Heat and buoyancy budgets and mixing rates in the upper thermocline of the Indian and Global Oceans

Abstract

Diapycnal and diathermal diffusivity values in the upper thermocline are estimated from buoyancy and heat budgets for water volumes bounded by isopycnals and isotherms, the air-sea interface, and coastline where applicable. Comprehensive analysis is given to the Indian Ocean, with an extended global general description. The Indian Ocean gains buoyancy in the north (especially in the northeast) and loses buoyancy in the subtropical south. Freshest and least-dense water appears in the Bay of Bengal and isopycnals outcrop southwestward from there and then southward. Computation of diapycnal diffusivity (Kp) starts from the Bay of Bengal, expanding southwestward and southward and with depth. As isopycnals extend equatorward from the northeast and with increasing depth. Kp remains at about 1.3 cm2 s-1 for 20.2 σθ (Bay of Bengal) to 22.0 σθ (northeast Indian Ocean). Farther south (poleward) and at greater depth, Kp decreases from 0.9 cm2 s-1 for 23.0 σθ (north of 20°S) to 0.5 cm2 s-1 for 25.0 σθ (north of 35°S). Isotherms outcrop poleward from the equator. Diathermal diffusivity values computed from the heat budget are large at the equator and near the surface (4.0 cm2 s-1 for 28.5°C isotherm) but decrease rapidly poleward and with depth (1.3 cm2 s-1 for 27.0°C). This indicates stronger mixing either near the equator or the surface, or a possible component in the diathermal direction of the larger isopycnal diffusivity, as isotherms do not follow isopycnals in the upper Indian Ocean north of 10°S. For the 21.0°C isotherm, which closely follows isopycnal 25.0 σθ, the heat budget yields a Kθ again of 0.5 cm2 s-1, the value of the diapycnal diffusivity. For the Indian-Pacific system, Kp decreases from 1.3 cm2 s-1 for 22.0 σθ (the warm pool water, depth ∼60 m) to 0.9 cm2 s-1 for 23.0 σθ (the tropical water between 20°N and 20°, depth ∼100 m), and to 0.1 cm2 s-1 for 25.0 σθ (40°N-40°S, depth ∼170 m). In the eastern tropical Pacific, Kp = 1.1 cm2 s-1 for 21.5 σθ (depth ∼25 m) while Kp = 0.6 cm2 s-1 for 22.0 σθ (depth ∼35 m). In the Atlantic, Kp = 0.6 cm2 s-1 for 24.0 σθ between 2O°N and 15°S (depth ∼80 m), and 0.2 cm2 s-1 for 25.0 σθ between 30°N and 35°S (depth ∼120 m). For the water volume bounded by 25.5 σθ farther south and north (50°N-40°S), air-sea buoyancy gain in the Tropics is about the size of the buoyancy loss in the subtropics, and the near-zero net flux may not have significance compared to the errors in the data. For 27.5 σθ, which encompasses the large region from about 65°N to the Antarctic (with midocean average depth of 400 m), Kp, is 0.2 cm2 s-1. The results indicate that mixing strength generally decreases poleward and with depth in the upper ocean.