Notes: Abstract An ocean general circulation model is used to study the influence of positive precipitation anomalies associated with El Nino and La Nina events. In this idealized model, the precipitation over the appropriate part of the equatorial Indo-Pacific region is doubled for one year. At the surface, salinity anomalies of up to −0.9 parts per thousand result from this anomalous precipitation. Perturbation surface currents ranging from 10–100% of the climatological values are induced in the tropical Indian and Pacific Oceans. A return flow is found beneath the thermocline with upwelling (downwelling) in (outside) the region of enhanced precipitation. The net effect of the precipitation anomalies is to generate a zonal overturning cell which transports fresher surface water away from the forcing region and replaces it with cooler, more saline water from below.

Notes: Abstract An ocean general circulation model of global domain, full continental geometry and bottom topography, is used to study the influence of the Bering Strait on the general circulation by comparing equilibrium solutions obtained with and without a land-bridge between Siberia and Alaska. The model is integrated with restoring boundary conditions (BC) on temperature and salinity, and later, with mixed BC in which a restoring BC on temperature is maintained but a specified flux condition on salinity is imposed. In both cases, the effect of the Bering Strait is to allow a flow of about 1.25–1.5 Sv from the North Pacific to the Arctic Ocean and, ultimately, back to the North Pacific along the western boundary current regions of the Atlantic and Indian Oceans. When a restoring BC on salinity is used, the overturning associated with North Atlantic Deep Water and Antarctic Intermediate Water formation are increased if the Bering Strait is present in the model geometry. The result of switching to a specified flux BC on salinity is to cause a transition in the THC in which the overturning associated with North Atlantic Deep Water formation increases from about 12 Sv to about 22 Sv. This transition occurs in an essentially smooth fashion with no significant variability and is about 12% smaller in magnitude if the Bering Strait is present in the model geometry. Because the Bering Strait appears to exert some influence on the general circulation and the formation of deep water masses, it is recommended that this Strait be included in the geometry of similar resolution models designed to study the deep ocean and potential changes in climate.

Notes: Summary Parameterisations of mixing induced through shear instability, internal wave breaking, and double diffusion are investigated in simulations of ocean climate using a global ocean general circulation model (OGCM). Focus is placed on the sensitivity of the large scale circulation, water mass formation and transport of heat as measures of the model's ability to represent current climate. The model resolution is typical of OGCMs being coupled to atmospheric. GCMs in climate models and the parameterisations investigated are all computationally inexpensive enough to allow for integrations on long time scales. Under the assumption of constant vertical eddy coefficients (the control case), the model climatology displays acceptable values of North Atlantic Deep Water formation, Antarctic Circumpolar Current (ACC) transport, and Indonesian through-flow but an excessively deep and diffuse pycnocline structure with weak stratification in the deep ocean. It is found that various circulation and water mass properties are sensitive to the choice of parameterisation of vertical mixing and that determining a scheme which works satisfactorily over all regions (tropical, mid-latitude, and polar) of the domain is not straightforward. Parameterisations of internal wave breaking or upper ocean shear instability lead to some improvements in the model water mass formation. ACC and poleward heat transport when compared to the control case whereas parameterisations of double diffusive processes did not. Based on these and other results, various recommendations are made for mixing parameterisations in ocean climate models.

Notes: Summary A general circulation model is used to study the response of the atmosphere to an idealised sea surface temperature (SST) anomaly pattern (warm throughout the southern midlatitudes, cool in the tropics) in the South Indian Ocean region. The anomaly imposed on monthly SST climatology captures the essence of patterns observed in the South Indian Ocean during both ENSO events and multidecadal epochs, and facilitates diagnosis of the model response. A previous study with this anomaly imposed in the model examined differences in the response between that on the seasonal scale (favours enhancement of the original SST anomaly) and that on the decadal scale (favours damping of the anomaly). The current study extends that work firstly by comparing the response on the intraseasonal, seasonal and interannual scales, and secondly, by assessing the changes in the circulation and rainfall over the adjoining African landmass. 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. Compared to the shorter scale response, the perturbation pressure and wind distribution on the interannual scale is shifted poleward, and is more reminiscent of the decadal response. Winds are now stronger over the warm anomaly in the southern midlatitudes suggesting enhanced surface fluxes, upper ocean mixing, and consequently, a damping of the anomaly. Examination of the circulation and rainfall patterns indicates that there are significant anomalies over large parts of southern Africa during the spring, summer and autumn seasons for both short (intraseasonal to interannual) and decadal scales. It appears that rainfall anomalies are associated with changes in the advection of moist tropical air from the Indian Ocean and its related convergence over southern Africa. Over eastern equatorial Africa, the austral autumn season (the main wet season) showed rainfall increases on all time scales, while parts of central to eastern subtropical southern Africa were dry. The signals during summer were more varied. Spring showed generally dry conditions over the eastern half of southern Africa on both short and decadal time scales, with wet areas confined to the west. 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. It appears that relatively modest SST anomalies in the South Indian Ocean can lead to sizeable rainfall anomalies in the model. Although precipitation in general circulation models tends to be less accurately simulated than many other variables, the model results, together with previous observational work, emphasize the need for ongoing monitoring of SST in this region.

Notes: Summary The performance of three different vertical mixing schemes embedded in a global coarse resolution OGCM under both annual and monthly mean forcing are compared. These schemes are the integral model of Kraus and Turner and the differential parametrisations of Pacanowski and Philander and Henderson-Sellers (EDD1 scheme). The simulations of mean climatological conditions suggest that, with respect to climate change studies, the Kraus-Turner and the EDD1 schemes are overall more robust than the Pacanowski and Philander parametrisation. With respect to anomalous climatic conditions (i.e. decay of imposed SST anomalies), all three schemes indicate that the lifespan and penetration depth of a cold anomaly is somewhat greater than for a warm one. Also, the EDD1 scheme portrays the evolution process of the SST anomalies somewhat differently than the other two schemes.

Notes: 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.

Notes: Summary  The southeast Australian coastally trapped disturbance (CTD) of 9–11 November 1982 that was previously studied by Holland and Leslie is re-visited. Additional observational data not considered by these authors and a numerical simulation using the Colorado State University Regional Atmospheric Modeling system (RAMS) are used for this purpose. Following initiation of the event on the south coast, mesoscale ridging propagated along the east coast to just north of Brisbane. Associated with the arrival of the event were a marked increase in surface pressure, drop in temperature and a shift and strengthening of the wind. While the simulation does not appear to capture the details of the boundary layer as well as one would like, it does represent the main features of the event, including the speed of propagation along the coast, reasonably well. Similar to the observed, the model event shows gravity current-like characteristics. The significance of topographic variability (e.g., large gap in the coastal mountains at the Hunter Valley) is considered. It is suggested that the topography and ambient stratification in southeastern Australia are less favourable for CTD occurrence than those in southern Africa and western North America where these systems have been extensively studied. Consequently, when CTD do occur in southeastern Australia, the less pronounced topography and weaker stratification may enable local effects to mask the CTD signal to some extent, thereby posing challenges in observation and forecasting.

Notes: Summary Orographically trapped disturbances (OTD) in the lower atmosphere propagate along sufficiently large mountain barriers and are trapped vertically by stable stratification and, laterally, by Coriolis effects against the mountains. Observed events have occurred along the coastal mountains of western North and South America, the Southern African coastal escarpment, the Australian Great Dividing Range, the Rockies and the Himalayas. Scale analysis indicates that, during generation, the fundamental parameters are the Rossby number and the ratio of the Rossby to the Froude number where the length scales in these numbers are the mountain half-width and height. Evaluation of these parameters indicates that only the Southern African case possibly remains in the quasi-geostrophic regime. For propagating disturbances, the observed asymmetry in the horizontal length scales implies that only the momentum balance in the across-mountain direction is likely to be geostrophic. In the along-mountain direction, the inertial, advective and frictional terms are also significant in the momentum balance.

Notes: Summary The response of the coastal atmosphere in subtropical Western Australia and south-western Africa to easterly flow is considered. Easterly flow, arising from the ridging of a large scale anticyclone near the southern extremity of these land-masses, is a common synoptic pattern, particularly during the summer half of the year. Despite similar synoptic forcing, coastline orientation and latitude, there are significant differences in the response. In Western Australia, the typical response to this easterly flow is a synoptic, non-propagating feature (the West Coast Trough) which may be located on- or offshore. The response in southern Africa is typically a mesoscale, propagating feature (the coastal low) which is trapped against the coastal mountains. It is argued that the steep coastal mountain ranges (about 1 km height) in southern Africa compared to the gentle, low-lying Western Australian topography combined with the mean coastal stratification contribute significantly towards the differences between the coastal low and West Coast Trough. A secondary feature associated with the regional topography is the existence of an oceanic throughflow north of Western Australia from the western equatorial Pacific Ocean with associated flow of the warm Leeuwin Current polewards along the Western Australian coast. It is suggested that this current and the associated lack of coastal upwelling may play a role in the location and intensity of the West Coast Trough.

Notes: Summary An Australian circular mesoscale convective system (MCS) is examined using available surface and upper air analyses as well as satellite imagery. The MCS formed over central South Australia on 5 February 1997 and lasted approximately nine hours. It is found that MCS generation occurred following anomalous southward penetration of the monsoon trough over Australia. This penetration into southern Australia resulted in an input of extremely moist and unstable tropical air over the region which, together with the development of complex of shallow lows and troughs within the main monsoon trough, led to generation of the MCS. During the lifespan of the MCS, rainfall amounts in excess of 100 mm (and up to 175 mm over a four hour period at certain locations) were recorded with accompanying flash flooding and severe damage. A low in the middle levels of the atmosphere was responsible for the eventual decay of the storm. North to north-westerly winds winds around this low continually advected cloud away from the MCS towards the south and south east. This removal of cloud mass eventually led to dissipation of the MCS as it tracked away from the zone of maximum surface heating. Despite this storm just failing to meet the size criterion for mesoscale convective complex (MCC) status, it is very similar to “typical” MCCs found elsewhere in the world in terms of its lifetime and nocturnal nature. Although mesoscale storms of this type are not rare in Australia, MCS’s in South Australia make up only a small proportion of the total number of systems over south eastern Australia. These factors, in conjunction with the anomalous southward penetration of the monsoon trough and associated synoptic conditions, make this storm somewhat unusual.