1. The Caribbean Low Level Jet variability during August and September and its relation with the regional hydroclimate
Ernesto Muñoz

2. JunJul & AugSep averages of 925 mb wind
NARR
ERA40
Strong easterly flow in Caribbean Sea during summer.
CALLJ an extension of the North Atlantic trade winds amplified by regional gradients in SLP and temperature

3. Few tidbits Low land areas of Central America are jungle.
The Netherland Antilles (Aruba, Curazao) are dry and windy.
Before the Pacific was called the South Ocean and the Atlantic the North Ocean

4. The Caribbean and Central America

5. Southern Gulf of Mexico, Isthmus and Gulf of Tehuantepec

6. Motivation
Previous studies of Caribbean hydroclimate mostly on precipitation from land stations.
Lack of knowledge of role of Caribbean Low Level Jet on regional hydroclimate.
Other LLJs are associated with moisture transport and downwind convection.

7. Background (Giannini et al 2000, 2001)
Giannini et al (2000, 2001) observe anomalous tropical inter-basin SLP gradient and SST gradient.
Associated with less precip over Caribbean and Central America are warm Pacific and low SLPA vs cool Atlantic and high SLPA.
Make association with drier Carib’n and divergence over region (except east of Costa Rica).
Positive NAO in winter cools TNA SSTs that when followed by onset of ENSO in summer combines effect on less precip over Carib’n (4/6 for ).

8. Background (Taylor et al 2002)
Taylor et al (2002) partitioned summer rainy season in a early rainy season (MJJ) and a late rainy season (ASO).
Indicate that late rainfall season not affected so much by SSTA but by vertical wind shear (VWS).
VWS inhibits convection.

9. Data and Methods NARR and ERA40 reanalyses GPCP precipitation
HadSST and ERSST sea surface temperature
August and September anomalies from climatology
Calculated Caribbean Low Level Jet index as area average 12N-16N, 80W-70W.
Calculated regressions and correlations

10. Summer climatological conditions

11. Jun-Jul and Aug-Sep averages of wind at 925 mb
NARR
ERA40
Strong easterly flow during summer across the Caribbean Sea
Magnitude of wind in colors
Maximum at 925 mb
Presence of meridional shear

12. AugSep sfc temp (at 2m) and SLP and annual cycle of zonal average

13. Precip and SST for AugSep
East-west precipitation gradient across Central America with minimum below CALLJ and maximum west of Central America.
Meridional gradient of SST in CALLJ region
SSTs are convective (i.e., greater than 28°C)

14. GPCP and NARR Precip for AugSep
Differences in precipitation between GPCP and NARR

16. Aug-Sep clim & stdev of zonal wind at 925 mb
Climatology
Stdev
NARR
ERA40
CALLJ index: 80W-70W, 12N-16N at 925 mb
stronger ERA40 CALLJ but more NARR CALLJ variability

17. Annual cycle of column with CALLJ
Stronger in June and July
Second maximum in February (in NARR)
Presence of vertical shear
ERA40 stronger than NARR during summer

18. Stdev of column with CALLJ
Maximum variability in August (& September).
Maxima in variability at 925 mb mb

19. Interannual variability
NARR and ERA40 CALLJ show similar variability throughout the period.
Correlation of 0.93 between NARR and ERA40 from 1979 to 2001.

20. Autocorrelation of CALLJ with subsequent month
July and August, August and September are most highly autocorrelated.
Weaker autocorrelations indicate sudden month-to-month changes in CALLJ intensity.

21. CALLJ regress to GPCP precip in Aug-Sep
CALLJ from NARR and ERA40 show very similar anomalous precipitation patterns (from GPCP precipitation).
Stronger easterly CALLJ related with less precipitation in Caribbean Sea, the Antilles and Central America.

22. NARR CALLJ correlated to SSTs
Variability associated with tropical inter-basin gradient of SSTA.
Stronger easterly CALLJ when tropical Pacific warmer and tropical Atlantic cooler SSTs.

23. Closer look of previous plot
East-west gradient of SSTAs across Central America.
Stronger CALLJ associated with cooler SSTs in Caribbean Sea and warmer SSTs to the west of Central America.

24. Low level winds & SLP regressed with NARR CALLJ
Sharp northward gradient of SLP anomalies over Central America and Caribbean region

25. ERA40 CALLJ regress to ERA40 winds and SLP
Similar pattern to NARR anomalies in Inter-Americas Sea region.
Inter-basin pattern of SLP anomalies

26. Is there difference in precip correlated to Niño3 SSTAs and that correlated to CALLJ?
Precipitation over the Antilles, the Caribbean Sea and Central America is more anti-correlated with easterly CALLJ than with Niño3 warm SSTAs.

27. GPCP & NARR precip regressed on NARR CALLJ
Similar pattern of precipitation between GPCP and NARR associated with strengthening of CALLJ.
Less precipitation on Pacific coast of Central America and offshore; Isthmus and Gulf of Tehuantepec; central and eastern Caribbean; and east-northeast of Caribbean.
Close to normal precipitation over and offshore of Caribbean side of Central America.

28. Partition of NARR total precip onto convective and stratiform
Less convective precip northeast of Caribbean and eastern half of Caribbean and southern Gulf of Mexico.
More convective precip east and west of Central America and Yucatan.
Less stratiform precip downwind of CALLJ over western half of Caribbean, Pacific coast and to the west.

29. Anomalous vertical wind shear (VWS), convective precip
Anomalous VWS in consonance with anomalous convective precip over eastern half of Caribbean and to the west of Central America.
Not in consonance over western half of Caribbean (close to Central America) and to northeast beyond Caribbean.

30. Vertically integrated moisture flux
Caribbean moisture fluxes directing west-southwestard to Central America.
MFs increase in magnitude to and over Central America and to the west.

31. Moisture Flux Convergence (MFC) and NARR Horizontal Moisture Convergence
MF divergence over eastern half of Caribbean.
MF convergence over western half of Caribbean.
Moisture convergence over eastern side of Central America.
Moisture divergence over western side of Central America.

32. Summary
In August and September a strengthening of the CALLJ is associated with:
a strengthening of the meridional SLP gradient in Caribbean region,
an interbasin SST gradient,
less precipitation over western Central America and eastern Caribbean,
MFDiv over eastern Caribbean and MFConv over western Caribbean.
An increase of vertical wind shear over Caribbean and a decrease to the west of Central America.
The CALLJ seems to be a principal atmospheric modulator of Caribbean and Central American precipitation from year to year.