No abstract

ABSTRACT:The current paper is an observational study that investigates the October to December (OND) rainfall variability over the east Southern African Development Community (SADC) mainland region in relation to El Niño-Southern Oscillation (ENSO) and the Indian Ocean dipole zonal mode (IODZM) for the period 1950-1999. An empirical orthogonal function (EOF) analysis of OND rainfall field revealed that the north-south aligned areas of the eastern SADC are located in different covariability regions. This meridionally aligned dipole rainfall anomaly configuration is captured only in the dominant principal component (PC1), making it possible for the opposing rainfall anomalies of the two regions to have a common trigger. However, ENSO which is the standard attribute for regional rainfall variability failed dismally to adequately explain this dipole rainfall anomaly pattern. Instead, there appears to be consistent evidence through statistical techniques which strongly indicates the likelihood of the participation of only the positive IODZM phase events in the creation of the positive dipole rainfall phase (i.e. simultaneous floods over the northeast and droughts over the southeast of the SADC region). Since the negative IODZM phase events can hardly be linked to the reverse rainfall pattern, it implies that the positive and negative rainfall dipoles have fundamentally different causes. Thus, contrary to convectional knowledge, the ENSO association to this dipole rainfall anomaly pattern is by no means robust and could be symptomatic of the well-known ENSO-IODZM connection. Interestingly, however, when analysed over the 31-year overlapping segments, IODZM's once significant independent influence on this dipole rainfall seems to be diminishing gradually as from the early 1990s, whereas that of ENSO is correspondingly being reinforced.

The decoupling of the Indian Ocean Dipole/zonal mode (IODZM) from southern Africa's dominant mode of rainfall variability since 1997 is investigated. An approach of using variability as the major descriptive parameter for the observational time series is adopted. This approach enables the identification of epochs differentiated by the examination of low‐frequency signals in the climate data of the Indian Ocean basin. The period of analysis stretches from 1922 to 2008 and focuses on the October to December season so as to capture the most active component of the IODZM events that coincide with the early summer rainfall. The period around 1997/1998 is noted as an important climate regime transition in the Indian Ocean climate system whereas the well known 1976/1977 climate shift appears to be of lesser significance in the region. The temporal manifestation of statistical parameters of the IODZM matches significantly with those of basic atmospheric variables in the western subtropical South Indian Ocean and consistently identifies 1961 and 1997 as important turning points in the Indian Ocean climate system. However, although the relatively high variability of the atmospheric variables quickly decreases after 1997, a corresponding fall in the rainfall and IODZM time series variability is not found. Instead, an abrupt reduction in the correlation between the two time series to insignificant values occurs after 1997. Coincidentally, notable cooling over the western South Indian Ocean is observed during the pre‐1997 positive IODZM events, followed by a synchronized anomalous warming and cooling over the respective western and eastern subtropical South Indian Ocean during the events after 1997. This change is associated with an eastward displacement of the South Indian Ocean anticyclone after 1997. Thus, 1997 is a significant year in the Indian Ocean climate system when a discontinuity acts to decouple the IODZM from the dominant mode of regional rainfall variability. Copyright © 2011 Royal Meteorological Society

The Standardized Precipitation Index (SPI) was computed for October to December (OND) and January to March (JFM) summer subseasons for Free State Province, South Africa, to assess the influence of altitude on drought severity and frequency. The observed spatiotemporal heterogeneity in the SPI variability revealed that factors governing drought interannual variability varied markedly within the region for the two subseasons. Strong correlations between = 0.76 and 0.93 across the clusters in both subseasons were observed. Significant shift in average SPI, towards the high during the OND subseason, was detected for the far western lowlying and central regions of the province around the 1990s. An ANOVA test revealed a significant relationship between drought severity and altitude during the OND subseason only. The impact of altitude is partly manifested in the strong relationship between meridional winds and SPI extremes. When the winds are largely northerly, Free State lies predominantly in the windward side of the Drakensberg Mountains but lies in the rain shadow when the winds are mostly southerly. The relationship between ENSO and SPI indicates stronger correlations for the early summer subseason than for the late summer subseason while overall presenting a diminishing intensity with height over the province.

We use reanalysis and observational data to link the lower stratospheric ozone regulation of the ultraviolet radiation (UV-B) component of solar energy to ENSO modulation. Results indicate that during ENSO extremes, the Walker Circulation (WC) and Brewer Dobson Circulation are related to lower stratospheric ozone alterations east of the date line over the Pacific. These in turn are linked to upper tropospheric anomalous dipole temperature patterns on either side of the equator. The ensuing changes in geopotential height values do not only drive equatorial zonal wind anomalies in the upper troposphere that are reversed at the equatorial surface, but also impact on the intensity of the South Pacific High circulation. When the WC is enhanced, a La Nina type of circulation is indentified but if the circulation cell is inverted, the anomalous circulation results in an El Nino. Though the anomalous lower stratospheric ozone peaks during austral summer it is significant throughout the ENSO lifecycle. Hence, ENSO structure and variability are mainly linked to the lower stratospheric ozone instigated internal dynamics of the Pacific atmosphere. The ENSO forcing most likely originates from the ozone related regulation of the incoming solar UV-B radiation rather than the Pacific Ocean surface.

Zimbabwe's homogeneous precipitation regions are investigated by means of principal component analysis (PCA) with regard to the underlying processes related to ENSO and/or Indian Ocean Dipole zonal mode (IODZM). Station standardized precipitation index rather than direct rainfall values represent the data matrix used in the PCA. The results indicate that the country's rainfall is highly homogeneous and is dominantly described by the first principal mode (PC1). This leading PC can be used to represent the major rainfall patterns affecting the country, both spatially and temporarily. The current practice of subdividing the country into the two seasonal rainfall forecast zones becomes irrelevant. Partial correlation analysis shows that PC1 is linked more to the IODZM than to the traditional ENSO which predominantly demonstrates insignificant association with PC1. The pure IODZM composite is linked to the most intense rainfall suppression mechanisms, while the pure El Niño composite is linked to rainfall enhancing mechanisms.

No abstract

No abstract