Non-linearity in the pathway of El Niño-Southern Oscillation to the tropical North Atlantic

Abstract

El Niño-Southern Oscillation can influence the Tropical North Atlantic (TNA), leading to anomalous sea surface temperatures (SST) at a lag of several months. Several mechanisms have been proposed to explain this teleconnection. These mechanisms include both tropical and extratropical pathways, contributing to anomalous trade winds and static stability over the TNA region. The TNA SST response to ENSO has been suggested to be nonlinear. Yet the overall linearity of the ENSO-TNA teleconnection via the two pathways remains unclear. Here we use reanalysis data to confirm that the SST anomaly (SSTA) in the TNA is nonlinear with respect to the strength of the SST forcing in the tropical Pacific, as further increases in El Niño magnitudes cease to create further increases of the TNA SSTA. We further show that the tropical pathway is more linear than the extratropical pathway by sub-dividing the inter-basin connection into extratropical and tropical pathways. This is confirmed by a climate model participating in the CMIP5. The extratropical pathway is modulated by the North Atlantic Oscillation (NAO) and the location of the SSTA in the Pacific, but this modulation insufficiently explains the nonlinearity in TNA SSTA. As neither extratropical nor tropical pathways can explain the nonlinearity, this suggests that external factors are at play. Further analysis shows that the TNA SSTA is highly influenced by the preconditioning of the tropical Atlantic SST. This preconditioning is found to be associated with the NAO through SST-tripole patterns.

Modulation of the ENSO teleconnection to the North Atlantic by the tropical North Atlantic and Caribbean

“…As a result of an altered Kelvin wave, this may limit the accuracy of the Secondary Gill response. Nonetheless, given that the location of both the Secondary Gill and asymmetric Gill responses match those from reanalysis (Casselman et al, 2021), we believe that our results are representative of the observations. Although from our results, it is clear that the Pacific dominates the Walker circulation during boreal spring, the Atlantic modulates the Walker index considerably, whereby the correlation with the Caribbean RWS nearly doubles when adding the tropical Atlantic forcing to the Pacific forcing (AP response).…”

“…We use multiple linear regression (MLR) analysis to isolate the linear influence from the Atlantic and Pacific, respectively, and plot the pointwise MLR coefficients following the method in Izumo et al (2010) and Casselman et al (2021). We also utilize partial correlation between three variables, following Wang et al (2006).…”

“…Finally, we focus on boreal spring and boreal summer for all fields as we are interested in the impact the TNA SSTA peak has following an ENSO event (MAM, Casselman et al (2021)), as well as the time period when the Atlantic warm pool has the largest influence onto the Walker circulation (JJA). Both MAM and JJA are also related to wave trains towards the NAE region (Lim, 2015;Jung et al, 2017;Wulff et al, 2017;Li et al, 2018;Choi and Ahn, 2019;Neddermann et al, 2019;Lledó et al, 2020).…”

“…2019; Rieke et al, 2021). Furthermore, different regions modulate the regional response to ENSO teleconnections, such as the tropical North Atlantic (Sung et al, 2013;Casselman et al, 2021), and the Indian Ocean (Zhong et al, 2005;Fletcher and Cassou, 2015;Joshi et al, 2021). The teleconnections are, furthermore, sensitive to aspects such as the longitudinal position of Pacific sea surface temperatures (SSTs) (Zhang et al, 2019), non-stationarity (Rieke et al, 2021), and trends over time (Garfinkel et al, 2019).…”

“…The TNA region is salient for its strong connection to ENSO, whereby TNA SST anomalies (SSTA) are positively correlated with the Niño3.4 index and peak in boreal spring (March-May, MAM) following an ENSO event (Enfield and Mayer, 1997;Lee et al, 2008). This robust teleconnection travels through both a tropical and an extratropical pathway, influencing the TNA SSTs through either moist convection/stability processes or changes in the trade winds (Jiang and Li, 2019;Casselman et al, 2021). There is also a connection between the TNA and NAE region.…”