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1.
An index of ENSO in the Pacific during early boreal winter is shown to account for a significant part of the variability of coastal SST anomalies measured a few months later within the wind driven West African coastal upwelling region from 10°N to 26°N. This teleconnection is thought to result from an atmospheric bridge between the Pacific and Atlantic oceans, leading to warm (cold) ENSO events being associated with a relaxation (intensification) of the Atlantic trade winds and of the wind-induced coastal upwelling. This ENSO related modulation of the wind-driven coastal upwelling appears to contribute to the connection observed at the basin-scale between ENSO and SST in the north Atlantic. The ability to use this teleconnection to give warnings of large changes in the West African upwelling several months in advance is successfully tested using data from the 1998 and 1999 ENSO events. 相似文献
2.
Mahlalela P. T. Blamey R. C. Hart N. C. G. Reason C. J. C. 《Climate Dynamics》2020,55(9-10):2743-2759
Much of the Eastern Cape province in South Africa has been experiencing a severe drought since 2015. This drought has had major socio-economic effects particularly on the large impoverished rural population as well as on some urban areas where supplied water services have broken down in several cases. The region is influenced by both midlatitude and tropical systems leading to a complex regional meteorology that hitherto has not been much studied compared to other parts of South Africa. Here, the ongoing drought is examined in the context of long-term trends and the interannual rainfall variability of the region. Although the region has experienced drought in all seasons since 2015, focus here is placed on the spring (September–November) which shows the most consistent and robust signal. On average, this season contributes between about 25–35% of the annual rainfall total. Based on CHIRPS data, it is found that this season shows a significant decreasing trend in both rainfall totals as well as the number of rainfall days (but not heavy rainfall days) for spring over most of the province since 1981. On interannual time scales, the results indicate that dry (wet) springs over the Eastern Cape are associated with a cyclonic (anticyclonic) anomaly southeast of South Africa as part of a shift in the zonal wavenumber 3 pattern in the midlatitudes. Over the landmass, a stronger (weaker) Botswana High is also apparent with increased (decreased) subsidence over and near the Eastern Cape which is less (more) favourable for cloud band development and hence reduced (enhanced) rainfall during dry (wet) springs. Analysis of mid-century (2040–2060) CMIP5 rainfall projections suggests that there may be a flattening of the annual cycle over the Eastern Cape with the winter becoming wetter and the summer drier. For the spring season of interest here, the multi-model projections also indicate drying but less pronounced than that projected for the summer.
相似文献3.
Nicolas Fauchereau B. Pohl C. J. C. Reason M. Rouault Y. Richard 《Climate Dynamics》2009,32(4):575-591
A cluster analysis of daily outgoing longwave radiation (OLR) anomalies from 1979 to 2002 over the Southern Africa/Southwest
Indian Ocean (SWIO) region for the November to February season reveals seven robust and statistically well separated recurrent
patterns of large-scale organized convection. Among them are three regimes indicative of well defined tropical–temperate interactions
linking the hinterland parts of Southern Africa to the mid-latitudes of the SWIO. Preferred transitions show a tendency for
an eastward propagation of these systems. Analysis of daily rainfall records for South Africa shows that six of the OLR regimes
are associated with spatially coherent and significant patterns of enhanced or reduced daily rainfall over the country. Atmospheric
anomalies from the NCEP/DOE II reanalysis dataset show that the OLR regimes are associated with either regional or near-global
adjustments of the atmospheric circulation, the three regimes representative of tropical–temperate interactions being in particular
related to a well-defined wave structure encompassing the subtropical and temperate latitudes, featuring strong vertical anomalies
and strong poleward export of momentum in the lee of the location of the cloud-band. The time-series of OLR regimes seasonal
frequency are correlated to distinctive anomaly patterns in the global sea-surface-temperature field, among which are shown
to be those corresponding to El Nino and La Nina conditions. The spatial signature of El Nino Southern Oscillation’s (ENSO)
influence is related to the combination of an increased/decreased frequency of these regimes. It is shown in particular that
the well-known “dipole” in convection anomalies contrasting Southern Africa and the SWIO during ENSO events arises as an effect
of seasonal averaging and is therefore not valid at the synoptic scale. This study also provides a framework to better understand
the observed non-linearities between ENSO and the seasonal convection and rainfall anomalies over the region. 相似文献
4.
Holton L. Deshayes J. Backeberg B. C. Loveday B. R. Hermes J. C. Reason C. J. C. 《Climate Dynamics》2017,48(7-8):2107-2121
Climate Dynamics - Investigating the variability of Agulhas leakage, the volume transport of water from the Indian Ocean to the South Atlantic Ocean, is highly relevant due to its potential... 相似文献
5.
A regional ocean model was used to simulate the annual cycle of the upper-ocean dynamics and its influence on ocean properties in the tropical western Indian Ocean. Surface winds and heat fluxes from the National Centers for Environmental Prediction (NCEP) reanalysis forced the model (Model_NCEP) with initial and lateral boundary conditions derived from the Simple Ocean Data Assimilation (SODA). The model findings were in good agreement with previous research, satellite and observational data as well as another model configuration forced by Comprehensive Ocean and Atmosphere Data Sets (COADS). The initial and lateral boundary conditions for Model_COADS were extracted from World Ocean Atlas 2001. Anticyclonic wind stress curl occurred to the north of Madagascar, and extended towards the Tanzanian coast throughout the year, leading to Ekman convergence and downwelling in that region. The lowest sea-surface height values during the year occurred between 5° and 12° S with an elongated and contracted shape. The East African Coastal Current (EACC) was in phase with the westward North-East Madagascar Current (NEMC) throughout the year with volume transports peaking in June through July in the model forced by NCEP reanalysis. The variability of the volume transport, ocean currents, temperature and salinity to the north of Madagascar on the path of the NEMC mirrored those in the middle Tanzanian shelf on the path of the EACC throughout the year. The NEMC seemed to influence the water masses on the Tanzanian shelf, with cooler and lower-salinity water in the South-West Monsoon, and warmer and saltier water during the North-East Monsoon. 相似文献
6.
Summary This study investigates the onset and cessation dates of the main summer rainy season over Zambia, their interannual variability,
and potential relationships with ENSO and regional circulation anomalies. Focus is placed on onset and cessation dates because
these rainy season characteristics are often of more relevance than seasonal rainfall totals to user groups such as farmers,
water resource managers, health and tourism officials.
It is found that there is substantial interannual variability in these parameters with some indications of a relationship
between anomalies in onset date and those in Ni?o3.4 SST, particularly over the northern part of the country. A strong gradient
exists between the south and the north in terms of rainfall amount, mean onset date and mean cessation date and all areas
of the country experience significant variability.
Analysis of circulation anomalies for early (late) onset seasons over northern Zambia shows that they are characterised by
anomalous ridging (troughing) over and south of South Africa, a weaker (stronger) Angola heat low and enhanced (reduced) low
level moisture flux into eastern Zambia from the Indian Ocean. The connection with ENSO during the onset season of austral
spring appears to arise both through changes in the amount of subsidence over southern Africa as well as via the so-called
Pacific South America pattern that extends across the South Pacific and South Atlantic towards southern Africa.
Authors’ address: S. Hachigonta, C. J. C. Reason, M. Tadross, Department of Oceanography, University of Cape Town, Private
Bag, Rondebosch 7701, South Africa. 相似文献
7.
Analysis of three-dimensional wind profiles recorded by an acoustic sounder near Cape Town has indicated that extreme subsidence (-35 cm s-1) is a mean feature throughout the atmospheric boundary layer (50–1000m) during summertime southerly winds. Over the SW Cape coast, the atmospheric subsidence translates into a N-S gradient of the mean summer water deficit (-20 to -32 cm month-1). The rapid drying out of the air mass along a northward trajectory is linked to a number of factors including synoptic-scale divergence of the surface wind and the effects of the local orography which produce a hydraulic jump of the southerly wind. The along-coast reduction in sea surface temperature provides a major constraint on the height of the moist marine layer. As the depth of the marine air mass shrinks, its potential for inland penetration becomes limited. In addition, dry air is entrained towards the surface as evidenced by aerial survey data. A model is formulated which indicates the importance of the surface heat fluxes in reducing the depth of the Agulhas air mass as it passes northward over the SW tip of Africa during summer. 相似文献
8.
9.
C. J. C. Reason 《Meteorology and Atmospheric Physics》1998,69(1-2):49-65
Summary An important pattern of interannual variability in the southern African region is one where sea surface temperature (SST) in neighbouring waters, particularly in the Agulhas Current, its retroflection region and outflow across the southern midlatitudes of the Indian Ocean, is anomalously warm or cool. Evidence exists of significant rainfall anomalies over large parts of southern Africa during these warm or cool SST events. Here, a general circulation model is used to study the response of the atmosphere in the region to an idealised representation of these SST anomalies. The induced atmospheric circulation and precipitation anomalies over the adjacent southern African landmass on intraseasonal through to interannual time scales are investigated.A nonlinear response to the SST anomalies is found in that the changes to the model atmosphere when warm SST forcing is used are not the reverse (in either pattern or magnitude) to that when cold SST forcing is imposed. For the warm SST anomaly, it is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and including, annual. However, as the scale becomes interannual (i.e., 15–21 months after imposition of the anomaly), the model response suggests that damping of the original SST anomaly becomes likely. However, no such coherent timescale dependent response is found when the cold SST anomaly is impose. It is suggested that the relationship of the SST anomaly to the background seasonal climatology may help explain this fundamental difference in the response.Examination of the circulation and rainfall patterns under warm SST forcing indicates that there are significant anomalies over large parts of southern Africa on all scales from intraseasonal through to interannual. On the south coast, rainfall anomalies result from enhanced evaporation of moisture off the SST anomaly. Over the interior, changer in the convergence of moist air streams together with suggestions of a shift in the Walker circulations between southern Africa and the bordering tropical South Atlantic and Indian Oceans appear to be associated with the rainfall anomalies. Similar mechanisms of rainfall perturbation are found when the cold SST anomaly is imposed; however, there is a significant response only on intra-annual to interannual scales. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90–180 mm, and therefore represent a significant fraction of the annual total of many areas. These model results re-inforce previous observational work suggesting that SST anomalies south of Africa, particularly in the retroflection region of the Agulhas Current, are linked with significant rainfall anomalies over the adjacent subcontinent.With 12 Figures 相似文献
10.
Theoretical and Applied Climatology - Variability in summer rainfall over tropical Australia, defined here as that part of the continent north of 25° S, and its linkages with regional... 相似文献