Global Land Precipitation: A 50-yr Monthly Analysis Based on Gauge Observations

Self Cite

333

207

Using station rainfall data extracted from two comprehensive data sets, we show that large decreasing rainfall trends were widespread in the Sahel (10-20°N and 18°W-20°E) from the late 1950s to the late 1980s. Thereafter, Sahel rainfall has recovered somewhat through 2003, although the drought conditions have not ended in the region. These results confirm the findings of many previous studies. We also found that large multi-year oscillations appear to be more frequent and extreme after the late 1980s than previously. Analyses of Sahel regional rainfall time series derived from a fixed subset of stations and from all available stations show that the decreasing trend in Sahel rainfall is not an artifact of changing station networks. The rainfall model used by Chappell and Agnew (2004 International Journal of Climatology 24: 547-554) is incorrect and their modelled rainfall time series is totally unrepresentative of Sahel average rainfall. Their conclusion about the Sahel rainfall trends being an artifact of changing station locations is emphatically wrong and their speculative statements about the implications of their results for other studies and other regions of the world are completely unfounded.

Section: Effects Of Changing Station Networkmentioning

confidence: 99%

Section: Updated Sahel Rainfall Time Series: 1920-2003mentioning

confidence: 99%

The recent Sahel drought is real

Dai

Lamb

Trenberth

et al. 2004

Self Cite

333

207

“…The 1950-98 monthly land precipitation data on a 2.5°× 2.5°grid (Chen et al, 2002) are used as a basic dataset of precipitation for this paper. This gridded field of monthly precipitation is defined by interpolating gauge observations at over 15 000 stations in the world using the optimum interpolation algorithm.…”

Section: Datamentioning

confidence: 99%

Influence of Eurasian spring snow cover on Asian summer rainfall

Liu

Yanai

2002

125

The Eurasian snow cover anomaly in spring has been considered as one of the important factors affecting Asian summer monsoon variability. Using the long time series of Eurasian spring (March-April) snow cover (ESSC) reconstructed by Brown (2000. Journal of Climate 13: 2339) and snow cover (1973-98) and depth (1979-87) data from satellite observation, the influences of ESSC on the all-India monsoon (June-September) rainfall (AIMR) and the summer rainfall over all parts of Asia are examined. It is found that the statistical relation between AIMR and ESSC changes over a multi-decadal time scale. The negative correlation between them has increased markedly since the mid 1970s. The region where the summer rainfall has the strongest and most stable negative correlation with the preceding ESSC is located in northern Mongolia, south of Lake Baikal. The correlation between the summer rainfall and ESSC increases after the data are treated with a low-pass filter, showing that the impact of snow cover may be seen more clearly with the removal of the effect of El Niño-southern oscillation. Comparative analyses for contrasting years with excessive and deficient snow cover show that the anomalies of ESSC occur mainly in northwestern Eurasia. In the years of excessive ESSC anomalies, cooling and a cyclonic circulation anomaly in the lower troposphere appear over the northern part of Eurasia, leading to a Rossby-wave-train-like circulation response, then a weakened East Asia summer monsoon and deficient rainfall with an anticyclonic circulation anomaly south of Lake Baikal. Anomalies with opposite signs occur in the years of deficient snow cover.

“…The station data were extracted from NCDC/GHCN, and gaps were filled using gridded rainfall estimates from NCEP, based on the CMAP reconstruction technique (Xie and Arkin, 1997;Chen et al, 2002;Yin et al, 2004), available at the website http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/. In the study by Jury et al (2007), rainfall data were truncated at 1981, but here, we employ a record extending from 1951 to 2005.…”

Section: Methodsmentioning

confidence: 99%

Joint modes of climate variability across the inter‐Americas

Jury

Malmgren

2011

Surface temperature (Ts), sea-level pressure (SLP), and zonal wind (U) fields from NCEP/NCAR reanalysis spanning the period 1949-2006 have been jointly analysed by means of principal component analysis to assess the dominant modes of climate variability in the inter-American region, including the eastern Pacific and Atlantic Oceans. Two aspects of these time-series were analysed: the annual cycle and its residual. Seasonal modes 1 and 2 take the form of north-south dipoles of Ts over oceanic and terrestrial environments. Seasonal mode 3 derives from SLP changes over the Amazon with a Caribbean wind response. Temporal fluctuations in the residual (ANOM) fields are resolved by six modes. ANOM1-3 is dominated by SLP and Ts changes across the equator in the Atlantic and east Pacific. The ANOM4 pattern is an isolated wind mode in the NE Atlantic. ANOM5 and 6 are dominated by sea temperature patterns in the South Atlantic and in the path of African easterly waves crossing the tropical Atlantic. The residual time scores are marked by significant warming (Ts) trends and co-located lower pressure. Cycles in the anomaly time scores tend to occur at 2-to 5-year periods. Modes dominated by atmospheric variables exhibit greater high-frequency 'noise' than oceanic modes. Predictability is assessed through development of multivariate regression models trained on Caribbean rainfall and related target time-series. The seasonal dipole between North and South America is found to modulate Pacific El Niño-Southern Oscillation and tropical Atlantic influence on Caribbean rainfall.