5:15 PM - 7:15 PM
[ACG43-P02] Extratropical precursors to the Atlantic Zonal Mode and ENSO
Keywords:Atlantic zonal mode, ENSO, precursor, internal variability, GCM experiments
Westerly wind anomalies over the equatorial Atlantic and Pacific play an important role in initiating the Atlantic Zonal Mode (AZM; also known as Atlantic Niño) and El Niño/Southern Oscillation (ENSO). Both the AZM and ENSO tend to develop in boreal spring and may interact through the Walker circulation. For example, it has been suggested that a negative AZM event can contribute to the development of El Niño in the Pacific. Conversely, it has also been suggested that an early developing El Niño event can trigger a negative AZM event. Disentangling these causalities has proven difficult because both variability patterns tend to develop around the same time in spring.
Here we use multi-centennial atmosphere-only simulations with prescribed climatological SSTs to evaluate the role of atmospheric internal variability as a precursor to both the AZM and ENSO. Despite the absence of SST anomalies, the simulations produce substantial wind variability over the equatorial basins, highlighting the role of internal atmospheric variability. Composite analysis indicates that westerly wind events over the equatorial Atlantic are accompanied by easterly anomalies over the maritime continent and western equatorial Pacific. These anomalies are preceded by a North Atlantic Oscillation (NAO)-like pattern during later winter. Our results suggest that atmospheric internal variability tends to produce opposite-signed wind anomalies over the western equatorial Pacific and Atlantic. This could mean that an external factor, such as the NAO, seeds the development of the AZM and ENSO. Their further evolution will be determined by interbasin interaction but also by the pre-conditioning of the two basins. To what extent such processes act in nature awaits further investigation.
Here we use multi-centennial atmosphere-only simulations with prescribed climatological SSTs to evaluate the role of atmospheric internal variability as a precursor to both the AZM and ENSO. Despite the absence of SST anomalies, the simulations produce substantial wind variability over the equatorial basins, highlighting the role of internal atmospheric variability. Composite analysis indicates that westerly wind events over the equatorial Atlantic are accompanied by easterly anomalies over the maritime continent and western equatorial Pacific. These anomalies are preceded by a North Atlantic Oscillation (NAO)-like pattern during later winter. Our results suggest that atmospheric internal variability tends to produce opposite-signed wind anomalies over the western equatorial Pacific and Atlantic. This could mean that an external factor, such as the NAO, seeds the development of the AZM and ENSO. Their further evolution will be determined by interbasin interaction but also by the pre-conditioning of the two basins. To what extent such processes act in nature awaits further investigation.