Chap 5 Conceptual models of synoptic Tropical disturbances

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’.

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

Chap. 5 Conceptual models of synoptic 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

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

5. 1 African Easterly waves On NIMBUS software, look after the streamline at 850 h. Pa (closed circulation) or at 700 h. Pa (sinusoide circulation) but easterly waves are not visible in upper troposphere.

5. 1 African Easterly waves T= Trough R= Ridge Composite structure : rain, clouds on the western side of the trough : divergence at 200 h. Pa and convergence at 850 h. Pa on the western side of the trough

5. 1 Carribean Easterly waves • 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 • Maximum signal of the wave = between 500 and 700 h. Pa • Vertical tilt of the trough : below 700 h. Pa, westward tilt with altitude (not shown on the above figure) above 700 h. Pa, eastward tilt with altitude

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 h. Pa), first peak of fluctuation of wind as large as 3 to 4 m/s and strong convergence - in upper troposphere (175 h. Pa), second peak of fluctuation of wind as large as 2 to 3 m/s and strong divergence - in mid- troposphere (500 to 300 h. Pa) : 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

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

5. 2 Trough of Upper Tropical Troposphere (TUTT) 200 h. Pa streamline in august : • 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)

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). h. Pa Anomaly of VV, in microbars/s • Vertical velocity : in vortex, subsidence max. at 300 h. Pa whence sky clear (or isolated cumulus) and radiational cooling • Cold core max. at 300 h. Pa, • Cyclonic vorticity max. at 200 h. Pa • 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)

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’ ?

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 12 m/s between the surface and 200 h. Pa. ⇨ 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

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

5. 3 Indian monsoon : monsoon dépression 200 h. Pa : Streamline and isotach ‣ upper tropo : H light anticyclonic circulation and light signal of divergence 500 h. Pa: Streamline and isotach ‣ mid-tropo : moderate signal of convergence C Shaded area=wind > 40 kt 850 h. Pa: Streamline and isotach ‣ low tropo : Maximum signal of wind and convergence from 600 to 800 h. Pa C

5. 3 Indian monsoon : Monsoon depression Main Key features : • 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 h. Pa • 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 h. Pa max. intensity (wind, convergence) from 600 to 800 h. Pa • Temperature ‣ cold core between surface-600 h. Pa ‣ above, between 500 -200 h. Pa, 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)

5. 3 Indian monsoon : Monsoon depression Location of rain and vertical velocity : • Deep convection and vertical velocity max. in the SW quadrant • Rain : in the SW quadrant between 100 mm to 300 mm per day

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

5. 4 Indian monsoon Mid-tropospheric cyclone 925 h. Pa 600 h. Pa Streamline and isotach at (left) 925 h. Pa, (right) 600 h. Pa • Closed circulation between 700 -300 h. Pa • Maximum of intensity (wind, convergence) in mid-troposphere from 500 to 600 h. Pa • At low and upper troposphere : absence or light signature in wind (manifested as a trough in the streamline) whence risk of ‘messing- up’ forecasters

5. 4 Indian monsoon Mid-tropospheric cyclone Main Key features : • 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 h. Pa ‣ above between 500 -200 h. Pa, 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 h. Pa ‣ for growth, release of latent heat

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

5. 2 Rossby Wave Breaking (RWB) Event of RWB from 27/05 06 TU to 29/05/98 18 TU Source : ERA 15 CEP : PVU on the surface 350°K high PV RWB high PV Cutt-off of high PV

5. 2 Rossby Wave Breaking Event of RWB : zoom on 27/05/98 18 TU Source : ERA 15 CEP : PVU on the surface 350°K Cut-off of high PV : Stratospheric air Cut-off of low PV : tropospheric air • Deep convection occur upstream the RWB, northeastward the ‘TUTT cell’, in the upper warm and divergent flow

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