CGD's Dr. Jian Lu

Abstract

The width of the Hadley cell is studied over a wide range of climate regimes using both simple and comprehensive atmospheric general circulation models. Aquaplanet, fixed sea surface temperature lower boundary conditions are used in both models to study the response of the Hadley cell width to changes in both globalmean temperature and pole-to-equator temperature gradient. The primary sensitivity of both models is a large expansion of the Hadley cell with increased mean temperature. The models also exhibit a smaller increase in width with temperature gradient. The Hadley cell widths agree well with a scaling theory by Held which assumes that the width is determined by the latitude where baroclinic eddies begin to occur. As surface temperatures are warmed, the latitude of baroclinic instability onset is shifted poleward due to increases in the static stability of the subtropics, which is increased in an atmosphere with higher moisture content.

Figure caption: Held 2000 scaling for the Hadley cell width versus the actual cell width for the idealized GCM (left) and the full GCM (right). Simulations are color-coded based on their global mean temperature value (T_m.

Support: NSF.

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Lu, J., G. Vecchi, T. Reichler, 2007: Expansion of the Hadley cell under global warming. Geophys. Res. Lett., 34, L06805, doi: 10.1029/2006GL028443.

Abstract

Over the last century hydraulic engineering works on a massive scale have transformed the arid western region of the United States into a complex ur- ban, industrial, primary extraction and agricultural society (1-3) . Many of the fastest growing urban centers in the United States are now located in the arid West even as agriculture remains the dominant water user across the region. How precipitation, evaporation, groundwater storage and river flow change in the West as a consequence of climate change are, therefore, matters of signif- icant concern with widepsread implications for the allocation of limited water resources and the course of regional development. Here we show that there exists a broad consensus amongst climate models participating in the Intergov- ernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) that this region will dry significantly in the 21st Century and that the transition to a more arid climate should already be underway. According to these models, the levels of aridity seen in the recent multiyear drought that began in 1998, or the devastating droughts of the Dust Bowl and the 1950s, will, within the next years to decades, become the new climatology of the American Southwest.

Support: NSF.

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Seager, R., M. Ting, I. Held, Y. Kushnir, J. Lu, G. Vecchi, H.-P. Ping, N. Harnik, A. Leetmaa, N.-C. Lau, C. Li, J. Velez, N. Naik, 2007: Model projection of an imminent, anthropogenically-induced, transition to a climate of greater aridity in southwestern North America. Science, 316, 1184, doi: 10.1126/science.1139601.

Abstract

A consistent weakening and poleward expansion of the Hadley circulation is diagnosed in the climate change simulations of the IPCC AR4 project. Associated with this widening is a poleward expansion of the subtropical dry zone. Simple scaling analysis supports the notion that the poleward extent of the Hadley cell is set by the location where the thermally driven jet first becomes baroclinically unstable. The expansion of the Hadley cell is caused by an increase in the subtropical static stability, which pushes poleward the baroclinic instability zone and hence the outer boundary of the Hadley cell.

Figure caption: Modeled changes in annual mean precipitation minus evaporation over the American Southwest (125 °W to 95°W and 25°N to 40°N, land areas only), averaged over ensemble members for each of the 19 models. The historical period used known and estimated climate forcings, and the projections used the SResA1B emissions scenario. The median (red line) and the 25th and 75th percentiles (pink shading) of the P - E distibution among the 19 models are shown, as are the ensemble medians of P (blue line) and E (green line) for the period common to al models (1900-2098). Anomalies (Anom) for each model are relative to that model's climatology from 1950-2000. Results have been 6-year low-pass Butterworth-filtered to emphasize low-frequency variability that is of most consequence for water resources. The model ensemble mean P - E in this region is around 0.3 mm/day.

Support: NSF.