Improved Sea Surface Temperature (SST) Analyses for Climate

1
Improved Sea Surface Temperature (SST) Analyses
for Climate

• Thomas M. Smith
• Richard W. Reynolds
• Kenneth S. Casey
• Dudley Chelton

NOAAs National Climatic Data Center Asheville, NC
2
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

3
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

4
Jan 2003 Pathfinder Number of days

• Top panel Daytime
• Bottom Panel Nighttime
• From the figure note
• Regions north of 40N and south of 40S have
  roughly only 5 days of data.
• Number increases toward the tropics to roughly 15
  days except for ITCZ and SPCZ regions

5
Jan 2003 AMSR Number of days

• From the figure note
• Better coverage than AVHRR
• Regions north of 40N and south of 40S have more
  than 20 days of data
• Drop offs occur from
• Sun glint near 60S in day
• Definition of day near 160W for day 20E for
  night
• Toward equator due to swath width Earth sphere
• In ITCZ and SPCZ regions due to precipitation

6
First Focus on SST Gradients

• Examine Daily OI (1/4grid)
• 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)

7
Jan 2003 Mean SST Gradient

• 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

8
Magnitude of GradientGulf Stream

• Daily OI using AMSR for January - March 2003
• Gradients are quasi stationary due to topography
• Thus, limited AVHRR data are useful

9
Jan 2003 Mean Gradient for 3 Daily OIs and Data

• 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

10
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

11
1 Oct 2003 SST Daily Gradient

• From the figure note
• Very Sparse AVHRR data
• AMSR data missing near coast between swaths and
  in ITCZ
• Some features lost in AVHRR

12
Second Focus on Means and Standard Deviations

• Examine Daily OI (1/4grid)
• Variable e-folding spatial scales noise to
  signal ratios
• 3 versions
• Pathfinder AVHRR
• Operational Navy AVHRR
• AMSR
• Compare versions with and without bias correction

13
18 Month SST Average DifferencePathfinder - 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

14
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?

15
Jan 2003 Standard Deviation
Top Pathfinder Bottom AMSR

• 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

16
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 30N Dateline
• suggests an in situ data problem
• Differences also suggest ship tracks e.g.,
  Australia to Panama

17
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