Reef-cloud-radiation-SST interactions over the Great Barrier Reef (GBR) during the trade wind season

Abstract:

The Great Barrier Reef (GBR) is known to face a number of threats including widespread coral bleaching events. With corals already living close to their thermal maxima, increases in Sea Surface Temperatures (SSTs) are of great concern for the survival of coral reefs. Cloud is known to have the potential to modulate the regional SST by reflecting and/or absorbing part of the solar radiation, and therefore potentially promoting the survival of coral reefs.

In this study, total cloud cover (TCC), net shortwave radiative flux at the surface (SW-RF) and SST anomalies during past major coral bleaching events over the GBR (140°-155°E, 10°-25°S) are examined using long-term (1996-2018) monthly Cloud-CCI product and ERA-5 reanalysis product. Coral bleaching months are found to be experiencing positive SST anomalies, accompanied by negative TCC anomalies and positive SW-RF anomalies. Large magnitudes of these anomalies are found to be occurring around severe coral bleaching areas.

Direct correlations between these variables are examined during the coral bleaching seasons (Dec. to May) across the GBR. As ENSO is known to affect the climate of much of the tropics and subtropics, ENSO-induced relationships between TCC, SSTs and SW-RF anomalies across the GBR were calculated using a simple regression method. After the ENSO signal is removed, a statistically significant negative correlations between TCC anomalies and SST anomalies, while positive correlations between SW-RF and SST anomalies is identified across the mid and sub-tropics regions of the GBR (i.e. the lower two thirds of the domain), where south-easterly trade winds dominate. The deep tropics, dominated by deep convection, displays a weaker relationship.

Given the greater heat capacity of the ocean than that of the atmosphere, lagged correlations between TCC, SW-RF and SSTs anomalies are also examined. Overall, the correlations are found even stronger across the GBR in the results with a one-month time lag. In particular, the significant negative correlation between TCC and SST anomalies extended further north to parts of the deep tropics. The whole GBR domain is characterized by strong widespread positive correlations between SW-RF and SST anomalies including deep tropics.