Abstract
Abstract. Version-4 of the Goddard Earth Observing System (GEOS-4) General Circulation Model (GCM) was employed to assess the influence of potential changes in aerosols on the regional circulation, ambient temperatures, and precipitation in four selected regions: India and Africa (current paper), as well as North and South America (companion paper). Ensemble-simulations were carried out with the GCM to assess the aerosol direct and indirect effects, hereafter ADE and AIE. Each simulation was started from the NCEP-analyzed initial conditions for 1 May and was integrated through May-June-July-August of each year: 1982–1987 to provide an ensemble set of six simulations. In the first set, called experiment (#1), climatological aerosols were prescribed. The next two experiments (#2 and #3) had two sets of simulations each: one with 2X and other with 1/2X the climatological aerosols over each of the four selected regions. In experiment #2, the anomaly regions were advectively restricted (AR), i.e., the large-scale prognostic fields outside the aerosol anomaly regions were prescribed while in experiment #3, the anomaly regions were advectively Interactive (AI) as is the case in a normal GCM integrations, but with the same aerosols anomalies as in experiment #2. Intercomparisons of circulation, diabatic heating, and precipitation difference fields showed large disparities among the AR and AI simulations, which raised serious questions about the proverbial AR assumption, commonly invoked in regional climate simulation studies. Consequently AI simulation mode was chosen for the subsequent studies. Two more experiments (#4 and #5) were performed in the AI mode in which ADE and AIE were activated one at a time. The results showed that ADE and AIE work in concert to make the joint influences larger than sum of each acting alone. Moreover, the ADE and AIE influences were vastly different for the Indian and Africa regions, which suggest an imperative need to include them rationally in climate models. We also found that the aerosol induced increase of tropical cirrus clouds would potentially offset any cirrus thinning that may occur due to warming in response to CO2 increase.
Highlights
From the sizzling hot tropics to the frigid cold high latitudes, one finds large temperature extremes that are greatly exacerbated by intrinsic natural variability of weather and climate systems and the annual cycle of solar irradiation
We examined the regional influences of aerosol-perturbations in advectively restricted (AR) simulations vis-a-vis the corresponding advectively interactive (AI) simulations
A six-member ensemble was produced in which each run started from the analyzed initial conditions for 1 May of six different years: 1982–1987, and was integrated through 31 August of the same year
Summary
From the sizzling hot tropics to the frigid cold high latitudes, one finds large temperature extremes that are greatly exacerbated by intrinsic natural variability of weather and climate systems and the annual cycle of solar irradiation. Lau et al (2006) and Kim et al (2009) showed that the manner in which aerosols influence the surface evaporation, convective precipitation, diabatic heating and circulation is quite different from that of the well-mixed greenhouse gases. Kim et al (2009) showed how ADE over West Africa and eastern Atlantic Ocean lead to a Walker-like circulation forced entirely by aerosol induced warming (cooling) of the atmospheric temperature structure over land (ocean). How do these changes effect the largescale circulation and precipitation? Several competing factors influence the effects of aerosols on atmospheric circulation and precipitation These include ADE, AIE, cloud-radiative forcing, precipitation efficiency and cloud life-time. The successes cited above motivated us to examine the influence of aerosol on the tropical and subtropical atmosphere as well as to determine the relative contributions of ADE and AIE on the circulation and precipitation
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