Balanced dynamics of deep and shallow Hadley circulations in the tropics

Abstract

This paper examines the dynamics of large-scale overturning circulations in the tropical atmosphere using an idealized zonally symmetric model on the equatorial b-plane. Under certain simplifications of its coefficients, the elliptic partial differential equation for the transverse circulation can be solved by first performing a vertical transform to obtain a horizontal structure equation, and then using Green's function to solve the horizontal structure equation. When deep diabatic heating is present in the Intertropical Convergence Zone (ITCZ), the deep Hadley circulation is of first-order importance. In the absence of deep diabatic heating, the interior circulation associated with Ekman pumping cannot penetrate deep into the troposphere because the resistance of fluid parcels to horizontal motion (i.e., inertial stability) is significantly smaller than their resistance to vertical motion (i.e., static stability). In this scenario, only a shallow Hadley circulation exists. The shallow overturning circulation is characterized by meridional velocities as large as 7 m s 21 at the top of the boundary layer, in qualitative agreement with observations in the tropical easternPacific. The meridional asymmetry between the winter and summer deep and shallow Hadley cells is attributed to the anisotropy of the inertial stability parameter, and as the ITCZ widens meridionally or as the forcing involves higher vertical wave numbers, the asymmetry between the winter and summer cells increases.

Transient aspects of the Hadley circulation forced by an idealized off‐equatorial ITCZ

“…More specifically, the inertia‐gravity wave amplitude peaks when km and 950 km in the southern and northern cell, respectively. These ITCZ locations nearly coincide with the ITCZ location where there is the maximum asymmetry between the southern and northern Hadley cells [ Hack et al ., ; Gonzalez and Mora Rojas , ]. Also, the inertia‐gravity waves in the southern cell are about twice as large in amplitude as those in the northern cell, similar to the time average asymmetry between the two cells.…”

“…Values of the parameter in the switch‐on function , presented in the next section, are plotted in the four horizontal dashed lines, h. Notice how h does not intersect with any of the inertia‐gravity wave frequencies. As will be seen later, when diabatic heating is switched on at this slow rate, inertia‐gravity wave activity is minimal and the transient solutions are approximately equal to the balanced solutions derived in Gonzalez and Mora Rojas []. In the next section, we solve (40) and (41) for a particular forcing.…”

“…When h, the southern and northern Hadley cells are almost symmetric in size and magnitude at t = 12 h, but by t = 36 h, the northern cell, which is typically thermally direct, has become thermally indirect. At t = 60 h, the northern cell is both wider and stronger than the southern cell, and the opposite occurs at t = 84 h. In addition to this transient activity there is an inherent asymmetry between the Hadley cells in a time averaged sense due to the anisotropy of the inertial stability [ Hack et al ., ; Gonzalez and Mora Rojas , ]. This is even more clear in Figure , where the transient activity is not excited as effectively, implying that the zonal winds in the Hadley cells are essentially in geostrophic balance.…”

“…The total mass flux in the ITCZ, shown in the black curves, also contains high frequency behavior due to the time lag between the two cells. Note that the time average asymmetry between the southern and northern cells is approximately 2 to 1 for all ITCZ switch on rates, similar to the balanced solutions shown in Gonzalez and Mora Rojas [].…”

“…On the other hand, the northern Hadley cell is dominated by 40 h oscillations when the ITCZ is 0–2.5° off of the equator and 70 h oscillations when the ITCZ is 2.5–20° off of the equator. A possible explanation for this behavior is that as the forcing involves higher vertical wave numbers, the asymmetry between the southern and northern cells increases, as discussed in Gonzalez and Mora Rojas [].…”