Sanchez, S. C., D. J. Amaya, A. J. Miller, S.-P. Xie and C. D. Charles, 2019:
The Pacific Meridional Mode over the last millennium
Climate Dynamics, 53, 3547-3560.
Abstract.
The Pacific Meridional Mode, a coupled ocean-atmospheric interaction responsible for propagating subtropical anomalies
to the tropics via thermodynamic mechanisms, features prominently in discussions of the response of climate variability to
climate change. However, it is presently unclear how and why the variance in PMM might change, or even if greenhouse gas
forcing might lead to heightened activity. Here, PMM variance over the last millennium is assessed in the Community Earth
System Model Last Millennium Ensemble (LME). The model reproduces the main spatial characteristics of the PMM in
the modern ocean in agreement with observations. With this basis, we assess the magnitude of the PMM variance over the
past millennium, subject to forcing from a variety of sources. Internal (unforced) variability dominates the PMM variance
in the LME, but prolonged periods of strong or weak PMM variance are found to be associated with characteristic spatial
patterns, consistent across ensemble members and forcing experiments. The pattern of strong PMM variance features a cooler
north Pacific, weaker Walker circulation, and a southward-shifted ITCZ. Comparison with a slab ocean model suggests that
equatorial ocean dynamics are necessary to sustain the statistically significant multidecadal variability. With respect to the
last millennium, present greenhouse forcing does not promote exceptional PMM variance. However, the PMM variability
projected in the RCP8.5 scenario exceeds the thresholds expressed with the forcings applied over the Last Millennium. Aside
from multidecadal variability, the model simulations also bear on ENSO variability and the sensitivity of climate variability
to external forcing.
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