an overview of the satellite chlorophyll patterns in the north atlantic. andré valente ccmmg,...

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 chlorophyllGreen water – High chlorophyll

The hypothesis fails in coastal waters (other coloring material)

How to measure chlorophyll from space?

WATERWATER

AIR Lwat

Lsup

Eletromagnetic Radiation

Ltraj

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.

CCchlchl

LLww(443nm)(443nm)

LLww(551nm)(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 2009/05/25 -

2009/06/01)

1 month composite (all images 2009/05/01 -

2009/05/31)

What does a chl satellite image look like?

We have:

Daily, high resolution, global images of chlorophyll patterns.

Huge amount of data (since 1997... 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)

Aug Nov

MayFeb

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 layerWarmer 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?



Coastal upwelling

Subpolar Gyre Coastal

waters

Different surface chlorophyll patterns and therefore productivity.

Why?

Subtropical Gyre



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.



Coastal upwelling:

Eastern boundaries

Winds induce the upwelling of

deeper cold, nutrient-rich waters.

Very productive regions.


Annual chl mean for 1998 Subpolar gyre:

High productivity.

Strong seasonal blooms of

phytoplankton.

http://web.me.com/uriarte/Earths_Climate/Appendix_4._Ocean_currents.html

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


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.


2 – Different winter mixing

light

mld


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

Aug Nov

MayFeb

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 1998-2007

Maximum MLDs ~ 100mWeak bloom in winterNutrient limited

Maximum MLDs > 200mStrong 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 moreintense 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.


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).


Interannual time-scale

Henson et al, 2009


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


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:10.1029/2004JC002378.

Follows, M., Dutkiewicz, S., 2002. 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:10.1016/j.dsr.2006.07.009.

Henson, S. A., J. P. Dunne, and J. L. Sarmiento (2009), Decadal variability in North Atlantic phytoplankton blooms, J. Geophys. Res., 114, C04013, doi:10.1029/2008JC005139.

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:10.1029/2006JC003927

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: 10.1029/2004JC00277

Lévy, M. (2008). The modulation of biological production by oceanic mesoscal turbulence, Lect. Notes Phys., 744, 219-261, DOI 10.1007/978-3-540-75215-8_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, 2006. What have we learned about ocean variability from satellite ocean color imagers? Oceanography, 19(1), 152-171

an overview of the satellite chlorophyll patterns in the north atlantic. andré valente ccmmg,...

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