Western Tropical Indian Ocean Research Articles

Interdecadal Indian Ocean Basin Mode Driven by Interdecadal Pacific Oscillation: A Season-Dependent Growth Mechanism

The interdecadal variability of basinwide sea surface temperature anomalies (SSTAs) in the tropical Indian Ocean (TIO), referred to as the interdecadal Indian Ocean basin mode (ID-IOBM), is caused by remote forcing of the interdecadal Pacific oscillation (IPO), as demonstrated by the observational datasets and tropical Pacific pacemaker experiments of the Community Earth System Model (CESM). It is noted that the growth of the ID-IOBM shows a season-dependent characteristic, with a maximum tendency of mixed layer heat anomalies occurring in early boreal winter. Three factors contribute to this maximum tendency. In response to the positive IPO forcing, the eastern TIO is covered by the descending branch of the anomalous Walker circulation. Thus, the convection over the southeastern TIO is suppressed, which increases local downward shortwave radiative fluxes. Meanwhile, the equatorial easterly anomalies to the west of the suppressed convection weaken the background mean westerly and thus decrease the upward latent heat fluxes over the equatorial Indian Ocean. Third, anomalous westward Ekman currents driven by the equatorial easterly anomalies advect climatological warm water westward and thus warm the western TIO. In summer, the TIO is out of the control of the positive IPO remote forcing. The ID-IOBM gradually decays due to the Newtonian damping effect.

Impacts of large-scale teleconnections on climate variability over Southwest Asia

The atmospheric low frequency variability at a regional or global scale is represented by teleconnection. Using monthly dataset of the Climatic Research Unit (CRU) for the period 1971–2016, the impacts of four large-scale teleconnection patterns on the climate variability over Southwest Asia are investigated. The large-scale features include the El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) and the East Atlantic (EA) teleconnection patterns, as well as western tropical Indian Ocean (WTIO) sea surface temperature anomaly index. Results indicate that ENSO and EA are the first leading modes that explain variation of Southwest Asian precipitation, with positive (negative) anomalies during El Niño (La Niña) and the negative (positive) phase of EA. Variation of Southwest Asian near-surface temperature is most strongly related to WTIO index, with above-average (below-average) temperature during the positive (negative) phase of WTIO index, although the negative (positive) phase of NAO also favours the above-average (below-average) temperature. On the other hand, temperature (precipitation) over Southwest Asia shows the least response to ENSO (WTIO). ENSO and EA individually explain 13 percent annual variance of precipitation, while WTIO index explains 36 percent annual variance of near-surface temperature over Southwest Asia. Analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis Interim (ERA-Interim) data indicated establishments of negative (positive) geopotential height anomalies in the middle troposphere over Southwest Asia during El Niño (La Niña) or the negative (positive) phase of NAO, EA and WTIO. The response of precipitation variability over Southwest Asia to NAO is opposite to that expected from the geopotential height anomalies, but the correlation between precipitation and NAO is not statistically significant. Due to predictability of large-scale teleconnections, results of this study are encouraging for improvement of the state-of-the-art seasonal prediction of the climate over Southwest Asia.

New real-time indices for the quasi-biweekly oscillation over the Asian summer monsoon region

The quasi-biweekly oscillation (QBWO), with a periodicity of 10–30 days, is one of the most important modes of intraseasonal variability over the Asian summer monsoon region. Unlike the lower frequency (30–60 days) Madden–Julian Oscillation, which has had several real-time indices proposed for monitoring and predicting its state, there are very few equivalents for the QBWO. Given that extended empirical orthogonal function (EEOF) analysis is arguably more suitable than empirical orthogonal function analysis for capturing the canonical spatial and temporal structure of a propagating wave system, we introduce two new indices for real-time monitoring of the QBWO over the tropical western North Pacific (WNP) and Indian Ocean (IO), separately, by projecting outgoing longwave radiation (OLR) anomalies onto the two leading modes of EEOF analysis for the 10–30-day-filtered OLR anomalies over the two basins during 1980–2012. The newly proposed QBWO indices faithfully represent the vigorous QBWO activities over the WNP and IO, which are underestimated and/or not necessarily well represented by existing indices. It is also shown that the indices are able to reveal the connections between the QBWO and other pronounced phenomena, such as the onset of the South China Sea and Indian monsoons, the occurrence of extreme rainfall events, and tropical cyclone genesis. Thus, monitoring and predicting the evolution of the QBWO in these two basins based on the new QBWO indices should help in the quest to mitigate future damage caused by weather-related disasters at an extended lead time.

Dynamics of 2015 positive Indian Ocean Dipole

Evolution of typical positive Indian Ocean Dipole (pIOD) event was dominated by a significant sea-surface temperature (SST) cooling in the south-eastern tropical Indian Ocean. Interestingly, during the evolution of 2015 pIOD event, the SST in the south-eastern tropical Indian Ocean did not reveal significant cooling, instead anomalous strong SST warming took place in the western tropical Indian Ocean off the East African coast. This anomalous SST warming was associated with a weakening of the Asian summer monsoon. Furthermore, analysis on the mixed layer heat budget demonstrated that the evolution of the 2015 pIOD event could be attributed mainly to the air-sea heat flux. By decomposing the air-sea heat flux, it is found that reduced latent heat loss plays an important role on the SST warming in the western pole and keeping SST warm in the eastern pole. We note that a residual term also may play a role during the initial development of the event. In contrast to the SST pattern, the subsurface temperature revealed a clear positive dipole pattern. Shallow (deep) 20°C isothermal layer in the eastern (western) equatorial Indian Ocean was observed during boreal summer. This robust subsurface dipole pattern indicated that the subsurface ocean response was largely wind driven through the equatorial wave dynamics as previously suggested.

Southern Hemisphere Origins for Interannual Variations of Snow Cover over the Western Tibetan Plateau in Boreal Summer

The climate response to the Tibetan Plateau (TP) snow cover (TPSC) has been receiving extensive concern. However, relatively few studies have been devoted to revealing the potential factors that can contribute to the TPSC interannual variability, especially during boreal summer. This study finds that the May Southern Hemisphere (SH) annular mode (SAM), the dominating mode of atmospheric circulation variability in the SH extratropics, exhibits a significant positive relationship with the interannual variations in western TPSC during boreal summer. Observational analysis and numerical experiments manifest that the signal of the May SAM can be “prolonged” by a meridional Indian Ocean tripole (IOT) sea surface temperature anomaly (SSTA) via atmosphere–ocean interaction. The IOT SSTA pattern persists into the following summer and excites anomalous local-scale zonal–vertical circulation. Subsequently, a tropical dipole rainfall (TDR) mode is induced with precipitation anomalies between the tropical western Indian Ocean and the eastern Indian Ocean–Maritime Continent. Rossby wave ray tracing diagnosis reveals that the wave energies, generated by the latent heat release of the TDR mode, can propagate northward into the western TP. As a response, abnormal cyclone (or anticyclone) and upward (or downward) movement are triggered over the western TP, providing favorable dynamical conditions for more (or less) TPSC. Moreover, the strong May SAM is usually followed by a cold air temperature anomaly over the western TP in summer, which is unfavorable for snow-cover melting, and vice versa. In brief, the IOT SSTA plays an “ocean bridge” role and the TDR mode plays an “atmosphere bridge” role in the process of the May SAM impacting the following summer TPSC variability. The results may provide new insight into the cross-equatorial propagation of the SAM influence.

A spurious positive Indian Ocean Dipole in 2017

The Indian Ocean Dipole (IOD) is an intrinsic ocean-atmosphere coupled mode in the tropical Indian Ocean (IO). It features a sea surface temperature (SST) anomaly gradient, defined by the dipole mode index (DMI), the difference of area-averaged SSTanomaly between the western tropical IO (50°–70°E, 10°N–10°S) and the southeastern tropical IO (90°–110°E, Eq.-10°S). The IOD event tends to develop and mature in the boreal summer and fall seasons. When a positive IOD (pIOD) occurs, SST warms up in the west and cools down in the east, which sets up an east-west pressure gradient. The atmosphere hence responds to the anomalous SST gradient. Anomalous easterly winds prevail along the equator driven by the zonal pressure gradient, causing low sea level anomaly (SLA) in the eastern tropical IO. Rainless weather dominates over the Maritime Continent due to a weakened convection. Anomalies of an opposing polarity are generated during a negative IOD event.

Modelling the precipitation response over southern Africa to the 2009–2010 El Niño using a stretched grid global atmospheric model

The 2009–2010 El Nino event was a moderately strong Central Pacific type ENSO and, as such, expected to lead to a substantial drought over southern Africa during austral summer. However, many parts of the region experienced average to well above average rainfall during this season. Given that seasonal forecasting skill tends only to be good during ENSO seasons and many parts of the sub-continent rely on rain-fed agriculture, understanding why the rainfall was so anomalous during this El Nino event is important. To this end, a global stretched grid atmospheric general circulation model with relatively high horizontal resolution of 0.5° × 0.5° is applied over the region with observed and idealised SST forcing. In the latter experiments, SSTs are kept as observed over one ocean basin (e.g. Pacific) and as climatology over the other two basins (e.g. Atlantic, Indian), or as climatology over one basin and as observed over the other two basins in order to help understand which of these three basins may have contributed most to the observed rainfall anomalies in summer 2009–2010. It is found that Pacific Ocean forcing contributed to the summer rainfall anomalies over most regions of southern Africa with the Atlantic acting to oppose the effects of the Pacific. However, over Mozambique, and to some extent Zimbabwe, the Indian Ocean was more important with the Atlantic again acting in opposition. The increased rainfall over most of southern Africa appears mainly due to a stronger Angola Low circulation system and to anomalously strong moisture flux from the tropical western Indian Ocean and, to lesser extent, the tropical South East Atlantic Ocean.

Loss of coral reef growth capacity to track future increases in sea level.

Sea-level rise (SLR) is predicted to elevate water depths above coral reefs and to increase coastal wave exposure as ecological degradation limits vertical reef growth, but projections lack data on interactions between local rates of reef growth and sea level rise. Here we calculate the vertical growth potential of more than 200 tropical western Atlantic and Indian Ocean reefs, and compare these against recent and projected rates of SLR under different Representative Concentration Pathway (RCP) scenarios. Although many reefs retain accretion rates close to recent SLR trends, few will have the capacity to track SLR projections under RCP4.5 scenarios without sustained ecological recovery, and under RCP8.5 scenarios most reefs are predicted to experience mean water depth increases of more than 0.5 m by 2100. Coral cover strongly predicts reef capacity to track SLR, but threshold cover levels that will be necessary to prevent submergence are well above those observed on most reefs. Urgent action is thus needed to mitigate climate, sea-level and future ecological changes in order to limit the magnitude of future reef submergence.

Contrasting the impacts of the 1997–1998 and 2015–2016 extreme El Niño events on the East Asian winter atmospheric circulation

This study investigates how the Indian Ocean Dipole (IOD) modulated the influence of the 1997–1998 and 2015–2016 El Nino events on the East Asian winter atmospheric circulation. Due to the extreme IOD in the fall of 1997, the tropical Indian Ocean (TIO) displayed higher sea surface temperature (SST) anomalies in the western basin than in the eastern basin during the following winter. As a result, the Walker circulation weakened in the TIO and tropical Pacific, leading to a zonal rainfall dipole in the Indo-Western Pacific (IWP), which triggered a wave train pattern over East Asia with positive geopotential height anomalies in the upper-level troposphere over northeastern China and Japan during the winter of 1997–1998. In contrast, there was no IOD in the fall of 2015, and the winter rainfall anomaly in the western TIO was weak that year. The geopotential height anomalies were negative around northeastern China and the Sea of Japan—the opposite of those during the winter of 1997–1998. Comparing the historical El Nino events with the high IOD index and low IOD index indicates that the IOD is a crucial contributor to the dipole rainfall in the IWP and, thus, the circulation anomalies in East Asia.

Severe Drought Event in Indonesia Following 2015/16 El Niño/positive Indian Dipole Events

During boreal fall and winter 2015/16, Indonesia experienced catastrophic drought event causing many environmental problems. This study explored dynamical evolution of drought event in Indonesia associated with those two climate modes. Based on the Niño3.4 index, the evolution of the El Niño has started in April 2015, reached its peak in January 2016 and terminated in April 2016. Meanwhile, the Dipole Mode Index (DMI) revealed that the evolution of positive Indian Ocean Dipole has started in August, reached its peak in September and terminated in November 2015. It is shown that during those two events, Indonesia experienced severe drought in which the precipitation was extremely decreased. During the peak drought condition co-occurring with the dry season, the anomalous of precipitation reached −450 mm/month in September 2015. The peak of the drought was associated with the El Niño and positive Indian Ocean Dipole sea surface temperature anomaly (SSTA) patterns, in which negative SSTA covered the eastern tropical Indian Ocean and the western Pacific Ocean including Indonesia seas. Meanwhile, positive SSTA observed in the western tropical Indian Ocean and Eastern Pacific Ocean.

The Influence of Air‐Sea Fluxes on Atmospheric Aerosols During the Summer Monsoon Over the Tropical Indian Ocean

AbstractDuring the summer monsoon, the western tropical Indian Ocean is predicted to be a hot spot for dimethylsulfide emissions, the major marine sulfur source to the atmosphere, and an important aerosol precursor. Other aerosol relevant fluxes, such as isoprene and sea spray, should also be enhanced, due to the steady strong winds during the monsoon. Marine air masses dominate the area during the summer monsoon, excluding the influence of continentally derived pollutants. During the SO234‐2/235 cruise in the western tropical Indian Ocean from July to August 2014, directly measured eddy covariance DMS fluxes confirm that the area is a large source of sulfur to the atmosphere (cruise average 9.1 μmol m−2 d−1). The directly measured fluxes, as well as computed isoprene and sea spray fluxes, were combined with FLEXPART backward and forward trajectories to track the emissions in space and time. The fluxes show a significant positive correlation with aerosol data from the Terra and Suomi‐NPP satellites, indicating a local influence of marine emissions on atmospheric aerosol numbers.

Effect of Aerosols on Ocean Parameters in India by Using Satellite Data

The rapid industrial and economic development in India leads to high level of pollution in environment. Due to increased level of aerosols oceans are warming. This paper highlights on aerosols effect on sea surface wind and sea surface temperature by using remote sensing data. The windplays important role in Aerosol Optical Depth (AOD) and radiative forcing and is analyzed using National Centers for Environmental Prediction (NCEP) monthly wind data. The radiative forcing observed is much higher and up to 60% of total AOD during summer monsoon. SeaSurface Temperature (SST) is another important parameter in ocean atmosphere system and a key variable in coupling the atmosphere and ocean. The SST has changed during the change of atmospheric pattern and it plays an important role in aerosols mechanism. The study observed that during summer monsoon SST over the South Eastern Tropical Indian Ocean (SETIO) much higher than Western Tropical Indian Ocean (WTIO) and it plays important role in aerosols mechanism.

Industrial-age doubling of snow accumulation in the Alaska Range linked to tropical ocean warming

Future precipitation changes in a warming climate depend regionally upon the response of natural climate modes to anthropogenic forcing. North Pacific hydroclimate is dominated by the Aleutian Low, a semi-permanent wintertime feature characterized by frequent low-pressure conditions that is influenced by tropical Pacific Ocean temperatures through the Pacific-North American (PNA) teleconnection pattern. Instrumental records show a recent increase in coastal Alaskan precipitation and Aleutian Low intensification, but are of insufficient length to accurately assess low frequency trends and forcing mechanisms. Here we present a 1200-year seasonally- to annually-resolved ice core record of snow accumulation from Mt. Hunter in the Alaska Range developed using annual layer counting and four ice-flow thinning models. Under a wide range of glacier flow conditions and layer counting uncertainty, our record shows a doubling of precipitation since ~1840 CE, with recent values exceeding the variability observed over the past millennium. The precipitation increase is nearly synchronous with the warming of western tropical Pacific and Indian Ocean sea surface temperatures. While regional 20th Century warming may account for a portion of the observed precipitation increase on Mt. Hunter, the magnitude and seasonality of the precipitation change indicate a long-term strengthening of the Aleutian Low.

Sporolithon indopacificum sp. nov. (Sporolithales, Rhodophyta) from tropical western Indian and western Pacific oceans: First report, confirmed by DNA sequence data, of a widely distributed species of Sporolithon

Sporolithon indopacificum sp. nov. is characterized from Zanzibar Island, Tanzania. rbcL and psbA markers indicate that the species is unique compared to all other Sporolithon species that have thus far been sequenced. The species previously had been collected at Nukulau, Suva, Fiji and identified as S. episporum. The two species bear a close morpho-anatomical resemblance differing only by degree in three measured tetra/bisporangial characters. This is the first Sporolithon species confirmed to be widely distributed in the Indo-West Pacific based on DNA sequences.

Insolation forcing of coccolithophore productivity in the western tropical Indian Ocean over the last two glacial‐interglacial cycles

AbstractWe present a new coccolithophore productivity reconstruction spanning the last 300 ka in core GeoB12613‐1 retrieved from the western tropical Indian Ocean (IO), an area that mainly derives its warm and oligotrophic surface waters from the eastern IO. Application of a calibrated assemblage‐based productivity index indicates a reduction in estimated primary productivity (EPP) from 300 ka to the present, with reconstructed EPP values ranging from 91 to 246 g C/m2/yr. Coccolithophore assemblages and coccolith fraction Sr/Ca indicate three main phases of productivity change, with major changes at 160 and 46 ka. The productivity and water‐column stratification records show both dominant precession and obliquity periodicities, which appear to control the paleoproductivity in the study area over the last two glacial‐interglacial cycles. Shallowing of the thermocline due to strengthening of the trade winds in response to insolation maxima resulted to peaks in EPP. Comparison with the eastern IO productivity and stratification coccolithophore data reveals good correspondence with our records, indicating a strong tropical Pacific influence in our study area. Both of these records show high productivity from 300 to 160 ka, interpreted to be due to stronger Walker Circulation while the declining productivity from 160 ka to the present day is a consequence of its weakening intensity.

Why was the western Pacific subtropical anticyclone weaker in late summer after the 2015/2016 super El Niño?

The western Pacific subtropical anticyclone (WPSA), which bridges the El Niño events and the East Asian summer monsoon, is generally enhanced in boreal summer after El Niño events. However, it becomes weaker in boreal late summer (July–August) of 2016, following the 2015/2016 super El Niño event, indicating failure of the previous experience. Our results suggest that the weaker WPSA in late summer 2016 is associated with the extinction of the Indian Ocean capacitor effect, presenting the cold sea surface temperature anomalies (SSTAs) in the western tropical Indian Ocean (WTIO) after the fast decaying of the 2015/2016 super El Niño in spring. Compared with the case in 1982/1983 and 1997/1998, both data analysis and numerical experiments indicate that the cold WTIO SSTAs could induce an anomalous zonal circulation, with its descending branch over the WTIO and the ascending branch over the western North Pacific (WNP). Afterwards, the thermal structure in the upper troposphere of the WNP is altered to enhance the local wave guide, which facilitate the equatorward propagation of the mid‐latitudinal wave train into the WNP. Consequently, the abnormal ascending over the WNP weaken the WPSA in conjunction with the anomaly of the upper‐tropospheric cyclone due to the southward intrusion of the wave train during late summer 2016. This implicates the possible influence of El Niño evolution in spring on the WPSA intensity in the subsequent summer.

On the shortening of Indian summer monsoon season in a warming scenario

Assessing the future projections of the length of rainy season (LRS) has paramount societal impact considering its potential to alter the seasonal mean rainfall over the Indian subcontinent. Here, we explored the projections of LRS using both historical and Representative Concentration Pathways 8.5 (RCP8.5) simulations of the Coupled Model Intercomparison Project Phase5 (CMIP5). RCP8.5 simulations project shortening of the LRS of Indian summer monsoon by altering the timing of onset and withdrawal dates. Most CMIP5 RCP8.5 model simulations indicate a faster warming rate over the western tropical Indian Ocean compared to other regions of the Indian Ocean. It is found that the pronounced western Indian Ocean warming and associated increase in convection results in warmer upper troposphere over the Indian Ocean compared to the Indian subcontinent, reducing the meridional gradient in upper tropospheric temperature (UTT) over the Asian summer monsoon (ASM) domain. The weakening of the meridional gradient in UTT induces weakening of easterly vertical wind shear over the ASM domain during first and last phase of monsoon, facilitate delayed (advanced) monsoon onset (withdrawal) dates, ensues the shortening of LRS of the Indian summer monsoon in a warming scenario.

Changes in shallow-reef community composition along environmental gradients on the East African coast

Abiotic determinants of community composition on shallow reefs are seldom examined at a scale transcending biogeographic regions, and knowledge for the western Indian Ocean is sparse. To address this deficit, turnover in reef community composition was quantified, using gradient forest analyses, along gradients of nine abiotic variables collected in situ or via remote sensing at 54 sites across and within three bioregions: Tropical Western Indian Ocean (Tropical WIO), Delagoa and Natal, spanning 4800 km of east African coastline (5.2–31.1°S). All oceanographic variables differed significantly among bioregions. At a scale across bioregions, wave height declined northwards, while temperature increased. Values for suspended inorganic sediments, chlorophyll-a concentration [chl-a], turbidity and nutrients were all lowest in Delagoa, in association with the scarcity of rivers there. The most novel and unexpected finding was that the northwards decline in wave action was more strongly associated with changes in community composition across bioregions than any other variable. Temperature was the second-most important variable at that scale. Within bioregions, temperature and [chl-a] had strongest relationships with community composition in the Tropical WIO. By contrast, in Delagoa, reef profile, susceptibility to sand inundation and reef heterogeneity were most strongly associated with community composition; and in Natal, depth, turbidity and suspended sediment were foremost. Variables of greatest importance within bioregions distilled to (1) those influencing food/energy supply ([chl-a], turbidity, depth and suspended sediments), which were of particular importance in the Tropical WIO and Natal bioregions, and (2) those altering sand inundation and small-scale current dynamics of reefs (profile, heterogeneity and depth), which were leading factors in the Delagoa Bioregion.

Millennial-scale hydroclimate variations in southwest China linked to tropical Indian Ocean since the Last Glacial Maximum

Indian summer monsoon (ISM) variations have been linked to the orbital-scale boreal summer insolation and millennial-scale North Atlantic climates. Recent studies show the critical role of Indian Ocean sea-surface temperatures (SSTs) in affecting deglacial millennial-scale monsoon oscillations. However, it is unclear whether SSTs can affect monsoon rainfall and terrestrial hydroclimate during the Holocene. Here we report multiproxy evidence of hydroclimate changes in southwest China since the Last Glacial Maximum. Similar to the often-documented gradual decrease in Holocene monsoon rainfall with superimposed millennial-scale variations, our records particularly show pronounced hydroclimate fluctuations including wet conditions at ∼5000-4000 yr ago, and perhaps over the past 1000 yr. We also find coherent variations between our records and sea-surface salinities in the eastern Indian Ocean, suggesting that terrestrial hydroclimate and resultant continental drainage have affected surface ocean conditions. These fluctuations are likely linked to changes in SSTs downstream of the monsoon source in the tropical western Indian Ocean, i.e., a warmer ocean and more monsoon rainfall. We conclude that the influence of both insolation and tropical SSTs on the ISM has persisted from the last deglaciation into Holocene.

The Niger River Basin Moisture Sources: A Lagrangian Analysis

The Niger River basin (NRB) is located in the important climatic region of the African Sahel. In this study we use the Lagrangian tridimensional model FLEXPART v9.0 to identify and characterise the moisture sources for the NRB. This method allows the integration of the budget of evaporation minus precipitation over 10-day backward trajectories, thereby identifying the origins of the air masses residing over the NRB. The analysis was performed for the 35-year period from 1980 to 2014, which allowed us to identify the main semi-annual climatological moisture sources of the NRB, for November–April (NDJFMA) (dry season) and May–October (MJJASO) (wet season), and to quantify the respective moisture uptakes. Throughout the year, the NRB main moisture sources are located on the tropical eastern North Atlantic Ocean near Africa, the tropical eastern South Atlantic Ocean in the Gulf of Guinea, in the regions surrounding the Sahel and in the Mediterranean Sea. The extents of these sources vary between dry and wet seasons. In NDJFMA two regions appear in the east of the basin, which then join up, forming a larger source to the northeast of the basin in MJJASO, when three other less important moisture sources can be seen in central-equatorial Africa, the tropical western Indian Ocean and the Persian Gulf. In NDJFMA the majority of the moisture uptake comes from the NRB itself but then, later in MJJASO, when the precipitation increases over the basin the greatest uptake of moisture occurs over the tropical eastern South Atlantic Ocean, northeast Africa and the NRB, which suggests that these are the effective sources of precipitation in the basin in overall terms. The seasonal moisture uptake quantification over the moisture sources of the NRB, reveals that largest fraction of moisture income to the basin from outside its boundaries. Despite providing moisture to the NRB the source located in the tropical eastern North Atlantic Ocean does not contribute that much to precipitation in the basin. A daily (ten-day) backward analysis shows the importance of the moisture uptake within the NRB and from near moisture sources during the first few (backward) days, while the Atlantic Ocean sources and the Mediterranean became more important during the last five (backward) days of the analysis.