2021 Articles

Quantifying atmosphere and ocean origins of North American precipitation variability. Climate Dynamics

Zhang, Honghai; Seager, Richard; He, Jie; Diao, Hansheng; Pascale, Salvatore

How atmospheric and oceanic processes control North American precipitation variability has been extensively investigated,
and yet debates remain. Here we address this question in a 50 km-resolution fux-adjusted global climate model. The high
spatial resolution and fux adjustment greatly improve the model’s ability to realistically simulate North American precipitation, the relevant tropical and midlatitude variability and their teleconnections. Comparing two millennium-long simulations
with and without an interactive ocean, we fnd that the leading modes of North American precipitation variability on seasonal
and longer timescales exhibit nearly identical spatial and spectral characteristics, explained fraction of total variance and
associated atmospheric circulation. This fnding suggests that these leading modes arise from internal atmospheric dynamics and atmosphere-land coupling. However, in the fully coupled simulation, North American precipitation variability still
correlates signifcantly with tropical ocean variability, consistent with observations and prior literature. We fnd that tropical
ocean variability does not create its own type of atmospheric variability but excites internal atmospheric modes of variability
in midlatitudes. This oceanic impact on North American precipitation is secondary to atmospheric impacts based on correlation. However, relative to the simulation without an interactive ocean, the fully coupled simulation amplifes precipitation
variance over southwest North America (SWNA) during late spring to summer by up to 90%. The amplifcation is caused by
a stronger variability in atmospheric moisture content that is attributed to tropical Pacifc sea surface temperature variability.
Enhanced atmospheric moisture variations over the tropical Pacifc are transported by seasonal mean southwesterly winds
into SWNA, resulting in larger precipitation variance