Heat Stress Gauge and Time Series Product Description

The purpose of these Regional Coral Bleaching Heat Stress Gauges is to provide coral reef ecosystem managers with a comprehensive summary of current satellite-monitored and model-projected bleaching thermal stress conditions to help facilitate timely and effective management actions pertaining to mass coral bleaching.

NOAA Coral Reef Watch (CRW) has developed a set of Heat Stress Gauges to reflect the observed and forecasted bleaching alert level surrounding select islands or reefs. The areas chosen for monitoring are designed to match the 5 km Regional Virtual Station boundaries outlined in black. These gauges are based on CRW's Regional Virtual Station time series data, updated daily, and 0.5-degree Climate Forecast System (CFS)-based Four-Month Coral Bleaching Thermal Stress Outlook, updated weekly.

CRW provides four gauges per Virtual Station that include the current near-real-time coral bleaching thermal stress alert level and the projected alert level for three consecutive 4-week time periods (i.e., the upcoming 1-4 weeks, 5-8 weeks, and 9-12 weeks, hereafter referred to as 4, 8, and 12 weeks, respectively). A map of the region surrounding the island or coastline of interest accompanies the gauges. The image(s) show either the current 7-day maximum Bleaching Alert Area, the current Four-Month Coral Bleaching Outlook for 4, 8, or 12 weeks out, or a composite of the four depending on the user's selection. The gauge corresponding to the map is outlined in black.

Due to the regional nature of the virtual stations, the level displayed on each gauge is a derived value that is not taken from any one data point within the virtual station area. To derive alerts, the daily 90th percentile HotSpot values within the station boundary are used to calculate new Degree Heating Week Values unique to each Regional Virtual Station. From the 90th percentile HotSpot values and newly calculated DHW values, a new Bleaching Alert Area value can be calculated using the same heat stress level table as the standard 5 km Bleaching Alert Area product. The daily 90th percentile HotSpot value and unique DHW and BAA values are stored in the time series data files accessible on the all stations and products page. This methodology keeps the Heat Stress Gauges, Time Series Graphs and Bleaching Email Alerts internally consistent and prevents a few stray warm pixels from exaggerating bleaching risk while reflecting the greatest thermal stress impacting the reefs in the region.

The CFS-based Outlook product used is the 60% likelihood of bleaching thermal stress from the latest Four-Month Coral Bleaching Thermal Stress Outlook. Outlook values displayed are the maximum of a composite of each corresponding 4-week period (i.e., the maximum value from weeks 1-4, 5-8, or 9-12 of the Outlook). A small gray arrow is sometimes present on a gauge indicating a change from the previous gauge reading upon updating. If the arrow is not present then there has been no change since the last update. The current alert gauge and image updates daily while the outlook gauges and images update weekly.

For the current alert level, the area of interest is specified by a black outline surrounding the island(s) or known reef locations on the map. For the 4-, 8-, and 12-week Outlooks, the area of interest is the entire extent of the corresponding map. This increases the area used for the gauge level as one moves from the Bleaching Alert Area to the 4-, 8-, and 12-week Outlooks. The areas covered by the 8- and 12-week Outlooks increase by 2 degrees in every direction from the previous Outlook. The increase in area coverage as the outlook lead-time increases for the gauge calculation is designed to help coral reef managers understand the extent of potentially approaching offshore thermal stress, with increased projection uncertainty factored in over longer time scales. Incorporating an increase in the area at these predefined, projected, time-dependent spatial scales should capture thermal stress events that may contribute to bleaching risk at the depicted coral reefs. In some special cases (i.e, Central America) the extent of the outlook map may encompass two ocean basins that should not collectively be considered when determining the outlook alert. In these cases, a dark gray mask is drawn over the basin that is excluded when calculating the outlook alert to display on the gauges.

The time series graph provides a record of temperature, thermal stress, and bleaching potential and allows for comparisons between years. The SST trace shown depicts the SST value where the HotSpot value is equal to the 90th percentile HotSpot. The minimum and maximum SST range within the Regional Virtual Station boundary is shown as a light gray area behind the SST trace. Click here to access a more detailed description of the time series graphs and data.