Interannual-to-interdecadal changes in the atmospheric moisture over the global tropics

Interannual to interdecadal changes in the atmospheric moisture over the global tropics are investigated using precipitable water content (PWC) obtained from the National Centers for Environmental Prediction - National Center for Atmospheric Research (NCEP/NCAR) Reanalysis for a 51-year period (1948-1998). Climatology of annual mean PWC agrees well with the radiosonde-based and satellite-based (NVAP) climatologies of atmospheric moisture. In general, total year-to-year variability of PWC, expressed as a standard deviation to the annual mean value, is also consistent with previously found patterns. Exception is North Africa region, where substantial biases are found. Empirical orthogonal functions (EOF) analysis revealed dominant modes of PWC variability in the global tropics. The first EOF mode accounts for 25.4 % of the total variance of annual mean PWC and reflects interdecadal changes, characterized by a climate regime shift in the early 1970s. The spatial pattern of this mode shows four major action centers over the South America, equatorial central Pacific, Indonesian maritime continent, and North Africa. The second EOF has essentially different structure and explains 11.9 % of the total variance of annual mean PWC. Its spatial pattern is dominated by coherent variations over almost the entire tropics. This mode is associated with the El Nino-Southern Oscillation (ENSO), which is the major climatic signal in the global tropics. The third EOF mode of annual mean PWC accounts for 9.8 % of the total variance. Major centers of action include a meridionally symmetric pattern in the central-eastern tropical Pacific, and centers of opposite polarity over the maritime continent and South America. Temporal behavior of the corresponding PC-3 is characterized by a quasi-biennial variability. Analysis of regional changes in annual mean PWC demonstrated that long-term variations are presented realistically over Malaysia, central tropical Pacific, and Brazil, and are consistent with changes of other hydrological variables in these regions. Changes of PWC over North Africa are probably biased during the earlier period of records (1950s- 1960s).