Water Mass Anomalies Research Articles

Two Pathways of Subsurface Spiciness Anomalies in the Subtropical South Pacific

Subduction and migration of density-compensated (warm/salty or cool/fresh) temperature and salinity water-mass anomalies on isopycnals, referred to as spiciness anomalies, are examined in the subtropical gyre of the South Pacific using an observational dataset. The present results demonstrate that the spiciness anomalies are found to follow two pathways from the subtropical region to the tropical area on the 25–25.5σθ isopycnals. The water masses of one pathway subduct south of 20°S and mainly flow westward via the mean geostrophic current to the western boundary region. The water masses of this pathway correspond to the salinity maximum on these isopycnals as well as the bottom of the South Pacific Tropical Water. Positive temperature and salinity trends were prominent along this pathway during the study period. In the other pathway, the water masses subduct north of 20°S and go directly to the tropics through the interior region. Decadal variability of the spiciness anomalies is prominent along this pathway. In both pathways, sea surface salinity variability likely plays an important role in generating the spiciness anomalies on the isopycnals. A passive tracer experiment revealed that the advection by the South Equatorial Countercurrent (SECC) divides these two pathways. Hence, SECC plays a key role in determining whether a spiciness anomaly propagates through the interior region or the western boundary region.

Water scarcity and social conflict

Climate change and the increasing demand of water intensify the global water cycle, altering the distribution of water in space and time. This is expected to result in wet areas getting wetter and dry areas getting drier. As water is key to life, water scarcity is likely to provoke conflict. Using grid-cell data for Africa and central America over the years of 2002 to 2017, we provide empirical evidence for a link between the likelihood of local conflict and water mass declines. We measure water mass anomalies based on changes in Earth’s gravity field recorded by GRACE and link them to social conflict events recorded in the SCAD data. To account for potential endogeneity in the demand for water, we instrument water mass change by the interaction of the number of drought months per year with yearly global average temperature changes. Our results show that a one standard deviation decrease in local water mass that follows from droughts and an intensifying water cycle more than triple the local likelihood of social conflict. Access to groundwater and surface water help to mitigate these effects substantially. Water demand factors contribute to a quicker depletion of water mass in case of drought shocks, but do not intensify the link between water decline and conflict itself.

Mesoscale and Submesoscale Shelf‐Ocean Exchanges Initialize an Advective Marine Heatwave

AbstractObservations and high‐resolution numerical modeling are used to investigate the dynamical processes related to the initiation of an advective Marine Heatwave in the Middle Atlantic Bight of the Northwest Atlantic continental shelf. Both the observations and the model identify two significant cross‐shelf intrusions in November 2016 and January 2017, with the latter inducing large‐magnitude water mass anomalies across the shelf. Model prognostic fields reveal the importance of the combination of cyclonic eddies or ringlets and upwelling‐favorable winds in producing the large‐distance cross‐shelf penetration and temperature/salinity anomalies. The cyclonic eddies in close proximity to the shelfbreak set up local along‐isobath pressure gradients and provide favorable conditions for the intensification of the shelfbreak front, both processes driving cross‐isobath intrusions of warm, salty offshore water onto the outer continental shelf. Subsequently, strong and persistent upwelling‐favorable winds drive a rapid, bottom intensified cross‐shelf penetration in January 2017 composed of the anomalous water mass off the shelfbreak. The along‐shelf settings including realistic representation of bathymetric features are essential in the characteristics of the cross‐shelf penetration. The results highlight the importance of smaller scale cyclonic eddies and the intricacy of the interplay between multiple processes to drive significant cross‐shelf events.

Water Scarcity and Social Conflict

Climate change and the increasing demand of water intensify the global water cycle, altering the distribution of water in space and time. This is expected to result in wet areas getting wetter and dry areas getting drier (Pan et al., 2015). As water is key to life, water scarcity is likely to provoke conflict. Using grid-cell data for Africa and central America over the years of 2002 to 2017, we establish a causal empirical link between the likelihood of local conflict and water mass declines. We measure water mass anomalies based on changes in Earth's gravity field recorded by GRACE and link them to social conflict events recorded in the SCAD data. To account for potential endogeneity in the demand for water, we instrument water mass change by the interaction of the number of drought months per year with yearly global average temperature changes. Our results show that a one standard deviation decrease in local water mass that follows from droughts and an intensifying water cycle more than triple the local likelihood of social conflict. Access to groundwater and surface water helps to mitigate these effects substantially. Water demand factors contribute to a quicker depletion of water mass in case of drought shocks, but do not intensify the link between water decline and conflict itself.

Seasonal-to-Interannual Response of Southern Ocean Mixed Layer Depth to the Southern Annular Mode from a Global 1/10° Ocean Model

Abstract The relationship between the southern annular mode (SAM) and Southern Ocean mixed layer depth (MLD) is investigated using a global 0.1° resolution ocean model. The SAM index is defined as the principal component time series of the leading empirical orthogonal function of extratropical sea level pressure from September to December, when the zonally symmetric SAM feature is most prominent. Following positive phases of the SAM, anomalous deep mixed layers occur in the subsequent fall season, starting in May, particularly in the southeast Pacific. Composite analyses reveal that for positive SAM phases enhanced surface cooling caused by anomalously strong westerlies weakens the stratification of the water column, leading to deeper mixed layers during spring when the SAM signal is at its strongest. During the subsequent summer, the surface warms and the mixed layer shoals. However, beneath the warm surface layer, anomalously weak stratification persists throughout the summer and into fall. When the surface cools again during fall, the mixed layer readily deepens due to this weak interior stratification, a legacy from the previous springtime conditions. Therefore, the spring SAM–fall MLD relationship is interpreted here as a manifestation of reemergence of interior water mass anomalies. The opposite occurs after negative phases of the SAM, with anomalously shallow mixed layers resulting. Additional analyses reveal that for the MLD region in the southeast Pacific, the effects of salinity variations and Ekman heat advection are negligible, although Ekman heat transport may play an important role in other regions where mode water is formed, such as south of Australia and in the Indian Ocean.

Dynamics of water mass in the Central Siberia permafrost zone based on gravity survey from the grace satellites

The GRACE gravimetric survey is applied to analyze the equivalent water mass anomalies (EWMAs) in the permafrost zone of Central Siberia. Variations in EWMAs are related to precipitation, air temperature, potential evapotranspiration, and soil composition (drainage conditions). The EWMA dynamics demonstrates two periods. The period of 2003–2008 is characterized by a positive trend. The one of 2008–2012 shows a decrease in the trend with a simultaneous increase by 30–70% of EWMA dispersion in the background of growth (up to 40%) of precipitation variability. The rate of water mass increment demonstrates a positive correlation with the sand and gravel contents in soil (r = 0.72) and a negative one with clay content (r =–0.69 to–0.77). For Taimyr Peninsula, there is a deficit of residual water mass (~250 mm for the period of 2012–2013) indicating the deeper thawing of permafrost soils. In the Central Siberian Plateau, the indicator of more intensive permafrost thawing (and that of an increase in active layer thickness) is a considerable trend of water mass increase (2003–2008). The increasing trend of the largest Siberian rivers (Yenisei and Lena) is revealed in the period of 2003–2012.

Subthermocline eddies observed by rapid‐sampling Argo floats in the subtropical northwestern Pacific Ocean in Spring 2014

AbstractIn Spring 2014, two subthermocline eddies (STEs) were observed by rapid‐sampling Argo floats in the subtropical northwestern Pacific (STNWP). The first one is a warm, salty, and oxygen‐poor lens, with its temperature/salinity /dissolved oxygen (T/S/DO) anomalies reaching 1.16°C/0.21 practical salinity unit (psu)/−29.9 µmol/kg, respectively, near the 26.62σ0 surface. The other is a cold, fresh, and oxygen‐rich lens, with its T/S/DO anomalies reaching −1.95°C/−0.34 psu/88.0 µmol/kg, respectively, near the 26.54σ0 surface. The vertical extent of the water mass anomalies in the warm (cold) STE is about 190 m (150 m), and its horizontal length scale is 22 ± 7 km (18 ± 10 km). According to their water mass properties, we speculate that the warm and cold STEs are generated in the North Pacific Subtropical and Subarctic Front region, respectively. The observed STEs may play an important role in modifying the intermediate‐layer water properties in the STNWP, and this needs to be confirmed by more focused observations in the future.

Formation of Intrathermocline Lenses by Eddy–Wind Interaction

AbstractMesoscale intrathermocline lenses are observed throughout the World Ocean and are commonly attributed to water mass anomalies advected from a distant origin. An alternative mechanism of local generation is offered herein, in which eddy–wind interaction can create lens-shaped disturbances in the thermocline. Numerical simulations illustrate how eddy–wind-driven upwelling in anticyclones can yield a convex lens reminiscent of a mode water eddy, whereas eddy–wind-driven downwelling in cyclones produces a concave lens that thins the mode water layer (a cyclonic “thinny”). Such transformations should be observable with long-term time series in the interiors of mesoscale eddies.

Numerical simulations of global‐scale high‐resolution hydrological crustal deformations

High‐resolution load‐induced crustal deformations calculated from numerical models are tested for their ability to predict hydrologically‐induced station height variability, as they are known to be large enough to affect epoch‐wise parameters obtained from the analysis of global geodetic networks. Loading contributions due to terrestrial water storage as given by global hydrological models are calculated on a 0.5° global regular grid with daily temporal resolution. Apart from the dominant seasonal variations, the hydrological loading signal discloses also rapid changes exceeding 1 mm in several regions that can be associated with major precipitation events and river floods. Locally strong loading signals with exceptionally high amplitudes, in many cases even with nonseasonal nature, occur along the major river channels. Only high‐resolution loading calculations considering also the water mass anomalies stored in the model river flow can resolve the correct amplitudes in the surrounded area up to 100 km distance. The comparison of the modeled hydrological surface deformation with GPS station time series shows that high‐resolution hydrological loading estimates based on global‐scale models are able to explain a considerable fraction (up to 54%) of the observed vertical station movements caused by continental water storage variations.

Independent patterns of water mass anomalies over Australia from satellite data and models

The Gravity Recovery and Climate Experiment (GRACE) products allow the quantification of total water storage (TWS) changes at global to regional scales. However, the quantity measured by GRACE represents mass signals integrated over vertical columns, requiring their separation into their original sources. Such a separation is vital for Australia, for which GRACE estimates are affected by leakage from the surrounding oceans. The independent component analysis (ICA) method that uses higher-order statistics, is implemented here to separate GRACE-derived water storage signals over the Australian continent from its surrounding oceans, covering from October 2002 to May 2011. The performance of ICA applied to GRACE is then compared to the ICA of WaterGAP Global Hydrology Model (WGHM) and the ICA of the Australian Water Resources Assessment (AWRA) system. To study the influence of rainfall variability on the derived independent patterns, use is made of Tropical Rainfall Measuring Mission (TRMM) data set, from January 2000 to May 2011. Implementing ICA on GRACE-TWS showed a remarkable improvement in separating the continental hydrological signals from the surrounding oceanic anomalies, which was not achievable using a conventional principle component analysis. Reconstructing the continental TWS changes using only those independent components of GRACE that were located over the continent showed a high correlation with WGHM-TWS and AWRA-TWS. Mass concentrations over the oceans and particularly S2 semi-diurnal aliased pattern were separated as independent modes. Correlation analysis between the independent components of GRACE and climate teleconnections showed that the mass anomalies over the northern ocean, Gulf of Carpentaria and north-eastern parts of Australia were significantly correlated with the El Niño-Southern Oscillation, while those over south and south-eastern parts of Australia were mainly linked to the Indian Ocean Dipole.

Suppression of the 2010 Alexandrium fundyense bloom by changes in physical, biological, and chemical properties of the Gulf of Maine.

For the period 2005-2009, the abundance of resting cysts in bottom sediments from the preceding fall was a first-order predictor of the overall severity of spring-summer blooms of Alexandrium fundyense in the western Gulf of Maine and southern New England. Cyst abundance off mid-coast Maine was significantly higher in fall 2009 than it was preceding a major regional bloom in 2005. A seasonal ensemble forecast was computed using a range of forcing conditions for the period 2004-2009, suggesting that a large bloom was likely in the western Gulf of Maine in 2010. This did not materialize, perhaps because environmental conditions in spring-summer 2010 were not favorable for growth of A.fundyense. Water mass anomalies indicate a regional-scale change in circulation with direct influence on A. fundyense's niche. Specifically, near-surface waters were warmer, fresher, more stratified, and had lower nutrients than during the period of observations used to construct the ensemble forecast. Moreover, a weaker-than-normal coastal current lessened A. fundyense transport into the western Gulf of Maine and Massachusetts Bay. Satellite ocean color observations indicate the 2010 spring phytoplankton bloom was more intense than usual. Early-season nutrient depletion may have caused a temporal mismatch with A. fundyense's endogenous clock that regulates the timing of cyst germination. These findings highlight the difficulties of ecological forecasting in a changing oceanographic environment, and underscore the need for a sustained observational network to drive such forecasts.

On the capabilities of the multi-channel singular spectrum method for extracting the main periodic and non-periodic variability from weekly GRACE data

We study the capabilities of the method of multi-channel singular spectrum analysis (MSSA) to extract periodic signals from the Gravity Recovery and Climate Experiment (GRACE) gravity field solutions. As a non-parametric method and because of the data-adaptive nature of the base functions, the MSSA allows for modelling of non-periodic variations. In addition, it can identify modulated oscillations in the presence of noise. In our study, we analyze a complete 6-year weekly time series of the GFZ-produced GRACE spherical harmonic coefficients of degree and order 30. The MSSA filtering reduces the average root-mean-square (RMS) of the mass variability over the oceans by more than 60% when all but the annual, semi-annual and long-term variations in the spherical harmonic coefficients are filtered out. While the high variance annual signal can be extracted from the GRACE data straightforwardly, the semi-annual and long-term modes identified in the low variance portion of the data eigen-spectrum are mixed. Moreover, the semi-annual variability is contaminated by the S2 tidal alias signal shown in an example for the Hudson Bay region to possibly alter the typical two-peak seasonal cycle of the water mass anomalies. Also, some long-term modes indicate that a variation exists with a 2.1–2.5 years period, possibly of a hydrology origin, which was found by previous studies in the GFZ GRACE data. On land, we analyze time series of basin averages in Amazon, Congo, Mississippi and Nelson rivers basins computed from the filtered GRACE and Global Land Data Assimilation System (GLDAS)/Noah model coefficients. The analysis is performed in two steps: (1) an approximation of the series of basin average is computed from the identified annual, semi-annual and long-term variations in the spherical harmonic coefficients and (2) to improve the approximation, the residual variability in the basin average series is analyzed by means of singular spectrum analysis. Time lags between the hydrology model and observations are found for the Amazon (4 weeks) and Mississippi and Nelson (2 weeks) basins, where GLDAS is generally ahead of GRACE. Significant differences in the water content for particular years, for example, during the 2007–2008 time period in Congo, are observed, as well. The use of the MSSA method in the analysis of the GRACE mass variations is not pertinent to the low resolution weekly solutions. The same approach can be applied to the monthly GRACE solutions of a much higher degree and order in the search for local signals. Here, we traded off the spatial resolution for the time resolution and we determined the time lag of the model water mass anomalies and observations with a nominal weekly resolution.

GRACE-derived surface water mass anomalies by energy integral approach: application to continental hydrology

We propose an unconstrained approach to recover regional time-variations of surface mass anomalies using Level-1 Gravity Recovery and Climate Experiment (GRACE) orbit observations, for reaching spatial resolutions of a few hundreds of kilometers. Potential differences between the twin GRACE vehicles are determined along short satellite tracks using the energy integral method (i.e., integration of orbit parameters vs. time) in a quasi-inertial terrestrial reference frame. Potential differences residuals corresponding mainly to changes in continental hydrology are then obtained after removing the gravitational effects of the known geophysical phenomena that are mainly the static part of the Earth’s gravity field and time-varying contributions to gravity (Sun, Moon, planets, atmosphere, ocean, tides, variations of Earth’s rotation axis) through ad hoc models. Regional surface mass anomalies are restored from potential difference anomalies of 10 to 30-day orbits onto 1◦ continental grids by regularization techniques based on singular value decomposition. Error budget analysis has been made by considering the important effects of spectrum truncation, the time length of observation (or spatial coverage of the data to invert) and for different levels of noise.

Observed characteristics of Mozambique Channel eddies

The flow in the Mozambique Channel is dominated by large, southward propagating, anti‐cyclonic eddies, as opposed to a steady western boundary current. These Mozambique Channel eddies feed their waters into the Agulhas Current system, where they are thought to have a significant influence on the formation of the Natal Pulse and Agulhas Ring shedding. Here we use in situ hydrographic and nutrient data, together with satellite altimetry and surface velocity profilers to provide a detailed characterization of the Mozambique Channel eddies. Two warm eddies in the Channel at 20°S and 24°S had diameters of over 200 km. They rotated anti‐cyclonically with a tangential velocity of over 0.5 m.s−1. Vertical sections show that the eddies reached to the bottom of the water column. Relative to the surrounding waters, the features were warm and saline. The total heat and salt anomalies for the southernmost eddy were computed relative to a reference station close by. At 24°S the total anomalies were 1.3 × 1020 J and 6.9 × 1012 kg, respectively, being on par with Agulhas rings. Mozambique Channel eddies thus have the potential to form a major contribution to the southward eddy heat flux in the Agulhas Current system. The feature also had positive nutrient and negative oxygen anomalies. The large magnitude of the water mass anomalies within the eddy suggests that interannual variability in Mozambique Channel eddy numbers would have a significant impact on downstream water mass characteristics.

Labrador Sea convection and subpolar North Atlantic Deep Water export in the SODA assimilation model

Labrador Sea convection was most intense and reached the greatest depths in the early 1990s, followed by weaker, shallower, and more variable convection after 1995. The Simple Ocean Data Assimilation (SODA) version 2.0.2/2.0.4 assimilation model is used to explore convective activity in the North Atlantic Ocean for the period from 1992 to 2007. Hydrographic conditions, which are relatively well observed during this period, are used to compare modeled and observed winter mixed-layer depths and water mass anomalies in relation to Deep Western Boundary Current transports and meridional overturning circulation (MOC) changes at the exit of the subpolar basin. The assimilation differs markedly from local observations in the March mixed-layer depth, which represents deep convection and water mass transformation. However, mean MOC rates at the exit of the subpolar gyre, forced by stratification in the mid-latitudes, are similar to estimates based on observations and show no significant decrease during the 1992–2007 period. SODA reproduces the deep Labrador Sea Water formation in the western North Atlantic without any clear indication of significant formation in the Irminger Sea while the lighter upper Labrador Sea Water density range is reached in the Irminger Sea in the 1990s, in agreement with existing assumptions of deep convection in the Irminger Sea and also supported by computed lag correlations with the Labrador Sea. Deep Water transformation mainly takes place in the eastern North Atlantic. The introduction of CFC-11 into the SODA model as a tracer reproduces the mean and multiyear variations of observed distributions.

Causes for Large-Scale Hydrographic Changes at the Hawaii Ocean Time Series Station

Abstract Results from Estimating the Circulation and Climate of the Ocean (ECCO)–Scripps Institution of Oceanography (SIO) global ocean state estimate, available over the 11-yr period 1992 through 2002, are compared with independent observations available at the Hawaii Ocean time series station ALOHA. The comparison shows that at this position, the estimated temporal variability has some skill in simulating observed ocean variability and that the quality of future syntheses could benefit from additional information available from the Argo network and from the time series observations themselves. On a decadal time scale, the influence radius of the station ALOHA T–S time series covers large parts of the tropical and subtropical Pacific Ocean and reaches even into the Indian Ocean through the Indonesian Throughflow. Estimated changes in sea surface height (SSH) result largely from thermosteric changes; however, nonsteric (barotropic) variations on the order of 1–2 cm also contribute to SSH changes at station ALOHA. Moreover, changes of similar magnitude can be caused by changes in the salinity field because of a quasi-biennial oscillation in the horizontal flow structure and heaving of the mean salinity structure on seasonal and interannual time scales. The adjoint modeling framework confirms westward-propagating Rossby waves (due to wind forcing) and subduction of water-mass anomalies (due to surface buoyancy forcing) as the primary mechanisms leading to observed changes of T–S structures at station ALOHA. Specifically, the analysis identifies surface freshwater fluxes along the wintertime outcrop of intermediate waters as a primary cause for salinity changes at station ALOHA and wind stress forcing east of the station position as another forcing mechanism of salinity variations around the Hawaiian Archipelago.

Climatological coupling of the thermohaline decadal changes in Central Water of the Eastern North Atlantic

Data collected at 42oN, 10oW in the intergyre region of the Northeast Atlantic show significant year to year variability in the T-S characteristics of the upper 800m of the water column. Taking salinity values on the sq = 27.1 kg m-3 isopycnal as representative of the Eastern North Atlantic Central Water mass it was found that the variability correlates well with the wind stress at 43oN, 11oW, with cumulative river discharge (which we take as an index of precipitation over the ocean) and with the NAO (which is an index of the strength and position of storm tracks and the state of the evaporation-precipitation balance). The covariation illustrates the close coupling between water mass formation and climate in the North Atlantic, where climate changes affect the deep ventilation by which ENACW is formed and the evaporation-precipitation balance from which the T-S signature results. Hints of a 20 year cycle in the ocean correlate with a 20 year periodicity in the NAO. It remains to be established whether there is a feedback mechanism by which water mass anomalies affect the climate and the intensity and variation of the NAO pattern, and the extent to which upper ocean observations can be used as an indicator of future climate trends.

Observations and modeling of the 1991–1992 El Niño signal off central California

Five research cruises were conducted over the continental shelf and slope near the Farallon Islands, California, in February, May, August, and October/November 1991 and February 1992. The observations consisted of shipboard hydrographic and acoustic Doppler current profiler data and moored current meter measurements. Water mass anomalies were calculated for each cruise by subtracting seasonal means based on historical data. In general, the maximum anomalies were observed subsurface in the 100‐to 150‐m range. In May 1991, equatorward, upwelling favorable winds elevated the thermocline resulting in cold, salty anomalies nearshore, with cold, fresh anomalies offshore associated with the advection of Pacific Subarctic Water into the region from the north. Warm, fresh anomalies and a strongly depressed thermocline were observed during the February 1992 cruise. A combination of coastal sea level and wind stress data and output from the Los Alamos National Laboratory parallel ocean program model was used to explain the cause of these anomalies. The February 1992 anomalies were shown to be due to both the deepening of the Aleutian low in the North Pacific associated with the 1991–1993 El Nino/Southern Oscillation event in the equatorial Pacific and poleward propagating intraseasonal coastal trapped Kelvin waves also arising from this event. The anomalous poleward wind forcing produced onshore flow, deepening of the thermocline, and downwelling at progressively southward locations. The “downwelling” Kelvin waves propagated northward with the two signals meeting somewhere near the cruise region. Both the model and the coastal sea level data showed the phase speed of the waves to slow by about 50% after passing the Gulf of California. This may be due to the scattering of energy from the fastest baroclinic mode into a slower mode. The strongest wave signal in the equatorial Pacific did not necessarily produce the strongest anomalies off central California.

Mesoscale eddies, jets, and fronts off Point Arena, California, July 1986

A broad (about 40 km wide) cold filament observed off Point Arena in advanced very high resolution radiometer images at the end of June 1986 evolved into a narrow (10–20 km wide) cold filament embedded in a large‐scale (about 200 km alongshore) cool anomaly by mid July. The cool anomaly was part of a much larger‐scale anomaly, stretching from north of Cape Mendocino to south of San Francisco, in which several cold filaments were embedded. The filament off Point Arena, surveyed by ship from July 7 to 19, 1986, extended offshore around the edge of a cyclonic center. The horizontal temperature gradient was sharpest (up to 1.5°C/2.2 km) across the southern edge of the filament, and a salinity front was located within the cold filament; temperature and salinity fronts were not coincident. The surface salinity was high (33.2 psu (practical salinity unit)) in the cyclonic center and decreased across the cold filament to about 32.6 psu. The entire cool anomaly was denser than surrounding water, and there was a sharp density front (1 sigma‐t unit/10 km) at its northern edge. At the inner (cyclonic) edge of the filament the temperature and salinity variations were density compensating. The cold filament was advected offshore by a jet which was symmetric about its axis, located about 10 km north of the cold filament core. The surface manifestation of the cold filament was highly variable in space and time: the location of the filament core changed by about 5 km in 14 hours along one transect; the velocity field was less variable. Several narrow (10 km wide) water mass anomalies were evident, including a salinity subduction zone in the northern segment of the cold filament. Thermohaline intrusions 50–100 m thick may have been caused by complex frontal interactions upstream of the anomaly and offshore in a region of jet confluence. From a second survey (July 27 to August 5, 1986) the offshore jet had reoriented to alongshore, possibly related to an observed increase in the poleward flow over the continental slope. Prior to the second survey there were changes in the nearshore wind stress and the large‐scale wind stress curl.