The Effect of El Niño-Southern Oscillation on the Upper Troposphere and Stratosphere PDF Download

Author: Kristin Marie Raisanen
Publisher:
ISBN: 9781124908113
Category :
Languages : en
Pages :

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Book Description
El Niño-Southern Oscillation (ENSO) is a well known coupled ocean-atmosphere phenomenon which occurs on a cyclical basis in the Pacific Ocean. In order to further understand the extent of the ENSO signal propagation into the upper troposphere and stratosphere, the anomalies from the monthly mean caused by El Niño and La Niña were determined and examined. Four El Niño and three La Niña events were chosen using the Niño 3.4 index. Data from the seven events were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). Both zonal means (Latitude vs. Pressure) and meridional cross sections (Longitude vs. Pressure) of the temperature, ozone and wind components were analyzed. The anomalies were analyzed for the event maximum, which was used as the base month (lag month 0), and several months following the event maximum at one month intervals. To determine the anomaly from the monthly mean for the variables, the long-term monthly average was removed from the event month or lagging month, leaving only the signal of the anomaly related to ENSO. The El Niño and La Niña anomalies were then averaged and the averaged La Niña anomalies were subtracted from the averaged El Niño anomalies, leaving the ENSO related anomalies. A t-test was used to determine statistical significance to the 90% and 95% confidence level. The results show that ENSO has a statistically significant impact on the wind, temperature and ozone in the troposphere and stratosphere during the maximum event month and for many months following the ENSO event maximum. The zonal mean temperature and the zonal mean zonal winds show some similarities around 30° in both hemispheres during months 3 and 4. Both the zonal mean temperature and the zonal mean zonal wind are positive, and are both significant at the 90% level though the zonal mean temperature anomalies increase in strength through the months while the zonal mean zonal wind anomalies weaken. These differences suggest a dependence of the zonal mean zonal wind anomalies to changes in the zonal mean temperature anomalies, though there is a possible delay between the two signals. The zonal mean omega anomalies show consistent areas of upward anomalies at the equator and downward anomalies just outside of the equatorial region, showing a possible northward shift of the Hadley cell. The zonal mean meridional wind and omega anomalies suggest some weak strengthening of the northern branch of the Hadley cell in the upper troposphere though any strengthening would be weak. Though the anomalies are not statistically significant, this conclusion is in agreement with several past studies. Anomalies in the zonal mean omega field and the zonal mean meridional winds also suggest that ENSO may initially strengthen the Polar cell during months 0 and 1 and then weaken it during month 2. Negative zonal mean omega anomalies and southward zonal mean meridional anomalies are seen north of 60°N in the upper troposphere during months 0 and 1 suggesting a weakened polar cell. These anomalies reverse during month 2 suggesting a strengthened polar cell, though none of these anomalies are statistically significant. Many hypotheses for the consistencies between the temperature and ozone anomalies are discussed, including ozone layer compression and decompression, radiative reactions and vertical and horizontal advection. It was found that these hypotheses are all weak and are not consistent throughout the analysis.