The Walker Circulation, tropical cyclones, and global warming

Abstract

The Walker Circulation (WC) is one of the world’s most prominent and important atmospheric systems. The WC weakened during the 20th century, reaching record low levels in recent decades. This weakening is thought to be partly due to global warming and partly due to internally generated natural variability. There is, however, no consensus in the literature on the relative contribution of external forcing and natural variability to the observed weakening of the WC. Here we examine changes in the strength of the WC in both the observations and in WCRP/CMIP3 climate model simulations for the 20th and 21st centuries. The analysis we will present leads us to conclude that: (i) both external forcing and internally generated variability contributed to the observed weakening of the WC over the 20th century and (ii) external forcing accounts for approximately 50% +/- 20% of the observed weakening, with internally generated climate variability making up the rest. We then examine changes in the Southern Oscillation Index (SOI), an index that is often used to track and predict changes in both the El Niño-Southern Oscillation (ENSO), and the WC. During El Niño, for example, the WC weakens and the SOI tends to be negative. In stark contrast to expectations the SOI increases in response to global warming during the 21st century integrations, even though the WC weakens. The reasons for and implications of this stark contrast between the character of ENSO-driven natural variability and anthropogenically-forced climate change will be discussed. Finally, we describe and use a new data base (Callaghan and Power, Climate Dynamics, 2010) of severe land-falling tropical cyclones (TCs) for eastern Australia derived from numerous historical sources that has taken many years to develop. It allows us to document changes in TC activity over much longer periods than has been done previously for the Southern Hemisphere. Land-fall numbers are well-simulated as a Poisson process. They occurred almost twice as often in La Niña years as they did in El Niño years, and multiple land-falls only occurred during La Niña years. Decadal variability in ENSO drives some of the decadal variability in land-fall numbers. Other links between decadal variability in land-falls and climate variability in the Pacific more broadly will be examined. The number of severe TCs making land-fall over eastern Australia declined from about 0.45 TCs/yr in the early 1870s to about 0.17 TCs/yr in recent times (p=0.1 only). This decline can be partially explained by a weakening of the Walker Circulation, and a natural shift towards a more El Niño-dominated era. References Power, S.B., and G. Kociuba, 2010: The impact of global warming on the Southern Oscillation Index, Climate Dynamics, DOI 10.1007/s00382-010-0951-7. Power, and G. Kociuba, 2011: What caused the observed 20th century weakening of the Walker Circulation? J. Climate, (in press). Callaghan, J. and S. Power, 2010: Variability and decline in severe tropical cyclones making land-fall over eastern Australia since the late 19th century. Climate Dynamics, DOI 10.1007/s00382-010-0883-2.