Investigating the Influence of Anthropogenic Forcing on Observed Mean and Extreme Sea Level Pressure Trends over the Mediterranean Region

Armineh Barkhordarian

doi:10.1100/2012/525303

Abstract

We investigate whether the observed mean sea level pressure (SLP) trends over the Mediterranean region in the period from 1975 to 2004 are significantly consistent with what 17 models projected as response of SLP to anthropogenic forcing (greenhouse gases and sulphate aerosols, GS). Obtained results indicate that the observed trends in mean SLP cannot be explained by natural (internal) variability. Externally forced changes are detectable in all seasons, except spring. The large-scale component (spatial mean) of the GS signal is detectable in all the 17 models in winter and in 12 of the 17 models in summer. However, the small-scale component (spatial anomalies about the spatial mean) of GS signal is only detectable in winter within 11 of the 17 models. We also show that GS signal has a detectable influence on observed decreasing (increasing) tendency in the frequencies of extremely low (high) SLP days in winter and that these changes cannot be explained by internal climate variability. While the detection of GS forcing is robust in winter and summer, there are striking inconsistencies in autumn, where analysis points to the presence of an external forcing, which is not GS forcing.

Highlights

Applying formal detection methods, observed patterns of global December-to-February sea level pressure changes have been shown to be inconsistent with simulated internal variability but consistent, after rescaling its amplitude, with simulations driven by anthropogenic forcings
If externally forced changes are detectable, in the second step we assess whether the observed trends are consistent with what models simulate as response of sea level pressure (SLP) to anthropogenic forcing
In winter (DJF) the observed trends show an area of increased SLP centred over the Mediterranean, which extends to the north in spring (MAM).This area of high pressure is reproduced in response of SLP to gases and sulphate aerosols (GS) forcing in all 17 models and, in the multimodel mean

Summary

Introduction

Applying formal detection methods, observed patterns of global December-to-February sea level pressure changes have been shown to be inconsistent with simulated internal variability but consistent, after rescaling its amplitude, with simulations driven by anthropogenic forcings. Gillett and Stott [6] applied for the first time a formal attribution analysis and demonstrated that externally forced SLP trends are observed in all four seasons, with simulated and observed decreases in SLP at high latitudes and increases elsewhere. They found that the observed pattern of seasonal and zonal mean SLP changes is inconsistent with simulated internal variability and that the anthropogenic influence on SLP is detectable, regardless of other natural influences. The anthropogenic influence could not be detected at mid- and high-latitude regions of either hemisphere

Methods

Results

Conclusion