WIO climatology regions
Since 2014, bleaching reporting regions are used as defined by SST analysis from the last decade (2003-2009 as ‘background’ years, and 2010 as a year of high bleaching), rather than a longer historical record that includes pre-1998 SSTs. Five SST zones were identified:
A – the hottest region; the East Madagascar Channel and Comoros (reported as SW Madagascar and the NE Madagascar Channel) and the NW Seychelles islands
B – the second hottest region; East African mainland coast from 7-18oS (Zanzibar/Dar es Salaam to Primeiras/Segundas islands), and including the NW Madagascar Channel
C – a moderate/intermediate region; the South Equatorial Current region, comprising the Mascarene Banks, southern Seychelles islands and NE Madagascar
D – the southern cooler regions; SW Indian Ocean islands, E and S Madagascar and S Mozambique and South Africa
E – the cooler northern but highly variable region; the Kenya-Somali coast, including Pemba island and N Tanzania coast (Tanga).
Because of latitudinal variation (e.g. in A) and splitting geographically (e.g. A and D), we report in 8 sub-regions.
Statistical analysis of alerts from 2007-13 indicated that low confidence is attached to an alert of ‘low’ bleaching risk (ie. not zero risk, but not severe). By contrast, predictions of ‘mid’ and ‘high’ risk of bleaching were more reliable. Accordingly, the alert is being presented as:
- ‘watch’ – there indications of warmer conditions that will result in some bleaching
- ‘1’ – medium alert for bleaching
- ‘2’ – high alert for bleaching.
These findings match the categories used by NOAA, with ‘watch’, ‘bleaching level 1’ and bleaching level 2’ categories.
Sea Surface Temperatures (SST)
The surface of the sea heats up by direct insolation, causing stress to corals and other shallow water organisms. Satellites directly measure the skin-temperature of the sea, providing these maps and coral bleaching products for early warning.
The Bleaching Thermal Stress Outlook is based on sea surface temperature (SST) forecasts generated by the Linear Inverse Model from the NOAA Earth System Research Laboratory. In a normal year, the Outlook forecasts no potential for bleaching.
Wind is an important physical factor influencing conditions conducive to coral bleaching. Wind-driven mixing reduces temperature stress and wind generated waves can scatter harmful levels of incoming solar radiation.
- Cyclones – cause strong mixing, reducing SST.
- Doldrums – periods of sustained low wind promote stratification, and heating of the upper layers of water. They therefore promote environmental conditions adverse to corals experiencing thermal and/or light stress.
El Niño/Southern Oscillation (ENSO)
The El Niño/Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon to cause global climate variability on interannual time scales.
- Multivariate ENSO Index (MEI) – Negative values of the MEI represent the cold ENSO phase (La Niña), while positive MEI values represent the warm ENSO phase (El Niño).
- The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin (Note, negative SOI is equivalent to positive MEI).
- The Nino 3.4 index is similar to the SOI, but focused on the central Pacific Niño region, straddling the equator and from 170-120oW. It has been found to be most strongly associated with climatic consequences in the African region, so is used here.
Indian Ocean Dipole
The Indian Ocean Dipole is analogous to the ENSO, but for the Indian Ocean. It is calculated using the Dipole Mode Index (DMI), which calculates the gradient between the western equatorial Indian Ocean (50E-70E and 10S-10N) and the southeastern equatorial Indian Ocean (90E-110E and 10S-0N).
Local temperatures are affected by global and regional trends. With global warming, temperatures are expected to rise over longer periods (decades), but significant variation can occur between years, and under the influence of regional factors such as ocean-atmosphere interactions across the Pacific and Indian Ocean. From 2014 the UK Meteorological Office puts out an outlook around January 1 for the coming year, discussing whether it is likely to be hot or not.
Some data sources