scholarly journals The Relationship of Rainfall Variability in Western Equatorial Africa to the Tropical Oceans and Atmospheric Circulation. Part II: The Boreal Autumn

2013 ◽  
Vol 26 (1) ◽  
pp. 66-84 ◽  
Author(s):  
Amin K. Dezfuli ◽  
Sharon E. Nicholson
Keyword(s):  
Atmospheric Circulation ◽  
Rainfall Variability ◽  
Sea Level Pressure ◽  
Tropical Oceans ◽  
Remote Forcing ◽  
Equatorial Africa ◽  
The Pacific ◽  
Dry Conditions ◽  
Relationship Of ◽  
East West

Abstract This paper examines the mechanisms controlling the year-to-year variability of rainfall over western equatorial Africa during the rainy season of October–December. Five regions with distinct behavior are analyzed separately. Only two show strong associations with the ocean and atmospheric features in the global tropics. These two regions, in the east (the eastern Zaire basin) and west (Angolan coast) of the study area, respectively, demonstrate strikingly opposite relationships with the anomalies of sea surface temperatures (SSTs), sea level pressure (SLP), and east–west atmospheric circulation. The wet (dry) conditions in the eastern Zaire basin are associated with El Niño(La Niña)–like phases. The inverse pattern is apparent for the Angolan coast. The other three regions, lying between these two poles of variability, represent a transition zone with a weak linear relationship to the circulation features. The vital impact of the east–west circulation cells on rainfall variability results in a stronger association with zonal wind than with SSTs or SLP. In addition to the zonal shift, changes in intensity of the zonal cells also play a crucial role. Variability in both magnitude and location of the circulation cells appear to be modulated by the remote forcing from the Pacific via an atmospheric bridge. However, the eastern sector is impacted mainly when synchronous changes occur in the Indian Ocean, and the western sector is impacted mainly when synchronous changes occur in the Atlantic Ocean.

scholarly journals The Relationship of Rainfall Variability in Western Equatorial Africa to the Tropical Oceans and Atmospheric Circulation. Part I: The Boreal Spring

2013 ◽  
Vol 26 (1) ◽  
pp. 45-65 ◽  
Author(s):  
Sharon E. Nicholson ◽  
Amin K. Dezfuli
Keyword(s):  
Large Scale ◽  
Rainfall Variability ◽  
Atlantic Coast ◽  
Atmospheric Parameters ◽  
Zonal Circulation ◽  
Remote Forcing ◽  
Equatorial Africa ◽  
The Pacific ◽  
Dry Conditions ◽  
The Relationship

Abstract This paper examines the factors governing rainfall variability in western equatorial Africa (WEA) during the April–June rainy season. In three of the five regions examined some degree of large-scale forcing is indicated, particularly in the region along the Atlantic coast. Interannual variability in this coastal sector also demonstrates a strong link to changes in local sea surface temperatures (SSTs) and the South Atlantic subtropical high. To examine potential causal mechanisms, various atmospheric parameters are evaluated for wet and dry composites. The results suggest that the intensity of the zonal circulation in the global tropics is a crucial control on rainfall variability over WEA. A La Niña (El Niño)–like signal in both SSTs and zonal circulation over the Pacific is apparent in association with the wet (dry) conditions in the western sector. However, remote forcing from the Pacific modulates the circulation over Africa indirectly by way of synchronous changes in the entire Indian or Atlantic Ocean. Anomalies in the local zonal winds are similar in all three regions: the wet (dry) composite is associated with an intensification (weakening) of the upper-tropospheric easterlies and low-level westerlies, but a weakening (intensification) of the midlevel easterlies. This work also suggests that, in most cases, the relationship between local SSTs and rainfall reflects a common remote forcing by the large-scale atmosphere–ocean system. This forcing is manifested via changes in the zonal circulation. Thus, the statistical associations between rainfall and SSTs do not indicate direct forcing by local SSTs. One point of evidence for this conclusion is the stronger association with atmospheric parameters than with SSTs.

scholarly journals Rainfall Response in Northeast Brazil from Ocean Climate Variability during the Second Half of the Twentieth Century

2011 ◽  
Vol 24 (23) ◽  
pp. 6174-6184 ◽  
Author(s):  
Daisy Beserra Lucena ◽  
Jacques Servain ◽  
Manoel Francisco Gomes Filho
Keyword(s):  
Twentieth Century ◽  
Rainfall Variability ◽  
Low Frequency ◽  
Northeast Brazil ◽  
Drought Impact ◽  
The North ◽  
Tropical Oceans ◽  
The Pacific ◽  
The North Atlantic Oscillation

Abstract The authors investigated the rainfall variability response in northern Northeast Brazil (NNEB) from El Niño/La Niña (EN/LN) events and from the meridional sea surface temperature gradient (MGRAD) over the tropical Atlantic during the period 1948–97. The diagnostic analysis was stratified according to four climatic series of scenarios associated with EN, LN, and positive and negative MGRAD. During ENs, which were more numerous and more intense after the 1970s, the MGRAD was generally not noticeable, and the drought impact in NNEB was mainly due to the warm Pacific influence. Conversely, during LNs, the MGRAD signal was important, but there was an inverse relationship between the third and the fourth quarters of the twentieth century. Thus, before the 1970s the LNs were associated with positive MGRAD, which led to an inverse influence inducing minor changes in seasonal rainfall in NNEB. After the 1970s the LNs were linked to negative MGRAD, which induced a cumulative wet influence in NNEB. The positive MGRADs were generally associated with ENs, which reinforced the drought impact in NNEB. The well-marked negative MGRADs, which all occurred after the beginning of the 1970s, were generally linked with large LNs that induced very consistent wet episodes in NNEB. Interestingly, the two low-frequency variations in the tropical oceans observed during the second half of the twentieth century (i.e., from a few to several strong ENs and from none to numerous strong negative MGRADs) occurred concomitantly with symmetric long-term changes in the Pacific decadal oscillation (PDO) and the North Atlantic Oscillation (NAO). This symmetrical long-term climate behavior during the second half of the twentieth century could have lead to an inverse influence on the climate over the north Northeast Brazil, in agreement with a quasi-null long-term trend of the rainfall observed in that region all along this period. Such symmetrical behavior seems to have been unique during the last 150 years.

scholarly journals Java-Sumatra Niño/Niña and associated regional rainfall variability

2021 ◽  
Author(s):  
Sang-Ki Lee ◽  
Hosmay Lopez ◽  
Gregory Foltz ◽  
Dongmin Kim ◽  
Sarah Larson ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Coastal Upwelling ◽  
Rainfall Variability ◽  
Equatorial Indian Ocean ◽  
Tropical Indian Ocean ◽  
Remote Forcing ◽  
The Pacific ◽  
Sea Surface Temperature Anomalies ◽  
The Indian Ocean ◽  
Asian Summer

Abstract A phenomenon referred to here as Java-Sumatra Niño/Niña (JSN or JS Niño/Niña) is characterized by the appearance of warm/cold sea surface temperature anomalies (SSTAs) in the coastal upwelling region off Java-Sumatra in the southeastern equatorial Indian Ocean. JSN develops in July-September and sometimes as a precursor to the Indian Ocean Dipole, but often without corresponding SSTAs in the western equatorial Indian Ocean. Although its spatiotemporal evolution varies considerably between individual events, JSN is essentially an intrinsic mode of variability driven by local atmosphere-ocean positive feedback, and thus does not rely on remote forcing from the Pacific for its emergence. JSN is an important driver of climate variability over the tropical Indian Ocean and the surrounding continents. Notably, JS Niña events developing in July-September project onto the South and Southeast Asian summer monsoons, increasing the probability of heavy rainfall and flooding across the most heavily populated regions of the world.

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.

scholarly journals Solar Cycle Signals in the Pacific and the Issue of Timings

2012 ◽  
Vol 69 (4) ◽  
pp. 1446-1451 ◽  
Author(s):  
Indrani Roy ◽  
Joanna D. Haigh
Keyword(s):  
Solar Activity ◽  
Solar Cycle ◽  
Aleutian Low ◽  
Positive Signal ◽  
Tropical Eastern Pacific ◽  
Sea Level Pressure ◽  
The Pacific ◽  
Strong Positive Signal ◽  
The Relationship ◽  
The Tropical Pacific

Abstract The solar cycle signal in sea level pressure during 1856–2007 is analyzed. Using composites of data from January–February in solar cycle peak years the strong positive signal in the region of the Aleutian low, found by previous authors, is confirmed. It is found, however, that signals in other regions of the globe, particularly in the South Pacific, are very sensitive to the choice of reference climatology. Also investigated is the relationship between solar activity and sea surface temperatures in the tropical eastern Pacific. A marked overall association of higher solar activity with colder temperatures in the tropical Pacific that is not restricted to years of peak sunspot number is noted. The ENSO-like variation following peak years that has been suggested by other authors is not found as a consistent signal. Both the SLP and SST signals vary coherently with the solar cycle and neither evolves on an ENSO-like time scale. The solar signals are weaker during the period spanning approximately 1956–97, which may be due to masking by a stronger innate ENSO variability at that time.

scholarly journals Rossby Waves Mediate Impacts of Tropical Oceans on West Antarctic Atmospheric Circulation in Austral Winter

2015 ◽  
Vol 28 (20) ◽  
pp. 8151-8164 ◽  
Author(s):  
Xichen Li ◽  
David M. Holland ◽  
Edwin P. Gerber ◽  
Changhyun Yoo
Keyword(s):  
Atmospheric Circulation ◽  
Rossby Wave ◽  
Rossby Waves ◽  
Ocean Warming ◽  
Austral Winter ◽  
West Antarctica ◽  
Tropical Ocean ◽  
Amundsen Sea ◽  
Tropical Oceans ◽  
Simulation Results

Abstract Recent studies link climate change around Antarctica to the sea surface temperature of tropical oceans, with teleconnections from the Pacific, Atlantic, and Indian Oceans making different contributions to Antarctic climate. In this study, the impacts of each ocean basin on the wintertime Southern Hemisphere circulation are identified by comparing simulation results using a comprehensive atmospheric model, an idealized dynamical core model, and a theoretical Rossby wave model. The results herein show that tropical Atlantic Ocean warming, Indian Ocean warming, and eastern Pacific cooling are all able to deepen the Amundsen Sea low located adjacent to West Antarctica, while western Pacific warming increases the pressure to the west of the international date line, encompassing the Ross Sea and regions south of the Tasman Sea. In austral winter, these tropical ocean basins work together linearly to modulate the atmospheric circulation around West Antarctica. Further analyses indicate that these teleconnections critically depend on stationary Rossby wave dynamics and are thus sensitive to the background flow, particularly the subtropical/midlatitude jet. Near these jets, wind shear is amplified, which strengthens the generation of Rossby waves. On the other hand, near the edges of the jets the meridional gradient of the absolute vorticity is also enhanced. As a consequence of the Rossby wave dispersion relationship, the jet edge may reflect stationary Rossby wave trains, serving as a waveguide. The simulation results not only identify the relative roles of each of the tropical ocean basins in the tropical–Antarctica teleconnection, but also suggest that a deeper understanding of teleconnections requires a better estimation of the atmospheric jet structures.

Intraseasonal-to-Interannual Variability of South Indian Ocean Sea Level and Thermocline: Remote versus Local Forcing

2012 ◽  
Vol 42 (4) ◽  
pp. 602-627 ◽  
Author(s):  
Laurie L. Trenary ◽  
Weiqing Han
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Sea Level ◽  
Time Scales ◽  
Ekman Pumping ◽  
Remote Forcing ◽  
South Indian ◽  
The Pacific ◽  
The Indian Ocean ◽  
Thermocline Variability

Abstract The relative importance of local versus remote forcing on intraseasonal-to-interannual sea level and thermocline variability of the tropical south Indian Ocean (SIO) is systematically examined by performing a suite of controlled experiments using an ocean general circulation model and a linear ocean model. Particular emphasis is placed on the thermocline ridge of the Indian Ocean (TRIO; 5°–12°S, 50°–80°E). On interannual and seasonal time scales, sea level and thermocline variability within the TRIO region is primarily forced by winds over the Indian Ocean. Interannual variability is largely caused by westward propagating Rossby waves forced by Ekman pumping velocities east of the region. Seasonally, thermocline variability over the TRIO region is induced by a combination of local Ekman pumping and Rossby waves generated by winds from the east. Adjustment of the tropical SIO at both time scales generally follows linear theory and is captured by the first two baroclinic modes. Remote forcing from the Pacific via the oceanic bridge has significant influence on seasonal and interannual thermocline variability in the east basin of the SIO and weak impact on the TRIO region. On intraseasonal time scales, strong sea level and thermocline variability is found in the southeast tropical Indian Ocean, and it primarily arises from oceanic instabilities. In the TRIO region, intraseasonal sea level is relatively weak and results from Indian Ocean wind forcing. Forcing over the Pacific is the major cause for interannual variability of the Indonesian Throughflow (ITF) transport, whereas forcing over the Indian Ocean plays a larger role in determining seasonal and intraseasonal ITF variability.

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