ATMS 310 Tropical Dynamics

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1 ATMS 310 Tropical Dynamics Introduction Throughout the semester we have focused on mid-latitude dynamics. This is not to say that the dynamics of other parts of the world, such as the tropics, are any less important. The mid-latitudes are particularly compelling because motions of the region are driven by horizontal temperature gradients, which can be conveniently analyzed through quasigeostrophic theory. In the tropics, horizontal temperature gradients are weak or nonexistent. The energy harnessed by synoptic-scale systems comes from diabatic heating due to latent heat release, rather than potential energy from temperature gradients. A unique aspect to tropical systems is that there are strong cross-scale interactions. That is, heating created from cumulonimbi development can drive synoptic and even planetary motion. In addition, the Coriolis force is very small in the tropics, creating a unique set of disturbances and waves that are not found elsewhere. Intertropical Convergence Zone (ITCZ) The ITCZ is an area of low-level convergence near the Equator by the trade winds. It is most easily seen by a narrow (few hundred km), linear form of thunderstorms that rings around the tropics in a broken fashion. See the satellite image below: Figure 1. Infra-red GOES satellite image. Note the line of bright clouds near 10 N, which shows the location of the ITCZ. The ITCZ is where the upward mass flux of the Hadley Cell is located. The ITCZ is strongest around 5-10 N, especially during the Northern Hemisphere summer. Note that

2 the wintertime Hadley Cell dominates over the summertime cell. Thus, the southern Hadley cell is strongest during the months of May September. Equatorial Waves Much of the convection seen along the ITCZ originates from equatorial waves, which are disturbances that propagate along it. They derive their energy from the conversion of diabatic heating into kinetic energy. Equatorial waves have been observed to move 8-10 m/s toward the west. Their period is 4-5 days. It is believed that equatorial waves play a role in enhancing the large-scale convergence into the ITCZ. A large area of deep cumulonimbi convection creates a regional heating source, which induces larger-scale upward motion. This will lower the surface pressure, inducing a stronger low-level convergence into the area. The cycle then continues. This process is called Convective Instability of the Second Kind, or CISK. It was first proposed by Charney and Eliassen in the 1960 s, and has been used as a system for describing tropical cyclogenesis. However, one requirement of CISK is that the tropical atmosphere be conditionally unstable. Observations have shown that this requirement is usually not met. Therefore, alternative hypothesis have been put forward, including Wave-CISK and Wind Induced Surface Heat Exchange (WISHE). Equatorial waves have a deep layer of low-level convergence:

This implies that very dry mid-level air is entrained into the cumulonimbi clouds. Usually, this will entail a killing off of the convection as strong evaporative cooling creates strong downdrafts.

3 This implies that very dry mid-level air is entrained into the cumulonimbi clouds. Usually, this will entail a killing off of the convection as strong evaporative cooling creates strong downdrafts. However, the core updrafts of the equatorial waves are typically sheltered from the entrainment, allowing the waves to persist for many days. African Easterly Jet During the Northern Hemisphere summer, strong heating of the Saharan region in North Africa creates a situation in which the usual north-south horizontal temperature gradient is reversed (i.e. temperature increasing toward the north). The thermal wind relation calls for a strong change in geostrophic wind with height over such an area of steep horizontal temperature gradient. Observations show an easterly jet stream in this region during the summer. This regional jet stream is called the African Easterly Jet (AEJ). A cross section of the zonal wind shows a jet maximum located around mb, centered around 16 N:

4 African Wave Disturbances One important impact of the AEJ is the creation of African wave disturbances (AWDs). These are synoptic-scale features that form within the cyclonic shear zone on the south side of the AEJ. The have a phase speed of about 8 m/s and a period of 3.5 days. Over the African continent, these disturbances tap the AEJ for most of their energy. After they move offshore (where the AEJ disappears), African waves must transition to a diabatic heating energy source. This transition happens successfully most often in the late summer/autumn, when sea surface temperatures (SSTs) off the west coast of Africa are warm enough to sustain the intense convection of AWDs. As the disturbance continue their westward march, they may form into tropical cyclones, assuming other favorable atmospheric conditions are in place (e.g. low vertical wind shear). As shown in the satellite image below, AWDs may continue on into the Pacific basin sometimes the low-level convergence will do so without the existence of larger-scale convection. AWDs are also commonly called African Easterly Waves.

Tropical Monsoons A monsoon is a wind circulation that reverses itself seasonally. It is forced by thermal contrasts between land and sea. They are quite common around the world.

5 Tropical Monsoons A monsoon is a wind circulation that reverses itself seasonally. It is forced by thermal contrasts between land and sea. They are quite common around the world. The most widely known are monsoons in Asia, Australia, and North America. The following steps occur during one cycle of the monsoon circulation (summer cycle): 1) Land heats rapidly, creates higher thickness 2) Sets up pressure gradient at upper levels, induces offshore flow there 3) Lowers mass in column over land, creates low pressure and onshore flow 4) Moisture convergence over land produces cumulus convection/diabatic heating Walker Circulation The Walker Circulation is an east-west circulation in the tropical Pacific forced by differential diabatic heating in the zonal plane. The diabatic heating asymmetries are caused by a SST distribution brought on by warm/cold ocean currents. The rising branch of the Walker cell is centered over the Maritime continent in the tropical west Pacific. This is where the warm pool is located (warmest SSTs in the world). Other local areas of rising motion are over the South America and African continents. See the figure below.

Mixed Rossby Waves Equatorial Kelvin Waves A Kelvin Wave is any wave in the ocean or atmosphere that balances the Coriolis Force with a topographic boundary.

6 Mixed Rossby Waves Equatorial Kelvin Waves A Kelvin Wave is any wave in the ocean or atmosphere that balances the Coriolis Force with a topographic boundary. For example, as an ocean current flow along a coastline in the Northern Hemisphere, it will be turned to the right by the Coriolis force. However, pressure forces by water running into the coastline will cause the wave to propagate up the coastline, with the coastline on its right. In the southern hemisphere, Kelvin waves move with the coastline on its left. Kelvin waves are in geostrophic balance with the pressure gradient forces created by the coastline. (from An equatorial Kelvin wave is a special instance of a Kelvin wave in which the wave is trapped along the equator. Here, the Coriolis force reverses sign as the equator is crossed. This keeps the flow confined to the equator.

7 Equatorial Kelvin waves must propagate eastward. They have important implications for El-Nino. It is though that a change in wind speed in the western Pacific spawns a series of Kelvin waves, which then advect the thicker western Pacific water eastward. Examine the time lapse analysis of ocean temperature depth anomalies below: These Kelvin waves can be detected by buoys in the ocean (through density differences in the water). Stronger Kelvin waves can indicate the coming of a strong El-Nino event in the Pacific. See for more information. Atmospheric Kelvin waves are not forced by wind anomalies at the surface. Rather, convective outbreaks are thought to be responsible. The height and velocity perturbations due to a Kelvin wave are shown below (figure in Holton):

Chapter 6: Atmospheric Pressure, Wind, and Global Circulation

Discovering Physical Geography Third Edition by Alan Arbogast Chapter 6: Atmospheric Pressure, Wind, and Global Circulation Factors That Influence Air Pressure Air Pressure is the measured weight of air