How does the Indian Ocean subtropical dipole trigger the tropical Indian Ocean dipole via the Mascarene high?

The variation in the Indian Ocean is investigated using Hadley center sea surface temperature（SST） data during the period 1958–2010.All the first empirical orthogonal function（EOF） modes of the SST anomalies（SSTA） in different domains represent the basin-wide warming and are closely related to the Pacific El Ni o– Southern Oscillation（ENSO） phenomenon.Further examination suggests that the impact of ENSO on the tropical Indian Ocean is stronger than that on the southern Indian Ocean.The second EOF modes in different domains show different features.It shows a clear east-west SSTA dipole pattern in the tropical Indian Ocean（Indian Ocean dipole,IOD）,and a southwest-northeast SSTA dipole in the southern Indian Ocean（Indian Ocean subtropical dipole,IOSD）.It is further revealed that the IOSD is also the main structure of the second EOF mode on the whole basin-scale,in which the IOD pattern does not appear.A correlation analysis indicates that an IOSD event observed during the austral summer is highly correlated to the IOD event peaking about 9 months later.One of the possible physical mechanisms underlying this highly significant statistical relationship is proposed.The IOSD and the IOD can occur in sequence with the help of the Mascarene high.The SSTA in the southwestern Indian Ocean persists for several seasons after the mature phase of the IOSD event,likely due to the positive wind–evaporation–SST feedback mechanism.The Mascarene high will be weakened or intensified by this SSTA,which can affect the atmosphere in the tropical region by teleconnection.The pressure gradient between the Mascarene high and the monsoon trough in the tropical Indian Ocean increases（decreases）.Hence,an anticyclone（cyclone） circulation appears over the Arabian Sea-India continent.The easterly or westerly anomalies appear in the equatorial Indian Ocean,inducing the onset stage of the IOD.This study shows that the SSTA associated with the IOSD can lead to the onset of IOD with the aid of atmosphere circulation and also explains why some IOD events in the tropical tend to be followed by IOSD in the southern Indian Ocean.

Cyclones and Societies in the Mascarene Islands 17th-20th Centuries

The recent IPCC-SREX report focuses on the impact of extreme weather events on societies and underlines the absence of reliable data to assert a solid link between them and the current global climate change. Thanks to the unpublished materials that are contained in historic archives, this article suggests studying the cyclones which affected the Mascarene islands between 1654 and 2007 and which supply us with a catalog of hitherto unpublished events. Inspired by the Simpson-Saffir hurricanes Wind Scale, the research proposes a relative evaluation of the extremes of the region. It underlines the big fluctuations in the last three centuries and partially answers the current debate on the reliability of the data in relation to hurricanes and their link with the contemporary climate. The available archives show that this type of meteorological event has occurred frequently during the relevant historical period and that for that reason, has given rise to original strategies of adaptation on the part of the societies affected. The results presented here constitute new and reliable data which could make an important contribution to the decision-makers and to climatologists trying to design strategies which the populations of small islands facing the climatic hazards of the future will have to adopt.

Circulation Pattern Controls of Summer Temperature Anomalies in Southern Africa

This study investigates the relationship between circulation patterns and austral summer temperature anomalies in southern Africa. The results show that the formation of continental lows tends to increase the thickness of the lower atmosphere. Further, the distinct variabilities of high and low pressure under the circulation types, influence air mass advection from the adjacent oceans, as well as atmospheric stability over land. Stronger anticyclonic circulation at the western branch of the Mascarene high-pressure system enhances the low-level cold air advection by southeast winds,decreases the thickness, and lowers the temperature over a majority of the land in southern Africa. Conversely, a weaker Mascarene High, coupled with enhanced cyclonic activity in the southwest Indian Ocean increases low-level warm air advection and increases temperature anomalies over vast regions in southern Africa. The ridging of a closed South Atlantic anticyclone at the southern coast of southern Africa results in colder temperatures near the tip of southern Africa due to enhanced low-level cold air advection by southeast winds. However, when the ridge is weak and westerly winds dominate the southern coast of southern Africa, these areas experience temperature increases. The northward track of the Southern Hemisphere mid-latitude cyclone, which can be linked to the negative Southern Annular Mode, reduces the temperature in the southwestern part of southern Africa. Also, during the analysis period, El Ni?o was associated with temperature increases over the central parts of southern Africa;while the positive Indian Ocean dipole was linked to a temperature increase over the northeastern, northwestern, and southwestern parts of southern Africa.

A GIS Based Approach to Back Trajectory Analysis and Mass Concentration &Dispositions of Aerosols in Nairobi, Kenya

The origins and pathways of atmospheric aerosols in Kenya are examined in Nairobi for the three years (2015-2017) using HYSPLIT model and meteorological data. Aerosol concentrations in Kenya depended on the frequency of the air mass from the northeastern Asian Continent and southeastern Madagascar Island. There was a direct correlation of more pollutants during dry and hot seasons (JJA) of the year. To study the reasons for the seasonal variations, the origins of the air mass flowing to Nairobi were analyzed by backward air mass trajectory analysis. Monthly variations revealed that aerosols are transported from different directions due to variation of winds flowing and peak values were during July because of the increased emissions due to higher temperature and stronger solar radiation during heating, while the lowest value found in April was probably due to rain wash.