| The Argentine Gyre is an energetic and consistent oceanic gyre locating in the South Atlantic Ocean,extending about 1000 km in the zonal and 500 km in the meridional directions.The gyre itself is a particular feature among the world ocean,for its intensity as well as its location.It is circulating counterclockwise around the Zapiola Rise.It locates in the east of the Brazil-Malvinas Convolution,one of the most energetic regions of the world ocean,and in the north of the Antarctic Circumpolar Current,which plays a decisive role in the mid-to high-latitude Southern Hemisphere climate.One particular feature about Argentine Gyre is,that there is a-25-day oscillation superimposed on it.The physical mechanisms responsible for the persistence and variability of the Argentine Gyre are inadequately known up to now,yet generally accepted to be associated with the interaction of mesoscale eddies with seafloor topography.The ocean altimetry mission TOPEX/Poseidon,which was launched in 1993,allows the first opportunity for observing large scale oceanic variabilities.With the advent of the Gravity Recovery and Climate Experiment(GRACE)mission in 2002,it is possible to study geostrophic currents from the perspective of gravity changes.In this study,we analyze the time-variable gravity measurements from the GRACE satellite mission in conjunction with sea level variations from the satellite ocean altimetry,to investigate the non-seasonal and high-frequency variations of the Argentine Gyre by means of empirical orthogonal functions(EOF)and complex EOF(CEOF),respectively.We adopt the 10-day time-variable GRACE gravity field with the Nyquist period is 20 days released by the Group de Research de Geodesie Spatiale,which allows the detection of a 25-day signal in principle.We use multi-satellite altimetric sea level variations provided by the Copernicus Marine Environment Monitoring Service in comparison with GRACE measurements.The methods of EOF and CEOF are effective in detecting standing oscillations and propagating waves,respectively,extracting spatial modes as well as corresponding temporal revolutions.Our results demonstrate that GRACE is effective in observing time-variable gravity signals at temporal resolutions higher than practiced hitherto.In particular we find good agreement between time-variable gravity and altimetry observations,confirming the barotropic structure of the Argentine Gyre.The leading EOF modes of the overall up-and-down undulation in time-variable gravity and altimetry variations are found to be in pace temporally with the Antarctic Oscillation index with correlation as high as 0.69 at zero time shift.Possible mechanism of the variability of the Argentine Gyre in relation to the Antarctic Oscillation is linked to mesoscale eddies and meanders excited by the Antarctic Circumpolar Current.We find that the Argentine Gyre varies in different time scales,including the 0.25-0.5 year,the quasi-biannual and an interannual(6-7 years)periodicity.On the other hand the leading CEOF mode signifies a counterclockwise dipole pattern of-25-day periodicity within the overall gyre with multi-scale modulation in the amplitude.Prominent periodicities of the modulation in amplitude include an annual and a quasi-biannual one.We find possible evidence supporting the energy exchange between the mean gyre and the-25-day oscillation.We also use GRACE time-variable gravity daily solutions resolved by Graz University of Technology to study the-25-day oscillation of the Argentine Gyre.The second CEOF mode demonstrates a counterclockwise circulating pattern,albeit the spatial pattern is different from that resolved from the above GRACE and altimetry measurements.Our study proves the efficiency of using GRACE time-variable gravity field to study high-frequency ocean currents and provide more evidence about the Argentine Gyre's dynamics.Future research can be focused on polar gyres,where altimetry measurements are incomplete or unavailable. |
PDF Full Text Request