Info

topography [m]

Sandwell seafloor topography

Fig. 5.76 Fine structure of seafloor topography (based on Smith and Sandwell, 1997). See color plate section.

Treating the bottom topography accurately It is of vital importance to have the deep channels between different sub-basins connected for low-resolution models. Although a low-resolution model can provide useful information about deep circulation in the world's oceans, results from such numerical simulations can be rather sensitive to the horizontal resolution of the model. As seen from a fine-resolution map of the sea floor topography near the mid-ocean ridge (Fig. 5.76), flow can be very complicated due to the rough topography associated with the newly formed seafloor on both sides of the mid-ocean ridge. In particular, due to the existence of deep valleys perpendicular to the axis of the mid-ocean ridge, the geostrophic flow parallel to the axis of the mid-ocean ridge may be interrupted near the bottom topography. For example, the equatorward geostrophic flow shown in Figure 5.72 was not observed in the field experiments involving a tracer released near the mid-ocean ridge. Instead, deep floats deployed in the Brazil Basin indicated that flows at the mid-depth are dominated by alternating zonal jets, as discussed in the next section.

Using the best estimate for the mixing coefficient A spatially varying mixing coefficient is clearly one of the most critical parts of ocean models for simulating the deep circulation. In particular, extremely strong localized mixing associated with overflow through sills connecting different basins has been omitted in most previous basin-scale simulations; however, such strong localized mixing may be crucial in simulating the deep circulation accurately.

5.2.5 Mid-depth circulation

According to the classical theory of Stommel and Arons (1960a) circulation in the deep ocean interior is dominated by broad eastward and poleward flows driven by the uniform upwelling (Fig. 5.56). Deep western boundary currents are required for completing the mass balance in individual basins. The deep western boundary currents predicted by their theory are probably the most robust feature in the ocean, and these have been confirmed through observations in the world's oceans. On the other hand, the broad interior flows predicted by the theory have never been observed.

Much effort has been made to explore the deep circulation through observation. For example, a large-scale field observation program, the Deep Basin Experiment, was specially designed to observe the deep circulation in the Brazil Basin. A large number of neutrally buoyant floats were released within the Brazil Basin during the 1990s in an attempt to measure directly the circulation in the deep ocean interior (Hogg and Owens, 1999).

It appears that the flow in the deep Brazil Basin is unlike the prediction from the classical theory. Although the deep western boundary currents observed confirm the theory, the interior flow inferred from the neutrally buoyant floats is dominated by zonal flows with unexpected small meridional scales, as shown in Figure 5.77.

It is not surprising that zonal flows dominate the deep circulation because potential vortic-ity contours in the deep ocean are primarily zonally oriented (O'Dwyer and Williams, 1997). As an example, a potential vorticity map for the abyssal ocean is shown in Figure 5.78. Weak diapycnal mixing in the basin interior gives rise to small vertical velocity divergence; thus, the corresponding meridional velocity is small and flows should be primarily zonal.

The study of the deep circulation may require basin-scale high-resolution data, which are currently not available from observations; thus, our discussion here is limited to results from eddy-permitting numerical models. According to a study by Nakano and Hasumi (2005), the zonal currents in the sub-surface oceans can be classified into two categories. First, there are broad-scale zonal flows which have a poleward slanting pattern in the meridional section. Second, there are fine-scale zonal jets, which have a meridional scale of 3°-5°, formed in each broad zonal flow.

There are many possible origins for the zonal flows. First, baroclinic instability may induce such flows. Treguier et al. (2003) showed that although the mean flow in the Brazil Basin is baroclinically unstable, the corresponding growth rates are small; thus, this process is unlikely to be the sole source of the zonal jets observed there. Second, the topographic feature may also be responsible for the existence of zonal jets. However, the primary mechanism for the zonally alternating jets may be the response to wind stress. Using numerical experiments for the Pacific Ocean, Nakano and Suginohara (2002)

2500m, 600-800 day displacements

2500m, 600-800 day displacements

Fig. 5.77 600-800 days displacement of floats at 2,500 m in the Brazil Basin (Hogg and Owens, 1999).

Fig. 5.77 600-800 days displacement of floats at 2,500 m in the Brazil Basin (Hogg and Owens, 1999).

demonstrated zonal flows driven by wind. The basic idea is that wind-driven circulation in the ocean is established through Rossby waves. The first few vertical modes move across the basin within a few decades and establish the barotropic and first baroclinic components of the wind-driven circulation. Higher modes take a much longer time to move across the basin. Due to dissipation, the Rossby waves of high modes can never complete their

Fig. 5.78 Potential vorticity (in units of 10 12 m/s) at a depth of 2,750 m in the world's oceans, based on Levitus et al.'s (1998) Climatology.

00E 600E 1200E 1800 1200W 600W 00W

Fig. 5.78 Potential vorticity (in units of 10 12 m/s) at a depth of 2,750 m in the world's oceans, based on Levitus et al.'s (1998) Climatology.

Fig. 5.79 A zonal velocity section along 180°, from a high-resolution (1/4° x 1/6°) model with contour intervals of 2 cm/s; shaded areas indicate westward velocity bands (Nakano and Hasumi, 2005).

journey to the western coast, thus leaving a steady zonal flow with the characteristic meridional and vertical structure of the equatorial waves. Numerical experiments for the Brazil Basin produced results that seem quite consistent with the Lagrangian float data (Treguier et al., 2003).

Numerical experiments with high resolution produced both types of current in the North Pacific Ocean (Fig. 5.79). Apparently, baroclinic instability plays the major role in producing the fine-scale zonal jets embedded in the broad-scale zonal flows. In fact, in both the atmosphere and oceans, there are zonal jet-like features which resemble the flow pattern observed from other planets, such as Jupiter and Saturn.

5.3 Haline circulation

5.3.1 Hydrological cycle and poleward heat flux

Poleward heat flux in the climate system has been one of the main focuses of climate study. According to traditional classification, the atmospheric component of the poleward heat flux dominates the total poleward heat flux at high latitudes. For example, at 35° latitude, where the poleward heat flux is maximal, the atmospheric transport accounts for 78% of the total in the Northern Hemisphere and 92% in the Southern Hemisphere. The oceans seem unimportant for mid-latitude and high-latitude climate, in terms of carrying the heat flux that is much needed for warming up high latitudes. But is that true? In particular, the Southern Hemisphere is covered primarily by oceans, especially between 35° S and 70° S. Therefore, it is almost impossible to believe that the oceans play such a minor role in poleward heat flux.

Definition of poleward heat flux Oceanic heat flux has been discussed in many papers and textbooks. As both instruments and numerical models improve, poleward heat fluxes are becoming better diagnosed. For up-to-date information, the reader is referred to the review by Bryden and Imawaki (2001).

The main point is that although heat flux data may be further improved, there seems to be a fundamental problem in the definition of poleward heat fluxes that gives rise to misconceptions and similar mistakes by many people. From thermodynamics, it is well known that heat flux cannot be well defined for a system that has net mass gain (or loss) through the lateral boundary of the system. For example, Trenberth and Caron (2001) made the point that the poleward heat fluxes in the South Pacific and Indian Oceans are not well defined because of the existence of the Indonesian Throughflow.

The same caution should apply to both the oceans and atmosphere, because neither the ocean nor the atmosphere is a closed system in terms of mass - there is water exchange between them. In fact, the hydrological cycle of evaporation and precipitation is an essential ingredient for heat flux in the atmosphere - the latent heat flux. The mass flux associated with evaporation and precipitation has been traditionally ignored in heat flux calculations by oceanographers, for the following reasons.

First, such a small mass flux is rather hard to identify from the classical dynamical calculation based on either a reference level or other means. Second, most oceanic general circulation models have been based on the Boussinesq approximations; thus, the dynamical effects associated with evaporation and precipitation are simulated in terms of the virtual salt flux condition on the upper surface, while the mass flux associated with evaporation and precipitation is totally ignored. Improvements in oceanic general circulation models include a new generation of models formulated under the natural boundary condition (Huang, 1993b). As a result, the mass flux associated with evaporation and precipitation is exactly accounted for. A more accurate definition of heat flux is therefore necessary. A simple definition of heat flux is found in the Appendix to this chapter, which can be used for an oceanic section with a net mass flux exchange.

Dry air flux Ma

Water vapor flUX Mao

Water flux Mo

Atmosphere

Ocean

Equator 60°N

Fig. 5.80 Sketch of the atmosphere-ocean coupled climate system (Huang, 2005b).

Strictly speaking, the poleward heat flux in the climate system should be defined as three terms: sensible heat flux in the oceans, sensible heat flux in the atmosphere, and latent heat flux in the atmosphere-ocean coupled system, as shown in Figure 5.80. Over the latitudinal range from equator to pole there is a continuous exchange of heat between these three loops.

As an example, we calculate the freshwater flux through the air-sea interface. We use the evaporation and precipitation rates over the world's oceans as reported by Da Silva et al. (1994) (Fig. 5.81). This dataset has taken river run-off into consideration, so the globally integrated evaporation and precipitation rates are balanced. Near the equator, precipitation dominates, especially between 0° N and 10° N. However, evaporation dominates over the subtropics and thus produces a net water vapor flux that is transported by the atmosphere to high latitudes. At high latitudes (roughly beyond 40° off the equator) precipitation dominates.

However, this dataset seems to give rise to a poleward freshwater flux that is too large in the Southern Hemisphere. As an alternative, we use the water vapor flux calculated from atmospheric circulation models (Gaffen et al., 1997). The freshwater flux reported in their study is based on the average of 25 atmospheric general circulation models. This poleward water vapor flux, Mw, is the major mechanism of poleward heat flux in the climate system (Fig. 5.82).

The poleward heat flux associated with the water vapor cycle is closely related to the latent heat content of water vapor

where Lh = 2,500 J/g is the latent heat content for water vapor, and hw is the enthalpy of the return water in the ocean, which is much smaller than Lh and is neglected in the

Fig. 5.81 Water vapor source and sink due to evaporation and precipitation, integrated zonally (Huang, 2005b).
Urban Latent Heat Flux Diagram
Fig. 5.82 Northward water vapor flux (Sv) and associated latent heat flux (PW) due to evaporation and precipitation, as calculated from atmospheric general circulation models (Huang, 2005b).
Table 5.5. Mass transport maxima in the climate system

Current systems

Gulf Stream

Kuroshio

0 0

Post a comment