5-km Bleaching Thermal Stress Gauges
Gauge and Time Series Product Description
The purpose of these Regional Coral Bleaching Thermal 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 thermal 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 new 5-km Virtual Station boundaries outlined in black. These gauges are based on CRW's 5-km 7-day maximum
Bleaching Alert Area composite product, 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.
The level displayed on each gauge is calculated using the warmest 10% of the pixels in the displayed region to determine
thermal conditions (No Stress, Bleaching Watch, Bleaching Warning, Alert Level 1, and Alert Level 2) and the values presented
in the time series. For example, if 5% of the pixels for Vanuatu have reached Alert Level 2 and 8% have reached Alert Level 1,
the status for Vanuatu will be Alert Level 1. This methodology also 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 (Version 3) 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 black crosshatch is drawn over the basin that is excluded when calculating the outlook alert to display on the gauges.
The time series graph below the gauge images provides a record of temperature, thermal stress, and bleaching potential and allows for comparisons between years. It also uses the warmest 10% of the pixels within the black outline surrounding the island(s) or known reef locations on the map. Click here for a more detailed description of the time series graphs.