Distinct Impacts of the Central and Eastern Atlantic Niño on the European Climate

The Atlantic Niño is the primary interannual variability mode in the tropical Atlantic, with far‐reaching impacts on global climate. A recent study identified two types of the Atlantic Niño, each with its maximum warming centered in the central and eastern equatorial Atlantic, respectively. Through analysis of observational data and numerical model experiments, we find that the two Atlantic Niño types have distinct climatic impacts on Europe. This is because the central Atlantic Niño is associated with a pronounced increase in precipitation in the western tropical Atlantic, while the positive precipitation anomalies during the eastern type are mainly located in the eastern basin with weaker amplitudes. Consequently, compared to the eastern Atlantic Niño, the extra‐tropical atmospheric waves and the associated precipitation and temperature anomalies in Europe during the central type are stronger and shifted westward. Therefore, distinguishing between the two Atlantic Niño types may help improve seasonal climate predictions in Europe.


Introduction
The Atlantic Niño is the dominant mode of interannual climate variability in the equatorial Atlantic (Carton et al., 1996;Keenlyside & Latif, 2007;Wang et al., 2009;Xie & Carton, 2004;Zebiak, 1993).It is characterized by prominent changes in sea surface temperature (SST), wind, and precipitation in the region, and typically peaks in boreal summer (Carton & Huang, 1994;Ding et al., 2009;Nnamchi et al., 2021;Picaut, 1983;Zhang & Han, 2021).During the Atlantic Niño, warm SST anomalies occur in the central and eastern tropical Atlantic basin, resulting in westerly wind anomalies.Additionally, these anomalies enhance convection and lead to increased precipitation in the Atlantic intertropical convergence zone.

Supporting Information:
Supporting Information may be found in the online version of this article.(Drevillon et al., 2003;Peng et al., 2005).The Atlantic Niño may also influence the ocean-atmosphere coupling processes in the North Atlantic, which can impact the climate conditions in Europe as well (Lu et al., 2019;Xie et al., 2023;Yang & Wang, 2023).Zhang et al. (2023) recently identified two types of the Atlantic Niño, that is, the central Atlantic Niño (CAN) and the eastern Atlantic Niño (EAN).The two Atlantic Niño types exhibit distinct patterns and diverse climatic impacts on local and remote regions.For instance, they found that the CAN has a more prominent influence on the subsequent El Niño-Southern Oscillation (ENSO) evolution compared to the EAN, especially after ∼2,000.This is because the SST warming associated with the CAN is embedded in a higher background SST in the central basin compared with the EAN.Consequently, the SST changes during the CAN can more effectively affect local precipitation and thereby create more prominent influences on remote regions.Observations also show that the EAN has been weakening substantially in the past few decades, while the strength of the CAN remains relatively stable.Such changes seem consistent with recent changes in the Atlantic Niño characteristics, which have been attributed to the Atlantic Multidecadal Oscillation (Martín-Rey et al., 2018).
The classification of the Atlantic Niño is in a way similar to the central Pacific (CP) and eastern Pacific (EP) ENSO (Capotondi et al., 2015;Freund et al., 2019;Kao & Yu, 2009;Kug et al., 2009), which are also classified based on their different locations of warming center.Previous research has shown that the CP and EP ENSO events have distinct effects on global climate (Boucharel et al., 2016;Chen et al., 2019;Guo et al., 2021;Hong et al., 2011;Lopez-Parages et al., 2016;Park et al., 2020), which are associated with different positions and intensities of the atmospheric Rossby (Pacific-North American) wave trains triggered by them (Cai et al., 2021;Chen et al., 2018;Ge & Luo, 2023;Graf & Zanchettin, 2012;Yu et al., 2012).In the tropical Atlantic Ocean, the Atlantic Niño is also associated with prominent anomalous heating source in the tropics, which may excite atmospheric waves propagating to the mid-latitudes and influence the climate conditions in the region as well.Given that the two Atlantic Niño types exhibit distinct SST and precipitation changes in the tropical Atlantic Ocean, whether they can also excite different atmospheric teleconnection patterns and yield distinct climatic impacts requires further investigation, which may help improve the seasonal climate predictions for European countries.
In this study, we explore the remote influences of the CAN and the EAN on Europe climate conditions during boreal summer (the peak season of the Atlantic Niño) by analyzing multiple sources of observational data sets and performing numerical experiments using a Linear Baroclinic Model (LBM).Our results indeed reveal that the two Atlantic Niño types have distinct climatic impacts on Europe, which are associated with the different tropical heating sources during the CAN and the EAN.

Data Sets
The monthly SST data utilized in this study are from the Hadley Centre Sea Surface Temperature data set (Rayner et al., 2003a), with a horizontal resolution of 1°× 1°.The precipitation, surface air temperature, winds, specific humidity, geopotential, mean surface net short-wave radiation flux, mean surface net long-wave radiation flux, mean surface latent heat flux, mean surface sensible heat flux and vertical integral of moisture flux divergence are obtained from the fifth generation of the European Center for Medium-Range Weather Forecasts atmospheric reanalysis data set (ERA5) (Hersbach et al., 2020a).The data set has a spatial resolution of 0.25°× 0.25°and a vertical resolution of 37 levels for winds and geopotential.The analysis period is from 1970 to 2020.The longterm linear trends in all anomaly fields have been removed to exclude the global warming signal.The significance test is based on the two-tailed p-value of the Student's t-test.

Indices of the CAN, EAN and Atlantic Niño
To characterize the CAN and EAN, we perform an empirical orthogonal function (EOF) analysis of the SST anomalies in the tropical Atlantic region (10°S-10°N, 40°W-20°E) from 1970 to 2020 (Figure S1 in Supporting Information S1).The first EOF mode mainly characterize the Atlantic Niño as a whole (Figure S1a in Supporting Information S1), while the third EOF mode depicts the east-west shift of the warming center (Figure S1c in Supporting Information S1).Hence, following Zhang et al. (2023), we define indices for the two types of the Atlantic Niño using the time series of the first and third principal component.The CAN index is obtained as .Atlantic Niño is defined by the Atl3 index as area-averaged SST anomalies over 3°S-3°N, 0°-20°E (Zebiak, 1993).

Atmospheric Wave Activity Flux and Linearized Rossby Wave Source
This study uses atmospheric wave activity flux to represent the horizontal propagation of quasi-stationary Rossby wave (Takaya & Nakamura, 2001).Under the quasi-geostrophic assumption, the horizontal component of wave activity flux can be expressed in the pressure coordinate system as: where a is the radius of the Earth (6.37 × 10 6 m), p is the normalized atmospheric pressure (pressure/1,000-hPa), and (φ, λ) denote the latitude and longitude, respectively.U c → = (U c ,V c ) represents the climatological horizontal winds, and prime denotes their anomalies.ψ′ = ϕ′ f is the stream function anomalies under the quasi-geostrophic assumption, where ϕ′ is the geopotential anomalies and f is the planetary vorticity.
To explore the influence of the Atlantic Niño on the generation of atmospheric Rossby waves, we further analyze the Rossby wave source (RWS) (Sardeshmukh & Hoskins, 1988) expressed as: where f and ζ represent planetary vorticity and relative vorticity, respectively.V χ is the divergent component of the horizontal winds and ∇ H is the horizontal gradient.The overbar indicates climatological mean and prime represents anomalies.Equation 2shows that RWS can be decomposed into two terms: the first term (RWS1) represents the component related to divergent winds anomalies and climatological absolute vorticity, while the second term (RWS2) represents the component related to relative vorticity anomalies and climatological divergent winds.

Linear Baroclinic Model Experiments
The LBM employed in this research is derived from the primitive equations linearized at a specific state (Watanabe et al., 2002;Watanabe & Jin, 2003).The model has a horizontal resolution of T42 (∼2.8°× 2.8°) and 20 sigma levels.Linear Rayleigh friction and Newtonian damping are incorporated into the model to simulate dynamic and thermodynamic dissipation.The heating source added to the model in this study is based on the location and intensity of the precipitation anomalies associated with the two types of Atlantic Niño events in observations.The damping time is set to 1 day in the lowest 3 layers and the top most 2 layers, and 30 days in the remaining layers.
The model is integrated for 30 days to ensure the stable generation of atmospheric waves resulting from the heating source forcing (Watanabe & Jin, 2003), and the average of the last 10 days is used for analysis.

Characteristics of the Two Atlantic Niño Types
We first characterize climate anomalies associated with the two types of the Atlantic Niño in the tropical Atlantic during boreal summer (June, July and August, JJA), the peak season of the Atlantic Niño.Observational data show that the CAN features the largest positive SST anomalies in the central equatorial Atlantic near 20°W (Figure 1a), while the largest positive SST anomalies of the EAN are situated in the eastern basin extending to the western African coasts (Figure 1b).Hence, the CAN and the EAN are associated with notably different SST gradient anomalies, which may further affect the local wind changes.Indeed, the low-level wind anomalies during the CAN are mainly characterized by westerly anomalies in the western equatorial Atlantic and easterly anomalies in the eastern basin (Figure 1a).In contrast, the EAN is dominated by westerly anomalies across the entire equatorial Atlantic (Figure 1b).
Both SST and wind anomalies may affect moisture supply and thereby induce precipitation changes.It is shown that the locations of the changes in the tropical Atlantic precipitation during the two Atlantic Niño types also differ.While the CAN precipitation anomaly center is situated near 40°W in the equatorial Atlantic (Figure 1c), the precipitation changes associated with the EAN are centered near 5°W with weaker amplitudes (Figure 1d).Hence, our results suggest that the two types of the Atlantic Niño indeed exhibit distinct characteristics in the equatorial Atlantic Ocean during boreal summer, consistent with the finding in Zhang et al. (2023).

Distinct Impacts on European Climate
The Atlantic Niño has a significant impact on the European climate, particularly on precipitation and temperature changes (Figures S2a and S2d in Supporting Information S1).Furthermore, as the two types of Atlantic Niño exhibit distinct characteristics in the equatorial Atlantic Ocean, it is worth further examining whether they have different impacts on the European climate through atmospheric teleconnections and exploring the underlying physical mechanisms.
The contrasting precipitation anomalies associated with the two types of the Atlantic Niño indeed lead to their distinct remote influences on European climate conditions.For instance, a regression analysis using reanalysis data reveals that the CAN induces low-level cyclonic wind anomalies over western Europe and anticyclonic wind anomalies over eastern Europe (Figures 2a and 2c).As a result, the European region at 20˚E is dominated by pronounced southerly anomalies.By contrast, the wind anomalies caused by the EAN are shifted eastward compared with the CAN with overall weaker amplitudes, and the associated southerly anomalies are located at 40˚E (Figures 2b and 2d).
The atmospheric circulation anomalies also lead to prominent changes in precipitation and temperature.The CAN enhances (suppresses) precipitation over Europe at 20˚E (50˚E) (Figure 2a), which is associated with the anomalous convergence (divergence) of moisture flux primarily due to the meridional moisture transport (Figure S3a in Supporting Information S1).Meanwhile, the cyclonic wind anomalies over western Europe causes cold anomalies, whereas the anomalous anti-cyclone over eastern Europe tends to warm the land surface through changes in the surface short-wave radiation (Figures 2c and S4a in Supporting Information S1).On the other hand, precipitation and temperature anomalies changes induced by the EAN are evidently weaker and shifted eastward by ∼20°longitude (Figures 2b and 2d).Overall, the two Atlantic Niño types both explains more than 40% of the variability in precipitation and temperature anomalies in the specific regions of Europe (Figures S2b, S2c, S2e and S2f in Supporting Information S1) and the spatial distribution is similar to the regression analysis (Figure 2).Note that the CAN and the EAN cause opposite precipitation anomalies at ∼40°E over Europe due to the zonal displacement of the associated circulation changes, highlighting the distinct impacts of the two Atlantic Niño types.The same zonal shift in the impact on temperature and precipitation over Europe is evident in the composite difference between CAN and EAN events (Figure S5 in Supporting Information S1).The differences are found to be significant at the 90% confidence levels over parts of western and eastern Europe (highlighted in Figure S5 in Supporting Information S1).

Atmospheric Waves Induced by the Central and Eastern Atlantic Niño
Previous studies have suggested that the remote influences of the Atlantic Niño are through exciting atmospheric Rossby waves originated from the tropics (García-Serrano et al., 2008;Hoskins & Karoly, 1981;Lübbecke et al., 2018).Indeed, the CAN induces alternating positive and negative geopotential height anomalies in the upper-level propagating from the North Atlantic Ocean to Europe (Figure 3a).The EAN excites a similar wave pattern, but with weaker magnitudes and eastward-shifted locations compared with the CAN (Figure 3b).Moreover, both types of the Atlantic Niño have a strong correspondence between the spatial distribution of upperlevel geopotential height anomalies and the lower-level atmospheric circulation anomalies (Figures 2,3a and 3b).
Consistent with the geopotential height anomalies, the atmospheric wave activity flux, which represents the Rossby wave energy propagation, also exhibit evident discrepancies between the two Atlantic Niño types (Figures 3a and 3b).The CAN induces strong wave activity fluxes originated from the mid-latitude North Atlantic Ocean, which then propagate northeastward toward Europe and deflected southward at ∼55°E, aligning well with the alternating pressure anomalies.The wave activity fluxes associated with the EAN are weaker and extended further to the east, which results in different wave patterns compared to the CAN.
The Atlantic Niño excites the atmospheric Rossby wave trains through inducing the tropical heating anomalies, which can be examined by analyzing the RWS.The RWS induced by both types are mainly located north of 20°N near the westerly jet stream (Figures 3c and 3d), as evident from the zonal mean profiles (Figure S6 in Supporting Information S1).Moreover, the RWS is mainly contributed by RWS1 associated with divergent winds anomalies Notably, the RWS induced by the CAN is stronger and extends further to the west compared with the EAN (Figures 3c and 3d).This difference is also due to the more prominent precipitation anomalies in the western tropical Atlantic during the CAN (Figures 1c and 1d), which cause stronger wind anomalies that further excite a more pronounced Rossby wave train.
To confirm the distinct roles of the two types of the Atlantic Niño in inducing the atmospheric teleconnection patterns, we further performed the LBM experiments forced with the heating sources that mimic the precipitation anomalies associated with the CAN and the EAN in observations.The tropical Atlantic precipitation anomaly center during the CAN is mainly located near 40°W in the equatorial Atlantic, while the EAN precipitation anomalies are mainly located at near 5°W.In addition, the intensity of the CAN precipitation anomalies is higher than that the EAN.Based on these precipitation anomaly patterns, we have respectively set up forcings in the tropical Atlantic for the two types in the LBM (Figures 4a and 4b).The model results show that both types can cause alternating positive and negative geopotential height anomalies propagating from the North Atlantic Ocean to Europe, which are similar to observational results despite some noticeable discrepancies in the locations of the pressure anomalies centers.More importantly, the waves induced by the CAN are stronger and shifted westward compared to the EAN (Figures 4c and 4d).Hence, the LBM results are overall consistent with observations and thus support our hypothesis that the two types of the Atlantic Niño can induce distinct atmospheric teleconnection patterns.
In addition, in order to further validate the physical processes of the impact of the two Atlantic Niño types on the European climate, we also conduct a case study (Figure S8 in Supporting Information S1).When the CAN or EAN events occurred, the atmospheric circulation associated with the alternating positive and negative geopotential height anomalies thereby affects precipitation and temperature anomalies in Europe which were similar to the findings from the regression analysis (Figures 2,3a,and 3b).This provides compelling evidence that both types of Atlantic Niño events have the potential to impact the climate in Europe through their influence on atmospheric circulation patterns.

Summary and Discussion
In this study, we examined the remote impacts of the CAN and EAN on the European climate through analyzing observational data, reanalysis product and performing LBM experiments.Observations show that the two types of Both types can influence the European climate conditions through inducing atmospheric Rossby wave trains, but with notable discrepancies.In particular, the RWS induced by the CAN is stronger and extends further westward compared to the EAN, which results in the stronger and westward shifted atmospheric waves generated by the CAN.This can be attributed to the positive precipitation anomalies being located in the western tropical Atlantic Ocean associated with the CAN.As a result, the CAN induces prominent changes in the European climate, producing positive (negative) precipitation anomalies and negative (positive) temperature anomalies in western (eastern) Europe, which are stronger and located further to the west compared to those caused by the EAN.The LBM experiments effectively reproduced the distinct teleconnection patterns associated with the two types of the Atlantic Niño, which aligns well with the observations.These findings provide support for the need to separate the Atlantic Niño into the CAN and the EAN types.The two types not only have different impacts on ENSO (Zhang et al., 2023), but also exhibit distinct effects on the climate conditions of Europe.It is therefore important to distinguish the two Atlantic Niño types as well as their remote influences on the other region.Gaining deeper insights and understanding of the patterns and mechanisms by which the two types of the Atlantic Niño events influence the climate of other regions globally will contribute to a better understanding of the processes driving global climate change, which may help improve seasonal climate predictions.
It has also been noted that the impacts of the two Atlantic Niño types on remote regions have diverged after ∼2,000, with stronger impacts of the central type and overall weaker influences from the eastern type.This is due to the substantial weakening of the EAN in recent decades (Zhang et al., 2023).Further research that examines whether there are decadal variations in the influence of the two Atlantic Niño types on European climate conditions is warranted.Additionally, it is also worth investigating whether the impacts of the two types of the Atlantic Niño on the climate of Europe will change under future scenarios of global warming, which may provide guidance for future climate projections of the region.While the statistical analysis conducted here does not distinguish between cold and warm events, further composite analysis revealed that the regression patterns are dominated by the warm phase and that the response to cold Atlantic Niña events is actually slightly different (not shown).This aspect of asymmetry will be investigated in a future study.

Figure 1 .
Figure 1.Characteristics of the two types of the Atlantic Niño.(a, b) Regression of JJA sea surface temperature anomalies (shading; °C) and 850-hPa wind anomalies (vector; m s 1 ) on the normalized JJA central Atlantic Niño index (CANI) and eastern Atlantic Niño index (EANI), respectively.Shading and vectors denote results that are statistically significant at the 90% confidence level.(c, d) JJA precipitation anomalies (mm day 1 ) regressed on the normalized JJA CANI and EANI, respectively.The black dots denote results that are 90% statistically significant.

Figure 2 .
Figure 2. Impacts of the two Atlantic Niño types on Europe.(a, b) Regression of JJA precipitation anomalies (shading; mm day 1 ) and 850-hPa wind anomalies (vector; m s 1 ) on the normalized JJA central Atlantic Niño index (CANI) and eastern Atlantic Niño index (EANI), respectively.(c,d) The JJA surface air temperature anomalies (SAT; shading; K) and 850-hPa wind anomalies (vector; m s 1 ) regressed on the normalized JJA CANI and EANI, respectively.Stippling and blue vectors represent anomalies that are statistically significant at the 90% confidence level.

Figure 3 .
Figure 3. Atmospheric waves induced by the two Atlantic Niño types.(a, b) Regression of JJA 200-hPa geopotential height anomalies (GPH; shading; m) and wave activity fluxes (WAF; vector; m 2 s 2 ) on the normalized JJA central Atlantic Niño index (CANI) and eastern Atlantic Niño index (EANI), respectively.(c, d) 200-hPa Rossby wave source (RWS; shading; s 2 ) regressed on the normalized JJA CANI and EANI, respectively.Gray lines denote the zonal wind climatology (contour; m s 1 ) during JJA.The black dots indicate the signals are statistically significant at the 90% confidence level.

Figure 4 .
Figure 4. Linear baroclinic model experiment results.(a, b) Idealized heating pattern based on the distribution of precipitation anomalies in the tropics caused by the central Atlantic Niño (CAN) and eastern Atlantic Niño (EAN), respectively.(c, d) 200-hPa GPH (shading; m) and wind anomalies (vector; m s 1 ) in response to the CAN and EAN idealized heating.Shown are average of 20-30 days of the LBM runs.
or modulating the North Atlantic Oscillation