Interhemispheric joint behavior of temperature and geopotential height between the thermal tropopause and the 500 and 100 hPa levels

ADRIÁN YUCHECHEN; EDUARDO AGOSTA

Montevideo

Conferencia; WCRP Conference for Latin America and Caribbean: developing, linking and applying climate knowledge; 2014

World Climate Research Programme

The goal of this research is to determine the modes of variability for the joint behavior of the the thermal tropopause with two mandatory levels, connecting remote regions in southern South America, the South Atlantic Ocean, South Africa, the Indian subcontinent and the Indian Ocean, Oceania and the Pacific Ocean. Radiosonde data spanning the period 1973-2012 are used. The thermal tropopause is calculated according to the World Meteorogical Organization definition, whereas data for the two mandatory levels are provided in each sounding and have been taken as is from them. A selection procedure is applied to each level analyzed so as to avoid the inclusion of either wrong information or extreme events. Monthly means are calculated, and these time series are Fourier analyzed. Monthly anomalies are built by subtracting from the monthly means both their grand time mean and those harmonics explaining over 25% of the variance of each series. An Emp irical Orthogonal Functions (EOF) analysis is applied to the monthly anomalies of temperature and geopotential height for the single thermal tropopause and 500 hPa. A similar analysis is carried out using 100 hPa data. The first unrotated leading mode (EOF1) for the coupling between the thermal tropopause and 500 hPa explains 15% of the total variance. The EOF1 time series has an overall significant negative trend with stepped negative slopes that apparently present two breaking points by the end of the 1970s and the 1990s. The power wavelet analysis of the detrended time series shows significant peaks at band periods that are centered at about half a year (semiannual oscillation) and 3 years, also showing significant signals at decadal scales. For some atmospheric variables, the heterogeneous maps associated with EOF1 suggest linkages with features of the global circulation relating to the Walker cell. The EOF2 time series explains 8% of the total variance, and un like the previous EOF, it shows a significant positive trend. Significant quasi-cycles are present at the semiannual scale as well as at intradecadal and decadal scales. The EOF3 time series explains 7% of the total variance. It presents a strong trend during the first four years. When this trend is removed, the time series shows a significant quasi-cycle around 4 years, likely related to El Niño events. Nonetheless, EOF2 and EOF3 seem to be degenerated; hence, further analysis will consider the rotation of the EOFs.