Changes in the angular momentum of the atmosphere, oceans, and the solid Earth are linked, while the mass balance between the ocean and atmosphere is related to the overall gravity field of the Earth.
Within the Global Geophysical Fluids Center of the International Earth Rotation and Reference Systems Service (IERS) the Special Bureau for the Atmosphere (SBA) is a joint effort of Atmospheric and Environmental Research (AER) and the U.S. National Centers for Environmental Prediction (NCEP) to provide atmospheric data relevant to the study of the Earth's variable rotation. The SBA is a successor to the Sub-bureau for Atmospheric Angular Momentum, operating within the IERS since 1989.
The figure above shows the zonal (east-west) wind. Measured in meters per second, negative values indicate easterly winds (from the east), and positive values are westerly winds. The much larger westerly than easterly winds, overall, sampled in this January month, show that the atmosphere superrotates around the Earth, and so has a net positive angular momentum. The angular momentum in the northern hemisphere summer months, like August, have overall weaker westerly winds and thus the atmosphere's angular momentum is decreased. Due to overall conservation of angular momentum in the Earth-atmosphere system, this decrease in angular momentum in the atmosphere from January to August is compensated by the Earth by increasing its angular momentum. Thus our planet itself spins faster, and hence the length of day decreases by a small amount on the order of a millisecond.
Our principal data consist of atmospheric angular momentum (AAM) and related quantities from four major meteorological centers: NCEP, the Japan Meteorological Agency (JMA), the United Kingdom Meteorological Office (UKMO), and the European Centre for Medium-Range Weather Forecasts (ECMWF). Also included are long-term series derived from atmospheric reanalysis efforts. Analysis values up to the most recent month are available at AER. Analysis values and forecasts of AAM are available for retrieval from NCEP in near-real time, for the most recent 10 days.
Other products contributed to the Special Bureau for the Atmosphere:
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