4.7. Deep Tropical Circulation

The issue of geostrophy in the tropics was mentioned above. A vast amount of successful work has been done in the past decade on circulation, variability, and feedback with the atmosphere in the upper tropical ocean, focused primarily on the El Niño problem. Deeper in the ocean there have been fewer observations, but those which are emerging are indicative of a regime within 15 to 20 of the equator which may be significantly different from mid-latitudes. Typical horizontal scales of disturbances are given by a quantity called the ``Rossby deformation radius,'' which has a value of several thousand kilometers for the barotropic mode (that which is independent of depth and which depends on sea surface deformation only), and tens to several hundred kilometers for the baroclinic mode (that which depends on the vertical stratification within the water column). At the equator, the baroclinic deformation radius is a few degrees latitude. Firing [1987, 1989] has shown that the long-term flow within the equatorial baroclinic deformation radius is dominated by a set of stacked jets, whose vertical scale increases with distance from the equator, with mean flows off the equator at depth (Fig. 5). Talley and Johnson [1994] infer zonal flows at about 2500 meters to the west at about 7-10N and 12-15S and to the east near the equator (Fig. 3). The strong zonality of the tracer patterns suggests strongly zonal flows. Preliminary results from subsurface floats in the mid-latitude and tropical Pacific at about 1000 meters depth show strongly zonal flows within this broad tropical regime [Davis, personal communication].

An issue of abiding interest is the location of cross-equatorial flow for the intermediate to abyssal water masses, since the deep North Pacific is a cul de sac for the world ocean's abyssal waters. Tracer patterns, particularly of silica, suggest that northward flow occurs broadly in the western Pacific north of Samoan Passage, with southward flow in the central/eastern Pacific along the western flank of the East Pacific Rise [ Reid, 1986; Talley and Joyce, 1992; Johnson and Toole, 1993].