scholarly journals Opposite Effects of ENSO on the Rainfall over the Northern and Equatorial Great Horn of Africa and Possible Causes

2020 ◽  
Vol 2020 ◽  
pp. 1-16
Author(s):  
K. Abebe Kiflie ◽  
Li Tao
Keyword(s):  
Rainfall Variability ◽  
Summer Rainfall ◽  
Western Indian Ocean ◽  
Annual Rainfall ◽  
Horn Of Africa ◽  
Tropical Africa ◽  
Low Level ◽  
Tropical Easterly Jet ◽  
Rainfall Anomalies ◽  
Upper Level

In this study, we explore the possible mechanism of opposite ENSO effects on summer rainfall in the JJAS region (northern GHA) and autumn rainfall in the OND region (equatorial GHA). The two regions are identified based on the spatial distribution of high seasonal fractions of annual rainfall for the period 1979–2016. The summer rainfall over the JJAS region is negatively correlated with ENSO. It is because the warm Niño3.4 SST triggers zonal wave one pattern in tropics and forces upper-level westerly anomaly and the low-level easterly anomaly over tropical Africa. Thus, the weakened upper-level Tropical Easterly Jet (TEJ) and the low-level westerly over the JJAS region result in deficient rainfall during JJAS over the northern GHA. For the autumn rainfall variability over the equatorial GHA, IOD is a pivotal factor. But, autumn rainfall anomalies are far greater in ENSO and IOD coexisting years than those in IOD alone years. In other words, ENSO has a significant impact on the autumn rainfall over the equatorial GHA by means of IOD. It is because the warming SST, which is fully developed over western Indian Ocean (IO) in autumn of ENSO developing year, causes low-level convergence over the equatorial GHA and enhances upper-level easterly over tropical Africa. Those conditions are favorable for abundant rainfall over the equatorial GHA in autumn.

Enhanced Tropical Eastern Indian Ocean Rainfall Breaks down the Tropical Easterly Jet-Indian Rainfall Relationship

2021 ◽  
pp. 1-44
Author(s):  
Sihua Huang ◽  
Bin Wang ◽  
Zhiping Wen ◽  
Zesheng Chen
Keyword(s):  
Indian Ocean ◽  
Rainfall Variability ◽  
Summer Monsoon Rainfall ◽  
Eastern Indian Ocean ◽  
Sst Variability ◽  
Tropical Easterly Jet ◽  
Tight Connection ◽  
Indian Rainfall ◽  
Rainfall Anomalies ◽  
Upper Level

AbstractPrevious studies found a tight connection between the tropical easterly jet (TEJ) and Indian summer monsoon rainfall (ISMR). Here we show that the TEJ-ISMR relationship is nonstationary and breaks down from 1994–2003 (epoch P2), in contrast to the significant positive correlation during the epoch P1 (1979–1993) and P3 (2004–2016). The breakdown of the TEJ-ISMR relationship concurs with the increased rainfall variability over the tropical eastern Indian Ocean (TEIO). The enhanced TEIO rainfall anomalies excite a significant lower-level cyclonic circulation that reduces the ISMR, meanwhile, strengthens the upper-level divergence and excites a pair of upper-level anticyclone to the west of the TEIO as Rossby wave responses, both accelerating the TEJ. Thus, the TEIO rainfall plays a more important role than the ISMR in the TEJ variability during P2, causing the breakdown of the TEJ-ISMR relationship. In contrast, a relatively weak amplitude of the TEIO rainfall during P1 and P3 was unable to change the positive TEJ-ISMR relationship. The changes in the TEIO rainfall variability is mainly attributed to the increased SST variability over the tropical southeastern Indian Ocean, but the cause of it remains elusive.

The Influence on Summer Rainfall in the Lesotho Lowlands from Indian Ocean SSTs

2002 ◽  
Vol 33 (4) ◽  
pp. 305-318 ◽  
Author(s):  
Lars Hydén
Keyword(s):  
Indian Ocean ◽  
Southern Africa ◽  
Rainfall Variability ◽  
Austral Summer ◽  
Equatorial Indian Ocean ◽  
Summer Rainfall ◽  
Significant Negative Correlation ◽  
Annual Rainfall ◽  
Ocean Area ◽  
The Indian Ocean

Lesotho is located approximately at latitude 30 degrees south in the interior of Southern Africa. The mesoscale climate is complicated and governed by various weather systems. The inter-annual rainfall variability is great, resulting in low food security, since the growing of crops in the Lesotho Lowlands is almost exclusively rain-fed. Reliable forecasts of austral summer rainfall are thus valuable. Earlier research has shown that the sea surface temperatures (SST) in the Indian Ocean to some extent govern rainfall in Southern Africa. The research presented is part of an on-going project to find suitable oceanographic and meteorological predictors, which can be used in a forecast model for summer rainfall, to be developed later. The first part of this paper investigates the correlation between the average SSTs in the Equatorial Indian Ocean, the Central Indian Ocean, and the Agulhas Gyre, respectively, and rainfall two months later in the Lesotho Lowlands during early austral summer, October until December for the period 1949-1995. No significant correlations have been found, probably because the three ocean areas are too large. In the second part of this paper the monthly SST in 132 grid squares in the Indian Ocean were investigated and found to be correlated with rainfall in the Lesotho Lowlands two months later, October until March. Significant correlations have been found between the SSTs and certain ocean areas and December, January, and February rainfall, respectively. There is significant negative correlation between December rainfall and October SST in an ocean area between Kenya and Somalia across the Indian Ocean to Sumatra. In the area where the Somali Current flows there is also significant correlation between December SST and December rainfall. January rainfall is significantly negatively correlated with November SST in an ocean area, northeast of Madagascar. February rainfall is significantly, but weakly, negatively correlated with SST in a narrow north-south corridor in the Eastern Indian Ocean from the equator down to latitude 40 degrees south.

scholarly journals Orographically Anchored El Niño Effect on Summer Rainfall in Central China

2017 ◽  
Vol 30 (24) ◽  
pp. 10037-10045 ◽  
Author(s):  
Kaiming Hu ◽  
Shang-Ping Xie ◽  
Gang Huang
Keyword(s):  
El Niño ◽  
Large Scale ◽  
El Nino ◽  
Rainfall Variability ◽  
Eastern China ◽  
Summer Rainfall ◽  
Central China ◽  
Northwest Pacific ◽  
The Tibetan Plateau ◽  
Rainfall Anomalies

Year-to-year variations in summer precipitation have great socioeconomic impacts on China. Historical rainfall variability over China is investigated using a newly released high-resolution dataset. The results reveal summer-mean rainfall anomalies associated with ENSO that are anchored by mountains in central China east of the Tibetan Plateau. These orographically anchored hot spots of ENSO influence are poorly represented in coarse-resolution datasets so far in use. In post–El Niño summers, an anomalous anticyclone forms over the tropical northwest Pacific, and the anomalous southwesterlies on the northwest flank cause rainfall to increase in mountainous central China through orographic lift. At upper levels, the winds induce additional adiabatic updraft by increasing the eastward advection of warm air from Tibet. In post–El Niño summers, large-scale moisture convergence induces rainfall anomalies elsewhere over flat eastern China, which move northward from June to August and amount to little in the seasonal mean.

scholarly journals The Dominant Patterns of Intraseasonal Rainfall Variability in May–October and November–April over the Tropical Western Pacific

2019 ◽  
Vol 147 (8) ◽  
pp. 2941-2960 ◽  
Author(s):  
Sunil Kumar Pariyar ◽  
Noel Keenlyside ◽  
Bhuwan Chandra Bhatt ◽  
Nour-Eddine Omrani
Keyword(s):  
Rainfall Variability ◽  
Surface Wind ◽  
Daily Rainfall ◽  
Moisture Flux ◽  
Feedback Mechanism ◽  
Western Pacific ◽  
Moisture Flux Convergence ◽  
Low Level ◽  
Northward Propagation ◽  
Rainfall Anomalies

Abstract The space–time structure of intraseasonal (10–90 day) rainfall variability in the western tropical Pacific is studied using daily 3B42 TRMM and ERA-Interim reanalysis data for the period 1998–2014. Empirical orthogonal function (EOF) analysis of 10–90-day filtered daily rainfall anomalies identifies two leading modes in both May–October and November–April; together these modes explain about 11%–12% of the total intraseasonal variance over the domain in both seasons and up to 60% over large areas of the western Pacific in both climatological periods. The two leading modes in May–October are linearly related to each other and both are well correlated with the Madden–Julian oscillation (MJO) indices. Although the two leading EOF modes in November–April are linearly independent of each other, both show statistically significant correlations with the MJO. The phase composites of 30–80-day filtered data show that the two leading modes are associated with strong eastward and northward propagation of rainfall anomalies in May–October, and eastward and southward propagation of rainfall anomalies in November–April. The eastward propagation of rainfall anomalies in both seasons and southeastward propagation related with EOF2 in November–April is linked to the development of low-level moisture flux convergence ahead of the active convection. Similarly, the northward propagation in May–October is also connected with low-level moisture flux convergence, but surface wind and evaporation variations are also important. The wind–evaporation–SST feedback mechanism drives the southeastward propagation of rainfall anomalies associated with EOF1 in November–April. The different mechanisms for southeastward propagation associated with two leading modes in November–April suggest dynamically different relations with the MJO.

scholarly journals Regional Differences in the Response of Rainfall to Convectively Coupled Kelvin Waves over Tropical Africa

2019 ◽  
Vol 32 (23) ◽  
pp. 8143-8165 ◽  
Author(s):  
Lawrence S. Jackson ◽  
Richard J. Keane ◽  
Declan L. Finney ◽  
John H. Marsham ◽  
Douglas J. Parker ◽  
...  
Keyword(s):  
Regional Differences ◽  
Large Scale ◽  
Climate Model ◽  
Rainfall Variability ◽  
Regional Climate ◽  
Kelvin Waves ◽  
Tropical Africa ◽  
Equatorial Africa ◽  
Phase Cycle ◽  
Rainfall Anomalies

Abstract The representation of convection remains one of the most important sources of bias in global models, and evaluation methods are needed that show that models provide the correct mean state and variability, both for the correct reasons. Here we develop a novel approach for evaluating rainfall variability due to convectively coupled Kelvin waves (CCKWs) in this region. A phase cycle was defined for the CCKW cycle in OLR and used to composite rainfall anomalies. We characterize the observed (TRMM) rainfall response to CCKWs over tropical Africa in April and evaluate the performance of regional climate model (RCM) simulations: a parameterized convection simulation (P25) and the first pan-Africa convection-permitting simulation (CP4). TRMM mean rainfall is enhanced and suppressed by CCKW activity, and the occurrence of extreme rainfall and dry days is coupled with CCKW activity. Focusing on regional differences, we show for the first time that there is a dipole between West Africa and the Gulf of Guinea involving onshore/offshore shifts in rainfall, and the transition to enhanced rainfall over west equatorial Africa occurs one phase before the transition over east equatorial Africa. The global model used to drive the RCMs simulated CCKWs with mean amplitudes of 75%–82% of observations. The RCMs simulated coherent responses to the CCKWs and captured the large-scale spatial patterns and phase relationships in rainfall although the simulated rainfall response is weaker than observations and there are regional biases that are bigger away from the equator. P25 produced a closer match to TRMM mean rainfall anomalies than CP4 although the response in dry days was more closely simulated by CP4.

scholarly journals Mechanisms for the Dipolar Patterns of Rainfall Variability over Large Islands in the Maritime Continent Associated with the Madden–Julian Oscillation

2020 ◽  
Vol 77 (6) ◽  
pp. 2257-2278
Author(s):  
Jian-Hua Qian
Keyword(s):  
Large Scale ◽  
Rainfall Variability ◽  
Reanalysis Data ◽  
Mountain Range ◽  
Madden Julian Oscillation ◽  
Maritime Continent ◽  
Wake Effect ◽  
Low Level ◽  
Rainfall Anomalies ◽  
Normal Rainfall

Abstract Before the eastward-propagating rainy envelope of a Madden–Julian oscillation (MJO) arrives at the Maritime Continent (MC), some islands in the MC experience dipolar patterns of rainfall variability with opposite signs of rainfall anomalies in two neighboring regions within an island. Similar incoherent rainfall anomalies are also observed after the MJO passed the MC. The mechanisms for these dipolar patterns of rainfall anomalies are investigated by using observed and reanalysis data. It is found that the response of rainfall in the MC depends on the direction of wind anomalies and the availability of atmospheric moisture in different phases of the MJO. The low-level wind anomalies over the MC are easterlies in MJO phases 1–3, which cause above-normal rainfall over the mountainous areas in Java, and in western Borneo, western Sumatra, and western Malay Peninsula, respectively. In phases 5–6, the low-level wind anomalies are westerlies and the positive rainfall anomalies are over the eastern part of the islands. Two physical mechanisms are responsible for this phenomenon of the dipolar patterns of rainfall anomalies: 1) the monsoonal damping effect on rainfall over elongated narrow islands—an inverse relationship between the intensity of the diurnal cycle of sea breezes and valley breezes and the large-scale monsoonal wind speed, and 2) the wake effect on rainfall over large and wide islands—above-normal rainfall on the downwind wake side of an island or mountain range with respect to large-scale wind anomalies.

Teleconnection between Summer NAO and East China Rainfall Variations: A Bridge Effect of the Tibetan Plateau

2018 ◽  
Vol 31 (16) ◽  
pp. 6433-6444 ◽  
Author(s):  
Ziqian Wang ◽  
Song Yang ◽  
Ngar-Cheung Lau ◽  
Anmin Duan
Keyword(s):  
Tibetan Plateau ◽  
Rainfall Variability ◽  
Lower Troposphere ◽  
Summer Rainfall ◽  
East China ◽  
The Tibetan Plateau ◽  
Thermal Forcing ◽  
Low Level ◽  
The Impact ◽  
Eurasian Teleconnection

Although the impact of the North Atlantic Oscillation (NAO), especially the antecedent NAO in winter and spring, on East Asian summer climate has been studied extensively, the possible connection from the summer NAO (SNAO) and then the Tibetan Plateau (TP) to East China summer rainfall remains unclear. This study reveals that on interannual time scales the SNAO is significantly correlated with the variations of East China summer rainfall and the thermal forcing of the TP provides an intermediate bridge effect in this Eurasian teleconnection. The SNAO primarily regulates the rainfall variability over the TP through large-scale wave trains and the TP rainfall anomalies in turn lead to a change in local diabatic heating, which excites Rossby waves to the downstream regions. To the northeast of the TP, an anomalous barotropic cyclone is formed in the nearly entire troposphere, generating low-level northerly flow anomalies over northern China. Meanwhile, the TP heating also induces low-level southerly flow anomalies over southern China. The anomalous northerly and southerly winds converge in the lower troposphere, enhancing the summer rainfall over central East China. Compared to the SNAO, the TP thermal forcing exerts a more direct impact on the variations of East China summer rainfall in the Eurasian teleconnection discussed.

scholarly journals The Role of Mesoscale Convective Complexes in Southern Africa Summer Rainfall

2013 ◽  
Vol 26 (5) ◽  
pp. 1654-1668 ◽  
Author(s):  
R. C. Blamey ◽  
C. J. C. Reason
Keyword(s):  
South Africa ◽  
Southern Africa ◽  
Rainfall Variability ◽  
Warm Season ◽  
Tropical Rainfall Measuring Mission ◽  
Summer Rainfall ◽  
Spatial And Temporal Variability ◽  
Eastern Region ◽  
Southwest Indian Ocean ◽  
Mesoscale Convective

Abstract A combination of numerous factors, including geographic position, regional orography, and local sea surface temperatures, means that subtropical southern Africa experiences considerable spatial and temporal variability in rainfall and is prone to both frequent flooding and drought events. One system that may contribute to rainfall variability in the region is the mesoscale convective complex (MCC). In this study, Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) data is used to document the precipitation produced by MCCs over southern Africa for the 1998–2006 period. Most of the rainfall associated with MCCs is found to occur over central Mozambique, extending southward to eastern South Africa. High precipitation totals associated with these systems also occur over the neighboring southwest Indian Ocean, particularly off the northeast coast of South Africa. MCCs are found to contribute up to 20% of the total summer rainfall (November–March) in parts of the eastern region of southern Africa. If the month of March is excluded from the analysis, then the contribution increases up to 24%. In general, the MCC summer rainfall contribution for most of the eastern region is approximately between 8% and 16%. Over the western interior and Botswana and Namibia, the MCC contribution is much less (<6%). It is also evident that there is considerable interannual variability associated with the contribution that these systems make to the total warm season rainfall.

scholarly journals On the Relationship between Winter Sea Ice and Summer Atmospheric Circulation over Eurasia

2013 ◽  
Vol 26 (15) ◽  
pp. 5523-5536 ◽  
Author(s):  
Bingyi Wu ◽  
Renhe Zhang ◽  
Rosanne D'Arrigo ◽  
Jingzhi Su
Keyword(s):  
Sea Ice ◽  
Atmospheric Circulation ◽  
Rainfall Variability ◽  
Summer Rainfall ◽  
Eastern Coast ◽  
Northern Eurasia ◽  
Sea Ice Concentration ◽  
The North ◽  
Circulation Anomalies ◽  
Atmospheric Circulation Anomalies

Abstract Using NCEP–NCAR reanalysis and Japanese 25-yr Reanalysis (JRA-25) data, this paper investigates the association between winter sea ice concentration (SIC) in Baffin Bay southward to the eastern coast of Newfoundland, and the ensuing summer atmospheric circulation over the mid- to high latitudes of Eurasia. It is found that winter SIC anomalies are significantly correlated with the ensuing summer 500-hPa height anomalies that dynamically correspond to the Eurasian pattern of 850-hPa wind variability and significantly influence summer rainfall variability over northern Eurasia. Spring atmospheric circulation anomalies south of Newfoundland, associated with persistent winter–spring SIC and a horseshoe-like pattern of sea surface temperature (SST) anomalies in the North Atlantic, act as a bridge linking winter SIC and the ensuing summer atmospheric circulation anomalies over northern Eurasia. Indeed, this study only reveals the association based on observations and simple simulation experiments with SIC forcing. The more precise mechanism for this linkage needs to be addressed in future work using numerical simulations with SIC and SST as the external forcings. The results herein have the following implication: Winter SIC west of Greenland is a possible precursor for summer atmospheric circulation and rainfall anomalies over northern Eurasia.

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