8th Nov – Anomaly dipole


The SST anomaly pattern has amplified in recent weeks. A general distribution –  cool anomalies in the west and neutral to warm anomalies in the east – has further diverged.

SST Anomaly for 7th Nov – RAMSSA

warmer in the east

The warm anomaly off the coast of NSW is particularly strong, with most areas 1 to 2 deg C above the average.

SST Anomaly for NSW on 7th Nov – RAMSSA

This is also clearly seen in the IMOS Ocean Current products (below – SST, SST Anomaly and SST percentile). 6-day SST, 30th October – OceanCurrent

In the middle of the maps is a cooler area centred at 34 N 153 E. In the RAMSSA product this is presents a neutral anomaly, while the Ocean Current 6 day SST anomaly is strongly negative by 2 to 3 deg C. The positive anomaly nearby is also larger in Ocean Current, up to 3 deg C!

SST Anomaly against SSTAARS climatology, 30th October – OceanCurrent

Daily percentiles show the cool anomaly is in the lowest 10th percentile  while the warm anomaly is in the highest 10th percentile for this day of the year. However, across a yearly non-seasonal distribution the temperatures are not extreme (see OceanMAPS webviewer percentiles which come from BRAN).

SST Percentiles against SSTAARS climatology, 30th October – OceanCurrent

SST comparison

This region provides an interesting case study of analysis input and resolution. For NSW let us compare four SST products:

  1. RAMSSA – 0.083 degree analysis, with correlation length scale of 12 km
  2. 4-hour composite satellite SST from Ocean Current – 2 km grid
  3. OceanMAPS – 0.1 degree grid
  4. NCEP’s product shown on Weatherzone – presumably the G-RTOFS on 0.083 degree grid
Comparison of NSW SST products. a) RAMSSA for 7th Nov b) Ocean Current for 4th Nov c) OceanMAPS for 9th Nov d) NCEP SST for 8th Nov – Click to expand

Despite the slight differences in analysis time, other differences are evident. The satellite based Ocean Current image shows finer detail but is hampered by cloud cover. RAMSSA is smoother with less detail. OceanMAPS is even smoother and is a forecast rather than an analysis.

The NCEP/Weatherzone image has the least information in it. However, its easy access online (Google-able) means it is sometimes used by forecasters (according to inside sources).

A similar story is seen in the anomaly images.

Comparison of NSW SST Anomaly products. a) RAMSSA for 7th Nov b) L3S-6day Ocean Current for 4th Nov c) OceanMAPS for 8th Nov, showing 50% percentile exceedence d) NCEP SST for 8th Nov – Click to expand

As always, which product you use depends on the application. The 4-hourly satellite SST shows the ‘real anomalies’ which can be large in amplitude. localised and short-lived, as seen in the images earlier. RAMSSA and other broader analyses can show regional patterns more useful for seasonal trends. OceanMAPS provides a projection into the future.

Which SST products do you use?

cooler in the west

SST Anomaly for North West WA on 7th Nov – RAMMSA

The cool anomalies in the west have persisted from mid-winter (from at least 10th August) and into spring.

The northwest region is linked to the Indian Ocean Dipole, where a positive event is now officially underway as the anomaly threshold has been sustained for the required 8 weeks. A positive event tends to reduce rainfall for southern and central Australia.

2018 will be considered a positive IOD year.  – BOM

However the IOD is expected to dissipate imminently when the monsoon trough moves southward in summer.

It begs the question, if it can only be announced after it has happened, can the IOD be used as a prognostic at all?

Indian Ocean Dipole ensemble forecast – BOM

Hot water and hailstones

A cold core eddy is expected to consolidate to the east-southeast of Sydney. The associated clockwise rotating currents will encourage the very warm EAC water to travel south-southeastward from the separation point near Port Macquarie. This will keep most of the extra heat well offshore from Sydney.

Sea level anomaly forecast for NSW waters for 9th and 14th Nov, arrow showing current direction on eastern side of cold eddy – OceanMAPS
Sea surface temperature forecast for NSW waters for 9th and 14th Nov, large red dot showing location of Port Macquarie – OceanMAPS

However, further up the coast where the EAC is along the shore, conditions are ripe for convection.

On the 7th of November the hot water off Port Macquarie supported the development of large thunderstorm with “life-threatening hail” the size of tennis balls.

Huge hail stones falling in Port Macquarie on 7th Nov – ABC

A sequence of infrared images show the storm’s development. The lightning activity intensified as the convection moved northeastward over the EAC water.

Infrared and lightning images for central NSW coast on 7th Nov 2018, showing four timestamps in local time (LT), black arrow shows location of Port Macquarie – BOM Panther

Now that the ‘Severe Weather Season’ has started (October-April each year), forecasters around the country are on the lookout for warm waters as sources of moisture and heat for convective storms.

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