Upwelling linked to warm summers and bleaching on the Great Barrier Reef

Berkelmans, Ray, Scarla J. Weeks, and Craig R. Steinberg

Limnol. Oceanogr., 55(6), 2010, 2634-2644 | DOI: 10.4319/lo.2010.55.6.2634

ABSTRACT: We investigate a range of indices to quantify upwelling on the central Great Barrier Reef (GBR), Australia, so that environmental and biological relationships associated with upwelling in this area can be explored. We show that "Upwelling days" (the number of days of upwelling) and diurnal variation in subsurface temperature (maximum-minimum, 20-m depth) are satisfactory metrics to describe the duration and intensity of upwelling events, respectively. We use these to examine key characteristics of shelf-break upwelling in the central GBR. Our results show, somewhat paradoxically, that although upwelling involves cold water being brought near to the surface, it is linked to positive thermal anomalies on the GBR, both locally and regionally. Summers (December to February) with strongest upwelling occurred during the GBR-wide bleaching events of 1997-1998 and 2001-2002. Upwelling in the GBR is enhanced during doldrums conditions that were a feature of these summers. During these conditions, the poleward-flowing East Australian Current flows faster, lifting the thermocline closer to the surface, spilling more sub-thermocline waters onto the shelf. Doldrums conditions also result in intense local heating, stratification of the water column, and, when severe, coral bleaching. Upwelling intrusions are spatially restricted (central GBR), generally remain subsurface, and are often intermittent, allowing GBR-wide bleaching to occur despite conditions resulting in enhanced upwelling. Intense upwelling events precede anomalous seasonal temperature maxima by up to 2 months and bleaching by 1-3 wk, leading to the prospect of using upwelling activity as a seasonal forecasting index of unusually warm summers and widespread bleaching.

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