Role of Land Surface Processes in South American Monsoon Development

Abstract

This study explores the role of vegetation biophysical processes (VBPs) in the structure and evolution of the South American monsoon system (SAMS) with an emphasis on the precipitation field. The approach is based on comparing ensemble simulations by the National Centers for Environmental Prediction general circulation model (GCM) in which the land surface parameterization in one ensemble includes an explicit representation of vegetation processes in the calculation of surface fluxes while the other does not [GCM/ Simplified Simple Biosphere Model (SSiB) and GCM/Soil, respectively], but with similar monthly mean surface albedo and initial soil moisture. The ensembles consist of five pairs of 1-yr integrations differing in the initial conditions for the atmosphere. The results show that, during the austral summer, consideration of explicit vegetation processes does not alter the monthly mean precipitation at the planetary scale. However, at continental scales, GCM/SSiB produces a more successful simulation of SAMS than GCM/Soil. The improvement is particularly clear in reference to the seasonal southward displacement of precipitation during the onset of the SAMS and its northward merging with the intertropical convergence zone during the monsoon mature stage, as well as better monthly mean austral summer precipitation over the South American continent.The changes in surface water and energy balances and circulation in October (monsoon onset) and December (the start of the monsoon mature stage) were analyzed for a better understanding of the results and mechanisms involved. It was found that the major difference between the simulations is in the partitioning of latent heat and sensible heat fluxes (i.e., different Bowen ratio), which produced different latitudinal and longitudinal thermal gradients at the surface. A stronger sensible heat flux gradient between continent and ocean in the GCM/SSiB simulation helped generate an enhanced ventilation effect, which lowered moist static energy (MSE) over the northeast coast of South America leading to stronger counterclockwise turning of the low-level wind from the Atlantic Ocean toward the continent during the premonsoon and early monsoon stages, modifying moisture flux convergence (MFC). It was further iden-* Current affiliation: National Climate Center, Beijing, China. tified that the seasonality of savanna and shrublands to the south and east of the Amazon rain forest contributed to the variability of heating gradients and influenced the SAMS onset and its northward merge with the ITCZ at the early monsoon mature stage. The comparison of the differences between precipitation, evaporation, advection of MSE, and MFC based on simulations using two different land parameterizations suggested that the VBP modulated the surface water budget, but its impact on precipitation was determined by the changes in circulation via changes in heat gradient and MSE.

A review on regional dynamical downscaling in intraseasonal to seasonal simulation/prediction and major factors that affect downscaling ability

“…They have been listed as one of the major sources to generate small scale features in RCMs (Denis et al, 2002). Although at intraseasonal and seasonal scales, oceanic forcing may be the main source of climate variability over many regions, in places where land-atmosphere coupling is strong (Koster et al, 2006;Xue et al, 2004Xue et al, , 2006Xue et al, , 2010b, soil moisture and vegetation biophysical processes could make significant contributions to dynamic downscaling. Extreme climate events, such as droughts and flooding, are an important focus in RCM land/atmosphere interaction studies (e.g., Seth and Giorgi, 1998;Bosilovich and Sun, 1999;Zhang et al, 2003;Gao et al, 2011;Liu et al, 2013;Stefanon et al, 2013).…”

New Insights on Land Surface-Atmosphere Feedbacks over Tropical South America at Interannual Timescales

“…In this sense, such feedbacks can be understood as the degree to which the atmosphere responds to anomalies in the land surface state [15]. Soil moisture has been identified as a key variable in several studies and especially in the terrestrial segment of the hydrological cycle i.e., the evaporation-soil moisture interaction [4,[8][9][10]15,16]. Using Graph Theory, we verify the interdependences of several variables in the context of these complex interactions at interannual timescales.…”

“…The study of LAFs has been approached from physical models and numerical experiments [7][8][9][10][11][12][13]; analysis of observations and models with statistical tools [3,[14][15][16][17]; and traces of moisture trajectories [18,19], among others. An important body of literature has focused on the role of vegetation and land uses in the dynamics of LAFs [20][21][22][23][24], and the conditions under which LAFs determine the stability of the lower atmosphere [25][26][27].…”

<strong>New insights on land surface-atmosphere feedbacks over tropical South America at interannual timescales</strong>

“…Traditionally, the study of LAFs have been approached from physical modelling and numerical experiments [7][8][9][10][11][12][13]; analysis of observations and models with statistical tools [3,[14][15][16][17]; traces of moisture trajectories [18,19], among others. An important body of literature has focused on the role Figure 1.…”