Influence of elevated heating on the Zagros Plateau (Iran) on circulations in Southwest Asia

The summertime atmospheric circulation in Southwest Asia is characterized by persistent subsidence in the free troposphere. This subsidence is a product of global circulations involving the Hadley Cell and the Asian Monsoon, but the distribution and intensity of descent within the region is influenced by local geography. The Zagros Plateau of Iran, in particular, is a large elevated heat source that may affect patterns of subsidence and associated atmospheric stability. This effect would be greatest in the summertime, when both plateau heating and regional subsidence are at a maximum. A regional climate model was used to simulate the influence of plateau heating on summertime circulations in Southwest Asia for 1999 and 2003, a particularly dry year and a moderately wet year, respectively. It was found that heating on the Zagros plateau leads to a persistent temperature anomaly throughout the troposphere in the summer months. This anomaly is associated with a steady heat-driven circulation tendency that causes southerly (warming) advection over the Zagros and northerly (cooling) advection over the Tigris-Euphrates Basin, relative to the circulation that would exist in the absence of the elevated heat source. Cool advection in the Fertile Crescent and Levant is associated with adiabatic descent on the order of 0.2 – 0.3 Pa s-1 in the summer months. In a simulation with suppressed plateau heating several well-known features of summertime climatology in the region—the “Persian Trough” low pressure system, a persistent low-level inversion in the Levant, and the near-surface northwesterly Etesian winds—were reduced or eliminated. Plateau heating also inhibits cloud formation within the basin and has an influence on summertime vapor flux and precipitation events. In these respects the diurnal component of plateau heating is also significant, as a diurnally oscillating plain-plateau circulation that circulates water vapor out of the Tigris-Euphrates Basin is weakened when Zagros heating is absent.

The plateau influence on circulations can be reasonably well simulated by a linear heating model driven by an elevated heat source that oscillates diurnally around a non-zero mean. Near the surface the diurnal component of heating dominates, resulting in strong inflow at the time of heating maximum and outflow at heating minimum. This circulation develops under conditions of weak stability and moderate friction, and the wind speed maxima correlate with local maxima in pressure gradient. Further aloft the steady component of heating is dominant, stability is large, and friction is small. Under these conditions the pressure gradient and coriolis forces approach geostrophy, leading to an anticyclonic circulation with broad horizontal extent. The near-surface, oscillating circulation acts to export water vapor from the basin in the afternoon and also plays an important role in determining the location of convection along the slopes of the Zagros. The circulation aloft interacts with the large-scale flow, bending the path of dominant westerlies and causing warm advection over the Zagros and cold advection in the Levant. The intensity and location of these heat-driven circulations change with the intensity of surface heating, reaching a maximum in both extent and intensity in August. Implications for climate sensitivity are significant when considering time periods when the heating contrast between basin and plateau may have been reduced—such would be the case, for example, when permanent glaciers were widespread on the plateau, or at a time when the basin was less arid. Under the current large-scale climate regime, in which summertime aridity is enforced by hemispheric as well as regional circulations, the removal of the Zagros heat source has only a modest impact on precipitation.