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  • Moetasim Ashfaq

ENSO Jani, what happened?

ENSO's positive phase (El Niño) reached its maximum strength in early winter and is predicted to calm down in the next few months. It was anticipated to be the most powerful El Niños in recent records, although it fell short of breaking any records set in 1982, 1997, and 2015. Despite that, for some regions, this may be one of the oddest ENSO years and one to remember for a long time.


Central Southwest Asia (CSWA) is one of those regions where ENSO and the winter precipitation relationship are among the most faithful, especially during the early winter season. During strong El Niño conditions in the past, the excess snow in the high mountains of western Asia sometimes reached such an anomaly level that it was thought to impact the subsequent summer monsoon season in South Asia. This year, given the strong El Niño forcing, seasonal forecasts for CSWA also suggested an above-normal winter precipitation across the region. However, this winter has been remarkable, with many mountains in CSWA not seeing their first snowfall until the end of January and no major weather systems penetrating the region during this time.


Top: ECMWF winter (December-January-February; DJF) precipitation probability above median using Nov 1 initial conditions. Bottom: ECMWF winter (DJF) prediction for CSWA (left) and actual (CPC) precipitation anomaly over CSWA in December-January.

What caused this ENSO's flip-flop from faithful to unfaithful this year is a question that may attract the attention of the scientific community and flood journals with various hypotheses. It wouldn't be even surprising to witness attribution studies in the months ahead linking the absence of winter precipitation in a strong ENSO year over CSWA to the overarching effects of climate change. After all, the narrative aligns neatly with prevailing discourse.


Disappointingly for many, the precise reason behind the failure of the ENSO-CSWA relationship and poor predictability in seasonal forecasting systems this year is not shrouded in mystery or attributable to climate change. Instead, it stems from the delicate equilibrium between tropical and extratropical forcings that drives precipitation variability over CSWA – an understanding we have only recently begun to fathom and which challenges long-held beliefs about how teleconnections work in this region.


Winter precipitation variability in the CSWA region is influenced by two main factors: tropical and extratropical forcings. ENSO drives the tropical forcing, but this forcing does not originate directly from the equatorial Pacific. Instead, it comes indirectly through the Indian Ocean, where ENSO yields its influence through inter-basin interaction.


ENSO excites the leading mode of precipitation variability, a zonal dipole of precipitation, in the Indian Ocean during winter when it is at its peak strength. The atmospheric diabatic heating anomalies caused by this precipitation dipole propagate a Rossby wave response in the extratropics, establishing a positive teleconnection with CSWA. Direct ENSO forcing emanating from the equatorial Pacific establishes similar but weaker teleconnection.


Tropical forcing: the leading mode of precipitation variability in the Indian Ocean in winter. Taken from Horan et al., 2024

The extratropical forcing represents an internal atmospheric mode that resembles a mix of the East Atlantic Mode and the East Atlantic West Russia Pattern, among the modes of atmospheric variability in the northern hemisphere.


Extratropical forcing: the second leading mode of 200mb geopotential height after removing tropical forcings. Taken from Horan et al., 2024

The variability in the strength of the tropical forcing - precipitation dipole in the Indian Ocean - does not consistently match the intensity of ENSO events each year. For instance, while the El Niño forcing and its associated precipitation dipole in the Indian Ocean were similarly robust in the 1997/98 winter, the strength of the dipole was notably diminished in the 2015/16 winter despite El Niño being the most intense in modern records. This inconsistency bears significant consequences for CSWA, as the impact of ENSO on the region is primarily mediated by the strength of the Indian Ocean's precipitation dipole rather than the magnitude of sea surface temperature (SST) anomalies in the equatorial Pacific.


Furthermore, the synchronization of tropical and extratropical forcings plays a crucial role. When these forces align, CSWA experiences their maximum impact, whereas when they are out of phase, their effects can either dampen or reverse each other depending on their respective strengths.


Consider the cases of the 1997/98 and 2015/16 El Niños. In the 1997/98 winter, not only was the ENSO-induced precipitation dipole comparable in strength to the Pacific SST forcing (ENSO), but the tropical and extratropical influences were also in sync, resulting in significantly above-average winter precipitation over CSWA. In contrast, in the 2015/16 winter, not only the strength of the ENSO-forced precipitation dipole in the Indian Ocean was weaker compared to El Niño, but the extratropical forcing was also out of phase, leading to below-average winter precipitation over CSWA despite the exceptional intensity of the El Niño event.


Standardized anomalies in tropical forcing (leading mode of precipitation anomaly in the Indian Ocean), extratropical forcing (second leading mode of 200mb geopotential height over Europe/Asia), Nino3.4 (ENSO), and CSWA precipitation.

In the winter of 2023/24, the situation parallels 2015/16. While El Niño is one of the strongest in recent records, the ENSO-induced precipitation dipole in the Indian Ocean is less potent than the El Niño event. Additionally, the extratropical forcing is not only out of phase but also considerably stronger in the opposite phase. These background conditions collectively create an unfavorable environment for the normal winter season across CSWA.


Preliminary results for the 2023/24 winter season: standardized anomalies in tropical, extratropical, ENSO forcings, and CSWA precipitation.

Answering why the ECMWF seasonal winter outlook was poor for CSWA requires evaluating its forecast data. However, seasonal forecasting systems generally can reasonably represent the interannual variability of tropical forcing and its teleconnection with CSWA. However, they do not perform well in representing the characteristics of extratropical forcing, which represents internal atmospheric variability. These models exhibit poor skill in simulating the interannual variability of extratropical forcing and its teleconnection with CSWA. As a result, in years when extratropical forcing is substantially stronger and not accurately represented in models, poor predictability over CSWA is expected.


So, in essence, "ENSO Jani" remains faithful, but our evolving understanding struggles to grasp its intricate relationship with CSWA's winter climate.


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