October 10, 2009

The Pathfinder program was jointly created by the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) through the Earth Observing System (EOS) Program Office. The focus of the Pathfinder Program was to determine how existing satellite based data sets could be reprocessed and used to study global change. The data sets were designed to be long time-series data processed with stable calibration and community consensus algorithms to better assist the research community. The Pathfinder SST program was extremely successful in reprocessing efforts from the original version 1 to version 4 (Kilpatrick et al., 2001). Through continued funding from NOAA and the support of its National Oceanographic Data Center, the program was extended to create a Version 5 Pathfinder data set. Each reprocessing effort has led to a higher quality SST.  Improvements include the calculation of monthly versus yearly coefficients, as well as two sets of coefficients covering different water vapor regimes.  Additionally, the Pathfinder SST (PFSST) V5 data sets represent the longest satellite derived climate data record (CDR), currently spanning 1981-2008. This 27-year record presents a unique opportunity for comparing climatological means across a variety of parameters, including SST gradients (Mesias et al., 2007).  Other than spatial resolution, differences between Version 5 and Version 4 include an improved land mask and reference field and the enhanced use of sea ice information in quality flag determination (Vazquez et al., 2009). The latest version of the algorithm (Version 5.1) includes processed data from 1981 to end of 1984, spanning the NOAA-7 platform. For the first time a satellite based 4 km CDR has observed the two largest recent El Nino Events 1982-1983 and 1997-1998. Data sets such as the Reynolds Optimally Interpolated SSTs (OISSTv2) (Reynolds et al., 2002) have also resolved the 1982-1983 El Nino, but at a much coarser resolution. The new V5.1 PFSST from 1981-1985 provides, for the first time, the derivation of global SST anomalies at a much higher 4 km spatial resolution.

SST anomalies for both the OISSTV2 and PFSST were calculated by removing their respective climatologies (Casey and Cornillon, 1999).  PFSST and OISSTV2 anomalies were then averaged and calculated for each of the El Nino regions 1 and 2 (combined); 3; 3.4; and 4. Figure 1 a,b,c,d shows the anomalies in each of the El Nino Regions for both the OISSTV2 and the PFSST.

As expected, large area averages like the Nino regions show strong similarity between the OISSTv2 and PFSST data.  However, new regional insights can be gained by the higher resolution PFSST data.  See, for example, Figures 2 and 3 for the Gulf of California, in which  higher resolution gradients are identified in the PFSST.

The recently released Version 5.1 of the PFSST extends the data set back to September of 1981. This presents an advantage over previous SST data sets in that it provides global maps of SST at 4 km for both the 1982-1983 El Nino and the 1997-1998 El Nino.  Comparisons with the OISSTV2 at 1 degree (100km) for four of the El Nino Regions indicate the PFSST and OISSTV2 have high correlations (0.9-1.0)  for each of the indices. Analysis on a regional scale for an example area off of Baja California and the Gulf of California show the advantage of the higher resolution.

As part of the Group for High Resolution Sea Surface Temperature (GHRSST) project (Donlon et al, 2007), the weekly, 1-degree resolution OISSTv2 described here has been updated to create a daily, 0.25-degree resolution product that uses the Version 5 Pathfinder as its base (Reynolds et al., 2007). This daily analysis now also extends back to 1981 using the Version 5.1 data from Pathfinder.  It is anticipated that the PFSST and related GHRSST reprocessing efforts will provide users with new opportunities for understanding the impacts of the 1982-1983 and 1997-1998 events on even higher regional to coastal scales.

Jorge Vazquez
Project Scientist Sea Surface Temperature for the Physical Oceanography Distributed Active Archive Center

Ken Casey
Technical Director National Oceanographic Data Center

Robert Evans
Professor University of Miami Rosenstiel School of Marine and Atmospheric Science


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