Arctic sea ice–air interactions weaken El Niño–Southern Oscillation

This genre of paper is interesting. Underlying the research is the assumption that climate model simulations are data or reality. The researchers then perform an in-depth analysis of these PlayStation worlds as if the simulations represented physical reality.

A new study, published in Science Advances by researchers at the University at Albany and Nanjing University of Information Science and Technology in China, has found that these events, which typically occur once every few years, might become even stronger due to melting Arctic sea ice.

Using a combination of climate model simulations and observational data, the researchers found that the current interaction of Arctic sea ice with the atmosphere reduces the strength of El Niño events by up to 17%, compared to when the interaction is removed.

https://phys.org/news/2024-04-atmospheric-scientists-link-arctic-sea.amp

To reach their findings, the researchers performed and analyzed two global climate model simulations for 500 years using the Community Earth System Model from the National Center for Atmospheric Research. The simulations, carried out on a computer hosted at the UAlbany Data Center, had fixed atmospheric CO₂ levels, one with sea ice-air interactions in the Arctic, and another without it.

By examining the difference between the two simulations, the researchers found that Arctic sea ice-air interactions weaken El Niño-related variations in the tropical Pacific Ocean by about 12 to 17%, compared to when the interaction was removed.

“The difference between the two model simulations represents the impact of the Arctic sea ice-air coupling, which led to significant changes in tropical Pacific Ocean mean climate states and El Niño–Southern Oscillation strength. This was mainly due to asymmetric impacts of positive and negative sea-ice anomalies on surface fluxes, the exchange of heat crossing the surface between the ocean and the atmosphere,” said Jiechun Deng, an associate professor at Nanjing University of Information Science and Technology and the study’s lead author.

“Our findings highlight the crucial role of sea ice-air interactions in regulating El Niño activity over the tropical Pacific. It calls for a more realistic representation of such interactions in current climate models, to better project El Niño and its various impacts in a warming future.”

https://phys.org/news/2024-04-atmospheric-scientists-link-arctic-sea.amp

Here is the abstract

Abstract

El Niño–Southern Oscillation (ENSO) over the tropical Pacific can affect Arctic climate, but whether it can be influenced by the Arctic is unclear. Using model simulations, we show that Arctic sea ice–air interactions weaken ENSO by about 12 to 17%. The northern North Pacific Ocean warms due to increased absorption of solar radiation under such interactions. The warming excites an anomalous tropospheric Rossby wave propagating equatorward into the tropical Pacific to strengthen cross-equator winds and deepen the thermocline. These mean changes dampen ENSO amplitude via weakened thermocline and zonal advective feedbacks. Observed historical changes from 1921–1960 (with strong sea ice–air interactions) to 1971–2000 (with weak interactions) are qualitatively consistent with the model results. Our findings suggest that Arctic sea ice–air interactions affect both the mean state and variability in the tropical Pacific, and imply increased ENSO amplitude as Arctic sea ice and its interactions with the atmosphere diminish under anthropogenic warming.

https://www.science.org/doi/10.1126/sciadv.adk3990

Here is a “summation” of what the researchers “found”.

Arctic–to–tropical Pacific teleconnection

How would the above mean-state changes over the Arctic and northern North Pacific (mainly the BOS region) cause remote mean-state changes over the tropical Pacific? During JJAS, the increased surface net upward energy over the BOS in FC due to the sea ice–air coupling causes tropospheric warming that is largest near the surface and extends to ~200 hPa (Fig. 4A and fig. S8A), which leads to a positive 200-hPa geopotential height (Z200) anomaly (Fig. 4B and fig. S8B) and a negative sea level pressure (SLP) anomaly over the BOS (mainly the Sea of Okhotsk) (Fig. 4C) and thus ascending anomalies there (fig. S8C). The concurring anomalous divergence in the upper troposphere over most of the BOS region can locally generate a negative Rossby wave source (RWS) through the vortex squeezing (see Materials and Methods and fig. S9) and, thus, an anomalous Rossby wave propagating equatorward into the tropical Pacific. Concurrently, decreased air temperature (T) and Z200 with increased SLP are seen over a southwest-northeast zone just south of the BOS, while a negative SLP anomaly is located along ~20°N with weak positive Z200 anomaly (Fig. 4, A to C), featuring a northward tilted structure of such an anomalous Rossby wave (fig. S8B). Note that the increased SLP over the midlatitude North Pacific due to sea ice–air interactions is nearly opposite to that induced by Arctic sea ice loss (i.e., a negative SLP anomaly) noticed previously (24, 25), implying their competing effects on the tropical Pacific mean state and, thus, ENSO variability. The above T changes over the North Pacific north of ~20°N reduce the south-north T gradient and thus upper-level zonal wind along 40°N (fig. S8D), which decelerates the southern part of the mean westerly jet to facilitate the southeastward propagation of the anomalous Rossby wave.

Along with increased SLP over the midlatitude North Pacific, the anticyclonic surface circulation anomaly extends from the east of Japan to the west of North America (Fig. 4C). On one hand, the southwesterly wind anomaly on its northwestern side can transport more warm and moist airmass poleward (fig. S7E) to further warm the SSTs in the BOS region through increased downward LW radiation (Fig. 3I); on the other hand, the northeasterly wind anomaly in the south would strengthen the prevailing trade wind along ~30°N over the North Pacific (Fig. 4C), thus causing cooler SSTs there (Fig. 4D) via the WES feedback (34). Meanwhile, anomalous cyclonic circulation is found in the western tropical Pacific along ~20°N together with a negative SLP anomaly there (Fig. 4C) and anomalous subsidence to its south near ~10°N (fig. S8C). Thus, the anomalous southwesterly wind along its southern flank (north of 10°N) could weaken the prevailing northeasterlies (Fig. 4C), while the anomalous northerly wind to its south (south of 10°N) decreases the mean cross-equator southerlies in the western equatorial Pacific (Fig. 2A). These mean surface wind changes ultimately lead to warmer mean SSTs near the warm pool (Fig. 4D) via the WES feedback. This increases zonal SST gradients that strengthen mean trade winds over the central-eastern tropical Pacific to induce colder SSTs in the east (Fig. 2A), which would, in turn, further intensify surface winds and thus form a positive feedback loop (i.e., the Bjerknes feedback). Such an SST response pattern over the tropical and North Pacific resembles that induced by winter SIC anomalies over the Sea of Okhotsk through increased surface heat fluxes (36), although they appear in different seasons. In addition, the concurring negative anomalies of wind stress curl (fig. S4E) also deepen the thermocline over the central tropical Pacific via Ekman pumping. These mean changes in equatorial trade winds and thermocline depth can substantially weaken ENSO amplitude through dampening the TH and ZA feedbacks as discussed above. The southeastward propagating anomalous Rossby wave (Fig. 4B), excited by anomalous heating over the BOS and northern North Pacific due to sea ice–air interactions, serves as a key bridge that connects the Arctic and tropical Pacific mean-state changes.

https://www.science.org/doi/10.1126/sciadv.adk3990

The Discussion (conclusion)

DISCUSSION

In summary, our model experiments reveal that Arctic sea ice–air interactions can remotely weaken ENSO’s amplitudes and associated variability in other fields in the tropical Pacific by 12 to 17% by modulating the tropical Pacific mean state. The main processes, summarized in Fig. 6, are as follows. During JJAS, sea ice–air interactions would lead to enhanced surface SW absorption, warmer SST, and increased upward energy fluxes over the northern North Pacific (mainly the BOS region) near the MIZ, causing anomalous warming from the surface to around 200 hPa there. Such a heating anomaly forces an anomalous cyclone at the surface over the BOS and an anticyclonic anomaly in the upper troposphere locally; it also excites an anomalous atmospheric Rossby wave propagating equatorward into the tropical Pacific. The Rossby wave creates alternating SLP patterns toward the tropical Pacific, with an anticyclonic response in the midlatitude North Pacific and a cyclonic anomaly around 20°N in the western-central Pacific. The anomalous southwesterly winds over the western-central tropical Pacific act to weaken the mean northeasterly trades, leading to warmer SSTs in the western-central tropical Pacific via the WES feedback and, thus, the enlarged zonal SST gradients over the tropical Pacific. This causes stronger mean trade winds and colder SSTs in the CEP through both Bjerknes and WES feedbacks, together with a slightly deeper mean thermocline in the western-central tropical Pacific by an accumulation of warm waters and Ekman pumping there. As a result, the ENSO amplitude is dampened because of the weakened TH feedback by the deeper mean thermocline and the weakened ZA feedback by the enlarged zonal SST gradient under stronger mean trade winds.

https://www.science.org/doi/10.1126/sciadv.adk3990

The discussion continues with the following paragraph, emphasis mine.

Historical changes from P1 (with weak ENSO, strong SIC variations, and likely strong sea ice–air interactions) to P2 (with strong ENSO, weak SIC variations, and likely weak sea ice–air interactions) based on observational and reanalysis data are qualitatively consistent with the model results, such as the surface heat flux–induced teleconnection between the Arctic and tropical Pacific. The historical changes between P1 and P2 appear to be linked to changes in the strength of the sea ice–air interactions over the Pacific sector of the Arctic. Compared with P1, ENSO activity has already strengthened in P2 when the SIC variations become much weaker during JJAS (implying a weakening of the sea ice–air interactions) (fig. S5), although many other factors may also play a role in the real world. Note that the SIC variations have also become relatively stronger after the 1990s (but much weaker than P1 due to large sea ice loss). This implies that the associated stronger sea ice–air interactions might lead to weakened ENSO amplitude and increased CP El Niño occurrence in recent decades (24, 31), which is qualitatively consistent with our model results. However, this observed multidecadal relationship of the SIC variations with ENSO activity and diversity needs further investigation in multiple coupled climate models.

When I read this paper, especially paragraphs such as the above, all I could think of was this.

For brave souls who wish to dive in further, the paper can be found here.