The central American mid-summer drought (MSD, veranillo or canicula )

1. The central American mid-summer drought (MSD, veranillo or canicula) Brian Mapes University of Miami

2. Key West midsummer drought 30-year climatology Miami

3. 1999 MSD

4. ( ?

5. What causes the MSD? • Local, fussy: • The ingredients for convection get worse (moisture, instability, lifting mechanisms). • Global, vague: • Solar declination angle ultimately drives all aspects of the seasonal cycle. • Continent-ocean contrasts involved • Somewhere in between lies a useful truth...A

6. What causes the MSD? • Regional: • North Atlantic Subtropical High (NASH or Bermuda High) & tradewinds are enhanced and extended west in mid-summer.

7. Midsummer nose on N. Atlantic Subtropical High (NASH) Cayman Veracruz late May- early June Valparaiso wet July Carib easterly jet dry wet

8. upwind slopes wetter inmidsummer

9. Key West Valparaiso, FL midsummer drought Miami

10. CaymanMSD broader in time than Veracruz CMAP

11. Individual years - still discernable (in big regional Central American box average) intraseasonal can mask it a bit 9 year mean

12. TAIWAN 25N MSD happens in other places too May CMAP rainfall CUBA 25N May SE BRAZIL 25S Nov. MADAGASCAR 25S Nov.

13. High High High High Where are those boxes? W. subtropical oceans

14. What causes the MSD? • Regional: • NASH (Bermuda High) & tradewinds are enhanced and extended west in mid-summer. • Locally, less rain is due to: • frictional divergence or other subsidence mech.? • flow direction brings drier airmass? • wind leads to lower SST (albeit via enhanced LHF)...? • capping inversion strength/height? • different meso/synoptic disturbance characteristics? • But what causes the NASH/trades to vary?

15. A broader view of the summertime oceanic subtropical highs • Meteorology 101: • Hot air rises, cool air sinks. • Hydrostatic sinking is all about PGF to push horizontal branch of circulation (divergence) • e.g. Hadley cell: eq. trough, subtropical H belt • In summer, subtrop. oceans cooler than land =>Highs over the ocean basins

16. Annual mean SLP

17. July SLP (same color scale)

18. cooling cooling January SLP (same color scale)

19. Meteorology 301: stratified • Heatedair rises, cooledair tends to sink. • heating rate = rate of flow across isentropes = vertical velocity (in  coordinates) • E. subtropical oceans: cooling(even in winter) • little latent heat release (rain), much rad. cooling (stratocu decks under dry upper troposphere)

20. “The Usual Explanation” • Hoskins (1996): • “the usual explanation for the subtropical anticyclones (radiative cooling)” in summer is “inadequate” • “monsoon latent heat release over the neighboring continent” is essential

21. 2001 Model experiments, forced by realistic heating (above), in realistic JJA zonal mean flow, with real topography

22. Y887 (Q_Asia, mts) Pure heating effect: Y887 (Q_Asia, mts)-Y887 (mts only) ?? Rossby Kelvin Rodwell and Hoskins 2001 Asian Heating Q_Asia

24. ?? Rossby Kelvin Red: global Q Try NAm and NPac Heating: Y887 forced by mountains + NAm Q Y887 forced by mtns + NAm Q + NPac Q local heating is most of it

25. Small et al. 2007 concur: imposed heating anomalies in the region can explain most of the flow anomalies • some westward influence from E. Atl • MSD negative heating drives NASH/trade enhancement • positive feedback

26. positive feedback • Easterlies / High drive (negative) rain anomalies • (negative) latent heating anomalies drive easteries/ High • An amplified, twitchy system - but what drives it, to make this climatological feature?

27. One hypothesis:

28. One hypothesis: MSD wet midsummer tradewind surge

29. Drivers: 2. Mean flow changes in midsummer? Chen, Hoerling & Dole 2001 Heating Eddy Z1000 w/o shear Eddy Z1000 July u(y,p)

30. time slice u300, zonal mean Jul-Aug time slice 60N Westerlies retreat to >30N in midsummer <0 Eq J F M A M J J A S O N D easterlies protrude to 30N suddenly in mid summer • WHY, in terms of [u] budget? • Not f[v]: ~barotropic; [v](t) wrong • [u’v’]: Tilted TUTTs, Tibetan High, Transients?

31. Hudson Bay contributes too west coast thermal gradient Drivers: 3. Influences via the midlatitude westerlies? 2006

32. Japan’s midsummer drought mechanism: the Bonin High“Formation mechanism of the Bonin High in August”(Enomoto, Hoskins, & Matsuda 2003)

33. positive feedback • An amplified, twitchy system - what drives it, to make this climatological feature? • Local SST cycle hypothesis? • not well supported by obs • Global easterlies in midsummer? • but where do they come from? • Midlatitude wave pathways? • Asian monsoon? • via Rodwell-Hoskins’ wraparound Kelvin wave?

34. Clues from climate models • Do free-running, full-physics climate models have the MSD? • Short answer: yes • What drives it there? • Global, vague: • solar declination, like all aspects of seasons • continent-ocean contrasts • Experiment!

35. 5 CMAP rain ObservationsIPRC/ ECHAM model: quite good 1 SLP July Aug 8 IPRC GCM rain 2 GCM SLP

36. Experiment: global monsoon system on steroids • Increase continent-ocean contrasts • perpetual June (and every other month): • fix SST and solar decl. for each day of year • run model for many days, so atm+land come into equilibrium • Warmest NH now in June, not July • Continents hotter in summer (dried out?) • What happens to monsoons (and MSD)?

37. Northern continents get hotter in summer Dec Jan

38. Annual mean rainfall change Our MSD is a major part of the global rainfall response Americas drier Indian Ocean wetter Dec Jan in MSD early onset

39. Conclusions (so far) • The Central American MSD is a significant aspect of the seasonal cycle, both regionally and globally. • Useful for understanding phys.mechanisms • how exactly does the NASH affect rainfall/convection • U. for u. summer teleconnections • from Asia, zonal [u] easterlies, waves on westerlies... • A “natural mode” of the regional system - relevant to other types of S-I climate variability?

41. Future work • Starting fall 07 in earnest (thanks NSF) • Atlantic summer climate reading group • me, 2-3 students on project, other experts (please!) • where to start?

43. software demo

46. u200 HF anomaly plot, July-Aug HF anomalies(total - annual - semiannual):Yes, the Bonin High and 2 other stationary waves are evident, but also a zonally elongated u anomaly spanning all Asia-WPAC Japan 2 -4 0 m/s -2 time slice HF total Jul Aug HF wavelet analysis 10d 100d

47. Interannual consistency of the Asian HF jet anomaly ~1 Aug

48. Low-level heating dominant. Influences from monsoons and planetary waves, but mainly local feedbacks. Eastern-basin oriented.

49. EASM ENASM? EASM ISM NAM WNPM (2nd active) Jun 1 Jul 1 May 1 Jun 1 Jul 1 May 1