Physical Oceanography, Vol.
EXPERIMENTAL AND FIELD RESEARCH
INTRAYEAR VARIABILITY OF GEOSTROPHIC ZONAL CURRENTS
IN THE TROPICAL ZONE OF THE SOUTH ATLANTIC
Yu. V. Artamonov, N. P. Bulgakov, and P. D. Lomakin
UDC 551.465 (261)
On the basis of the results of calculation and analysis of the characteristics of space variability of
the geostrophic component of large-scale zonal currents in the tropical zone of the South Atlan-
tic, its intrayear oscillations, and the components of tangential wind stresses, we establish the
regularities of seasonal variability of the system of gradient currents on the ocean surface de-
pending on wind conditions. We describe the specific features of seasonal meridional fluctua-
tions of the midstreams of the geostrophic component of zonal large-scale currents and the intra-
year variations of their velocity.
The available data on gradient currents in the south part of the Tropical Atlantic [1–5] obtained mainly by
analyzing the data of synoptic sections made in the 70s and early 80s do not give a reliable picture of the season-
al variability of circulation of waters in this region. In the last decade, the results of large-scale oceanographic
investigations performed according to the WOCE project substantially enlarged the archives database. This
makes it possible to study the intrayear behavior of the parameters of the main currents in this region and come
closer to the solution of the posed problem.
In what follows, we present the results of investigation of the seasonal variability of large-scale geostrophic
circulation of waters in the south part of the Tropical Atlantic. The aim of the present work is to specify the pa-
rameters of intrayear variability of large-scale zonal gradient currents in the tropical zone of the South Atlantic
more precisely and estimate the influence of winds on the meridional displacements of these currents and the
variations of their intensity within the annual cycle.
To do this, on the basis of the array of monthly average climatic data presented in , we computed the dy-
namic heights and geostrophic velocities on the ocean surface at the nodes of a one-degree grid for each calendar
month in a year. As a reference surface, we took a level of
m. Further, at reference meridians of
according to the extrema of the zonal component of the velocity
we determined the mid-
of the large-scale currents and constructed plots of intrayear variations of the maxima of current ve-
and their latitudinal displacements.
The intensity of currents in the equatorial zone, where the geostrophic balance is violated, was estimated by
using an indirect parameter—the meridional drop of dynamic heights across one-degree zonal strips on both
sides of the equator. The idea of detection and evaluation of the intensity of flow along the equator with the help
of indirect methods is not new. Thus, the Equatorial Undercurrent was detected by analyzing the meridional in-
clinations of isosurfaces to the equator and a core of high salinity in the upper thermocline [1, 2, 7–9]. The sea-
sonal variations of the intensity of the Equatorial Undercurrent were determined either according to the intrayear
variations of the zonal pressure gradient  or according to the variations of the total content of high-salinity
waters along the equator . In the present work, we study the structure of the thermohaline fields in the neigh-
Marine Hydrophysical Institute, Ukrainian Academy of Sciences, Sevastopol. Translated from Morskoi Gidrofizicheskii Zhurnal,
46–57, November–December, 2002. Original article submitted August 2, 2000; revision submitted June 18, 2001.
0928-5105/02/1206–0341 $27.00 © 2002 Plenum Publishing Corporation 341