Abstract
We examine interhemispheric transport processes that occurred over the central Pacific during the PEM‐Tropics B mission (PTB) in March‐April 1999 by correlating the observed distribution of chemical tracers with the prevailing and anomalous windfields. The Intertropical Convergence Zone (ITCZ) had a double structure during PTB, and interhemispheric mixing occurred in the equatorial region between ITCZ branches. The anomalously strong tropical easterly surface wind had a large northerly component across the equator in the central Pacific, causing transport of aged, polluted air into the Southern Hemisphere (SH) at altitudes below 4 km. Elevated concentrations of chemical tracers from the Northern Hemisphere (NH) measured south of the equator in the central Pacific during PTB may represent an upper limit because the coincidence of seasonal and cold phase ENSO conditions are optimum for this transport. Stronger and more consistent surface convergence between the northeasterly and southeasterly trade winds in the Southern Hemisphere (SH) resulted in more total convective activity in the SH branch of the ITCZ, at about 6°S. The middle troposphere between 4–7 km was a complex shear zone between prevailing northeasterly winds at low altitudes and southwesterly winds at higher altitudes. Persistent anomalous streamline patterns and the chemical tracer distribution show that during PTB most transport in the central Pacific was from SH to NH across the equator in the upper troposphere. Seasonal differences in source strength caused larger interhemispheric gradients of chemical tracers during PTB than during the complementary PEM‐Tropics A mission in September‐October 1996.
Highlights
A major atmosphericcirculationfeature of the equatorialPacific is a zonal band of surfaceconvergencebetween theThe NASA Global TroposphericExperiment(GTE) spon- northern and southern hemisphericlow-level easterly trade sored a two aircraft mission(Pacific Exploratory Mission- winds, called the Intertropical ConvergenceZone (ITCZ), TropicsB) during March-April 1999 to sampletropospheric which is described in detail
Extensivechemicalmeasurements were made from the NASA DC-8 and P-3B aircraft acrossthe Pacific Ocean in both hemispheres.Experimental objectivesand aircraftinstrumentationduringPEM-Tropics B (PTB) aredescribedbyRaperetat. [thisissue].PEM-TropicsB was designed to complement the earlier PEM-Tropics A (PTA) experiment[Hoettet at., 1999],whichoccurredduring August and September1996 in the tropical Pacific.Flow patterns over the tropical PacificduringPTA were more represcientificobjectiveof PTB wasto studythe role of the ITCZ as a barrier to interhemispherictransport[Raperetat., thisissue]
Convectiveactivityduring March-April 1999 appearedto alternatebetweenbranchesof the ITCZ in the Northern Hemisphere (NH) and SH [Hu et al, this issue].SSM/I imagesof averagerainfall duringMarch and April suggesthat convectionin the SH was greaterthan in the NH duringboth months.For comparison, Plate lc showsthe SSM/I averagerainfall duringPTA (Sep- relaxingof the southeastrade winds,which are affectedby tember1996).Tropicalrainfallassociatedwith the ITCZ in the both seasonal and interannual variations in the location of the NH is greaterduringPTA than duringPTBbut confinedto a narrowzonalbandbetween5øN [Waliserand Gautier, 1993]
Summary
The NASA Global TroposphericExperiment(GTE) spon- northern and southern hemisphericlow-level easterly trade sored a two aircraft mission(Pacific Exploratory Mission- winds, called the Intertropical ConvergenceZone (ITCZ), TropicsB) during March-April 1999 to sampletropospheric which is described in detail . Extensivechemicalmeasurements were made from the NASA DC-8 and P-3B aircraft acrossthe Pacific Ocean in both hemispheres.Experimental objectivesand aircraftinstrumentationduringPEM-Tropics B (PTB) aredescribedbyRaperetat. [thisissue].PEM-TropicsB was designed to complement the earlier PEM-Tropics A (PTA) experiment[Hoettet at., 1999],whichoccurredduring August and September1996 in the tropical Pacific.Flow patterns over the tropical PacificduringPTA were more represcientificobjectiveof PTB wasto studythe role of the ITCZ as a barrier to interhemispherictransport[Raperetat., thisissue]. Our specificgoalsare to locate and describethe altitudedependent dynamical and chemical boundary between the Northern Hemisphere (NH) and the SouthernHemisphere (SH) in thisregionof the Pacific,andto describetheprocesses that controlthe meridionaltransportof chemicaltracersacross this boundary.We examinehow the double ITCZ structure andthe uppertroposphericwind patterninfluencedinterhemispheric chemical gradients during PTB. Tween 25øNto 25øSand 175øEto 140øW,becausethis region has the most abundant chemical measurements from both hemispheres
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