Figure 1 describes two coral bleaching models, the TEL-like and our combined model. The former has SST (Sea Surface Temperature), ENSO, and longitude that directly fed into an MR (Multiple Regression) model to predict bleaching categories. Our model has both local forcing such as seawater temperature (SST) and salinity (SSS – Sea Surface Salinity), and a couple of remote ocean-atmospheric phenomena used as model inputs or predictors. The remote factors are chosen specifically depending on the area where the bleach occurs. They are: El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), Indian Ocean Dipole (IOD), Madden Julian Oscillation (MJO), Monsoon and Sunspot numbers (SSN). These remote forcing, known to affect rainfall patterns over the studied areas, may induce sea water temperature and salinity changes within the areas where corals live (Malmgren et al. 1998; Giannini et al. 2000; Taylor et al. 2002; Rodriguez et al. 2014; Fan et al. 2019; Rensch et al. 2019; Zhuang and Duan 2019; Naidu et al. 2020; Ratnam et al. 2020). These predictors are then processed through a stepwise multiple regression (SWMR), a Principal Component Analysis (PCA), and a decision tree (DT) supervised learning in order to predict coral bleaching categories (see Methods). There are three type of bleaching severities (Donner et al. 2017). They are: category ‘1’ or ‘Mild’ (1–10% bleached), category ‘2’ or ‘Moderate’ (11–50% bleached), and category ‘3’ or ‘Severe’ (> 50% bleached).
Table 1 shows 2 (two) important results. First, significant factors, determined by their standardized coefficient β’s, responsible for bleaching events are different for each bleaching region (Fig. 2). For example, all remote factors but seawater temperature play significant role in the Caribbean coral bleaching events. Seawater temperature has a minor contribution in the Great Barrier Reef and the Coral Triangle bleaching events. It is also found that ENSO, MJO and PDO from distant past and seawater salinity near the time of bleach are strong correlates to coral bleaching. It is important to note that seawater salinity, ENSO and seawater temperature responsible for the Great Barrier Reef bleaching events have different sign at different time lag – this seems to be a trigger mechanism for a bleaching to occur.
Second, using the HSS (Heidke Skill Score) as prediction skill metric, our statistical model developed here remarkably has a much higher skill than that of the extended TEL model except for the Coral Triangle bleaching case (Table 1 and Fig. 2). Our statistical model performance for classifying coral bleaching severities are shown in Fig. 3.
Table 1
Predictors used in the extended TEL and combined models presented in Fig. 1. The models are used for predicting the Caribbean, Coral Triangle, and the Great Barrier Reef (GBR) bleaching event categories. The subscript in each predictor is the lagged in month from the time when the bleaching occurs. For instance ENSO− 12 and ENSO0 means the ENSO value at 12 months before the month of bleaching and the ENSO value at the month of bleaching, respectively. The model’s skill is determined using three metrics: the determination coefficient R2, the forecast accuracy ACC, and the Heidke Skill Score HSS described in section 2.4.
Region/ Events | Predictors | Selected predictors | Prediction skill |
| | | R2 | ACC [%] | HSS |
Caribbean: 67 Mild, 23 Moderate, and 26 Severe events | ENSO0,…, ENSO− 12, SSTo,…,SST− 12, Longitude. | All selected | 0.23 | 50.00 | 0.22 |
ENSO0,…, ENSO− 12, PDO0,…, PDO− 12, NAO0,…, NAO− 12, SSTo,…,SST− 12, SSSo,…,SSS− 12, SSNo, …,SSN− 12. | ENSO− 11 (β = 0.84), ENSO− 10 (β = 0.63), PDO− 11 (β = 0.32),NAO− 3 (β=-0.32), NAO0 (β = 0.21), ENSO0 (β = 0.09), SSS0 (β=-0.04), SSN0 (β=-0.01) | 0.39 | 76.72 | 0.58 |
Coral Triangle: 11 Mild, 14 Moderate, and 27 Severe events | ENSO0,…, ENSO− 12, SSTo,…,SST− 12, Longitude. | All selected | 0.67 | 78.85 | 0.66 |
ENSO0,…, ENSO− 12, IOD0,…, IOD− 12, PDO0,…, PDO− 12, MJO0,…, MJO− 12, Monsoon0,…, Monsoon− 8, SSTo,…,SST− 12, SSSo,…,SSS− 12, SSNo,…,SSN− 12. | ENSO− 12 (β = 0.37), MJO− 11 (β=-0.34), SST− 1 (β=-0.15) | 0.60 | 76.92 | 0.63 |
Great Barrier Reef: 28 Mild, 16 Moderate, and 39 Severe events | ENSO0,…, ENSO− 12, SSTo,…,SST− 12, Longitude. | All selected | 0.78 | 81.93 | 0.72 |
ENSO0,…, ENSO− 12, SSTo,…,SST− 12, SSSo,…,SSS− 12, SSNo,…,SSN− 12. | SSS− 2 (β = 287.20), SSS− 1 (β=-248.20), ENSO− 12 (β=-0.5395), SST0 (β=-0.06), SSN− 1 (β = 0.02), SSN− 4 (β= -0.02) | 0.74 | 85.54 | 0.77 |
Figure 3 shows the classification map based on the DT (decision Tree) algorithm using PCA-1 and PCA-2 scores as inputs for predicting the Caribbean, the Coral Triangle and the Great Barrier Reef coral bleaching events, respectively. Some misclassifications are found in Fig. 3. The Caribbean has 27 misclassifications while GBR and Coral Triangle both have 12 misclassifications. The skill of these combined models could be improved by adding more physical quantities as inputs. They are: water surface currents (DeCarlo and Harrison 2019), water turbidity (Sully and Woesik 2019), water depth (Schmarek et al. 2018), internal waves (Wyatt et al. 2020), the Atlantic Multi-decadal Oscillation (AMO; Mendez and Magaña 2010), and the Atlantic and Pacific ITCZ (Inter Tropical Convergence Zone; Martinez et al. 2019). In addition, biological aspects are also incorporated as model inputs. They are: the composition of corals and their occupants such as symbionts and bacterias (Thornhill et al. 2006; Baker et al. 2013; Smith et al. 2017; Yang et al. 2017; Cunning et al. 2018; Carballo et al. 2019; Rouze et al. 2019).
In spite of its poor predictive capability in terms of the seawater temperature DHW index (McClanahan et al. 2019), the index is still considered as the prime factor for coral bleaching (Palmer 2018; Skirving et al. 2019; Smith and Spillman 2019; Sully et al. 2019). The metric has also been used in coral monitoring (Heron et al. 2016) and coral bleaching early warning (Liu et al. 2018). Beside seawater temperature, there are other stressors responsible for bleaching such as: light (Bellworthy and Fine 2017), seawater salinity (Aguilar et al. 2019) and ocean current (DeCarlo and Harrison 2019). In this work, seawater salinity, ENSO, MJO, PDO, and NAO (see Table 1 and Fig. 2) are more important than seawater temperature on determining coral bleaching severities. It is important to note that these factors are shown to be affected by global warming (Hahn et al. 2018; Llovel et al. 2019; Bui and Maloney 2019; Yan et al. 2020; Li et al. 2020).