| Meridional Criculations |
Zonal Circulations |
Meridional
Circulations (Figure 4)
The most well known thermally direct circulation cell is the Hadley Cell. The Hadley Cell temperature maximum developes at the point of maximum incoming solar radiation, also associated with the Inter-Tropical Convergence Zone (ITCZ). Hence, the axis of the Hadley Cell moves along with the seasons. The Hadley Cell rises at the ITCZ, and the diverges aloft as a two cell structure, with descending branches (or cooling columns) occuring in the mid-latitudes.
A lesser known thermally direct cell in the Monsoon Cell. While the monsoon circulation is also associated with the seasonal cycle of solar heating; local heating processes concerning the topography, land-sea heating gradiant, and moist processes are important for setting up the associated temperature maximum. In simplest form, the Monsoon Cell consists of rising air in the warm summer hemisphere diverging aloft, and subsiding in the cooler winter hemisphere.
The Hadley and Monsoon circulations can be combined into a Mean Meridional Circulation (MMC). Figure 4 illistrates the MMC and shows how the Hadley Cell in the summer hemisphere is weakend by contrasting Monsoon flow, whereas the in the winter hemipshere the Hadley and Monsoon Cells overlap resulting with a dominant cooling column (subsidence) occuring in the winter hemisphere.
Figure 4. A skematic showing the combined Hadley and Moonsoon circulations, also referred to as the MMC.
Zonal Circulations (Figure 5)
The most well-know thermally direct zonal circulation is the Walker Cell, after which the nomenclature Walker-type follows. In the Walker circulation, low-level, cold, heavy air from over the east Pacific flows west toward the Pacific warm pool where it is heated and there participates in large-scale moist-adiabatic ascent. The upper-level outflow then moves toward the east where it converges and subsides largely over the southern hemisphere subtropical eastern Pacific. The Walker cell also exhibits clear interaction with the El Nino Southern Oscillation (ENSO) phenomena, where it is generally perceived that the ascending branch of the Pacific Walker cell adjusts in accordance with the region of positive anomalous SST. A similar, but smaller circulation exsists over the Atlantic Ocean, and is refered to as the Atlantic Walker Cell.
Another Walker-type circulation is the Transverse Monsoon Cell. Here, deep convection associated with the East Asain Monsoon results in a strong warming column with 200 mb outflow over the Himalayan Plateau. This divergence aloft flows west and subsides over north Africa (see Example: Sahara Desert).
Figure 5. A skematic showing the combined zonal Walker-type circulations.
The most well known thermally direct circulation cell is the Hadley Cell. The Hadley Cell temperature maximum developes at the point of maximum incoming solar radiation, also associated with the Inter-Tropical Convergence Zone (ITCZ). Hence, the axis of the Hadley Cell moves along with the seasons. The Hadley Cell rises at the ITCZ, and the diverges aloft as a two cell structure, with descending branches (or cooling columns) occuring in the mid-latitudes.
A lesser known thermally direct cell in the Monsoon Cell. While the monsoon circulation is also associated with the seasonal cycle of solar heating; local heating processes concerning the topography, land-sea heating gradiant, and moist processes are important for setting up the associated temperature maximum. In simplest form, the Monsoon Cell consists of rising air in the warm summer hemisphere diverging aloft, and subsiding in the cooler winter hemisphere.
The Hadley and Monsoon circulations can be combined into a Mean Meridional Circulation (MMC). Figure 4 illistrates the MMC and shows how the Hadley Cell in the summer hemisphere is weakend by contrasting Monsoon flow, whereas the in the winter hemipshere the Hadley and Monsoon Cells overlap resulting with a dominant cooling column (subsidence) occuring in the winter hemisphere.
Figure 4. A skematic showing the combined Hadley and Moonsoon circulations, also referred to as the MMC.
Zonal Circulations (Figure 5)
The most well-know thermally direct zonal circulation is the Walker Cell, after which the nomenclature Walker-type follows. In the Walker circulation, low-level, cold, heavy air from over the east Pacific flows west toward the Pacific warm pool where it is heated and there participates in large-scale moist-adiabatic ascent. The upper-level outflow then moves toward the east where it converges and subsides largely over the southern hemisphere subtropical eastern Pacific. The Walker cell also exhibits clear interaction with the El Nino Southern Oscillation (ENSO) phenomena, where it is generally perceived that the ascending branch of the Pacific Walker cell adjusts in accordance with the region of positive anomalous SST. A similar, but smaller circulation exsists over the Atlantic Ocean, and is refered to as the Atlantic Walker Cell.
Another Walker-type circulation is the Transverse Monsoon Cell. Here, deep convection associated with the East Asain Monsoon results in a strong warming column with 200 mb outflow over the Himalayan Plateau. This divergence aloft flows west and subsides over north Africa (see Example: Sahara Desert).
Figure 5. A skematic showing the combined zonal Walker-type circulations.
| Introduction |
Dynamics |
Examples |
Observations |
Summary |
References |