ISOS Miniproposal: An assessment of the Bjerknes Stability Index

March 2017

In 2016, Christian Wengel applied for a Miniproposal to go to Australia. Here, he reports about his work on El Niño/Southern Oscillation.


The El Niño/Southern Oscillation (ENSO) is the dominant mode of interannual climate variability in the tropics. ENSO is characterized by sea surface temperature anomalies of a few centigrade in the equatorial Pacific and has an environmental and socioeconomic impact on a global scale.
The Bjerknes Stability Index (BJ index) presents a widely used tool for the quantification of coupled atmosphere-ocean feedbacks that produce ENSO. It is a measure of the growth rate of sea surface temperature anomalies. The BJ index is therefore a popular tool to explain different ENSO behaviors among coupled climate models and to assess ENSO stability changes in future climate scenarios. Despite its widespread application, there are also cautionary notes on the usage of the BJ index. In my recent work I found that an overestimation of ENSO amplitude in the Kiel Climate Model, with respect to observations, is counter-intuitively linked to an underestimation of the BJ index. In a similar context, there appears to be an inconsistent relationship between the BJ index and the ENSO amplitude between models. A positive correlation is expected, i.e. a larger BJ index (more unstable conditions) favors increased ENSO amplitude. However, there appears to be no correlation of these two quantities among climate models that participate in the CMIP5.
The Miniproposal offered me the chance to investigate this relationship in more detail in collaboration with Dietmar Dommenget at the Monash University in Melbourne.
I was able to produce some interesting results in this matter. There are indications that it may not be sufficient to use the BJ index alone as a reliable measure of ENSO amplitude, as other parameters play an important role as well. One example is the damping effect of equatorial heat content in the ocean, which is highly linked to ENSO amplitude as well and yet has been left unconsidered in the BJ index framework.
This research stay strongly contributed to my personal development as a doctoral researcher. It presents a great opportunity for extending ones international scientific network. Also, experiencing to be part in another working group during this time comes along with new perspectives on how to deal with scientific questions. I can only recommend this opportunity to everybody.