Interactions Between Tropics & Midlatitudes in Winter Season

1. Chap. 6Interactions between the tropics and midlatitudes Intro : Interactions between the tropics and midlatitudes occurred the most frequently during the winter season 6.1 Cold front or pseudo-cold front 6.2 Winter subtropical cyclone or winter mid-tropospheric cyclone 6.3 Waves in the winter easterlies 6.4 Shear lines 6.5 Cold surge 6.6 Dust storm 6.7 Influence of the extratropics on tropical convection over N.Pacific general content

2. 6.1 Westerly troughs : Key features 30°N Move of the Cold front W. E. Upper westerly trough Move of the pseudo-cold front equator Schematic airflow about a retarded cold front in the tropics. Broken stremalines for upper flow; solid lines for lower levels. Source : d’après Met Office College • Synoptic Features : • In winter, the extension of the upper trough, which is a mid-latitude feature, into the tropics occurs when the cold front advances equatorwards and then becomes retarded in its southward (northen hemisphere) movement. Then, the cold front is often represented as a ‘pseudo cold-front’ • The upper through, however, continues to dig south and moves ahead of the retarted portion of the front which soon loses its identity as a surface feature

3. 6.1 Westerly troughs : Key features • Weather associated : ‣ In the southward flow ahead of the cold front, trades winds vanish and convection by diurnal evolution enhances day after day ‣ Pre-frontale convergence with moist and warm air ahead of the front (high instability) which may induce heavy rains (100mm) ‣ Strenghtening of northeasterlies behind the cold front with drop of temperature • Frequency : 3 to 5 times per month over Carribean, • Mascareign, Polynesian islands • Tracking the convection : • ‣ Z, theta on 1.5 PVU since vertical velocity • are enhanced ahead of a stratospheric intrusion • ‣ divergence at 200 hPa and convergence at 925 hPa • Further possible scenario : • If the upper through extends equatorward far enough to • interact with the ITCZ, then marked intensification of convection • within the latter occurs. On occasions, this may cause a large band • of cloud to move out of the ITCZ, caught up in the SW’ lies ahead • of the upper trough and extend to mid-latitude. Sommaire chap.6

4. Infra-rouge Proximité d'un pseudo-front --> appel chaud et humide de Sud (9 Mai 2001) WV intégrée DMSP Pmer + Théta'700 6.1 Westerly troughs : Carribean Islands Source : images du satellite GOES E. Météo-France Source : Analyse Arpège; Météo-France Sommaire chap.6

5. A A 6.1 Westerly troughs :Reunion Islands : example1 of 11/04/2001 image satellite du 11/04/2001. Source : Météo-France Satellite image of 11/04/2001 • Cold front affecting Mascareign Islands and taking place in a trough beetween two ridges of the Mascareign high • This cold front has generated heavy rains at Reunion , from 60 mm to 300 mm over Volcano! Sommaire chap.6

6. 6.1 Westerly troughs :Reunion Islands : example2 of 09/04/2003 Z sur la surface 2 PVU + image vapeur d’eau du 09/04/03. Source : Météo-France Z on the 2 PVU + Water Vapor image of 09/04/03 ⇒ intrusion of stratospheric air : with subsidence behind and ascending motion enhanced ahead Sommaire chap.6

7. 6.1 Westerly troughs :Reunion Islands : example2 of 09/04/2003 vitesse de vent seuillée à 40 kt sur la 2 PVU (CEP) image vapeur d’eau du 09/04/03 du satellite INSAT. Source : Météo-France. Speed Wind > 40 kt on the 2 PVU (CEP) Water Vapor image of 09/04/03 ⇒ Convection enhanced at the left entry (we are in the Southern hemisphere ) of the STJ Sommaire chap.6

8. 6.1 Westerly troughs : French Polynesian Les dépressions d ’origine polaire le 31 mai 2000. Source : Météo-France Marquises A Tahiti A Rikitea Tubuai D 140°W • Cold front generally not active over Polynesian Islands, except for Gambier and Austral Islands, since cold front go generally polewards, eastwards of 140°W • Over Tahiti, and nortwards regions, the weather associated is only • a line of Stratocumulus, but ahead of the cold front a convergence • line very unstable may develop Sommaire chap.6

9. 6.1 Westerly troughs : New Caledonia • Cold front • occur in winter , • from mid-may to • mid-september • Cold front • may induce • westerlies up to • 60 km/h and • heavy rains • (100 mm/day) Source : Météo-France Sommaire chap.6

10. Chap. 6Interactions between the tropics and midlatitudes Intro : Interactions between the tropics and midlatitudes occurred the most frequently during the winter season 6.1 Cold front or pseudo-cold front 6.2 Winter subtropical cyclone or winter mid-tropospheric cyclone 6.3 Waves in the winter easterlies 6.4 Shear lines 6.5 Cold surge 6.6 Dust storm 6.7 Influence of the extratropics on tropical convection over N.Pacific general content

11. 6.2Winter subtropical cyclone or winter mid-tropospheric cyclone Initiation : • In winter, as a rule, cold front move northeastward out of tropics region before any major deepening takes place. • However, if the upper through in the westerlies, located behind the cold front, has a considerable meridional extension, then a major deepening can take place in low latitudes. • If high pressure build across the trough to its north, a cut-off is created and the vigorous system has a slow moving. It is commonly referred to as a subtropical cyclone. To sum-up, the winter subtropical cyclone originate from cut-off lows in the upper-level subtropical westerlies but the heart of the system is in mid-troposphere (400-600 hPa)

12. 6.2Winter subtropical cyclone or winter mid-tropospheric cyclone Divergence représentée par +, convergence par – Source : d’après Ramage, 1971 Main features : • Closed circulation between 700 and 300 hPa (mid-tropospheric cyclone). Light signature at surface and 200 hPa at its early stage. • Cold core (but subtropical cyclone is not marked as a front cold) • Occurrence (northern hemisphere) : from november to january • Location : ‣ in the Atlantic generally found btw 15°N/35°N and 30°W/60°W (SW of the Azores) but may exist everywhere, only 1 or 2 per year ‣ more common in Pacific btw 15 and 35°N and 175°E and 140°W (next to Hawaii), called as ‘Kona cyclone’

13. 6.2Winter subtropical cyclone or winter mid-tropospheric cyclone - = convergence + = divergence Source : d’après Ramage, 1971 Main features : • Maximum of wind, between 400 and 600 hPa, on the eastern side (about 500 km from the center) of the cyclone • Max. of convergence = 600 to 500 hPa • Divergence at 300 hPa and at 800 hPa (trade winds inversion not vanished !) • Maximum of rain on the eastern side (about 500 km from the center) of the cyclone. Sky clear at the center. • In surface, light wind and slack pressure gradient but it may exist a great variability with the time

14. 6.2Winter subtropical cyclone or winter mid-tropospheric cyclone tropopause Source : d’après Ramage, 1971 • Life span = several days to several weeks ! • High PV (i.e. dry air) and low tropopause at center of cyclone : • A great variability of structure with the time : Usually, subtropical cylone remain cold core, but sometimes, the release of latent heat is filling litlle by little the ‘cold pool’ and subtropical cyclone become a warm core and it can develop an eye, as a hurricane. Should the occasion arise, the MSPL fall and the surface wind can become violent !! Sommaire chap.6

15. 6.2Wintersubtropical cyclone Carribean Islands :example of 23/05/04 23/05/04 at 00UTC Source : Météo-France Z + VV>0 on 1.5 PVU Wind > 40 kt on 1.5 PVU Water vapor

16. 6.2Winter subtropical cyclone Carribean Islands :example of 23/05/04 Source : Météo-France 23/05/04 at 18UTC Cutoff VV>0 at Righ entry of STJ and upstream the cutoff Cutoff Z + VV>0 on 1.5 PVU Wind > 40 kt on 1.5 PVU Water vapor

17. 6.2Winter subtropical cyclone Carribean Islands :example of 23/05/04 Source : Météo-France 23/05/04 at 18UTC Cutoff VV >0 Right entry of STJ and upstream the cutoff Cutoff Z + VV>0 on 1.5 PVU Wind > 40 kt

18. 6.2Winter subtropical cyclone Carribean Islands :example of 23/05/04 Source : Météo-France 24/05/04 at 00 UTC 30°N 25°N 1008 hPa 20°N Location of the convection in subtropical cyclone: • right entry of STJ • upstream the cutoff Pmer + IR image Sommaire chap.6

19. Chap. 6Interactions between the tropics and midlatitudes Intro : Interactions between the tropics and midlatitudes occurred the most frequently during the winter season 6.1 Cold front or pseudo-cold front 6.2 Winter subtropical cyclone or winter mid-tropospheric cyclone 6.3 Waves in the winter easterlies 6.4 Shear lines 6.5 Cold surge 6.6 Dust storm 6.7 Influence of the extratropics on tropical convection over N.Pacific general content

20. 6.3 Waves in the winter easterlies North Pole 30°N Upper westerly trough W. E. trough in easterlies equator equator Schematic airflow about a retarded cold front in the tropics. Broken stremalines for upper flow; solid lines for lower levels Source : D’après Met Office College • Occasionally a trough in the upper westerlies forms and induces • a surface trough in the trade wind easterlies, which moves • eastward against the surface flow • The trough of the easterly waves induce relative vorticity and so • it may enhance convection

21. 6.3 Waves in the winter easterlies : illustration in Guyane D A Westerlies trough A Trough in easterlies Streamline 700 Hpa Source : Météo-France

22. 6.3 Waves in the winter easterlies : illustration in Guyane D A Westerlies trough A Trough in easterlies Source : Météo-France Infra-Red Image Heavy rains : 200 to 250 mm in 12h. on Guyana coast !!! Sommaire chap.6

23. Chap. 6Interactions between the tropics and midlatitudes Intro : Interactions between the tropics and midlatitudes occurred the most frequently during the winter season 6.1 Cold front or pseudo-cold front 6.2 Winter subtropical cyclone or winter mid-tropospheric cyclone 6.3 Waves in the winter easterlies 6.4 Shear lines 6.5 Cold surge 6.6 Dust storm 6.7 Influence of the extratropics on tropical convection over N.Pacific general content

24. 6.4 Shear lines : the origins 25°N 20°N Source : Atkinson, 71 ; d’après Palmer et al.,55 Origins of the shear line : When a cold front move into the tropics, the cold air behind it is much modified by passage over the warm water. In addition, warming by sudsidence takes place in the cold air aloft and so the front ceases to exist as a density discontinuity : at about 20° of latitude, surface air temperature and dew point difference across the front are small or zero.

25. 6.4 Shear lines : the origins a b c d Source : d’après Met Office College Origins of the shear line (following …): Conditions favouring the transformation of a cold front into a shear line rather than for simple frontolysis to take place, occur when the cold front is followed by an intensifying high which moves or builds equatorwards behind the front as represented at above figure

26. 6.4 Shear lines : definition 25°N 20°N Source : Atkinson, 71 ; d’après Palmer et al.,55 • Definition : • However, instead of decaying as a synoptic feature, the • front may retain its identity weather wise, by virtue of cyclonic shear induced by the continuance os strong winds in what was the cold air. • Ashear line is defined as a line or narrow zone where • there is an abrupt vector change in horizontal wind component parallel to the line. In other words, shear lines • are defined as a line of maximal horizontal shear.

27. 6.4 Shear lines : main features Main features : • Lifespan : may persist several days, sometimes even longer • Period and location : all tropical regions in winter months • move : generally, slowly equatorward and penetrating deep into • the tropics. Occasionally, the equatorwards portion may • merge with the ITCZ and if the ITCZ is in the other • hemisphere, the shear line may even cross the equator. • Weather associated : • narrow band of convective clouds of varying development. • Cloud tops are usually not excessively high (10000 to 15000 ft) • although isolated cumulonimbus may exist. • The weather starts to deteriorates in the lights winds just ahead • the shear line, while along its axis there may be low cloud and • poor visibility associated with the showers

28. 6.4 Shear lines : tracking Streamline in low tropo (unbroken line) ; isotach in kt (dashed line) Source : Atkinson, 1971, d’après Palmer et al., 1955 • Tracking ‣ through streamline, speed wind, vorticity in low • troposphere or/and in surface • ‣ satelitte cloud imagery makes identification and • tracking easy • Foreraster be careful ! The activity can wax and wane and so • there should be no hurry to ‘drop’ a shear line from the analysis on • the strenght of one image without strong supporting evidence • that the high is weakening or mowing away. Sommaire chap.6

29. Chap. 6Interactions between the tropics and midlatitudes Intro : Interactions between the tropics and midlatitudes occurred the most frequently during the winter season 6.1 Cold front or pseudo-cold front 6.2 Winter subtropical cyclone or winter mid-tropospheric cyclone 6.3 Waves in the winter easterlies 6.4 Shear lines 6.5 Cold surge 6.6 Dust storm 6.7 Influence of the extratropics on tropical convection over N.Pacific general content

30. 6.5 Cold surges : Definition Schematic diagram of a left) synoptic gravity wave right) cold front. Source : Li et Ding, 1989 and from Monsoons over China, chap.2, p.165. • Definition : • During winter seasons, disturbances of midlatitude origin • may penetrate deep into the tropics. Characteristics of cold surges • is the build-up of a cold air reservoir in the midlatitudes, the • establishement of a steep meridional pressure gradient in low • troposphere, and a subsequent abrupt cold-air outbreak • (southward of 40°N over East Asia).

31. A A D D 6.5 Cold surges : The winter Asian cold surge Schematic diagram of a left) synoptic gravity wave right) cold front. Source : Li et Ding, 1989 and from Monsoons over China, chap.2, p.165. • Stage 1 : Initiation of a cold surge over the East Asia • ‣ Simultaneous cyclogenesis on the Eastern Sea China • (called ‘extratropical cyclone’) and anticyclogenesis over Siberia • and North China. • ‣ Following an increase of pressure force and the cold • airstream in low troposphere burst out of the anticyclone initiating • a cold surge • ‣ In addition, it must be outlined thr role played by the • Himalaya which provide a suitable setting for the generation of a • vast pool of cold air and its outpout eastward of 90°E.

32. 6.5 Cold surges : The winter Asian cold surge Schematic diagram of a left) synoptic gravity wave right) cold front. Source : Li et Ding, 1989 and from Monsoons over China, chap.2, p.165. Stage 2 : Southward propagation of the cold surge divided itself into 2 distinct phases : 1) the fisrt phase (left figure) is a southward propagation of a synoptic gravity wave coupled with an increase of pressure, with a speed velocity of 40 m/s. This is not associated with a change in the synoptic weather. 2) the second phase (right figure) is caracterised by a drop of the dew point which is in phase with the passage of the surface cold front (speed velocity of about 10 m/s). Heaviest rainfall over Malaysia-Indonesia area.

33. 6.5 Cold surges : The winter Asian cold surge Streamline (solid line) and isotherm (dashed line) at 900 hPa on the a) 10 dec. b) 11 dec. C) 12dec. Source : Johnson et Zimmerman, 1986 • The increase of northeasterlies • may be associated with both phases • (gravity wave and cold front ) with • about : • ‣ 20/30 kt over the North China Sea • ‣ 30/40 kt over the South China Sea • The maximum of wind speed is • observed at 900/950 hPa, just • above the boundary layer Sommaire chap.6

34. Chap. 6Interactions between the tropics and midlatitudes Intro : Interactions between the tropics and midlatitudes occurred the most frequently during the winter season 6.1 Cold front or pseudo-cold front 6.2 Winter subtropical cyclone or winter mid-tropospheric cyclone 6.3 Waves in the winter easterlies 6.4 Shear lines 6.5 Cold surge 6.6 Dust storm 6.7 Influence of the extratropics on tropical convection over N.Pacific general content

35. 6.6 Dust storm: in sudan the 27/12/2004 The airborne dust has a cooling effect on the observed temperature (30°c cooler than surrounding desert ) Egypt Sudan Nile River Source : images from MODIS (spectroradiometer of 1 km per pixel) left image : natural color right image : temperature

36. 6.6 Dust storm Definition : Strong and sustained winds, capable of raising large amounts of dry opaque particles, causing a serious reduction in visibility (< 1000 m) ‣ in Sudan Area (Sudan and Lybia), they are called ‘Haboobs’ ‣ in Middle-East area (Iraq, Saudi Arabia), they are called ‘Shamal’

37. 3 kinds of synoptic situations in Sudan Area : 6.6 Dust storm 1) instability type associated with thunderstorm activity, for example ahead a squall line. ‣ between May and August ‣ short-lived and localized, with a width of 10-80km 2) pressure gradient type related with to the steepening of the large-scale meridional pressure gradient ‣ between May and August during the advancing summer monsoon or early monsoon period. ‣ widespread and blow for long periods 3) pressure gradient type, associated with cold front and strong southward pressure gradient ‣ between February and May with the passage of Mediterranean depression ‣ widespread and blow for long periods (24-36 h) and 2-3 times a month Statistics over Central Sudan : 20 dust storms per year, more than 70% of which are of types 1) and 2) between May and August Over middle-East Area : Dust Storms occur only with the third above synoptic situation depicted

38. 6.6 Dust storm: over the Red Sea the 30/06/2003 Thick plumes of tan-colored dust are blowing across the Red-sea This dust storm is related to strong post-frontal north-westerly winds and has persisted 24-36 h The dust storms also severely affect the Darfur region in Sudan and MSG, with its capacity to observe dust storm in the whole area at a frequency of 15 mn, plays an essential role in helping United nations to coordinate their aid. Source : images from MODIS :natural color

39. Chap. 6Interactions between the tropics and midlatitudes Intro : Interactions between the tropics and midlatitudes occurred the most frequently during the winter season 6.1 Cold front or pseudo-cold front 6.2 Winter subtropical cyclone or winter mid-tropospheric cyclone 6.3 Waves in the winter easterlies 6.4 Shear lines 6.5 Cold surge 6.6 Dust storm of the Sudan 6.7 Influence of the extratropics on tropical convection over N.Pacific general content

40. 30°N 30°N 6.7 Influence of the extratropics on tropical convection (over North Pacific in winter) Schematic showing the sequence of events describing tropical- extratropical interaction processes over the Pacific Ocean as simulated by an AGCM . Source : Slingo, 98. 1) Cold surge in low troposphere at 110°E initiated by the increasing Of the Siberian high associated with the passage of deepening mid-latitude weather system 2) This cold surge enhances convection over the maritime continent intensifying the local Hadley circulation 3) which in turns interacts with the extratropics by accelerating the Subtropical Westerly Jet over Eastern Asia Sommaire chap.6

41. 30°N 30°N 6.7 Influence of the extratropics on tropical convection(over North Pacific in winter) Source : Slingo, 98. Reminder ‣ Upper easterlies generally observed between 15°N/15°S vanish Rossby wave propagation ‣ On the contrary, Upper westerlies observed at 30° of latitude and over Equatorial East Pacific are considered as a Rossby wave duct or waveguide 4) The subsequent eastwards extension of the Subtropical Westerly Jet considered as a Rossby canal duct lead to an amplification of a Rossby wavetrain 5) The westerly canal duct observed over Equatorial East Pacific allow the intrusion of an upper tropospheric through in deep tropics over the East Pacific some 3-4 days later resulting in a flaring of convection Sommaire chap.6

42. 30°N 30°N 6.7 Influence of the extratropics on tropical convection (over North Pacific in winter) Source : Slingo, 98. Schematic showing the sequence of events describing tropical- extratropical interaction processes over the Pacific Ocean as simulated by an AGCM (Slingo, 98) 6) The final link in the chain is the excitation of equatorial easterly waves by the enhanced convection in the East Pacific. These easterly waves, propagating across the Pacific with a speed phase of 7m/s, may contribute to convective activity in the West Pacific and can, if the phasing is correct, subsequently interact with later cold surge event. Meehl et al. (96) describe such an interaction btw easterly wave and cold surge can contribute to extreme weather event in that region, such as tropical cyclone and associated westerly wind burst Sommaire chap.6

43. Atkinson, G. D. 1971 : Forecaster guide to tropical meteorology. Rapport technique 240,U.S. Air Weather Service. • Chang, C. P. , Millard, J. E.,Chen, G.T. J., 1983 :’Gravitationnal character of cold surges during winter MONEX’. Mon. Wea. Rev., Vol.111, p.293-307 • Ding Yihui, 1994 : ‘Monsoons over China’. Kluwer Academic Publishers,419 p. • Johnson, R. H. and J. R. Zimmerman, 1986 : Modification of the boundary layer over the South China Sea during a winter MONEX cold surge event. Mon. Wea. Rev., Vol.114, p.2004-2005 • Lau, K.-M., and H. Lim, 1984 :’ On the dynamics of equatorial forcing of climate teleconnections’. • J. of the Atm. Sci., Vol.41, p.161-176. • Li, C. and Ding, Y. H., 1989 :’A diagnostic study of an explosively deepening oceanic cyclone over the northwest Pacific Ocean’. Acta Meteorological Sinica, Vol.47, p.180-190 • - Malardel S. , 2005 : ‘Fondamentaux de Météorologie : à l’école du temps’. Cépadues Editions, • 708 p. • - Matthews, A. J. and G. N. Kiladis, 2000 : ‘ A model of Rossby waves linked to submonthly convection over the Eastern Tropical Pacific’. J. of the Atm. Sci., Vol.57, p. 3785-3798 • Meteorological Office College, FitzRoy Road, Exeter, Devon –Courses Note : Introduction to tropical Meteorology • Palmer, C. E., C.W. Wise, L. J. Stempson et G. H. Duncan, 1955 : The practical aspect of tropical meteorology. AWS Manual, Vol.1, p. 105-148 References (1)

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