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  • Author or Editor: K. E. Trenberth x
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Kevin E. Trenberth and Won-Tae K. Shin


An investigation is made into the presence of quasi-biennial oscillations (QBOs) in sea-level pressure fields over the Northern Hemisphere. Using 55 years (1925–79) of seasonally averaged sea-level pressure anomalies, a series of analyses has been performed in order to systematically isolate and describe the QBOs. A standard empirical orthogonal function (EOF) analysis of the seasonal anomalies reveals significant nonrandomness at QBO periods. The data were then band-pass filtered in order to focus on the QBO, and a second EOF analysis was performed. Finally, a new complex EOF analysis technique was applied to the data. Complex EOFs have the advantage of permitting both standing and propagating modes. The QBO variance in six standard EOFs is essentially confined to four EOFs in the filtered data set and compressed into three complex EOFs. The latter account for 56% of the filtered variance.

The dominant mode of the complex EOF analysis was common to all analyses and has been found in many other studies. It is essentially a standing wave pattern corresponding to a high-latitude zonal index with departures in pressure of opposite sign in low and high latitudes. This mode includes elements of the North Atlantic Oscillation, the Pacific–North American teleconnection pattern and the Southern Oscillation, but also differs somewhat from all three. It tends to be phase locked to the annual cycle.

The second complex EOF mode C2 exhibits clear propagating characteristics as it evolves in time. The third mode C3 also has some propagating characteristics and, at times, both modes strongly resemble wave trains of quasi-stationary Rossby waves. Neither C2 nor C3 is phase locked to the annual cycle.

It appears likely that all three complex EOFs are normal mode responses of the atmosphere to different kinds of forcing. Statistical evidence for phase locking of all three complex EOFs to the QBO in zonal winds in the equatorial stratosphere is not convincing, and the origin of the preferred QBO periodicity remains to be determined.

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R. A Anthes, P. A Bernhardt, Y. Chen, L. Cucurull, K. F. Dymond, D. Ector, S. B. Healy, S.-P. Ho, D. C Hunt, Y.-H. Kuo, H. Liu, K. Manning, C. McCormick, T. K. Meehan, W J. Randel, C. Rocken, W S. Schreiner, S. V. Sokolovskiy, S. Syndergaard, D. C. Thompson, K. E. Trenberth, T.-K. Wee, N. L. Yen, and Z Zeng

The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO data are of better quality than those from the previous missions and penetrate much farther down into the troposphere; 70%–90% of the soundings reach to within 1 km of the surface on a global basis. The data are having a positive impact on operational global weather forecast models.

With the ability to penetrate deep into the lower troposphere using an advanced open-loop tracking technique, the COSMIC RO instruments can observe the structure of the tropical atmospheric boundary layer. The value of RO for climate monitoring and research is demonstrated by the precise and consistent observations between different instruments, platforms, and missions. COSMIC observations are capable of intercalibrating microwave measurements from the Advanced Microwave Sounding Unit (AMSU) on different satellites. Finally, unique and useful observations of the ionosphere are being obtained using the RO receiver and two other instruments on the COSMIC satellites, the tiny ionosphere photometer (TIP) and the tri-band beacon.

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