Abstract ID
climch13
Type
Oral
Theme
climch
Full title
What causes the variability in Okavango River flow?
Abstract text
The annual cycle of the Okavango River discharge is the “heart beat― of the Okavango delta pulse flood
system. However, the river flow exhibits pronounced variability at a range of timescales beyond the annual
cycle. This has long been recognised both in the timeseries of river flow above the delta panhandle and in
records of the extent of delta flood. The low flow and suppressed delta flooding in the mid 1990s is a well
known expression of the variability. This variability in the extent of floodplain inundation contributes to high
habitat diversity across the Okavango Delta. However, to date, little systematic evaluation has been undertaken
to determine which climatic processes over the upstream basin actually cause the variability in river flow.
Understanding variability in tropical convective rainfall over the basin region in Angola is important for
interpreting climate model projections of future changes in rainfall and hence the Okavango River flow. In this
paper we use the long term record of peak season (Jan-June) river flow at Mohembo from 1933-2008 to
determine the climatic drivers of variability by focussing on the role of the global oceans and large scale
atmospheric circulation patterns. Results indicate that variability in Okavango river flow is the result of a
complex set of time-varying climate processes operating at different timescales. The very highest frequency
component of Okavango River flow variability (less than 4 years) is negatively associated with Sea Surface
Temperatures (SSTs) over the equatorial Atlantic during the early austral summer period. This is consistent
with behaviour of the Atlantic Nino II mode of ocean variability and associated precipitation variability over the
adjacent African continent. The influence of this mode has, however, declined in recent decades. In addition,
individual Benguela Nino warm events can result in increased river discharge, as in 2004, but the relationship is
inconsistent. Generally, low flow years tend to associate with an anomalous westerly circulation around a
strengthened Angola Low, which reduces moisture transport from the east, thereby limiting convective rainfall
over the basin. This is particularly apparent during the dry 1990s. Lower frequency variability at multi-annual
timescales (~3-8 yrs) has a weak association with SST patterns characteristic of the El Nino/Southern
Oscillation (ENSO) phenomenon, such that ENSO warm events weakly associate with low river flow, consistent
with most of the rest of Southern Africa. There is evidence of lower frequency variability in the Okavango river
flow but the short record renders robust analysis of these timescales problematic. The lowest frequency cycle
with a period of about 60 years (not statistically significant) is out of phase with the Atlantic Multidecadal
Oscillation (AMO), known to influence rainfall over the Sahel and Northeast Brazil, and lags the Pacific
Decadal Oscillation by about 5 years. Overall, the absence of a clear, dominant control on river flow is partly
the result of the location of the river basin, which straddles a sharp rainfall gradient, under the influence of both
the Atlantic and Indian oceans which exhibit independent patterns of variability as well as modulation from the
Pacific sector. Paucity of observational data over the region limits analysis that can be done.
Submission date
2009-09-24
Keywords
Okavango River, climate, variability, ENSO, decadal, cyclicity
Will be submitting paper?
Yes