Relationships between intraseasonal rainfall variability of coastal Tanzania and ENSO

Summary The influence of ENSO on intraseasonal variability over the Tanzanian coast during the short (OND) and long (MAM) rainy seasons is examined. In particular, variability in the rainfall onset, peak and end dates as well as dry spells are considered. In general, El Niño appears to be associated with above average rainfall while La Niña is associated with below average rainfall over the northern Tanzanian coast during OND, and to lesser extent MAM. Over the southern coast, the ENSO impacts are less coherent and this region appears to be a transition zone between the opposite signed impacts over equatorial East and southern Africa. The increased north coast rainfall during El Niño years is generally due to a longer than normal rainfall season associated with early onset while reduced rainfall during La Niña years tends to be associated with a late onset, and thus a shorter than average rainfall season. Wet conditions during El Niño years were associated with enhanced convection and low-level easterly anomalies over the equatorial western Indian Ocean implying enhanced advection of moisture from the Indian Ocean while the reverse is true for La Niña years. Hovmöller plots for OLR and zonal wind at 850 hPa and 200 hPa show eastward, westward propagating and stationary features over the Indian Ocean. It was observed that the propagating features were absent during strong El Niño years. Based on the Hovmöller results, it is observed that the convective oscillations over the Tanzanian coast have some of the characteristic features of intraseasonal oscillations occurring elsewhere in the tropics.     

Modelling the dominant climate signals around southern Africa

In this study, statistical techniques are employed to decompose climate signals around southern Africa into the dominant temporal frequencies, with the aim of modelling and predicting area-averaged rainfall. In the rainfall time series over the period 1900–1999, the annual cycle accounts for 83% of variance. Residual spectral energy cascades from biennial (42%) to interannual (20%) to decadal bands (3%). Regional climate signals are revealed through a multi-taper singular value decomposition analysis of sea surface temperature and sea level pressure fields over the Atlantic and Indian Oceans, in conjunction with southern Africa rainfall. Rossby wave action in the South Indian Ocean dominates the biennial scale variability. El Niño-Southern Oscillation (ENSO) and related Indian Ocean dipole patterns are important for interannual variability. Significant sea temperature and pressure fluctuations occurring 6–12 months prior to rainfall contribute biennial and interannual indices to a multi-variate model that demonstrates useful predictive skill.     

Stability dependent parameterisations of vertical mixing in ocean general circulation models

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.With 8 Figures     

On the relative roles of El Nino and Indian Ocean Dipole events on the Monsoon Onset over Kerala

Interannual variations of the monsoon onset over Kerala (MOK) have been studied using data from over 60?years (1948?C2009) of NCEP/NCAR reanalysis and outgoing long-wave radiation. The sea surface temperature fields over the North Indian Ocean associated with the MOK have been examined in association with El Nino and Indian Ocean Dipole (IOD) events which originate in the Pacific and Indian Ocean, respectively. An analysis of the tropical convective maximum showed significant differences in its strength and location during the El Nino, IOD, early, normal, and delayed MOK composites. Further, we also looked into the role of the convective systems formed over the Arabian Sea and Bay of Bengal on MOK. The most significant features during early (delayed) MOK years is the abnormal persistence of westerlies (easterlies) several days prior to MOK and enhanced (suppressed) deep convection over the southeastern Arabian Sea and the southern Bay of Bengal. Moisture builds up over peninsular India several pentads prior to MOK during La Nina, negative IOD, and concurrent La Nina and negative IOD years as compared to the El Nino, positive IOD, and concurrent El Nino and positive IOD years, indicating its significant role on MOK. The monsoon Hadley cell and Walker circulations are weaker (stronger) during a delayed (early) MOK. Further, the sea surface temperature anomalies in the western Pacific are negative (positive) during delayed (early) MOK.     

Variability of rainfall characteristics over the South Coast region of South Africa

Thirty years of daily rainfall data are analysed for the South Coast region of South Africa, a region which experiences substantial rainfall variability and frequent severe drought and flood events, but whose climate variability has not been much researched. It is found that El Niño–Southern Oscillation (ENSO) exerts an influence since most wet years correspond to mature phase La Niña years. ENSO also influences South Coast rainfall via increases in the number of cut-off lows in southern South Africa during mature phase La Niña years. A statistically significant correlation between the Niño 3.4 index and monthly rainfall totals, and between this index and the frequency of wet days, exists for two summer months and also for June. There are also changes in the heavy rainfall day frequencies from one decade to another. Examination of NCEP re-analyses indicates that wet (dry) years result from an equatorward (poleward) shift in the subtropical jet, cyclonic (anticyclonic) pressure anomalies over the South Atlantic and South Africa, and increased (decreased) density of mid-latitude cyclonic systems.     

On the effect of ENSO precipitation anomalies in a global ocean GCM

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.     

A model investigation of recent ENSO impacts over southern Africa

Summary This study investigates the impacts of five recent ENSO events on southern Africa, the associated circulation anomalies and the ability of an atmospheric general circulation model (UKMO HadAM3) to represent these impacts when forced by observed sea-surface temperature (SST). It is found that the model is most successful for the 1997/8 El Niño but does less well for the 1991/2 and 2002/3 El Niños and the 1995/6 and 1999/00 La Niña events. Diagnostics from the model and NCEP re-analyses suggest that modulations to the Angola low, an important centre of tropical convection over southern Africa during austral summer, are often important for influencing the rainfall impacts of ENSO over subtropical southern Africa. Since the model has difficulty in adequately representing this regional circulation feature and its variability, it has problems in capturing ENSO rainfall impacts over southern Africa. During 1997/8, modulations to the Angola low were weak and Indian Ocean SST forcing strong and the model is relatively successful. The implications of these results for dynamical model based seasonal forecasting of the region are discussed.Current affiliation: CSIR Centre for Mathematical Modelling and Computer Simulation, Bangalore, India.     

The influence of the Bering Strait on the circulation in a coarse resolution global ocean model

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. Correspondence to: CJC Reason     

Interactions between synoptic, intraseasonal and interannual convective variability over Southern Africa

After removing the annual cycle, a principal component analysis is applied to the daily outgoing longwave radiation anomaly field, used here as a proxy for atmospheric convection. The analysis is carried out over the southern African region (7.5°E–70°E, 10°S–40°S) for austral summer (November through February) for the period 1979–1980 to 2006–2007. The first five principal components (PC) are retained. The first two PCs describe spatial patterns oriented north-west to south-east from tropical southern Africa (SA) to the mid-latitudes. They are interpreted to be different possible locations for synoptic-scale tropical–temperate troughs (TTT), one dominant rainfall-producing synoptic system in the region. The phase relationship between these two PCs describes a tendency for these TTT to propagate eastwards from SA to the Mozambique Channel and southern Madagascar. The next three PCs describe convective fluctuations, respectively, located over the north-west, the south and the centre of SA. Their time series are significantly associated with Madden–Julian oscillation (MJO) activity in the tropics. However, we find that TTT systems are statistically independent of the MJO, i.e. they are equally liable to occur during any phase of the MJO. Three PCs out of five also show a significant association with El Niño southern oscillation, confirming that El Niño years mostly coincide with suppressed convection at the intraseasonal time-scales, a result consistent with its impact on seasonal averages diagnosed in previous studies.