Interannual Variability In Onset Research Articles

Inter Annual Variability of Onset and Cessation of the Long Rains in Kenya

This study examined the spatial and temporal variability of the onset and cessation of long rains (MAM). The daily rainfall data used in the study was obtained from the Kenya Meteorological Department (KMD. Both graphical and statistical methods were applied in the analyses. Most of the stations indicated the onset month to be March. However, a few stations indicated February as their onset month while a few others had April as the onset month. Further analysis indicated a link between performance of the seasonal rainfall and the onset date. Thus positive (negative) rainfall anomaly was associated with early (late) onset. The results from the analysis showed that over most stations, the mean onset is the 74th day (14th march) with an average standard deviation of 24 days. The mean cessation day was day 150 (29th of May) with an average standard deviation of 21 days. The highest variability was observed in dry areas. The study showed that over most parts of the country there is very high likelihood of below normal rainfall when the onset is late. However, over the north western part of the country (region 1) which receive it seasonal rainfall over a short period, the rainfall performance may not be affected by the late the onset It was also noted that over most regions there is high chances of near normal rainfall occurring when the onset is normal. The chances of below normal rainfall occurring when the onset is normal are low. It was noted that whereas the chances of below normal rainfall occurring when there is an early onset were minimal, The western region (region 12) represented by Elgon had substantive chance of having below normal rainfall even with early onset. The onset over this region is associated with the low level westerlies from Congo, while the performance of the seasonal rainfall over this region is largely linked with the characteristics of the ITCZ.

The onset, retreat and the length of growing season in the north-eastern region of Sri Lanka

The onset, retreat and the length of growing season in the north-eastern region of Sri Lanka were investigated using daily rainfall data for the period 1961 to 2000. Data from three weather stations situated in the coastal belt in the northern and eastern parts of Sri Lanka (Jaffna, Trincomalee and Batticaloa) that receive rainfall predominantly from the northeast monsoon were selected for this study. A method based on cumulative rainy days was utilized in the determination of the onset and retreat dates. It is shown that there is substantial interannual variability in onset and retreat dates. The mean onset and retreat dates fall on the standard week 38.3 ± 2.7 and 53.0 ± 2.9, respectively. The mean duration of the growing season is 14.7 ± 3.4 weeks. The retreat date and thus the length of growing season could be extended by 2 weeks if the probability of occurrence of rain during the onset is favourable for the retreat. The results indicate that there has been no significant trend in the onset and retreat dates during the last 40 years in the dry zone of Sri Lanka. The onset date and the length of growing season are weakly correlated with early onset dates leading to longer growing seasons. The study concludes that rainy days could be used successfully to determine the mean rainfall onset and retreat dates in the dry zone of Sri Lanka.

Application of daily rainfall principal component analysis to the assessment of the rainy season characteristics in Senegal

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 23:159-169 (2003) - doi:10.3354/cr023159 Application of daily rainfall principal component analysis to the assessment of the rainy season characteristics in Senegal Pierre Camberlin1,*, Mbaye Diop2 1UMR 5080 CNRS, Centre de Recherches de Climatologie, Université de Bourgogne, 6 Bd Gabriel, 21000 Dijon, France 2Laboratoire d¹Enseignement et de Recherche en Géomatique (LERG), Campus universitaire de l¹ESP, BP 25275 Dakar-Fann, Sénégal *Email: camber@u-bourgogne.fr ABSTRACT: The interannual variability of the onset and cessation dates of the rainy season (RS) in Senegal is analyzed over the 43 yr period 1950-1992, using daily rainfall data for 34 stations. The use of principal component analysis, based on rainfall only, is explored to identify aggregate, regional indexes for the onset and cessation of the rains. The minimum and maximum values of the cumulative scores of principal component 1, for each year, are used to locate the onset and cessation dates, respectively. Very distinct spatial rainfall patterns are found before and after the onset/cessation dates. Mean dates compare favorably with those based on other definitions, though our method is not meant to provide dates with a local, rigorous agronomic significance. Over 1950-1992, the time-series for the cessation shows a significant trend towards earlier dates, with an abrupt shift occurring around 1970. There is also a small trend for a delayed onset of the rains. However, the interannual variability of onset is greater than that of cessation dates. Correlation between onset, cessation, duration and total rainfall amounts is generally significant, but not as high as expected. 1988 and 1989 are good illustrations, with nearly identical rainfall amounts but an onset delayed by 1.5 mo in 1988 as compared to 1989. Graphs obtained using this method provide other interesting information about the RS, such as the general performance of the RS in terms of total rainfall amount. Pronounced dry spells are easily located: 29 cases were found between 1950 and 1992, concentrated in the early part of the RS (June-July). ŒFalse starts¹ of the rains can also be monitored; they were reported in 16 out of the 43 years. Frequencies of dry spells and false starts do not exhibit any particular long-term trends. KEY WORDS: Interannual rainfall variability · Onset and cessation of the rains · Senegal · Spatial patterns · Dry spells Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 23, No. 2. Online publication date: January 31, 2003 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2003 Inter-Research.

A continental phenology model for monitoring vegetation responses to interannual climatic variability

Regional phenology is important in ecosystem simulation models and coupled biosphere/atmosphere models. In the continental United States, the timing of the onset of greenness in the spring (leaf expansion, grass green‐up) and offset of greenness in the fall (leaf abscission, cessation of height growth, grass brown‐off) are strongly influenced by meteorological and climatological conditions. We developed predictive phenology models based on traditional phenology research using commonly available meteorological and climatological data. Predictions were compared with satellite phenology observations at numerous 20 km × 20 km contiguous landcover sites. Onset mean absolute error was 7.2 days in the deciduous broadleaf forest (DBF) biome and 6.1 days in the grassland biome. Offset mean absolute error was 5.3 days in the DBF biome and 6.3 days in the grassland biome. Maximum expected errors at a 95% probability level ranged from 10 to 14 days. Onset was strongly associated with temperature summations in both grassland and DBF biomes; DBF offset was best predicted with a photoperiod function, while grassland offset required a combination of precipitation and temperature controls. A long‐term regional test of the DBF onset model captured field‐measured interannual variability trends in lilac phenology. Continental application of the phenology models for 1990–1992 revealed extensive interannual variability in onset and offset. Median continental growing season length ranged from a low of 129 days in 1991 to a high of 146 days in 1992. Potential uses of the models include regulation of the timing and length of the growing season in large‐scale biogeochemical models and monitoring vegetation response to interannual climatic variability.

Recent changes in productivity and climate of Lake Ontario detected by isotopic analysis of sediments

Cores from Lake Ontario provide a record of lake responses to increased phosphorus loading that began after early forest clearance in the mid‐1800s. We measured 13C/12C ratios of organic and inorganic C (calcite) in the sediments to infer recent trends in primary production. The δ13C of organic C increased in response to historic increases in P enrichment until the mid‐1970s and then decreased in response to reduced P loading. The δ13C of calcite increased after 1940 but did not decrease in response to reduced P loading. We conclude that the results differ because δ13C of organic C is determined by primary production early in the seasonal cycle when flux of organic C to the sediments is greatest, whereas δ13C of calcite depends on the isotopic composition of dissolved inorganic C during late summer when calcite is precipitated.Calcite accumulation and its δ13C ratio oscillated after 1940 with peaks estimated at 1948, 1966, 1977, and 1983. These peaks correspond to minima in δ18O values, indicating calcite precipitation at higher temperature, and to years of early thermal stratification. We hypothesize that calcite accumulation and its stable isotopic ratios are controlled by interannual variability in onset of thermal stratification of the lake. Our results thus suggest a direct link from climate (i.e. thermal stratification) to interannual variation in lake productivity and to carbonate precipitation and its δ13C signature.