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12

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T (

C)

Distance (km)

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Temporal and Spatial Variability of Satellite Sea Surface Temperature and Ocean ColorRichard W. Gould, Jr, Robert A. Arnone, and Christine O. Chan

Naval Research Laboratory, Code 7333, Stennis Space Center, MS 39529

ABSTRACT

INTRODUCTION

OBJECTIVES

METHODS

The Japan/East Sea is a semi-enclosed basin with restricted inflow and outflow and complex circulation. The northern Japan basin is generally 2500 – 3700 m deep, whereas the southern basin is shallower with more complex bottom topography, including the shallow Yamato Rise which separates the Ulleung and Yamato Basins. Warm, oligotrophic water enters the basin from the south through the Tsushima/Korea Strait and bifurcates into the Tsushima Current that flows eastward along the coast of Japan and the East Korean Warm Current (EKWC) that flows northward along the Korean Peninsula. A central branch through the central southern basin may also be present (Naganuma, 1977). The EKWC breaks away from the Korean coast near 37 N and meanders eastward, bisecting the basin and forming the Subpolar Front around 40 N (Figure 1). The front separates the cold, dense, weakly-stratified northern water from the warmer, stratified water to the south. The Subpolar Front is also a region of very active eddy formation and exhibits strong thermal and bio-optical gradients. Flow exits the basin through the Tsugaru and Soya Straits.

By coupling AVHRR and SeaWiFS imagery we define the SST and bio-optical properties and begin to characterize and interpret the variability. A long-term image climatology enables us to examine temporal and spatial patterns in the region. By comparing individual SST and chlorophyll images collected at nearly the same time, we can begin to assess whether thermal and bio-optical fronts are spatially co-located.

Examine the spatial and temporal variability of sea surface temperature (SST) and chlorophyll:

• Compare locations of thermal and optical fronts in satellite imagery.• Examine SST and chlorophyll variability at the Subpolar Front.• Describe timing and spatial distribution of the spring bloom in the

Japan/East Sea.• Describe regional variability in chlorophyll concentration, by month.

We have compiled a two-year climatology of daily AVHRR (SST) and SeaWiFS (bio-optics) imagery of the Japan/East Sea, at 1 km spatial resolution. Some of the imagery was collected at sea during two cruises to the region (May/June 1999 and January/February 2000), using a shipboard receiving system. The real-time satellite thermal and ocean color imagery enabled us to optimize station locations and cruise tracks, provided the large-scale context of the circulation, and provided a broader context to aid interpretation of cruise data. In addition we obtained archived imagery from NASA/Goddard Distributed Active Archive Center, NOAA Satellite Active Archive Center, and Dr. Ichio Asanumai at JAMSTEK. The imagery provides a long-term data base to characterize the spatial and temporal variability of the region. Products from the SeaWiFS imagery include chlorophyll concentration and absorption and scattering coefficients at six wavelengths. In addition to the daily imagery, weekly and monthly composites have been created to reduce cloud-covered pixels.

To assess how closely SST and chlorophyll fronts are co-located, we selected three individual AVHRR and SeaWiFS scenes (covering a three-month period from April-June, 1999) that were collected at nearly the same time (within 3-8 hrs). We subdivided the Japan/East Sea into four regions of interest: the northern basin, the Subpolar Front, the southern basin, and the Korean Coastal waters. For each subregion, we then overlaid chlorophyll contours on the SST image and SST contours on the chlorophyll image, and produced scatter plots of chlorophyll vs. SST to help extract patterns. We also calculated summary statistics and histograms using monthly chlorophyll composite images, to describe regional variability over a nine month period in the Japan/East Sea (January-August, 1999).

1.) Comparison of Frontal Locations: SST and Chlorophyll

• The spring bloom started in late March – early April in the southern basin and progressed northward through late May, ending in all areas by early June.

• There was a shift in maximum chlorophyll concentrations from south of the Subpolar Front in April to north of the front in June.

• SST at the Subpolar Front increased about 7° C over a 1.5 month period from late-April to early-June.

• Chlorophyll concentrations at the Subpolar Front decreased from maximum values of > 30 g/l in April to 10 g/l in May and 1 g/l in June.

• SST/Chlorophyll relationship is complex, with regional and temporal dependencies.

• In May, highest chlorophyll at intermediate SST, in mixing regimes.• At Subpolar Front, highest chlorophyll associated with highest SST in April,

lowest SST in June.

• April – highest chlorophyll in Korean Coastal and South Basin; lower at Subpolar Front and North Basin.

• May – Decrease at Korean Coast and South Basin (sharp); no change at Front; highest at North Basin (sharp increase).

• June – All areas very low, particularly South Basin; North Basin is the highest.

• Regionally, in the January – August 1999 time period, the mean chlorophyll concentration was highest in March across the entire Japan/East Sea Basin, and at the Korean Coast, South Basin, and Subpolar Front; it was highest in May in the North Basin.

Summary

Comparison of Thermal/Optical Front Locations

SST/Chlorophyll Variability at the Subpolar Front

Timing and Location of the Spring Bloom

Regional Chlorophyll Variability

2.) Variability at the Subpolar Front: SST and Chlorophyll

4.) Spatial/Temporal Variability: Chlorophyll

3.) Timing and Location of the Spring Bloom

We examine temporal and spatial variability of sea surface temperature (SST) and ocean color in the Japan/East Sea during winter and spring, using satellite data from AVHRR and SeaWiFS sensors. The Japan/East Sea is a semi-enclosed basin divided roughly in half by the east/west oriented Subpolar Front. During the winter, cold-air outbreaks sweep off Siberia and cause rapid cooling of the surface waters, resulting in convective overturning of the upper layers. In the spring, the water column stratifies and a phytoplankton bloom develops. A complex circulation pattern along the front generates a series of mesoscale eddies visible in both the thermal and bio-optical fields. We relate the timing of the bloom and the locations of the chlorophyll fronts to changes in the thermal field and the locations of the temperature fronts. We examine daily images of SST and chlorophyll concentration to determine whether the bio-optical and thermal fronts coincide.

We also define four regions of interest: the northern basin, the southern basin, the Subpolar Front, and the South Korean coast. Then, using weekly and monthly composite images (to reduce cloud-contaminated pixels), we extract SST and chlorophyll values from all pixels in each region and derive summary statistics. SST at the Subpolar Front increased about 6 over a 1.5-month period from late April to early June in 1999. During this same period, elevated chlorophyll values near the Korean coast and in the southern basin decreased sharply as the phytoplankton bloom that first developed in the southern basin progressed to the front and northward. The SST/chlorophyll relationship is complex; high chlorophyll values correspond more closely with mixing regimes, such as areas of divergence at the edges of meanders, than to specific water mass distributions.

• Define regions of interest (ROIs)

• Extract ROI pixels from SST (AVHRR) and chlorophyll (SeaWiFS) imagery - daily scenes and monthly composites

• Examine thermal/bio-optical relationships (scatter plots, image overlays)

• Examine temporal and spatial changes (time-series analysis, histograms)

North Basin

Subpolar Front

South Basin

KoreanCoast

Tsushima/KoreaStraits

Japan

Russia

SouthKorea

TsugaruStrait

Circulation

EastKoreanWarmCurrent Tsushima

Current

Subpolar Front

Bathymetry

Japan Basin

YamatoBasin

YamatoRise

UlleungBasin

0 50 100 1507

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11

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loro

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rophyl

l (g

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0 50 100 150 200 250 300 35013

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SS

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loro

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SST Chlorophyll

SS

T (

C)

Distance (km)

0

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loro

ph

yll (g

/l)

1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 70

5

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15

20

25

30

35

8090

100April 1999

Chlorophyll (g/l)

North BasinSubpolar FrontSouth BasinKorean Coast

% T

ota

l RO

I A

rea

1 0.0 - 0.52 0.5 - 1.03 1.0 - 1.54 1.5 - 2.05 2.0 - 2.56 2.5 - 3.07 > 3.0

1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 70

10

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Chlorophyll (g/l)

1 0.0 - 0.52 0.5 - 1.03 1.0 - 1.54 1.5 - 2.05 2.0 - 2.56 2.5 - 3.07 > 3.0

May 1999

North BasinSubpolar FrontSouth BasinKorean Coast

% T

ota

l RO

I A

rea

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May 21, 1999AVHRR SST and SeaWiFS chlorophyll overpasses approximately 6 hours apart

North Basin

Chlorophyll and SST pixel valuesextracted along image transect

south north

SST imagewith

chlorophyllcontours

Chlorophyllimage with

SST contours

SST imagewith

highlightedpixels fromscatter plot

SST

Ch

loro

ph

yll

(g

/l)

0

2.76

5.52

8.28

2.88 5.76 8.65 11.53

red pixels: high chlor, low SST

blue pixels: high SST

Polar Front

south north

SST

Ch

loro

ph

yll

(g

/l)

04.17 8.33 12.50 16.66

2.66

6.10

9.53

12.96red pixels: high chlor, low SST

blue pixels: high chlor, high SST

Chlorophyll and SST pixel valuesextracted along image transect

SST imagewith

chlorophyllcontours

SST imagewith

highlightedpixels fromscatter plot

Chlorophyllimage with

SST contours

SST imagewith

chlorophyllcontours

SST imagewith

highlightedpixels fromscatter plot

Chlorophyllimage with

SST contours

Chlorophyll and SSTpixel values extractedalong image transect

South Basin

west east

Ch

loro

ph

yll

(g

/l)

SST

09.02 13.52 18.03

1.35

2.71

4.06

5.42

red pixels: high chlor

blue pixels: low chlor, high SST

blue pixels: low SST

SST imagewith

chlorophyllcontours

SST imagewith

highlightedpixels fromscatter plot

Chlorophyllimage with

SST contours

Korean Coast

north south

Ch

loro

ph

yll

(g

/l)

SST

09.35 14.02 18.70

1.58

3.93

6.28

8.63red pixels: high chlor,low SSTblue pixels: high chlor, high SST

SST Chlorophyll

4/21/997.0 – 9.0 CSST range 1-15° C

Chlor range 0-7 g/l

white pixels indicate SST front overlaidon SST and chlorophyll images

5/21/9911.5 – 13.5 C

SST range 5-20° CChlor range 0-5 g/l

6/09/9914.0 – 16.0 C

SST range 8-20° CChlor range 0-1 g/l

SST images with highlighted pixelsfrom scatter plot (high chlorophyll)

4/21/99

5/21/99

6/09/99SST

4.17 8.33 12.5 16.7

Ch

loro

ph

yll (

g/l

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2.7

6.1

9.5

13.0

Ch

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yll (

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0 3.09 6.17 9.26 12.35

16

32

48

64

SST

SSTC

hlo

rop

hyll (

g/l

)

9.05 13.6 18.10

0.37

0.74

1.12

1.49

Chlor > 2.0

Chlor > 2.0

Chlor > 0.75

Shift in chlorophylldistribution

south north

north

north

south

south

Chlorophyll and SSTpixel values extractedalong image transect

Chlorophyll MonthlyComposite – April, 1999

Chlorophyll MonthlyComposite – May, 1999

Subpolar Front defined by 2° C gradientSST increased by 7° C over 1.5 months

Regional Comparison:April bloom in south basinMay bloom in north basin

Regional binning by month,Calculate areal percentages,

Calculate average concentrations.Plot summary histograms

Highest chlorophyll at intermediateSST, in mixing regimes

1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 70

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North BasinSubpolar FrontSouth BasinKorean Coast

Chlorophyll (g/l)

June 1999

% T

otal

RO

I A

rea

1 0.0 - 0.52 0.5 - 1.03 1.0 - 1.54 1.5 - 2.05 2.0 - 2.56 2.5 - 3.07 > 3.0

JFMAMJ JA JFMAMJ JA JFMAMJ JA JFMAMJ JA JFMAMJ JA0.0

0.5

1.0

1.5

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3.0

Monthly Mean Chlorophyll Concentration, by Region

Entire BasinNorth BasinSubpolar FrontSouth BasinKorean Coast

Mea

n C

hlor

ophy

ll (

g/l)