Description for 5-km Regional Virtual Station Time Series Data & Graphs
(Updated May 19, 2015*)

Click to see the full-sized two-year time series graph   Click to see the full-sized multi-year time series graph

The recent development of Coral Reef Watch's (CRW) 5-km Regional satellite-based coral bleaching product suite has led to changes in the way we represent time series data for each station. The new Regional Virtual Stations have been designed to take advantage of higher resolution data while simultaneously providing comprehensive and collective information on all reefs in a jurisdiction or predetermined sub-region. Rather than constructing Virtual Stations based on single 5-km pixels as we did in our heritage 50-km Virtual Stations, we created the Regional Virtual Stations based on data from all of the 5-km grids within reef containing areas of each individual jurisdiction (e.g., Main Hawaiian Islands). You can see those pixels on the region's Thermal Stress Gauge page or download the shape file from the 5-km Regional Virtual Station page.

To represent the status of each Regional Virtual Station through time we needed to first calculate a representative climatological value for that region. This was done by using the average value of all of the maximum monthly mean SSTs for reef areas (primarily pixels within 20 km of known reefs) within the region. The same approach had to be done for Sea Surface Temperature as well since one representative value is needed to track changes through time. For this, we used the SST value where the HotSpot value was equal to the 90th percentile HotSpot for a particular region. In other words, every day we first look at the HotSpot values within a particular region and find the index of the 90th percentile HotSpot value. We apply that index to the SST values for the same region and day to get one single SST value out of the entire region. This allowed for the location of the SST data value to potentially migrate around the region depending on where the 90th percentile HotSpot fell on each day. Degree Heating Week values were selected as the 90th percentile value through time. Using this method, the data in the ASCII text file and corresponding time series graph are new and do not correspond directly with any single data pixel on a map through time. This is a recognized limitation to represent regions of differing sizes in a consistent way across the globe. Actual data from all pixels can be downloaded from our dataset page in netCDF format.

ASCII Data File Description:

The time series data file for the Regional Virtual Stations is in ASCII text format. Time series data are updated daily in the afternoon (U.S. Eastern Time) concurrently with CRW's 5-km products. The first 16 rows of each file is the header containing the name of the region, the latitude and longitude representing the icons on the Google Maps and Google Earth interfaces, and the region-specific climatological data described below:

Maximum Monthly Mean:
The maximum value across the twelve monthly mean SSTs for pixels contained in a Regional Virtual Station. This value is represented as a dashed light blue line on the time series graphs. The bleaching threshold (solid light blue line) is 1 degree Celcius greater than this value.

Monthly Mean (Jan-Dec):
The average of the monthly mean SSTs for pixels contained in a Regional Virtual Station. These values are represented as light blue "+" symbols on the time series graphs. The maximum of these values represents the Maximum Monthly Mean for a particualr Regional Virtual Station.

The last line of the header describes the time series data that follows. There are 9 fields in each record described in detail below:

These are the year, month, and day, respectively, of the data.

The daily SST value where the HotSpot value is equal to the 90th percentile HotSpot value for pixels contained in a Regional Virtual Station. This acts as a moving pixel within a region that is free to migrate with each data update. It follows thermal stress as indicated by the HotSpot value for that day. These values are represented on the time series graphs as a solid dark blue line.

These values are the 90th percentile (or highest 10% cutoff) values of each of the corresponding 5-km products (SST, SST Anomaly, HotSpot, DHW) within each Regional Virtual Station region. They are used in the information balloons in the Google Maps and Google Earth interfaces. DHW is represented by the red line on the time series graph and read using the right axis.

The Bleaching Alert Area when 10% or more of the pixels in a region reach a certain thermal stress level. For example, if 8% of the 5-km satellite pixels in a predetermined region are at Alert Level 1 and 3% are at Alert Level 2, the Bleaching Alert Area is set to Alert Level 1. These values are calculated from the 7-day maximum Bleaching Alert Area product (updated daily) and are what drive the Bleaching Thermal Stress Gauges. The stress level is indicated by the colors below the DHW line on the time series graph.

Virtual Station Time Series Graph Description

The Virtual Station time series graphs display the representative SST and DHW at CRW's Virtual Stations from March 12, 2013 to the present. Currently, two types of graphs are provided: two-year graphs (click on the sample graph on the left, above) and multi-year graphs (click on the sample graph on the right, above). Both graphs were created from the same data accessible from CRW's Virtual Stations web page. The multi-year graphs provide a convenient way to compare the changes in SST and DHW through the annual cycle, among years, while recent SST and DHW variations can be examined easily on the two-year graphs. These graphs and corresponding data (accessible from the Virtual Station web pages) depict the historical record of regionally representative SST and coral bleaching thermal stress measurements at each Virtual Station.

The representative SST value is shown in the center of graphs and read using the vertical axis on the left. The DHW is shown in the bottom portion of the graphs and read using the vertical axis on the right. In the multi-year graphs, the SST and DHW time series for the current year are plotted in thick black solid lines; the past year is plotted as a thin gray line. All other years are plotted on a spectrum of colors from blue to red where red is more recent and blue the start of the data.

The coral bleaching thermal stress level (or Bleaching Alert Level) is color-coded and plotted along the horizontal axis. On the multi-year graphs, the bleaching thermal stress level is plotted for the current year only. Bleaching thermal stress is categorized into five bleaching risk levels (defined below) based on the Coral Bleaching HotSpot and Degree Heating Week values experienced at the Virtual Stations.

Stress Level

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

At Bleaching Alert Level 1, significant bleaching is expected within a few weeks of the alert. At Bleaching Alert Level 2 and above, widespread bleaching and significant coral mortality are likely. Red dashed lines across the graphs indicate DHW threshold values of 4- and 8-degree Celsius-weeks (triggers for Bleaching Alert Levels 1 and 2, respectively). The color of the alert level represents the maximum level of the last 7 days.

Three types of information describing the long-term mean conditions for a Regional Virtual Station site are also provided on each graph: the monthly mean climatology, the MMM, and the bleaching threshold SST. Monthly mean SST climatologies (blue crosses) are also plotted to show "normal" SST conditions at each Virtual Station; the peak of the bleaching season typically falls during, or immediately following, the warmest months of the climatology. The time of peak bleaching varies among ocean basins and hemispheres. Generally, for the northern Atlantic and Pacific Oceans, it is July-September; for the southern Atlantic and Pacific Oceans, January-March. The peak season for the northern Indian Ocean is April-June and for the southern Indian Ocean, January-April.

The newly calculated MMM SST climatology (Averaged Maximum Monthly Mean in the data file) is featured as a dashed light-blue line on the graphs. It is the warmest of the twelve monthly mean SST climatology values (Averaged Monthly Mean in the data file). The Coral Bleaching Threshold SST (solid light-blue line) is defined as the MMM SST plus one degree Celsius. The climatology, MMM SST, and Coral Bleaching Threshold SST are region-specific in the 5-km products.

The Virtual Station time series were extracted and derived from 5-km satellite data pixels found primarily within 20 km of reef sites identified in available reef location references. These included the Millennium Coral Reef project maps, the IUCN Coral Reefs of the World three-volume set, the UNEP/WCMC World Atlas of Coral Reefs, several country scale atlas publications, and a few other resources. These references were also used to develop the outline (in black) for each new 5-km Virtual Station. Each Virtual Station outline is based on a global 5-km reef pixel mask developed by NOAA CRW, with the addition of a 20-km buffer around each 5-km reef mask. If we have missed a coral reef that you know of, please let us know the name and coordinates of the missing reef.

Note that all 5-km Regional Virtual Stations also can be accessed using Google Earth.

*Updates to virtual station products
May 19, 2015
- Changed name of "Averaged Maximum Monthly Mean" to "Maximum Monthly Mean".
- Updated description of Maximum Monthly Mean to better represent the definition.
- Changed name of "Averaged Monthly Mean (Jan-Dec)" to "Monthly Mean (Jan-Dec)".
Mar 27, 2015
- Changed representative climatological value from 90th percentile to the average.
- Use 90th percentile DHW value in time series graphs.
- Use the maximum 10% of the 7-day maximum running bleaching alert area product on
  the time series figures.
- Changed ASCII text file headers and removed old columns HS_TS and DHW_TS.
  "MMM_TS" is now "Averaged Maximum Monthly Mean"
  "MONTHLY_TS (JAN-DEC)" is now "Averaged Monthly Mean (Jan-Dec)""
  "SST_TS" is now "SST_AT_HS_90TH"
  "SST_10", ... "DHW_10" are now "SST_90TH", ... "DHW_90TH"
  "BAA_10" is now "BAA_MAX_10"
- Changed bleaching alert color scale on 2-year and multi-year time series graphs.
- Changed past year color scheme for multi-year time series graphs.
Feb 17, 2015
- Initial release.