Low Level Jets in the
Tropical Americas
Department of Atmospheric Sciences
Colorado State University
Gabriela Mora
Motivation and Introduction

Understand the dynamics of the low level jets in the tropical Americas

Study the convection, vorticity, divergence and sea surface temperature
associated with these jets

Climatology of the wind influences the weather and its prediction

Main weather fields are strongly correlated with each other

Jets play an important role inhibiting or enhancing convection
Background
Relevant features
1) Cold tongue:
 located on the west coast of Ecuador and Peru
 explained by wind-driven Ekman transport (July-November)
2) Double ITCZ:
- two simultaneous ITCZ's
- caused when the cold tongue is present and inhibits convection at the
equator and splitting the ITCZ in two
3) Gaps in the topography:
- Isthmus of Tehuantepec
- Papagayo Gap
- Isthmus of Panama
Background
Relevant features
4) Jets:
- Caribbean Low Level Jet (CLLJ): JJA, DJF
- Tehuantepec: ONDJF, JA
- Papagayo: DJFMA, JA
- Panama: DJFMA
- Choco: SON
5) Western Hemisphere Warm Pool (WHWP):
- SST's > 28.5°C
- Divided into four regions: Eastern North Pacific
Gulf of Mexico
Western Tropical North Atlantic
Caribbean
6) Costa Rica Dome:
- Dome in the thermocline
- 20°C isotherm lies 30 m below the sea surface
Data
Wind
 QUIKSCAT 2000-2007
-- spatial resolution: 0.25°, temporal resolution: twice daily
 NCEP Re-analysis 1948-2006
-- spatial resolution: 2.5°, temporal resolution: monthly
Precipitation
 Tropical Rainfall Measuring Mission (TRMM) 2000-2007
-- spatial resolution: 0.25°, temporal resolution: monthly
Sea Surface Temperature
 NCEP/NOAA Optimum Interpolation (OI) Analysis 2000-2007
-- spatial resolution: 1°, temporal resolution: monthly
ENSO Index
 ENSO Years – NOAA 1998-2005
Main features: Cold Tongue
July
September
August
October
Main features: Western Hemisphere Warm Pool
March
May
April
June
Main features: Western Hemisphere Warm Pool
July
September
August
October
Main features: Low-Level Jets
January
July
September
Main features: Caribbean Low Level Jet
Primary maximum
December
January
February
Secondary maximum
June
July
August
Main features: Tehuantepec Jet
Primary maximum
October
January
November
February
December
Main features: Tehuantepec Jet
Secondary maximum
July
August
Main features: Papagayo Jet
Primary maximum
December
January
February
Secondary maximum
July
August
Main features: Panama Jet
January
February
March
Main features: Choco Jet
September
October
November
Main features: Double ITCZ
February
Divergence
March
April
Main features: Double ITCZ
March
February
Precipitation
April
Main features: Double ITCZ
Precipitation
(from TRMM)
April during La Nina event
2001
April during El Nino event 1998
Main features: Costa Rica Dome
Dome in the thermocline explained by wind forcing
 20°C isotherm lies 30 m below the sea surface
 Produces a “dry hole” in the ITCZ, which can reach 500 km in diameter

Strong Papagayo jet
 Cyclonic wind stress curl

January
Vorticity
Anticyclonic
Cyclonic
Dome: annual cycle
Feb-Apr: coastal shoaling of
the thermocline
May-Jun: separation from the
coast
Jul-Nov: thermocline ridge
shoals, dome expands to the
west
Dec-Jan: deepening
Cloud frequency
May-Nov, 2006
CIRA/RAMM
Main features: Costa Rica Dome
Sverdrup relation:
f2
h

h
=

w
dx

E
e
g'
H
β
h: thermocline depth anomaly
H: mean thermocline depth
E: eastern boundary
g': reduced gravity
we: Ekman pumping velocity
df
β
dy
Conclusions




The Caribbean Low Level Jet, Tehuantepec Jet, Papagayo Jet and Panama
Jet, respond mostly to pressure gradients between the Atlantic and Pacific
Ocean, accelerating the winds toward the Pacific.
These pressure gradients have been explained by:
1) cold fronts from the North Hemisphere, push high pressure systems from
midlatitudes to the tropics.
2) the annual cycle of the North Atlantic Subtropical High, which is more
intense during the boreal summer.
The Choco Jet owes its existence to the meridional temperature gradient
between the Cold Tongue and western Colombia.
The Papagayo Jet has cyclonic vorticity on the left side of its exit, which
generates Ekman pumping and upwelling in the ocean, producing a dome in
the thermocline (the Costa Rica dome).
Conclusions

Double ITCZ is found from February to April during neutral and La Nina
ENSO conditions
Future work

Use of pilot balloons to study the wind vertical profile in the continental
Papagayo region

Study the correlation of cold fronts and the CLLJ

Study the correlation of the North Atlantic Subtropical High with the CLLJ

Compare the wind results obtained with the NCEP data, with another
database
Extra slides
Cold tongue
Western Hemisphere Warm Pool
Region I
Gaps in the topography
Main features: Caribbean Low Level Jet
Vertical profile: zonal wind, meridional wind, wind speed
Main features:
Tehuantepec Jet
Vertical profile: zonal wind, meridional wind, wind speed
Main features: Papagayo Jet
Vertical profile: zonal wind, meridional wind, wind speed
Main features: Panama Jet
Vertical profile: zonal wind, meridional wind, wind speed
Main features: Choco Jet
Vertical profile: zonal wind, meridional wind, wind speed
Double ITCZ
Main features: Double ITCZ
January
April
February
May
March
June
July
October
August
November
September
December
Costa Rica Dome
20˚C isotherm depth (m)
Data from World Ocean Database 1998 (Conkright et al., 1999)
Conkright, M.E., Levitus, S., O’Brien, T., Boyer, T.P., Stephens, C., Johnson,
D., Baranova, O., Antonov, J., Gelfeld, R., Rochester, J., Forgy, C., 1999.
World ocean database 1998 CD-ROM data set documentation, Version 2.0.
NODC Internal Report 14, 116pp.