17 Aug 2023
 | 17 Aug 2023
Status: this preprint is currently under review for the journal CP.

Technical Note: A best-practice approach to calculating the Southern Annular Mode index

Laura Velasquez-Jimenez and Nerilie J. Abram

Abstract. The Southern Annular Mode (SAM) strongly influences climate variability in the Southern Hemisphere. The SAM index describes the phase and magnitude of the SAM and can be calculated by measuring the difference in mean sea level pressure (MSLP) between mid- and high-latitudes. This study investigates the effects of calculation methods and data resolution on the SAM index, and subsequent interpretations of SAM impacts and trends. We show that the normalisation step that is traditionally used in calculating a non-dimensional SAM index leads to substantial differences in the magnitude of the SAM index calculated at different temporal resolutions, and that the equal weighting given to MSLP variability at the mid and high southern latitudes artificially alters temperature and precipitation correlations and the interpretation of climate change trends in the SAM. These issues can be overcome by instead using a dimensional formulation of the SAM based on MSLP anomalies, resulting in consistent scaling and variability of the SAM index calculated at daily, monthly and annual data resolutions. The 10 dimensional version of the SAM index has improved representation of SAM impacts in the high southern latitudes, including the asymmetric (zonal wave-3) component of MSLP variability, whereas the increased weighting given to mid-latitude MSLP variability in the non-dimensional SAM incorporates a stronger component of tropical climate variability that is not directly associated with SAM variability. We conclude that a best-practice approach of calculating the SAM index as a dimensional index derived from MSLP anomalies would aid consistency across climate studies and avoid potential ambiguity in the SAM 15 index, including SAM index reconstructions from paleoclimate data, and enable more consistent interpretations of SAM trends and impacts.

Laura Velasquez-Jimenez and Nerilie J. Abram

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on cp-2023-64', Elio Campitelli, 23 Aug 2023
    • AC1: 'Reply on CC1', Laura Velasquez Jimenez, 10 Oct 2023
  • RC1: 'Comment on cp-2023-64', Anonymous Referee #1, 25 Sep 2023
    • AC2: 'Reply on RC1', Laura Velasquez Jimenez, 10 Oct 2023
  • RC2: 'Comment on cp-2023-64', Anonymous Referee #2, 01 Jan 2024
    • AC3: 'Reply on RC2', Laura Velasquez Jimenez, 15 Jan 2024
Laura Velasquez-Jimenez and Nerilie J. Abram
Laura Velasquez-Jimenez and Nerilie J. Abram


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Short summary
The Southern Annular Mode (SAM) influences climate in the Southern hemisphere. We investigate the effects of calculation method and data used to calculate the SAM index, and how it influences the relationship between the SAM and climate. We propose a method to calculate a dimensional SAM index that facilitate consistency between studies, including when using different data resolutions, avoiding distortion of SAM impacts and allow more reliable results of past and future SAM trends.