Abstract
This paper highlights some caveats in using composite analyses to form physical hypotheses on the associations between environmental variables. This is illustrated using a specific example, namely the apparent links between heat waves (HWs) and sea surface temperatures (SSTs). In this case study, a composite analysis is performed to show the large-scale and regional SST conditions observed during summer HWs in Perth, southwest Australia. Composite results initially point to the importance of the subtropical South Indian Ocean, where physically coherent SST dipole anomalies appear to form a necessary condition for HWs to develop across southwest Australia. However, sensitivity tests based on pattern correlation analyses indicate that the vast majority of days when the identified SST pattern appears are overwhelmingly not associated with observed HWs, which suggests that this is definitely not a sufficient condition for HW development. Very similar findings are obtained from the analyses of 15 coupled climate model simulations. The results presented here have pertinent implications and applications for other climate case studies, and highlight the importance of applying comprehensive statistical approaches before making physical inferences on apparent climate associations.
Highlights
Statistical analyses are commonly used in climate science as they provide deep insights into the variability of climate occurrences at various spatial and time scales
This work highlights some caveats in using composite analyses to form physical hypotheses on climate connections, and we have presented a concrete example of this for the case of the relationship between heat waves (HWs) in Perth, southwest Australia and surface ocean temperatures
Results from our composite analysis initially suggest that sea surface temperatures (SSTs) dipole conditions in the South Indian Ocean may form a necessary condition for summer HWs to develop across southwest Australia
Summary
Statistical analyses are commonly used in climate science as they provide deep insights into the variability of climate occurrences at various spatial and time scales. This study highlights the caveats of using composite analyses to form physical hypotheses on the associations between environmental variables These caveats are illustrated using the example of the apparent links between heat waves (HWs) and regional sea surface temperatures (SSTs). The limited evidence provided so far by observations and the lack of agreement in models simulating these observed SST patterns[28] suggest that further work is needed to understand the role of SSTs for Australian HWs, especially as the global ocean continues to warm[29] In light of these considerations, we explore the association between summer HWs in Perth, southwest Australia, and large-scale and regional SSTs, as an illustrative case study of the use of composite analyses. The final section discusses some caveats when interpreting the composite results, and more generally highlights the importance of combining different statistical approaches before forming physical hypotheses regarding climate connections
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