Operational Satellite Coral Bleaching Monitoring Products Methodology

SST anomaly | HotSpot | DHWs | Coral Bleaching Virtual Stations | Satellite Bleaching Alerts | References

I. Sea Surface Temperature (SST) anomalies

NOAA/NESDIS has been producing sea surface temperatures from satellite data since 1972. Monitoring of SST from earth-orbiting infrared radiometers has had a wide impact on oceanographic science. Beginning in mid-1996, a new satellite-only climatology (for 1985-1993) became available and made it possible to generate more accurate SST anomaly products from the operational 50-km daily SST field. The NOAA/NESDIS operational SSTs are provided twice a week in near real-time and use both day and night retrievals. Since the satellite-only SST monthly mean climatology is derived only from nighttime SST observations to eliminate the diurnal variation caused by diurnal solar heating at the sea surface (primarily at the "skin" interface, 10-20 µm), only nighttime SST analyses are used to ensure consistency between the satellite SST observations and the climatology.

The original satellite-only SST monthly mean climatology data is at 36 km resolution and was derived from the Multi-Channel SSTs (MCSSTs) reprocessed by the Rosenstiel School of Marine and Atmospheric Science (RSMAS) of the University of Miami (Gleeson and Strong, 1995). This climatology is based on nighttime observations from 1985-1993, with SST observations from the years 1991 and 1992 omitted due to aerosol contamination from the eruption of Mt. Pinatubo. In-situ SSTs from drifting and moored buoys are used to remove any biases, and statistics are compiled with time. The monthly mean climatology data is then interpolated to 50 km resolution to match the operational SST field. To obtain the SST climatology for a specific date, the linear interpolation method is applied on the two SST monthly mean climatology files that are closest to that date, with the assumption that the SST monthly climatology is more accurate on the 15th of the month. So, the linear interpolation is done based on the number of days away from the 15th of the first month's climatology. The SST anomaly is calculated as follows:

SST_anomaly = SST_analysis - SST_interpolated_climatology
with

SST_interpolated_climatology = day_fraction*(b2-b1) + b1

where SST_analysis is the 50km nighttime only SST analysis, b1 is the first month's climatology data, b2 is the second month's climatology data, and the day_fraction is the ratio of the current number of days away from the fifteenth of the first month to the number of days between the two closest climatology files. The SST anomalies are then derived by subtracting the interpolated 'daily' climatology from the operational nighttime 50 km SST analysis.

The anomaly field is produced twice a week as the updated nighttime-only 50 km SST analysis is produced. Initially, the experimental SST anomalies were derived from the operational day/night 50 km SST analysis, but were discontinued as of September 1, 1996. Currently, the operational SST anomaly charts shown are derived from the NOAA/NESDIS 50 km operational night-only SST analysis and climatology. The range of temperatures displayed is -5.0 to +5.0 K. Each color gradation on the color bar is 0.5 K. The operational SST anomaly charts are useful in assessing ENSO (El Niño-Southern Oscillation) development, monitoring hurricane "wake" cooling, and even major shifts in coastal upwelling. Note that these anomalies are somewhat less reliable at high latitudes where more persistent clouds limit the amount of satellite data available for the computation of the monthly climatology.

Monthly SST anomalies are also provided here through our Office of Research and Applications (ORA) partners. These data are available at 36 km resolution.

An ice mask, courtesy of the National Center for Environmental Prediction, has been incorporated as of April 28, 1998. Regions where there is more than 15% sea-ice are shown in white.

II. Coral Bleaching HotSpots

The night-only 50 km SST anomaly was primarily developed for producing our thermally-induced coral bleaching early warning products, which include SST anomalies, coral bleaching HotSpots, Degree Heating Weeks, and Tropical Ocean Coral Bleaching Indices.

Coral bleaching HotSpots highlight SST anomalies that are one degree greater than the SST maximum monthly climatology. Satellite-only SST Maximum Monthly Climatology (also called MMM) was provided at 36 km resolution by ORA. The climatology was then interpolated to 50 km resolution and serves as the input threshold for the Coral Bleaching HotSpot charts, i.e.,

HotSpots = SST_analysis - SST_interpolated_MMM

In the HotSpot charts, only regions where the SST is one degree greater than the maximum expected summer time temperature are highlighted. A color table is used which highlights anomalies greater than 1.0 degree C in yellow to red, with anomalies between 0.25 to 1.0 degrees C in purple to blue. The yellow to red colors usually indicate potential coral bleaching.

Coral bleaching events have been noted in areas where the HotSpots are greater than 1 degree C. Several examples of this phenomenon occurred in 1998 and animations can be seen on the 1998 Coral Bleaching HotSpots webpage.

III. Degree Heating Weeks

Degree Heating Weeks (DHWs) indicate the accumulation of thermal stress that coral reefs have experienced over the past 12 weeks. One DHW is equivalent to one week of sea surface temperatures one degree Celsius greater than the expected summertime maximum. Two DHWs are equivalent to two weeks at one degree above the expected summertime maximum OR one week of two degrees above the expected summertime maximum.

The half-week approach is used here considering the twice-a-week nature of the coral data. With this approach, the DHWs are accumulated based on half-weekly HotSpot data, i.e.,

DHWs = Sum (over the last 12 weeks) 0.5* half-weekly HotSpot (HotSpot > 1.0 C)

We realize that this approach is slightly different than the whole-week approach where the weekly mean HotSpots are calculated first and then accumulated over 12 weeks for DHWs.

These charts, which are produced twice a week at a 50km resolution, are being used to depict the duration and strength of thermal stress that results in bleaching. The orange to purple colors (where DHWs are greater than 10) indicate the existence of high and persistent sea surface temperatures. Under these conditions, coral reefs may be undergoing severe bleaching, and possibly mortality. We recognize that a single parameter such as thermal stress may not have the ability to force bleaching events in some areas without other factors (low winds, high solar irradiance, etc.). Warnings about these conditions are provided to reef site managers through the Tropical Ocean Coral Bleaching Indices web page. A coral bleaching data file is archived at NCDC.

IV. Coral Bleaching Virtual Stations

The Coral Bleaching Virtual Stations web page (formerly known as the Tropical Ocean Coral Bleaching Indices web page) is well known in the coral reef community. It is designed to provide near real time information on thermal stress that induces coral bleaching, for 24 selected reef sites around the globe. For each reef site, the closest 50 km ocean pixel data is extracted and listed on the web page. This data includes current DHWs, DHW charts, maximum DHWs, current SST, and maximum SST climatology. Site names are followed by a status level and a warning image when the current SST exceeds or equals the maximum expected summertime temperature. If the current SST exceeds the temperature by 1 degree Celsius or more, DHWs begin to accumulate. In addition, links to local reef maps, coral bleaching monitoring products, current ocean surface winds, and SST time-series are available by clicking on the name of each reef site. The Virtual Stations page, as well as the current operational SST anomalies, HotSpots and DHWs, are very useful to reef scientists and managers for monitoring and predicting potential coral bleaching events.

V. Satellite Bleaching Alerts

NOAA's Coral Reef Watch (CRW) Satellite Bleaching Alert (SBA) system is an automated coral bleaching e-mail alert system designed to monitor the status of thermal stress conducive to coral bleaching via the use of the CRW global satellite near-real time HotSpot suite of products. The SBA was developed by the NOAA CRW satellite team as a tool for coral reef managers, scientists and other interested people. The SBA became operational in July 2005. Currently, the alert messages are available for a number of coral reefs around the world (see the subscription page for a list of available reef sites). A sample of the SBA message can be viewed here.

An automated e-mail will be sent to a subscriber for a reef site when and only when the status level of thermal stress changes regardless of the current status level. The status level of thermal stress at selected reef sites is updated twice per week.

There are five status levels: No Stress, Bleaching Watch, Bleaching Warning, Bleaching Alert Level 1, and Bleaching Alert Level 2. These levels are defined in terms of the CRW HotSpot and Degree Heating Weeks (DHW) products:



The existence of a HotSpot value indicates that thermal stress potentially conducive to bleaching is present, and the value of the HotSpot quantifies the intensity of the thermal stress. A Degree Heating Week (DHW) is a measure of accumulated thermal stress over a consecutive 12-week period.

Please note that since the DHW is a 12-week accumulated anomaly, it is possible for a location to have a non-zero DHW value when the HotSpot value is already less than one and even zero. Hence, at a status level of No Stress or Bleaching Watch, it is possible for the corresponding DHW value to be greater than zero.

To receive an automated e-mail alert for any of the available reef sites, go to the subscription page, where you can also view and alter your selection(s) or unsubscribe from the e-mail list.

Your report of any bleaching observations (including observations of no bleaching) will be greatly appreciated. Bleaching observations can be sent to the NOAA/ReefBase Collaborative online bleaching report form.

Should you have any questions or feedback on the Satellite Bleaching Alert system, please send e-mail to _NESDIS_CoralReefWatch@noaa.gov.

VI. References

(4) Strong, A. E. and C. Duda, 1997: New AVHRR product - Coral Reef HotSpots.

(5) Strong, A. E., C. S. Barrientos, C. Duda and John Sapper, 1996: Improved Satellite Techniques for Monitoring Coral Reef Bleaching.

(6) Liu, G., W. Skirving, and A.E. Strong. 2003. Remote sensing of sea surface temperature during 2002 Barrier Reef coral bleaching. EOS, 84(15), 137-144.

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For more references, please refer to our research partner's web site.

Last Modified March 03, 2008 (jw)

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