Regional atmospheric angular momentum contributions to polar motion excitation

We focus on a regional analysis of equatorial components of the effective a tmospheric angular momentum (EAAM) functions that measure the excitation of polar motion. These functions are computed from National Centers for Envir onmental Prediction and National Center for Atmospheric Research (NCEP/NCAR ) reanalysis data both globally and in 108 geographic sectors for the perio d 1968-1997. We investigate the relationship between the regional sector EA AM and the global functions responsible for polar motion excitation. We exa mine two excitation terms in parallel, with and without the inverted barome ter (IB) approximation, which adjusts the atmosphere to account for an isos tatic equilibrium response of the ocean to overlying pressure. In the case of pressure terms without IB the largest contributions to the equatorial co mponents of polar motion excitation functions originate in the South Pacifi c, North Atlantic, and North Pacific regions. Applying the IB correction re sults in the dominance of Eurasia and North America instead, with nearly al l Southern Hemisphere contributions disappearing. When comparing the region al functions to their global sum, such fluctuations are mainly coherent wit h variations over northern midlatitude land areas. In some sectors, oscilla tions in these functions tend to occur broadly across two frequency bands: 25-75 and 75-125 days in both prograde and retrograde directions, correspon ding to counterclockwise and clockwise polar motion, respectively. Other se ctors contain more continuous spectral bands, which are centered at similar to 70 days. Coherence and cross-spectral analyses lead us to identify a re gion over Eurasia that contributes importantly to exciting polar motion; we also note an eastwardly propagating signal toward this region in these exc itation terms.