The influence of synoptic scale transport mechanisms on trace gas relationships above the western North Atlantic Ocean

Contributed by
Owen R. Cooper, and Jennie L. Moody, Department of Environmental Sciences, University of Virginia, Charlottesville, and Andreas Stohl, Lehrstuhl für Bioklimatologie und Immissionsforschung, Tech.University of Munich, Germany.

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A goal of the North Atlantic Regional Experiment (NARE) is to determine the synoptic scale meteorological mechanisms that transport pollutants from North America to the western North Atlantic Ocean (WNAO). Mid-latitude cyclones tracking from west to east are believed to be responsible for the bulk of the trace gas transport throughout the year, even in summer when these systems are weaker [Merrill and Moody, 1996]. Over the past 30-40 years precipitation and clouds have been studied in terms of the component airstreams of mid-latitude cyclones [Carlson, 1998]. Recently, both the origin and evolution of individual airstreams have been shown to influence trace gas mixing ratios and relationships in the troposphere [Bethan et al., 1998; Cooper et al., 2001a]. This article interprets in situ trace gas measurements and their relationships in terms of the airstreams of mid-latitude cyclones.

Figure 1. Model of a mid-latitude cyclone. The scalloped lines indicate the border of the comma-cloud about the cyclone center (L). The numbers on the warm (WCB) and cold (CCB) conveyor belts indicate the pressure at the top of these airstreams, while the numbers on the dry air stream (DA) indicate the pressure at the bottom of this airstream. The post cold front (PCF) airstream flows beneath the DA.

The typical mid-latitude cyclone is composed of four major airstreams (Figure 1). The warm conveyor belt (WCB), cold conveyor belt (CCB) and dry airstream (DA) produce the distinctive comma cloud of a mature mid-latitude cyclone [Browning and Monk, 1982; Browning and Roberts, 1994; Bader et al. 1995; Bethan et al., 1998; Carlson, 1998]. To a first approximation, these airstreams move along sloping isentropic surfaces. The CCB originates north of the cyclone's warm front, and relative to the cyclone center, ascends as it heads westward, with a component heading eastward at higher altitudes. The WCB is located on the eastern side of the cyclone, ahead of the surface cold front. The air originates at low altitudes in the warm sector of the cyclone and travels northward, ascending into the mid- and upper troposphere, above the CCB. In contrast the DA originates at high altitudes on the poleward side of the cyclone and descends into the mid- and lower troposphere west of the cold front. Cooper et al. [2001a] introduced the concept of the post cold front airstream (PCF) to explain the lower and mid-tropospheric flow that follows a cold front, beneath the DA.

Initially, NARE focused on two synoptic scale transport mechanisms, the tropopause fold and the cyclone warm sector of mid-latitude cyclones. The cyclone warm sector constitutes the lower portion of the WCB. In eastern North America, this feature develops on the western side of surface anticyclones where stable and often stagnant conditions allow for the accumulation of photochemically active trace gases in the lower troposphere [Comrie and Yarnal, 1992; Comrie, 1994; Cooper and Moody, 2000]. As the cyclone moves off shore it carries the warm sector with it, exporting the polluted air mass to the WNAO [Berkowitz et al., 1996; Merrill and Moody, 1996; Moody et al., 1996].

Table 1. Key results from Stohl et al. [2001].

Tropopause folds embody the isentropic decent of stratospheric air on the polar side of upper level cold fronts. They occur within the dry airstream and are associated with every mid-latitude cyclone, their depth of penetration roughly proportional to the strength of the cyclone [Danielsen, 1968; Shapiro, 1980; Johnson and Viezee, 1981]. These intrusions of stratospheric air into the mid- and upper troposphere lead to irreversible stratosphere/troposphere exchange (STE), enriching the ozone content of the DA. Tropopause folds are easily identified when isentropic potential vorticity, a quasi-conservative dynamic tracer of stratospheric air, is contoured on an altered water vapor image [Moody et al., 1999; Wimmers and Moody, 2000]. This product is derived by correcting the GOES satellite 6.7 mm water vapor channel for temperature and zenith angle bias, resulting in a depiction of specific humidity, rather than relative humidity [Soden and Bretherton, 1993] in the mid- to upper troposphere. Tropopause folds have their greatest impact during spring when cross-tropopause exchange and ozone mixing ratios in the lower stratosphere reach their seasonal peaks; their weakest impact occurs in late summer/autumn [Appenzeller et al., 1996; Fahey and Ravishankara, 1999; Monks, 2000]. Several NARE [Berkowitz et al., 1995; Merrill and Moody, 1996; Moody et al., 1996; Oltmans et al.; Parrish et al., 2000; Cooper et al., 2001a] and Atmosphere Ocean Chemistry Experiment (AEROCE) studies [Moody et al., 1995; Cooper et al., 1998; Prados et al., 1999] have described the influence of tropopause fold events on the chemistry of the WNAO atmosphere. NARE aircraft and ozonesondes have routinely detected mid- and upper troposphere ozone enhancements on the polar side of cold fronts during all seasons; ample meteorological and chemical data indicate that tropopause folds produced these enhancements. Moody et al. [1995] found that post-frontal transport, descending from the mid-troposphere above North America, produced surface ozone enhancements at Bermuda with greatest frequency and intensity during spring.

The interpretation of in situ trace gas measurements and their relationships in terms of the airstreams of mid-latitude cyclones requires two components: a climatology of the airstreams, including their frequency, origin and transport routes, and an understanding of the chemical and physical processing of the trace species in each airstream. Stohl [2001] has established a climatology of WCBs and DAs by calculating a large set of trajectories with initialization points equally distributed over the Northern Hemisphere over a year's period. The trajectories were classified by airstream according to certain criteria [Wernli and Davies, 1997; Wernli, 1997]. The results indicate that WCB inflow occurs only at latitudes below 50 N; during winter and spring inflow only occurs below 40 N. Inflow frequency maxima occur at the southeastern seaboards of North America and Asia. The proximity to the continental east coasts is significant since the highest global anthropogenic emissions of sulfur, nitrogen oxides, and other pollutants are located there. The air masses entering WCBs traverse both the high-emission continental regions and the neighboring, relatively source free Gulf of Mexico, Caribbean, and tropical Pacific. Thus the chemical characteristics of WCB air masses range from very clean [Grant et al., 2000] to highly polluted [Stohl and Trickl 1999], depending on their exact path. DAs originate from a belt around the Northern Hemisphere between approximately 30 and 60 N, in the vicinity of the polar front jet stream. The most striking feature is the seasonal cycle with high DA activity during the winter and very low activity in the summer.

The chemical and physical processing of the trace species in each airstream has been characterized by Cooper et al. [2001a,b,c]. They 1) give examples of trace gas signatures from all four major mid-latitude cyclone airstreams, 2) develop a conceptual model for systematically investigating the trace gas relationships in the airstreams, and 3) discuss the seasonal differences in these relationships. Cooper et al. [2001a] describe how modeled, remotely sensed and in situ meteorological data were used to identify airstreams during the late summer-early autumn 1997 NARE campaign above the Canadian Maritimes, and present a limited number of case studies. Cooper et al. [2001b] analyze the chemical measurements of all the airstreams sampled during NARE 1997, amalgamate the data into a single composite cyclone, and develop a conceptual model of the chemical and physical processing within a mid-latitude cyclone. The model separates the meteorological influences on airstream trace gas signatures from the influence of surface emissions heterogeneity. The meteorological influences on airstream trace gas composition during late summer–early autumn above the WNAO are: 1) The DA always has a stratospheric component that brings ozone into the mid- and upper troposphere. 2) The PCF originates to the northwest, is unaffected by wet deposition and the sunny conditions may allow for some photochemical ozone production. 3) Both the CCB and WCB experience wet deposition, but because of its southerly origin and association with the western side of surface anticyclones the WCB draws from regions more favorable for photochemical activity than either the CCB or PCF. 4) The CCB is generally cloudy and does not show signs of significant photochemical production of ozone. 5) Very little of the oxidized nitrogen species (NO_Y) is exported from the lower troposphere, due to wet and dry deposition. 6) Airstreams in the mid-troposphere are the most susceptible to mixing with other air masses, which blurs the distinction between airstream trace gas signatures. The remainder of the chemical variation between and within airstreams is largely controlled by surface emission heterogeneity; for example a WCB that draws from the polluted mixed layer will have higher mixing ratios of ozone and CO than one that draws from the clean marine boundary layer.

Stohl et al. [2001] have investigated the extent and time scale of the removal of NO_Y by wet and dry deposition during transport from the North American continental surface, where the sources are located, to the free troposphere of the WNAO. Their analysis utilized aircraft-based measurements of NO_Y and CO from the NARE 1997 and early spring NARE 1996 studies, combined with a particle transport model. Extensive and rapid removal of NO_Y was found in both seasons. The major findings of the study are summarized in Table 1.

Figure 2. Spring 1996 ozone vs. CO for all four cyclone airstreams at three levels of the troposphere. The lighter (darker) shading corresponds to relatively higher (lower) data density. The range of the late summer-early autumn 1997 data for each airstream is outlined in blue. The linear regression lines for each airstream are shown for spring 1996 (gray lines) and summer-autumn 1997 (blue dashed lines), with the slope and r-squared values labeled in black (spring) and blue (summer-autumn).

Cooper et al. [2001c] show that seasonal variation of photochemistry and meteorology affect the characteristic trace gas mixing ratios of the conceptual cyclone. During spring background ozone and CO are at their maximum, cyclones track farther south where continental surface emissions are greater, and STE injects more ozone into the troposphere. Ozone and CO are compared by airstream for early spring and late summer-early autumn conditions over the WNAO in Figure 2. Using the positive (negative) O₃/CO slope as an indicator of photochemical ozone production (destruction), ozone production during late summer-early autumn is associated with the lower troposphere PCF and all levels of the WCB, especially the lower troposphere. During early spring, significant ozone production is not associated with any airstream at any level, with the lower troposphere CCB associated with ozone destruction. The negative slopes of the DA indicate STE increases ozone in the mid- and upper troposphere.

Building a more complete picture of the seasonal and spatial variation of the trace gas relationships in mid-latitude cyclones will require additional analyses. Emmons et al. [2000] have compiled an extensive data set of chemical measurements from many aircraft campaigns over the past 20 years, indicating regions of the globe where additional composite cyclones could be constructed. The upcoming Intercontinental Transport and Chemical Transformation (ITCT) aircraft campaign will be conducted over the eastern North Pacific Ocean and western North America in spring, 2002. The differences in chemical composition of cyclones entering North America from the Pacific and exiting the continent into the WNAO will provide a more comprehensive picture of North American cyclones. Ultimately the trace species relationships in cyclone airstreams, and their variation with season and region will be very useful for comparison to the output of chemical transport models that have the ability to resolve cyclone structure.

References

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