Northern Hemisphere Subtropics Research Articles

Interannual Variability of Trace Gases in the Subtropical Winter Stratosphere

Abstract Measurements of water vapor and methane from the Halogen Occultation Experiment instrument on board the Upper Atmosphere Research Satellite are used to study the interannual variability of trace gas distributions in the atmosphere. Particular attention is paid to the mechanisms influencing trace gas distributions in the subtropics. The study highlights the quasi-biennial oscillation (QBO) dependence of subtropical tracer distributions more clearly than in previous studies. There is a strong correlation between the equatorial wind QBO and the slope of the tracer isolines in the Northern Hemisphere subtropics, with steeper subtropical isoline slopes in the easterly phase compared with the westerly phase. This is particularly so in the lower stratosphere. Two possible mechanisms for the QBO signal in subtropical isoline slopes are identified: advection by the mean circulation and isentropic mixing. A comparison between the QBO signal in the slope of the tracer isolines and the isentropic tracer grad...

Reaction of N2O5 on tropospheric aerosols: Impact on the global distributions of NOx, O3, and OH

Using a three‐dimensional global model of the troposphere, we show that the heterogeneous reactions of NO3 and N2O5 on aerosol particles have a substantial influence on the concentrations of NOx, O3, and OH. Due to these reactions, the modeled yearly average global NOx burden decreases by 50% (80% in winter and 20% in summer). The heterogeneous removal of NOx in the northern hemisphere (NH) is dominated by reactions on aerosols; in the tropics and southern hemisphere (SH), with substantial smaller aerosol concentrations, liquid water clouds can provide an additional sink for N2O5 and NO3. During spring in the NH subtropics and at mid‐latitudes, O3‐concentrations are lowered by 25%. In winter and spring in the subtropics of the NH calculated OH concentrations decreased by up to 30%. Global tropospheric average O3 and OH burden (the latter weighted with the amount of methane reacting with OH) can drop by about 9% each. By including reactions on aerosols, we are better able to simulate observed nitrate wet deposition patterns in North America and Europe. O3 concentrations in springtime smog situations are shown to be affected by heterogeneous reactions, indicating the great importance of chemical interactions resulting from NOx and SO2 emissions. However, a preliminary analysis shows that under present conditions a change in aerosol concentrations due to limited SO2 emission control strategies (e.g., reductions by a factor of 2 in industrial areas) will have only a relatively minor influence on O3 concentrations. Much larger reductions in SO2 emissions may cause larger increases in surface O3 concentrations, up to a maximum of 15%, if they are not accompanied by a reduction in NOx or hydrocarbon emission.

Linkages between 200-mb Tropical and Extratropical Circulation Anomalies during the 1986–1989 ENSO Cycle

NMC operational wind analyses have been used to document the 200-mb circulation anomalies during the solstice seasons of the 1987–1989 ENSO cycle. Using the one-level vorticity balance diagnostic approach, the 200-mb Rossby wave anomalous source distribution was derived as departures from a three-year mean for the single warm-phase season (January–March 1987). The effective Rossby wave anomaly sources were found to be generally quite small within 15° of the equator. Poleward of about 15°, the anomalous 200-mb divergence, through the planetary vorticity-divergence anomaly stretching term, is the dominant factor in determining the anomalous 200-mb vorticity source distribution. Its JFM standard deviation around 30°−35°N was an order of magnitude larger than that found in the deep tropics. The January–March and June–August 200-mb circulation differences between the warm and cold phase of the 1987–1989 ENSO cycle are described. The ENSO cycle linkages between the equatorial Pacific convection anomalies and the extratropical circulation are viewed in terms of regional Hadley component anomalies and tropical-extratropical divergence anomaly zones (TEDAZs), which typically extend eastward and poleward from the equatorial convection anomalies into the subtropics and extratropics. The larger Coriolis parameter at higher latitudes can result in significant vorticity anomaly sources in the subtropical branches of the anomalous Hadley circulations and in the extratropical extensions of the TEDAZS. The information obtained from this case study is synthesized into a coherent description of linkages between the eastern equatorial Pacific and the Northern Hemisphere circulation during the northern winter. The prime longitudinal belt of coupling extends from near the date line eastward to the central or eastern Atlantic. The primary ingredients of the coupling are 1) the subsiding branch of the anomalous Hadley circulation in the North Pacific, 2) enhanced downstream subtropical westerlies across the Gulf of Mexico into the Atlantic, 3) an equatorward enhancement of 200-mb synoptic-scale variability associated with the enhanced westerlies, 4) coupling between the anomalous eddy activity in the eastern subtropical Pacific and the anomalous equatorial convection. The resulting northeastward extension of the anomalous equatorial convection across Mexico into the Atlantic gives rise to a time-averaged anomalous divergent circulation whose upward branch coincides with the region of enhanced convection and whose primary subsiding branch extends from northeastern South America northeastward across the low-latitude North Atlantic. This linkage with the Northern Hemisphere subtropics contrasts with most previous descriptions, which associate the low-latitude South American-Atlantic anomalies more directly with the Pacific anomalies through an anomalous Walker circulation.