and M. J. Follows (2003), Physical transport of nutrients and the maintenance of biological production. In : Ocean Biogeochemistry : a JGOFS synthesis, ed by Springer Yentsch C. S. (1989), AN OVERVIEW OF MESOSCALES DISTRIBUTION OF OCEAN COLOR IN THE NORTH ATLANTIC Adv. Space Res. Vol. 9, No. 7, pp. (7)435-(7)442, 1989 Yoder, J.A., and M.A. Kennelly, What have we learned about ocean variability from satellite ocean color imagers? Oceanography, 19(1),