7th Feb – Fresh water deluge


SST Anomaly for Australia – RAMSSA

The marine heatwave continues in the Tasman Sea, Bass Strait and Great Australian Bight. Temperatures are 2 to 3 degrees above normal through large areas.

In northern QLD  (where much of the recent extreme weather has occurred) surface temperatures are neutral to slightly cooler than normal. However there has been a lot of cloud cover in the area and no infra-red satellite estimates have been possible for many days.

Observation input into RAMSSA analysis. Black circle shows data gap over cloud-affected QLD – BOM

Salinity plummets in QLD deluge

An active monsoon trough formed a convergence line over northern QLD

Record rain has fallen through QLD, with Townsville amongst the worst hit. A years’ worth of rainfall has come in just 10 days. The Ross River Dam reached 200% capacity before the dam gates were opened, flooding several suburbs. Several locations have recorded over 1.5 m of rain and thousands of houses have been severely damaged.

Flood warnings current for 7th of Feb – BOM

So how is the ocean being affected? A lot of rain has fallen directly into the ocean and along the coasts there are large rivers pouring out into the sea.

The salinity field in OceanMAPS shows an area of fresher water off Townsville. This model is at 10 km resolution and includes rainfall into the ocean from ACCESS-G but uses climatological rivers at the boundaries. Salinity values here range from 32.8 to 34 psu. The greater Coral Sea sits at 34 to 35 psu.

Salinity field with four depth profiles for 5th Feb – OceanMAPS – click to expand

Now let’s look at a 4 km ROMS model that uses real-time river flow data. This model is being run in research mode at NCI. It is a lot fresher! Values around 29 to 33 psu inshore.

Salinity field with four depth profiles for 5th Feb – ROMS – click to expand

The trouble with salinity is that real-time measurements are few and far between.

There is at least one functional salinity sensor in the area. It is at 26 m depth at a place called Yongala. The station is run by AIMS and data can be accessed through the data centre.

Position of salinity sensor – AIMS
Salinity measurements at Yongala. Red – near surface sensor; blue – near seabed sensor. Source: AIMS

Earlier in the month, Yongala measurements showed a freshwater influx at the surface (0.6 m) on the 9th of January that then mixed down to the seabed (26 m) two days later. The surface sensor broke shortly afterwards, but the seabed sensor continued to measure. Then there was a plummet in salinity from the 28th of January. The latest values for the 6th of February show 34.15 psu. This is higher than the 4 km ROMS model which reads 33.4 psu at the 0.95 sigma level near the seabed.

Hang on, does that mean the 4 km ROMS has too much fresh water?

Craig Steinberg, oceanography and shelf processes team leader at AIMS, says that the relative coarseness of the 4 km model may be to blame. The fresh water plume is probably hugging the coast and rounding Cape Bowling Green more than the model suggests (personal communication).

The image below is a comparison of salinity fields from 1 km and 4 km resolution CSIRO models from a few weeks ago. For animation, click here.

Salinity fields for 4 km model (left) and 1 km model (right) – AIMS/CSIRO

We can see that the 4 km model smooths out the salinity fronts along the coast so that lower salinity values reach further out to sea towards Yongala. The 1 km model keeps low salinity very close to the coast. If this pattern is still occurring, then despite the fresh water influx, Yongala could indeed stay quite salty. The fine details of these fronts show how critical model resolution is to making valid verification comparisons.

Putting the exact psu values aside for now, we know that the water will be fresher than usual in this part of the Great Barrier Reef. How does that affect the fish and the coral?

The Sydney Morning Herald speculates that the rainfall and cloud cover will cool sea-surface temperatures and prevent a likely coral bleaching event. Certainly the cloud cover will have reduced the insolation.

Fluxes can tell us a thing or two. OceanMAPS has shown latent heat loss under the convergence line as some of the ocean heat contributed to atmospheric convection.

Total Heat Flux during daylight hours at 06 UTC on 3rd Feb – ACCESS-G

SST anomalies are neutral to mildly cool. However this is not new: there is evidence of SSTs being on the cool side in QLD since as least early January. The impact of the extreme rainfall on the reef is more likely to be related to increased sediment and chemical runoff as described by an AIMS study.

“Flood plumes can have a number of effects on coral reefs. Large scale flooding can carry land-based pollutants such as insecticides, fertilisers and herbicides out to the reef. Fresh water can kill corals at shallow depths where mixing is low, and sediment in the water blocks light which the coral needs to survive.” – Dr Katharina Fabricius, AIMS.

Some clues to look for potential negative impact on the reef will be big plumes of sediment on visual satellite images, and algal blooms in Chlorophyll-a images.

Let’s see what we can see when the cloud finally clears …

Good upwelling vs bad upwelling

SST for central NSW – OceanMAPS

Upwelling is hitting the mainstream news with beachgoers in northern NSW still complaining about the chilly waters. Local media argues that persistent north-easterly winds and cold waters have badly affected marine businesses such as dive charters, surf schools and fishers.

Dive charters are affected by  increased turbidity which makes visibility poor. Surf schools are affected by onshore winds causing choppy conditions. But fishing? How can fishers not like upwelling? Local fishermen claim that they are catching less fish because of the cold waters.

Combining this with the complaints from swimmers of ‘icecream headaches‘ stretching back to early January, we could call this unpopular or bad upwelling.

Is this whinging or is it actually unusual? SSTs inshore range from 16 to 19 deg C. This is in the lowest 10th percentile of the SSTAARS climatology which suggests the mean is 21 to 22 deg C. It is certainly chillier than normal.

MSLP anomaly for January 2019 – BOM

The northeasterly winds have been more persistent this summer than in previous years. A large positive MSLP anomaly in the Tasman Sea has caused stronger and longer periods of northeasterly winds and associated upwelling. Southerly changes have also been blocked from moving up the eastern coast. The cold-water complaints are not without basis.

Meanwhile, the famous Bonney Coast upwelling has quietly resurged.

SST for SA. Black circle showing upwelling location – OceanMAPS

There is no media fanfare about this upwelling. However, the prayers of the Portland community last November have obviously been answered, and anecdotes overheard in the corridors of BOM describe fishers flocking to the nutrient-rich water. We could call this good upwelling.

In northeastern Victoria, a third upwelling event is quietly taking place. Northeasterly winds on the eastern side of an inland trough have caused water temperatures to drop to 17 deg C, surrounded by otherwise balmy MHW conditions of 21 deg C.

SST for VIC and TAS. Black circle showing upwelling location – OceanMAPS

There is no media fanfare and no fishing industry dependent on this one.

So what do you think – good or bad? A similar event in 2017 caused fish kills, will this one do the same?

Underwater forests dying

Underwater kelp forests (Source:ABC – Matthew Doggett)

Ninety-five percent of eastern Tasmania’s kelp forests have gone. Ocean warming and pollution are the main culprits. Dive operators say the kelp is almost extinct and it’s “too late”. The Institute of Marine and Antarctic Studies (IMAS) describes the loss as “staggering” and worthy of public outrage and rioting on the streets.

Sea temperatures have been rising near Tasmania over the past few decades, as detailed in the recent State of the Climate Report. Multiple marine heatwaves have not helped.

Trends in sea surface temperatures in the Australian region from 1950 to 2017 (Source: BOM)

Studies show that kelp struggles when forests become ‘patchy’, which may explain the rapid decline over the last two decades. However there are good signs that the remaining 5% of kelp is healthy and more thermally tolerant.

Disappearing kelp (Source: ABC / Mick Baron)

IMAS and the Climate Foundation are trialling ways to restore the kelp. For example, planting juvenile kelp selected from the remaining healthy stock. But Dr Cayne Layton said this was only “buying time” and what is really needed to to reduce carbon emissions and ocean warming.

In the meantime, if you want to see or dive on some of the last kelp, it may still be found in patches near Eaglehawk Neck.

3 thoughts on “7th Feb – Fresh water deluge”

  1. Hi Jess, in addition to SST observations from in situ platforms and infra-red radiometers on satellites, RAMSSA ingests SSTs from a microwave satellite sensor (AMSR-2 on GCOM-W), so it ingests SST observations over oceans obscured by cloud, but not in cloudy regions within ~100 km of land or in the presence of rain. It also ingests in situ SST data, although this is much sparser than satellite observations of course.

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