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

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

• Ernesto Muñoz

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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

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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

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The Caribbean and Central America
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Southern Gulf of Mexico, Isthmus and Gulf of
Tehuantepec
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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.

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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 Caribn 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 Caribn (4/6 for
  1979-1999).

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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.

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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

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Summer climatological conditions
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Jun-Jul and Aug-Sep averages of wind at 925 mb
JunJul
AugSep
NARR
ERA40

• Strong easterly flow during summer across the
  Caribbean Sea
• Magnitude of wind in colors
• Maximum at 925 mb
• Presence of meridional shear

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AugSep sfc temp (at 2m) and SLP and annual cycle
of zonal average
T_2m
SLP
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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 28C)

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GPCP and NARR Precip for AugSep

• Differences in precipitation between GPCP and NARR

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(No Transcript)
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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

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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

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Stdev of column with CALLJ

• Maximum variability in August ( September).
• Maxima in variability at 925 mb - 950 mb

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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.

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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.

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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.

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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.

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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.

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Low level winds SLP regressed with NARR CALLJ

• Sharp northward gradient of SLP anomalies over
  Central America and Caribbean region

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ERA40 CALLJ regress to ERA40 winds and SLP

• Similar pattern to NARR anomalies in
  Inter-Americas Sea region.
• Inter-basin pattern of SLP anomalies

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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.

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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.

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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.

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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.

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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.

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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.

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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.