Upscale Effects of Deep Convection during the North American Monsoon

Abstract

The ability of deep monsoon convection to influence the larger-scale circulation over North America is investigated for a 6-day-long case study during the 2006 North American monsoon. Results from Rossby wave ray tracing and numerical simulations using the Advanced Research Weather Research and Forecasting model indicate that North American monsoon convection provides a source region for stationary Rossby waves. Two wave trains are seen in the numerical model simulations, with behaviors that agree well with expectations from theory and ray tracing. The shorter and faster-moving wave train moves eastward from the source region in Mexico and reaches the western Atlantic within 4 days. The longer and slower-moving wave train travels northeastward and reaches the coastal New England region within 6 days. An upstream tail of anticyclonic vorticity extends westward from the source region into the central Pacific Ocean.The monsoon convection appears to help cut off the low-level anticyclonic flow by developing low-level southerly flow in the Gulf of Mexico and northerly flow in the eastern Pacific, as suggested in earlier global model studies. However, the stationary Rossby wave trains further alter the location and intensity of deep convection in locations remote from the monsoon. These results suggest that unless a numerical model can correctly predict monsoon convection, the ability of the model to produce accurate forecasts of the large-scale pattern and associated convective activity beyond a few days is in question. This result may be important for global climate modeling, since an inaccurate prediction of monsoon convection would lead to an inaccurate Rossby wave response.

A partial mechanistic understanding of the North American monsoon

“…Upper level winds are dominated by the NAM anticyclone at 200 hPa [e.g., Stensrud, 2013], so that NAM anvil clouds tend to orient SE-NW.…”

“…This makes the OLR gradient somewhat ambiguous for discriminating between convective and dry regions, but it is still useful if one keeps this caveat in mind. Upper level winds are dominated by the NAM anticyclone at 200 hPa [e.g., Stensrud , ], so that NAM anvil clouds tend to orient SE‐NW.…”

“…It is also observed that the anticyclone center at 500 hPa is to the north or northeast of deep convection, with a correspondence in the temporal evolution of the 240–250 W m −2 gradient. Stensrud [] used the Weather Research and Forecasting (WRF) regional model for a 6 day simulation during the 2006 NAM season and concluded that deep convection is essential in the formation of the NAM anticyclone. Latent heat release, as a result of deep convection, lifted the upper level geopotential height (as demonstrated by model runs with and without deep convection), forming a pronounced anticyclone at 200 hPa.…”

“…Also observed in Figure are regions of high OLR (that are associated with strong subsidence) to the west of NAM convection over the eastern Pacific and also to the northwest of NAM convection over the Baja California peninsula beginning in mid‐May. Stensrud [] has demonstrated that these subsidence regions are linked to NAM deep convection, and our study shows that these regions of strong subsidence progress northward from mid‐May to mid‐July (consistent with the observed poleward evolution of deep convection). It was also shown by Rodwell and Hoskins [] that such subsidence regions are formed by the NAM core.…”

An algorithm for identifying the initiation of synoptic‐scale Rossby waves on potential vorticity waveguides

“…In agreement with the findings of Rodwell et al . () and Stensrud (), this case highlights the importance of diabatic processes in large convective systems to modulate upper‐level waveguides. It illustrates the pathway from strong subtropical convection to a Rossby‐wave response on the extratropical jet.…”