Using the mean pressure gradient and NCEP/NCAR reanalysis to estimate the strength of the South Atlantic Anticyclone

• We proposed a new estimate (AWPG) of the South Atlantic Anticyclone (SAA) strength.
• The AWPG is computed from NCEP/NCAR reanalysis data (sea level pressure, SLP).
• A good correlation with significant wave height (SWH) and wind speed (WS) was observed.
• Comparatively, the AWPG correlates better with SWH and WS than the maximum SLP does.
• Limitations and advantages of this estimation are discussed.

A new methodology is proposed to estimate the strength of the South Atlantic Anticyclone (SAA), using the gridded sea level pressure (SLP) of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data. The top quartile (1017.3 hPa) of the SLP data was found a reasonable criterion to delimit the SAA area. Consequently, we defined the SAA area as the quadrangle containing 80% of the observations with pressure >1017.3 hPa. In this quadrangle, an area weighted pressure gradient (AWPG) was computed for the whole area and for the north–south and west–east halves. When compared with maximum pressure, the AWPG showed a better correlation with the significant wave height (SWH) and wind speed (WS) derived from altimetry. The mean value of the AWPG was 8 × 10−4 Pa/m, with representative values of 9.1 × 10−4 Pa/m and 7.4 × 10−4 Pa/m for austral winter and summer, respectively. The phase difference between the monthly AWPG in the north and south sub-quadrangles accounts for the evolution of the spatial pattern of the anticyclone throughout a year. This quantitative approach proved to be a useful estimate of the strength of South Atlantic Anticyclone. Further improvements of this approach are discussed.