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Published on November 7, 2007

Author: Dixon

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Sea Surface Temperature (SST) Analyses for Climate:  Sea Surface Temperature (SST) Analyses for Climate Richard W. Reynolds National Oceanic and Atmospheric Administration National Environmental Satellite, Data & Information Service National Climatic Data Center Asheville, NC E-mail: Richard.W.Reynolds@noaa.gov National Climatic Data Center Introduction:  Introduction Why should we care about SST? What are the types of SST data? In situ (measured from ships and buoys) Satellite How are the data distributed? How are SST analyses computed? What are the SST errors? National Climatic Data Center Climate Change Detection:  Climate Change Detection Surface Temperature Land plus SST Marine air temperatures not used because biased & noisier than SST Smoothed annual anomalies of global combined land-surface air and sea surface temperatures (oC). Vertical axis: temperature anomaly Horizontal axis: time SSTs for El Niño and La Niña:  SSTs for El Niño and La Niña Total SST oC Anomaly SST Anomaly is departure from normal El Niño vs. Normal:  El Niño vs. Normal Differences occur in Sea level pressure Surface winds * Precipitation * Sea surface salinity *? Sea surface temperature * Thermocline depth Sea level * * Measured by satellite Typical January-March Weather Anomalies and Atmospheric Circulation:  Typical January-March Weather Anomalies and Atmospheric Circulation Moderate to Strong El Niño Moderate to Strong La Niña North Atlantic Oscillation (NAO):  North Atlantic Oscillation (NAO) Positive NAO Negative NAO More frequent & stronger storms Warmer & wetter Europe Colder & drier Greenland Milder & wetter US East Coast Fewer & weaker storms Colder & dryer Europe Warmer & wetter Greenland Colder & dryer US East Coast Impact in winter In Situ SST Data:  In Situ SST Data SHIP: ~1800-present Uninsulated bucket (common before WWII) Insulated bucket (common after WWII) Engine intake (common after WWII, roughly 50%) Hull contact (relatively new & rare, most accurate) BUOY: ~1979-present Moored (thermistor and hull contact, more accurate) Drifter (hull contact) Biases (differences between observation & truth) Uninsulated biased cold due to evaporation Engine intake biased warm due to engine room Decadal In Situ SST Coverage:  Decadal In Situ SST Coverage Number of months per decade with at least one observation in 2o grid cell Range: 0-120 Scaled: 0-100% In Plot Ship Buckets for SSTs:  1. Ship’s wooden bucket, 1891 Courtesy of Scottish Maritime Museum Other buckets German metal & leather bucket UK Met Office, black insulated bucket UK Met Office Mk II canvas sea bucket Ship Buckets for SSTs 2. Pictures courtesy of David Parker Tropical Moored Buoy Array:  Tropical Moored Buoy Array Buoy measures Surface wind speed and direction Air temperature SST Ocean temperature with depth Data available in real time via satellite In Situ & Satellite Analyses 1982-present:  In Situ & Satellite Analyses 1982-present In Situ Data (ship & buoy) Satellite IR: Advanced Very High Resolution Radiometer (AVHRR) Began November 1981 Satellite microwave: Tropical Rainfall Measuring Mission (TRMM) Microwave Instrument (TMI) Began December 1979 Balance between high coverage of satellite and ground truth of in situ data Climatologists Hate Biases Ship & Buoy Data:  Ship & Buoy Data Buoy distribution complements ship distribution Few observations south of 40oS Week of 2-8 January 2000 Satellite: Infrared (IR) SST Data:  Satellite: Infrared (IR) SST Data Advantages Improved coverage over in situ data Multiple channels available to eliminate atmospheric water vapor High resolution ~ 4 km Problems View must be cloud free Cold biases with cloud or volcanic aerosol contamination Satellite: AVHRR Data:  Satellite: AVHRR Data Missing data due to cloud cover Missing daytime data north of 40oN due to orbit of NOAA-14 Week of 2-8 January 2000 Satellite: Microwave SST Data:  Satellite: Microwave SST Data Advantages Improved coverage over in situ data Improved coverage over satellite IR data Error characteristics different from IR data Problems View must be precipitation free Lower resolution than IR (~50 km vs. ~4 km) Wind speed dependent Satellite: Microwave TMI Data:  Satellite: Microwave TMI Data Data limits of 38oS to 38oN due to orbit Additional diurnal limits due to time of orbit No impact of cloud cover Week of 2-8 January 2000 Comments on Errors (1):  Comments on Errors (1) There are three types of errors Sampling Random Ship ~ 1.3oC Buoy ~ 0.5oC Day Satellite ~ 0.5oC Night Satellite ~ 0.3oC Bias: differences between observation & truth Satellites can help with Sampling & Random errors Not biases Schematic profiles:  Schematic profiles Profile A: Nighttime or Daytime with moderate to strong winds Profile B: Daytime with light (<3m/s) winds T(1) is the surface skin temperature IR satellite T(2) is the bulk temperature just below the skin Buoy T(3) is the bulk mixed layer temperature. Ship Comments on Errors (2) SST Analyses:  SST Analyses Optimum interpolation (OI) Requires error statistics for data & analyses Weights data by distance and relative error Best with “lots” of data Preliminary bias correction of satellite on relatively coarse spatial scales SST Data and Analysis for 1 Week 2-8 January 2000: La Niña:  SST Data and Analysis for 1 Week 2-8 January 2000: La Niña Objective comparisons:  Objective comparisons National Climatic Data Center Withhold a random 20% of buoy SSTs from analyses Define set as buoy IDs ending in 4 or 9 Use withheld buoy data as independent data Compute weekly OI analyses WITH & WITHOUT satellite bias correction Using different satellite data AVHRR only TMI only and TMI + AVHRR Weekly OI Analyses:  Weekly OI Analyses OI Analyses use AVHRR only TMI only and TMI + AVHRR Upper panel: OI without satellite bias correction Lower panel: OI with satellite bias correction NOAA-14 biases at end of life: mid 2001 Vertical axis: SST anomaly Horizontal axis: time Bias of OI – Independent Buoy Data:  Bias of OI – Independent Buoy Data OI Analyses use AVHRR only TMI only and TMI + AVHRR Upper panel: OI analyses without satellite bias correction Lower panel: OI analysis with satellite bias correction Vertical axis: SST difference Horizontal axis: time SST Anomaly Movie (1997-1998):  SST Anomaly Movie (1997-1998) Jan 5 1997 Sep 5 1997 Apr 30 1998 Dec 30 1998 SST Anomaly Movie (1997-1998):  SST Anomaly Movie (1997-1998) Jan 5, 1997 Sep 5, 1997 Apr 30, 1998 Dec 30, 1998 Jan 5, 1997 - Dec 30, 1998 By Dave Wuertz Summary Satellite SST for Climate:  Summary Satellite SST for Climate Satellite data greatly reduces SST sampling and random errors over in situ data alone Satellites must be corrected for SST bias errors Correction done by use of in situ data Because in situ data are relatively sparse, scales of bias corrections are limited to 5o spatial scales Weekly temporal scales National Climatic Data Center

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