A Taste of the Tropics Easterly waves Tropical Cyclones Many Images in this lecture are courtesy of Kerry Emanuel

The Coupled Monsoon System Key features of the WAM Climate System during Boreal summer Cold Tongue SAL ITCZ Heat Low AEJ

Schematic of African Easterly Jet AEJ 90oC 50oC θ θe θe θ 60oC 20oC

The Mean State over West Africa Thorncroft and Blackburn 1999

Zonal Variations in the Mean State Mean 700hPa U wind, 16th July – 15th August 2000 Berry and Thorncroft 2005

Zonal Variations in the Mean State 925hPa q 315K PV PV ‘strip’ present on the cyclonic shear side of AEJ. Strong baroclinic zone 10o-20oN 925hPa qe High qe strip exists near 15oN

Observations of African Easterly Waves Burpee (1970) Eastward tilt beneath the AEJ – Westward tilt above the AEJ Northerlies dry and warm Southerlies wet and cold

Observations of African Easterly Waves Reed et al, 1977 Composite AEW structures from phase III of GATE (after Reed et al, 1977). (a) and (b) are relative vorticity at the surface and 700hPa respectively with a contour interval of 10-5s-1. (c) and (d) show percentage cover by convective cloud and average precipitation rate (mm day-1) respectively. Category 4 is location of 700hPa trough and the “0” latitude is 11oN over land and 12oN over ocean.

Observations of African Easterly Waves Thorncroft and Hodges (2001)

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day 0 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day-4 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day-3 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day-2 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day-1 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day 0 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day+1 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day+2 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day+3 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day+4 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September 1979-1993 Day+5 Streamfunction (contours 1 X 105 m2 s-1) Wind (vectors, largest around 2 m s-1) OLR (shading starts at +/- 6 W s-2), negative blue

Diagnostics for highlighting multi-scale aspects of AEWs 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery. Berry et al 2006

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

315K Potential Vorticity (Coloured contours every 0 315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

Conceptual framework (ii) Baroclinic growth. q’ Max 700hPa Trough

Conceptual framework (ii) Baroclinic growth. q’ Max 700hPa Trough

Conceptual framework (ii) Baroclinic growth. q’ Max 700hPa Trough

Synoptic-Mesoscale Interactions PV-theta analysis of AEWs – Scale Interactions Synoptic-Mesoscale Interactions

Synoptic-Mesoscale Interactions PV-theta analysis of AEWs – Scale Interactions Synoptic-Mesoscale Interactions

Synoptic-Mesoscale Interactions PV-theta analysis of AEWs – Scale Interactions Synoptic-Mesoscale Interactions From a PV-theta perspective, the heating rate profiles are crucial to know and understand.

Synoptic-Mesoscale Interactions PV-theta analysis of AEWs – Scale Interactions Synoptic-Mesoscale Interactions From a PV-theta perspective, the heating rate profiles are crucial to know and understand. Mesoscale-Microscale Interactions Ultimately these profiles are influenced by the nature of the microphysics!

Hurricanes The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).

Hurricanes: Definitions Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called “tropical depressions”. Once the tropical cyclone reaches winds of at least 17 m/s (34 kt, 39 mph) they are typically called a “tropical storm” and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called: "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east160E) "typhoon" (the Northwest Pacific Ocean west of the dateline) "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E) "severe cyclonic storm" (the North Indian Ocean) "tropical cyclone" (the Southwest Indian Ocean) (Neumann 1993)

Hurricanes: Atlantic Names 2007 2008 2009 2010 2011 2012 Andrea Arthur Ana Alex Arlene Alberto Barry Bertha Bill Bonnie Bret Beryl Chantal Cristobal Claudette Colin Cindy Chris Dean Dolly Danny Danielle Don Debby Erin Edouard Erika Earl Emily Ernesto Felix Fay Fred Fiona Franklin Florence Gabrielle Gustav Grace Gaston Gert Gordon Humberto Hanna Henri Hermine Harvey Helene Ingrid Ike Ida Igor Irene Isaac Jerry Josephine Joaquin Julia Jose Joyce Karen Kyle Kate Karl Katia Kirk Lorenzo Laura Larry Lisa Lee Leslie Melissa Marco Mindy Matthew Maria Michael Noel Nana Nicholas Nicole Nate Nadine Olga Omar Odette Otto Ophelia Oscar Pablo Paloma Peter Paula Philippe Patty Rebekah Rene Rose Richard Rina Rafael Sebastien Sally Sam Shary Sean Sandy Tanya Teddy Teresa Tomas Tammy Tony Van Vicky Victor Virginie Vince Valerie Wendy Wilfred Wanda Walter Whitney William

Hurricane: Categories The USA utilizes the Saffir-Simpson hurricane intensity scale (Simpson and Riehl 1981) for the Atlantic and Northeast Pacific basins to give an estimate of the potential flooding and damage to property given a hurricane's estimated intensity: Saffir-Simpson Scale Saffir-Simpson Category Maximum sustained wind speed Minimum central pressure Storm surge mph m/s kts mb ft m 1 74-95 33-42 64-82 > 980 3-5 1.0-1.7 2 96-110 43-49 83-95 979-965 6-8 1.8-2.6 3 111-130 50-58 96-113 964-945 9-12 2.7-3.8 4 131-155 59-69 114-135 944-920 13-18 3.9-5.6 5 156+ 70+ 136+ < 920 19+ 5.7+ Note : Classification by central pressure was ended in the 1990s, and wind speed alone is now used. These estimates of the central pressure that accompany each category are for reference only.

Tropical Cyclones are Important! "The death toll in the infamous Bangladesh Cyclone of 1970 has had several estimates, some wildly speculative, but it seems certain that at least 300,000 people died from the associated storm tide [surge] in the low-lying deltas." (Holland 1993) The largest damage caused by a tropical cyclone as estimated by monetary amounts has been Hurricane Katrina (2005) as it struck the Bahamas, Florida and Louisiana: US $40.6 Billion in insured losses, and an estimated $81 billion in total losses. However, if one normalizes hurricane damage by inflation, wealth changes and coastal county population increases, then Katrina is only the third worst, after the 1926 Great Miami Hurricane and the lethal 1900 Galveston Hurricane. If the 1926 storm hit in 2005, it is estimated that it would cause over $140 billion in damages, and the 1900 storm about $92 billion (Pielke, Gratz, Landsea, Collins, Saunders, Musulin 2006).

Climatology and Structure: Regions of Formation low latitudes, 5-20o (not at the equator): f important none in SE Pacific or S. Atlantic: SSTs important more appear in the Pacific than in the Atlantic: SSTs important

Climatology and Structure: Seasonality Most Tropical Cyclones occur in Summer and early Fall

Climatology and Structure: Tracks Tropical cyclones generally recurve polewards from their easterly track

Climatology and Structure: Conditions for occurrence Gray (1979) Warm ocean waters (of at least 26.5°C [80°F], some say 26oC) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m [150 ft]), and area. Why is this important? Warm waters are necessary to fuel the heat engine of the tropical cyclone. Warm SSTs allows cumulus convection to reach the tropopause and give maximum warming in the troposphere.

Climatology and Structure: Conditions for occurrence 2. A minimum distance of at least 500 km [300 mi] from the equator. Why is this important? Must have background rotation (f > 0) large enough to convert inflow to tangential flow (cf conservation of angular momentum). Consider vorticity equation.

Climatology and Structure: Conditions for occurrence Gray (1979) 3. Low values (less than about 10 m/s [20 kts 23 mph]) of vertical wind shear between the surface and the upper troposphere. Why is this important? From thickness considerations the minimum surface pressure is related to the mean temperature of the column above. If shear tilts the column of warm air over then the surface pressure will rise. Other possible reasons include “ventilation” effects or more simply understood “shearing out”. Some shear may actually be beneficial!

Climatology and Structure: Conditions for occurrence Gray (1979) 4. A pre-existing near-surface disturbance with sufficient vorticity. Why is this important? Tropical cyclones cannot be generated spontaneously. They must be triggered. What can serve as a seedling? Easterly wave, MCSs, Upper-level midlatitude troughs. Problem: these are often cold core!

Climatology and Structure: Conditions for occurrence Gray (1979) 4. A pre-existing near-surface disturbance with sufficient vorticity. Why is this important? Tropical cyclones cannot be generated spontaneously. They must be triggered. What can serve as a seedling? Easterly wave, MCSs, Upper-level midlatitude troughs. Problem: these are often cold core!

6.2 Climatology and Structure: Conditions for occurrence In Summary: Environment: Need significant f, SSTs and low shear. Seedling: Need finite amplitude disturbance.

Climatology and Structure: Observed Structure

Climatology and Structure: Observed Structure

Climatology and Structure: Observed Structure

Climatology and Structure: Observed Structure

Climatology and Structure: Observed Structure

Climatology and Structure: Observed Structure

Climatology and Structure: Observed Structure

Climatology and Structure: Observed Structure The Thetae Structure Key is the strong surface radial gradient of thetae Mature hurricanes are nearly moist adiabatic in the eye wall. Thus the strong surface radial gradient in thetae leads to the large gradient in T in the troposphere. Hydrostatically this leads to the large surface pressure gradient and associated large surface winds.

AEWs and Tropical Cyclones Key weather systems in the West African and Tropical Atlantic regions An ideal region to study scale interactions SAL AEWs TC MCSs

AEWs and Tropical Cyclogenesis AEWs often get a “boost” before they leave Africa; associated with mergers of PV from upstream and in situ generation. The Guinea Highlands region is one of the wettest regions of tropical North Africa. GPCP rainfall (mm/day) for Aug-Sep, 1997-2007)

Importance of Guinea Highlands Region Coherent cyclonic centers are tracked within the ITCZ at 700hPa and in the low-level baroclinic zone at 850hPa Average vorticity tracking statistics for June-July-August at 700hPa and 850hPa based on ERA40 using methodology of Thorncroft and Hodges (2001).

Importance of Guinea Highlands Region Composites of East Atlantic Developing and Non-Developing AEWs (1979-2001) Developing Non-Developing Hopsch , Thorncroft and Tyle 2010

Importance of Guinea Highlands Region Composites of East Atlantic Developing and Non-Developing AEWs (1979-2001) Developing (33) Non-Developing (512) Hopsch , Thorncroft and Tyle 2010

Importance of Guinea Highlands Region Composites of East Atlantic Developing and Non-Developing AEWs (1979-2001) Developing (33) Non-Developing (33 most intense) Hopsch , Thorncroft and Tyle 2010

Most Intense Non-Developing AEWs Importance of Guinea Highlands Region Most Intense Non-Developing AEWs DAY 0 DAY 1 DAY 2 Hopsch , Thorncroft and Tyle 2010