Tracking Tropical Waves: Structure, Nature and Propagation
Since 92L is a few days away from land and we do seem to have any imminent areas of concern I will post a full tropical update at 5PM this afternoon after the full 12Z and 18Z models are updated. In the meantime, here is the blog on tropical waves that I had to postpone on Sunday. You can enjoy the read while you wait for 92L to do something. It is one my personal best researches all in the name of Tropical Meteorology and I assure you it will cover everything you need to understand about African Easterly Waves. Thanks for all the get well wishes, I am evidently getting there.
Tracking Tropical Waves: Structure, Nature and Propagation
Last week we looked at the conditions that set the stage for the development of the African Easterly Jet (AEJ) and found that mid-level African waves developed to the south of the jet and that low-level waves developed north of the jet. Today we are going to take a further look at African Easterly Waves (AEWs), particularly the mid-level waves south of the jet. Before we do so we are going look at an overview of low-level waves and the genesis discussed last week. Much of the information you see here was made possible by numerous studies done on African Easterly Waves, most importantly the African Monsoon Multidisciplinary Analyses Project (AMMA).
Low Level Waves
These waves develop to the north of the African Easterly Jet (AEJ) in a region of static instability associated with the low-level thermal trough below the Saharan High as suggested by Chang (1993) and Thorncroft (1995). These waves that develop north of the AEJ and lack any convective activity due to the availability of moisture. They often head westward with uncertainty as to where they will go. Some may move towards the southwest and eventually are located below the AEJ, while others continue northwards neutral Rossby waves. For this reason, we will stress more on the classic mid-level waves that we recognise over tropical North Africa.
Overview of Mid-Level African Easterly Waves (AEWs)
For these tropical waves to develop, you need instability within a fast flowing stream of air at the mid-levels and a rapid reversal of the potential vorticity (PV) gradient as suggested by Charney and Stern in 1969. The African Easterly Jet (AEJ) satisfies these conditions. But how? Well the AEJ is a fast flowing stream of air typically near 600mb over Northern Africa. The flow near the jet axis is going faster than that of flow further way in the surroundings. What happens, is that eddies begin to develop due to differences in speed of the flow within the AEJ (barotropic instability/horizontal shear). A cyclonic eddy develops south of the jet and in the northern hemisphere, this is positive vorticity (+PVU). Anticyclonic eddies develop north of the wave and in the northern hemisphere, this is negative vorticity (-PVU). In the natural world, positive vorticity is stronger and thus will bulge the axis of the AEJ northwards resulting in a kink or wave. The wave continues to amplify until the jet weakens and an AEW is born. Now if we look closely, we see the PV gradients as the positive eddy to the south and the negative to the north. This is the rapid change in PV gradient that we often discuss. Figure 1 summarizes the stages of development and figure 2 shows the vertical profile of this PV gradient.
Figure 1. Schematic diagram of the stages of tropical wave development. From the upper left the AEJ is established as we looked at last week, vortexes begin to develop within the fast flowing stream which leads to a kink in the AEJ axis. The last stage of development is the initiation of convection due divergence aloft near 700-600 mb.
Figure 2. Vertical cross section of the PV gradient associated with tropical waves. Notice the +PVU is south of the -PVU as described by Charney and Stern in 1969.
Development of Mesoscale Convective Systems (MSCs)
These can develop along wave disturbances by a combination of factors. First, the supply of moisture is provided by the moist monsoon southwesterlies from the Gulf of Guinea. Daytime heating and the topography aids in the spatial and temporal scale of these MSCs. The last and most important factor is divergence ahead of the 700 mb wave as described in figure 1. MCSs reach maximum intensity usually in the late afternoon, evening and early morning local time, and die during the day due to diurnal influences over land. These MCSs are important features of the rainy season over Africa. Figure 3 shows the propagation of a strong AEW in September 2006 that became Hurricane Helene. If you notice, the convection normally is initiated by a developing disturbance in the AEJ over Eastern Africa, most commonly known as the Ethiopian Highlands and propagates westward with diurnal variations and much of the convection ahead of the wave axis.
Figure 3. METEOSAT infrared image loop of Northern Africa between September 05 and September 13 2006. The wave is initiated over the Eastern Ethiopian Highlands and propagates westward with diurnal variations and convection mainly ahead of the axis.
African easterly waves normally have a wavelength of 2500 km and transit period of 3-5 days. They tend to develop the most intense convection during the late afternoon, night and early morning local time. It takes about 6.5 days for a wave to move from Eastern Africa to the Eastern Atlantic going at a speed of 8 m/s or 20 mph. They often reach maximum amplitude just before exiting the coast. They propagate westward much slower than the environmental flow, which they were first created in. The main steering factor is the mid-level Saharan High to the north.
Horizontal and Vertical Structure
Horizontally, AEWs are characterized by kinks in the mid-low level wind field with a sharp PV gradient along the axis. As we discussed before, much of the bad weather occurs west of the axis where divergence aloft near 600 mb is greatest. Strong tropical waves often amplified north into the Saharan Thermal Low giving them twin vorticity centers north and south along the axis. Winds flowing along this kink are normally from the northeast (ahead of the axis) to southeast (behind the axis). At the surface, this may not be as pronounce (see the section: Weather).
Figure 4. GFS analysis of West Africa around 2:00pm June 5 showing a strong tropical wave about to emerge off the coast of Africa. The top left image shows the 650 mb winds with the African Jet axis nicely illustrated with the wave-like disturbance within the jet axis. The top right image shows the potential vorticity showing the greatest PV is often found south of the jet axis as we discussed. The bottom image is another illustration of the moisture gradient over Northern Africa and the position of the jet relative to this gradient. Without these gradients, there would be no jets and consequently no African waves.
Vertical – Winds
Typically, the greatest change in winds will occur near 600 mb and this is from northeast (ahead of the axis) to southeast (behind the axis). Below the waves are the monsoon southwesterlies that extend to about 900 mb and above the waves the prevailing westerlies from 300 mb-150mb. The African Easterly Jet (AEJ) is often found at the core of each wave as an easterly jet max that accompanies. Figure 5 shows the vertical structure of a wave that passed Dakar, Senegal on Friday.
Figure 5. Upper air time cross section of Dakar, Senegal showing the passage of two African waves on the 8 of June and 12 of June 2010. The waves normally have their greatest wind shifts near 700 mb with southwesterly dominating the surface and the AEJ above at 600 mb and normal westerlies much higher up.
Vertical - Potential Vorticity
Figure 2 shows a simplistic overview of the vertical profile of the PV gradient found along tropical waves. The positive vorticity is found south of the wave axis and the negative vorticity is found north of the wave axis.
The changes in surface conditions over Africa are much more different from the changes in weather over the Atlantic and Caribbean. First, there are no interruptions of the trades since the prevailing low-level winds over Africa are southwesterlies. Before the passage of the wave, the temperature is normally high, the dew points are low and the wind is out if the southwest. As the wave approaches, cloudiness increases, the winds turn easterly and the temperature drops causing relative humidity to increase. As the axis approaches, maximum bad weather is experience. Behind the axis, pressure begins to drop and the winds turn southeasterly. After the wave passes, weather returns to normal and southwesterly prevailing winds return.
Figure 6. Meteogram of Ouagadougou, Burkina Faso in Central West Africa showing the passage of an African Easterly Wave on 10 June 2010. A shows that air temperature decreases as the wave approaches causing relative humidity to rise. B shows the weather which is characterized by thundershowers ahead of the axis as appose to behind. C shows the cloud cover and winds shifting from the normal southwesterly flow so southeasterly. D shows the visibility decreases and finally E shows the increase in pressure ahead (likely due to the mesoscale highs behind outflow boundaries ahead of the thunderstorms) leading to a drop in pressure behind the axis.
After emerging off the coast of West Africa, tropical waves go through a number of influences and changes but the original trough axis is maintained. The changes are induced by sea surface temperatures, the tradewind inversion, the inter-tropical convergence zone (ITCZ), the tropical upper tropospheric trough (TUTT), among others.
The Saharan Air Layer
In May of 2009, I posted a blog, which discussed the influences of AEWs and the Saharan Air Layer (SAL). Numerous studies done on the relationship of these two phenomenons revealed the negative vorticity north of the AEWs is what disturbs the dust particles over the Saharan Desert. This is why emerging SAL has that unique anticyclonic signature. Excellent examples of these are shown in video 1 and figure 11, which show a very large dust event succeeding a tropical wave in July 2005 and July 2007.
Dust is often entrained into the convection associated with tropical waves.
Figure 7. SAL conceptual model illustrating the following properties: geographic location of the African continent, ICTZ, and dust plume, surface flow (solid yellow arrows), particle trajectories (dashed yellow arrows), mid-level easterly jet (thick red arrow), 700 hPa wave axis (thin red arrow), regions of convection, and the rise of the SAL base to the west (Karyampudi et al. 1999).
Video 1. Large African dust plume from July 2005 showing the anticyclonic rotation of SAL caused by the negative vorticity north of AEWs. There is always a tropical wave at the leading edge of such intense outbreaks.
The Oceanic Environment
Lapse rates over the ocean are much gentler than those over land and this is because of the properties of water - it absorbs, and releases heat much slower than rocky land. Over the Eastern Atlantic, there are two regions of extreme tropical sea surface temperatures. Just off the coast of Africa, there is a region of high sea surface temperatures due to the downwelling of monsoon southwesterlies. Further west near 30W-50W, the ocean temperatures drop sharply due to 1) cooler air coming from the Canary Current and 2) surface divergence south of the Azores High. This cooler ocean cools the air above it and is capped by an inversion due to 1) the subsidence warming from the Azores High and 2) dry stable air from SAL. This is called the Tradewind Inversion. Consequently, the lapse rate will support deep convection once the wave emerges (warm air near the sea surface below cooler mid-level air). However, as the wave tracks further west, increase stability results in loss of convection. Sometimes a wave will not generate any convection between 30W and the Caribbean. Thus, we see warm sea surface temperatures are important for the sustainability of convection along waves but the tradewind inversion more than often results in the loss of this convection.
Figure 8. Sea surface temperatures taken June 11 2010 of the Eastern Atlantic overlaid in Google Earth showing the monsoon southwesterlies, NE trades, ITCZ/Monsoon Trough, the cold Canary Current and two major areas of SST extremes.
Figure 9. GFS soundings taken at two locations (A and B) in figure 8. Notice the differences in stability and winds between each location. The left is region A and the right is region B.
The Inverted-V Pattern
Easterly waves in the tropical Atlantic have been found to be associated with a characteristic cloud pattern, which has the appearance of an “Inverted V.” Cloud bands are aligned approximately parallel to the lower tropospheric winds and change orientation along the wave axis. We have already established how the wind changes but unlike over Africa, maximum convergence and band weather is behind the axis. This is because the wind behind the axis slows down to compensate for the sub-geotropic flow. When air slows down, the flow behinds piles up and causes convergence. As the air flows around the axis to other side, it speeds up and diverges. Lower level convergence promotes rising air and cloudiness, while lower level divergence promotes subsidence and fair weather.
Figure 10. Location of convergence and divergence in an easterly wave in relation to the trough axis.
This wave model was developed by Frank et al in 1969-1970 and thus is dubbed the Frank’s Inverted V Wave Model. They have average wavelengths of 2000-2500 km and mean speed of 10-15 knots, much slower than AEWs. The reduce speed is the result of the lack of the AEJ further west. The deep easterlies are much slower. It takes about 6-7 days for a wave to travel between the West Coast of Africa to the Caribbean Islands.
Diurnal variations in convection are opposite to over land and the waves reach convective maxes during the early morning and convective minimums during the late afternoon.
Figure 11. GOES-12 visible image of the Tropical Atlantic taken July 17 2007 of a large Inverted-V wave approaching the islands at the leading edge of a large African Dust Plume. This is one of the largest dust outbreaks I have ever observed so it was easier to relocate the exact date.
Riehl's Easterly Wave Model
This is one of the first wave models develop for the Eastern Caribbean and it largely involves interaction with the tropical upper tropospheric trough (TUTT). As waves transit the tradewind inversion with little or no convection, they suddenly explode in convective activity as we saw last week when they approach the Eastern Caribbean. This is due to the interaction with the TUTT among other factors like warmer sea surface temperatures and a deeper moisture field. Westerly upper winds keep the wave vertically tilted and much of the convection remains east of axis. As the wave passes a station in the Eastern Caribbean, the northeast trades veer easterly then south easterly. Relative humidity increases, pressure drops and maximum cloudiness is found along and behind the wave axis. This is quite the opposite of what we see at Dakar further east across the ocean.
Figure 12. GOES-13 water vapor image taken of the Caribbean on June 9 2010 showing the interaction of a tropical wave and the TUTT. Notice how the ITCZ is pulled northward by the wave as discussed in the next section.
Figure 13. Meteogram of Point, Grenada from June 8-9 showing the passage of the wave. It is more or less the same as over Africa except that the bad weather lies behind the axis (5).
Interaction with the ITCZ, Lake Maracaibo, Tropical South America and the Colombian Low
As waves interact with the TUTT they amplify northwards and with that, the ITCZ follows. Strong tropical waves can pull the ITCZ northwards. It has happen before and has been proven in studies. This mostly happens in the Southeastern Caribbean. Early seasonal tropical waves are often embedded within the circulation of the ITCZ and they create classic inverted-V patterns in the otherwise linear/flat cloud band.
As waves across over into South America, they often revert to diurnal forces they were once under over Africa. Tropical waves will enhance afternoon thunderstorms over northern South America and the availability of moisture from Lake Maracaibo and the northern portions of the Amazon aides in the development of these thunderstorms.
The third point of interactions is the Panama Low or Colombian Low, which enhances cloudiness along wave just like the monsoon environment over Africa.
Here we see there are positive interactions between tropical waves and the ITCZ, Amazon, Colombian Low and small water bodies.
Figure 14. Visible image of the Caribbean and Northern South America showing the major contributors of tropical wave's positive interactions. This was taken 3 days after the wave in figure 11.
Development into Tropical Cyclones
Under favourable environmental conditions, (warm ssts, deep layer of moisture, low vertical shear and enough positive vorticity). MCSs along tropical waves can develop into tropical cyclones through a process called CISK. CISK, or "Convective Instability of the Second Kind", is a popular theory that explains how thunderstorms can evolve and organize into hurricanes. CISK is a positive feedback mechanism, meaning that once a process starts, it causes events, which enhance the original process, and the whole cycle repeats itself over and over.
The theory of CISK states that as thunderstorms develop, condensation within the thunderstorm releases heat (heat of condensation). This warms the air and causes it to rise. As air rises, the pressure heights lower and a low-pressure area forms. Air rushes into this low-pressure area while picking up moisture from the sea, converges and rises. Rising air condenses and releases heat of condensation and the process continues repeatedly. If conditions are favourable, then air rises higher and higher with each cycle and pressure lowers and lowers. Thus, we see a thunderstorm develops into an organized storm.
While only about 60% of the Atlantic tropical storms and minor hurricanes ( Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves (Landsea 1993). It is suggested, though, that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa (Avila and Pasch 1995).
The study of weather was my first drive but began to pay more attention to Tropical Meteorology due the fact that I reside in the Caribbean. My desire and longing to learn about tropical cyclones drove me years ago to explain where they came from and it led me back to Africa. This is where my interest in tropical waves sparked. I have studied more on tropical waves than any other synoptic feature in the tropics, with tropical cyclones and the ITCZ close behind. I have been look at synoptic maps of tropical waves and the associated weather since the summer of 1998 when I was only 9 years old, but the peak of actually getting into where they came from occurred from 2004-to the present. No tropical met book is completely without ever mentioning tropical waves, these fascinating creatures of nature.
African Monsoon Multidisciplinary Analyses Project (AMMA)
Hurricane Research Division FAQ – Easterly Waves
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Mesoscale Convective Systems and African Easterly Waves, Doug Paker, 2007
Three-Dimensional Structure and Dynamics of African Easterly Waves. Part I: Observations GEORGE N. KILADIS Earth System Research Laboratory, NOAA, Boulder, Colorado CHRIS D. THORNCROFT Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York NICHOLAS M. J. HALL Laboratoire d’e´tude des Transferts en Hydrologie et Environment, Grenoble, France, 2005.
Generation of African Easterly Wave Disturbances: Relationship to the African Easterly Jet; JEN-SHAN HSIEH AND KERRY H. COOK; Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, 2004.
Energetics of Easterly Waves; M.A. Estogue and M.S. Lin; Rosentiel School of Marine and Atmospheric Science, University of Miami, Coral Gables, Florida, 1977.
Characteristics of African Easterly Waves Depicted by ECMWF Reanalyses for 1991–2000 TSING-CHANG CHEN Atmospheric Science Program, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, 2006.
Characteristics of North African Easterly Waves During the Summers 1968 and 1969, Robert W. Burpee, Laboratory of Atmospheric Research, University of Illinois, Urbana, 1973.
Three-dimensional structure of easterly wave disturbances over Africa and the tropical north Atlantic, George N. Kiladis and Chris D. Thorncroft, NOAA Aeronomy Laboratory, Boulder, Colorado Department of Earth and Atmospheric Sciences, SUNY Albany, NY, Unknown.
The Low-Level Structure of African Easterly Waves in 1995 Ioannis Pytharoulis and Chris Thorncroft, Department of Meteorology, University of Reading, Reading, United Kingdom, 1998.