Variation of Antarctic circumpolar current and its intensification in relation to the southern annular mode detected in the time-variable gravity signals by GRACE satellite

Jen-Ru Liau,Benjamin F Chao

doi:10.1186/s40623-017-0678-3

Abstract

The southern annular mode (SAM) in the atmosphere and the Antarctic circumpolar current (ACC) in the ocean play decisive roles in the climatic system of the mid- to high-latitude southern hemisphere. Using the time-variable gravity data from the GRACE satellite mission, we find the link between the space–time variabilities of the ACC and the SAM. We calculate the empirical orthogonal functions (EOF) of the non-seasonal ocean bottom pressure (OBP) field in the circum-Antarctic seas from the GRACE data for the period from 2003 to 2015. We find that the leading EOF mode of the non-seasonal OBP represents a unison OBP oscillation around Antarctica with time history closely in pace with that of the SAM Index with a high correlation of 0.77. This OBP variation gives rise to a variation in the geostrophic flow field; the result for the same EOF mode shows heightened variations in the zonal velocity that resides primarily in the eastern hemispheric portion of the ACC and coincided geographically with the southernmost boundary of the ACC’s main stream. Confirming previous oceanographic studies, these geodetic satellite results provide independent information toward better understanding of the ACC–SAM process.Graphical .

Highlights

Connecting the major Southern Hemisphere oceans, the Antarctic circumpolar current (ACC) plays a decisive role in the mid- to high-latitude SH climate and its variability (e.g., Nowlin and Klinck 1986; Olbers et al 2004)
It should be pointed out that this variation, controlling the entire ocean area where the ACC resides, represents one dominant meteorological signal that is not captured by the present atmosphere–ocean general circulation models (Atmosphere and Ocean De-aliasing Level-1B products adopted by the Gravity Recovery and Climate Experiment (GRACE) project to yield the GAD data and removed from the GRACE observations as stated)
High correlation is again found between the empirical orthogonal functions (EOF) time series and the southern annular mode (SAM) Index: The correlation coefficient is 0.66 at zero time shifts, far exceeding, say, the 0.1% significant level of 0.27 given the pertinent degree of freedom of the broadband GSM data

Summary

Introduction

Connecting the major Southern Hemisphere oceans, the Antarctic circumpolar current (ACC) plays a decisive role in the mid- to high-latitude SH climate and its variability (e.g., Nowlin and Klinck 1986; Olbers et al 2004). It is known that the ACC transport depends on the eastward wind stress over the Southern Ocean (Thompson and Wallace 2000). In the positive phase of the SAM, the region of the largest westerly surface wind stress shifts poleward giving rise to a poleward intensification of the ACC, which in turn enhances the equatorward Ekman transport across the ACC (Hall and Visbeck 2002). The ACC can show intrinsic variability independent of changes in wind forcing, through positive feedback between the generation of mesoscale eddies through baroclinic instability and the dynamics of the mean circulation (Hogg and Blundell 2006). That relationship has been demonstrated in situ across the Drake Passage based on bottom pressure and tide gauge records there (Meredith et al 2011), and in an episode of anomalously high eddy kinetic energy (EKE) in the ACC via satellite altimeter data (Meredith and Hogg 2006)

Objectives

Results

Conclusion