Upwelling Off Research Articles

Can Coastal Upwelling Trigger a Climate Mode? A Study on Intraseasonal‐Scale Coastal Upwelling Off Java and the Indian Ocean Dipole

AbstractCoastal upwelling along the southern coast of Java brings cold and nutrient‐rich subsurface water to the surface. We explored whether the upwelling could trigger the onset of the Indian Ocean Dipole (IOD) by supplying cold water to the southeastern tropical Indian Ocean. We used satellite‐based daily chlorophyll‐a concentration (Chl‐a) data during 2003–2020 as a proxy of the coastal upwelling. We focused on first Chl‐a bloom that occurred in April‐June, the onset phase of the positive IOD (pIOD). We found that the timing and strength of the upwelling signals were significantly correlated with the subsequent IOD peaks. We diagnosed processes associated with the upwelling affecting sea surface temperature (SST) in the southeastern Indian Ocean. Results indicate that after the cold‐water upwelling south of Java, westward surface temperature advection plays a role in anomalously cooling the SST in the southeastern Indian Ocean and setting a favorable condition for the subsequent pIOD development.

Impact of Typhoon on Coastal Upwelling Off The Eastern Hainan Island: A Case Study of Typhoon Rammasun (2014)

The Hainan Island coastal upwelling (HIU hereafter) off the eastern Hainan Island (EHI) is an important phenomenon in the northern South China Sea. Though this phenomenon has been extensively studied, the fast responses of its inner oceanic processes to typhoon passage remain unclear. Based on satellite data and a validated three-dimensional numerical model, the HIU response to the passage of Typhoon Rammasun (2014) are explored in this study. Impacted by Rammasun, the climbing of deep water off the EHI was depressed and the bottom cold water moved seaward for at least 10 km. Though the HIU was weakened, a cooling event and Chl-a bloom in the HIU were formed after the passage of Rammasun. Diagnostic analyses revealed that both advection and diffusion terms contributed to controlling the local rate of temperate change during the typhoon passage and the advection terms showed significant internal wave signals along the shelf break off southeastern Hainan Island. At the same time, strong near-inertial waves (NIWs) with 0.705 cpd were triggered in the HIU. The NIWs lasted for about two weeks and caused strong vertical mixing in the HIU after Rammasun. Overall, typhoon wind and NIWs combined to dominate temperature variation in the HIU during the typhoon period.

Distribution and Oxidation Rates of Ammonia-Oxidizing Archaea Influenced by the Coastal Upwelling off Eastern Hainan Island.

Coastal upwelling causes variations in temperature, salinity and inorganic nutrients in the water column, consequently leading to the shift of microbial populations and their metabolic activities. Impacts of the eastern Hainan upwelling (EHU) on the ammonia-oxidizing archaea (AOA) were investigated based on the amoA gene using pyrosequencing and quantitative PCR at both DNA and cDNA levels, together with the determination of the ammonia oxidation (AO) rate measured with 15N-labelled ammonium. By comparing stations with and without upwelling influence, we found that coastal upwelling correlated with an increase in amoA gene abundance, the dominance of distinct clades for AOA communities at the respective gene and transcript levels, and a large increase in the proportion of the SCM1-like (Nitrosopumilus maritimus-like) cluster as well. The AO rates were generally higher in the deeper water (~25 m), which was in significant positive correlation with the proportion of cluster Water Column A (WCA) at the transcript level, indicating the potential contribution of this cluster to in situ ammonia oxidization. Our study demonstrated that coastal upwelling had a significant impact on the AOA community and ammonia oxidization rate; therefore, this physical forcing should be considered in the future assessment of the global nitrogen budgets and biogeochemical nitrogen cycles.

Impacts of Wind and Current on the Interannual Variation of the Summertime Upwelling Off Southern Vietnam in the South China Sea

AbstractSummertime upwelling off the southern Vietnamese coast is one of the most essential oceanographic features in the South China Sea. Based on analyzing the 38‐year (1982–2019) sea surface temperature (SST) images, locations of summertime Vietnamese upwelling centers are found to be classified into three sub‐regions: the Southern Coastal Upwelling (SCU; south of 12.5°N), the Northern Coastal Upwelling (NCU; north of 12.5°N), and the Offshore Upwelling (OU; east of 110°E). Variations of upwelling intensities in the three sub‐regions are further quantified via an adaptive SST‐based upwelling index, and possible processes relevant to wind field (including wind stress and its curl) and currents are, respectively, proposed. The analyses show that the local along‐shore wind stress, inducing offshore Ekman transport, can produce the coastal upwelling‐favorable condition but is not the only one of main factors in controlling the interannual variability of the coastal upwelling. The enhancement of wind stress curl dipole off the southern Vietnamese coast, which accompanies the scaling up of the double‐gyre structure east of Vietnam and results in the reinforcement and southward shift of eastward‐flowing jet, and the exaggeration of the cyclonic gyre‐associated southward along‐shore current are responsible for the intensification of upwelling in the SCU, but suppress the development of upwelling in the NCU. The well‐developed double‐gyre structure serves as the essential condition for the OU occurrence and the OU is much more sensitive to the change of cyclonic gyre to the north of the jet.

Coherent Response of Vietnam and Sumatra-Java Upwellings to Cross-Equatorial Winds

Upwelling off Vietnam in the South China Sea (SCS) and the Sumatra–Java upwelling in the Indian Ocean significantly modulate regional variation in climate. Although located in different hemispheres, these upwellings nearly concur during the boreal summer; both are the result of wind-induced Ekman divergence. Beyond seasonal time scales, the two upwellings were not synchronous in 1998. In the summer of 1998, upwelling off Vietnam was almost absent, generating the warmest summer on record in the SCS. We demonstrated that the El Niño–Southern Oscillation, which was highly correlated with the upwelling in previous studies, was not solely responsible for this variability. Wind trajectory analyses revealed that cross-equatorial winds, which had passed over the Sumatra–Java upwelling site about 2 weeks earlier, were a rapid force acting on SCS summer upwelling. In the summer of 1998, SCS winds were greatly perturbed due to an anomalous wind path that blew toward the SCS through the Sulu Sea. Our findings suggest that not only the resulting weakening but also the perturbation of the SCS winds prevented the formation of summer upwelling off central Vietnam in that year.

Short‐Term Variations in the Surface Upwelling off Northeastern Taiwan Observed via Satellite Data

AbstractShort‐term biweekly variations in the surface upwelling off northeastern Taiwan are well documented. However, due to limitations in the spatiotemporal resolution and coverage of the observed data, the lifecycle and associated dynamics are poorly understood. In this study, a gradient‐based edge detection algorithm is proposed to detect the surface upwelling. The sea surface temperature data of geostationary satellite Himawari‐8 were used to explore a complete short‐term process as a case study. The evolution was analyzed in terms of an upwelling index measured via the temperature difference between upwelled and surrounding water and the area and shape of the surface upwelling. The process lasted approximately 17 days as the upwelling moved northeastward under the advection of the Kuroshio Current and included two stages: intensification and decay. The last 8 years (2010–2017) of multisatellite combined sea surface temperature data were examined to verify the results obtained in the case study. A statistical analysis shows that most of the short‐term processes occur in the summer with an average lifecycle of approximately 15 ± 5 days. The trajectories of the upwelling center can be grouped into three types: quasi‐stationary near the shelf break, moving northward across the shelf break, and moving northeastward along the shelf break. A preliminary discussion suggests that the observed variations may be caused by the collective effects of multiple dynamical mechanisms, such as fluctuations in the Kuroshio Current, surface wind curl, and typhoons. This study furthers our knowledge of the surface upwelling off northeastern Taiwan and provides constraints for numerical simulations.

Coastal Upwelling Off Southwest Nova Scotia Simulated With a High‐Resolution Baroclinic Ocean Model

AbstractA high‐resolution baroclinic coastal ocean model is applied to study seasonal circulation and upwelling off South West Nova Scotia (SWNS) based on 1 year simulations for 2010. The model reasonably reproduces tidal currents, seasonal hydrography, and circulation from multiyear observations, in consistence with the observed strong seasonal variations of these properties in the study area. The main physical processes affecting circulation are analyzed using numerical experiments, with focus on the effect of tidal and density induced currents on topographic upwelling. It is confirmed that the shoreward near‐bottom currents and associated upwelling are tidally induced and persistent throughout the year. It is revealed that these currents have seasonal variability, with cross‐isobath component being strong in summer throughout a large area, but weaker and confined to deeper regions in winter. The seasonal variability of Scotian Current is the dominant forcing affecting the variability of onshore bottom currents. Lagrangian particle tracking identifies two major pathways of source waters arriving at the SWNS upwelling region. A large proportion of particles come from the east with the Scotian Current, mostly from the surface layer; and a small portion of water parcels come from a mean depth exceeding 100 m from the Gulf of Maine and Northeast Channel.

English

English

Rapid 20th-Century Increase in Coastal Upwelling off Northwest Africa

Near-shore waters along the northwest African margin are characterized by coastal upwelling and represent one of the world's major upwelling regions. Sea surface temperature (SST) records from Moroccan sediment cores, extending back 2500 years, reveal anomalous and unprecedented cooling during the 20th century, which is consistent with increased upwelling. Upwelling-driven SSTs also vary out of phase with millennial-scale changes in Northern Hemisphere temperature anomalies (NHTAs) and show relatively warm conditions during the Little Ice Age and relatively cool conditions during the Medieval Warm Period. Together, these results suggest that coastal upwelling varies with NHTAs and that upwelling off northwest Africa may continue to intensify as global warming and atmospheric CO2 levels increase.

Modeling of subsurface calcite dissolution, including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off Gabon

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O 2, pH, pCO 2 and Ca 2+ (in situ), pore-water concentration profiles of NO 3 −, NH 4 +, Fe 2+, and Mn 2+ and SO 4 2− (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O 2, NO 3 −, Fe(OH) 3 and SO 4 2−, reoxidation reactions involving Fe 2+ and Mn 2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm −2 yr −1, of which 77% were due to O 2, 17% to NO 3 − and 3% to Fe(OH) 3 and 3% to SO 4 2−. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d −1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO 3 flux to the sediment was estimated to occur at a rate of 42 g m −2 yr −1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m −2 yr −1, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.

Topographic Upwelling off Southwest Nova Scotia

The waters off Cape Sable in southwestern Nova Scotia have anomalously low temperatures and high nutrients during summer. Using a three-dimensional tidal model as well as field observations obtained between 1978 and 1985, a new mechanism is proposed to explain the origin of these anomalous properties. The numerical model predicts several areas of strong tidally induced residual upwelling and downwelling. On the submarine ridge off Cape Sable, upwelling occurs over the eastern flank and downwelling over the western flank. This upward vertical transport is very effective in supplying cold, saline, and nutrient-rich water from deep to shallow layers. The predicted upwelling and downwelling are induced by a tidal rectification process resulting from tidal currents flowing over complex bottom topography. Hence, the process is named “topographic upwelling and downwelling.” The upwelling (downwelling) is generated by the residual currents flowing from deep to shallow (shallow to deep) waters. This process is three-dimensional so that the water parcels advected onshore from deep to shallow zones in the upwelling regime may remain in the lower layer and be carried away from the upwelling region by a longshore coastal current. Thus, the upward transfer of cold and saline water in the three-dimensional topographic upwelling may not be as efficient as the classical upwelling associated with two-dimensional (on-off shore) circulation. However, efficient vertical transfer can be accomplished in conjunction with the topographic upwelling if the cross-isobath transport associated with the upwelling carries deep water to a shallow region where there is strong vertical mixing. This combination of topographic upwelling and strong tidal mixing provides the mechanism for producing the observed cold water anomaly off Cape Sable. Topographic upwelling occurs on the eastern side of the cape, and strong tidal mixing produces a well-mixed area off the cape. These processes are verified by hydrographic, current meter, and Lagrangian drift data collected in the surrounding area. The mechanism is expected to be important in other coastal regions where strong tidal currents and large variations of bottom topography are found.

Biological indications of active upwelling in the Northwestern Indian Ocean in 1964 and 1979, and a comparison with Peru and Northwest Africa

When surface properties of the northwestern Indian Ocean observed during the southwest monsoon of 1964 (R.R.S. ‘DISCOVERY’) are compared with those of 1979 (R.V. ‘ISELIN’), the area of upwelling around Ras Hafun (near 10°N) showed the clearest similarity in the two years. The highest surface concentration of chlorophyll a between Mombasa (4°S) and Cabo Guardafui (12°N) occurred near Ras Hafun in both years and was between 4 and 5 mg m −3. Sea-surface temperatures around Ras Hafun were lower than elsewhere along the coast, and surface nitrate was higher. The abundance of the copepod Calanoides carinatus was >100 m −3 there and near 5°N in both years. The age-structure of this dominant copepod suggests active reproduction in both years. Major dissimilarities between 1964 and 1979 were observed in the vertical distribution of temperature and nitrate near 5°N, which suggested that upwelling there in August 1964 was less intense than in July 1979. The intensity of upwelling near 5°N probably reached a peak in July both years, implying a relatively stable annual cycle for the upwellings off Somalia. The relationship of primary productivity and chlorophyll a observed off Somalia is very similar to that observed off Peru, and standing stocks of net zooplankton are also similar, but both are quite different from northwestern Africa. Perhaps the most striking difference, however, is the areal of extent of upwelling off Somalia which is considerably larger than that of the other two upwelling areas.

The Influence of Bottom Topography on Upwelling off Peru

Abstract The x-y-t, two-layer β-plane numerical model developed by Hurlburt (1974) is used to examine the upwelling system off Peru. The region off Peru from 14 to 15°30's is one of strong and persistent upwelling. The most distinctive feature of the Peruvian upwelling circulation is a predominant poleward flow. A local area model, when forced by wind stress only, cannot account for the observed Peruvian circulation. When an additional barotropic forcing is applied in the model, a dominating poleward flow results. The effects of wind stress are felt on the upper layer and by the third day of integration an equatorward flow develops near the coast. Two functional representations of the actual topography are used in the model and compared to a flat-bottom case. Model results, when compared to observations, show that the observed upwelling maximum ∼40 km south of 15°S is the result of a mesoscale topographic feature, a seamount. Variations of longshore and cross-shelf flow in cases with sloping topography ar...

The ratios of nitrate, phosphate, and silicate during uptake and regeneration phases of the Moroccan upwelling regime

Upwelling off Morocco was investigated in the summers of 1972 and 1973. In the northern zone, superficial waters result from the mixture of three water masses. The southern area is characterized by a ‘bell’ of cold water brought to the continental shelf, where nutrient regeneration is intense. Using models of water mass formation, nutrient anomalies (nitrogen, phosphorus, and silicon) are calculated in both uptake and regeneration phases and corresponding ratios are evaluated. Deviations from Redfield's ratios were observed in superficial North Atlantic central waters bathing the continental shelf, where inorganic phosphorus and silicon appear to regenerate faster than nitrogen. In the photic layer no limitation on phytoplankton growth by silicon was detected.

A Three-Dimensional Simulation of Coastal Upwelling off Oregon

Abstract The wind driven, x-y-t, two-layer β-plane numerical model developed by Hurlburt (1974) is used to investigate the effects of a bottom topography and coastline configuration, like that off Oregon, on the onset and decay of the ocean upwelling circulation. The digitized nearshore Oregon bathymetry is analyzed for dominant scales, and a smoothed version is used in model cases with several different initial states and wind stresses. Cases with topography are compared to cases with plane sea beds. Topographic variations are found to dominate over coastline irregularities in determining the longshore distribution of upwelling. Results indicate that stronger upwelling observed near Cape Blanco is primarily due to the local bottom topography and not the cape itself. Observed variations in the meridional and zonal flow are attributed to the topographic β-effect. In particular, during spin-up with an equatorward wind stress, a nearshore poleward undercurrent is most likely to develop in regions where topog...

Large-Scale Upwelling off Northwestern Mexico

The seasonal variation of large-scale upwelling off northwestern Mexico is investigated by means of the divergence of the Ekman transport, computed for a 1° latitude-longitude grid. It is found, by using such a close grid, that the vertical velocity is resolved in sufficient detail to bring out many important features of upwelling that agree favorably with the temperature distribution. The limitations of the method are discussed.