by P. Gosselin, Apr 19, 2023 in NoTricksZone
We’ve had a La Niña for nearly three years. But now it has officially ended, and ENSO has moved into its neutral phase, the “La Nada”. The La Niña event lasted three winters in a row, something that has only occurred twice before in modern times: 1973–1976 and 1998–2001. Both of these followed in response to a very strong El Niño.
The La Niña that has now ended, on the other hand, came after the more neutral winter of 2019/20.
Figure 1. Number of months with each ENSO phase for five-year periods from 1950 – 2023
The number of months that we have had each ENSO phase in the last 74 years is shown in Figure 1. The La Niñas are more frequent than the El Niños. Interestingly, the opposite was true during the rapid warming we had from 1975 – 1999, when El Niños were more common. But then it reverts back again around 1998/99. Is there a pattern here?
It is established in climate science that the climate underwent a profound shift in 1976/77, related to the ocean currents. In IPCC AR4, they write: “The 1976–1977 climate shift in the Pacific, associated with a phase change in the PDO from negative to positive, was associated with significant changes in ENSO evolution.” The Pacific Decadal Oscillation (PDO) shifted from a ‘cold’ to a ‘warm’ phase during these two years. The ENSO also became dominated by the El Niño phase, which is characterized by warmer temperatures. These changes affected the global climate, and a rapid warming began.
But what happened in the years just before 2000? As seen in Figure 1, ENSO reverts to being dominated by the ‘colder’ La Niña at that time.
Figure 2. The PDO index according to NOAA/ESRL.
Interestingly, the PDO also reverts to its cold phase in 1998–1999.
Are we seeing a shift to a colder phase in the climate here?