CRW Experimental Bleaching Outlook Demonstration

Managers and researchers frequently request some indication of whether ocean waters are likely to warm or cool during upcoming weeks or if the upcoming season is likely to result in bleaching. CRW’s bleaching outlook will serve as one of such tools to the coral reef community.

Our retrospective bleaching outlook for the 2005 Caribbean mass coral bleaching event is used here as a demonstration. In the demonstration, we will focus on the eastern Caribbean region where the record-breaking thermal stress event was concentrated. First, the archived CRW 2005 near-real-time satellite coral bleaching HotSpot and Degree Heating Week observations are examined for identifying the onset and development of the bleaching thermal stress in the eastern Caribbean region throughout the summer of 2005. Then the bleaching outlooks with lead times of 2, 4, and 12 weeks are presented and compared with the HotSpot and DHW observations.

The CRW operational HotSpot and DHW are at 0.5 by 0.5 degree spatial resolution and twice-weekly temporal resolution. The bleaching outlook is currently produced at 2x2 degree spatial resolution and weekly temporal resolution to agree with the resolution of the SST forecasts

1. Onset of the thermal stress

CRW’s operational twice-weekly HotSpot observation shows that well organized HotSpot started to appear in the southern and eastern Caribbean regions around May 28, 2005 (Fig. 5). On May 31, twice-weekly DHW started to appear over these regions (Fig. 5).

Two-week bleaching outlook shows the thermal stress would start to spread in the southern and eastern Caribbean regions during the 7-day period centered on June 5 (Fig. 6). The 4-week outlook shows the thermal stress would start to appear during the 7-day period centered on June 19 (Fig. 6). Although the appearance of the large-scale stressed area in the two outlooks is one week and three weeks later than the observation, isolated stressed grids start to appear in the two outlooks from May 15 and 29, respectively.

The 12-week outlook shows the initiation of the thermal stress at various locations across the whole region with a large patch in the eastern Caribbean during the 7-day period centered on July 24 (Fig. 6). This is much later than the onset of the thermal stress outlined by the observation. By July 24, CRW’s near-real-time observation shows that the region was close to the edge of wide-spread coral bleaching. After reading the rest of this demonstration, reader will find that the 12-week outlook, as a seasonal outlook, does capture well the occurrence, major development stages, and the epicenter of the 2005 record-breaking thermal stress event in the Caribbean.

2. Beginning of the thermal stress for wide-spread bleaching

The DHW observation of July 9, 2005 shows that DHW value of around 4 degree-weeks (green grids) started to show in multiple locations east of the eastern Caribbean. This indicates that a wide-spread bleaching was just over the horizon if the thermal stress continued to build up (Fig. 7). Both the 2-week and 4-week outlooks for the 7-day period centered on July 3 show the initiation of the orange-colored areas in the eastern Caribbean region indicating the likelihood for wide-spread bleaching (Fig. 8).

By Aug 21, DHW values of above 4 degree-weeks covered a large area east of the Caribbean in the Atlantic Ocean as well as a significant area in the eastern Caribbean (Fig. 9). This is when the orange-colored area starts to appear in the 12-week outlook (Fig. 10). This 12-week outlook also shows that the bleaching thermal stress covers the whole Caribbean region and the Gulf of Mexico as the observation indicates.

3. Beginning of the thermal stress for severe bleaching

By Sept 12, the twice-weekly DHW observation shows that the accumulation of the thermal stress in a large area of the eastern Caribbean reached to 8 degree-weeks and above, a level related to wide-spread severe bleaching (Fig. 11). All three outlooks for the 7-day period centered on Sept 11 show a large area in the eastern Caribbean is cover in red color for likelihood of severe bleaching (Fig. 12). The spatial distribution of the highest thermal stress levels in the outlook outlines well the pattern shown in the observation. Red-colored grids start to appear in the two- and four-week outlook as early as late-July.

4. Maximum spatial coverage and intensity of the accumulated thermal stress

The maximum spatial coverage of the DHW values of 8 degree-weeks or above in the eastern Caribbean was reached in early-Oct while the DHW value continued to climb up in the region until late-Oct (Fig. 13). All the three outlooks show the maximum spatial coverage of the red-colored severe bleaching area is reached in early-Oct as well (Fig. 14).

5. Dissipation of the thermal stress

The HotSpot started to dissipate around Nov 1 and by Nov 8, all HotSpot values went below the threshold for accumulating DHW (Fig. 15). By Nov 28, the HotSpot observations essentially disappeared in the region. The red- and orange-colored areas disappear on the 7-day period centered on Nov 13 for two- and four-week outlooks and on Nov 6 for the 12-week outlook (Fig. 16). All stress areas disappear in the whole Caribbean region by Nov 27 in all the three outlooks.

6. Gulf of Mexico and the Bahamas

The DHW observation shows that the accumulative thermal stress reached to its highest levels in the Gulf of Mexico and the southern Florida around the end of August and reached to its highest levels in the Bahamas in early September. DHW map of Sept 17 (Fig. 17) shows the maximum levels of DHW both in the Gulf of Mexico and the Bahamas, as DHW is a 12-week accumulation. Fig. 18 shows the three outlooks at their peak 7-day periods around late-August and early-September for the region. The two- and four-week outlooks capture the peak time and the spatial coverage of most of the observed thermal stress in the region, although predicted thermal stress is relatively low for the southern Florida region, including Florida Keys. The 12-week outlook does not capture the high thermal stress level as the observation indicates, but it does outline the thermal stress area quite well.

7. Summary

This demonstration clearly shows that the CRW bleaching outlook products are able to predict the major stages of the occurrence, development, and dissipation of the thermal stress that were conducive to the record-breaking 2005 Caribbean mass coral bleaching event in a great accuracy in terms of both timing and location of the bleaching event. The 12-week (seasonal) outlook tends to underestimate the thermal stress but this may indicates that a different set of thresholds may be need to calculate thermal stress indices for the season outlook or a different interpretation may be needed.