The tropical Atlantic Ocean is characterized by a large seasonal cycle around which there are climatically significant interannual and decadal timescale variations. The most pronounced of these interannual variations are equatorial warm events, somewhat similar to the El Nino events for the Pacific, and the so-called Atlantic sea surface temperature dipole. Both of these phenomena in turn may be related to El Nino-Southern Oscillation variability in the tropical Pacific and other modes of regional climatic variability in ways that are not yet fully understood. PIRATA (Pilot Research Moored Array in the Tropical Atlantic) will address the lack of oceanic and atmospheric data in the tropical Atlantic, which limits our ability to make progress on these important climate issues. The PIRATA array consists of 12 moored Autonomous Temperature Line Acquisition System buoy sites to be occupied during the years 1997-2000 for monitoring the surface variables and upper-ocean thermal structure at key locations in the tropical Atlantic. Meteorological and oceanographical measurements are transmitted via satellite in real time and are available to all interested users in the research or operational communities. The total number of moorings is a compromise between the need to put out a large enough array for a long enough period of time to gain fundamentally new insights into coupled ocean-atmosphere interactions in the region, while at the same time recognizing the practical constraints of resource limitations in terms of funding, ship time, and personnel. Seen as a pilot Global Ocean Observing System/Global Climate Observing System experiment, PIRATA contributes to monitoring the tropical Atlantic in real time and anticipates a comprehensive observing system that could be operational in the region for the 2000s. 1 • A scientific rationale The seasonal cycle is the largest ocean-atmosphere signal in the tropical Atlantic. The timing and characteristics of the seasonal evolution of the location of the

A striking feature of the South Indian Ocean circulation is the presence of the eastward South Indian Countercurrent (SICC) that flows in a direction opposite to that predicted by the classical theories of wind-driven circulation. Several authors suggest that the SICC resembles the subtropical countercurrents (STCCs) observed in other oceans, which are defined as narrow eastward jets on the equatorward side of subtropical gyres, where the depth-integrated flow is westward. These jets are associated with subsurface thermal fronts at thermocline depths by the thermal wind relation. However, the subsurface thermal front associated with the SICC has not been described to date. Other studies conjecture an important role for salinity in controlling the SICC. In the present work, we analyze three Argo-based atlases and data from six hydrographic cruises to investigate whether the SICC is accompanied by permanent thermal and density fronts including salinity effects. The seasonal cycle of these fronts in relation to the SICC strength is also investigated. We find that the SICC is better described as composed of three distinct jets, which we name the northern, central, and southern SICC. We find that the southern SICC around 26 S has an associated thermal front at subsurface depths around 100-200 m with salinity being of secondary importance. The southern branch strength is related to mode waters poleward of the front, similar to a STCC-like current. However, the SICC multiple jet structure seems to be better described as resulting from PV staircases.

[1] A new filtering method based on Singular Spectrum Analysis has been devised to extract high resolution (0.25°Â 0.25°grid) satellite-only Mean Dynamic Ocean Topography(MDT) constructed by differencing of the GGM02 GRACE Gravity Model from the GSFCMSS00 Mean Sea Surface. This data-adaptive interpolation-type filter is adequate for the finite domain MDT processing since it minimizes smoothing and does not lose boundary

[1] The unexpected evolution of the first recorded South Atlantic Hurricane Catarina over waters with homogeneous sea surface temperatures (SST) of 24°C in March 2004 was a challenge to the weather forecast community. This work concentrates on a thorough data-driven comparative analysis to make reliable diagnostics of the role of the ocean in the genesis and evolution of Catarina. We used several high-resolution multisatellite-derived products, including three microwave-based SST data sets, multisatellite collinear data of sea surface height (SSH) anomalies, significant wave heights and wind speeds, four QuikSCAT ocean surface wind vector products (including the 12.5 km resolution swath data), daily fields of absolute objectively analyzed SSH and corresponding geostrophic currents, and Argo floats. The synergic use of these data sets showed that Catarina interacted strongly with four warm core rings (WCRs), forcing upwelling of isotherms and mixed layer waters. These interactions minimized the known negative SST feedback, as attested by the SST differences being less than 1.2°C. Although the SST in the region was around 24°C, below the Palmén threshold, the surface air temperatures were 14°C which still furnished a large air-sea temperature gradient capable of extracting large enthalpy fluxes from the WCRs influenced by Ekman pumping. It is shown here that Catarina achieved category 1 over the ocean on 26 March with its maximum intensity of 34 m/s seen in the 12.5 km swath winds.

In the present work, we investigate the interannual variability of the South Indian Countercurrent (SICC), a major and still understudied current of the Indian Ocean circulation. To characterize the interannual variability of the SICC, four different data sets (altimetry, GLORYS, OFAM3, and SODA) are analyzed using multiple tools, which include Singular Spectrum Analysis and wavelet methods. The quasi‐biennial band dominates the SICC low‐frequency variance, with the main peak in the 1.5–1.8 year interval. A secondary peak (2.1–2.5 year) is only found in the western basin. Interannual and decadal‐type modulations of the quasi‐biennial signal are also identified. In addition, limitations of SODA before the 1960s in the SICC region are revealed. Within the quasi‐biennial band, the SICC system presents two main patterns with a multiple jet structure. One pattern is characterized by a robust northern jet, while in the other the central jet is well developed and northern jet is weaker. In both patterns, the southern jet has always a strong signature. When the northern SICC jet is stronger, the northern cell of the subtropical gyre has a triangular shape, with its southern limb having a strong equatorward slant. The quasi‐biennial variability of the SICC is probably related to the Indian Ocean tropical climate modes that are known to have a strong biennial characteristic.