Introducing the Relative Ocean Nino Index
Oldenborgh et al have observed that the crux of the issue is that the traditional Nino 3.4, from which the ONI is derived, "not only describes ENSO but also has a trend due to global warming, which has very different impacts from that of el Nino" (Oldenborgh et al, 2021). The piece goes onto assert that this is problematic because the "spacial pattern of the observed SST trend over the past century does not resemble the ENSO pattern" (Oldenborgh et al, 2021). The suggested solution is to use the Relative Ocean Nino Index (RONI), which is a value derived from the difference of the original index with the contemporaneous tropical mean SST anomalies. According to Oldenborgh et al, subtracting the tropical mean SST anomaly not only removes the trend, but also results in an SST anomaly (RONI) that is of more direct relevance to changes in tropical convection driven by SST anomalies. Removing the trend in order to distinguish between ENSO related warming and climate change attributed background warming is important because tropical convection is more sensitive to the warming of ENSO relative to the rest of the tropics. Thus convection will concentrate in areas that warm the most relative to the tropical mean background warming. This is why the current generally warm Pacific basin is reducing the ability of ENSO to assert itself in the hemisphere to a level commensurate with the current 1.5 ONI value, as reflected by the ASO tri-monthly RONI value of 1.05, which is weak-moderate and somewhat more in line with the current 0.3 MEI value. This explains why el Nino has thus far failed to redistribute the tropical convection denoted by the negative velocity potential away from the central Pacific to the eastern Pacific, co-located with the strongest ENSO anomalies in regions 1.2 and 3, as was the case in 1997 and 2015. In comparison, the ASO RONI values were 2.15 and 1.87, which explains why the tropical forcing was much more co-located with the greatest anomalies to the east. The ocean-atmosphere coupling was much stronger. This is reflected by the AS bi-monthly MEI values of 2.2 and 2.1 as compared to the current value of 0.3. Theoretically speaking, should the el Nino become intense enough and the warmest anomalies remain strongly biased to the east, at some point the convective forcing should realign itself with the strongest positive ENSO anomalies to the east and away from the residual warmest absolute SSTs to the west. The key is what is this critical threshold and will it be reached.