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El Niño-Southern Oscillation

Geosciences Column: How El Niño triggered Indonesia corals die-off

Geosciences Column: How El Niño triggered Indonesia corals die-off

In the glistening waters of Indonesia, shallow corals – the rain forests of the sea – teem with life.  Or at least they did once. Towards the end of 2015 the corals started to die, leaving a bleak landscape behind. An international team of researchers investigated the causes of the die-off. Their findings, published recently in the EGU’s open access journal, Biogeosciences, are rather surprising.

Globally, corals face tough times. Increasing ocean-water temperatures (driven by a warming climate) are disrupting the symbiotic relationship between corals and the algae that live on (and in) them.

The algae, known as zooxanthellae, provide a food source for corals and give them their colour. Changing water temperatures and/or levels, the presence of contaminants or overexposure to sunlight, put corals under stress, forcing the algae to leave. If that happens, the corals turn white – they become bleached – and are highly susceptible to disease and death.

Triggered by the 2015-2016 El Niño, water temperatures in many coral reef regions across the globe have risen, causing the National Oceanic and Atmospheric Administration (NOAA) to declare the longest and most widespread coral bleaching event in recorded history. Now into its third year, the mass bleaching event is anticipated to cause major coral die-off in Australia’s Great Barrier Reef for the second consecutive year.

The team of researchers studying the Indonesian corals found that, unlike most corals globally, it’s not rising water temperatures which caused the recent die-off, but rather decreasing sea level.

While conducting a census of coral biodiversity in the Bunaken National Park, located in the northwest tip of Sulawesi (Indonesia), in late February 2016, the researchers noticed widespread occurrences of dead massive corals. Similar surveys, carried out in the springs of 2014 and 2015 revealed the corals to be alive and thriving.

In 2016, all the dying corals were found to have a sharp horizontal limit above which dead tissue was present and below which the coral was, seemingly, healthy. Up to 30% of the reef was affected by some degree of die-off.

Bunaken reef flats. (a)Close-up of one Heliopora coerula colony with clear tissue mortality on the upper part of the colonies; (b)same for a Porites lutea colony; (c) reef flat Porites colonies observed at low spring tide in May 2014. Even partially above water a few hours per month in similar conditions, the entire colonies were alive. (d) A living Heliopora coerula (blue coral) community in 2015 in a keep-up position relative to mean low sea level, with almost all the space occupied by corals. In that case, a 15 cm sea level fall will impact most of the reef flat. (e–h) Before–after comparison of coral status for colonies visible in (c). In (e), healthy Poritea lutea (yellow and pink massive corals) reef flat colonies in May 2014, observed at low spring tide. The upper part of colonies is above water, yet healthy; (f) same colonies in February 2016. The white lines visualize tissue mortality limit. Large Porites colonies (P1, P2) at low tide levels in 2014 are affected, while lower colonies (P3) are not. (g) P1 colony in 2014. (h) Viewed from another angle, the P1 colony in February 2016. (i) Reef flat community with scattered Heliopora colonies in February 2016, with tissue mortality and algal turf overgrowth. Taken from E. E. Ampou et al. 2016.

The confinement of the dead tissue to the tops and flanks of the corals, lead the scientists to think that the deaths must be linked to variations in sea level rather than temperature, which would affect the organisms ubiquitously. To confirm the theory the researchers had to establish that there had indeed been fluctuations in sea level across the region between the springs of 2015 and 2016.

To do so they consulted data from regional tide-gauges. Though not located exactly on Bunaken, they provided a good first-order measure of sea levels over the period of time in question. To bolster their results, the team also used sea level height data acquired by satellites, known as altimetry data, which had sampling points just off Bunaken Island. When compared, the sea level data acquired by the tidal gauges and satellites correlated well.

Sea-level data from the Bitung (east North Sulawesi) tide-gauge, referenced against Bako GPS station. On top, sea level anomalies measured by the Bitung tide-gauge station (low-quality data), and overlaid on altimetry ADT anomaly data for the 1993– 2016 period. Note the gaps in the tide-gauge time series. Middle: Bitung tide-gauge sea level variations (high-quality data, shown here from 1986 till early 2015) with daily mean and daily lowest values. Bottom, a close-up for the 2008–2015 period. Taken from E. E. Ampou et al. 2016.

The data showed that prior to the 2015-2016 El Niño, fluctuations in sea levels could be attributed to the normal ebb and flow of the tides. Crucially, between August and September 2015, they also showed a sharp decrease in sea level: in the region of 15cm (compared to the 1993-2016 mean). Though short-lived (probably a few weeks only), the period was long enough that the corals sustain tissue damage due to exposure to excessive UV light and air.

NOAA provides real-time Sea Surface Temperatures which identify areas at risk for coral bleaching. The Bunaken region was only put on alert in June 2016, long after the coral die-off started, therefore supporting the crucial role sea level fall played in coral mortality in Indonesia.

The link between falling sea level and El Niño events is not limited to Indonesia and the 2015-2016 event. When the researchers studied Absolute Dynamic Topography (ADT) data, which provides a measure of how sea level has change from 1992 to 2016, they found sea level falls matched with El Niño years.

The results of the study highlight that while all eyes are focused on the consequences of rising ocean temperatures and levels triggered by El Niño events, falling sea levels (also triggered by El Niño) could be having a, largely unquantified, harmful effect on corals globally.

By Laura Roberts Artal, EGU Communications Officer

References and resources

Ampou, E. E., Johan, O., Menkes, C. E., Niño, F., Birol, F., Ouillon, S., and Andréfouët, S.: Coral mortality induced by the 2015–2016 El-Niño in Indonesia: the effect of rapid sea level fall, Biogeosciences, 14, 817-826, doi:10.5194/bg-14-817-2017, 2017

Varotsos, C. A., Tzanis, C. G., and Sarlis, N. V.: On the progress of the 2015–2016 El Niño event, Atmos. Chem. Phys., 16, 2007-2011, doi:10.5194/acp-16-2007-2016, 2016.

What are El Niño and La Niña? – a video explainer by NOAA

Coral Reef Watch Satellite Monitoring by NOAA

Global sea level time series – global estimates of sea level rise based on measurements from satellite radar altimeters (NOAA/NESDIS/STAR, Laboratory for Satellite Altimetry)

El Niño prolongs longest global coral bleaching event – a NOAA News item

NOAA declares third ever global coral bleaching event – a NOAA active weather alert (Oct. 2015)

The 3rd Global Coral Bleaching Event – 2014/2017 – free resources for media and educators

What is coral bleaching? – an infographic by NOAA
The ENSO (El Niño–Southern Oscillation) Blog by Climate.gov (a NOAA resource)

GeoSciences Column: The ‘dirty weather’ diaries of Reverend Richard Davis

GeoSciences Column: The ‘dirty weather’ diaries of Reverend Richard Davis

Researching the Earth’s climate of the past, helps scientists make better predictions about how the climate and our environment will continue to be affected by, change and adapt to rising temperatures.

One of the most invaluable sources of data, when it comes to understanding the Earth’s past climate, are historical meteorological records.

Accounts of weather and climate conditions for the Southern Hemisphere, prior to the 1850s, are particularly sparse. This makes the recently discovered, painstakingly detailed and richly descriptive weather diaries of a 19th Century missionary in New Zealand, incredibly valuable.

Researchers from the National Institute of Water and Atmospheric Research, In Auckland (New Zealand), poured over the contents of the diaries, which provide an eyewitness account to the end of the Little Ice Age (between 1300 and the 1870s winter temperatures – particularly in the Norther Hemisphere- were lower than those experienced throughout the 20th Century). The journals reveal that 19th century New Zealand experienced cooler winter temperatures and more dominant southerly winds when compared to the present day climatic conditions. The researchers present these and other findings in the open access journal of the EGU, Climate of the Past.

Print of a photomechanical portrait of Reverend Richard Davis taken ca. 1860, from the file print collection, Box 16. Ref: PAColl-7344-97, Alexander Turnbull Library, Wellington, New Zealand, sourced from http://natlib.govt.nz/records/23073407 (From A. M. Lorrey et al., 2016).

Print of a photomechanical portrait of Reverend Richard Davis taken ca. 1860, from the file print collection, Box 16. Ref:
PAColl-7344-97, Alexander Turnbull Library, Wellington, New Zealand, sourced from http://natlib.govt.nz/records/23073407 (From A. M. Lorrey et al., 2016).

The diaries were kept by Reverend Richard Davis, born in Dorset (England) in 1790. The Reverend was associated to the Church Mission Society (CMS) of England; an connection which lead him to settle in the blustery Northland Peninsula in the far north of New Zealand, back in 1831.

From 1839 to 1844, and then again from 1848 to 1851, Davis collected over 13,000 meteorological measurements and made detailed notes about the condition of the local environment.

The Reverend’s collection of data is remarkable, not only for its detail, but also because it is the earliest record of land-based meteorological measurements from New Zealand found to date.

He took twice daily temperature measurements – one at 9 a.m. and one at 12 noon – as well as noon pressure measurements. Qualitative observations included information about wind direction and strength, as well as detailed cloud cover descriptions, and notes on the occurrence of hail, frost, rainfall, snowfall, thunderstorms, lightning, sunsets and behavior of wildlife.

The journals also reveal that the Reverend was not keen on particularly gloomy days, when the winds were strong and blustery, cloud cover hung low and was often accompanied by rain.  On 67 separate occasions, Davis’ used the term “dirty weather” to described days like this.

It is important to assess the reliability of the measurements taken by Davis before drawing comparisons between 19th and 20th Century weather patterns for the island of the long white cloud; and especially if the data are to be integrated within past climate and weather reconstructions for New Zealand and the Southern Hemisphere.

To do so, the Auckland based researchers, compared the Reverend’s pressure measurements with observations made by ships travelling through New Zealand waters or stationed on the island (usually completing military operations), during the same time interval. The Reverend’s daily pressure observations are regularly lower (on average by -0:64 ± 0:10 inches of mercury) than those taken on board the ships. The offset is consistent with the change in altitude between the ships anchored in harbour versus the land-based measurements made by Davis; meaning the Reverend’s pressure measurements are robust.

Reverend Richard Davis pressure observation vs. expedition measurements (leader noted in parentheses) from USS Vincennes (Wilkes), the corvettes Astrolabe and Zelee (d’Urville) and the HMS Erebus (Ross). There are 29 pairs of daily observations and so the x axis simply shows the comparisons of Davis’ record to the three ships in a sequence with the specific intervals noted. (From A. M. Lorrey et al., 2016).

Reverend Richard Davis pressure observation vs. expedition measurements (leader noted in parentheses) from USS Vincennes (Wilkes), the corvettes Astrolabe and Zelee (d’Urville) and the HMS Erebus (Ross). There are 29 pairs of daily observations and so the x axis simply shows the comparisons of Davis’ record to the three ships in a sequence with the specific intervals noted. (From A. M. Lorrey et al., 2016).

There is no similar test which would verify the accuracy of Davis’ temperature measurements. However, the researchers argue that the (expected) annual cycles evident in his measurements, as well as the reliability of his other records mean that, at least some, of his readings are faithful to the local conditions. Not only that, Davis’ mean winter temperature anomalies are comparable to the temperatures reconstructed from tree rings and can be used by the researchers to gather information about the local atmospheric circulation at the time.

When compared to modern-day temperature measurements (from the Virtual Climate Station Network, VCSN), the journal data reveals that mid 1800s winters, at the far north of the island, were cooler. At present, atmospheric circulation over Northland means winds from the southwest are common, especially during the winter and spring. During the summer, easterly winds become dominant. There is a higher frequency of records of south and southwesterly winds in Davis’ diaries. Reconstructions of atmospheric flow over New Zealand in the 1800s, made with proxy tree-ring and coral data, also point towards more frequent south and southwesterly winds and cooler temperatures.

Not only that, the timing of monthly and seasonal climate anomalies, recorded both in tree-ring and the Davis diary data suggest that El Niño-Southern Oscillation (ENSO)-like conditions existed, in New Zealand, during the 1839-1851 time period. However, more work (and data) is needed in Australasia to corroborate the findings and define the extent of the ENSO conditions at the time.

With more data, better reconstructions of the atmospheric conditions in the southwest Pacific and Southern Hemisphere can be made. Combined with the newly found Davis’ records, these will make an important impact to the understanding of past weather and climate in the region.

By Laura Roberts Artal, EGU Communications Officer

The Waimate North mission house in the Far North of New Zealand where Davis lived (From: A. M. Lorrey et al., 2016).

The Waimate North mission house in the Far North of New Zealand where Davis lived (From: A. M. Lorrey et al., 2016).

References

Lorrey, A. M. and Chappell, P. R.: The “dirty weather” diaries of Reverend Richard Davis: insights about early colonial-era meteorology and climate variability for northern New Zealand, 1839–1851, Clim. Past, 12, 553-573, doi:10.5194/cp-12-553-2016, 2016.