Month: December 2018

7 Dec – QLD. Fires one day, floods the next.

 SST

SST Anomaly for Australia on 5th Dec – RAMSSA

western cool pool

As we’ve seen in recent weeks it remains much cooler than normal in the west, with most of the region in the bottom 10th percentile for for this time of year.

SST percentiles for 28th Nov against SSTAARS climatology – Ocean Current

iod as a driver

If this western cold anomaly is related to the positive Indian Ocean Dipole event, as this blog has been speculating, then it ‘should’ decay as the Australian monsoon starts – at least in northwestern parts.

A recent ABC article discusses the IOD as the main culprit for the 2018 NSW drought.

hot up the top

In contrast, it is warm in the north. The Gulf of Carpentaria is mostly above the 90th percentile. The Bay of Papua is similarly hot.

SST percentiles for 5th Dec against SSTAARS climatology – Ocean Current
SST percentiles for 2nd Dec against SSTAARS climatology – Ocean Current

The warm seas are contributing to a sustained heatwave in northern Australia. Hot overnight temperatures are causing stress to people and animals alike, with all-time temperature records at risk. For the latest heatwave warnings, check out the BOM website.

QLD – fires, floods and forecast divergence

Less than 10 days since eastern QLD was ravaged by bushfires, the same area is under threat from flooding.

A strong high pressure system in the Tasman and weak tropical lows in the north of the Coral Sea are directing moist onshore winds to the coast. The remains of short-lived TC Owen are to the north of New Caledonia and expected to travel westward as a tropical low.

ACCESS-G forecast for 7th Dec at 00 UTC – BOM

sea level and counter-currents

A positive sea level anomaly is developing along central QLD over the next few days as the strong easterly winds push up water levels and cause downwelling. The positive anomaly is already being measured at coastal tide monitoring stations like the one at Bowen (below). Luckily, the anomaly won’t trigger coastal inundation as the levels won’t reach HAT.

Bowen tide monitoring station showing a positive anomaly – Queensland Government

However, the positive SLA will cause a northward current along the coast in the opposite direction to usual southward-flowing EAC. This counter-current will carry up some cooler water from NSW. See the SST signature in the images below.

Prior to this week, SSTs were reaching up to 30 to 31 deg C in central QLD. Now the inshore counter-current will drop the temperatures drop back to 27 to 28 deg C.

SLA and surface currents 72 hr forecast for northern QLD – OceanMAPS
SST and surface currents 72 hr forecast for northern QLD – OceanMAPS

forecast divergence

Will the cooler water cause an atmospheric response?

The tropical low from ex-TC Owen will travel westward and cause significant rainfall along the QLD coast, but forecasting the intensity and duration of that rain is proving challenging.

There is large divergence in the global atmospheric models for early next week: ACCESS-G is forecasting the low to maintain strength and deliver heavy and sustained rain to the central coast, while the ECMWF model dissipates the system earlier.

The difference between the models could mean the difference between flooding and no flooding for QLD.

ACCESS-G forecast for MSLP and precipitation for 9th and 11th December – BOM
ECMWF forecast for MSLP and precip for 9th and 11th Dec – BOM

With this much uncertainty around a high impact situation, forecasters at the National Operations Centre are casting around for more guidance.

The ACCESS-G and ECMWF model each use a different SST analysis product at the bottom boundary, but both are static throughout the run.

The new seasonal prediction system at BOM – ACCESS-S – is a coupled model and could provide some insight on the interaction between atmosphere and ocean in this event. [ACCESS-S is comprised of the UKMO’s Unified Model at 60 km horizontal grid spacing for the atmosphere and the NEMO model at 25 km grid spacing for the ocean. Thirty-three ensemble members are produced daily.]

BNOC’s Ocean and Seasonal Prediction team produced some charts at the request of the forecasters, using the control member of the latest run. What do we see?

ACCESS-S forecast for MSLP and precipitation for 9th and 11th December – BOM

The ACCESS-S forecast from the 4th of December supports the ACCESS-G solution. It brings the low pressure system in close to the coast on the 11th of December. The results give us more confidence in this scenario.

It is hard to tease out the root cause of this solution. Is it the SST response? Or perhaps it is the upper air dynamics? We may never know for sure.

However, it does seem clear that coupled models will become increasingly useful in situations such as these: complex systems that rely on ocean surface feedback.

High model uncertainty will always drive forecasters to seek alternate sources of information. As coupled models increase in skill and resolution their operational use will increase too.