1. Hydrodynamics of the Caribbean Low-level Jet and its Relationship to Drought K.H. Cook and E.K. Vizy Jackson School of Geosciences University of Texas at Austin NOAA's 33rd Climate Diagnostics and Prediction Workshop/CLIVAR Drought Workshop (CDPW)October 20-24, 2008 in Lincoln, Nebraska

2. Seen in the NARR domain, the CLLJ is a prominent feature, with the highest low-level zonal wind speeds in the domain July, 925 hPa, m/s Caribbean low-level jet (CLLJ)

3. Zonal Wind Speed July, 925 hPa, m/s Cuba Hispaniola Puerto Rico Venezuela Columbia

4. Seasonality of the CLLJ Zonal wind speed (m/s) Month U wind averaged 80-60W and 12-18N, 925 hPa

5. Seasonality of the CLLJ 0 m/s to -10 m/s (easterly) Zonal wind speed (m/s) Month U wind averaged 80-60W and 12-18N, 925 hPa

6. Seasonality of the CLLJ Zonal wind speed (m/s) Strong in DJF Strongest in June/July Month U wind averaged 80-60W and 12-18N, 925 hPa

7. Seasonality of the CLLJ Weakest in October Zonal wind speed (m/s) Weak in May Month U wind averaged 80-60W and 12-18N, 925 hPa

8. October through April CLLJ largely disconnected from the flow (and moisture) entering the US and northern Mexico May through September Caribbean and Great Plains low-level jet system

9. There are also significant seasonal differences in the elevation and vertical structure of the CLLJ.

10. Zonal wind cross section in latitude and height at 70W January February March April May June

11. Zonal wind cross section in latitude and height at 70W July August September October November December Note that the minimum in Sept/Oct at 925 hPa shown previously is partly due to this elevation change in boreal fall

12. Diurnal Cycle July 925 hPa Daytime CLLJ ~ 2m/s weaker than the nighttime jet. Secondary minimum at 4 AM LT Local time at CLLJ region ~ GMT – 5 hr

13. Purpose What controls the CLLJ on seasonal and diurnal time scales? What is the relationship of the CLLJ to drought …. over the central U.S.? …. over central America and southern Mexico?

14. Dynamics of the CLLJ • Examine the horizontal momentum balance to understand • Diurnal cycle of the jet • Differences between seasons Start with July ….

16. First-order v-momentum balance: geostrophy

17. Note the diurnal cycle of the height gradient, weakening through the night and building up during the day.

18. Afternoon: 4 PM LT Late Night: 4 AM LT The diurnal cycle of the meridional height gradient is caused by the diurnal cycle of heating over far northern South America.

19. This diurnal cycle in height gradient does not directly generate the CLLJ’s diurnal cycle. The diurnal cycle of the jet has a different shape and the jet weakens during the day. Instead, the diurnal heating and cooling over northern South America generates meridional acceleration.

20. Daytime: Land warms and geopotential heights fall. This strengthens the meridional geopotential height gradients and leads to onshore flow (southward, negative local acceleration). Night: Offshore flow and northward (positive) acceleration. This solenoid circulation sets up because of the space scales involved and because of the low latitude – Coriolis accelerations are relatively weak.

21. July, u-momentum equation

22. The July u-momentum balance is more complicated than the v-momentum balance The only small terms are meridional and vertical advection.

23. Strong zonal geopotential height gradients associated with low heights over Central America/ southern Mexico to the west and the NASH to the east.

24. Strong westward frictional accelerations associated with the easterly jet.

25. jet decelerates Should have the weakest jet speeds at ~ 3 PM, and a secondary minima at ~3 AM. (We saw this above.)

26. jet decelerates The minimum at ~ 3 PM occurs mainly because of the Coriolis force, but also because the zonal pressure gradient force weakens. The northerly off-shore flow anomaly is strongest at 10 AM, after which it begins to weaken.

27. jet decelerates The minimum at ~ 3 AM is related to a weakening of the zonal pressure gradient force.

28. How is the momentum balance different in other months, and what does this tell us about how the seasonal cycle works?

29. Seasonality of the CLLJ Magnitude CLLJ Magnitude GPLLJ Jet has its maximum in July (which we just examined), and has minima in May and Oct, with a secondary max in December/January

30. V-momentum balance is about the same as in July, i.e., to first order geostrophic. So we need to understand the seasonality of meridional height gradients.

31. For example, why are the meridional geopotential height gradients similar in magnitude and, especially, sign in January and July, and weak in May and October?

33. Even though the CLLJ is strong in both January and July, it has very different downstream connections in those months.

34. October through April CLLJ largely disconnected from the flow (and moisture) entering the US and northern Mexico May through September Caribbean and Great Plains low-level jet system

35. The u-momentum equation is different because the Coriolis force in January is negative, while it was (mostly) positive in July.

36. 925 hPa geopotential heights and winds MJJAS ONDJFMA V > 0 V < 0

37. 925 hPa geopotential heights and winds Difference

39. What is the relationship between the CLLJ and drought?

40. 1. Is the CLLJ anomalous during dry years in the central US and/or Central America? • Is precipitation anomalous during • strong or weak CLLJ years?

41. Examine the NARR for 1978-1996 to identify dry years in these regions Central U.S, 100-90W and 30-50N, for JJA 1980, 1988, 2006 are anomalously dry

42. In June 1988, the CLLJ is quite weak at ~½ its climatological value. But not In 1980 and 2006.

43. In July 1980 the CLLJ is weak by ~¼ of its climatological value. But not in 1988 or 2006 – its even strong in 2006.

44. In August, the CLLJ is weak in 1980 and 1988, but strong in 2006. We don’t find a consistent relationship between the CLLJ and the 3 years of central US drought.

45. For S. Mexico/Central America? Only one year (2003) is identifiable as a drought year in the NARR.

46. 1. Is the CLLJ anomalous during dry years in the central US and/or Central America? • Is precipitation anomalous during • strong or weak CLLJ years?

47. Define a CLLJ Index 925 hPa u; JJA; 75 - 70W and 12 - 15N

48. Standard Deviations from the 1979-2006 JJA mean zonal wind WeakerCLLJ StrongerCLLJ

49. Drought Years in the U.S. Weaker CLLJ Stronger CLLJ

50. Drought Year in Central America Weaker CLLJ Stronger CLLJ