1. Chap. 5 Conceptual models of synoptic Tropical disturbances in summer Definition of tropical disturbances : A discrete tropical weather system of apparently organized convection generally 200 to 600 km in diameter, originating in the tropics or subtropics, having a nonfrontal migratory character, and maintening its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. Disturbances associated with perturbations in the wind field and progressing through the tropics from east to west are also known as ‘easterly waves’.

2. An example of evolution of tropical disturbances over Atlantic in summer: Chap. 5 Conceptual models of synoptic Tropical disturbances in summer

3. 5.1 Easterly waves (West Africa, Caribbean Sea, Pacific) 5.2 Troughs of Upper Tropical Troposphere (TUTT) 5.3 Monsoon Depression 5.4 Mid-tropospheric cyclones 5.5 Tropical storms 5.6 Hurricanes sommaire général Chap. 5 Conceptual models of synoptic Tropical disturbances in summer

4. 5.1 African Easterly waves Located at the southern flank (12°N) of AEJ (15°N) from June to October Initiate by barotropic instability (inflexion point of PV on the isentrope surface 315°K) Growth by release of latent heat Speed phase = -8 m/s Period = 3 to 5 days Scale lenght = about 2500 km Bad weather in general on the western side of the trough

5. Streamline and wind at 700 hPa : we observe 2 easterly waves at the southern flank of AEJ 5°N 10°N 15°N Troughs of easterly waves are visibles through the sharp shift of the wind 5.1 African Easterly waves

6. On NIMBUS software, look after the streamline at 850 hPa (closed circulation) or at 700 hPa (sinusoide circulation) but easterly waves are not visible in upper troposphere. 5.1 African Easterly waves

7. Composite structure : rain, clouds on the western side of the trough : divergence at 200 hPa and convergence at 850 hPa on the western side of the trough T= Trough R= Ridge 5.1 African Easterly waves

8. Origin : African easterlies wave which cross Atlantic Occurrence/ frequency over Carribean : from may to november once every 3 to 4 days, either about 60 over the season Speed phase = -5 to -8 m/s Scale lenght = about 3000 km Bad weather, generally, on the eastern side of the trough 5.1 Carribean Easterly waves

9. Maximum signal of the wave = between 500 and 700 hPa Vertical tilt of the trough : below 700 hPa, westward tilt with altitude (not shown on the above figure) above 700 hPa, eastward tilt with altitude 5.1 Carribean Easterly waves

10. 5.1 Pacific Easterly waves Eastern Pacific (80°E to 170°E) : - scale lenght = 3000 to 3500 km - period = 4 to 6 days - phase speed = -5 to -7 m/s - in surface, we observe a fluctuation of wind as large as 4 m/s - in upper troposphere, lighter fluctuation of wind Western Pacific (170°E to 120°E) : - scale lenght = 3500 to 4000 km - period = 6 days - phase speed = -9 m/s - in low troposphere (800 hPa), first peak of fluctuation of wind as large as 3 to 4 m/s and strong convergence - in upper troposphere (175 hPa), second peak of fluctuation of wind as large as 2 to 3 m/s and strong divergence - in mid- troposphere (500 to 300 hPa) : no fluctuation of wind The vertical tilt of the trough: It varies from eastern Pacific (eastward vertical tilt) to western Pacific (westward vertical tilt); it probably depends of the environment, either the vertical shear wind which is the opposite between Eastern and Western Pacific Chap 5.2

11. Chap. 5 Differents types of tropical disturbances in summer 5.1 Easterly waves (West Africa, Caribbean Sea, Pacific) 5.2 Troughs of Upper Tropical Troposphere (TUTT) 5.3 Monsoon Depression 5.4 Mid-tropospheric cyclones 5.5 Tropical storms 5.6 Hurricanes sommaire général

12. 5.2 Trough of Upper Tropical Troposphere (TUTT) In summer, over N. Atlantic, an upper through quasi-zonal, lying WSW/ENE, is visible on monthly maps (not necessarily daily !) Hypothesis of initiation This upper trough is a typical example of the interactions between tropics and mid-latitudes : ‣ we observe low ratio of water vapour along the TUTT linked to (?) the successive Rossby Wave Breakings (RWB) which occur in summer over mid-oceans (called ‘surf zone’). RWB result in stratospheric mid-latitude intrusion (strong PV and light humidity) into the tropical troposphere ‣ along the axis of TUTT, as the atmosphere is very dry, we observe a very strong radiative cooling (-6°K/day) which implies a strong synoptic-scale subsidence (- 2 cm/s) along the axis (cf. thermo equation in tropics) 200 hPa streamline in august :

13. 5.2 TUTT ‘cells’ or TUTT ‘lows’ Definition of a TUTT ‘cell’: Cold vortex observed in upper troposphere along the climatological TUTT disturbing the tropical atmosphere, sometimes, down to the surface (should the occasion arise, a subtropical disturbance occur and rarely tropical storm). Vertical velocity : in vortex, subsidence max. at 300 hPa whence sky clear (or isolated cumulus) and radiational cooling Cold core max. at 300 hPa, Cyclonic vorticity max. at 200 hPa Deep convection : located upstream the vortex in the warm and divergent upper tropo, either at SE of the vortex in northen hemisphere (resp. NE in southern hemisphere) Anomaly of VV, in microbars/s hPa

14. 5.2 TUTT ‘cells’ or TUTT ‘lows’ Other features of a TUTT ‘cell’: Move : Stationnary or westward move (in both hemisphere) from 0 to 6° of longitude per day Life span : 2 to 31 days !! MSLP : the fall of MSLP depends on the interaction between the upper tropo (intrusion of high PV) and the low tropo (line of convergence coupled with high tp’w..) Future study : a TUTT event seems to be linked with an intrusion of stratospheric air into tropical troposphere (linked with the Rossby wave Breaking ?) So, high values of PV (>2 PVU) on 350°K should be a good proxy to detect ‘TUTT cell’ ?

15. 5.2 TUTT ‘cells’ or TUTT ‘lows’ Relation between ‘TUTT cell’ and tropical cyclone: * ⇨ The ‘TUTT cell’ vanish the tropical cyclone when the vertical shear (induced by the TUTT ‘cell’) is greater than 12m/s between the surface and 200 hPa. ⇨ Under this threshold vertical shear, a tropical cyclone could be enhanced when it is located on the eastern side on the TUTT ‘cell’, that is to say in the Upper warm and ascent outflow Chap 5.3

16. Chap. 5 Differents types of tropical disturbances in summer 5.1 Easterly waves (West Africa, Caribbean Sea, Pacific) 5.2 Troughs of Upper Tropical Troposphere (TUTT) 5.3 Monsoon Depression 5.4 Mid-tropospheric cyclones 5.5 Tropical storms 5.6 Hurricanes sommaire général

17. 5.3 Indian monsoon : monsoon dépression ‣ mid-tropo : moderate signal of convergence Shaded area=wind > 40 kt ‣ low tropo : Maximum signal of wind and convergence from 600 to 800 hPa ‣ upper tropo : light anticyclonic circulation and light signal of divergence 200 hPa : Streamline and isotach 500 hPa: Streamline and isotach 850 hPa: Streamline and isotach H C C

18. 5.3 Indian monsoon : Monsoon depression Synoptic-scale : about 2000 km of diameter Location of initiation ‣ 80 % in tha Bay of Bengale, ‣ 10% in Arabian Sea ‣ 10 % over land (Bangladesh) MSLP ~ as low as 990 hPa Livespan : from 3 to 5 days Frequency : twice a month over Bay of Bengale Move : westward or northwestward at 2 or 3 m/s in direction of heat low at least as far as Central India before decaying Closed circulation between surface-300 hPa max. intensity (wind, convergence) from 600 to 800 hPa Temperature ‣ cold core between surface-600 hPa ‣ above, between 500-200 hPa, hot core ‣ but, some of them no cold core observed Evolution : no risk of development of tropical cyclone because of the strong vertical shear (TEJ in upper tropo and SW monsoon flow in low tropo) Origin hypothesis : baroclinic instability (eastward vertical tilt coupled with westward vertical shear ) + CISK (convergence linked to the surface trough) Main Key features :

19. 5.3 Indian monsoon : Monsoon depression Deep convection and vertical velocity max. in the SW quadrant Rain : in the SW quadrant between 100 mm to 300 mm per day Location of rain and vertical velocity :

20. Chap. 5 Differents types of tropical disturbances in summer 5.1 Easterly waves (West Africa, Caribbean Sea, Pacific) 5.2 Troughs of Upper Tropical Troposphere (TUTT) 5.3 Monsoon Depression 5.4 Mid-tropospheric cyclones 5.5 Tropical storms 5.6 Hurricanes sommaire général

21. 5.4 Indian monsoon Mid-tropospheric cyclone Streamline and isotach at (left) 925 hPa, (right) 600 hPa Closed circulation between 700-300 hPa Maximum of intensity (wind, convergence) in mid-troposphere from 500 to 600 hPa A t low and upper troposphere : absence or light signature in wind (manifested as a trough in the streamline) whence risk of ‘messing- up’ for forecasters 925 hPa600 hPa

22. Location of initiation : Mid-tropospheric cyclone occur in NE of Arabian Sea, South Vietnam, South China Sea from Period : from may to october Synoptic scale : ~ 3000 km Livespan : from 3 to 7 days, even 10 days ! Frequency : less than monsoon depression Move : stationnary or westward Temperature ‣ cold core between surface-600 hPa ‣ above between 500-200 hPa, hot core Heavy rains : in western quadrant (location of ascending motion) up to 200 mm per day Origin hypothesis ‣ for initiation, barotropic instability of the monsoon flow at 700 hPa ‣ for growth, release of latent heat Main Key features : 5.4 Indian monsoon Mid-tropospheric cyclone

23. Chap. 5 Differents types of tropical disturbances in summer 5.1 Easterly waves (West Africa, Caribbean Sea, Pacific) 5.2 Troughs of Upper Tropical Troposphere (TUTT) 5.3 Monsoon Depression 5.4 Mid-tropospheric cyclones 5.5 Tropical storms 5.6 Hurricanes sommaire général

24. Chap. 5 Differents types of tropical disturbances in summer 5.1 Easterly waves (West Africa, Caribbean Sea, Pacific) 5.2 Troughs of Upper Tropical Troposphere (TUTT) 5.3 Monsoon Depression 5.4 Mid-tropospheric cyclones 5.5 Tropical storms 5.6 Hurricanes sommaire général

25. 5.2 Rossby Wave Breaking (RWB) Event of RWB from 27/05 06TU to 29/05/98 18TU Source : ERA15 CEP : PVU on the surface 350°K high PV RWB Cutt-off of high PV Cutt-off of high PV

26. 5.2 Rossby Wave Breaking Event of RWB : zoom on 27/05/98 18TU Source : ERA15 CEP : PVU on the surface 350°K Deep convection occur upstream the RWB, northeastward the ‘TUTT cell’, in the upper warm and divergent flow Cut-off of high PV : Stratospheric air Cut-off of low PV : tropospheric air

27. 5.2 Example of a RWB over the Southern Atlantic 04/02/04 : Geopotential and wind on the surface 2 PVU : Deep convection occur upstream the RWB, northeastward the ‘TUTT cell’, in the upper warm and divergent flow