Methodology, Product Description, and Data Availability of
NOAA Coral Reef Watch's (CRW) Heritage Twice-Weekly Global 50km-Resolution
Satellite Coral Bleaching Heat Stress Monitoring Products

(Products Retired: April 2020)


1. Introduction

SUMMARY: Mass coral bleaching has become one of the most visible marine ecological impacts of persistently rising ocean temperatures. NOAA's satellites measure changes in the sea surface temperature (SST), in near real-time, that drive coral bleaching. Coral Reef Watch (CRW) uses this information to pinpoint areas around the world where corals are at risk for bleaching. CRW's suite of heritage global data products were produced at 0.5 degree (approximately 50km) spatial resolution twice-weekly. Data and images are available for free on our website, through the date of retirement of the 50km products.

Coral reefs are presently Earth's largest biological structures and have taken thousands of years to form. However, in recent decades, coral reef ecosystems have been declining at alarming rates worldwide. Coral bleaching has been one of the most significant contributors to this increased deterioration (Wilkinson, 2008).

Coral bleaching occurs when the symbiotic relationship between algae (zooxanthellae) and their host coral breaks down under certain environmental stresses. As a result, the host expels its zooxanthellae, exposing its white calcium carbonate skeleton and the affected coral colony becomes stark white or pale in color. This is known as "coral bleaching". Coral bleaching can be triggered and sustained under various environmental stresses. Anomalously warm water temperatures have been observed as one of the major causes of mass coral bleaching worldwide. Ambient water temperatures as little as 1 to 2 °C above a coral's tolerance level, indicated by summer monthly mean temperatures, can cause coral bleaching (Berkelmans and Willis, 1999; Reaser et al., 2000). Corals that are partially to totally bleached for long periods often die. Severe bleaching events have dramatic long-term ecological and social impacts, including loss of reef-building corals, changes in benthic habitat and, in some cases, changes in fish populations on the reef. Even under favorable conditions, it can take decades for severely bleached reefs to fully recover (Wilkinson, 2008).

The need for improved understanding, monitoring, and prediction of coral bleaching has become imperative. Satellite remote sensing, which provides synoptic views of the global oceans in near-real-time and monitors remote reef areas previously known only to wildlife, has become an essential tool for coral reef managers and scientists. As early as 1997, NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) began producing web-accessible, satellite-derived, global near real-time nighttime sea surface temperature (SST) products to monitor thermal conditions conducive to coral bleaching and to assess the intensity of bleaching stress around the globe. This activity evolved into a crucial part of NOAA's Coral Reef Watch (CRW) program in 2000 (Strong et al., 2004; Liu et al., 2005). CRW's earliest "experimental" products were the outgrowth of earlier work by Goreau and Hayes (1994) and by Montgomery and Strong (1995). Between September 2002 and February 2003, after successfully providing early warnings of coral bleaching to the global coral reef community for several years, most "experimental" products were transitioned to "operational" status. These "operational" products are now supported and delivered by NESDIS on a 24-hour/7-day basis, permitting almost constant global monitoring of environmental conditions that can cause coral bleaching. This satellite thermal stress monitoring technique has been successful in now-casting coral bleaching episodes around the globe (e.g., Goreau et al., 2000; Wellington et al., 2001; Strong et al., 2002; Liu et al., 2003; Coral Reef Watch, 2003; Liu et al., 2005; Skirving et al., 2006); and Eakin et al., 2010).

CRW's heritage twice-weekly global 50km satellite coral bleaching monitoring and assessment products include: SST, SST Anomaly, Coral Bleaching HotSpot, coral bleaching Degree Heating Week (DHW), Bleaching Alert Area, Virtual Stations, and a free, automated Satellite Bleaching Alert (SBA) e-mail system. These products were produced by CRW in near real-time, at 0.5-degree (approximately 50km) spatial resolution, until their retirement at NOAA.

Until January 31, 2016, CRW used data from the Advanced Very High Resolution Radiometer (AVHRR) instrument sensors onboard NOAA's Polar-orbiting Operational Environmental Satellites (POES) to derive its 50km satellite sea surface temperature (SST) product (discussed below). Each twice-weekly SST measurement was based on data from one AVHRR sensor onboard a single POES. However, as of February 1, 2016, and until the retirement of the 50km products on April 30, 2020, the twice-weekly 50km SST measurement was based on NOAA's operational daily global 5km Geostationary-Polar-orbiting (Geo-Polar) Blended Nighttime-only SST Analysis.

The 50km products were updated twice-weekly every Monday morning (using observations from the previous Thursday through Sunday) and Thursday morning (observations from the previous Monday through Wednesday), U.S. Eastern Time. Data and images are date-stamped with the end-date of the half-week period. These products are described in detail in the following sections. Data and images are available for free on the CRW website at https://coralreefwatch.noaa.gov/product/50km/index.php.

The CRW team at NOAA/NESIDS that developed, generated, operated, enhanced, and maintained these heritage coral bleaching monitoring products comprises scientists from the Center for Satellite Applications and Research (STAR, formerly the Office of Research and Applications (ORA)) and from the Office of Satellite and Product Operations (OSPO, formerly the Office of Satellite Data Processing and Distribution (OSDPD)).


2. Sea Surface Temperature (SST)

SUMMARY: The CRW heritage near real-time twice-weekly global 50km SST product was produced from nighttime-only data, to eliminate the effect of solar glare and reduce variability caused by heating during the day. Until January 31, 2016, SST data came from NOAA's polar-orbiting satellites, which measure infrared radiation from the ocean surface across the entire globe every day. As of February 1, 2016, and until the retirement of the 50km products at NOAA, the twice-weekly 50km SST measurement was based on NOAA's operational daily global 5km Geostationary-Polar-orbiting Blended Nighttime-only SST Analysis. The SST product discussed here was a twice-weekly composite at 0.5-degree (50km) resolution.

NOAA has been measuring SST via satellites since 1972. Monitoring of SST from earth-orbiting infrared radiometers has had a wide impact on oceanographic science. One of the principal sources of infrared data for SST measurement was the AVHRR carried on NOAA's POES satellites, beginning in 1978. AVHRR is a broad-band, four or five channel (depending on the model) scanner, sensing in the visible, near-infrared, and thermal infrared portions of the electromagnetic spectrum. The POES satellite system offers the advantage of daily global coverage, by making near-polar orbits roughly 14 times daily. In situ SSTs from buoys (drifting and moored) are used operationally to maintain accuracy of satellite SST by removing biases and compiling statistics with time (McClain et al., 1985; Strong, 1991; Montgomery and Strong, 1995; Strong et al., 2000).

Until January 31, 2016, the CRW near real-time nighttime SST product included the most recent satellite global nighttime composite AVHRR SSTs at 0.5-degree (50km) resolution, produced twice-weekly (see Introduction section for details on the update schedule). Nighttime-only satellite SST observations are used to eliminate daily warming caused by solar heating at the sea surface (primarily at the "skin" interface, 10-20 µm) during the day and to avoid contamination from solar glare. Compared with daytime SST and day-night blended SST, nighttime SST provides more conservative and stable estimates of heat stress conducive to coral bleaching. Nighttime SSTs also compare favorably with in situ SSTs at one meter depth (Montgomery and Strong, 1995). The 50km-resolution data were calculated by averaging multiple temperature observations (weighted by distance from pixel center, conditionally out to a maximum of 150km), and were based on 4km AVHRR Global Area Coverage (GAC) SST acquired daily.

With NOAA's retirement of the heritage AVHRR-based Main Unit Task (MUT) SST product in 2016, as of February 1, 2016, and until the retirement of CRW's 50km products on April 30, 2020, the twice-weekly 50km SST measurement was based on NOAA's operational daily global 5km Geostationary-Polar-orbiting (Geo-Polar) Blended Nighttime-only SST Analysis.

Data and images are available for free on the CRW website at https://coralreefwatch.noaa.gov/product/50km/index.php.

The color range of temperatures displayed on the SST charts is -2.0 to 34.0 °C. Each color gradation on the color bar is 1.0 °C. Any satellite pixels that have SST values greater than 34.0 °C are displayed in the same color as SST equal to 34.0 °C. An ice mask, courtesy of the NOAA National Center for Environmental Prediction (NCEP) was incorporated as of April 28, 1998.

Charts of the retrospective 1984-1998 monthly mean SSTs are available online at 36km-resolution.


3. Climatology

SUMMARY: Coral bleaching is caused by unusually warm sea surface temperatures. Therefore, the first step in looking for areas at risk for bleaching is to define the "usual" temperatures in the world's oceans. This is accomplished by calculating a long-term mean SST, or climatology. CRW monthly mean SST climatologies are calculated from 7 years of satellite data. The Maximum of the Monthly Mean SST climatology is then defined as the warmest monthly mean value for each pixel around the world, indicating the upper limit of "usual" temperature. These climatologies are available as images and HDF data files on the CRW website.

Beginning in mid-1996, more accurate monthly mean SST climatologies derived solely from satellite nighttime SST observations became available at a higher spatial resolution, 36km, than any previous global SST climatologies (at 60 to 100km) (Strong et al., 1997). This made it possible to generate more accurate, higher-resolution climatologies, which led to improved near real-time SST Anomaly and coral bleaching HotSpot products from the CRW nighttime SST field.

The original 36km satellite-only reprocessed SST data used for creating the climatologies were generated from the Multi-Channel SSTs (MCSSTs) by the Rosenstiel School of Marine and Atmospheric Science (RSMAS) at the University of Miami (Gleeson and Strong, 1995). In situ SSTs from drifting and moored buoys were used to remove any biases, and statistics were compiled with time to derive the reprocessed SSTs. The monthly mean SST climatologies were then derived by averaging these satellite SSTs during the time period of 1985-1993. Observations from the years 1991 and 1992 were omitted due to the aerosol contamination from the eruption of Mt. Pinatubo. These climatologies were developed at NOAA/NESDIS/STAR (formerly ORA) before being delivered to NOAA/NESDIS/OSPO (formerly OSDPD) for implementation. The 36km climatologies were finally interpolated into 0.5-degree (50km) resolution to match the resolution of the operational SST analysis field.

Daily SST climatologies were derived from these 12 operational monthly mean climatologies to produce our SST Anomaly product. First, the 12 monthly mean SST climatologies were set as the daily climatologies for the 15th days of the corresponding months. Daily climatology of any other day was derived by linearly interpolating between the two temporally closest values in the 12 monthly climatologies described above. For example, the daily climatology for June 30th was calculated by linear interpolation between June 15th and July 15th. If these two closest monthly climatologies are named A and B, with A the earlier one and B the subsequent one, the formula for deriving a daily SST climatology is

      Daily_SST_climatology = day_fraction*(B-A) + A,

where the day_fraction is the ratio of the number of days of the targeted day away from A to the number of days between A and B. For example, say we want to calculate the daily value for May 25th, the May climatology is 26 °C, and the June climatology is 30 °C. May 25th is ten days from May 15th, and there are 31 days between May 15th and June 15th.

      day_fraction = 10/31 = 0.32258
      Daily_SST_climatology = 0.323 * (30 °C - 26 °C) + 26 °C = 27.3 °C

To produce CRW's heritage satellite coral bleaching heat stress monitoring products (including Coral Blaching HotSpot and Degree Heating Week) a specialized Maximum of the Monthly Mean (MMM) SST climatology was derived from the 12 monthly mean climatologies by taking the highest monthly mean climatology value for each pixel. The MMM SST climatology is static in time but varies in space (Strong et al., 1997).

The 50km-resolution climatologies are available as images and HDF data files on the CRW website.


4. Sea Surface Temperature (SST) Anomaly

SUMMARY: CRW's heritage twice-weekly global 50km SST Anomaly was produced by subtracting the long-term mean SST (for that location in that time of year) from the current value. A positive anomaly meant that the current SST was warmer than average, and a negative anomaly means it was cooler than average. The spatial resolution was 0.5-degree (50km), and the data and images were updated twice-weekly.

CRW's near real-time twice-weekly global 50km SST Anomaly product made it possible to quickly pinpoint regions of elevated SSTs throughout the world oceans. It was especially valuable for the tropical regions where most of the world's coral reef ecosystems thrive. It also was very useful in assessing ENSO (El Niño-Southern Oscillation) development, monitoring hurricane "wake" cooling, observing major shifts in coastal upwellings, etc.

A twice-weekly SST anomaly at a 0.5-degree (50km) grid was calculated by subtracting the daily climatological SST of the last day of the twice-weekly period at that grid from the corresponding twice-weekly SST (described in Sea Surface Temperature Section). The formula for obtaining the anomaly is

      SST_anomaly = SST - Daily_SST_climatology

The color range of temperature anomalies displayed on the SST Anomaly charts is -5.0 to +5.0 °C (or Kelvin). Areas with SST anomaly values less than -5.0 °C are displayed as -5.0 °C, and areas with values greater than +5.0 °C are displayed as +5.0 °C. Note that these anomalies are somewhat less reliable at high latitudes where more persistent clouds limit the amount of satellite data available for deriving accurate SST analysis fields and climatologies.

Data and images, along with the 0.5-degree monthly mean SST climatologies, are available for free on the CRW website at https://coralreefwatch.noaa.gov/product/50km/index.php.

Charts of the retrospective 1984-1998 monthly mean SST anomalies are available online at 36km-resolution.


5. Coral Bleaching HotSpot

SUMMARY: Corals are vulnerable to bleaching when the SST exceeds the temperatures normally experienced in the hottest month. This is shown in the heritage twice-weekly global 50km-resolution Coral Bleaching HotSpot product, which highlighted regions where the SST was warmer than the highest climatological monthly mean SST for that location. The HotSpot value of 1.0 °C is a threshold for heat stress leading to coral bleaching. To highlight this threshold, HotSpot values below 1.0 °C are shown in purple, and HotSpots of 1.0 °C or greater range from yellow to red. Global images and datasets are at 0.5-degree (50-km) resolution and are updated twice-weekly.

CRW's near real-time twice-weekly global 50km Coral Bleaching HotSpot product measured the occurrence and magnitude of heat stress potentially conducive to coral bleaching. It was an anomaly product, but not a typical climatological SST anomaly which is based on the average of all SSTs. The HotSpot anomaly was based on the climatological mean SST of the hottest month (often referred to as the Maximum of the Monthly Mean (MMM) SST climatology) (Liu et al., 2003; Liu et al., 2005; Skirving, 2006). This MMM SST climatology is simply the highest of the monthly mean SST climatologies described in the Climatology section. The Coral Bleaching HotSpot product became available in 1997 (Strong et al., 1997), using a technique based on earlier work by Goreau and Hayes (1994). Glynn and D'Croz (1990) showed that temperatures exceeding 1 °C above the usual summertime maximum are sufficient to cause stress to corals. Based on this study, the MMM SST climatology was derived as a threshold for monitoring coral bleaching.

The HotSpot value shows the difference between the measured global 0.5-degree (50km) near real-time nighttime satellite SST analysis field and the MMM SST climatology:

      HotSpot = SST - MMM_SST_climatology

Only positive values were derived, since the HotSpot was designed to show the occurrence and distribution of heat stress conducive to coral bleaching. The range of HotSpots displayed on the charts is 0.0 to +5.0 °C. The HotSpot chart highlights (in yellow to red color) anomalies that are at least 1.0 °C greater than the MMM SST climatology, as studies have shown that bleaching stress occurs when the water temperatures exceed 1.0 °C above the maximum mean summertime temperature (Glynn and D'Croz, 1990). HotSpot values between 0 and 1.0 °C are displayed in light purple to light blue. Areas with HotSpot values greater than +5.0 °C are displayed in the same color as +5.0 °C.

Charts of 1998 50km HotSpots and their animations are available here.


6. Coral Bleaching Degree Heating Week (DHW)

SUMMARY: Mass coral bleaching has been shown to be caused by prolonged periods of thermal stress. The heritage twice-weekly global 50km DHW product accumulated any HotSpots greater than 1 °C over a 12- week window, thus showing how stressful conditions had been for corals in the prior three months. It was a cumulative measurement of the intensity and duration of heat stress, and was expressed in the unit °C-weeks. DHWs over 4 °C-weeks have been shown to cause significant coral bleaching; values over 8 °C-weeks have caused severe, widespread bleaching and significant mortality. The global data were at 0.5-degree (50km) resolution are were updated twice-weekly.

CRW's near real-time twice-weekly global 50km satellite coral bleaching DHW measured the accumulation of heat stress that coral reefs experienced over the prior 12 weeks (3 months), up to and including the most current product update. While the Coral Bleaching HotSpot provides an instantaneous measure of the thermal stress, there is evidence that corals are sensitive to an accumulation of thermal stress over time (Glynn and D'Croz, 1990)). In order to monitor this cumulative effect, a thermal stress index, the coral bleaching DHW, was developed by CRW in 2000 (Liu et al., 2003; Liu et al., 2005). Glynn and D'Croz (1990) showed that temperatures exceeding 1 °C above the usual summertime maximum are sufficient to cause stress to corals. This is commonly known as the bleaching threshold temperature. Based on our definition of Coral Bleaching HotSpot (see the Coral Bleaching HotSpot section above for more detail), only HotSpot values that were =>1 °C were accumulated. For example, two DHWs is equivalent to one week of HotSpot values at 2 °C, or two weeks of HotSpot values at 1 °C, etc.

Note that since the heritage 50km DHW was a 12-week accumulated HotSpot, it is possible for a location to have a non-zero DHW value when the HotSpot value was less than 1 °C and even 0 °C. This condition simply meant that there had been heat stress at that location within the last three months, but the local conditions were not currently stressful for corals. Exposure to the previous heat stress may still have had adverse impacts on the corals, although recovery also may have been underway.

A half-week approach was used because CRW's heritage near real-time 50km satellite coral bleaching monitoring products were updated twice-weekly. With this approach, the DHWs were accumulated based on twice-weekly HotSpots using the following formula,

      DHWs = 0.5 * Summation of previous 24 twice-weekly HotSpots,

where HotSpots have to be at least 1.0 °C to be accumulated. For example, if we have consecutive twice-weekly HotSpot values of 1.0, 2.0, 0.8 and 1.2 °C, the DHW value will be 2.1 °C-weeks because 0.8 °C is less than one and therefore does not contribute to the accumulated value.

The range of DHW displayed on the charts is 0.0 to 16.0 °C-weeks. In the chart, any area with DHW values greater than 16 °C-weeks is displayed in the same color as 16 °C-weeks.

Field observations (many of which are subjective measurements, presented as informal reports, and are not calibrated/validated with corresponding satellite data) have indicated that there is a correlation with significant bleaching in corals when DHW values of 4 °C-weeks have been reached. By the time DHW values reach 8 °C-weeks, severe, widespread bleaching is likely and significant mortality can be expected. Since its inauguration in 2000, the heritage 50km DHW product successfully generated satellite bleaching warnings and alerts (e.g., Goreau et al., 2000, Wellington et al., 2001; Strong et al., 2002; Liu et al., 2005; Coral Reef Watch, 2003; Liu et al., 2003; Skirving et al., 2006; and Eakin et al., 2010).

The timing of the peak bleaching season varies among ocean basins and hemispheres but it is generally during the local summertime. Thus, the peak season is July-September for the northern Atlantic and Pacific Oceans, and January-March for the southern Atlantic and Pacific. The peak is April-June for the northern Indian Ocean and January-April for the southern Indian Ocean.

Charts of retrospective 1998-1999 50km 3-month DHWs are also available here.


7. Bleaching Alert Area

SUMMARY: These heritage twice-weekly global 50km maps summarized the current DHW and HotSpot values at the time. At a glance, this product outlined the location, coverage, and potential risk level of the current bleaching heat stress. Alert levels use the same definition as our Satellite Bleaching Alert email system, but in the Bleaching Alert Area product every pixel had an alert level defined and color-coded. Global data were at 0.5-degree (50km) resolution and were updated twice-weekly.

CRW's near real-time twice-weekly global 50km Coral Bleaching Alert Area product outlined the areas where bleaching heat stress currently (at that time) reached various bleaching stress levels, based on satellite SST monitoring. The stress levels defined in the table below were based on current values (at that time) of the Coral Bleaching HotSpot and DHW products.

Stress Level     Definition     Effect
No Stress
Bleaching Watch
Bleaching Warning
Bleaching Alert Level 1
Bleaching Alert Level 2
    HotSpot <= 0
0 < HotSpot < 1
1 <= HotSpot and 0 < DHW < 4
1 <= HotSpot and 4 <= DHW < 8
1 <= HotSpot and 8 <= DHW
    --
--
Possible Bleaching
Significant Bleaching Likely
Severe Bleaching and Significant Mortality Likely

Note that if a location had a status level of "No Stress" or "Bleaching Watch," it was still possible for the corresponding DHW value to have been greater than 0 °C-week. This condition simply means that there had been heat stress at that location sometime over the prior 3 months, but local conditions were not currently stressful for corals. Previous heat stress exposure still may have had adverse impacts on the corals, although recovery also may have been underway.

As noted previously, the timing of the peak bleaching season varies among ocean basins and hemispheres but it is generally during the local summertime. Thus, the peak season is July-September for the northern Atlantic and Pacific Oceans, and January-March for the southern Atlantic and Pacific. The peak is April-June for the northern Indian Ocean and January-April for the southern Indian Ocean.


8. Coral Bleaching Virtual Stations

SUMMARY: CRW's heritage twice-weekly global 50km Coral Bleaching Virtual Stations focused the global satellite data products on select coral reef sites around the world. Users can think of these as virtual SST buoys; no instrumentation is needed in the water because all of the data are derived from satellite measurements. The Virtual Stations provided near real-time information for each site: current (at the time) heat stress status, current DHW value, historical maximum DHW value for the site, current SST, and the the MMM climatologies for that site. The data were updated twice-weekly. CRW produced 227 official Virtual Stations and 26 additional Stations in near real-time, prior to the product's retirement.

CRW's heritage twice-weekly global 50km Coral Bleaching Virtual Stations provided near real-time satellite monitoring information on heat stress conducive to coral bleaching for 227 reef sites (officially), as well as 26 additional reef sites around the world (Liu et al., 2001). The information was extracted from near real-time satellite global SST measurements and derived indices of coral bleaching-related heat stress (see the sections above on SST, SST Anomaly, Coral Bleaching HotSpot, and DHW for more details) from 0.5-degree (50km) water pixels surrounding or close to the reef sites. Information listed for each reef site included the reef site name, current (at that time) heat stress status, current DHW value in °C-weeks, historical maximum DHW, current SST value in degrees Celsius, and the MMM SST climatologies value. A map showing a particular reef site (Virtual Station) and its satellite pixel is accessible by clicking on the reef name. The map page also provided links to other coral bleaching heat stress monitoring products, inclduing the free, automated Satellite Bleaching Alert e-mail system.

The five status levels of heat stress shown on the Virtual Stations web page were the same as those for the Bleaching Alert Area product, defined as:

Stress Level     Definition     Effect
No Stress
Bleaching Watch
Bleaching Warning
Bleaching Alert Level 1
Bleaching Alert Level 2
    HotSpot <= 0
0 < HotSpot < 1
1 <= HotSpot and 0 < DHW < 4
1 <= HotSpot and 4 <= DHW < 8
1 <= HotSpot and 8 <= DHW
    --
--
Possible Bleaching
Significant Bleaching Likely
Severe, Widespread Bleaching and Significant Mortality Likely

These levels were defined in terms of the HotSpot and DHW values. When low heat stress was present at a reef site (0 °C < HotSpot < 1 °C) a Bleaching Watch was posted for the site. A triangular warning icon was also added to that reef site, and the status was displayed in red text. A Bleaching Warning was posted when the HotSpot => 1 °C. At this point, DHWs had been accumulating and a larger triangular icon was displayed. A DHW accumulation of 4 °C-weeks triggered a Bleaching Alert Level 1, and the status was displayed in bold red text. At Bleaching Alert Level 1, significant bleaching was expected at the site within a few weeks of the alert. An accumulation of 8 °C-weeks triggered a Bleaching Alert Level 2, at which point severe, widespread bleaching and significant coral mortality were likely.

Please note that since the DHW was a 12-week accumulation of Coral Bleaching HotSpots, it was possible for a location to have had a non-zero DHW value when the HotSpot value was less than 1 °C or even 0 °C. Hence, at a status level of "No Stress" or "Bleaching Watch," it was possible for the corresponding DHW value to have been greater than 0 °C-week. This condition simply meant that there has been heat stress at that location sometime within the prior 3 months, but the local conditions were not currently (at that time) stressful for corals. Previous heat stress exposure still may have had adverse impacts on the corals, although recovery also may have been underway.

Click here to access the heritage 50km Virtual Stations (both the official and additional Stations).


9. SST and DHW Time Series Graphs and Data for Virtual Stations

SUMMARY: Time series graphs show the satellite SST, DHW, and heat stress condition, since 2000 and until the date of retirement of the product, at CRW's Virtual Stations. The SST climatology for each month, the MMM SST Climatology, and the bleaching threshold temperature were plotted in the central portion of the graphs. In the bottom section of each graph, there was a separate plot of DHW and bleaching alerts for that reef location. There were two types of time series graphs available: single-year graphs and overlapping multi-year graphs. Also available were time series data for CRW's SST, SST Anomaly, HotSpot, and DHW products in ASCII text. These graphs and data were updated twice-weekly until retirement of the product.

Time series graphs show the CRW satellite SST, DHW, and heat stress condition, since 2000 and until the date of retirement, at CRW's twice-weekly global 50km Coral Bleaching Virtual Stations. The values were extracted from the same dataset used for making the Virtual Stations. Additional information can be found here.

In the single-year graphs, the monthly mean SST climatologies (light-blue crosses) were plotted on the charts to show the "normal" SST condition at the site and the time of climatologically warmest months. The MMM SST Climatology value (horizontal dashed light-blue line) was the warmest of the twelve monthly mean SST climatologies. The Coral Bleaching Threshold SST (horizontal solid light-blue line) was defined as the MMM SST + 1 °C. Both the MMM SST and Coral Bleaching Threshold SST are location-specific. DHWs, in units of °C-weeks on the right-hand axis, indicated the accumulation of heat stress whenever the SST equaled or exceeded the coral bleaching threshold SST at the pixel during the 12 weeks up to the given time of the data.

In each time series graph, the corresponding heat stress condition (see the table in the Coral Bleaching Virtual Stations section) related to coral bleaching was color-coded and plotted in a bar at the bottom of the time series graphs. The thermal condition was categorized in the five bleaching alert levels. The area below the DHW time series was also filled with colors corresponding to the color- coded bleaching alert levels whenever bleaching related heat stress was present. At Bleaching Alert Level 1, significant bleaching was expected at the site within a few weeks of the alert. An accumulation of DHW of 8 °C-weeks triggered a Bleaching Alert Level 2, at which point severe, widespread bleaching and significant coral mortality were likely.

The multi-year graphs, with time axis covering only 12 months, overlap time series from different years for a Virtual Station. In the multi-year graphs, the time series of the bleaching alerts was plotted only for the current year. They provided a convenient way to compare the time series between years.

The time series graphs and data integrated and delivered the most comprehensive site-specific information available from CRW's heritage twice-weekly global 50km satellite coral bleaching heat stress monitoring products.


10. Satellite Bleaching Alert (SBA) E-Mail System

SUMMARY: CRW's heritage twice-weekly global 50km SBA e-mail system was a free, automated system that alerted subscribers when coral reefs were at risk for bleaching. The alerts were available for the 227 official Virtual Stations only, and were based on CRW's measurements of stressfully high SSTs from satellites. The alerts were updated twice-weekly.

CRW's heritage twice-weekly global 50km SBA product was a free, automated e-mail alert system designed to monitor the status of heat stress conducive to coral bleaching via the use of CRW's heritage twice-weekly global 50km satellite coral bleaching heat stress monitoring products. The SBA was developed as a tool for coral reef managers, scientists, and other interested individuals. The SBA became operational in July 2005 for the original 24 operational 50km Virtual Stations. Before the SBA e-mail system was transitioned to provide alerts for CRW's next-generation daily 5km Regional Virtual Stations, it provided alerts for 227 official Virtual Stations around the world.

An automated e-mail was sent to a subscriber for one or more reef sites only when the heat stress status level changed, regardless of the current status level. The heat stress status, described in the Coral Bleaching Virtual Stations section, was evaluated twice-weekly. All of the information available on the Virtual Stations web page was included in the SBA emails; internet links to time series graphs and global/regional images were also provided, along with the previous bleaching alert levels experienced at the site.

The SBA was a convenient data delivery system that allowed critical information about CRW's heritage 50km products to reach a user's desktop as soon as the information became available, without the user having to manually check the CRW website for updated information.

Color-coded alert levels from the SBA e-mail system were also plotted on the SST/DHW time series graphs.


11. Heritage 50km Satellite Coral Bleaching Monitoring Source Data

The source data for CRW's heritage twice-weekly global 50km coral bleaching heat stress monitoring products are available on the CRW website for free download and use. For the 50km products, data are available in the Hierarchical Data Format (HDF) via FTP, HTTP, and OPeNDAP, and THREDDS servers. Preview images (graphic displays) of the data are also provided.

Additionally, a NOAA CoastWatch Utilities Tool can be used for visualizing the twice-weekly global 50km product data, viewing data information and values, calculating certain statistics, creating graphic output, etc. Click here to access the CoastWatch software tool. The software is easy to install and use and is customized for CRW's HDF data files. This software is not required to visualize and manipulate the data; many commonly used computer languages and software packages can read and process HDF files.


12. Tutorial for Heritage Twice-Weekly Global 50km Products

CRW developed a tutorial to provide background information on satellite remote sensing, coral bleaching, and its heritage suite of twice-weekly global 50km coral bleaching monitoring products. The tutorial was written in layman's language and aimed for both the coral reef management and research communities and the general public.


13. References

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