The Teleconnection of El Niño Southern Oscillation to the Stratosphere

Abstract

El Niño and La Niña events in the tropical Pacific have significant and disrupting impacts on the global atmospheric and oceanic circulation. El Niño Southern Oscillation (ENSO) impacts also extend above the troposphere, affecting the strength and variability of the stratospheric polar vortex in the high latitudes of both hemispheres, as well as the composition and circulation of the tropical stratosphere. El Niño events are associated with a warming and weakening of the polar vortex in the polar stratosphere of both hemispheres, while a cooling can be observed in the tropical lower stratosphere. These impacts are linked by a strengthened Brewer-Dobson circulation. Anomalous upward wave propagation is observed in the extratropics of both hemispheres. For La Niña, these anomalies are often opposite. The stratosphere in turn affects surface weather and climate over large areas of the globe. Since these surface impacts are long-lived, the changes in the stratosphere can lead to improved surface predictions on time scales of weeks to months. Over the past decade, our understanding of the mechanisms through which ENSO can drive impacts remote from the tropical Pacific has improved. This study reviews the possible mechanisms connecting ENSO to the stratosphere in the tropics and the extratropics of both hemispheres while also considering open questions, including nonlinearities in the teleconnections, the role of ENSO diversity, and the impacts of climate change and variability.Plain Language Summary El Niño and La Niña events, the irregular warming and cooling of the tropical Pacific that occurs every couple of years, have disrupting impacts spanning the entire world. These remote impacts, so-called "teleconnections", also reach the stratosphere, the layer of the atmosphere starting at around 10 km above the Earth's surface. El Niño leads to a warming of the stratosphere in both hemispheres, while the lower tropical stratosphere cools. These signatures are linked by a strengthened stratospheric circulation from the tropics to the polar regions. El Niño also leads to more frequent breakdowns of the stratospheric polar vortex, a band of strong eastward winds in the polar stratosphere. For La Niña, these effects tend to be opposite, though they are not always robust, suggesting nonlinear or nonstationary effects, long-term variability, and trends in the teleconnections. The observational data record is not yet long enough to make conclusions with certainty, and models that try to reproduce the teleconnections indicate that teleconnections might be more linear than the limited number of observations indicate. Further research will be needed to separate the El Niño and La Niña teleconnections from other effects and to determine to what extent nonlinearity and nonstationarity are indeed present.

Weakening of the Teleconnection From El Niño–Southern Oscillation to the Arctic Stratosphere Over the Past Few Decades: What Can Be Learned From Subseasonal Forecast Models?

“…The weakening of the ENSO‐vortex connection over the past few decades is also reflected in SSW frequency during each ENSO phase, with SSW preferentially occurring during LN recently (Butler & Polvani, ; Domeisen et al, ). In contrast, the S2S models simulated more frequent polar stratospheric easterlies during EN over this period (Figure ).…”

“…In addition, the North Pacific ridge associated with LN has shifted closer to North America since 1979 (Kumar et al, ; Yang et al, ) where it leads to an enhanced wave‐2 signal (Yang et al, ). This apparent decadal variability is unforced and rather occurred by chance. Model simulations run over a limited period can also simulate a relative weakening of the ENSO- vortex effect that is reminiscent of that observed, despite showing a robust connection over a longer period (Weinberger et al, ) The tendency of EN to be associated with more SSW and LN with fewer SSW events in the first half of this period only but not the second (Butler & Polvani, ; Domeisen et al, ). Over the entire period there is little difference in the frequency of SSW events between LN and EN winters (Butler & Polvani, ; Garfinkel et al, ), though there is sensitivity to how LN events and SSW events are classified (Polvani et al, ; Song & Son, ). It is possible that there may never have been a strong connection in the first place and the satellite network since 1979 better constrains reanalysis products.…”

“…The stratospheric response to EN events is driven by a deepened Aleutian low (e.g., Barnston & Livezey, ), which constructively interferes with the climatological stationary planetary wave pattern and leads to strengthened wave flux into the stratosphere (i.e., linear interference; Garfinkel & Hartmann, ; Garfinkel et al, ; Smith et al, ; Smith & Kushner, ), as recently reviewed by Domeisen et al (). Other modes of tropospheric variability (e.g., October Eurasian snow) can affect the vortex through a similar pathway but via variability over western Siberia/eastern Europe, where a ridge constructively interferes with the climatological stationary planetary wave pattern and leads to strengthened wave flux into the stratosphere (Bao et al, ; Cohen & Jones, ; Garfinkel et al, ; Kretschmer et al, ).…”

“…Over the past few decades the connection between ENSO and the Arctic stratospheric vortex has weakened, and if one examines only the period since 1979 it is not statistically significant in the lower stratosphere (Domeisen et al, ; Hu et al, ; Yang et al, ). Consistent with this, decadal variability is also present in the Atlantic sector response to ENSO, with a negative correlation between ENSO and the North Atlantic Oscillation from 1958 to 1978 and a weak response afterward (Domeisen et al, ; Greatbatch et al, ). This leads to differences in the precipitation response over Europe to ENSO (Ayarzagüena et al, ).…”

“…Several possible explanations for this decadal variability may exist: Decadal variability in the Pacific or Atlantic Ocean (Gershunov & Barnett, ; Hurwitz et al, ; Hu & Guan, ; Kren et al, ; Omrani et al, ; Woo et al, ). Modulation of the ENSO teleconnection by the quasi‐biennial oscillation (QBO): When westerly QBO occurs during EN, both the tropospheric and stratospheric teleconnections are stronger (Calvo et al, ; Garfinkel & Hartmann, ), and this alignment occurred preferentially before 1979 (Domeisen et al, ; Garfinkel & Hartmann, ; Hu et al, ). Changes in the North Pacific tropospheric anomalies during ENSO: The North Pacific low extended into northeast Asia during the period before 1979 but was confined to the northeast Pacific since 1979 (Domeisen et al, ; Hu et al, ). A low confined to the northeast Pacific is less efficient in weakening the vortex than a low that extends further west (Garfinkel et al, ).…”