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Improved Sea Surface Temperature (SST) Analyses for Climate

NOAA’s National Climatic Data CenterAsheville, NC

Thomas M. SmithRichard W. Reynolds

Kenneth S. CaseyDudley Chelton

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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?

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• 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

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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

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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