El Nino Watch
Accumulated heat stress on corals over the last 12 weeks, in °C-weeks. 4 = risk of reef-wide bleaching; 8 = bleaching with deaths of heat-sensitive corals likely.
Ocean
Corals are stressed when the water is more than 1 °C above the warmest monthly average they normally see. NOAA calls that excess a HotSpot.
DHW adds up the HotSpots over the last 12 weeks. Example: 2 weeks at +2 °C above the threshold = 4 °C-weeks. So DHW grows with both how hot and how long.
NOAA's alert levels (Bleaching Alert Area, updated with Levels 3-5 in December 2023) are in the table.
Bleaching hurts Samui's diving, snorkelling, fishing and tourism. It is not a direct safety threat, so it only counts as context in the risk level.
| NOAA level | Definition | Potential impact |
|---|---|---|
| No Stress | HotSpot <= 0 | No bleaching |
| Bleaching Watch | 0 < HotSpot < 1 | Bleaching unlikely |
| Bleaching Warning | HotSpot >= 1 and 0 < DHW < 4 | Risk of possible bleaching |
| Alert Level 1 | HotSpot >= 1 and 4 <= DHW < 8 | Risk of reef-wide bleaching |
| Alert Level 2 | HotSpot >= 1 and 8 <= DHW < 12 | Reef-wide bleaching with mortality of heat-sensitive corals |
| Alert Level 3 | HotSpot >= 1 and 12 <= DHW < 16 | Risk of multi-species mortality |
| Alert Level 4 | HotSpot >= 1 and 16 <= DHW < 20 | Severe multi-species mortality (> 50% of corals) |
| Alert Level 5 | HotSpot >= 1 and DHW >= 20 | Near complete mortality (> 80% of corals) |
HotSpot and DHW in °C and °C-weeks. In this app, DHW >= 4 gives the marine heat factor level 1 (the factor is capped at 1 in the overall level).