The building blocks of Northern Hemisphere summertime stationary waves and monsoonal circulations

Abstract:

An intermediate-complexity moist general circulation model is used to investigate the forcing of stationary waves in the Northern Hemisphere boreal summer by land–sea contrast, horizontal heat fluxes in the ocean, and topography. A particular focus is on the factors leading to the East-Asian monsoonal low and upper level anticyclone. The additivity of the response to these building blocks is investigated. While all three are important for the stationary wave pattern, the sum of the responses to each forcing does not resemble the response when all are imposed simultaneously. In the upper troposphere, the sum of the individual responses leads to a stationary wave pattern with incorrect horizontal structure as the monsoon is split into two disjointed parts. In the lower troposphere, the sum of the individual responses leads to a too-strong stationary wave pattern, and specifically the response to topography and land-sea contrast is weaker if the other forcings are already present. The source of the nonlinearity will be diagnosed using the zonally anomalous steady-state thermodynamic balance, as the background-state temperature field set up by each forcing dictates the stationary wave response to the other forcings. As all three forcings considered here strongly impact the temperature field and its zonal gradients, the nonlinearity and nonadditivity in our experiments can be explained, but only in a diagnostic sense.