Representation of the Tropical Stratospheric Zonal Wind in GlobalAtmospheric Reanalyses

摘要

This paper reports on a project to compare the representation of the monthly-mean zonal wind in the equatorialstratosphere among major global atmospheric reanalysis datasets. The degree of disagreement among the reanalyses ischaracterized by the standard deviation (SD) of the monthly-mean zonal wind and this depends on latitude, longitude, heightand 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 thegeographical distributions of SD are closely related to the density of radiosonde observations. The largest SD values are overthe eastern Pacific, where few in situ observations are available. At 10–20 hPa the spread among the reanalyses anddifferences with in situ observations both depend significantly on the QBO phase. Notably the easterly-to-westerly phasetransitions in all the reanalyses except MERRA are delayed relative to those directly observed at Singapore. In addition, thetiming of the easterly-to-westerly phase transitions displays considerable variability among the different reanalyses and thisspread is much larger than for the timing of the westerly-to-easterly phase changes. The eddy component in the monthlymean zonal wind near the equator is dominated by zonal wavenumber 1 and 2 quasi-stationary planetary waves propagatingfrom mid-latitudes in the westerly phase of the QBO. There generally is considerable disagreement among the reanalyses inthe details of the quasi-stationary waves near the equator. At each level, there is a tendency for the agreement to be best nearthe longitude of Singapore, suggesting that the Singapore observations act as a strong constraint on all the reanalyses. Ourmeasures of the quality of the reanalysis clearly show systematic improvement over the period considered (1979–2012). TheSD among the reanalysis declines significantly over the record, although the geographical pattern of SD remains nearlyconstant.