Does North American Monsoon Heating Influence the Low-level Pacific Subtropical Anticyclone?

Global circulation patterns during the summer are characterized by low-level subtropical highs over the oceans and monsoon rainfall over the continents. Rodwell and Hoskins (2001) hypothesized that northerly winds in the eastern Pacific, which help maintain the subtropical high, are largely a response to diabatic heating from monsoon convection over Mexico.  Using a simple global model, the heating due to monsoon convection was found to produce subsidence to the northwest of the monsoon region and enhanced low-level northerly winds below the region of subsidence.  If this mechanism is operating, increases in monsoon convection should lead to increases in low-level northerly flow over the eastern Pacific.  To further explore this proposed relationship between monsoon heating and the Pacific high, data from the NCEP/NCAR Reanalysis from 1979 – 2015 were used.  Rainfall data were first utilized to define a domain for the North American monsoon region. To the northwest of the monsoon region, off the coast of California, the reanalysis shows an area of strong subsidence and surface northerly winds as suggested by the theory.  Correlations between each of these components were calculated to explore if the hypothesized physical mechanism is supported by the data. Results indicated that monsoon rainfall was negatively correlated to omega at 500 hPa, as expected, however this upward motion was not correlated to subsidence off the coast of California.  Furthermore, the northerly winds underneath the region of subsidence were not correlated with the amount of monsoon precipitation received, as proposed.  The lack of strong correlations between the three components indicates that monsoon heating is not a driving mechanism in the maintenance of the Pacific subtropical anticyclone.