Transcript oceansfromspace

Temporal and Spatial Variability of Satellite Sea Surface Temperature and Ocean Color
Richard W. Gould, Jr, Robert A. Arnone, and Christine O. Chan
Naval Research Laboratory, Code 7333, Stennis Space Center, MS 39529
1.) Comparison of Frontal Locations: SST and Chlorophyll
May 21, 1999
ABSTRACT
AVHRR SST and SeaWiFS chlorophyll overpasses approximately 6 hours apart
North Basin
Polar Front
12.96
SST image
with
highlighted
pixels from
scatter plot
red pixels: high chlor, low SST
SST image
with
chlorophyll
contours
blue pixels: high SST
5.52
2.76
0
2.88
5.76
8.65
SST
5
11
9.53
6.10
2.66
4.17
SST
Chlorophyll
15
4
4
9
2
8
13
SST (C)
Chlorophyll
image with
SST contours
Chlorophyll (g/l)
14
3
Chlorophyll
image with
SST contours
3
12
2
11
10
1
1
9
0
7
0
50
south
Chlorophyll and SST pixel values
extracted along image transect
100
150
0
north
Distance (km)
0
8
50
south
SST image
with
highlighted
pixels from
scatter plot
Korean Coast
8.63
red pixels: high chlor
blue pixels: low chlor, high SST
blue pixels: low SST
2.71
1.35
9.02
13.52
SST
200
north
6.28
3.93
1.58
9.35
14.02
18.70
SST
18.03
6
19
5
18
250
red pixels: high chlor,low SST
blue pixels: high chlor, high SST
0
0
SST
Chlorophyll
SST
Chlorophyll
18
5
4
17
16
3
15
2
14
1
1
13
50
100
west
150
200
250
300
350
east
Distance (km)
2.) Variability at the Subpolar Front: SST and Chlorophyll
white pixels indicate SST front overlaid
on SST and chlorophyll images
SST
Russia
Chlorophyll
image with
SST contours
0
Chlorophyll
200
250
300
350
south
Distance (km)
Shift in chlorophyll
distribution
64
Chlorophyll (g/l)
SST range 1-15° C
Chlor range 0-7 g/l
Chlor > 2.0
48
8
9
32
8
6
7
4
6
16
Chlorophyll (g/l)
10
10
5
2
4
3.09
6.17
SST
9.26 12.35
9.5
0
3
0
south
13.0
SST range 5-20° C
Chlor range 0-5 g/l
SST
Chlorophyll
11
5/21/99
5/21/99
11.5 – 13.5 C
12
12
0
Tsushima/Korea
Straits
150
SST images with highlighted pixels
from scatter plot (high chlorophyll)
4/21/99
7.0 – 9.0 C
Japan
100
north
4/21/99
Subpolar Front
50
3.) Timing and Location of the Spring Bloom
Japan Basin
Tsugaru
Strait
0
12
SST (C)
0
SST image
with
highlighted
pixels from
scatter plot
50
100
150
200
Distance (km)
Chlor > 2.0
250
north
5
16
SST
Chlorophyll
15
4
14
13
6.1
SST (C)
Chlorophyll and SST
pixel values extracted
along image transect
SST image
with
chlorophyll
contours
0
13
Chlorophyll (g/l)
14
2.7
3
12
2
11
Chlorophyll (g/l)
2
15
SST (C)
Chlorophyll
image with
SST contours
Chlorophyll (g/l)
3
16
4
Chlorophyll (g/l)
17
Circulation
East
Korean
Warm
Current Tsushima
Current
150
Distance (km)
Chlorophyll (g/l)
4.06
100
Chlorophyll and SST pixel values
extracted along image transect
Highest chlorophyll at intermediate
SST, in mixing regimes
Chlorophyll (g/l)
SST image
with
chlorophyll
contours
SST (C)
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.
South
Korea
16.66
5
5.42
Ulleung
Basin
12.50
16
10
South Basin
Yamato
Rise
Yamato
Basin
8.33
SST
SST
Chlorophyll
INTRODUCTION
Bathymetry
blue pixels: high chlor,
high SST
0
SST image
with
highlighted
pixels from
scatter plot
11.53
red pixels: high chlor,
low SST
Chlorophyll (g/l)
SST image
with
chlorophyll
contours
Chlorophyll (g/l)
Chlorophyll (g/l)
8.28
SST (C)
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 cloudcontaminated 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.
10
1
9
4.17
6/09/99
Chlorophyll Monthly
Composite – April, 1999
Regional Comparison:
April bloom in south basin
May bloom in north basin
April 1999
100
90
80
150
1.12
19
Chlor > 0.75
% Total ROI Area
30
25
0.0 - 0.5
0.5 - 1.0
1.0 - 1.5
1.5 - 2.0
2.0 - 2.5
2.5 - 3.0
> 3.0
20
15
10
50
1
2
3
4
5
6
7
40
30
0.0 - 0.5
0.5 - 1.0
1.0 - 1.5
1.5 - 2.0
2.0 - 2.5
2.5 - 3.0
> 3.0
18
0.6
17
0.5
0.74
16
0.4
15
0.3
14
0.37
0.2
13
0.1
12
0
9.05
13.6
18.1
0.0
11
0
south
50
100
150
Distance (km)
Chlorophyll and SST
pixel values extracted
along image transect
Summary
Comparison of Thermal/Optical Front Locations
•
•
•
20
•
1 2 3 4 5 6 7
Korean Coast
1 2 3 4 5 6 7
South Basin
1 2 3 4 5 6 7
Subpolar Front
1 2 3 4 5 6 7
North Basin
1 2 3 4 5 6 7
Korean Coast
1 2 3 4 5 6 7
South Basin
1 2 3 4 5 6 7
Subpolar Front
1 2 3 4 5 6 7
North Basin
Monthly Mean Chlorophyll Concentration, by Region
June 1999
100
3.0
Chlorophyll (g/l)
90
1
2
3
4
5
6
7
80
% Total ROI Area
•
0
70
60
50
0.0 - 0.5
0.5 - 1.0
1.0 - 1.5
1.5 - 2.0
2.0 - 2.5
2.5 - 3.0
> 3.0
40
30
20
1 2 3 4 567
South Basin
1 23 45 67
Subpolar Front
1 23 456 7
North Basin
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.
Regional Chlorophyll Variability
2.0
•
1.5
•
•
•
1.0
0.5
0.0
1 23 4 56 7
Korean Coast
•
•
10
0
SST at the Subpolar Front increased about 7° C over a 1.5 month period from lateApril 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.
Timing and Location of the Spring Bloom
2.5
Mean Chlorophyll (g/l)
• Extract ROI pixels from SST
(AVHRR) and chlorophyll
(SeaWiFS) imagery - daily scenes
and monthly composites
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.
SST/Chlorophyll Variability at the Subpolar Front
5
J FMAMJ J A J FMAMJ J A J FMAMJ JA J FMAMJ JA
Korean Coast South Basin Subpolar Front North Basin
J FMAMJ JA
Entire Basin
north
0.7
Chlorophyll (g/l)
% Total ROI Area
1
2
3
4
5
6
7
35
250
0.8
SST
Chlorophyll
May 1999
Chlorophyll (g/l)
200
Distance (km)
60
0
Korean
Coast
100
Regional binning by month,
Calculate areal percentages,
Calculate average concentrations.
Plot summary histograms
• Define regions of interest (ROIs)
• Examine temporal and spatial
changes (time-series analysis,
histograms)
south
50
Chlorophyll Monthly
Composite – May, 1999
10
• Examine thermal/bio-optical
relationships (scatter plots,
image overlays)
0
SST
4.) Spatial/Temporal Variability: Chlorophyll
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 colocated, 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).
0
8
SST (C)
Chlorophyll (g/l)
Subpolar Front defined by 2° C gradient
SST increased by 7° C over 1.5 months
METHODS
South Basin
1.49
SST range 8-20° C
Chlor range 0-1 g/l
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.
Subpolar Front
16.7
20
6/09/99
14.0 – 16.0 C
Examine the spatial and temporal variability of sea surface temperature (SST)
and chlorophyll:
North Basin
12.5
SST
OBJECTIVES
•
•
•
•
8.33
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.
200
250
north