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11
Improved Sea Surface Temperature (SST) Analyses for Climate
NOAA’s National Climatic Data CenterAsheville, NC
Thomas M. SmithRichard W. Reynolds
Kenneth S. CaseyDudley Chelton
22
Optimum Interpolation (OI) SST Analysis
• A NOAA SST analysis, widely used for climate diagnostics and analysis
• Data: in situ (ship and buoy) and operational satellite (AVHRR) SSTs blended for global coverage
• Record: November 1981 to present• Resolution: weekly on 1o spatial grid• Analysis method: preliminary satellite bias
corrections followed by OI analysis
33
New Higher Resolution SST OI Analysis
• Resolution: Daily, 0.25o • Data: Satellite plus in situ (ship and buoy) data
– Satellite data to include• Infrared AVHRR: Longest satellite data set• Microwave AMSR: Not impacted by clouds so better
coverage than infrared
• 7-Day large-scale satellite bias correction for each satellite so analysis OK for climate
• Initial results shown for 2003– Separate analyses using AVHRR and AMSR to
examine impact of different satellite data
44
Top panel: Daytime
Bottom Panel: Nighttime
From the figure note:
• Regions north of 40°N and south of 40°S have roughly only 5 days of data.
• Number increases toward the tropics to roughly 15 days except for ITCZ and SPCZ regions
Jan 2003: Pathfinder Number of days
55
From the figure note:
• Better coverage than AVHRR– Regions north of 40°N and
south of 40°S have more than 20 days of data
• Drop offs occur from– Sun glint near 60°S in day
– Definition of day near 160°W for day & 20°E for night
– Toward equator due to swath width & Earth sphere
– In ITCZ and SPCZ regions due to precipitation
Jan 2003: AMSR Number of days
66
First Focus on SST Gradients • Examine Daily OI (1/4°grid)
– Constant e-folding spatial scale (100 km)
– Constant noise to signal ratio (1)
– 3 versions• Pathfinder AVHRR
• Operational Navy AVHRR
• AMSR
• Compare with– OI.v2 (weekly, 1°)
– RTG_SST (daily, 1/2° grid)
77
From the figure note:
• Sparse AVHRR data
• AMSR data missing near coast otherwise almost complete
• OI.v2 gradients very weak
• Daily OI and RTG gradients are similar
• AMSR OI has strongest gradients due to better data coverage than AVHRR
Jan 2003: Mean SST Gradient
88
Magnitude of Gradient:Gulf Stream
• Daily OI using AMSR for January - March 2003
• Gradients are quasi stationary due to topography– Thus, limited
AVHRR data are useful
99
From the figure note:
• OI gradients using Pathfinder AVHRR weaker the OI using Operational AVHRR
– Difference is due to data coverage
• AMSR data missing near coast otherwise almost complete
• Differences among products much smaller in summer because AVHRR coverage better
Jan 2003: Mean Gradient for 3 Daily OIs and Data
1010
Magnitude of Gradient: Tropical Eastern Pacific
• Daily OI using AMSR for August - October 2003
• Gradients propagate westward– Limited coverage
not as useful here– Monthly
averaging smooths out most of gradient signal
1111
From the figure note:
• Very Sparse AVHRR data
• AMSR data missing near coast between swaths and in ITCZ
• Some features lost in AVHRR
1 Oct 2003: SST Daily Gradient
1212
Second Focus on Means and Standard Deviations
• Examine Daily OI (1/4°grid)– Variable e-folding spatial scales & noise to signal ratios
– 3 versions• Pathfinder AVHRR
• Operational Navy AVHRR
• AMSR
• Compare versions with and without bias correction
1313
18 Month SST Average Difference:
Pathfinder - AMSR
Top: NO correction
• Pathfinder much colder than operations in tropics
– ITCZ & SPCZ: Pathfinder cloud bias?
– Aerosols in Atlantic and Indian?
Bottom: Bias corrected
• Differences much reduced but residual remains
• Unexplained bias increase off east coast US
1414
Jan 2003: Bias Corrections
Top: Pathfinder correction
Bottom: AMSR correction
• Algorithm and error characteristics independent for IR and microwave
• Note similar corrections in tropical Indian Ocean & Western Pacific plus Northern Mid-latitudes
– These locations include heavy ship traffic and may be due to warm ship biases
• Possible AMSR coastal warm bias?
1515
• AMSR has stronger standard deviations than Pathfinder
– Especially in mid-latitude winter
– Clouds reduce Pathfinder sampling
• This differences plus gradient differences suggest that separate Pathfinder OI and Pathfinder + AMSR OI needed
Jan 2003: Standard Deviation
Top: Pathfinder
Bottom: AMSR
1616
July 2003: Standard Deviation
Top: Pathfinder with - without Bias
Bottom: AMSR with- without Bias
• Bias correction typically adds a small extra standard deviation
• Note extra standard deviation at 30°N & Dateline
– suggests an in situ data problem
• Differences also suggest ship tracks: e.g., Australia to Panama
1717
Daily OI SST Work Planned
• Reexamine the daily OI statistics for both the bias correction and the SST analysis
• Test improved estimates of bias and errors – 2 separate satellites can help understand bias better and lead to
improved bias adjustment
• Preliminary analysis with Pathfinder AVHRR by the end of 2005
• Reanalysis efforts will extend – Daily OI analysis using Pathfinder AVHRR back to January 1985 – Daily OI analysis using Pathfinder AVHRR and AMSR back to
June 2002– Additional satellite data to be tested and possibly used