Discussion: A primary driver of climate forecasts is the El Nino southern oscillation (ENSO). Currently, La Nina is present in the equatorial East Pacific Ocean. La Nina initiated during spring 2020 and a “double dip” episode has followed peaking during late 2020 and again in late 2021. So where do we go from here? During the past 20 years “double dip” La Nina episodes are observed on 3 occasions each failing to produce a 3rd La Nina and sometimes reverse into El Nino. Consequently, analog ENSO forecasts for 2022-23 favor dissipation of La Nina with neutral ENSO to follow and possible emergence of El Nino by next winter. Previous Climate Impact Company ENSO forecasts identified El Nino as a possibility for later in 2022. But now, the El Nino option seems less likely while neutral ENSO is the safest “bet” and a 3rd La Nina is back on the table.
The latest ENSO climate diagnostics are revealing. The southern oscillation index (SOI) shifted into a stronger positive phase during March (Fig. 1). Strong +SOI indicates presence of a fortified La Nina climate. The tendency for +SOI throughout the 2020-22 La Nina presence has been a major contributor to the consistent La Nina signature of the multivariate ENSO index (Fig. 2). MEI identifies the response of the atmosphere to the equatorial SSTA pattern. While the Nino34 SSTA, a conventional measure of ENSO phase, has indicated the “double dip” La Nina of 2020-22 the MEI has stayed in La Nina phase the entire time. Consequently, the stronger +SOI and consistent La Nina-like MEI suggest that La Nina 2022 is likely to continue even though the Nino34 SSTA pattern may become neutral.
Another reason for the below average forecast confidence for 2022 is the Madden Julian oscillation (MJO) signature. During the past 6 months, a total of 5 weak-to-moderate surges of convection across the equatorial Pacific have occurred (Fig. 3). A sixth is starting now. The eastward surges of convection only occur because the La Nina system is breaking down. Typically, during La Nina the convection phase of MJO stays west of the Dateline (Fig. 4). When MJO convection surges eastward, trade winds weaken and the subsurface also warms. The most recent MJO event abled a subsurface warm Kelvin Wave to traverse the equatorial Pacific and remnants of that feature are now just-off the South America northwest coast (Fig. 5). However, in it’s wake the subsurface eastern equatorial Pacific has re-cooled and the weakened La Nina of February has regained strength.
“Double dip” La Nina episodes peaking in consecutive winter seasons historically have a nearly equal chance of the third winter being in neutral ENSO, El Nino or La Nina (Fig. 6). The restrengthening of La Nina in March 2022 strongly suggests that the El Nino analog is falling out of favor leaving neutral ENSO and La Nina with essentially equal chances for mid-to-late 2022.
What may be the leading clue as to where ENSO is heading for quarter 3 of 2022 is the expected evolution of negative phase Indian Ocean Dipole (-IOD). In this pattern, as forecast by the International Multi-model Ensemble (IMME) the eastern Indian Ocean warms (Fig. 7) and likely contributes to anomalous convection already present in the far West Pacific tropics due to warm SSTA. The heavy convection in this zone leads to widespread rising air which must be replaced by stronger trade winds from the east. The stronger trade winds maintain cool SSTA near the Dateline and La Nina (or La Nina Modoki) is sustained.
Climate Impact Company favors the IMME model (Fig. 8) for August which leans toward sustaining weak La Nina. If so, the chances of La Nina returning for a 3rd consecutive winter will increase. The Climate Impact Company season 1-3 ahead forecast is based on the presence of La Nina continuing. If the Nino34 SSTA becomes neutral, the La Nina-biased MEI will stay in cold ENSO mode.
Given increased confidence of a La Nina-biased climate for the middle third of 2022 a review of the modern day La Nina climate bias and adding optimum climate normal is reviewed. Typically, when a La Nina climate is present during the past 25 years during the middle third of the year a hot bias is centered over Texas while Washington State is cool (Fig. 9) while the Southeast U.S. has a dry bias and the northern states are wetter (Fig. 10). Due to acceleration of the shrinking polar ice cap and warming of the mid-latitude oceans of the past 10-15 years, an optimum climate normal (OCN) of the La Nina climatology indicates a warm-to-hot bias for Texas and the Northeast U.S. (Fig. 11) while Texas is very dry and the East U.S. is wet (Fig. 12). The anomalies indicated can be enhanced depending on regional soil moisture and SSTA patterns either side of the U.S. coastlines.
Fig. 1: A strengthening positive phase southern oscillation index in March 2022 strongly favors re-intensification of La Nina.
Fig. 2: The 2020-22 “double dip” La Nina is identified by the Nino34 SSTA index. However, note that during the entire time multi-variate ENSO index is in La Nina phase. MEI identifies the climate response to equatorial Pacific SSTA.
Fig. 3-4: Eastward surges of convection (blue) inspired by Madden Julian oscillation have been frequent the past 6 months (left). Normally, during La Nina, MJO convection pulses stay in the equatorial West Pacific (right).
Fig. 5: The equatorial East Pacific is re-cooling after passage of a Kelvin Wave which is now off the northwest coast of South America. A new kelvin Wave is forming west of the Dateline.
Fig. 6: NOAA/CPC depiction of the “double dip” La Nina and history of what follows during the third winter.
Fig. 7: IMME model indicates negative Indian Ocean Dipole developing mid-year and possibly peaking in August. This pattern inspires increased central equatorial Pacific trade winds which maintain La Nina (or La Nina Modoki).
Fig. 8: NOAA/CPC ENSO forecast models and based on trend which set is more likely to validate. The IMME or CON models are most likely correct.
Fig. 9-10: Modern-day La Nina temperature and precipitation anomalies for the middle third of the year (MAY-AUG). La Nina years are selected based on multi-variate ENSO index.
Fig. 11-12: Optimum climate normal of La Nina temperature and precipitation anomalies for the middle third of the year (MAY-AUG). La Nina years are selected based on multi-variate ENSO index.