0 50 100 150 200 25011
12
13
14
15
16
17
18
19
20
SST Chlorophyll
SS
T (
C)
Distance (km)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Ch
loro
ph
yll (g
/l)
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
8
9
10
11
SST Chlorophyll
SS
T (
C)
Distance (km)
0
1
2
3
4
5
Ch
loro
ph
yll (g
/l)
0 50 100 150 200 2508
9
10
11
12
13
14
15
16
SST Chlorophyll
SS
T (
C)
Distance (km)
0
1
2
3
4
5
Chlo
rophyl
l (g
/l)
0 50 100 150 200 250 300 35013
14
15
16
17
18
SST Chlorophyll
SS
T (
C)
Distance (km)
0
1
2
3
4
5
Ch
loro
ph
yll (g
/l)
0 50 100 150 200 250 300 35012
13
14
15
16
17
18
19
SST Chlorophyll
SS
T (
C)
Distance (km)
0
1
2
3
4
5
6
Ch
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
10
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
20
30
40
50
60
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
0 50 100 150 200 2503
4
5
6
7
8
9
10
11
12
SST Chlorophyll
SS
T (
C)
Distance (km)
0
2
4
6
8
10
12
Ch
loro
ph
yll (g
/l)
0 50 100 150 200 2508
9
10
11
12
13
14
15
16
SST Chlorophyll
SS
T (
C)
Distance (km)
0
1
2
3
4
5
Ch
loro
ph
yll (g
/l)
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
)
2.7
6.1
9.5
13.0
Ch
loro
ph
yll (
g/l
)
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
10
20
30
40
50
60
70
80
90
100
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
2.0
2.5
3.0
Monthly Mean Chlorophyll Concentration, by Region
Entire BasinNorth BasinSubpolar FrontSouth BasinKorean Coast
Mea
n C
hlor
ophy
ll (
g/l)