Meridional transport

Be should be a useful tracer in detailed studies of atmospheric transport and washout. It is not produced in significant quantities by nuclear tests, and therefore variations in its activity from year to year should reflect variations in atmospheric processes. Several investigators have measured 7Be at different altitudes and latitudes. As the latitude, altitude, and time variations become better known, it should be possible to calculate precisely the rates of vertical and meridional transport and the rates of deposition at the earth s surface. 

This tractability depends upon a rather unique and fortuitous combination of time constants which results in characteristic chemical response times being considerably shorter than dynamical time constants for many of the key mechanisms. Experimentally this means that pure chemical experiments can be executed in situ, to test photochemical hypotheses on the microscale within the atmosphere. On the other hand, dynamical experiments addressing the longer time scales can be conducted from Earth orbit via satellite to establish vertical and meridional transport maps. 

On balance, the shelves are not a net source of N to the open ocean. Instead, the North Atlantic has major exchanges with the Arctic Ocean and with the South Atlantic. Ganachaud and Wunsch (2002) estimate southerly nitrate fluxes of 2200 (+/- 3800) and 6600 (+/- 4700) x 10 mol N year- at 7.5°N and 4.5°S, respectively. We take their mid point value of 4400 (+/— 4000) X 10 mol N year as the net transport of nitrate from the North Atlantic to the South Atlantic. A significant uncertainty lies in the net meridional transport of DON in the basinwide N transport budgets in the Atlantic. Rintoul and Wunsch (1991) speculated that the imbalance they quantified in the poleward nitrate flux across subtropical sections may be compensated by unobserved fluxes of organic nitrogen. DON measurements are stiU too sparse and too imprecise to test this hypothesis. 

Finally, precise estimations of meridional transports provide powerful constraints of the basin scale nitrogen budgets. To provide better constraints, we require zonal sections including appropriately dense observations of the organic forms of dissolved nitrogen (measured at higher precision than now possible) to be used in quantification of the meridional transport of nitrogen. 

Carleton, A. M., 1988. Meridional transport of eddy sensible heat in winters marked by extremes of the North Atlantic Oscillation, 1948/49-1979/80. Journal of Climate, 1, 212-223. 

Time constants for transport by the meridional and zonal winds can be derived in similar fashion, but in these directions it is somewhat more difficult to characterize the typical gradients of chemical species. Assuming for these horizontal changes a typical length scale of about 1000 km in the zonal and meridional directions, the time constant for transport by the zonal winds is of the order of days throughout the middle atmosphere, while that for mean meridional transport is of the order of months in the stratosphere and days in the upper mesosphere. 

Dynamics and Meridional Transport in Two Dimensions A Conceptual View... 

As in the case of the potential temperature, the Eulerian mean and eddy components of meridional transport for the mixing ratio of chemical species tend to cancel, and the continuity equation for species i, written as a function of the TEM winds becomes... 

The formalism of the transformed Eulerian mean circulation shows that meridional transport in the middle atmosphere is generated primarily by non-local momentum forcing associated with wave dissipation. This forcing, represented by the Eliassen-Palm flux divergence in equation (3.67), acts as an extratropical pump producing strong upward air motions in the tropics and downward return ... 

Figure 3.26. Zonally averaged distribution of hydrogen fluoride (HF) mixing ratio (in ppbv) measured by the HALOE instrument on UARS. The heavy solid arrows denote the mean meridional transport and the horizontal arrows show the location of strong quasi-horizontal mixing by planetary waves. Courtesy of W. Randel, NCAR, 2001.

Figure 3.27. Schematic representation of the global diffuser model (upper panel) and tropical pipe model (lower panel). Gray arrows denote meridional transport by the transformed Eulerian mean circulation while the heavy solid arrows show quasi-horizontal mixing by large scale waves. The vertical lines in the lower panel represent dynamical barriers against meridional transport in the tropics. From Plumb and Ko (1992).

The polar vortex (near 60° latitude, above approximately 16 km) is another dynamical barrier against meridional transport. The isolation of the polar regions, and specifically of the Antarctic lower stratosphere... 

Figure 3.30. Schematic representation of the atmospheric circulation (arrows) and associated quasi-horizontal mixing between the surface and the middle stratosphere. Mixing processes leading to stratosphere-troposphere exchanges are also represented. The heavy vertical lines denote dynamical barriers against meridional transport. Note the large-scale ascent in the tropical stratosphere above intense convective systems in the tropical troposphere, and large scale descent associated with the polar vortex during winter (WMO, 1999).

Figure 5.32 shows the photochemical time constant of N2O and those appropriate to transport processes. Like CH4, N2O is an excellent tracer for transport in the middle stratosphere, where its lifetime is comparable to those for advection by the mean meridional circulation. At higher altitudes in the upper stratosphere and lower mesosphere, the N2O lifetime remains close to the mean meridional transport lifetimes, making it a more sensitive tracer than CH4 in this region. 

Some authors have argued that changes in stratospheric dynamics could have contributed to the observed mid-latitude ozone trends. A review is provided in Ravishankara et al. (1999 see references therein). In brief, some studies (e.g., Hood and Zaff, 1995 McCormack and Hood, 1997 Hood et al., 1997 Fusco and Salby, 1999) have argued for a component of purely dynamical change in mid-latitude ozone relating, for example, to changes in the meridional transport of ozone. It is well known that dynamical processes strongly influence ozone variability... 

Flohn, H. (1961). Meridional transport of particles and standard vector deviation of upper winds. Geophys. Pura Appl. 50, 229-234. 

These two equations state that, in each cell, the surface radiates away as much heat (right hand side) as it receives (left hand side). The term involving F on the left hand side is the solar radiation absorbed at the surface. The downwelling longwave radiation from the atmosphere is a fraction p of die energy radiated upward by the surface plus half the meridional transport D. This result is taken from a toy model of the radiative equilibrium between the atmosphere and the surface, Thorndike 1992. P is a measure of die longwave emissivity of the atmosphere. In today s atmosphere P is... 

The meridional transport is assumed to be proportional to the temperature contrast... 

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