A New Paper on the Importance of Regional Climate Forcings and Response

An important paper appeared in the March 22, 2006 issue of JGR-Atmosphere which further documents why we must focus on regional diabatic forcings and responses in order to provide a more scientifically sound understanding of climate variability and change. This paper by C. Erlick, V. Ramaswamy and L. Russell entitled “Differing regional responses to a perturbation in solar cloud absorption in the SKYHI general circulation model” has the following abstract,

“In this study we perform an idealized experiment to investigate the effect of solar absorption in clouds on climate using a general circulation model with prescribed sea surface temperatures, focusing on the manner of regional changes during the northern summer season. The response arising from this type of perturbation is akin to “semidirectâ€? effects of absorbing aerosols, namely, dissipation of clouds owing to a high aerosol absorption in the cloud layers. In the experiment, we apply a similar perturbation to all low-cloud layers, reducing their single-scattering albedo to a value of 0.99, which enables us to isolate the effect of such solar absorption from other aerosol related influences. We find that in both midlatitude and equatorial regions, the reduction in low-cloud single-scattering albedo causes a reduction in low-cloud amount and a warming of the surface. However, the dynamical response of the system varies from one continental region to another. In the midlatitude regions of the United States and Europe/east Asia, the diabatic heating perturbation leads to the dissipation of low clouds, an increase in shortwave flux to the surface, an increase in horizontal heat advection, and an increase in atmospheric stability. In the tropical region of North Africa, the diabatic heating perturbation translates into an increase in convection, a decrease in stability, an increase in middle- and high-level clouds, and a reduction in shortwave flux to the surface. In agreement with previous studies, these results demonstrate the distinctive response of the tropical versus midlatitude regions to a similar solar perturbation.”

In this study, while a uniform aerosol distribution is prescribed, the cloud cover is spatially heterogeneous such that the climate forcing is regional in scale.

The conclusion of the paper includes the following statement,

“…..the manner in which the diabatic warming input in any continental region translates into temperature increases versus increases in vertical velocity can vary between the tropics and midlatitudes, and this response can also be of differing magnitudes in different models. There can be considerable spatial heterogeneity as well, for example, subcontinental-scale changes can be quite different from continental-scale averages.

The results presented lend further support for the importance of taking into consideration the aerosol absorption effects on clouds in the lower troposphere and the ensuing effects on climate, including so-called semidirect effects. However, the global mean response of the climate is not an indication of the regional response, and a full understanding of the response of the climate to such a perturbation can only be gained by looking at the regional scale, and investigating both radiative and hydrologic budgets, where the mechanisms and effects prevailing in the tropical and midlatitude domains can be quite different from one another. Even when the radiative pattern of solar absorption in the atmosphere is well understood, as is the case in our idealized simulations, the responses of different regions may not follow in a similar manner. Our main conclusion is that even for an idealized uniform aerosol distribution, the response is not similar everywhere, not even qualitatively. Any departure from these globally uniform, idealized conditions could yield even more differences.”

A critically important conclusion in the above text is that,

“the global mean response of the climate is not an indication of the regional response, and a full understanding of the response of the climate to such a perturbation can only be gained by looking at the regional scale, and investigating both radiative and hydrologic budgets, ….”.

The importance of regional climate forcing and response has been discussed on the Climate Science weblog ; see

What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures?

Why We Need to Focus on Regional Tropospheric Temperature Trends

As reported on both of these Climate Science weblog postings,

““The 2005 National Research Council report concluded that:

‘regional variations in radiative forcing may have important regional and global climate implications that are not resolved by the concept of global mean radiative forcing.’

And furthermore:

‘Regional diabatic heating can cause atmospheric teleconnections that influence regional climate thousands of kilometers away from the point of forcing.’

This regional diabatic heating produces temperature increases or decreases in the layer-averaged regional troposphere. This necessarily alters the regional pressure fields and thus the wind pattern. This pressure and wind pattern then affects the pressure and wind patterns at large distances from the region of the forcing which we refer to as teleconnections.�

The new Erlick et al JGR paper provides further scientific evidence on the validity of the National Research Council’s findings.

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