Formation of summer precipitation dipole pattern over the Contiguous US: interplay between PDO and climate warming

Spatial modes of observed US summer precipitation are non-stationary according to running-Empirical Orthogonal Function (EOF) analysis. In the most recent 40 years, a dipole mode of summer precipitation (i.e., sea-saw pattern between the Southeastern (SE) US and the Great Lakes) emerges, which was absent in earlier precipitation records. By analyzing the atmospheric vorticity budget, this study attributes the recently formed precipitation dipole to the interplay between the Pacific Decadal Oscillation (PDO) and climate warming. Specifically, the positive PDO phase tends to induce anomalously positive (negative) vorticity over the Great Lakes (SE US) at 200hPa, which should be balanced by upper tropospheric divergence (convergence) in order to conserve atmospheric PV. The upper tropospheric divergence/convergence further induces an adjustment of circulation in the lower troposphere. According to our analysis, the adjustment of lower tropospheric circulation depends on the strength seasonal mean southerly wind over the SE US. When the southerly wind is weak (strong), the anomalous anticyclone, as forced by the positive PDO phase, is relatively weak (strong). In recent decades, the southerlies have intensified over the SE US due to the warming induced intensification and westward extension of the North Atlantic Subtropical High (NASH) western ridge. Consequently, the stronger response of atmospheric circulation and precipitation to the PDO positive phase will be expected in recent decades, contributing to the formation of the summer precipitation dipole. Our analysis suggests that the PDO influence on US summer precipitation will be further enhanced in a warming climate. This hypothesis is supported by the simulations of phase-5 of Coupled Model Intercomparison Project (CMIP5) models. The responses of US summer precipitation and large-scale circulation to PDO are analyzed with focus on the Preindustrial control, Historical, and Representative Concentration Pathway (RCP) 4.5 experiment. Consistent with observations, the response of US summer climate to the PDO is fairly weak in the Preindustrial control run; strengthens in the Historical run; and further intensifies under the RCP 4.5 scenario, as the NASH western ridge extends in a warmer climate. Thus, both observations and CMIP5 models suggest that the PDO influence on US summer climate depends on the background circulation that might be modulated by climate warming.