Wednesday, 23 October 2013

An explanation for the difference between twentieth and twenty-first century land–sea warming ratio in climate models

(c) NASA

This article has been published on Climate Dynamics on October 2013. The authors, M. M. Joshi, F. H. Lambert, M. J. Webb, are affiliated to different universities and research centers in the United Kingdom: University of Reading, University of East Anglia, University of Exeter and Met Office Hadley Centre in Exter. Here below the abstract of the article.

A land–sea surface warming ratio (or φ) that exceeds unity is a robust feature of both observed and modelled climate change. Interestingly, though climate models have differing values for φ, it remains almost time-invariant for a wide range of twenty-first century climate transient warming scenarios, while varying in simulations of the twentieth century. Here, we present an explanation for time-invariant land–sea warming ratio that applies if three conditions on radiative forcing are met: first, spatial variations in the climate forcing must be sufficiently small that the lower free troposphere warms evenly over land and ocean; second, the temperature response must not be large enough to change the global circulation to zeroth order; third, the temperature response must not be large enough to modify the boundary layer amplification mechanisms that contribute to making φ exceed unity. Projected temperature changes over this century are too small to breach the latter two conditions. Hence, the mechanism appears to show why both twenty-first century and time-invariant CO2 forcing lead to similar values of φ in climate models despite the presence of transient ocean heat uptake, whereas twentieth century forcing—which has a significant spatially confined anthropogenic tropospheric aerosol component that breaches the first condition—leads to modelled values of φ that vary widely amongst models and in time. Our results suggest an explanation for the behaviour of φ when climate is forced by other regionally confined forcing scenarios such as geo-engineered changes to oceanic clouds. Our results show how land–sea contrasts in surface and boundary layer characteristics act in tandem to produce the land–sea surface warming contrast.

Climate Dynamics October 2013, Volume 41, Issue 7-8, pp 1853-1869
NCAS Climate, Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB, UK
M. M. Joshi
Department of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK
M. M. Joshi
College of Engineering, Mathematics and Physical Sciences, University of Exeter, Harrison Building, North Park Road, Exeter, EX4 4QF, UK
F. H. Lambert

Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB, UK
M. J. Webb