Thread: Chasing Corals
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Unread 12/30/2017, 08:02 PM   #25
Scrubber_steve
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Quote:
Originally Posted by Megalonyx View Post
I apologize if I'm missing something but wouldn't sudden changes in current because of warmer oceans fall in line with climate change. I don't see why you're differentiating them.
Firstly, bleaching occurred in the most northern region of the GBR only.
What was the real cause of this bleaching event? Warmer water could have played a part. There was a record El Nino event after all, equal to the 1998 event, which was itself responsible for coral bleaching, directly due to the related rise in ocean surface temperature in that region, which is a measure of El Nino intensity. But, any related rise in ocean surface temperature is secondary to the most significant & critical factors responsible for coral bleaching. What are those?

As discussed by Ampou 2017 https://www.biogeosciences.net/14/817/2017/ Indonesian biologists had reported that a drop in sea level had bleached the upper 15 cm of the reefs before temperatures had reached NOAA’s Coral Reef Watch bleaching thresholds. 2015 altimetry data shows that sea level was at its lowest in the past 12 years, and this drop in sea level had likely been experienced throughout much of the Coral Triangle including the northern Great Barrier Reef (GBR), and then accelerated during the El Niño. They speculated sea level fall also contributed to the bleaching during the 1998 El Niño. Consistent with the effects of sea level fall, other researchers reported bleaching in the GBR was greatest near the surface then declined rapidly with depth.

Indeed the Great Barrier Reef had also experienced falling sea levels similar to those experienced by Indonesian reefs. Visitors to Lizard Island had reported more extreme low tides and more exposed reefs which is consistent with the extremely high mortality in the Lizard Island region during the 2016 El Niño. Of course reefs are often exposed to the air at low tide, but manage to survive if the exposure is short or during the night. However as seen in tide gauge data from Cairns just south of Lizard Island, since 2010 the average low tide had dropped by 10 to 15 cm. After previous decades of increasing sea level had permitted vertical coral growth and colonization of newly submerged coastline, that new growth was now being left high and dry during low tide. As a result shallow coral were increasingly vulnerable to deadly desiccation during more extreme sea level drops when warm waters slosh toward the Americas during an El Niño.

Furthermore, an El Niño in the Coral Triangle not only causes a sudden sea level fall, but it also generates a drier high-pressure system with clear skies, so that this region is exposed to more intense solar irradiance. In addition, El Niño conditions reduce regional winds that drive reef-flushing currents and produce greater wave washing that could minimize desiccation during extreme low tides. And as one would predict, these conditions were exactly what were observed during El Niño 2016 around Lizard Island and throughout the northern GBR.

Apology accepted!


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