Description for 5-km Regional Virtual Stations, Time Series Data, and Graphs
(Updated May 8, 2017*)

Product Overview:

NOAA Coral Reef Watch (CRW) has developed a new set of experimental 5-km Regional Virtual Stations (212 total). The Regional Virtual Stations represent a change in the way we have looked at Virtual Station data in the past. They 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. We realized early on that data from a single 5-km pixel (satellite data grid point) provide much higher spatial detail but are less representative of a region's thermal conditions. Also, many coral reef ecosystem managers wanted to know what was happening across their entire jurisdiction. Rather than constructing Virtual Stations based on single 5-km pixels as we did in our heritage 50-km Virtual Stations, we created Regional Virtual Stations based on data from all of the 5-km grids within each individual jurisdiction (e.g., Main Hawaiian Islands). An alert for a region is meant to inform users that they should look at the map products for more detail. These Regional Virtual Stations will be used in a series of our products including:

  • Regional Thermal Stress Gauges
  • Two-year time series graphics
  • Multi-year time series graphics
  • Virtual Stations map with data overlays in a Google Maps interface
  • Virtual Stations map with data overlays in a Google Earth file
  • Email Bleaching Alerts (one alert per region, sent out twice-weekly, free to subscribers)


Design of Regional Virtual Stations:

The Regional Virtual Stations were designed using a 5km global reef mask that was developed by CRW using available 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. The reef mask is updated as new information on reef locations are added periodically. Reef locations were separated into 212 individual stations covering most of the global coral reef regions and following country boundaries and island nation groups where possible. Reef pixels within each jurisdiction were given a 20km buffer indicating the boundary of the individual Regional Virtual Station. The size of these regions varies greatly, but the methodology for calculating alerts within each region is held consistent.


Calculating Bleaching Alerts:

The recent development of Coral Reef Watch's (CRW) 5-km satellite-based coral bleaching product suite (Version 3) has led to changes in the way we calculate Regional Virtual Station alerts. A time series of data is generated for each Regional Virtual Station based on data that fall within the boundary of the region. Rather than provide every value of every data point within the boundary of the station, we derive new Degree Heating Week values and Bleaching Alerts based on the 90th percentile HotSpot value. This allows for each station's data to be internally consistient and keeps a few stray warm pixels from falsely exaggerating the alert levels. These values, as well as sea surface temperature ranges, and climatolgoy information are stored in an ASCII text file and accessed by the Thermal Stress Gauge pages and email alert system.


ASCII Data File Description:

Click to see time series data

The time series data file for the Regional Virtual Stations is in ASCII text format. A sample file for the main Hawaiian Islands can be accessed by clicking the image above. Time series data are updated daily concurrently with CRW's 5-km products. These data are used to create the time series graphs found on the Regional Virtual Station pages. The first 22 rows of each file is the header containing the name of the region, the polygon middle latitude and longitude representing the icons on the Google Maps and Google Earth interfaces, the first valid Degree Heating Week and Bleaching Alert Area dates, and the region-specific climatological data described below:

Averaged Maximum Monthly Mean:
The maximum value across the twelve average 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.

Averaged 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.

First Valid DHW and BAA date:
The Degree Heating Week product for Regional Virtual Stations is an accumulation of the 90th percentile HotSpot values within each station. Beacuse it is an accumulation over a 12 week period, there can not be a DHW value until 12 weeks of HotSpots have occured. Therefore the first valid DHW date is later than the first valid SST and HotSpot date. The same goes for the Bleaching Alert Area product, which has a slightly later start date, due to the fact it is a 7-day maximum composite.

The last line of the header describes the time series data that follows. There are 10 fields in each record described in detail below:

YYYY, MM, DD:
These are the year, month, and day, respectively, of the data.

SST_MIN and SST_MAX:
The minimum and maximum daily SST value for pixels contained in a Regional Virtual Station. This shows the dynamic range in SST values within a Regional Virtual Station and is a reference for the size and variability of oceanographic conditions of a station.

SST@90th_HS:
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 that falls within the SST_MIN and SST_MAX range. This value is used in the information balloons in the Google Maps and Google Earth interfaces.

SSTA@90th_HS:
The daily SST anomaly 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 temperature anomalies as indicated by the HotSpot value for that day. This value is used in the information balloons in the Google Maps and Google Earth interfaces.

90th_HS>0:
The daily 90th percentile HotSpot value (positive only) 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. This value is used in the information balloons in the Google Maps and Google Earth interfaces. This value is accumulated over time to calculate the Degree Heating Weeks and Bleaching Alert Area values for each Regional Virtual Station.

DHW_from_90th_HS>1:
The daily Degree Heating Week value calculated by accumulating daily 90th percentile HotSpot values (greater than 1) for pixels contained in a Regional Virtual Station. This value is used in the information balloons in the Google Maps and Google Earth interfaces. This value is represented as a solid red line on the time series graphs.

BAA_7day_max:
The 7-day maximum Bleaching Alert value, calculated using the 90th percentile HotSpot value and newly calculated Degree Heating Week value. The algorithm for daily Bleaching Alert values follows CRW's existing algorithm, but uses the unique HotSpot and DHW value for each individual virtual station. From these daily Bleaching Alert values, the 7-day rolling maximum is calculated and used to color the area below the DHW trace on the time series graph. These alert values are used to change the thermal stress gauges and send email alerts to subscribers.

Virtual Station Time Series Graph Description

Click to see time series data

The Virtual Station time series graphs display the values from the ASCII text files described above from March 12, 2013 to the present. Currently, two types of graphs are provided: two-year graphs (click on the sample graph above) and multi-year graphs. Both graphs were created from the same data accessible from CRW's Virtual Stations data 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 drawn in thick black solid lines; the past year is drawn as a thin gray line. All other years are drawn 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 drawn along the horizontal axis. On the multi-year graphs, the bleaching thermal stress level is drawn 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
    Definition

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 drawn 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.

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


*Updates to virtual station products
May 08, 2017
- Release of Version 3 5km product suite.
- Change header of time series data file.
- Change DHW and BAA calculation method.
- Updated description page with new methodology.
- Added 5km SST range to 2-year time series graphs.
- Updated 2-year time series graph axes titles.
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.