I am carrying out research as a CASE (Collaborative Awards in Science and Engineering) student at the University of Reading, sponsored by NERC. The title of the project is
“The role of African Easterly Wave (AEW) variability and the large scale environment on Atlantic hurricanes: observations and models”.
The Background and Outline of the PhD Project
During the monsoon months (June to September) African Easterly Waves propagate from east to west across the Atlantic approximately every 3 days . The surface effect of the passage of an AEW is not severe – the surface winds back and then veer and increased convection brings increased rainfall and the possibility of strong but short-lived squalls. Over land the increased convection associated with AEWs can lead to an increase in the number of meso-scale convective events (MSCs) which have a direct link to the rainfall in the West African and Sahel regions and this can be extremely beneficial to the region’s rainfall and therefore its ability to support its population. Once over the Atlantic they change from being benign bringers of rainfall to the potential seeds of hurricanes. One of the criteria for cyclogenesis is that there should be an atmospheric disturbance as the trigger and analysis of the ERA-40 data set shows that over the period 1958 to 2002 approximately 40% of all AEWs in August and September become hurricanes .
Of these, the ones which have most human and economic impact are those that make landfall either in the Caribbean or continental Central and Northern America. Recent work has investigated the relationship between large scale effects on both the frequency of cyclogenesis and the ratio of landfalling hurricanes. This is particularly interesting as it brings together the concept of conditions that allow and AEW to develop into a hurricane, and then the conditions that will steer that hurricane towards landfall. Large scale indicators that are statistically important are the tropical Atlantic sea surface temperature (SST), the global tropical mean SST, the North Atlantic Oscillation (NAO) and the Southern Oscillation Index (SOI). In addition and specific to the North Atlantic is the presence (or not) of the Saharan Air Layer (SAL) and the associated dust carried within it. The radiative effects of the SAL has been shown in model experiments to move both the African Easterly Jet (AEJ) and the Inter-Tropical Convergence Zone (ITCZ) northwards , which would affect both the generation and subsequent steering of hurricanes. Also, SAL air can both hinder cyclogenesis by introducing dry air into the circulation, and encourage it along the southern and western edges of the SAL itself .
There is a need for a better understanding of the factors that affect AEW development over the Atlantic, and furthermore under what circumstances they develop into hurricanes, and then of the factors that affect the subsequent path of those hurricanes. By investigating the interaction of AEWs with large scale environmental factors the hope is that an enhanced seasonal forecast for both the number of hurricanes and the proportion of these that make landfall will be obtained. This would be of much use to both the scientific and business communities. The specific science questions to address these are:
1. What are the AEW spatial distributions and variability at intraseasonal and weather timescales and what AEW structures trigger cyclogenesis? Do these relate to the likelihood of a developing tropical cyclone making landfall in the Caribbean or United States?
2. What is the role of the AEJ-AEW dynamics relative to the large-scale environment for tropical cyclogenesis e.g. how do dry air (mid-latitude or Saharan origin) and dust impact AEWs and hurricane development? Can a better understanding of the mechanisms improve the predictability of Atlantic hurricanes?
3. How important are SST anomalies and associated large-scale modes of convection on intraseasonal and interannual time scales in determining the nature of the AEW variability? How does this impact the number of storms that make landfall in the Caribbean and United States?
I have recently completed a literature review, and will periodically update it.
I am building up an Excel database of the tracks of developing AEWs and the subsequent named systems. The idea behind this is to have an easily accessible database where the overall named storms records for each year can be seen and then the detailed track information for each individual AEW. The tracks themselves have been obtained using Dr Kevin Hodges’ TRACK (Thorncroft & Hodges, 2001) algorithm run at both T42 and T63 resolutions, and each AEW record has the observed best track data and so far the tracks obtained from the mean relative vorticity data averaged over the levels 850, 700 and 600 hPa at T42 and T63. These are shown for each named system from 1989 to 2010. this will be extended back to 1979, and forward as time goes on.
This is very much for my use, and is not particularly user-friendly - yet! It is password protected, please contact me if you would like access to it.
Hoffmuller plots for June through October each year from 50E (the Horn of Africa) to 70W (the Caribbean Sea) between 7.5N and 20N (the latitudes generally referred to as the MDR, the Main Development Region) are shown for the vertically averaged relative vorticity. This is in two diplays per year, one for the maximum RV for each longitude, the other for the average RV between 7.5N and 20N for each longitude. The maximum RV display, though noisy over land, gives better resolution.
The tracks can be accessed by years as below. In each one the observed best track line is in yellow, the T42 track in blue, and the T63 track in red.
1989 to 1992
1993 to 1996
1997 to 1999
2000 to 2002
2003 to 2004
2005 to 2006
2007 to 2009