OCEANIC CURRENTS IN THE SOUTHEASTERN OACIFIC OCEAN AS REVEALED BY SATELLITE ALTIMETRY DATA

Sirota A.M.¹, Lebedev S.A.^{2, 3} & Kostianoy A.G.⁴

1. Atlantic Research Institute for Fisheries and Oceanography, 5 Dm. Donskogo Str., Kaliningrad, 236000, Russia, E-mail: sirota@atlant.baltnet.ru
2 Geophysical Center, Russian Academy of Sciences, 3 Molodezhnaya Str., Moscow, 117236, Russia, E-mail: lebedev@wdcb.ru
3. State Oceanographic Institute, 6 Kropotkinsky Per., Moscow, 119838, Russia
4. P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, 36 Nakhimovsky Pr., Moscow, 117997, Russia, E-mail: kostianoy@mail.mipt.ru

ABSTRACT

Ten-days sea level anomalies (SLA) charts, based on the TOPEX/Poseidon (T/P) altimetry data for 1992-2003, as well as corresponding charts of sea surface dynamic heights (constructed by the superposition of SLA distributions over the climatic dynamic topography calculated from mean temperature and salinity data of WOA-1998 Atlas relative to 1000 m depth), were used to study main oceanic currents in the region 45-20°S, 110-70°W. Totally 377 T/P cycles for the period between 23 September 1992 and 25 February 2003 have been used for the analysis. Spatial and seasonal variability of the South Pacific Current has been investigated basing on the charts of dynamic heights gradients. The analysis allowed to distinguish zones with different degree of the current position variability, being minimal at 99°W where the current is most intense. Westward of 105°W the South Pacific Current may have bimodal structure and r.m.s. of its position may reach 3° of latitude. This is accompanied by a pronounced 35 T/P cycles peak (350 days) in its temporal variability. Eastward of 105°W there is no predominance in temporal variability of the current.

INTRODUCTION

The South Pacific Current is the eastward current that forms the southern part of the South Pacific Subtropical Gyre (Stramma et al., 1995; Tomczak and Godfrey, 2002). It is fed by the continuation of the East Australian Current, the East Auckland Current and East Cape Current, and follows the Subtropical Front. It feeds its water into the cold Peru/Chile Current, which travels northward along the coast of South America and feeds the South Equatorial Current (Fig.1). It is much weaker than the South Atlantic and South Indian Currents, carrying about 5 Sv. The South Pacific Current is an important heat source for the atmosphere. The South Pacific, South Indian and South Atlantic Currents are associated with the Antarctic Circumpolar Current (West Wind Drift), which encircles the globe, merging the waters of the Pacific, Atlantic, and Indian oceans.

Figure 1. Surface currents and fronts of the Eastern South Pacific (Tomczak and Godfrey, 2002) and the region of investigations (rectangular area).

The Eastern South Pacific Ocean is an important region where thermocline is ventilated through subduction. In the subduction regions the dominant features in the mean flow field in the mixed layer are the South Atlantic, South Indian and South Pacific Ocean Currents that are related to the Subtropical Front (Stramma et al.,1995; Karstensen and Quadfasel, 2002). High subduction rates are found between 15^o and 20^oS in Eastern South Pacific Ocean, associated with the bowl shape of the Subtropical Gyre and a northward component in the South Equatorial Current. Looking at the Southern Hemisphere, the strongest local subduction spirals southwestward from about 25^oS, 100^oW in the South Pacific through the South Atlantic and the South Indian Ocean to 55^oS, 50^oW near Drake Passage (Karstensen and Quadfasel, 2002).

Unfortunately, little is known about the South Pacific Current and Subtropical Front in this region. This paper presents the investigation of structure, and spatial and temporal variability of the South Pacific Current and Subtropical Front in the Eastern South Pacific Ocean (45-20S, 110-70W) (Fig. 1) based on the TOPEX/Poseidon satellite altimetry data set for the period of 1992-2003, with an emphasis of poorly known aspects of location and behavior of the frontal zone.

DATA AND METHODS

Analysis of structure and spatial and temporal variability of the Subtropical Front and the South Pacific Current in the Eastern South Pacific Ocean (region 45-20S, 110-70W) is based on the sea surface dynamic topography, which is calculated on the TOPEX/Poseidon (T/P) satellite altimetry data for the period between 1992 and 2003.

The sea surface height, calculated on the satellite altimetry data, contains the information about the dynamic topography. However, it differs from conventional results of the oceanographic data calculation or of simulation data, as it is measured over the reference ellipsoid, bound with the Earth mass center. Therefore the dynamic topography variability is studied on the sea level anomalies relative to the mean sea surface, which is calculated on the climatic satellite altimetry data (Satellite Altimetry, 2001). The synoptic dynamic topography field is calculated as a superposition of the mean climatic dynamic topography and sea level anomaly fields (Lebedev, 2002).

In the report we have used the sea level anomaly from the T/P satellite altimetry data of NASA Ocean Altimeter Pathfinder Project, which are produced in Goddard Space Flight Center (GSFC) (Koblinsky. et al., 1999). The anomalies are calculated relative to the GSFC00.1 model of the mean sea surface and include all standard geophysical corrections. The data are averaged in the regular one-degree grid on ten-days T/P cycles. Totally 377 T/P cycles for the period from 23 September 1992 to 25 February 2003 have been used.

Mean dynamic topography was calculated relative to 1000 mbar surface from mean temperature and salinity data of WOA-1998 Atlas (Antonov et al., 1998; Boyer et al., 1998). Also we used combined mean dynamic topography named RIO-03 (Rio and Hernandez, 2004), which is calculated for the 1993-1999 period using a multivariate analysis of hydrographic data, surface drifter velocities and satellite altimetry (relative to 1500 dbar surface). The synoptic dynamic topography maps were calculated as its superposition and the sea level anomaly field for each 10-days T/P cycle.

Since the position of the South Pacific Current coincides with the position of a maximum gradient of dynamic heights, the maps of dynamic heights gradients were plotted for each T/P cycle basing on the two-dimensional gradient operator.

The analysis shows, that values of dynamic heights gradients change from 0 to 0.10 cm/km, which corresponds to the geostrophic current velocities from 0 to 11.5 cm/s (according to model calculations). The gradient of 0.04 cm/km (5 cm/s) was selected as the threshold value for positioning of the South Pacific Current. Location of the current was determined as the position of maximum dynamic heights gradient along the fixed meridians between 110° and 90°W. The gradient values, as a rule, notably decrease eastward from 90W, where the current changes its direction to the north, passing on to the Peru/Chile Current.

RESULTS AND DISCUSSION

The obtained maps of dynamic heights gradients reveal the pronounced area of high gradient values stretched in a zonal direction along 36S between 110 and 90W (Fig. 2). According to previous investigations (Stramma et al., 1995) the South Pacific Current and Subtropical Front are located in this region (see Fig. 1).

Figure 2. Location of the Subtropical Front as defined from maps of the dynamics heights gradient for the period 1992-2003.

The ten-days gradients maps show an intense spatial variability of the front position and configuration. The front meanders with a wavelength of about 5 in latitude. The quasi-stationary cyclonic meander was observed between 105° and 100W.

Fig. 3 presents the mean position of the Subtropical Front together with associated standard deviations at every degree of longitude that represent seasonal variability of the front location. The mean position of the front occurs between 36° and 37S with the front being shifted from the north to the south while it passes from the west to the east.

Figure 3. Average position and standard deviation the Subtropical Front as inferred from T/P altimetry data (1992-2003).

Standard deviation of the frontal zone position made possible to compare the degree of variability between different parts of the front. Remarkable difference in the variability is seen between longitudes 101°-96W, where the frontal zone is more stable (r.m.s. are less than 1lat.) and the gradients are high, and the western part of the region between 110°-105W, where they exhibit larger northward and southward displacements (2-3lat.). Between 94° and 90W, at the eastern part of the region, variability of the front position is about 1.5lat.

Analysis of the dynamic heights gradients map and high degree of variability of the Subtropical Front location to the west of 105W reveals a double structure of the front in the western part of the region. The Subtropical Front is known to have a bimodal structure in the Southern Indian Ocean (Kostianoy et al., 2004).

Temporal variability of the frontal zone location demonstrates a pronounced 35 T/P cycles peak (350 days) in the western part of the region. Spectral density of the frontal zone location variability in the rest part of the region did not reveal any significant peaks. It indicates that to the west of the 105W location of the South Pacific Current and Subtropical Front has a strong seasonal variability with the period of approximately one year, while to the east of the 105W there is no predominance in temporal variability of the current and front position.

CONCLUSIONS

This study has demonstrated efficiency of the analysis of the satellite altimetry data in the investigation of the structure and spatial and temporal variability of the South Pacific Current in the Eastern South Pacific. The main advantage of this method is related to the use of regular sea level anomaly data set with a relatively high resolution in space and relatively high frequency (10 days).

A high degree of spatial and temporal variability of the South Pacific Current and Subtropical Front location was revealed in the western part of the investigated area. There is an annual fluctuation in the frontal zone shifts, with r.m.s. of its position being 3 latitude. To the west of 105W the South Pacific Current may have bimodal structure. The quasi-stationary cyclonic meander was observed in the central part of the region.

Further investigations of the dynamic heights field in the Eastern South Pacific may improve our knowledge about seasonal and interannual variability of the currents and fronts in this region.

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