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The surface wind stress curl is the forcing function in the equations of vertically integrated water transport of wind-driven ocean currents. Hence, it has become a basic quantity in theoretical oceanography. As the time dependence of all important surface quantities in the Indian Ocean is stronger than in other oceans, it is valuable to look particularly at the time variation in this region. This study presents monthly charts of the wind stress curl at the surface of the Indian Ocean from its land boundaries up to 50° S. and from 20° E. to 116° E.

Basic data were the monthly surface maps of the Koninklijk Nederlands Meteorologisch Instituut, derived from ship observations and given as 2° square means of the surface wind. The processing of the data is described in detail. In particular, small-scale fluctuations are objectively filtered out.

While earlier compilations are usually on a coarser grid (seasonal and 5° square averages), the present data have a refined time and space resolution. Therefore, they allow one to study more detailed structures. In particular, the charts show that the curl pattern in the Indian Ocean is not independent of longitude.

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Michael Hantel


The zonally averaged conservation equations for water, linear zonal momentum, and potential heat (gz+cpT) are written in a form analogous to the mass continuity equation. This is possible when atmospheric storage terms are negligible which is generally the case during the solstice seasons. It follows that the fluxes of these properties can be represented by Stokes streamfunctions. Patterns of streamfunctions in the vertical-meridional plane for mass, water (all 3 phases combined), momentum, and heat have been prepared from the “Atmospheric Circulation Statistics” of Oort and Rasmusson (1971). They are shown for the seasons December–January–February and June–July–August, and for the area between 10S-75N. The following details are of interest:

  1. The presented data are not new in any sense. Only the mode of presentation is new.

  2. Contrary to the total mass transport, which is almost entirely conservative, all other transports have sources and sinks. They are treated as vertical flux divergences and thus are amenable to the streamfunction concept.

  3. The streamfunction pattern can be considerably modified by linear combination with the mass continuity equation, characterized by a scale function: This function is zero for water transport, a 2Ω cos2ϕ for momentum transport, and gz0 +cpT 0 for heat transport (z 0, T 0 averages over total atmosphere). This choice minimizes the back-and-forth transport of properties by the cell circulation.

  4. Boundary conditions are that the upper surface of the atmosphere be a streamline for mass, water, and momentum transport. For potential heat, the value at the upper surface at a certain latitude is the net radiation flux across this surface, integrated between this latitude and the pole.

  5. The streamlines represent the total flux of the respective property in whatever form. For instance, vertical fluxes of water comprise mean and eddy components of all scales as well as net contributions of solid and liquid water flux.

  6. The streamlines of transports with sources and sinks begin and end at the earth's surface (water, momentum, and heat) or additionally at the upper surface of the atmosphere (heat). There are no closed isolines.

The mixed character of the various fluxes is qualitatively described. Fluxes of different properties cross each other or go in opposite directions. Further, fluxes of the same property on different scales may go in opposite directions, particularly in the vertical. The total horizontal flux divergences are compared with some independent flux estimates at the earth's surface. Although there are still significant imbalances, the general agreement is fair.

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