"The role of deep soil moisture in modulating climate in the Amazon rainforest"
Approximately half of the Amazon’s evergreen forests are subjected to dry seasons of at least three months [Nepstad et al., 1994], and yet the forest seems to thrive during the dry, sunny months. Understanding the mechanisms that enable the forest to live through extended dry periods is of particular importance considering that the effects of both global warming and land use change are predicted to cause a drier climate in this region [Oliveira et al., 2005].
The roots in the Amazon are well suited for dry season survival. Tap roots have been observed up to 11 m deep [Nepstad et al., 1994; Jipp et al., 1998]. Hydraulic redistribution (HR) allows the plants to access water from shallower soil layers, where most of a tree’s fine roots are located, and has been observed in three trees in the Tapajos National Forest in Brazil [Oliveira et al., 2005]. These adaptations increase a plant’s drought tolerance, enable the plants to maintain transpiration and carbon sequestration during seasonal droughts [Saleska et al. 2003; Oliveira et al., 2005], and have been shown to improve the seasonal cycles of evapotranspiration and carbon fluxes in land models [Lee et al., 2005; Baker et al., 2008, respectively]. However, few studies have looked at the effects of deep soils on climate in a coupled sense [e.g. Kleidon and Heimann, 1999; Lawrence and Chase, 2009]. This paper is a step in that direction.
By adding more realistic root and soil functions in the Simple Biosphere model, version three (SiB3), Baker et al. [2008] obtained more realistic results with regards to surface fluxes at certain sites in the Amazon. The goal of this paper is to examine the effects of these changes on the simulated hydrologic cycle when SiB3 is coupled to a single column version of a GCM. Ultimately, SiB3 will be coupled to a global GCM, and to a cloud resolving model which is then embedded as rows in GCM grid cells as a way of replacing typical cloud parameterizations. Therefore, this study lays the foundation for understanding the effects of certain soil biophysical properties on climate in the Amazon. The SCM is a useful tool for cutting the computational cost of coupled model development [Betts and Miller, 1986; Randall and Cripe, 1999] and facilitates testing parameters or diagnosing problems.
SiB, like other ecosystem models, previously had problems simulating fluxes of heat and moisture in the Amazon [Saleska et al., 2003; Randall et al., 1996; Liu, 2004]. In coupled runs of SiB2 and CSU’s GCM (BUGS5), Liu [2004] found that soil moisture stress led to decreased dry season transpiration and an overly dry and deep boundary layer, and ultimately to a complete shut-down of the hydrologic cycle in the Amazon. This result is analogous to the Amazon dieback found by Cox et al. [2004], where the forest transitioned to savannah due to decreased rainfall over western Amazonia in the 21st century [Cox et al., 2004]. Encouraged by the results of Baker et al. [2008], we investigate the climatic effects of including deep roots and more realistic ecosystem stress responses in SiB3. To study these effects, we coupled SiB3 to a single column version of BUGS5.
