Low-level stratiform clouds modulate California's coastal climate during the warm season. Previous work describing the seasonal and daily variability of coastal low cloudiness (CLC) suggests that in July, August, and September southern California's CLC is under the influence of an additional driver, which has less impact in northern California. In this work, we introduce the link in which free-tropospheric moisture dictated by North American Monsoon (NAM) processes can impact southern California CLC. We use in situ and remote sensing observations, as well as reanalysis and single column model simulations to identify and investigate this previously missing component. We find that monsoonal moisture advected by southeasterly flow from the core NAM region into southern California reduces CLC by diminishing cloud-top longwave cooling. To add to an already complex brew of known factors influencing coastal cloudiness, another one is hereby introduced and should be accounted for in future work.
Low-altitude marine layer clouds shade and cool coastal California in spring and summer. When these clouds are low enough that the base of the cloud intercepts terrain (which is known as fog), they additionally add moisture to the landscape during a typically dry time of year in California. Future trends in coastal low cloudiness (CLC) are uncertain. Although CLC impact the whole coast of California and beyond, previous studies have exposed differences in seasonal and daily CLC behavior in southern and northern California. The North American Monsoon (NAM), which becomes active in the US Southwest in summer, brings rain and thunderstorms to the desert southwest. Coastal southern California is on the northwest edge of the NAM influence and typically does not receive much rain from NAM. In this study, we show how low altitude coastal cloud cover in southern California and northern Baja California can be diminished by higher altitude moisture from the NAM. Dry and stable air above the top of low clouds helps to maintain the cloud layer, and higher altitude moisture interrupts this process. To better understand how CLC varies and may change, an accounting of all key drivers of CLC behavior, including the NAM, is needed.
