Ernesto Muñoz

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 JunJul AugSep 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. The Caribbean and Central America

4. 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.

5. 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 1979-1999).

6. 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.

7. Data and Methods • NARR and ERA40 reanalyses • GPCP precipitation • HadSST and ERSST sea surface temperature • August and September anomalies from 1979-2001 climatology • Calculated Caribbean Low Level Jet index as area average 12N-16N, 80W-70W. • Calculated regressions and correlations

8. Summer climatological conditions

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

10. AugSep sfc temp (at 2m) and SLP and annual cycle of zonal average T_2m SLP

11. 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)

12. GPCP and NARR Precip for AugSep • Differences in precipitation between GPCP and NARR

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

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

15. 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.

16. 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.

17. 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.

18. 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.

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

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

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.

27. 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.