Sensitivity of the atmospheric response to warm pool El Niño events to modeled SSTs and future climate forcings

Abstract

[1] Warm pool El Niño (WPEN) events are characterized by positive sea surface temperature (SST) anomalies in the central equatorial Pacific. Under present-day climate conditions, WPEN events generate poleward propagating wavetrains and enhance midlatitude planetary wave activity, weakening the stratospheric polar vortices. The late 21st century extratropical atmospheric response to WPEN events is investigated using the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM), version 2. GEOSCCM simulations are forced by projected late 21st century concentrations of greenhouse gases (GHGs) and ozone-depleting substances (ODSs) and by SSTs and sea ice concentrations from an existing ocean-atmosphere simulation. Despite known ocean-atmosphere model biases, the prescribed SST fields represent a best estimate of the structure of late 21st century WPEN events. The future Arctic vortex response is qualitatively similar to that observed in recent decades but is weaker in late winter. This response reflects the weaker SST forcing in the Niño 3.4 region and subsequently weaker Northern Hemisphere tropospheric teleconnections. The Antarctic stratosphere does not respond to WPEN events in a future climate, reflecting a change in tropospheric teleconnections: The meridional wavetrain weakens while a more zonal wavetrain originates near Australia. Sensitivity simulations show that a strong poleward wavetrain response to WPEN requires a strengthening and southeastward extension of the South Pacific Convergence Zone; this feature is not captured by the late 21st century modeled SSTs. Expected future increases in GHGs and decreases in ODSs do not affect the polar stratospheric responses to WPEN.

Signal of central Pacific El Niño in the Southern Hemispheric stratosphere during austral spring

“…During EPEN, there are no significant variations in SH polar temperatures before October (not shown). In addition, as suggested by Hurwitz et al [2013], the upper stratospheric anomalies are already well established by October in the two modeling experiments, likely due to heat flux anomalies earlier in the season. Figure 5 shows the composite anomalies of the zonal mean temperature and zonal wind during CP El Niño events from July to December, based on the ERA-Interim data set.…”

“…During EPEN, there are no significant variations in SH polar temperatures before October (not shown). In addition, as suggested by Hurwitz et al [], the upper stratospheric anomalies are already well established by October in the two modeling experiments, likely due to heat flux anomalies earlier in the season.…”

The Teleconnection of El Niño Southern Oscillation to the Stratosphere

“…(3) The impact of the teleconnections may be different for changes in the background state of the impact region. Several model studies predict that the nature of ENSO will be modified under a changing climate, with projections of increased amplitude and/or frequency of the events (e.g., Cai et al, , ; Latif et al, ; Timmermann et al, ; Zheng et al, ), though not all studies agree on the sign and amplitude of a possible change in ENSO (Chen et al, ; Collins et al, ; Hurwitz et al, ; Latif & Keenlyside, ; Stevenson, ). For a review of the changes in ENSO with climate change, see S. Yang et al ().…”

“…In addition, unforced coupled ocean‐atmosphere model runs spontaneously generate multidecadal epochs dominated by CP or EP events (Capotondi et al, ). While it remains unclear if the distribution between EP and CP El Niño will change in the future, it is possible that if the nature of ENSO events indeed changes, then changes in the stratospheric response could be expected with climate change (Hurwitz et al, ). Even if the character of ENSO events was to not appreciably change, trends in the tropospheric midlatitude mean state and the stratosphere may affect how teleconnections propagate to and impact the stratosphere. Changes are expected in the extratropical quasi‐stationary sea‐level pressure distribution—CMIP5 models simulate a deepening of the Aleutian low under climate change (Chang, ; Karpechko & Manzini, ), while other models predict an eastward shift of the ENSO impact in the North Pacific (Müller & Roeckner, )—and hence, a given teleconnection will be superimposed on locally altered climatological stationary waves.…”

Teleconnection between the central tropical Pacific and the Antarctic stratosphere: spatial patterns and time lags

“…Interestingly, that simulations of the atmospheric response to the El Niño events in the central equatorial Pacific in future warmer climate shows a change in the SH tropospheric teleconnections: The meridional wave train weakens and a zonal wave response appears in austral spring (Hurwitz et al 2013). Further analysis is necessary to determine whether the seasonal atmosphere warming in the austral spring could contribute to the teleconnection pattern change between October and November (Fig.…”