Fortnightly Climate Outlook-ENSO 3.4, IOD, SAM & More

Headlines: Negative Indian Ocean Dipole (IOD) and Weak LA NINA likely to drive Australian weather during the late Winter into Spring 2021 as LA NINA WATCH continues.

Discussion: The latest on the Nino 3.4 region in the western tropical pacific region nino 3.4 is that there is now a 50% chance of a LA NINA of weak intensity developing in the late winter/early spring period (late August-October). A LA NINA WATCH has therefore been issued for the last 4 weeks as sea sub-surface temperatures have cooled significantly over the past few months and weeks. Also adding to this is the fact that at least one-third of the surveyed models are showing cooling of more then -0.8c below average for the western tropical pacific ocean by spring which also warrants a WATCH contributing overall to a a shift from a neutral ENSO. a LOW chance of a El Nino developing throughout the Southern Hemisphere Winter to Spring. However a Positive IOD is likely at some stage likely peaking in September/October seeing below average rainfall to set in across parts of Southern and Southeastern Australia as we head into spring. While a weakening trend away from EL NINO is likely also peaking around spring during September before a slight increase towards EL NINO re-occurs near the end of the Forecast Period.

The current NINO 3.4 state of play sees the majority of the surveyed International Models indicating a weakening trend of EL NINO back into INACTIVE thresholds into spring and a POSITIVE IOD LIKELY going into spring bring a increasing chance of below average rainfall to Southern and Southeastern Australia into spring delaying the start of the storm season 2019/20 and a very dry start into the lead up of Summer in Southern/Southeastern Australia.

State of Play Breakdown:

So what is needed for a LA NINA ALERT to be issued? Below is a checklist of factors that must be satisfied before the Bureau will issue a ALERT for a LA NINA event in the tropical western pacific ocean:

All four of the following criteria need to be satisfied for a LA NINA WATCH to be issued:

1. Current climate state: ENSO phase is currently neutral or declining El Niño.
   • Either:
     SOI analogues: Of the 10 years that most closely resemble the current SOI pattern, 4 or more have shown La Niña characteristics. ?
   • Or:
     Sub-surface: Significant sub-surface cooling has been observed in the western or central equatorial Pacific Ocean.
2.  Models: One-third or more of surveyed climate models show sustained cooling to at least 0.8 °C below average in the NINO3 or NINO3.4 regions of the Pacific Ocean by late winter or spring.

All four of the following criteria need to be satisfied for a LA NINA to be DECLARED and issued:

1. Sea surface temperature: Temperatures in the NINO3 or NINO3.4 regions of the Pacific Ocean are 0.8 °C cooler than average. 
2. Winds: Trade winds have been stronger than average in the western or central equatorial Pacific Ocean during any three of the last four months. 
3. SOI: The three-month average SOI is +7 or higher. 
4. Models: A majority of surveyed climate models show sustained cooling to at least 0.8 °C below average in the NINO3 or NINO3.4 regions of the Pacific Ocean until the end of the year. 

=Criteria MET.

=Criteria NOT MET.

International Models:

Out of all the international models three of the seven (3 of 7) show a chance of a weak la Nina forming by at least October, been the UKMET, NOAA and the BoM which means we have already met one of the criteria under models to issue a La Nina Alert phase.