Assessing the Reliability of Satellite and Reanalysis Estimates of Rainfall in Equatorial Africa

This article examines the reliability of satellite and reanalysis estimates of rainfall in the Congo Basin and over Lake Victoria and its catchment. Nine satellite products and five reanalysis products are considered. They are assessed by way of inter-comparison and by comparison with observational data sets. The three locations considered include a region with little observational gauge data (the Congo), a region with extensive gauge data (Lake Victoria catchment), and an inland water body. Several important results emerge: for one, the diversity of estimates is generally very large, except for the Lake Victoria catchment. Reanalysis products show little relationship with observed rainfall or with the satellite estimates, and thus should not be used to assess rainfall in these regions. Most of the products either overestimate or underestimate rainfall over the lake. The diversity of estimates makes it difficult to assess the factors governing the interannual variability of rainfall in these regions. This is shown by way of correlation with sea-surface temperatures, particularly with the Niño 3.4 temperatures and with the Dipole Mode Index over the Indian Ocean. Some guidance is given as to the best products to utilize. Overall, any user must establish that the is product reliable in the region studied.

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CORRELATION AND COHERENCE ANALYSIS OF SEA SURFACE TEMPERATURE (SST) DISTRIBUTED BY THE SURFACE WIND IN WEST SUMATERA WATERS

Jurnal Meteorologi dan Geofisika ◽

10.31172/jmg.v19i2.572 ◽

2019 ◽

Vol 19 (2) ◽

pp. 81

Author(s):

Ulung Jantama Wisha ◽

Rahaden Bagas Hatmaja ◽

Ivonne Milichristi Radjawane ◽

Try Al Tanto

Keyword(s):

Wind Speed ◽

Indian Ocean ◽

Indian Ocean Dipole ◽

Surface Wind ◽

Surface Wind Speed ◽

Sea Surface ◽

Wavelet Coherence ◽

Dipole Mode ◽

The Indian Ocean ◽

Cross Wavelet

West Sumatera Waters have a tremendous dynamic in ocean characteristics. It directly faces the Indian Ocean exactly located below the equator. Consequently, West Sumatera waters are influenced by the tropical climatic factors such as monsoons, climate variability, and the Indian Ocean Dipole (IOD), controlling sea surface temperature (SST) fluctuation in the Indian Ocean. This study aims to review the correlation and coherence of SST distributed by surface wind in the West Sumatera waters. Wavelet method (cross wavelet transforms and wavelet coherence) was used to analyze the correlation and coherency between SST and surface wind. The annual variation of SST for 365 days period is the strongest event throughout the year caused by either monsoon or the changes of wind speed in the surface. Otherwise, the strongest intra-seasonal SST variation of 35 - 60 days observed from December 2012 to March 2013. The highest surface wind speed occurs in the southern and western waters. During the positive dipole mode in October 2015, the surface wind speed is slightly high resulting in the SST declination. Nevertheless, during the negative dipole mode in July 2016, the condition is inversely proportional. The surface wind plays a role in the SST distribution of 35 - 60 days period (intra-seasonal variability). Besides, surface wind with 6 months period (semi-annual variability) influences the SST distribution, identified only in the southern waters and the Indian Ocean regions. These conditions predicted as the influence of monsoon. Sumatera Barat merupakan wilayah perairan yang stategis dimana secara langsung berhadapan dengan Samudera Hindia dan tepat berada pada dibawah Garis Katulistiwa. Oleh karena itu, Perairan Sumatera Barat dipengaruhi oleh faktor-faktor iklim tropis seperti monsun dan variabilitas iklim, sangat terkait dengan Indian Ocean Dipole (IOD) yang mengendalikan fluktuasi suhu permukaan laut (SPL) di Samudera Hindia. Tujuan dari penelitian ini adalah menelaah korelasi dan koherensi antara parameter SPL dan komponen kecepatan angin di perairan Sumatera Barat. Metode wavelet (cross wavelet transform dan wavelet coherence) digunakan untuk menganalisa korelasi dan koherensi dari kedua parameter yang diuji. Variasi tahunan dari SPL pada periode 365 hari merupakan kejadian terkuat sepanjang tahun yang disebabkan oleh monsun atau perubahan pengaruh angin dipermukaan. Sebaliknya, variasi musiman terkuat dari SPL pada periode 35-60 hari ditemukan terjadi pada bulan Desember 2012 hingga Maret 2013. Kecepatan angin tertinggi terjadi di perairan selatan dan barat. Selama dipole mode positif pada bulan Oktober 2015, kecepatan angin permukaan sedikit meningkat yang mengakibatkan penurunan suhu perairan. Namun, selama dipole mode negatif pada bulan Juli 2016, kondisinya berbanding terbalik. Angin permukaan memainkan peran pada peningkatan distribusi suhu permukaan laut pada periode 35-60 hari (variabilistas musiman). Selain itu, angin permukaan dengan periode 6 bulan (tengah tahunan) sangat mempengaruhi distribusi suhu yang teridentifikasi pada wilayah selatan dan Samudera Hindia. Kondisi tersebut diperkirakan sebagai pengaruh dari monsun.

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The Influence of Indian Ocean Dipole on Production of Tembang Fishing (Sardinella fimbriata) at Sunda Strait Indonesia

Asian Journal of Fisheries and Aquatic Research ◽

10.9734/ajfar/2020/v7i330122 ◽

2020 ◽

pp. 51-58

Author(s):

Delima Mentari Amara ◽

Yuniar Mulyani ◽

Alexander M. A. Khan ◽

Herman Hamdani

Keyword(s):

Indian Ocean ◽

Surface Temperature ◽

Sea Surface Temperature ◽

Indian Ocean Dipole ◽

Pelagic Fish ◽

Sea Surface ◽

Dipole Mode ◽

Dipole Mode Index ◽

Mode Index ◽

The Indian Ocean

Tembang is a pelagic fish which is important in Indonesia and the development on the Sunda Strait. The Indian Ocean Dipole could affect oceanography and at the same time will affect the population of fishes. The aim of this study was to determine the effect of IOD and oceanographic factors on the catch of Tembang fish. This research was conducted in the Sunda Strait waters by looking at the Dipole Mode Index (DMI) and oceanographic ocean conditions such as sea surface temperature and chlorophyll as well as the production of fish catches for 11 years from 2008-2018. IOD affects the catch of Tembang fish by 35.8%. Temperature influences the catch of Tembang fish in the Sunda Strait by 9.48%. Klorofil-a influences the catch of Tembang fish in Sunda Strait by 38.6%. DMI, Temperature, and Chlorophyll affect fish catches by 26.9%.

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Customization and Validation of a Regional Climate Model Using Satellite Data Over East Africa

Atmosphere ◽

10.3390/atmos10060317 ◽

2019 ◽

Vol 10 (6) ◽

pp. 317

Author(s):

Masilin Gudoshava ◽

Fredrick H. M. Semazzi

Keyword(s):

Indian Ocean ◽

Surface Temperature ◽

East Africa ◽

Climate Model ◽

Lake Victoria ◽

Regional Climate ◽

Sea Surface ◽

Modes Of Variability ◽

The Indian Ocean ◽

Short Rains

This study focused on the customization of the fourth generation International Center for Theoretical Physics Regional Climate Model version 4.4 and its ability to reproduce the mean climate and most dominant modes of variability over East Africa. The simulations were performed at a spatial resolution of 25 km for the period 1998–2013. The model was driven by ERA-Interim reanalysis. The customization focus was on cumulus and microphysics schemes during the Short Rains for the year 2000. The best physics combinations were then utilized for the validation studies. The East Africa region and Lake Victoria Basin region are adapted to carry out empirical orthogonal function analysis, during the Short and Long Rains. Tropical Rainfall Measuring Mission data was utilized in the validation of the model. The first mode of variability from the model and observational data during the Short Rains was associated with the warming of the Pacific Ocean and the sea surface temperature gradients over the Indian Ocean. During the Long rains, the inter-annual rainfall variability over the Lake Victoria region was associated with the sea surface temperature anomaly over the Indian Ocean and for the East Africa region the associations were weak. The drivers during the Long Rains over East Africa region were then further investigated by splitting the season to the March–April and May periods. The March–April period was positively correlated to the West Pacific and Indian Ocean dipole index, while May was associated with the Quasi-Biennial Oscillation. In conclusion, although the model can reproduce the dominant modes of variability as in the observational data sets during the Short Rains, skill was lower during the Long Rains.

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Two Independent Triggers for the Indian Ocean Dipole/Zonal Mode in a Coupled GCM

Journal of Climate ◽

10.1175/jcli3478.1 ◽

2005 ◽

Vol 18 (17) ◽

pp. 3428-3449 ◽

Cited By ~ 114

Author(s):

Albert S. Fischer ◽

Pascal Terray ◽

Eric Guilyardi ◽

Silvio Gualdi ◽

Pascale Delecluse

Keyword(s):

Indian Ocean ◽

El Niño ◽

Indian Ocean Dipole ◽

El Nino ◽

Tropical Indian Ocean ◽

Sea Surface ◽

Dynamical Response ◽

Thermocline Depth ◽

Second Phase ◽

The Indian Ocean

Abstract The question of whether and how tropical Indian Ocean dipole or zonal mode (IOZM) interannual variability is independent of El Niño–Southern Oscillation (ENSO) variability in the Pacific is addressed in a comparison of twin 200-yr runs of a coupled climate model. The first is a reference simulation, and the second has ENSO-scale variability suppressed with a constraint on the tropical Pacific wind stress. The IOZM can exist in the model without ENSO, and the composite evolution of the main anomalies in the Indian Ocean in the two simulations is virtually identical. Its growth depends on a positive feedback between anomalous equatorial easterly winds, upwelling equatorial and coastal Kelvin waves reducing the thermocline depth and sea surface temperature off the coast of Sumatra, and the atmospheric dynamical response to the subsequently reduced convection. Two IOZM triggers in the boreal spring are found. The first is an anomalous Hadley circulation over the eastern tropical Indian Ocean and Maritime Continent, with an early northward penetration of the Southern Hemisphere southeasterly trades. This situation grows out of cooler sea surface temperatures in the southeastern tropical Indian Ocean left behind by a reinforcement of the late austral summer winds. The second trigger is a consequence of a zonal shift in the center of convection associated with a developing El Niño, a Walker cell anomaly. The first trigger is the only one present in the constrained simulation and is similar to the evolution of anomalies in 1994, when the IOZM occurred in the absence of a Pacific El Niño state. The presence of these two triggers—the first independent of ENSO and the second phase locking the IOZM to El Niño—allows an understanding of both the existence of IOZM events when Pacific conditions are neutral and the significant correlation between the IOZM and El Niño.

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Trapped Planetary (Rossby) Waves Observed in the Indian Ocean by Satellite Borne Altimeters

10.5194/os-2017-3 ◽

2017 ◽

Author(s):

Yair De-Leon ◽

Nathan Paldor

Keyword(s):

Indian Ocean ◽

Rossby Waves ◽

Low Frequency ◽

Propagation Speed ◽

Wave Theory ◽

Planetary Waves ◽

Sea Surface ◽

Trapped Wave ◽

The Indian Ocean ◽

Frequency Variations

Abstract. Using 20 years of accurately calibrated, high resolution, observations of Sea Surface Height Anomalies (SSHA) by satellite ‎borne altimeters we show that in the Indian Ocean south of the Australian coast the low frequency variations of SSHA are ‎dominated by westward propagating, trapped, i.e. non-harmonic, planetary waves. Our results demonstrate that the ‎meridional-dependent amplitudes of the SSHA are large only within a few degrees of latitude next to the South-Australian ‎coast while farther in the ocean they are uniformly small. This meridional variation of the SSHA signal is typical of the ‎amplitude structure in the trapped wave theory. The westward propagation speed of the SSHA signals is analyzed by ‎employing three different methods of estimation. Each one of these methods yields speed estimates that can vary widely ‎between adjacent latitudes but the combination of at least two of the three methods yields much smoother variation. The ‎estimates obtained in this manner show that the observed phase speeds at different latitudes exceed the phase speeds of ‎harmonic Rossby (Planetary) waves by 140 % to 200 %. In contrast, the theory of trapped Rossby (Planetary) waves in a ‎domain bounded by a wall on its equatorward side yields phase speeds that approximate more closely the observed phase ‎speeds.‎

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New Rainfall Datasets for the Congo Basin and Surrounding Regions

Journal of Hydrometeorology ◽

10.1175/jhm-d-18-0015.1 ◽

2018 ◽

Vol 19 (8) ◽

pp. 1379-1396 ◽

Cited By ~ 14

Author(s):

S. E. Nicholson ◽

D. Klotter ◽

A. K. Dezfuli ◽

L. Zhou

Keyword(s):

Congo Basin ◽

Annual Rainfall ◽

Reconstruction Method ◽

Validation Period ◽

Grid Data ◽

Statistical Reconstruction ◽

Equatorial Africa ◽

Spatial Reconstruction ◽

Gauge Data ◽

Dense Coverage

Abstract This paper describes three new rainfall datasets that have been developed for equatorial Africa. The development relies on acquisition of recent gauge data from the relevant countries and statistical methods to fill in gaps in coverage. Two of the three datasets are gridded with spatial resolutions of 2.5° and 5.0°, and the third is regionally aggregated and based purely on gauge data. The work is based on a total of 1826 gauge records in the analysis sector, of which only several hundred operate in recent years. The gridded datasets were produced and validated by using a period of dense coverage (1947–72) to “calibrate” a spatial reconstruction method, which is then utilized to grid data for the remaining years. The period 1973–2010 served as a validation period. The validation was carried out by comparing the gridded values with values obtained by simple averaging of station data in grid boxes with an adequate number of stations. This exercise clearly showed that the statistical reconstruction approach based on principal components produced far superior results than those from the more commonly used kriging. The gridded datasets cover each month of the year, six seasons, and annual rainfall, and they commence in 1921 and extend through 2014. In contrast, the gauge-only regional dataset covers varied time periods, depending on the geographical region in question. Records for several regions cover nearly all of the twentieth century and most extend to 2014.

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Assessing Sea Surface Salinity Derived by Aquarius in the Indian Ocean

IEEE Geoscience and Remote Sensing Letters ◽

10.1109/lgrs.2013.2277391 ◽

2014 ◽

Vol 11 (4) ◽

pp. 719-722 ◽

Cited By ~ 7

Author(s):

Smitha Ratheesh ◽

Rashmi Sharma ◽

Rajesh Sikhakolli ◽

Raj Kumar ◽

Sujit Basu

Keyword(s):

Indian Ocean ◽

Sea Surface ◽

Sea Surface Salinity ◽

Surface Salinity ◽

The Indian Ocean

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Variations of the Indian Ocean Walker circulation since the Last Glacial Maximum revealed by reconstructed and simulated zonal wind intensity

10.5194/egusphere-egu21-12762 ◽

2021 ◽

Author(s):

Xinquan Zhou ◽

Stéphanie Duchamp-Alphonse ◽

Masa Kageyama ◽

Franck Bassinot ◽

Xiaoxu Shi ◽

...

Keyword(s):

Indian Ocean ◽

Surface Temperature ◽

Equatorial Indian Ocean ◽

Walker Circulation ◽

Sea Surface ◽

The Indian Ocean ◽

Sst Anomaly ◽

Mean Circulation ◽

The Last Glacial Maximum ◽

The Last Glacial

Today, precipitation and wind patterns over the equatorial Indian Ocean and surrounding lands are paced by monsoon and Walker circulations that are controlled by the seasonal land-sea temperature contrast and the inter-annual convection over the Indo-Pacific Warm Pool, respectively. The annual mean surface westerly winds are particularly tied to the Walker circulation, showing interannual variability coupled with the gradient of Sea Surface Temperature (SST) anomaly between the tropical western and southeastern Indian Ocean, namely, the Indian Ocean Dipole (IOD). While the Indian monsoon pattern has been widely studied in the past, few works deal with the evolution of Walker circulation despite its crucial impacts on modern and future tropical climate systems. Here, we reconstruct the long-term westerly (summer) and easterly (winter) wind dynamics of the equatorial Indian Ocean (10&#176;S&#8722;10&#176;N), since the Last Glacial Maximum (LGM) based on i) primary productivity (PP) records derived from coccolith analyses of sedimentary cores MD77-191 and BAR94-24, retrieved off the southern tip of India and off the northwestern tip of Sumatra, respectively and ii) the calculation of a sea surface temperature (SST) anomaly gradient off (south) western Sumatra based on published SST data. We compare these reconstructions with atmospheric circulation simulations obtained with the general coupled model AWI-ESM-1-1-LR (Alfred Wegener Institute Earth System Model).Our results show that the Indian Ocean Walker circulation was weaker during the LGM and the early/middle Holocene than present. Model simulations suggest that this is due to anomalous easterlies over the eastern Indian Ocean. The LGM mean circulation state may have been comparable to the year 1997 with a positive IOD, when anomalously strong equatorial easterlies prevailed in winter. The early/mid Holocene mean circulation state may have been equivalent to the year 2006 with a positive IOD, when anomalously strong southeasterlies prevailed over Java-Sumatra in summer. The deglaciation can be seen as a transient period between these two positive IOD-like mean states.

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Interannual Variability in Sea Surface Height at Southern Midlatitudes of the Indian Ocean

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0279.1 ◽

2021 ◽

Vol 51 (5) ◽

pp. 1595-1609

Author(s):

Motoki Nagura ◽

Michael J. McPhaden

Keyword(s):

Indian Ocean ◽

Interannual Variability ◽

Sea Surface Height ◽

Wind Forcing ◽

Sea Surface ◽

The South ◽

South Indian Ocean ◽

Eastern Boundary ◽

South Indian ◽

The Indian Ocean

AbstractThis study examines interannual variability in sea surface height (SSH) at southern midlatitudes of the Indian Ocean (10°–35°S). Our focus is on the relative role of local wind forcing and remote forcing from the equatorial Pacific Ocean. We use satellite altimetry measurements, an atmospheric reanalysis, and a one-dimensional wave model tuned to simulate observed SSH anomalies. The model solution is decomposed into the part driven by local winds and that driven by SSH variability radiated from the western coast of Australia. Results show that variability radiated from the Australian coast is larger in amplitude than variability driven by local winds in the central and eastern parts of the south Indian Ocean at midlatitudes (between 19° and 33°S), whereas the influence from eastern boundary forcing is confined to the eastern basin at lower latitudes (10° and 17°S). The relative importance of eastern boundary forcing at midlatitudes is due to the weakness of wind stress curl anomalies in the interior of the south Indian Ocean. Our analysis further suggests that SSH variability along the west coast of Australia originates from remote wind forcing in the tropical Pacific, as is pointed out by previous studies. The zonal gradient of SSH between the western and eastern parts of the south Indian Ocean is also mostly controlled by variability radiated from the Australian coast, indicating that interannual variability in meridional geostrophic transport is driven principally by Pacific winds.

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