C. J. C. Reason

Relationships between south African rainfall and SST anomalies in the Southwest Indian Ocean

Previous work has indicated the importance of sea surface temperature (SST) anomalies in the Southwest Indian Ocean for rainfall variability over South Africa. Here, these links are further explored via bandpass filtering of the rainfall and SST data in several bands on the interannual and interdecadal scale that are known to be significant for global climate variability. This procedure suggests that warmer SST in the Southwest Indian Ocean tends to be associated with wetter conditions over eastern and central South Africa and vice versa. n n n nAn ensemble of experiments with an atmospheric general circulation model forced by an idealization of the warming in the Southwest Indian Ocean leads to statistically significant rainfall increases over large areas of eastern South Africa and neighbouring regions. The mechanism appears to involve changes in the convergence of moist air streams originating from the Indian Ocean and from tropical southern Africa. The magnitude of the rainfall anomalies accumulated over a 90 day season was of the order of 90–300 mm and, therefore, represent a significant fraction of the annual total. These model results reinforce the observational work suggesting that SST anomalies in the Southwest Indian Ocean are linked with significant rainfall anomalies over eastern South Africa. Copyright

Subtropical Indian Ocean SST dipole events and southern African rainfall

An atmospheric general circulation model (AGCM) is used to examine how the regional atmospheric circulation and rainfall over southern Africa respond to a recently observed dipole in subtropical sea surface temperature (SST) over the South Indian Ocean. Observations suggest that when SST is warm to the south of Madagascar and cool off Western Australia, increased summer rains occur over large areas of southeastern Africa. The model results suggest that this SST pattern leads to increased rainfall via enhanced convergence of moister than average air over the region. Increased evaporation occurs over the warm pole in the South West Indian Ocean and this moist air is advected towards Mozambique and eastern South Africa as a result of the low pressure anomaly generated over this pole which strengthens the onshore flow.

Links between the Antarctic Oscillation and winter rainfall over western South Africa

[1]xa0Relationships between the Antarctic Oscillation (AAO) and winter rainfall over western South Africa are investigated. This region receives most of its annual rainfall during austral winter and often experiences severe drought. It is found that 6 (6) of the 7 (8) wettest (driest) winters (JJA) during 1948–2004 occur during negative (positive) AAO phase, i.e., positive (negative) pressure anomalies over Antarctica and negative (positive) anomalies over the midlatitudes. The mechanisms by which the AAO appears to influence winter rainfall involve shifts in the subtropical jet, and changes in the low-level moisture flux upstream over the South Atlantic and in the mid-level uplift, low-level convergence and relative vorticity over the region. The anomalous circulation patterns extend into spring; thus, the springs following the identified winters also show similar rainfall anomalies.

The source of Benguela Niños in the South Atlantic Ocean

[1]xa0The intermittent occurrence of anomalous warm events in the upwelling regions of the Pacific – El Ninos – has been intensively studied. Pacific Ninos have striking effects on the local ecosystem, hence on the fisheries, and on rainfall. Similar dramatic events have been observed in the South Atlantic off the coasts of Angola and Namibia and named Benguela Ninos. They tend not to occur in unison with their Pacific counterpart and may thus have unrelated forcing mechanisms. Using an ocean general circulation model, forced by real winds and verified with satellite data, it is shown that Benguela Ninos are generated by specific wind stress events in the west-central equatorial Atlantic, and progress from there as subsurface temperature anomalies that eventually outcropped only at the south-west African coast. These results suggest that it now may be possible to predict the occurrence of these disruptive events with a lead-time of 2 months.

Annual cycle of the South Indian Ocean (Seychelles‐Chagos) thermocline ridge in a regional ocean model

[1]xa0The presence of an upwelling “dome”-like feature in the thermocline depth at 55°E–65°E, 5°S–12°S in the southwest tropical Indian Ocean (SWTIO) has been suggested in earlier work. However, the position, shape, and forcing mechanisms behind this upwelling region are not well understood. In this study, a regional ocean model is applied to the tropical South Indian Ocean. Experiments with monthly climatological winds from both Quick Scatterometer (QuikSCAT) and the National Centers for Environmental Protection (NCEP) reanalyses are performed. An annual and semiannual signal is present in the depth of the model thermocline. The model results suggest that SWTIO upwelling is focused in the west during austral spring and summer and forms a zonally elongated ridge during austral autumn and winter, termed here the Seychelles-Chagos thermocline ridge. Although the large-scale wind stress curl plays a major role in maintaining this upwelling ridge, local divergence between the southeasterly trade winds and the monsoon westerlies are shown to impact this region, as well as remote forcing through the arrival of both upwelling and downwelling annual Rossby waves from the eastern Indian Ocean.

ENSO‐like decadal variability and South African rainfall

[1]xa0Decadal to multidecadal ‘ENSO-like’ SST and circulation patterns in joint EOFs of global SST and MSLP, may be linked to South African rainfall variability. For eastern/northern South Africa (summer rainfall), the average of the 9–13 year and 18–39 year bandpass filtered EOFs tracks the rainfall well for 1903–1998. Higher (lower) pressure, warmer (cooler) SST in the Pacific and Indian Oceans and offshore (onshore) winds characterise the positive (negative) EOF signal and decreased (increased) rain. For southwestern South Africa (winter rainfall), the average of the 13–18 and 18–39 year filtered EOFs tracks the rainfall well from 1903–1982 with the 9–13 year EOF working better from 1983–1998. The suggested mechanism involves shifts in the upper level jet and in the strength of the subtropical high pressure belt and circumpolar trough.

On the roles of the northeast cold surge, the Borneo vortex, the Madden-Julian Oscillation, and the Indian Ocean Dipole during the extreme 2006/2007 flood in southern Peninsular Malaysia

The mid-December 2006 to late January 2007 flood in southern Peninsular Malaysia was the worst flood in a century and was caused by three extreme precipitation episodes. These extreme precipitation events were mainly associated with strong northeasterly winds over the South China Sea. In all cases, the northeasterlies penetrated anomalously far south and followed almost a straight trajectory. The elevated terrain over Sumatra and southern Peninsular Malaysia caused low-level convergence. The strong easterly winds near Java associated with the Rossby wave-type response to Madden-Julian Oscillation (MJO) inhibited the counter-clockwise turning of the northeasterlies and the formation of the Borneo vortex, which, in turn, enhanced the low-level convergence over the region. The abrupt termination of the Indian Ocean Dipole (IOD) in December 2006 played a secondary role as warmer equatorial Indian Ocean helped in the MJO formation.

Variability in satellite winds over the Benguela upwelling system during 1999–2000

[1]xa0Wind stress variability over the Benguela upwelling system is considered using 16 months (01 August 1999 to 29 November 2000) of satellite-derived QuikSCAT wind data. Variability is investigated using a type of artificial neural network, the self-organizing map (SOM), and a wavelet analysis. The SOM and wavelet analysis are applied to an extracted data set to find that the system may be divided into six discrete wind regimes. The wavelet power spectra for these wind regions span a range of frequencies from 4 to 64 days, with each region appearing to contain distinct periodicities. To the north, 10°–23.5°S, the majority of the power occurs during austral winter, with a 4–16 day periodicity. Further investigation of National Centers for Environmental Prediction reanalysis outgoing longwave radiation data indicates that the winter intensification of wind stress off the Angolan coast is linked with convective activity over equatorial West Africa. The summer activity appears to be linked with the intensification of the Angolan heat low. Convective activity over the Congo basin appears to impact upon wind stress variability, off the Angolan coast, throughout the year. Farther south, 24°–35°S, the majority of the power occurs in the summer. Here a bimodal distribution occurs, with peaks of 4–12 and 25–50 days. The southernmost regions appear to be forced at higher frequencies by both midlatitude cyclones (austral winter) and mesoscale coastal lows (austral summer). At lower frequencies, eastward propagating periodic wind events that originate over eastern South America appear to be important to the forcing of wind stress over the southern Benguela.

South Atlantic response to El Niño–Southern Oscillation induced climate variability in an ocean general circulation model

[1]xa0The response of the South Atlantic Ocean to El Nino-Southern Oscillation (ENSO) is investigated by means of an ocean general circulation model (ORCA2) forced with National Centers for Environmental Prediction (NCEP) reanalyses for the 1948–1999 period. Seasonal ENSO composites suggest that the ENSO-induced wind anomalies play a major role in driving upper ocean temperatures by altering the net surface heat fluxes, the meridional Ekman heat transport, and Ekman pumping. Model diagnostics indicate that the Ekman heat transport changes are in better agreement with the upper ocean temperature anomalies during the first half of the ENSO event whereas, in the latter half, the surface heat flux anomalies agree better. In general, the atmospheric forcing tends to lead to a coherent ocean response with a time lag of about one season. Subsurface temperatures evolve more slowly in response to ENSO forcing than the upper ocean. They receive time-filtered ENSO signals from mainly Ekman pumping (suction) and variations in thermocline depth that result in the poleward and equatorward margins of the subtropical gyre exhibiting temperature anomalies of the same sign but opposite to those in the central regions of the gyre.

A connection between the South Equatorial Current north of Madagascar and Mozambique Channel Eddies

[1]xa0Combining high resolution model output and geostrophic currents derived from satellite altimeter data, it is shown that the formation of mesoscale eddies in the Mozambique Channel (MZC) is connected to variability in the transport of the South Equatorial Current (SEC). Lagged cross-correlations of the currents north of Madagascar and vorticities in the MZC, combined with a composite analysis of the model output, show that eddies form in the narrows of the channel approximately 20 weeks following a westward transport pulse in the SEC. A relationship between MZC eddies and the large-scale variability of the South Indian Ocean may have downstream impacts on the Agulhas leakage, the Atlantic Meridional overturning circulation, and thus climate.