2:30 PM - 2:45 PM
[ACG33-03] The extratropical pacemaker effect on Pacific Walker circulation trend
Keywords:future projestion, the Walker circulation, the inter-ocean basin interaction
The Pacific Walker circulation variability is associated with El Niño-Southern Oscillation and Interdecadal Pacific Oscillation and affects the global climate. The Walker circulation has been strengthened since the comprehensive satellite observation. However, it is known that general circulation model (GCM) simulations driven by historical radiative forcing don’t reproduce the observed strengthened Walker circulation trend. The observed strengthened Walker circulation is out of range of the internal variability of the historical simulations by GCMs, suggesting the possibility that this discrepancy is caused by model biases.
The strengthening trend of the Walker circulation is associated with the sea surface temperature (SST) trend of greater warming in the tropical western Pacific Ocean than in the tropical eastern Pacific, the so-called ‘La Nina-like SST change’, in the same period. On the other hand, the historical simulations by GCMs tend to show larger eastern Pacific SST warming, the so-called ‘El Nino-like SST change’. These discrepancies between the observation and GCM simulations can influence the predictability of global climate, therefore, it is important to improve the physical-base understanding of dis reproducibility of the Walker circulation.
Regarding this observation-model inconsistency, recent studies argue the possible model bias in the reproducibility of SST change over the Southern Ocean and in the physical process of SST propagation from the Southern Ocean to the equatorward. Many other processes are known to determine the SST gradient in the tropical Pacific and the Walker circulation variabilities, including inter-basin interactions from other basins. However, all these discussions are qualitative only, and there is no quantitative physical understanding of the observation-model inconsistency in the Walker circulation trend.
In this study, we conducted several series of pacemaker experiments with GCM in which SST changes in specific ocean basins outside of the tropical Pacific are restored towards the observed values and examined remote influences from SST in extratropics to the Walker circulation and the tropical Pacific SST.
By restoring SST change towards observations poleward of 15º latitudes in the Pacific and 30º latitudes in the Indian and Atlantic Oceans (with a linearly tapering buffer zone between 10º and 15º, and between 25º and 30º), the GCM reproduced the observed Walker circulation trend very well. This experiment also reproduces the La Nina-like SST trend and interannual variabilities of the Walker circulation and SST in the tropical Pacific Ocean. This result suggests the importance of subtropical and midlatitude SST variability in attributing, predicting, and projecting the Walker circulation changes.
By varying ocean regions to which observed SST change is restored, the quantitative contributions of each ocean basin to the observed Walker circulation were evaluated. It is also suggested that the northern Pacific Ocean significantly affected the observed Walker circulation trend and there is a possibility that some model biases exist in equatorward SST propagation in the south Pacific around 20°S.
The strengthening trend of the Walker circulation is associated with the sea surface temperature (SST) trend of greater warming in the tropical western Pacific Ocean than in the tropical eastern Pacific, the so-called ‘La Nina-like SST change’, in the same period. On the other hand, the historical simulations by GCMs tend to show larger eastern Pacific SST warming, the so-called ‘El Nino-like SST change’. These discrepancies between the observation and GCM simulations can influence the predictability of global climate, therefore, it is important to improve the physical-base understanding of dis reproducibility of the Walker circulation.
Regarding this observation-model inconsistency, recent studies argue the possible model bias in the reproducibility of SST change over the Southern Ocean and in the physical process of SST propagation from the Southern Ocean to the equatorward. Many other processes are known to determine the SST gradient in the tropical Pacific and the Walker circulation variabilities, including inter-basin interactions from other basins. However, all these discussions are qualitative only, and there is no quantitative physical understanding of the observation-model inconsistency in the Walker circulation trend.
In this study, we conducted several series of pacemaker experiments with GCM in which SST changes in specific ocean basins outside of the tropical Pacific are restored towards the observed values and examined remote influences from SST in extratropics to the Walker circulation and the tropical Pacific SST.
By restoring SST change towards observations poleward of 15º latitudes in the Pacific and 30º latitudes in the Indian and Atlantic Oceans (with a linearly tapering buffer zone between 10º and 15º, and between 25º and 30º), the GCM reproduced the observed Walker circulation trend very well. This experiment also reproduces the La Nina-like SST trend and interannual variabilities of the Walker circulation and SST in the tropical Pacific Ocean. This result suggests the importance of subtropical and midlatitude SST variability in attributing, predicting, and projecting the Walker circulation changes.
By varying ocean regions to which observed SST change is restored, the quantitative contributions of each ocean basin to the observed Walker circulation were evaluated. It is also suggested that the northern Pacific Ocean significantly affected the observed Walker circulation trend and there is a possibility that some model biases exist in equatorward SST propagation in the south Pacific around 20°S.