African dry air outbreaks

[1] Dry air outbreaks from Africa into the tropical Atlantic region are documented using daily layered precipitable water (LPW) data of satellite remote sensing for 1993–1997. A dry air outbreak is defined as an abrupt decrease in middle-troposphere (700–500 hPa) LPW that is greater than 1 mm d−1 and remains below normal for at least two consecutive days in a reference box off the west coast of Africa. Pathways of individual dry air outbreaks thus identified are tracked both forward and backward in time. Dry air outbreaks intruding into the tropical Atlantic may originate from different source regions: the northern Atlantic Ocean, northern Africa, Sahel, and equatorial Africa. Some of these outbreaks can keep their identities up to 2 weeks, propagate across the Atlantic Ocean, and reach South America and the Caribbean Sea. Boreal summer is the overall peak season, with the majority of the outbreaks occurring north of the Intertropical Convergence Zone (ITCZ). In spring more outbreaks occur south of the ITCZ. Precipitation in the ITCZ tends to decrease during periods of the outbreaks in both spring and summer. Observed large-scale patterns of African dry air outbreaks are compared to known characteristics of African dust outbreaks. The results from this study suggest that African dry air outbreaks should and can be treated as prominent phenomena independent of African dust outbreaks. This study calls for systematic investigations on the similarities and differences between African dust and dry air outbreaks for a better understanding of their effects on weather and climate in the tropical Atlantic region.

[1]  Zev Levin,et al.  Modification of mineral dust particles by cloud processing and subsequent effects on drop size distributions , 2000 .

[2]  Toby N. Carlson,et al.  Atmospheric Turbidity in Saharan Dust Outbreaks as Determined by Analyses of Satellite Brightness Data , 1979 .

[3]  Kevin E. Trenberth,et al.  Atmospheric moisture recycling : Role of advection and local evaporation , 1999 .

[4]  H. Maring,et al.  Dust vertical distribution in the Caribbean during the Puerto Rico Dust Experiment , 2002 .

[5]  P. Xie,et al.  Global Precipitation: A 17-Year Monthly Analysis Based on Gauge Observations, Satellite Estimates, and Numerical Model Outputs , 1997 .

[6]  J. Prospero,et al.  Atmospheric transport of soil dust from Africa to South America , 1981, Nature.

[7]  J. P. Díaz,et al.  Radiative properties of aerosols in Saharan dust outbreaks using ground‐based and satellite data: Applications to radiative forcing , 2001 .

[8]  Chris Kidd,et al.  Rainfall Estimation from a Combination of TRMM Precipitation Radar and GOES Multispectral Satellite Imagery through the Use of an Artificial Neural Network , 2000 .

[9]  Sandy P. Harrison,et al.  The role of dust in climate changes today, at the last glacial maximum and in the future , 2001 .

[10]  Siri Jodha Singh Khalsa,et al.  A climatology of the water vapor band brightness temperatures from NOAA operational satellites , 1993 .

[11]  Larry L. Stowe,et al.  Characterization of tropospheric aerosols over the oceans with the NOAA advanced very high resolution radiometer optical thickness operational product , 1997 .

[12]  O. Boucher,et al.  A satellite view of aerosols in the climate system , 2002, Nature.

[13]  Stanley G. Benjamin,et al.  Radiative Heating Rates for Saharan Dust , 1980 .

[14]  V. Barros,et al.  Midsummer Low-Level Circulation and Precipitation in Subtropical South America and Related Sea Surface Temperature Anomalies in the South Atlantic , 2002 .

[15]  Chidong Zhang,et al.  Variability of Water Vapor, Infrared Radiative Cooling, and Atmospheric Instability for Deep Convection in the Equatorial Western Pacific , 1999 .

[16]  S. H. Melfi,et al.  Validation of the Saharan dust plume conceptual model using lidar, meteosat, and ECMWF Data , 1999 .

[17]  J. Prospero,et al.  Deposition rate of particulate and dissolved aluminum derived from saharan dust in precipitation at Miami, Florida , 1987 .

[18]  P. Chu,et al.  Large-Scale Circulation Features Associated with Decadal Variations of Tropical Cyclone Activity over the Central North Pacific , 2002 .

[19]  Ina Tegen,et al.  Climate Response to Soil Dust Aerosols , 1998 .

[20]  C. Thorncroft,et al.  THE JET2000 PROJECT Aircraft Observations of the African Easterly Jet and African Easterly Waves , 2003 .

[21]  S. Nicholson.  The Spatial Coherence of African Rainfall Anomalies: Interhemispheric Teleconnections , 1986 .

[22]  Toby N. Carlson,et al.  Saharan air outbreaks over the tropical North Atlantic , 1980 .

[23]  T. L. Wolfe,et al.  An assessment of the impact of pollution on global cloud albedo , 1984 .

[24]  X. Zeng.  The Relationship among Precipitation, Cloud-Top Temperature, and Precipitable Water over the Tropics , 1999 .

[25]  Michael Garstang,et al.  Saharan dust in the Amazon Basin , 1992 .

[26]  Ina Tegen,et al.  A comparison of seasonal and interannual variability of soil dust aerosols over the Atlantic Ocean as inferred by the TOMS AI and AVHRR AOT retrievals , 2001 .

[27]  Brian E. Mapes,et al.  Bimodality in tropical water vapour , 2003 .

[28]  Toby N. Carlson,et al.  Vertical and areal distribution of Saharan dust over the western equatorial north Atlantic Ocean , 1972 .

[29]  G. Gu,et al.  Westward-Propagating Synoptic-Scale Disturbances and the ITCZ , 2002 .

[30]  N. Middleton,et al.  Saharan dust storms: nature and consequences , 2001 .

[31]  Paul J. DeMott,et al.  Saharan dust storms and indirect aerosol effects on clouds: CRYSTAL‐FACE results , 2003 .

[32]  V. M. Karyampudi,et al.  Synoptic-Scale Influence of the Saharan Air Layer on Tropical Cyclogenesis over the Eastern Atlantic , 2002 .

[33]  Philippe Bousquet,et al.  Origins of African dust transported over the northeastern tropical Atlantic , 1997 .

[34]  A. Gruber,et al.  Discrepancy between Gauges and Satellite Estimates of Rainfall in Equatorial Africa , 2000 .

[35]  I. McCrea,et al.  Ohmic heating as evidence for strong field‐aligned currents in filamentary aurora , 2001 .

[36]  D. Parsons,et al.  A proposed mechanism for the intrusion of dry air into the Tropical Western Pacific Region , 1999 .

[37]  T. Gill,et al.  Long‐range transport of North African dust to the eastern United States , 1997 .

[38]  V. Pope,et al.  The representation of water vapor and its dependence on vertical resolution in the Hadley Centre Climate Model , 2001 .

[39]  T. Carlson,et al.  The Large-Scale Movement of Saharan Air Outbreaks over the Northern Equatorial Atlantic , 1972 .

[40]  P. Zuidema,et al.  Radiative-Dynamical Consequences of Dry Tongues in the Tropical Troposphere , 1996 .

[41]  Adrian M. Tompkins,et al.  Organization of Tropical Convection in Low Vertical Wind Shears: The Role of Water Vapor , 2001 .

[42]  V. Ramanathan,et al.  Aerosols, Climate, and the Hydrological Cycle , 2001, Science.

[43]  K. Tung,et al.  Water Vapor, Surface Temperature, and the Greenhouse Effect—A Statistical Analysis of Tropical-Mean Data , 1998 .

[44]  G. Stephens,et al.  A new global water vapor dataset , 1996 .

[45]  Richard Arimoto,et al.  Eolian dust and climate: relationships to sources, tropospheric chemistry, transport and deposition , 2001 .

[46]  C. Velden,et al.  ARTICLES: The Impact of the Saharan Air Layer on Atlantic Tropical Cyclone Activity. , 2004 .

[47]  Chidong Zhang,et al.  Variability of midtropospheric moisture and its effect on cloud-top height distribution during TOGA COARE , 1997 .