Wind-driven Upwelling Research Articles

Southern Ocean drives multidecadal atmospheric CO2 rise during Heinrich Stadials

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO2 measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO2 are concurrent with jumps in atmospheric CH4 and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.

Dynamic shifts in the southern Benguela upwelling system since the latest Miocene

This study reconstructs the upwelling history of the Southern Benguela Upwelling System (SBUS), examining its sensitivity to both regional and global climatic influences. We analyze planktic foraminiferal relative abundance and stable isotope data from ODP Site 1087 to investigate the history of SBUS since ∼ 6.1 Ma. During the latest Miocene-earliest Pliocene, the SBUS experienced increased productivity, fueled by wind-driven upwelling and an enriched global nutrient reservoir associated with the late Miocene biogenic bloom. The early Pliocene witnessed an apparent weakening of the SBUS due to the deepening of the global thermocline and weak trade winds. Significantly, our proxy records indicate strengthening of the SBUS beginning before the onset of intense northern hemisphere glaciation (iNHG) between 3.5 and 3 Ma. Despite contraction of upwelling cells after ∼ 3 Ma, our study challenges the notion of water column stability during the Matuyama Diatom Maximum event (3 to 2 Ma); we alternatively propose the existence of water column instability and probable presence of upwelling during this time. Post 2 Ma, the main upwelling cells shifted northward in response to the strengthening and equatorward migration of the Hadley cell, marked by long-term marginal upwelling. The SBUS achieved its modern state by ∼ 1 Ma with complete isolation from the coastal upwelling zone. Our observations indicate a strong correlation between the history of SBUS and global climatic events, with a strong correlation between them prior to the initiation of iNHG. Subsequently, local climatic influences have become more predominant, overshadowing the signals from global climatic events. The data also suggest linkages between shifts in the SBUS strengths and southwest African climate.

The southward migration of the Antarctic Circumpolar Current enhanced oceanic degassing of carbon dioxide during the last two deglaciations

Previous studies suggest that meridional migrations of the Antarctic Circumpolar Current may have altered wind-driven upwelling and carbon dioxide degassing in the Southern Ocean during past climate transitions. Here, we report a quantitative and continuous record of the Antarctic Circumpolar Current latitude over the last glacial-interglacial cycle, using biomarker-based reconstructions of surface layer temperature gradient in the southern Indian Ocean. The results show that the Antarctic Circumpolar Current was more equatorward during the ice ages and shifted ~6° poleward at the end of glacial terminations, consistent with Antarctic Circumpolar Current migration playing a role in glacial-interglacial atmospheric carbon dioxide change. Comparing the temporal evolution of the Antarctic Circumpolar Current mean latitude with other observations provides evidence that Earth’s axial tilt affects the strength and latitude range of Southern Ocean wind-driven upwelling, which may explain previously noted deviations in atmospheric carbon dioxide concentration from a simple correlation with Antarctic climate.

On the Wind-Driven Formation of Plankton Patches in Island Wakes

Using a three-dimensional coupled physical–biological model, this paper explores the effect that short-lived wind events lasting a few days in duration have on the creation of phytoplankton blooms in island wakes. Findings show that wind-induced coastal upwelling creates initial nutrient enrichment and phytoplankton growth near the island, whereas an oscillating flow, typical of island wakes, expels patches of upwelled water, including its nutrient and phytoplankton loads, into the ambient ocean. Dependent on the wind direction, a short-lived wind event can create one or more plankton patches with diameters of the order of the island diameter. Phytoplankton continues to grow within floating patches, each forming an individual marine ecosystem. While the ecological features of island wakes are well documented, this study is the first that describes the significance of short-lived, transient wind-driven upwelling in the process.

Impacts of Marine Heatwave Events on Three Distinct Upwelling Systems and Their Implications for Marine Ecosystems in the Northwestern South China Sea

Under global warming, the frequency and intensity of marine heatwaves are increasing. However, the inhibition of atmospheric-forcing marine heatwaves (AMHW) on upwelling and their impacts on marine ecosystems remain poorly understood. To address this issue, the satellite sea surface temperature and reanalysis data during 1998–2021 were analyzed in three distinct upwelling systems, in the northwestern South China Sea. The results showed that the coastal tide-induced upwelling in the west (W) of Hainan Island is primarily suppressed by enhanced stratification during the AMHW events, since the coastal tide-induced upwelling is insensitive to wind weakening. Contrarily, the wind-driven upwelling in the east (E) and northeast (NE) of Hainan Island are jointly regulated by wind and stratification during the AMHW. Specifically, the AMHW events have a stronger inhibitory effect on the upwelling and phytoplankton growth in the NE than that in the E. The causes could be the following: (1) the background upwelling in the NE region is stronger than in the E; thus, the NE region has a higher susceptibility to the wind weakening; (2) the wind-driven upwelling begins to be suppressed by AMHW when the high-pressure system is aligned with the coastline of the upwelling. In the NE region, the location of the high-pressure center during the occurrence of AMHW is positioned in closer proximity to the upwelling area. Moreover, the inhibitory effect of wind weakening and stratification enhancing on upwelling changes with the development of the AMHW. Before and during the mature phase of AMHW, stratification and wind jointly inhibit upwelling and phytoplankton growth, while a shift to stratification-dominated (>85%) occurs during the decline phase. This study suggests that MHW has a great impact on the upwelling ecosystem, especially the wind-driven upwelling, which should be given high attention under global warming (with increasing MHW events in the future).

Seasonal and Interannual Variability of Fronts and Their Impact on Chlorophyll-a in the Indonesian Seas

Abstract The spatiotemporal variability of oceanic fronts in the Indonesian seas was investigated using high-resolution satellite observations. The study aimed to understand the underlying mechanism driving these fronts and their impact on chlorophyll-a variability. A high value of frontal probability was found near the coasts of major islands, exhibiting a distinct seasonal cycle with peaks occurrences during austral winter. The distribution variability of chlorophyll-a was generally consistent with the presence of active frontal zones, although a significantly positive relationship between fronts and chlorophyll-a was limited to only some specific areas, e.g., south Java Island and the Celebes Sea. Wind-driven upwelling played a major role in front generation in the Java upwelling region and enhanced frontal activity can promote the growth of phytoplankton, leading to higher chlorophyll-a. Furthermore, the study demonstrated that wind patterns preceded variations in front probability and chlorophyll-a by approximately two months. This lag suggests that the spatiotemporal variability of fronts and chlorophyll-a in this region is primarily influenced by the monsoon system. In addition, the sea surface temperature (SST) simultaneously modulated the chlorophyll-a variability. Negative SST anomalies were typically associated with positive anomalies in front probability the chlorophyll-a in most areas. Notably, the interannual variability of fronts and chlorophyll-a are prominent in the Java upwelling region. During El Niño years, this region experienced an enhanced monsoon, resulting in a negative SST anomaly alongside positive anomalies in front probability and chlorophyll-a. A comprehensive description and underlying dynamics of frontal activity in the Indonesian seas are provided by this study. The findings are helpful to delineate the variability in chlorophyll-a, thereby facilitating the future understanding of local primary production and the carbon cycle. Significance Statement As typical mesoscale processes, oceanic fronts have significant impacts on biological processes and fisheries in marginal seas. The complex spatiotemporal variability of fronts and their effects on biological processes in the Indonesian seas remain poorly understood. This study aimed to address this knowledge gap by investigating the seasonal and interannual variability of fronts and their influence on chlorophyll-a, a key indicator of phytoplankton biomass and primary productivity. The study identified a high frontal probability in south Java Island during austral winter and El Niño years. Wind-driven upwelling was found to be a major factor in front generation and promoting phytoplankton growth. The findings of this study will improve the theoretical knowledge of regional dynamics, local primary production, and the carbon cycle in the Indonesian seas, benefiting fisheries management and ecosystem conservation efforts.

Biotic response of plankton communities to Middle to Late Miocene monsoon wind and nutrient flux changes in the Oman margin upwelling zone

Abstract. Understanding past dynamics of upwelling cells is an important aspect of assessing potential upwelling changes in future climate change scenarios. Our present understanding of nutrient fluxes throughout the world's oceans emphasizes the importance of intermediate waters transporting nutrients from the Antarctic divergence into the middle and lower latitudes. These nutrient-rich waters fuel productivity within wind-driven upwelling cells in all major oceans. One such upwelling system is located along the Oman margin in the western Arabian Sea (WAS). Driven by cross-hemispheric winds, the WAS upwelling zone's intense productivity led to the formation of one of the most extensive oxygen minimum zones known today. In this study covering the Middle to Late Miocene at Ocean Drilling Program (ODP) Site 722, we investigate the inception of upwelling-derived primary productivity. This study presents new plankton assemblage data in the context of existing model- and data-based evidence constraining the tectonic and atmospheric boundary conditions for upwelling in the WAS. With this research, we build upon the original planktonic foraminifer-based research by Dick Kroon in 1991 as part of his research based on the ODP LEG 117. We show that monsoonal winds likely sustained upwelling since the emergence of the Arabian Peninsula after the Miocene Climatic Optimum (MCO) ∼ 14.7 Ma, with fully monsoonal conditions occurring since the end of the Middle Miocene Climatic Transition (MMCT) at ∼ 13 Ma. However, changing nutrient fluxes through Antarctic Intermediate and sub-Antarctic Mode Waters (AAIW/SAMW) were only established after ∼ 12 Ma. Rare occurrences of diatom frustules correspond to the maximum abundances of Reticulofenestra haqii and Reticulofenestra antarctica, indicating higher upwelling-derived nutrient levels. By 11 Ma, diatom abundance increases significantly, leading to alternating diatom blooms and high-nutrient-adapted nannoplankton taxa. These changes in primary producers are also well reflected in geochemical proxies with increasing δ15Norg. values (> 6 ‰) and high organic carbon accumulation. These proxies provide further independent evidence for high productivity and the onset of denitrification simultaneously. Our multi-proxy-based evaluation of Site 722 primary producers provides evidence for a stepwise evolution of Middle to Late Miocene productivity in the western Arabian Sea for the first time. The absence of a clear correlation with existing deep marine climate records suggests that both local wind patterns and intermediate water nutrient changes likely modulated productivity in the western Arabian Sea during the Middle to Late Miocene. Finally, we show that using a multi-proxy record provides novel insights into how plankton responded to changing nutrient conditions through time in a monsoon-wind-driven upwelling zone.

Mean circulation and its seasonal cycle on the West Florida Shelf as evidenced by multi-decadal time series of moored currents and winds

Time series from a moored array of current velocity and surface meteorological sensors, some with record lengths as long as 25 years, are used to describe both the long-term mean circulation and its seasonal variations on the West Florida Continental Shelf (WFS). The moorings are part of the University of South Florida's Coastal Ocean Monitoring and Prediction System (USF-COMPS), a network of ocean observing assets along with numerical circulation models, all used to describe and understand physical and ecological processes on the WFS. These USF-COMPS observations reveal a coherent, shelf-wide mean circulation pattern with depth-averaged flow directed alongshore and down-coast. The vertical structure and the seasonal variations further describe an inner-shelf, wind-driven upwelling region separated from a deeper-ocean influenced offshore downwelling region by a coastal jet. By adding to the record lengths from previous analyses, the statistics are shown to be robust, with the inferences drawn from shorter records being borne out by the present longer-term analyses.

On the phenology of coastal upwelling off central-southern Chile

Coastal upwelling is the dominant physical process triggering high biological productivity in Eastern Boundary Upwelling Systems (EBUS). These regions are characterized by intense upwelling events driven by Equatorward alongshore winds. In the Humboldt current system off central-southern Chile (30–40°S) the coastal upwelling process has been studied from several approaches including biogeochemical, fisheries and physical studies. Yet, the phenology of wind-driven upwelling along the meridional gradient has been poorly inspected. Using reanalysis data from the ERA5 product (1966–2020), we calculated the Cumulative Upwelling Index (CUI, m2 s−1 × 1000 m) to characterize the phenology of coastal upwelling off central-southern Chile, identifying the beginning (STI), the maximum (MAX) and the end (END) of the upwelling season. In addition, we quantified the duration (LUSI) and the total magnitude (TUMI) of the upwelling season. The response of the water column to cumulative wind stress was determined using in situ hydrographic data (2002–2020) from a middle shelf station off Concepción, which showed marked seasonal and interannual variability. In general, the onset, duration, and intensity of Ekman transport were highly variable. At 36.5°S (off Concepción), the STI occurred on August 6 ± 25.4 days and the duration of the upwelling season (LUSI) was ∼9 months ± 32.5 days. On the other hand, the TUMI at this latitude was −1.97 × 108± 4.88 × 107. The CUI climatology during El Niño years showed weak and late upwelling (STI = August 29 ± 16.2 days) while upwelling was strong and early (STI = July 13 ± 30.6 days) during La Niña compared to the mean climatology. The water column showed a direct response to cumulative wind-driven upwelling conditions during El Niño 2015-2016 and La Niña 2007-2008. The rise of cold (≤11 °C), saline (34.5 isohaline), dense (>25.8 kg m−3), and oxygen-poor (≤ 1 ml L−1) subsurface waters corresponded to stronger upwelling winds during La Niña 2007–2008. In contrast, coastal upwelling was substantially weak, with a warmer water column and the isotherm of 11.5 °C located below 30 m depth during El Niño 2015–2016.

Voices from the deep: Odontocete occurrence patterns relative to oceanographic conditions in the northern California Current System

Odontocetes play an important ecological role as apex predators in the northern California Current System (CCS), which is characterized by seasonal rises in primary productivity fueled by wind-driven upwelling of cold nutrient-rich water. This productivity sustains higher trophic-level prey optimal for deep-diving odontocetes including beaked and sperm whales. Due to the cryptic ecology of these whales, there are few systematic studies of their occurrence across seasons in northern CCS waters. The Holistic Assessment of Living marine resources off Oregon (HALO) Project—a collaboration between Oregon State University and Cornell University—addresses this gap via quarterly vessel-based visual surveys and year-round passive acoustic monitoring (PAM). Here, we detect and classify species-specific odontocete acoustic signals using long-term spectral averages, automated click detection, and unsupervised clustering of semi-continuous PAM data from 10/2021 to 12/2022. Site-specific occurrence of beaked and sperm whales is revealed on hourly to seasonal temporal scales at three recording sites along a depth gradient spanning the continental slope (300 and 630 m water depth) and abyssal plain (2860 m). By relating odontocete detections to dynamic oceanographic conditions from nearby profiling moorings, we identify likely drivers of prey availability influencing predator distribution. These results will inform conservation efforts of cryptic odontocetes in the face of rapid environmental change.

Effects of basin-scale climate modes and upwelling on nearshore marine heatwaves and cold spells in the California Current

Marine heatwaves and cold spells (MHWs/MCSs) have been observed to be increasing globally in frequency and intensity based on satellite remote sensing and continue to pose a major threat to marine ecosystems worldwide. Despite this, there are limited in-situ based observational studies in the very shallow nearshore region, particularly in Eastern Boundary Current Upwelling Systems (EBUS). We analyzed a unique dataset collected in shallow waters along central California spanning more than four decades (1978–2020) and assessed links with basin-scale climate modes [Pacific Decadal Oscillation (PDO) and El Niño (MEI)] and regional-scale wind-driven upwelling. We found no significant increase/decrease in MHW/MCS frequency, duration, or intensity over the last four decades, but did observe considerable interannual variability linked with basin-scale climate modes. Additionally, there was a decrease in both MHW/MCS occurrence during the upwelling season, and the initiation of individual MHWs/MCSs coincided with anomalous upwelling. Most notably, the co-occurrence of warm (cold) phases of the PDO and MEI with negative (positive) upwelling anomalies strongly enhanced the relative frequency of positive (negative) temperature anomalies and MHW (MCS) days. Collectively, both basin-scale variability and upwelling forcing play a key role in predicting extreme events and shaping nearshore resilience in EBUS.

Regulation of a surface chlorophyll hotspot by wind-driven upwelling and eddy circulation in the Santa Barbara Channel, Southern California

While most of the U.S. Pacific coast is dominated by strong wind-driven upwelling in the spring and regular seasonal cycles of phytoplankton primary production, the Southern California Bight has weak intermittent wind-driven upwelling and low phytoplankton concentrations due to its irregular sheltered coastline. However, the Santa Barbara Channel (SBC), located in the northern Southern California Bight, contains an anomalous hotspot of phytoplankton biomass. We use 3D ocean circulation and particle tracking models, an empirical temperature-nitrate relationship, and satellite observations of surface chlorophyll from 1998 to 2007 to determine how wind-driven upwelling and cyclonic eddy circulation govern phytoplankton dynamics in the SBC. Our findings show that elevated surface chlorophyll in the spring is driven by the coupling of wind-driven upwelling and cyclonic eddy circulation and requires the presence of both high nitrate from wind-driven upwelling and prolonged residence times from eddy circulation. Long residence times, created by persistent cyclonic eddy circulation, allow nitrate transported into the surface layer by wind-driven upwelling to be retained in the SBC long enough to meet the required timescales for phytoplankton nutrient uptake and accumulation, overcoming the typically weak wind-driven upwelling of the Southern California Bight. Typical spring upwelling periods (SUPs) have high levels of surface chlorophyll, which are produced by the balanced coupling of upwelling strength and cyclonic eddy circulation. For two years of our study, 1998 and 1999, the SUP did not correlate with elevated surface chlorophyll. Corresponding to a strong El Niño event, the 1998 SUP had exceptionally weak wind-driven upwelling, which did not produce sufficient levels of nitrate to stimulate elevated surface chlorophyll. For the 1999 SUP, a strong La Niña event produced unusually strong wind-driven upwelling, which resulted in very high levels of nitrate, but suppressed eddy circulation and residence times to the point where phytoplankton biomass could not accumulate in the SBC. These anomalous SUPs illustrate that when wind-driven upwelling is too strong or too weak, the balance between wind-driven upwelling and cyclonic eddy circulation is disrupted, resulting in a dramatic reduction of surface chlorophyll, which may become more frequent with climate-driven upwelling changes in the future.

Upwelling processes driven by contributions from wind and current in the Southwest East Sea (Japan Sea)

The occurrence of coastal upwelling is influenced by the intensity and duration of sea surface wind stress and geophysical components such as vertical stratification, bottom topography, and the entrainment of water masses. In addition, strong alongshore currents can drive upwelling. Accordingly, this study analyzes how wind stress and ocean currents contribute to changing coastal upwelling along the southwest coast of the East Sea (Japan Sea), which has not yet been reported quantitatively. This study aims to estimate each geophysical factor affecting upwelling processes using the Upwelling Age index. The index assesses the major contributors to the upwelling process using the relationship between physical forcing and upwelling water fraction estimated from shipboard hydrographic data from January 1993 to October 2018. These findings reveal that wind-driven upwelling was dominant off the northern coast. In contrast, current-driven upwelling prevailed off the southern coast. These results suggest that persistent alongshore currents through the Korea Strait make the southern region a prolific upwelling area. Accordingly, it can shed light on the mechanisms of coastal upwelling in the study area, which is crucial for understanding the influence of physical forces on ocean ecosystems.

Projected cross-shore changes in upwelling induced by offshore wind farm development along the California coast

In California offshore waters, sustained northwesterly winds have been identified as a key resource that can contribute substantially to renewable energy goals. However, the development of large-scale offshore wind farms can reduce the wind stress at the sea surface, which could affect wind-driven upwelling, nutrient delivery, and ecosystem dynamics. Here we examine changes to upwelling using atmospheric and ocean circulation numerical models together with a hypothetical upper bound buildout scenario of 877 turbines spread across three areas of interest. Wind speed changes are found to reduce upwelling on the inshore side of windfarms and increase upwelling on the offshore side. These changes, when expressed in terms of widely used metrics for upwelling volume transport and nutrient delivery, show that while the net upwelling in a wide coastal band changes relatively little, the spatial structure of upwelling within this coastal region can be shifted outside the bounds of natural variability.

The new Kr-86 excess ice core proxy for synoptic activity: West Antarctic storminess possibly linked to Intertropical Convergence Zone (ITCZ) movement through the last deglaciation

Abstract. Here we present a newly developed ice core gas-phase proxy that directly samples a component of the large-scale atmospheric circulation: synoptic-scale pressure variability. Surface pressure changes weakly disrupt gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (86Krxs). The 86Krxs may therefore reflect the time-averaged synoptic pressure variability over several years (site “storminess”), but it likely cannot record individual synoptic events as ice core gas samples typically average over several years. We validate 86Krxs using late Holocene ice samples from 11 Antarctic ice cores and 1 Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong spatial correlation (r=-0.94, p<0.01) between site average 86Krxs and time-averaged synoptic variability from reanalysis data. The main uncertainties in the analysis are the corrections for gas loss and thermal fractionation and the relatively large scatter in the data. Limited scientific understanding of the firn physics and potential biases of 86Krxs require caution in interpreting this proxy at present. We show that Antarctic 86Krxs appears to be linked to the position of the Southern Hemisphere eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability. We present a 86Krxs record covering the last 24 kyr from the West Antarctic Ice Sheet (WAIS) Divide ice core. Based on the empirical spatial correlation of synoptic activity and 86Krxs at various Antarctic sites, we interpret this record to show that West Antarctic synoptic activity is slightly below modern levels during the Last Glacial Maximum (LGM), increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods, weakens abruptly at the Holocene onset, remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide 86Krxs record resembles records of monsoon intensity thought to reflect changes in the meridional position of the Intertropical Convergence Zone (ITCZ) on orbital and millennial timescales such that West Antarctic storminess is weaker when the ITCZ is displaced northward and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Niño–Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling. We emphasize that 86Krxs is a new proxy, and more work is called for to confirm, replicate, and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remain speculative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in 86Krxs. A list of suggested future studies is provided.

Wind-driven upwelling of iron sustains dense blooms and food webs in the eastern Weddell Gyre

The Southern Ocean is a major sink of anthropogenic CO2 and an important foraging area for top trophic level consumers. However, iron limitation sets an upper limit to primary productivity. Here we report on a considerably dense late summer phytoplankton bloom spanning 9000 km2 in the open ocean of the eastern Weddell Gyre. Over its 2.5 months duration, the bloom accumulated up to 20 g C m−2 of organic matter, which is unusually high for Southern Ocean open waters. We show that, over 1997–2019, this open ocean bloom was likely driven by anomalies in easterly winds that push sea ice southwards and favor the upwelling of Warm Deep Water enriched in hydrothermal iron and, possibly, other iron sources. This recurring open ocean bloom likely facilitates enhanced carbon export and sustains high standing stocks of Antarctic krill, supporting feeding hot spots for marine birds and baleen whales.

Coriolis effects on wind-driven upwelling in enclosed basins

Previous studies showed that the smaller the Coriolis parameter, the greater the upwelling that occurs in enclosed basins. This suggests that the Coriolis force suppresses the upwelling, but this has not been confirmed. Therefore, this study aims to clarify the Coriolis effect on upwelling using a three-dimensional hydrodynamic model. We used six different Rossby radius conditions for a two-layer fluid in a rectangular basin to investigate the relative importance of the Coriolis on upwelling. We found that, as expected, the effective upwelling area at the water surface was highest without the Coriolis. The same trend was also observed for a three-layer fluid in a rectangular basin. The theoretical analysis revealed that a geostrophic jet induced by the geostrophic balance along the lateral boundary suppresses the upwelling.

Effect of wind on summer chlorophyll-a variability in the Yellow Sea

Winds potentially affect primary production in shelf seas during the stratified season by enhancing upwelling and mixing. However, the exact extent and modalities of this effect in the Yellow Sea remain unclear. Here, based on the satellite and in situ observation data, statistical method, and wind-driven upwelling theory, we examined the wind effect on the chlorophyll-a (Chl-a) variability in the summer of 2002-2020 and the effect mechanism. The satellite data revealed a significantly positive correlation between anomalies of the monthly mean of the summer sea surface Chl-a and wind speed at the continental slope region (water depth of 20-60 m) in the southwestern Yellow Sea where strong wind-driven upwelling has been indicated by previous studies. The wind-driven upwelling along the continental slope was further verified using two summer in-situ observations. After a fortnight of southeasterly wind, the upwelling patterns of high salinity and rich nutrients from the Yellow Sea cold water mass were observed, and consequently, high Chl-a concentrations occurred in the upper layer of the slope region. The wind-driven upwelling occurred in the region at water depth of ~20-60 m, which is consistent with the result of the wind-driven coastal upwelling theory (0.5D < water depth < 1.25D, where D is the thickness of the Ekman layer). The dissolved inorganic nitrogen, phosphorus, and silicate fluxes contributed by wind-driven upwelling were estimated as 1345 ± 674 μmol/m2/d, 81 ± 45 μmol/m2/d and 1460 ± 899 μmol/m2/d, respectively, accounting for 30%-40% of total nutrient supply, and were several times larger than that contributed by the turbulent mixing, which can explain why the strong wind-Chl-a correlation only occurred at the upwelling region rather than the entire sea. In addition, in this region, the interannual variability of the summer mean Chl-a was negatively correlated to both the Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO) indexes, due to the opposite phase of the summer wind anomaly and the PDO/ENSO. This study revealed the wind effect on the shelf phytoplankton is regional and highlighted that wind could be a pivotal factor driving the climate variability of shelf primary production in the stratified season.

Modelled coastal circulation and Lagrangian statistics from a large coastal embayment: The case of Bay of Plenty, Aotearoa New Zealand

In this study, a high-resolution one-way nested, hindcast ROMS model was developed to analyse the coastal circulation and Lagrangian statistics within the Bay of Plenty (BoP) region in Aotearoa, New Zealand. The Bay of Plenty Model (BoPM) was statistically evaluated against a set of multiple remote sensing and in situ observations from 2003–2004, forming the analysis period for this study. Overall, the BoPM possesses good skill reproducing ocean water temperature, salinity, sea level and water column velocity over tidal and non-tidal timescales (Willmott skill ¿0.8 for most variables). Root-mean-squared errors of <1 °C for ocean water temperature, ≈0.15 for salinity (an exception present during winter), <8 cm s−1 for water column velocity and 0.09 m for non-tidal sea surface height are achieved. Over the 2-year period, nearshore modelled sea surface currents are correlated to local wind forcing on the western and central region of the BoP consistent with coastal wind-driven upwelling dynamics. Up to 30% of the cross-shelf current variability is explained by along-shore wind stress, consistent with previous observational studies. Meanwhile, the eastern region has no significant correlation to the along-shore winds, suggesting other forcings must be considered. Circulation patterns and Lagrangian statistics under two distinct atmospheric conditions over the two years were analysed: January, with predominantly moderate upwelling-favourable winds and July, with highly variable and stronger winds. January conditions show an eastward flowing large-scale boundary current, the East Auckland Current (EAUC), and a quasi-stationary eddy, the East Cape Eddy (ECE), close to the shelf break, while July conditions show the EAUC and ECE located further from shore. A series of particle release experiments from inner-, mid-shelf, and shelf break locations are used to identify trajectory and dispersion variability between western, central, and eastern sections of the BoP under January and July conditions. Particles released under January conditions tend to flow eastward following the EAUC. Under July conditions, inner- and mid-shelf releases converge towards the central BoP, where bathymetric changes and islands create pathways for particles to exit the continental shelf. Relative dispersion (R2) under January conditions shows a ballistic dispersion regime (R2≈t2) over the first 10 days followed by a diffusive regime (R2≈t1) after 15 days. Under July conditions, a ballistic-like regime is maintained throughout a 30-day period, likely due to strong mixing in the BoP associated with submesoscale processes. Particles released in proximity to East Cape headland that remain in the water column for >5 days show higher dispersion relative to releases from the central and western regions of the BoP. However, 70%–90% of particles released on the eastern inner-shelf strand to the coastline in <5 days. This suggests that the eastern region can act as a retention zone, constraining the particles towards the shore. Drift duration increases for particles released further from the shore, where offshore mesoscale currents influence them more, and islands become important receptor locations.

Variability of Summer Wind-Driven Upwelling along the Koryak Coast in the Northwestern Bering Sea Based on Satellite Data

Variability of Summer Wind-Driven Upwelling along the Koryak Coast in the Northwestern Bering Sea Based on Satellite Data