J. V. Ratnam

Remote Effects of El Niño and Modoki Events on the Austral Summer Precipitation of Southern Africa

AbstractRemote effects modulating the austral summer precipitation over southern Africa during El Nino/El Nino Modoki events are investigated by analyzing the observed events during December–February of the years from 1982/83 to 2010/11. Based on the composite analyses, it is found that southern Africa experiences significantly below normal precipitation during El Nino events compared to El Nino Modoki events. During these latter events, precipitation anomalies are not so significant although southern Africa as a whole receives below normal precipitations. The differences in the spatial distribution of precipitation over southern Africa are seen to be related to the sea surface temperature (SST) anomalies of the equatorial Pacific through atmospheric teleconnections.The low-level (850 hPa) Matsuno–Gill response to anomalously high precipitation over the Pacific during El Nino events results in an anomalous anticyclone extending from the equatorial to the subtropical South Indian Ocean. These anomalous ant...

Anomalous climatic conditions associated with the El Niño Modoki during boreal winter of 2009

The winter months from December 2009 to February 2010 witnessed extreme conditions affecting lives of millions of people around the globe. During this winter, the El Niño Modoki in the tropical Pacific was a dominant climatic mode. In this study, exclusive impacts of the El Niño Modoki are evaluated with an Atmospheric General Circulation Model. Sensitivity experiments are conducted by selectively specifying anomalies of the observed sea surface temperature in the tropical Pacific. Observed data are also used in the diagnostics to trace the source of forced Rossby waves. Both the observational results and the model simulated results show that the heating associated with the El Niño Modoki in the central tropical Pacific accounted for most of the anomalous conditions observed over southern parts of North America, Europe and over most countries in the Southern Hemisphere viz. Uruguay. Unlike those, the model-simulated results suggest that the anomalously high precipitation observed over Australia and Florida might be associated with the narrow eastern Pacific heating observed during the season.

A simple regional coupled model experiment for summer-time climate simulation over southern Africa

The main aim of this paper is to evaluate the Advanced Research Weather Research and Forecasting (WRF) regional model in simulating the precipitation over southern Africa during austral summer. The model’s ability to reproduce the southern African mean climate and its variability around this mean state was evaluated by using the two-tier approach of specifying sea surface temperature (SST) to WRF and by using the one-tier approach of coupling the WRF with a simple mixed-layer ocean model. The boundary conditions provided by the reanalysis-II data were used for the simulations. Model experiments were conducted for twelve austral summers from DJF1998-99 to DJF2009-10. The experiments using both the two-tier and one-tier approaches simulated the spatial and temporal distributions of the precipitation realistically. However, both experiments simulated negative biases over Mozambique. Furthermore, analysis of the wet and dry spells revealed that the one-tier approach is superior to the two-tier approach. Based on the analysis of the surface temperature and the zonal wind shear it is noted that the simple mixed-layer ocean model coupled to WRF can be effectively used in place of two-tier WRF to simulate the climate of southern Africa. This is an important result because specification of SST at higher temporal resolutions in the subtropics is the most difficult task in the two-tier approach for most regional prediction models. The one-tier approach with the simple mixed-layer model can effectively reduce the complicacy of finding good SST predictions.

Anatomy of Indian heatwaves

India suffers from major heatwaves during March-June. The rising trend of number of intense heatwaves in recent decades has been vaguely attributed to global warming. Since the heat waves have a serious effect on human mortality, root causes of these heatwaves need to be clarified. Based on the observed patterns and statistical analyses of the maximum temperature variability, we identified two types of heatwaves. The first-type of heatwave over the north-central India is found to be associated with blocking over the North Atlantic. The blocking over North Atlantic results in a cyclonic anomaly west of North Africa at upper levels. The stretching of vorticity generates a Rossby wave source of anomalous Rossby waves near the entrance of the African Jet. The resulting quasi-stationary Rossby wave-train along the Jet has a positive phase over Indian subcontinent causing anomalous sinking motion and thereby heatwave conditions over India. On the other hand, the second-type of heatwave over the coastal eastern India is found to be due to the anomalous Matsuno-Gill response to the anomalous cooling in the Pacific. The Matsuno-Gill response is such that it generates northwesterly anomalies over the landmass reducing the land-sea breeze, resulting in heatwaves.

Dynamical Downscaling of Austral Summer Climate Forecasts over Southern Africa Using a Regional Coupled Model

AbstractThe prediction skill of dynamical downscaling is evaluated for climate forecasts over southern Africa using the Advanced Research Weather Research and Forecasting (WRF) model. As a case study, forecasts for the December–February (DJF) season of 2011/12 are evaluated. Initial and boundary conditions for the WRF model were taken from the seasonal forecasts of the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F) coupled general circulation model. In addition to sea surface temperature (SST) forecasts generated by nine-member ensemble forecasts of SINTEX-F, the WRF was also configured to use SST generated by a simple mixed layer Price–Weller–Pinkel ocean model coupled to the WRF model. Analysis of the ensemble mean shows that the uncoupled WRF model significantly increases the biases (errors) in precipitation forecasted by SINTEX-F. When coupled to a simple mixed layer ocean model, the WRF model improves the spatial distribution of precipitation over southern Africa t...

Origin of extreme summers in Europe: the Indo-Pacific connection

Extreme summers of Europe are usually affected by blocking highs that shift between Western and Eastern Europe to cause regional variations in the surface temperature anomalies. Generally, the blocking high induces a regional temperature dipole with poles of warm and cold anomalies on two sides of Europe. The extreme summers of Western Europe, when the Eastern Europe is colder than normal, are usually associated with the teleconnections arising from positive Indian Ocean Dipole (IOD) events. In contrast, analogous warm events in Eastern Europe are usually associated with La Niña. The western Pacific conditions that prevail during the turnaround phase of El Niño to La Niña are found to be responsible for developing the extreme Eastern Europe events. The role of North Atlantic Oscillation (NAO) is not blatant for the Eastern Europe summers though it has a weaker influence on Western Europe summers for which IOD plays a dominant role: The seasonal July–August correlation for Western Europe temperature with IOD index is higher than that with the NAO index. The teleconnections for both types of extremes are associated with a Rossby wavetrain that travel around the globe to reach the Europe. This circumglobal teleconnection is largely determined by the location of the tropospheric heat source. For Western Europe warm events, major contributions come from the atmospheric convections/diabatic heating over northwest India and southern Pakistan. For the Eastern Europe events, the convections over northwest Pacific, south of Japan, are found to project the signals on to the mid-latitude wave-guide. These patterns of teleconnection are so robust that those can be seen on daily to seasonal time-scales of atmospheric anomalies. The wavetrains are found to set-in a couple of weeks prior to the development of blocking highs and extreme hot conditions over Europe.

Generation Mechanism of the South Pacific Subtropical Dipole

AbstractDistinct pattern of interannual variability in sea surface temperature (SST) in the South Pacific [i.e., the South Pacific subtropical dipole (SPSD)] is examined using outputs from a coupled general circulation model. The SPSD appears as the second empirical orthogonal function (EOF) mode of the SST anomalies in the South Pacific and is associated with a northeast–southwest-oriented dipole of positive and negative SST anomalies in the central basin. The positive and negative SST anomaly poles start to develop during austral spring, reach their peak during austral summer, and gradually decay afterward. Close examination of mixed-layer heat balance yields that the SST anomaly poles develop mainly because warming of the mixed layer by shortwave radiation is modulated by the anomalous mixed-layer thickness. Over the positive (negative) pole, the mixed layer becomes thinner (thicker) than normal and acts to enhance (reduce) the warming of the mixed layer by climatological shortwave radiation. This thin...

Improvements to the WRF Seasonal Hindcasts over South Africa by Bias Correcting the Driving SINTEX-F2v CGCM Fields

AbstractIn an attempt to improve the forecast skill of the austral summer precipitation over South Africa, an ensemble of 1-month-lead seasonal hindcasts generated by the Scale Interaction Experiment–Frontier Research Center for Global Change (SINTEX-F2v) coupled global circulation model is downscaled using the Weather Research and Forecasting (WRF) Model. The WRF Model with two-way interacting domains at horizontal resolutions of 27 and 9 km is used in the study. Evaluation of the deterministic skill score using the anomaly correlation coefficients shows that SINTEX-F2v has significant skill in precipitation forecasts confined to western regions of South Africa. Dynamical downscaling of SINTEX-F2v forecasts using the WRF Model is found to further improve the skill scores over South Africa. However, larger improvements in the skill scores are achieved when the WRF Model is forced by a form of bias-corrected SINTEX-F2v forecasts. The systematic biases in the original fields of the SITNEX-F2v forecasts are ...

A model study of regional air-sea interaction in the austral summer precipitation over southern Africa

The importance of air-sea interactions in the simulation of southern Africa precipitation is brought out using a fully coupled regional model and by forcing the atmospheric component of the coupled model with the coupled model-simulated sea surface temperature (SST). The coupled model has the Advanced Research Weather Research and Forecasting model as the atmospheric component and the Regional Ocean Modeling System as the oceanic component. The spatial and temporal distribution of the coupled model-simulated SST shows good agreement with observations. The coupled model shows a bias of about 0.25°C near the east coast of southern Africa. Comparison of the precipitation between the coupled and uncoupled model shows that the air-sea interactions play an important role in the simulation of the precipitation during the peak precipitation season January to March, when the precipitation is mostly due to tropical processes. It is found that the two-way specification of SST to the atmospheric model results in significant larger (smaller) precipitation depending on the spatial distribution of SST, due to the increase (decrease) of moisture from the surrounding oceans into the landmass. The results also show that the air-sea interactions are not so important during the initial phases of the precipitation from October to December.

Role of Cross-Equatorial Waves in Maintaining Long Periods of Low Convective Activity over Southern Africa

AbstractPeriods of low convective activity over southern Africa during the peak rainy season from December to February are known to be due to the northeastward displacement of the tropical temperate trough (TTT) systems from the landmass. In this study, using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data, the authors show that the displacement of the TTT systems during long periods of low convective activity has origins in the Northern Hemisphere. Using standardized area-averaged outgoing longwave radiation (OLR) daily anomalies over southern Africa, long periods of low convective activity are defined as periods of positive OLR anomalies lasting consecutively for 5 or more days with a standard deviation of 1 or more. An eddy streamfunction anomaly composite of the periods of low convective activity shows an upper-level anomalous wave originating in the Northern Hemisphere and extending to southern Africa from the eastern Pacific and displacing the tropic...