4.2.3. MLO Seasonal Transport Characteristics
Seasonal transport patterns for MLO as determined from isentropic trajectories for 1986-1997 are presented in Figure 4.9. Trajectories arrived at MLO at 3400 m, which is the approximate level of the observatory. Six average or cluster mean trajectories are also shown for each season with the percentage of trajectories that fall into each cluster. The cluster mean represents a group of trajectories that are similar in length and shape. There may be considerable variability within each cluster because of the variability of atmospheric conditions, but the cluster mean is a good way to visualize transport within that group. The four panels in Figure 4.9 show the progression of transport patterns throughout the year. During winter, westerly winds are strongest and about 40% of all trajectories have origins over Asia within 10 days. During spring, zonal flow is still prominent, although wind speeds are somewhat lighter and only 35% of trajectories have origins over Asia within 10 days. Transport during summer is dominated by easterly trade winds and MLO is mostly isolated from Asian influences. At this time of year, especially during August, some trajectories have origins over Central America within 10 days, but most easterly flow is light enough that air parcels remain over the tropical ocean. Fall is a season of transition between easterly trades and westerly flow. Every season does have a significant amount of trade wind flow, but it occurs most often in summer and least often in winter.
Fig. 4.9. Atmospheric flow patterns for MLO depicted by cluster-mean back trajectories for (a) Winter (December, January, February), (b) Spring (March, April, May), (c ) Summer (June, July, August), and (d) Fall (September, October, November). Isentropic trajectories used in this analysis arrived at MLO at 3400 m for 1986-1997.
When considering potential sources, it is important to keep in mind the elevation of air parcels. During a recent study we examined air parcel elevation during springtime continental transport. Trajectories appeared fairly homogeneous, with all showing a strong westerly flow from Asia. The range of elevation of air parcels as they crossed the coast of the continent was largeŸfrom 3 to 8 km. Some models and aircraft experiments have found that urban pollution is mostly confined to the lower troposphere [Banic et al., 1996], if not the boundary layer [Wu et al., 1997; Levy II et al., 1997]. At any rate, in order for Asian pollution to be detected at MLO, it would have to be mixed upward out of the boundary layer to considerable heights in the free troposphere. This may explain why a trajectory from Asia does not always result in larger trace gas amounts at MLO. The elevations of air parcels that originate over Central America are closer to surface sources ranging from 2 to 4 km. However, long transit times and photochemical modification en route to MLO could lessen the impact of sources in Central America.
Figure 4.10 shows the maximum elevation along 10-day trajectories as it varies by month. From December through April most air parcels originate above 5 km. This is largely due to subsidence around the North Pacific anticyclone. Maximum elevations during summer are close to the level of the observatory which is characteristic of trade wind flow. This transport feature is important when considering atmospheric constituents that have strong vertical gradients or, like ozone, have a source in the upper troposphere/stratosphere.
Fig. 4.10. Monthly mean maximum elevations attained by air parcels en route to MLO.
Figure 4.11 shows the northernmost latitude along 10-day trajectories by month. This transport characteristic may be important for trace gases that have a strong north-south gradient, such as ozone, methane, and CO during springtime. Transport from the most northerly latitudes occurs during April and December when the average northerly extreme is about 35°N. During July through September air parcels originate in more southerly latitudes because of the predominance of trade wind flow.
Fig. 4.11. Monthly means of the northernmost origins of air parcels en route to MLO.
A recent transport study investigated the cause of MLOs springtime maximum in tropospheric ozone [Harris et al., 1998]. Figure 4.12 shows contours of ozone mixing ratio over Hilo, Hawaii, constructed from ozonesonde flights for 1982-1997. The springtime ozone enhancement at the level of the observatory is clear. The remarkable feature of this plot is that the enhancement is not in a well-defined transport layer, but rather the entire troposphere has more ozone in spring than in other seasons. This picture along with transport characteristics and the relationships found among ozone, CO, methane, and water vapor during spring discount Asian pollution as the main source of the ozone enhancement. A contribution from the upper troposphere/stratosphere upwind of MLO, perhaps in the baroclinically active area near Japan, is indicated. Transport across ozone gradients from the north may also contribute to the seasonal ozone maximum.
Fig. 4.12. Contours of ozone mixing ratio over Hilo, Hawaii, constructed from ozonesonde flights for 1982-1997. The dark line near the top is the climatological tropopause according to the World Meteorological Organization definition (<2 K km-1 for at least 2 km).