Warm Season Response over North America to a Shutdown of the Atlantic Meridional Overturning Circulation and CO2 Increases

Abstract

Paleo-proxy and modeling evidence suggest that a shutdown of the Atlantic meridional overturning circulation (AMOC) would decrease North Atlantic Ocean sea surface temperatures and have far-reaching climate impacts. The authors use a regional climate model to examine the warm season response over North America to a hypothetical late-twenty-first-century shutdown of the AMOC with increased atmospheric CO 2 . In the future simulation, precipitation decreases over the western and central United States by up to 40% and over eastern Mexico by up to 50%. Over the eastern United States rainfall generally increases except during July. Variations in the moisture convergence associated with large-scale circulation changes dominate the rainfall variations, while evaporation plays a critical role over the northeastern United States in spring and the north-central United States in summer. During April-June the westward extension of the North Atlantic subtropical high enhances southwesterly moisture fluxes from the Gulf of Mexico into the eastern and southcentral United States. Increases in low-level moisture content reduce the stability of the atmosphere. Enhanced southerly winds promote convergence over the eastern United States through the Sverdrup vorticity balance and precipitation increases. In July-August anomalous anticyclonic moisture fluxes associated with an anomalous high over the Gulf of Mexico and eastern Pacific decrease the moisture supply into the United States and Mexico. Over the central United States decreases in evaporation support decreases in low-level moisture content and increases in atmospheric stability. Over the eastern United States the Sverdrup balance weakens in summer and anomalous moisture convergence is mainly located over the East Coast.

A Nonlinear Response of Sahel Rainfall to Atlantic Warming

“…The southerly monsoon flow from the Gulf of Guinea flows into the Guinean coast region and turns southwesterly over the continent under the influence of Coriolis accelerations. The West African westerly jet is in place on the west coast centered near 108N (Grodsky et al 2003;Pu and Cook 2010). The pronounced meridional gradients that characterize the region (Fig.…”

“…The position and strength of the North Atlantic subtropical high are realistically simulated, but the onshore flow along the Guinean and west coasts is stronger in the model simulation than in the reanalysis. The West African westerly jet (Grodsky et al 2003;Pu and near 88N is in place on the west coast in the model, earlier than its formation in the reanalysis. Figures 3c and 3d show reanalysis and model 925-hPa geopotential heights and winds from July through September.…”

“…Boundary conditions produced by this method include seasonality, but diurnal and synoptic time scales are suppressed. This ''climate mode'' approach works for large domains in the tropics (Cook and Vizy 2008;Hagos and Cook 2007;Patricola and Cook 2010) but fails for applications in middle latitudes where transients propagating into the domain are important for determining the time-mean state (Patricola and Cook 2013a,b;Pu et al 2012). Seven 231-day simulations (15 March-31 October) are produced, and 3-hourly output is averaged to generate daily and monthly means.…”

“…Some GCMs, such as the Community Climate System Model, version 4.0 (CCSM4.0; Cook et al 2012), produce an improved representation over West Africa compared with the CMIP3, but in other models the simulation is worse. Maynard et al (2002) used a coupled GCM to simulate African climate at the end of the twenty-first century under the Intergovernmental Panel on Climate Change (IPCC) B2 scenario and found that the summer monsoon precipitation over the Sahel increases.…”

Diurnal Spatial Variability of Great Plains Summer Precipitation Related to the Dynamics of the Low-Level Jet

“…6c), with anomalous positive (negative) vorticity tendency corresponding to the anomalous rising (sinking) motions. The diurnal oscillation of planetary vorticity transport is about one magnitude smaller but can become dominant with space and time averaging (e.g., Pu et al 2012), and the drag term is less than onehalf of the tendency term and so contributes less to the diurnal variations of convergence (but determines the daily mean). Changes of vorticity tendency can be inferred from the diurnal cycle of relative vorticity (Fig.…”