ioe 184 - the basics of satellite oceanography. 8. mesoscale variability and coastal pollution...

IoE 184 - The Basics of Satellite Oceanography. 8. Mesoscale variability and coastal pollution

Lecture 8Mesoscale variability and coastal pollution

This lecture includes the following topics:

1. Phytoplankton, the main contributor to ocean color:- Passive tracer- Active growing biomass

2. Horizontal transport of phytoplankton and pollutants offshore in the Black Sea

3. Spring bloom in Southern California Bight resulting from coastal upwelling


4. Stormwater plumes off southern California

Multi-discipline approach implies that the simultaneous measurements of distribution of phytoplankton and physical environment enable the studies of physical factors, which determine the distribution of phytoplankton.

SeaWiFS surface chlorophyll AVHRR Sea Surface Temperature


1. Phytoplankton is both a passive tracer transported by water circulation and an active biomass growing under favorable conditions (light, nutrients, etc.). It is important to distinguish between these two processes.

Horizontal circulation transports phytoplankton in horizontal direction, resulting in changes in its horizontal distribution.

Vertical stratification of water column regulates the growth of phytoplankton biomass.

So, the mesoscale variability of phytoplankton visible at satellite images results from both passive transport and active growth.



The illustrative example of mesoscale variability of phytoplankton is the Black Sea located in southeastern Europe.

The abyssal plain of depth more than 2000 m is separated from the margins by steep continental slopes, excluding the shallow northwestern part. The wide northwestern continental shelf (mean depth about 50 m) occupies the region between the Crimean peninsula and the west coast.



A basin scale cyclonic boundary Rim Current is the main feature of the Black Sea general circulation. The Rim Current is <75 km wide and has an average speed of 20 cm s-1. Along the coastal lines anticyclonic vorticity arises due to the Rim Current meandering, resulting in anticyclonic eddies in coastal zones.



High-resolution AVHRR images enabled studies of the formation and evolution of the cyclonic and near-shore anticyclonic eddies along the coast and their influence on distribution of remote-sensed chlorophyll.

aNAE-1

C1

C2

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08/10/97

oE

oN

a

m g/m3

NAE-1

NAE-2

NAE-3

NAE-4

C2

NAE-5



These AVHRR images illustrate the dynamics of the cyclonic eddies (C-1, C-2) and near-shore anticyclonic eddies (NAE-1, NAE-2, etc.) during the autumn 1997.

aNAE-1

C1

C2




bNAE-1

C1

C2




CN AE-1

C 2

C 1




d

N AE -1 NA E-2

N AE-3N AE-4

C 2N A E - 5



The propagation of the near-shore anticyclonic eddies resulted in change of the direction of currents over the continental slope.



Hydrographical observations support upwelling in the cyclonic eddies and downwelling in the anticyclonic eddies.

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3456789101112211917161514131220 St.



Torrential rains in the beginning of October 1997 resulted in increased freshwater discharge and accumulation of phytoplankton and pollutants in the near-shore anticyclonic eddies.

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The concentration of chlorophyll increased in the cyclonic eddies and decreased in the anticyclonic eddies of the open sea. Near-shore anticyclonic eddies accumulated high concentrations of chlorophyll.

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08/10/97

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The process of transport of phytoplankton from productive shelf region to the open sea was observed during summer 1998 over the continental slope in the northwestern part of the Black Sea.



Anticyclonic eddies slowly moved southwestward along the continental slope.



The largest anticyclone with diameter of 90 km displaced during three months southwestward with mean speed of about 3 cm/s.

2 8 o 2 9 o 3 0 o 3 1 o 3 2 o 3 3 o 3 4 o

2 8 o 2 9 o 3 0 o 3 1 o 3 2 o 3 3 o 3 4 o

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3.V I

5.V I

29.V I-7.VII

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21.V II26.V II

1.V III4.V III

8.V III

12.V III19.V III

28.V III

Tendrovskaya Spit

C .Hersones

C.Sarych

C.Em ine

C.Kaliakra

C .Tarhankut

Kalam itskyBay

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D anube Delta

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The eddies transported chlorophyll-reach coastal waters to the deep basin.



Circulation in the Southern California Bight is cyclonic, resulting from the interaction between California Current and Southern California Countercurrent.

123W 122W 121W 120W 119W 118W 117W

33N

34N

35N

36N

37N

Point Conception

Santa BarbaraChannel

San Nicolas Is.

Santa Rosa Ridge

Santa MonicaBasin

Monterrey Bay



Strong alongshore wind results in upwelling and phytoplankton bloom.


119°W 118°W

33°N

34°NLAX

200 m500 m

1000 m

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500 m

500 m

500 m

1000 m

500 m

200 m

500 m1000 m

Santa Monica Bay

San Pedro Basin

SantaBarbaraChannel

Upwelling index at 33N, 119W

Air tem perature at LAX

SeaW iFS chlorophyllAVHRR SST

119°W 118°W

33°N

34°NLAX

200 m500 m

1000 m

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500 m

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500 m1000 m

Santa Monica Bay

San Pedro Basin

SantaBarbaraChannel

Upwelling index at 33N, 119W

Air tem perature at LAX

SeaW iFS chlorophyllAVHRR SST

From Level 3 time-series of SeaWiFS chlorophyll concentration we study statistical correlations between phytoplankton dynamics and environmental factors.

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Phytoplankton blooms regularly occur in SCB, typically in spring.

1997 1998 1999 2000 20015

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30 (A ) - A it tem pera ture (oC )

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1997 1998 1999 2000 200110

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25(C ) - A V H R R S S T (oC )

1997 1998 1999 2000 2001-2

-1

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2 (D ) - Log S eaW iF S ch lorophyll (m g m -3 )

1997 1998 1999 2000 20015

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30 (A ) - A it tem pera ture (oC )

1997 1998 1999 2000 2001-400

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800(B ) - U pw elling Index (m 3 s -1 per 100 m )

1997 1998 1999 2000 200110

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25(C ) - A V H R R S S T (oC )

1997 1998 1999 2000 2001-2

-1

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2 (D ) - Log S eaW iF S ch lorophyll (m g m -3 )

Seasonal variations ofremote sensed SST andchlorophyll biomassaveraged over SMB,air temperature at LAX, and wind (upwelling indexat 33oN, 119oW).

Seasonal minima:wind - December 19 (winter solstice);air T - February 7 (+50 days)SST - March 2 (+23 days).

Seasonal maximum of Chl (February 27)coincides with SST minimum.

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Chlorophyll biomass growth results from decrease of air temperature and increase of upwelling-favorable wind stress with time lag 5-6 days.

4. Stormwater plumes in southern California

Mesoscale eddies transport offshore the rainstorm plumes with high suspended sediment and chlorophyll concentration.


Mesoscale eddies transport offshore the rainstorm plumes with high suspended sediment and chlorophyll concentration.


4. Stormwater plumes in southern California

On the basis of SeaWiFS observations collected over 7 years (1997-2004) the basic statistical characteristics of plumes in the Southern California Bight were estimated.

Normalized water-leaving radiation of 555 nm (nLw555) wavelength is highly correlated with the concentration of suspended sediments, resulting in brownish water color typical to stormwater plumes.

Plume size was assessed from nLw555 exceeding a certain threshold, estimated as 1.3 mW cm-2 µm-1 sr-1.

A primary factor regulating the plume size was rainstorm magnitude, i.e., the total volume of water precipitated over the coastal watershed.

Rain < 0.25 cm

5% 10% 20% 30% 40% 50%

Rain 0.25 - 1 cm

5% 10% 20% 30% 40% 50%

Rain 1 - 2.5 cm

5% 10% 20% 30% 40% 50%

Rain > 2.5 cm

5% 10% 20% 30% 40% 50%

121°W 120°W 119°W 118°W 117°W

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Plume dynamics was studied in four southern California regions: Ventura, Santa Monica Bay, San Pedro, and Orange County/San Diego.

Maximum plume size (150-200 km2) was observed from the outlets of the Santa Clara and Ventura rivers, because of higher sediment concentration resulting from highly erosive river beds.

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(A) - Ventura

121°W 120°W 119°W 118°W 117°W32°N

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In Santa Monica Bay, the optical signature of stormwater plumes was much weaker (0.8 mW cm-2 µm-1 sr-1) than in other regions (1.3 mW cm-2 µm-1 sr-1).

121°W 120°W 119°W 118°W 117°W32°N

33°N

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35°N

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(B) - Santa Monica

In San Pedro region, typical plume size was 10-80 km2. The time lag between rainstorm and maximum plume size in this area was 1 day, in contrast to 2 days in three other regions.

121°W 120°W 119°W 118°W 117°W32°N

33°N

34°N

35°N

VE

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Plume size (km 2)

(C) - San Pedro

In Orange County/San Diego region, typical plume size was small (10-40 km2), resulting from bimodal watershed physiography, where river flow is often retained in the inland alluvial valleys.

121°W 120°W 119°W 118°W 117°W32°N

33°N

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35°N

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Plume size (km 2)

(D) - Orange County/San Diego

In San Pedro region, the correlation between the precipitated rainwater and the plume size was almost linear, resulting from highly impervious surface in these developed watersheds.

121°W 120°W 119°W 118°W 117°W32°N

33°N

34°N

35°N

VE

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Accum ulated ra inw ater (cm )

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(km

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In three other regions, power function better described the correlation between rainwater and plume size, resulting from more natural watersheds where water is partly infiltrated.

121°W 120°W 119°W 118°W 117°W32°N

33°N

34°N

35°N

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(km

2 )(A ) - Ventura


121°W 120°W 119°W 118°W 117°W32°N

33°N

34°N

35°N

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(B ) - Santa M onica Bay


121°W 120°W 119°W 118°W 117°W32°N

33°N

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35°N

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2000(D ) - O range C ounty/ San D iego

The direction of plume propagation results from the near-shore circulation.

In particular, during spring transition typical to California Current System, equatorward currents associated with wind-driven upwelling can transport stormwater plumes downcoast.

ioe 184 - the basics of satellite oceanography. 8. mesoscale variability and coastal pollution...

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