Abstract
Abstract. This paper reports on a project to compare the representation of the monthly-mean zonal wind in the equatorial stratosphere among major global atmospheric reanalysis data sets. The degree of disagreement among the reanalyses is characterized by the standard deviation (SD) of the monthly-mean zonal wind and this depends on latitude, longitude, height, and the phase of the quasi-biennial oscillation (QBO). At each height the SD displays a prominent equatorial maximum, indicating the particularly challenging nature of the reanalysis problem in the low-latitude stratosphere. At 50–70 hPa the geographical distributions of SD are closely related to the density of radiosonde observations. The largest SD values are over the central Pacific, where few in situ observations are available. At 10–20 hPa the spread among the reanalyses and differences with in situ observations both depend significantly on the QBO phase. Notably the easterly-to-westerly phase transitions in all the reanalyses except MERRA are delayed relative to those directly observed in Singapore. In addition, the timing of the easterly-to-westerly phase transitions displays considerable variability among the different reanalyses and this spread is much larger than for the timing of the westerly-to-easterly phase changes. The eddy component in the monthly-mean zonal wind near the Equator is dominated by zonal wavenumber 1 and 2 quasi-stationary planetary waves propagating from midlatitudes in the westerly phase of the QBO. There generally is considerable disagreement among the reanalyses in the details of the quasi-stationary waves near the Equator. At each level, there is a tendency for the agreement to be best near the longitude of Singapore, suggesting that the Singapore observations act as a strong constraint on all the reanalyses. Our measures of the quality of the reanalysis clearly show systematic improvement over the period considered (1979–2012). The SD among the reanalysis declines significantly over the record, although the geographical pattern of SD remains nearly constant.
Highlights
The dynamics governing the circulation in the tropical stratosphere have attracted much interest over the years (e.g., Wallace, 1973; Baldwin et al, 2001)
The present paper reports on one of the studies contributing to the SPARC Reanalysis Intercomparison Project (S-RIP; Fujiwara and Jackson, 2013; Errera et al, 2015), which is focused on evaluating reanalysis output for the stratosphere
This paper reports on a project to compare the representation of the monthly-mean zonal wind in the equatorial stratosphere among major global atmospheric reanalyses data sets
Summary
The dynamics governing the circulation in the tropical stratosphere have attracted much interest over the years (e.g., Wallace, 1973; Baldwin et al, 2001). The tropical stratosphere is perhaps the only region of the atmosphere where most free-running GCMs have simulations with zero-order errors in the zonal-mean circulation (i.e., nothing even resembling a realistic QBO) This is because most GCMs display fairly steady, weak prevailing zonal winds due to failure to reproduce a spontaneous QBO. Randel et al (2004) indicated that QBO variations in temperature and zonal wind were underestimated in most reanalyses available at that time, as compared to Singapore radiosonde data They found that only ERA-40 had realistic zonal wind amplitudes above 30 hPa. Some centers have produced multiple reanalyses covering the same (or overlapping) periods; these more recent products are derived with updated data assimilation systems.
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