Sanchez, S. C., D. J. Amaya, A. J. Miller, S.-P. Xie and C. D. Charles, 2019:
Pacific Meridional Mode over the last millennium
Climate Dynamics, accepted pending revisions.
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 (CESM) 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.