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Warren J. Tennant
and
Chris J. C. Reason

Abstract

Large-scale atmospheric processes in the Southern Hemisphere are examined on both seasonal and daily time scales in order to seek associations between these and regional rainfall variability in the summer rainfall areas of South Africa and the winter rainfall regions of South Africa and Western Australia. The basis of the analysis is atmospheric energetics of the vertical mean and shear flow. Self-organizing maps (SOMs) are then used to find archetypical states of the daily flow and to assess how the frequency characteristics of these states change between wet and dry years.

The results show clear associations between the frequency of circulation archetypes on a hemispheric scale and regional rainfall for both summer and winter rainfall areas. Substantial changes in archetype frequencies between wet and dry years are found with as much as a doubling or halving of the number of days in which certain archetypes occur within a season. The physical reasons for observed teleconnections are shown by way of the atmospheric energy cycle, providing a deeper understanding of climate variability that may benefit extended-range prediction.

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Alice M. Grimm
and
Chris J. C. Reason

Abstract

Teleconnections between the South American monsoon and southern African rainfall are investigated for years with Benguela Niño or Niña events in the South Atlantic. During these events, it is found that substantial rainfall anomalies also occur over South America in addition to those previously known for southern Africa. The appearance of large rainfall anomalies in the South American monsoon region prior to the onset of the Benguela Niño proper suggests that anomalous convection over South America may influence the evolution of both the SST anomalies and the African rainfall anomalies associated with Benguela Niño events. This teleconnection between South America and southern African rainfall may occur directly, via atmospheric circulation anomalies induced by convection over South America, or indirectly, via the effect of induced circulation anomalies on regional SST.

To investigate these teleconnections, a vorticity equation model, which is linearized about a realistic basic state and which includes the divergence in this state and the advection of vorticity by the divergent wind, is applied to the events. The model is forced with anomalous divergence patterns observed during the events, and the steady-state solutions show that anomalies of convection during the South American monsoon produce the main circulation anomalies observed during the Benguela Niño events and hence influence rainfall and circulation patterns over Angola and other southern African countries. An influence function analysis confirms this result, indicating that South America is the most efficient source region to produce the observed anomalies, and also shows that there is no influence of convection over Africa on the South American monsoon. Based on these linear model and observational results, it is concluded that the South American monsoon can influence the evolution of Benguela Niños and associated rainfall anomalies in southern Africa.

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Robert J. Allan
,
Janette A. Lindesay
, and
Chris J. C. Reason

Abstract

Several independent historical studies of global atmospheric and oceanic parameters have identified low-frequency fluctuations in the global climate system. Much of this research has focused on Europe, the Atlantic Ocean, and North America. However, recent interest has begun to encompass decadal to multidecadal variability across the Indo-Pacific region. Such variability has been detected in sea surface temperature (SST), mean sea level pressure (MSLP), and surface wind fields over both the landmasses and the oceans.

Around the Indian Ocean basin, the broad periods before and after the 1940s show important differences in features such as Indian southwest monsoonal rainfall and circulation patterns, relationships between austral summer rainfall in southern Africa and the El Ni˜o–Southern Oscillation phenomenon, and Australasian MSLP. Very little is known about this variability, particularly during the austral summer. In an effort to isolate such fluctuations and work toward understanding the physical dynamics operating on such timescales, SST, MSLP, atmospheric circulation, vertical motion, and cloudiness anomalies are constructed and analyzed for austral summer (JFM) conditions over the Indian Ocean region during four 21-yr epochs since 1900.

The results of this research suggest that SSTs were cooler at midlatitudes and warmer in the subtropical southern Indian Ocean in the periods 1900–20 and 1921–41, compared with the 1942–62 and 1963–83 epochs. The most pronounced changes are found along the Agulhas Current outflow zone across the midlatitudes of the southwest Indian Ocean, with indications of coherent SST fluctuations in the northwest regions of the basin and in the northwest Pacific Ocean. Changes in surface wind anomalies are also observed. During 1900–20 and 1921–41, an anomalous atmospheric cyclonic feature is seen over the southern Indian Ocean, while in the later 1963–83 period, a distinct anticyclonic anomaly is evident in this region. This change suggests that the semi-permanent anticyclone in the mean flow field of the atmosphere over the southern Indian Ocean in JFM was weaker in the first 42 yr of this century. Concurrent variations are found in the trade wind regime over the western equatorial Pacific. Velocity potential field anomalin derived from the surface winds, show a strengthening of tropical-subtropical convergence over time. These observations, together with those of cloudiness and MSLP and a brief examination of near-global MSLP correlations and SST data back to 1879, point to a consistent fluctuation in ocean–atmosphere forcings during this century. Independent ocean general circulation model simulations involving modulations to global wind stresses or the Indonesian throughflow suggest that the sub-tropical/midlatitude southern Indian Ocean, and particularly the Agulhas outflow zone, is sensitive to low-frequency changes in wind and/or thermohaline forcing. Such long-term fluctuations in the mean state of the climate system may have ramifications for interannual variability and features such as the El Niño–Southem Oscillation phenomenon.

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Peter L. Jackson
,
Chris J. C. Reason
, and
Shucai Guan

Abstract

A detailed analysis of a simulation of a coastal trapped disturbance (CTD) using the Colorado State University Regional Atmospheric Modeling System (RAMS) is presented. The CTD considered (15–18 May 1985) represents an example of a strong mesoscale trapped event with abrupt gravity current–like transitions in many meteorological parameters, and which was closely tied to the synoptic forcing. Propagation of this event along the west coast of North America occurred from initiation in the Southern California Bight–Baja California coastal region to the northern tip of Vancouver Island, and the event appeared to have no difficulty in negotiating significant bends or gaps in the coastal mountains unlike some other events that have ceased or stalled near Cape Mendocino, Point Arena, and the mouth of the Columbia River.

It is found that warm offshore flow ahead of the CTD, and cool onshore flow in the Southern California Bight–northern Baja California coastal region, both driven by the westward tracking of a synoptic low, are very important for initiation, and subsequent propagation, of the model CTD, similar to observations. Convergence of the initial onshore cool flow in the south combined with warm offshore flow in the north lead to a northward-directed pressure gradient and, hence, a southerly wind transition. The adjustment timescale of the onshore flow to form the southerlies of the CTD is found to be consistent with expectations from theory.

During the propagating stage of the event, the pressure gradient and Coriolis terms were found to be most important for the meridional wind tendency, with advection and diffusion making smaller contributions. Consistent with semigeostrophic theory for CTD, the length scale in the across-mountain direction of the model CTD is much less than the along-mountain scale. Although the model transitions in winds, pressure, and temperature are not as sharp as observed (attributed to the lack of boundary layer structure in the NCEP fields used for model initialization), there is some signature of the gravity current nature of the event.

Decay of the event occurred when the favorable synoptic forcing related to the synoptic low moved to the northwest. There appeared to be no evidence of any topographic influence on the decay or termination of this particular event, unlike for several other cases.

Taken together, this and the previous validation study suggest that RAMS can be usefully employed to better understand the nature of at least these strong CTD events.

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Shucai Guan
,
Peter L. Jackson
, and
Chris J. C. Reason

Abstract

The coastal trapped disturbance (CTD) of 15–17 May 1985 represents an example of a strong mesoscale trapped event along the west coast of North America with abrupt transitions in many basic meteorological parameters. In this study, a comparison between observations and a numerical simulation of this event using the Regional Atmospheric Modeling System (RAMS) is presented. The model is shown to realistically reproduce CTD characteristics such as the coastal transition from northerly to southerly flow, as a mesoscale coastal ridge of higher pressure with associated drops in marine-layer temperature propagates northward along the west coast of North America. Simulated sea level pressure and temperature fields near the surface match well with observations, especially at the synoptic scale. The model realistically simulates mesoscale sea level pressure and 6-h pressure changes during the event. The modeled hourly time evolution of sea level pressure and the southerly transitions at a series of coastal stations and buoys also agree reasonably well with observations. The marine boundary layer is not well initialized or very well represented in the model, suggesting that, for this particular case, the details of the boundary layer are not crucial in the evolution of the CTD. It is suggested that the RAMS model can be usefully applied to investigate CTD evolution.

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Neil C. G. Hart
,
Chris J. C. Reason
, and
Nicolas Fauchereau

Abstract

An automated cloud band identification procedure is developed that captures the meteorology of such events over southern Africa. This “metbot” is built upon a connected component labeling method that enables blob detection in various atmospheric fields. Outgoing longwave radiation is used to flag candidate cloud band days by thresholding the data and requiring detected blobs to have sufficient latitudinal extent and exhibit positive tilt. The Laplacian operator is used on gridded reanalysis variables to highlight other features of meteorological interest. The ability of this methodology to capture the significant meteorology and rainfall of these synoptic systems is tested in a case study. Usefulness of the metbot in understanding event-to-event similarities of meteorological features is demonstrated, highlighting features previous studies have noted as key ingredients to cloud band development in the region. Moreover, this allows the presentation of a composite cloud band life cycle for southern Africa events. The potential of metbot to study multiscale interactions is discussed, emphasizing its key strength: the ability to retain details of extreme and infrequent events. It automatically builds a database that is ideal for research questions focused on the influence of intraseasonal to interannual variability processes on synoptic events. Application of the method to convergence zone studies and atmospheric river descriptions is suggested. In conclusion, a relation-building metbot can retain details that are often lost with object-based methods but are crucial in case studies. Capturing and summarizing these details may be necessary to develop a deeper process-level understanding of multiscale interactions.

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Neil C. G. Hart
,
Richard Washington
, and
Chris J. C. Reason

Abstract

The Southern Hemisphere subtropical convergence zones are important regions of rainfall in the subtropics. The south Indian Ocean convergence zone (SICZ) has the strongest seasonality and exhibits substantial interannual variability in strength and position during austral summer. On synoptic time scales, the SICZ is a preferred region for the formation of tropical–extratropical (TE) cloud bands with local maxima over the southern African mainland and Madagascar. This study investigates how the seasonality in satellite-observed cloud band frequency emerges from the interplay between the asynchronous seasonal cycles in convective instability and upper-level flow, as represented by reanalysis data. These atmospheric mean states are diagnosed with a gross convective instability metric and a method to distinguish between subtropical and eddy-driven jet axes. Month-by-month analysis of these diagnostics elucidates how mean-state perturbations during ENSO events modify cloud band likelihood. Typically, 150%–200% more cloud bands develop during La Niña seasons supported by 5°–10° latitudinal separation between the local subtropical and eddy-driven jets and higher values of convective instability, especially in semiarid parts of mainland southern Africa. During El Niño events, fewer cloud bands develop over southern Africa in a more convectively stable environment without a distinct subtropical jet. However, east of Madagascar cloud bands are 150% more likely. Plausible teleconnection pathways based on these ENSO-related perturbations are discussed. The paper concludes with a conceptual framing of the seasonal cycle in the mean-state pertinent to TE cloud band likelihood.

Open access
Joke F. Lübbecke
,
Natalie J. Burls
,
Chris J. C. Reason
, and
Michael J. McPhaden

Abstract

Previous studies have argued that the strength of the South Atlantic subtropical high pressure system, referred to as the South Atlantic anticyclone (SAA), modulates sea surface temperature (SST) anomalies in the eastern equatorial Atlantic. Using ocean and atmosphere reanalysis products, it is shown here that the strength of the SAA from February to May impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial Atlantic via anomalous tropical wind power. This modulation in the timing and amplitude of seasonal cold tongue development manifests itself via SST anomalies peaking between June and August. The timing and impact of this connection is not completely symmetric for warm and cold events. For cold events, an anomalously strong SAA in February and March leads to positive wind power anomalies from February to June resulting in an early cold tongue onset and subsequent cold SST anomalies in June and July. For warm events, the anomalously weak SAA persists until May, generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm SST anomalies. Mechanisms by which SAA-induced wind power variations south of the equator influence eastern equatorial Atlantic SST are discussed, including ocean adjustment via Rossby and Kelvin wave propagation, meridional advection, and local intraseasonal wind variations.

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Caroline C. Ummenhofer
,
Alexander Sen Gupta
,
Matthew H. England
, and
Chris J. C. Reason

Abstract

Links between extreme wet conditions over East Africa and Indian Ocean sea surface temperatures (SST) are investigated during the core of the so-called short rain season in October–November. During periods of enhanced East African rainfall, Indian Ocean SST anomalies reminiscent of a tropical Indian Ocean dipole (IOD) event are observed. Ensemble simulations with an atmospheric general circulation model are used to understand the relative effect of local and large-scale Indian Ocean SST anomalies on above-average East African precipitation. The importance of the various tropical and subtropical IOD SST poles, both individually and in combination, is quantified. In the simulations, enhanced East African “short rains” are predominantly driven by the local warm SST anomalies in the western equatorial Indian Ocean, while the eastern cold pole of the tropical IOD is of lesser importance. The changed East African rainfall distribution can be explained by a reorganization of the atmospheric circulation induced by the SST anomalies. A reduction in sea level pressure over the western half of the Indian Ocean and converging wind anomalies over East Africa lead to moisture convergence and increased convective activity over the region. The pattern of large-scale circulation changes over the tropical Indian Ocean and adjacent landmasses is consistent with an anomalous strengthening of the Walker cell. The seasonal cycle of various indices related to the SST and the atmospheric circulation in the equatorial Indian Ocean are examined to assess their potential usefulness for seasonal forecasting.

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Benjamin R. Loveday
,
Jonathan V. Durgadoo
,
Chris J. C. Reason
,
Arne Biastoch
, and
Pierrick Penven

Abstract

The relationship between the Agulhas Current and the Agulhas leakage is not well understood. Here, this is investigated using two basin-scale and two global ocean models of incrementally increasing resolution. The response of the Agulhas Current is evaluated under a series of sensitivity experiments, in which idealized anomalies, designed to geometrically modulate zonal trade wind stress, are applied across the Indian Ocean Basin. The imposed wind stress changes exceed plus or minus two standard deviations from the annual-mean trade winds and, in the case of intensification, are partially representative of recently observed trends. The Agulhas leakage is quantified using complimentary techniques based on Lagrangian virtual floats and Eulerian passive tracer flux. As resolution increases, model behavior converges and the sensitivity of the leakage to Agulhas Current transport anomalies is reduced. In the two eddy-resolving configurations tested, the leakage is insensitive to changes in Agulhas Current transport at 32°S, though substantial eddy kinetic energy anomalies are evident. Consistent with observations, the position of the retroflection remains stable. The decoupling of Agulhas Current variability from the Agulhas leakage suggests that while correlations between the two may exist, they may not have a clear dynamical basis. It is suggested that present and future Agulhas leakage proxies should be considered in the context of potentially transient forcing regimes.

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