Eastern Boundary Research Articles

Low confidence in multi-decadal trends of wind-driven upwelling across the Benguela Upwelling System

Abstract. Like other eastern boundary upwelling systems, in the Benguela Upwelling System, the upwelling along the coastline is primarily alongshore-wind-driven. In contrast, it is mainly driven by the wind stress curl farther offshore. The surface wind regime across the Benguela Upwelling System is strongly related to the South Atlantic Anticyclone that is believed to migrate poleward in response to anthropogenic global warming. Using the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis for 1979–2021, we investigate multi-decadal changes in the South Atlantic Anticyclone and their impacts on coastal upwelling driven by alongshore winds, wind-stress-curl-driven upwelling within the coastal zone and total upwelling as the sum of both across the Benguela Upwelling System. Even though the detailed structure of surface wind over the coastal zone matters for both alongshore-wind-driven coastal upwelling and wind-stress-curl-driven upwelling, we show that it is not of major importance for the total amount of upwelled water. We found a robust connection between the anticyclone intensity and the zonally integrated wind-stress-curl-driven and total upwelling. However, such connection for the alongshore-wind-driven coastal upwelling is weak. The upwelling in the equatorward portion of the Benguela Upwelling System is significantly affected by the anticyclone intensity. In contrast, the poleward portion is also influenced by the meridional position of the anticyclone. In general, the impacts of the anticyclone on the local upwelling are more robust during the austral winter. The multi-decadal trend in the sea level pressure across the South Atlantic renders a considerable heterogeneity in space. However, this trend features a small signal-to-noise ratio and can be obscured by interannual to decadal climate variability. This view is further supported by the coastal and wind-stress-curl-driven upwelling in several upwelling cells showing hardly any significant multi-decadal trends.

Spatial and temporal control of the volcanic activities over the distribution of sediments in the Magadi basin, Kenya Rift

High-velocity layers in basins with significant sub-basalt coverage, such as the Magadi basin, can impede geophysical imaging of potential subsurface hydrocarbon deposits. Using magnetic and gravity methods, this study examines the influence of volcano-tectonic processes and the basement on the evolution of the Magadi basin. Regional-residual separation analyses identified key structures and signatures. Lineaments and depths of the causative sources were determined using Euler deconvolution. Butterworth low-pass filters on aeromagnetic data isolated depth-varying magnetic layer responses. The study demonstrates a westward displacement of volcanic flows in the Magadi basin, originating from nearly all volcanic plugs. Depocenters near the western boundary fault are more affected by westerly movements of the volcano-tectonic flows than depocenters near the eastern boundary fault. The Rift underwent internal compartmentalization, resulting in depocenters with unique and intermittent tectonic and sedimentary occurrences. Rift segmentation led to modifications in catchment areas, which caused sudden variations in sediment flux, ultimately leading to the isolation of the depocenters. This study identifies the initial compartmentalized depocenters of Nguruman, Koora, and Forty-six. The initial Nguruman depocenter is oriented SW-NE and spans approximately 2512 sq. km with a sediment thickness range of 2000–4000 m. The presence of horsts in the southern and northern parts precludes volcaniclastics from the depocenter. Active tectonics and inversion caused the initial Nguruman depocenter to split into two halves: the northern Musenke depocenter and the southern Pakase. Euler depth analysis indicates sedimentary formations are thicker in compartmentalized depocenters in Magadi's western and southern regions than near the eastern boundary fault. The sediment thickness range in the Koora basin is approximately 1000–3000 m, while Pakase on the western flank has a sediment thickness range of 2000–3800 m.

Interior Route and Seasonal Dynamics of the Meridional Current in the Eastern Indian Ocean Tropical Gyre

AbstractThe eastern boundary current of the tropical Indian Ocean (IO) connects the equatorial/south tropical IO, contributing to the meridional heat and salinity exchange. As a part of the IO tropical gyre, the meridional current in the eastern IO tropical gyre (East Sumatra Current, ESC) flows southward in the mean state. However, the route of this current varies with months. Drifter trajectories display an exceptional route that turns southeastward before reaching Sumatra in the austral winter and spring, considered the interior ESC. This study uses observations and reanalysis data sets to investigate the seasonal structure and forcing mechanisms behind the interior ESC. The velocity is decomposed into geostrophic and wind‐driven components. The results suggest that the geostrophic component plays a dominant role in the ESC recirculating early from April to June. The southward geostrophic currents are mainly caused by positive zonal temperature gradients, which are induced by both the cold anomaly of the Chagos Dome and the warm water carried by the Indonesian Throughflow, particularly in April and May. Meanwhile, the annual Rossby wave contributes to the temperature anomaly of the Chagos Dome. During the monsoon that prevails after July, the wind‐driven component increases and becomes the primary forcing, accompanying the slowdown of geostrophic velocity. Therefore, the local wind stress, wave propagation, and water mass transport account for the formation of the seasonal interior ESC.

Intraseasonal oscillations of the Andaman Sea thermocline

Variations in the upper ocean thermal structure have significant implications for air-sea interaction and upper-ocean ecosystem processes. Vertical profiles of temperature measured by a moored buoy located at 10.5°N, 94°E in the Andaman Sea and simulation by an Indian Ocean model are used in this study to characterise the intraseasonal variations (ISV) in the Andaman Sea (AndS) thermal structure and identify their sources. The seasonal variations in the upper ocean thermal structure show a strong semi-annual cycle driven by monsoons. The sub-surface temperature shows significant intraseasonal oscillations within a band of 40–110 days, which are de-coupled from that in the mixed layer, which has dominant periodicity in the band of 90–120 days. Thermocline ISV are seasonally modulated with a primary peak during August–September and a secondary peak during February–March, with significant year-to-year variations. A cross-wavelet analysis shows that ISV in the 40–60 days period is in phase with that at the eastern boundary and they are locally forced by the passage of eddies. The 60–110 day band is out of phase with the eastern boundary and is forced by Rossby waves radiated from the equatorially generated intraseasonal coastal Kelvin waves.

Mechanisms and drivers controlling spatio-temporal evolution of pCO2 and air-sea CO2 fluxes in the southern Java coastal upwelling system

Upwelling systems are known for their complex dynamic processes spanning a wide variety of spatio-temporal scales, due to strong coupling between the ocean and atmosphere. One such upwelling system is found at the eastern boundary of the Indian Ocean along the southern coast of Java during the southeast monsoon (SM). This study examines the mechanisms and drivers involved in the spatial and temporal evolutions of surface pCO2 in this upwelling system over seasonal to inter-annual timescales using a coupled, high-resolution regional model. A decomposition analysis was used to quantify the changes in pCO2 in response to changes in surface-temperature (T), surface-salinity (S), dissolved inorganic carbon (DIC), and total alkalinity (ALK). The upwelling of deeper, cooler waters decreases the surface pCO2 by 50 ± 1.48 μatm from June to September. However, the presence of DIC-rich waters at significantly shallower depths increases the surface pCO2 by 27 ± 0.9 μatm. The surface pCO2 increases by 5 ± 0.50 μatm due to changes in salinity, which exerts less control than T-driven changes. Biological changes increase the surface pCO2 by 17.54 ± 2.74 μatm. The maximum seasonal amplitudes of the variations in pCO2 caused by seawater solubility and biology are in counterbalance during the SM. Therefore, the upwelling-driven combined effect of physical mechanisms dominates the effect of biological mechanisms in reducing surface pCO2 under these conditions. During the SM, the southern coast of Java acts as a seasonal CO2 sink (−1.36 ± 6.48 g C m−2 year−1), although consideration of the mean flux over 2006–2017 shows that the region acts as a source of CO2 (1.31 ± 5.03 g C m−2 year−1). Furthermore, following a negative Indian Ocean Dipole (IOD) year, a strong sink of atmospheric CO2 is observed during the SM.

Spatiotemporal Characteristics of Horizontal Crustal Deformation in the Sichuan–Yunnan Region Using GPS Data

Based on various velocity fields from Global Positioning System (GPS) data over nearly 20 years in the Sichuan–Yunnan region, this paper calculated the strain rate field and its spatiotemporal characteristics by using an improved least squares collocation method. We evaluated the calculated strain field by extensively discussing the impact of non-tectonic factors on the calculation. Subsequently, we described the present-day strain rate features and their spatiotemporal variations. The results indicate the necessity of considering the influence of non-tectonic factors when calculating the strain rate field by using GPS velocity data. Widespread strain accumulation is observed in the Sichuan–Yunnan region, and significant second strain rate invariant with an average value of 33.1 nanostrain/yr primarily occurs along the eastern boundary faults of the Sichuan–Yunnan rhomboid block, specifically the Xianshuihe–Anninghe–Zemuhe–Xiaojiang fault systems. These fault systems also demarcate the zones with negative and positive dilation strain. According to the spatiotemporal variations of strain rate fields, the northern and southern segments of the Xianshuihe Fault, the Anninghe Fault and its eastern adjacent faults, and the Xiaojiang Fault are undergoing intensifying strain. Consequently, these zones should be paid more attention due to their relatively higher seismic risk in the Sichuan–Yunnan region.

Historical biogeography supports Point Conception as the site of turnover between temperate East Pacific ichthyofaunas.

The cold temperate and subtropical marine faunas of the Northeastern Pacific meet within California as part of one of the few eastern boundary upwelling ecosystems in the world. Traditionally, it is believed that Point Conception is the precise site of turnover between these two faunas due to sharp changes in oceanographic conditions. However, evidence from intraspecific phylogeography and species range terminals do not support this view, finding stronger biogeographic breaks elsewhere along the coast. Here I develop a new application of historical biogeographic approaches to uncover sites of transition between faunas without needing an a priori hypothesis of where these occur. I used this approach to determine whether the point of transition between northern and southern temperate faunas occurs at Point Conception or elsewhere within California. I also examined expert-vetted latitudinal range data of California fish species from the 1970s and the 2020s to assess how biogeography could change with the backdrop of climate change. The site of turnover was found to occur near Point Conception, in concordance with the traditional view. I suggest that recent species- and population-level processes could be expected to give signals of different events from historical biogeography, possibly explaining the discrepancy across studies. Species richness of California has increased since the 1970s, mostly due to species's ranges expanding northward from Baja California (Mexico). Range shifts under warming conditions seem to be increasing the disparity between northern and southern faunas of California, creating a more divergent biogeography.

Physical mechanisms affecting phytoplankton variability along the Chilean coast

Chile has high phytoplankton production due to being a classic example of an Eastern Boundary Upwelling System. Monthly averaged chlorophyll-α (Chl) and physical parameters (sea surface temperature, precipitation rate, southerly and westerly winds) were studied off the Chilean coast from 2002 to 2018, in order to understand the primary production along this important ocean margin. The coastal margin was split into three zones and ten sub-sections. The Northern Zone had a low phytoplankton production with small seasonal variability, except in its north. This pattern is due to a narrow shelf, weak winds, lack of precipitation and relatively stable weather conditions driven by the Southeast Pacific Subtropical Anticyclone (SPSA). The Central Zone presented a seasonally varying production, with a high Chl concentration in summer and early spring. This is linked to the SPSA movement and sunlight reduction during the winter. A high Chl activity is seen in the Southern Zone despite this Zone being at the limits of the SPSA effect, leading to weak longshore winds only during the warm season. Overall, this study has demonstrated the importance of shelf width and the upwelling driven by the presence or absence of the SPSA for ocean primary production. Thus, the most productive region is from 35°S to 45°S owing to both variables being present.

Detection and attribution of intra-annual mass component of sea-level variations along the Norwegian coast

Reliable sea-level observations in coastal regions are needed to assess the impact of sea level on coastal communities and ecosystems. This paper evaluates the ability of in-situ and remote sensing instruments to monitor and help explain the mass component of sea level along the coast of Norway. The general agreement between three different GRACE/GRACE-FO mascon solutions and a combination of satellite altimetry and hydrography gives us confidence to explore the mass component of sea level in coastal areas on intra-annual timescales. At first, the estimates reveal a large spatial-scale coherence of the sea-level mass component on the shelf, which agrees with Ekman theory. Then, they suggest a link between the mass component of sea level and the along-slope wind stress integrated along the eastern boundary of the North Atlantic, which agrees with the theory of poleward propagating coastal trapped waves. These results highlight the potential of the sea-level mass component from GRACE and GRACE-FO, satellite altimetry and the hydrographic stations over the Norwegian shelf. Moreover, they indicate that GRACE and GRACE-FO can be used to monitor and understand the intra-annual variability of the mass component of sea level in the coastal ocean, especially where in-situ measurements are sparse or absent.

Seismological research in Yunnan Province, China, and its tectonic implication between the Xianshuihe-Xiaojiang fault system and the Red River fault zone

The tectonic crosscutting relationship between the two most tectonic important Xianshuihe-Xiaojiang fault system (XFS) and the Red River fault zone (RRF) is a key basic problem but it is controversial now. These obscure further leads to a hot argument about the geodynamic model of the southeast margin of the Tibetan Plateau. In order to answer whether the XFS has cut across the RRF and extended southwardly, multiple seismological methods, including seismic relocation, b-value analysis, seismic energy, density study, focal mechanism inversion, and regional stress field research, are applied in the Yunnan Province area, China, where the XFS and the RRF intersects with each other. The results comprehensively demonstrate that the southern segment of the XFS has not been affected by the RRF, and it has continued for a length after crossing the RRF, but the Dian Bien Phu fault zone should not be an extension fault of the XFS. Along the SW direction, starting from the middle segment of the XFS, and cutting across the Qujiang fault, Shiping-Jianshui fault zone, RRF, Ailaoshan fault zone, Wuliangshan fault zone, and the southern section of the Daluo fault, the belt should be treated as the eastern boundary of the clockwise rotational in geodynamics model of the Tibetan Plateau in this study area. Based on these conclusion above and previous recognitions, a new geodynamic evolution model is proposed.

Weather system classification and water vapor transport characteristics of extreme value rainstorm in northwestern Hunan

Abstract Using meteorological analysis, composite analysis and water vapor trajectory analysis, the extreme value rainstorm process in northwest Hunan was analyzed. The results show that three types are summarized: the Southwest Vortex and Warm Shear Line Pattern (SVWSLP), the Subtropical High Edge Pattern (SHEP) and the Cold Trough and Shear Line Pattern (CTSLP). The main influence systems are upper trough, southwest vortex, shear line, low-level jet and subtropical high edge. For SVWSLP, the water vapor transport channels are only from the low-latitude ocean whether it is affected by long-distance typhoons. For SHEP, the main water vapor channel comes from the long-distance ocean and is finally transported to northwestern Hunan around 650 hPa in the form of warm and wet airflow, whether it is affected by long-distance typhoons. The CTSLP appears a significant water vapor confrontation between the north and the south and the baroclinicity of the atmosphere in the rainstorm area. The southern and western boundaries are the input boundary, while the eastern and northern boundaries are the outflow boundary. Therefore, one of the three types of weather systems appears in northwestern Hunan in May–August, with strong water vapor transport from the ocean surface, which is likely to cause extreme rainstorm.

Most eastern boundary upwelling regions represent thermal refugia in the age of climate change

Eastern Boundary Upwelling Systems (EBUS) are regions where wind-driven coastal upwelling brings deep cold, nutrient-rich water to the surface and may be characterized by a coastal ‘footprint’ of sea surface temperature (SST) cooler than their surroundings. Previous studies have shown that EBUS coastal temperatures are responding differently to global ocean warming, warming slowly or not at all. However, the spatial dynamics of coastal upwelling footprints have yet to be investigated. In this paper, we use 20 years of high-resolution SST data derived from satellites (MUR SST) to test the null hypothesis that the extent of coastal upwelling footprints have remained stable over the period 2002–2022, consistent with the idea that these regions are thermal refugia. We investigate linear trends at different time scales, finding that the Humboldt and Iberian/Canary EBUS show no contraction of this footprint on annual or seasonal scales. The Benguela EBUS shows no change in its central and poleward subregions, but it exhibits contraction of the footprint in the equatorward subregion in the austral winter and spring. The California EBUS behaves differently: on the annual scale only the equatorward subregion shows contraction of the SST footprint, while on the seasonal scale, the entire EBUS show contraction during the fall or summer/fall. Summarizing the last two decades, most coastal habitats of EBUS (>80% of the areas tested) are remaining cool and may be acting as regional refugia from global warming, but this is true for some regions only during certain seasons. However, the declines in areal extent of upwelling in subregions of the California and Benguela EBUS indicate potential consequences for marine life and may help to explain changes in abundance, productivity, and redistributions of populations in these regions.

Seasonal predictability of summer north african subtropical high in operational climate prediction models

Seasonal predictability of summer North African Subtropical High (NASH) is investigated in this study by utilizing the hindcast data from four operational climate prediction models, including BCC_CSM1.1(m), NCEP CFSv2, ECMWF System 4, and JMA CPSv2. By reconstructing indices describing the variations in intensity, area, eastern boundary and ridge line of the NASH, it is shown that the intensity and area indices present high prediction skills compared to the relatively low prediction skills of position indices. The multi-model ensemble (MME) mean, calculated as the arithmetic average of the four models, presents relatively higher and stabler skills than individual models. Further investigation indicates that the prediction skill of the NASH is largely reliant on the models’ ability in reproducing the relationship between the NASH indices and the tropical-to-subtropical sea surface temperature (SST) anomalies associated with the El Niño/Southern Oscillation (ENSO). The pattern of atmospheric circulation anomaly over the North Africa in response to ENSO is well captured by the models, which suggests the dominant source of predictability of the NASH.

ORTHOPTERA OF AMURSKAYA OBLAST

An annotated list of 73 species from 50 genera and 7 families of orthopteran insects (Orthoptera) reliably noted for the Amurskaya oblast is given based on collection materials and literature data. Grasshoppers (Acrididae) and bush-crickets (Tettigoniidae) are best represented (38 and 19 species, respectively) while other families included of 1–7 species each. The Transpalaearctic (30%), Eastern-Palaearctic (30%) and Palaearchearctic (25%) species are dominated in the fauna of the Amurskaya oblast. Here lies the eastern boundary of the ranges of Transpalaearctic and Eastern-Palearctic species (Arcyptera fusca, Gomphocerippus rufus, Myrmeleotettix palpalis, and Psophus stridulus) and the northwestern boundary of 13 species known from the Pacific regions of East Asia (Atlanticus brunneri, Chizuella bonneti, Tettigonia uvarovi, Eobiana engelhardti, Ruspolia yezoensis, Shagniana ussuriana, Uvarovites inflatus, Oecanthus longicauda, Dianemobius csikii, Pteronemobius gorochovi, Xya japonica, Gryllotalpa orientalis, and Arcyptera orientalis).

Variability and controls of the ocean acidification metrics pH and pCO2 in a large embayment of an Eastern Boundary Upwelling System (EBUS)

An assessment was undertaken of the spatial and temporal variability of pCO2 and pH in St Helena Bay, a highly productive, wide-open bay located in the Benguela, a major eastern boundary upwelling system (EBUS). Mechanisms controlling their patterns of variability and their linkage to the distribution of oxygen are explored. The mean surface pCO2 was 385 μatm, below the mean atmospheric concentration of 410 μatm, indicative of a system serving as a CO2 sink. The corresponding mean pH of bay surface waters was 8.3. The greatest variation from these means was driven by deep winter mixing causing a seasonal reversal of the air-sea pCO2 gradient. Mean surface pCO2 was also elevated above mean atmospheric levels in the nearshore (<15 m depth), a likely consequence of higher microbial respiration in the upper water column due to regular resuspension of detrital material. The nearshore of the bay is also sometimes subject to high biomass dinoflagellate blooms referred to as red tides, and their decay leads to exceptional pCO2 concentrations. Discharge from the Berg River and the associated degradation of terrestrially derived organic matter also contributes to higher pCO2 in the nearshore particularly during winter when rainfall is highest. Through the water column the important role of photosynthesis in determining the vertical distribution of pCO2 is evident in that pCO2 is shown to be a function of both Chl a and PAR. In St Helena Bay nearly 50% of water column biomass is located below the community compensation depth (i.e. where respiration carbon loss exceeds gross community production) leading to rapidly declining O2 and increasing pCO2 concentrations with depth. Resulting conditions of bottom water hypoxia/anoxia are therefore accompanied by more corrosive waters (pH declining to ∼7.4) and conditions of severe hypercapnia (pCO2 exceeding 2000 μatm) subjecting marine life to multiple stressors. St Helena Bay and similar bays in EBUS are likely to provide extrema in the deoxygenation and acidification of coastal waters.

Characteristics of Water Vapor Transport for Extreme Summer Precipitation in the Eastern Southwest China and Its Impact Mechanism

To improve understanding of the characteristics of extreme summer rainfall and its water vapor transport in the eastern part of southwestern China (ESWC), this study analyzed data on daily precipitation from 118 meteorological stations in the ESWC from 1979 to 2020, as well as daily reanalysis data from ERA5 and daily reanalysis data from NCEP/NCAR. The study employed polynomial fitting, correlation, regression, clustering, and mixed single-particle Lagrangian trajectory (HYSPLITv5.0) modeling methods to simulate extreme summer precipitation and its water vapor transport characteristics in the ESWC and its possible formation mechanism. The results show that: (1) The contribution rate of extreme precipitation in the ESWC from 1979 to 2020 varied significantly on the interannual time scale. When the number of extreme precipitation days is high (low), the contribution rate of extreme precipitation is also high (low), while the contribution rate of general precipitation (the percentage of the sum of general precipitation to the total summer precipitation of that year) is often low (high). (2) When extreme precipitation occurs in the ESWC, compared with general precipitation, the high-level potential vortices are stronger, and the cold air from higher latitude is more likely to move southward. Meanwhile, the amount of water vapor input to the region is significantly larger than that of general precipitation. (3) There are four channels of water vapor sources in the ESWC during the period of extreme precipitation: the Bay of Bengal, the Arabian Sea, the western Pacific, and the northwest. The contribution of water vapor from the Bay of Bengal is the highest. The number of extreme summer precipitation days in the ESWC is significantly negatively correlated with the water vapor budget of the eastern boundary and positively correlated with Indian Ocean Basin-Wide (IOBW) index in the previous winter. (4) When the winter SST is high in the IOBW mode, it can cause the western Pacific subtropical high and the South Asian high to be stronger and shifted southward in summer, resulting in an increase in the number of extreme precipitation days in the ESWC.

Correction to: Modeling the potential genesis of tsunamis from below an accretionary prism and their potential impact: a case study along the eastern boundary of the Caribbean Plate

Correction to: Modeling the potential genesis of tsunamis from below an accretionary prism and their potential impact: a case study along the eastern boundary of the Caribbean Plate

Data Processing of Surface Wave Tomography in Australia using Automated Surface Wave Phase-Velocity Measuring System (ASWMS)

The Australian continent is divided into tectonic blocks, including Archaean and Proterozoic units in the western and central part, and Phanerozoic units in the eastern part. Surface wave tomography is an effective method of getting information about velocity variations in a particular region. Surface wave tomography was used in this project to map the subsurface of the Australian continent down to a depth of 100 km with the ASWMS (Automated Surface-Wave Phase-Velocity Measuring System). From 2010 to 2015, we gathered waveform data from 767 events and 219 permanent and temporal seismic stations dispersed across Australia. We selected a set of events that has a minimum magnitude of 6. Rayleigh wave extraction and cross-correlation between two nearby stations are among the first processing processes. Following that, the apparent phase-velocity value is calculated using the Eikonal and tomographic inversion. To generate a tomographic map with a true structural phase velocity, each phase velocity data is stacked with amplitude correction. According to current tomographic imaging studies, low-velocity values under Australia tend to be dispersed easterly, with Rayleigh velocity ranging from 3.7 to 3.9 km/s. The tectonic history and evolution of eastern Australia are related to a sequence of orogenic events that are pushing toward the continent’s eastern boundary.

Increased southerly and easterly water vapor transport contributed to the dry-to-wet transition of summer precipitation over the Three-River Headwaters in the Tibetan Plateau

The Three-River Headwaters (TRH) region in the Tibetan Plateau is vulnerable to climate change; changes in summer (June–August) precipitation have a significant impact on water security and sustainability in both local and downstream areas. However, the changes in summer precipitation of different intensities over the TRH region, along with their influencing factors, remain unclear. In this study, we used observational and ERA5 reanalysis data and employed a precipitation categorization and water vapor budget analysis to quantify the categorized precipitation variations and investigate their possible linkages with the water vapor budget. Our results showed an increasing trend in summer precipitation at a rate of 0.9 mm per year (p < 0.1) during 1979–2020, with a significant dry-to-wet transition in 2002. The category ‘very heavy precipitation’ (≥10 mm d−1) contributed 65.1% of the increased summer precipitation, which occurred frequently in the northern TRH region. The dry-to-wet transition was caused by the effects of varied atmospheric circulations in each subregion. Southwesterly water vapor transport through the southern boundary was responsible for the increased net water vapor flux in the western TRH region (158.2%), while southeasterly water vapor transport through the eastern boundary was responsible for the increased net water vapor flux in the central TRH (155.2%) and eastern TRH (229.2%) regions. Therefore, we inferred that the dry-to-wet transition of summer precipitation and the increased ‘very heavy precipitation’ over the TRH was caused by increased easterly and southerly water vapor transport.

Outgassing of CO2 dominates in the coastal upwelling off the northwest African coast

The Eastern Boundary Upwelling System off northwest Africa is among the most productive regions of the ocean. In 2019, two merchant ships equipped with an underway CO2 system sampled the region following exactly the same track from 10°N to 36°N. We determine the sources and sinks of CO2 and the seasonal cycle along the track. A weak permanent upwelling (WPU), a permanent upwelling (PU) and an open ocean regions are identified. The WPU (26°N to 33°N) is a source of CO2 in summer and autumn, and a sink of CO2 in winter and spring. Thermodynamic warming and cooling processes mainly drive the CO2 variations in this region. The PU (20°N to 26°N) is a sink of CO2 in spring and a source of CO2 in other seasons. This region is the most productive and exhibits the largest variability of the CO2 flux. The supply of CO2 from subsurface waters dominate over the carbon uptake by biology, which leads to a strong outgassing, especially in winter and autumn. A sink of CO2 occurs in spring only in the PU. Near Cape Blanc (20°N) in July 2019, a source of CO2 is observed around 20°N within ± 1° of latitude and becomes a sink of CO2 a few days later when the ship samples back the same area. South of 18°N, out of the influence of the coastal upwelling, the region is a sink of CO2 in winter only and the region is mainly controlled by physical processes. Using voyages from 2010 to 2022, seawater fCO2 significantly increases at a rate ranging from 1.82 μatm yr−1 to 2.10 μatm yr−1 close to the atmospheric increase. This is associated with a pH decrease between 0.0016 and 0.0022 yr−1. Nevertheless, there is no clear trend of the CO2 flux in any region.