Written by Sig Silber
NOAA updates the status of ENSO on the second Thursday of each month. This presumably plays a role in the issuance of the Seasonal Outlook which is issued on the following Thursday. We thought the status should have been updated to La Nina Advisory last month but it was a close call and this month they took the leap. There is not a huge difference between a weak La Nina and ENSO Neutral with a strong bias towards being La Nina. So this is not a major event but it solidifies the forecast to some extent. It also reminds us that La Nina is favorable for hurricane development in the Atlantic.
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Some useful information from the NOAA ENSO Blog. You can find this article here.
Now I am deviating from Emily Beckers’s presentation to show the forecast from the Australian Bureau of Meteorology (BOM). It is about ten days old issued in late August.
NOAA has their own proprietary model which they rarely use. It is not exactly clear why they shun their own model.
I am going to introduce another graphic which is the current view of the subsurface along the Equator.
Generalized World Impact
North America Impact
Here is another way to look at it.
Here are three generalized graphics of the Walker Circulation during the various phases of ENSO. Generalized or idealized is the keyword as there is a lot of variability from one event to the next but it provides a good framework for discussion. I have presented these or similar graphics many times before. These were produced by Fiona Martin.
To summarize, the Maritime Continent is north of Australia. And you can see that during La Nina, with the Indo-pacific Warm Pool strong, there is a lot of convection there. With ENSO Neutral there is less; and with El Nino it reverses. Prior to the satellite imagery, the Southern Oscillation Index (SOI) was used to estimate the relative strength of this process between Darwin Australia and Tahiti.
B. Conclusion
NOAA says we are in La Nina Conditions and were in August which they have denied until today. I have a little problem with NOAA re being late to the party on some occasions. I wonder why.
C. How significant is a pronouncement of the state of ENSO.
What we are going to do now is look at all the historical La Ninas (their winter effects only) and while we are at it (since I found a good source of information, we are going to also look at all the historical El Nino’s. I might be missing one as this data source in one case goes up to 2017 and in another case goes up to 2018 and early next year the normals for the Equator will be updated and some weak El Nino’s and La Nina’s may drop off the list and some others may be added. But those adjustments do not diminish the value of what I will present now.
The purpose is to introduce the reader to the concept of the statistical average. Generally speaking the statistical average La Nina and the statistical average El Nino are used to make forecasts. NOAA adjusts for the predicted strength of the La Nina or El Nino. JAMSTEC does the same but also adjusts for the Modokishness of the El Nino and also to a lesser extent the La Nina. NOAA will not admit to doing that.
In all cases, everyone is hampered by the small number of events in the historical record. Statistics is based on having a large number of events. You will see why that is important now.
NOAA provides the tools to create graphic displays like below. In theory, I could do that myself. But like most things involving software those who work with it frequently learn how to use it. So I am happy to find these graphics. The reference provided is where I found the graphic. In some cases that person produced it. In other cases, they got it from someone else and provide attribution. In all cases, they provide a discussion that assists with understanding what is being shown.
It is important to understand what the importance of the state of ENSO is. It probably is the most important factor in forecasts other than possibly decadal trends in temperature. The stronger the event (the deviation of the NINO 3.4 value from 0, the more significant we would expect the impacts to be. In the current case, we are expecting a pretty weak La Nina.
First, we look at La Nina
Temperature Record. Source and a discussion: Tom Di Liberto
Precipitation Record Source and a discussion: Tom Di Liberto
Now even though it does not apply right now we will look at El Nino….mostly for my benefit so I can find these graphics when I will need them.
Temperature Record Source and a discussion: Rebecca Lindsey
Precipitation Record. Source: Rebecca Lindsey
