dc.contributor.author | Gachahi, L W | |
dc.date.accessioned | 2013-05-09T09:06:31Z | |
dc.date.available | 2013-05-09T09:06:31Z | |
dc.date.issued | 2007 | |
dc.identifier.citation | Master of Science in Meteorology. | en |
dc.identifier.uri | http://erepository.uonbi.ac.ke:8080/xmlui/handle/123456789/20680 | |
dc.description.abstract | The onset of the March-May seasonal rainfall season has been observed to be
highly variable. There are years of early, normal and late onset. Prior knowledge of
whether there is going to be early or late onsets can be gained if the atmospheric
circulation patterns associated with the onsets are understood.
In this study the onset of the March-May rainfall season was examined with the
aim of identifying atmospheric circulation patterns, which could influence early and late
onsets.
Forty years of daily and monthly rainfall data for stations spread over the
Equatorial East Africa (EEA) region were used in the study. The onset was determined
through graphical analysis, use of pentad rainfall and monthly-standardized rainfall
anomaly indices.
In general the month of March was selected as the onset month of this season.
Positive (negati~ve) standardized anomaly indices for the month of March were an
indication of an early (late) onset. Analysis of pentad rainfall showed that years of early
onset were those that had rainfall starting on or before the 14th pentad. While the years of
late onset were those whose rainfall started in the is" pentad or later.
Results from the analysis of the QBO phase showed that early (late) onset was
favored by the changing phase from negative (positive) at about the months of
July/August of the year prior to the rainfall season. It was also noted that onset occurred
during the peak negative· phase of the SOL
The results from the wind showed some unique circulation patterns during the
early/late onset. A veering of the southeasterlies to southwesterlies and northeasterlies to
III
northwesterlies with a closed circulation near the equator at about 50S, 400E during the
months of February which intensified in March at the 700mb level was a common feature
in all the selected months of early onset. This mid-level circulation is accompanied by
equatorial easterlies with no meridional component at the 200mb level. These circulations
suggest low level convergence and upper level divergence within the region and with
sufficient moisture could lead to convective activities thus early onset.
The prominent features observed during early onset were either very weak or
absent during years with late onset especially at 700mb level. Instead the northeasterlies
in February were observed in some years to pass over the EEA region and to change to
northwesterlies further south of the equator at about 100S. These NE winds were strong
and almost parallel to the EEA coast. In other years (for example 1984, 1999) the winds
were seen to converge further into the ocean at about 100S, 55°E. The overlying easterlies
at 200mb level have a southerly component.
One other salient feature in the wind circulation influencing late onset is an
anticyclonic circulation in the Arabian region at 700mb level which is conspicuously
absent during early onset. These circulations suggest that Arabian high pressure system is
closer to the equator and stronger in February and March during late onset. This also
explains the presence of the NE observed in the EEA region at this level during this time.
The low level divergence occasioned by the presence of easterlies during the month of
February could explain the delayed onset in the month of March. | en |
dc.description.sponsorship | University of Nairobi | en |
dc.language.iso | en | en |
dc.title | Onset and performance of the March- May seasonal rainfall over the equatorial east African region | en |
dc.type | Thesis | en |
local.publisher | Department of Meteorology University of Nairobi | en |