Variability In Ocean Dynamics Research Articles

Global sea surface salinity via the synergistic use of SMAP satellite and HYCOM data based on machine learning

Sea surface salinity (SSS) provides information on the variability of ocean dynamics (global water cycle and ocean circulation) and air-sea interactions, thereby contributing to the identification and prediction of significant changes in the global climate. Monitoring global SSS via satellite observations has been possible using L-band microwave radiometers since 2010; however, their performance is limited by their retrieval algorithms under conditions such as radio frequency interference, low sea surface temperatures, and strong winds. This study proposes a new global SSS model using multi-source data based on seven machine learning approaches: K-nearest neighbor, support vector regression, artificial neural network, random forest, extreme gradient boosting, light gradient boosting model, and gradient boosted regression trees (GBRT). Five Soil Moisture Active Passive (SMAP) products, Hybrid Coordinate Ocean Model (HYCOM) SSS, and four ancillary data were used as input variables. All models produced better performance than either SMAP or HYCOM SSS products, with the top performing GBRT model reducing the root mean square difference for the validation dataset from 1.062 to 0.259 practical salinity units compared to the SMAP SSS product. The improved SSS products had increased correlation with the in-situ data for both low- and high-salinity waters across all global oceans, thus further advancing the understanding and monitoring of global SSS.

Global sea level reconstruction for 1900–2015 reveals regional variability in ocean dynamics and an unprecedented long weakening in the Gulf Stream flow since the 1990s

Abstract. A new monthly global sea level reconstruction for 1900–2015 was analyzed and compared with various observations to examine regional variability and trends in the ocean dynamics of the western North Atlantic Ocean and the US East Coast. Proxies of the Gulf Stream (GS) strength in the Mid-Atlantic Bight (GS-MAB) and in the South Atlantic Bight (GS-SAB) were derived from sea level differences across the GS. While decadal oscillations dominate the 116-year record, the analysis showed an unprecedented long period of weakening in the GS flow since the late 1990s. The only other period of long weakening in the record was during the 1960s–1970s, and red noise experiments showed that is very unlikely that those just occurred by chance. Ensemble empirical mode decomposition (EEMD) was used to separate oscillations at different timescales, showing that the low-frequency variability of the GS is connected to the Atlantic Multi-decadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC). The recent weakening of the reconstructed GS-MAB was mostly influenced by weakening of the upper mid-ocean transport component of AMOC as observed by the RAPID measurements for 2005–2015. Comparison between the reconstructed sea level near the coast and tide gauge data for 1927–2015 showed that the reconstruction underestimated observed coastal sea level variability for timescales less than ∼5 years, but lower-frequency variability of coastal sea level was captured very well in both amplitude and phase by the reconstruction. Comparison between the GS-SAB proxy and the observed Florida Current transport for 1982–2015 also showed significant correlations for oscillations with periods longer than ∼5 years. The study demonstrated that despite the coarse horizontal resolution of the global reconstruction (1∘ × 1∘), long-term variations in regional dynamics can be captured quite well, thus making the data useful for studies of long-term variability in other regions as well.

Arctic tidal current atlas

Tidal and wind-driven near-inertial currents play a vital role in the changing Arctic climate and the marine ecosystems. We compiled 429 available moored current observations taken over the last two decades throughout the Arctic to assemble a pan-Arctic atlas of tidal band currents. The atlas contains different tidal current products designed for the analysis of tidal parameters from monthly to inter-annual time scales. On shorter time scales, wind-driven inertial currents cannot be analytically separated from semidiurnal tidal constituents. Thus, we include 10–30 h band-pass filtered currents, which include all semidiurnal and diurnal tidal constituents as well as wind-driven inertial currents for the analysis of high-frequency variability of ocean dynamics. This allows for a wide range of possible uses, including local case studies of baroclinic tidal currents, assessment of long-term trends in tidal band kinetic energy and Arctic-wide validation of ocean circulation models. This atlas may also be a valuable tool for resource management and industrial applications such as fisheries, navigation and offshore construction.

Pathways of Oil Spills from Potential Cuban Offshore Exploration: Influence of Ocean Circulation

The DeepWater Horizon oil spill in the Gulf of Mexico (GoM) in 2010 raised the public awareness on potential spills from offshore exploration activities. It became apparent that knowledge of potential oil pathways in the case of a spill is important for preparedness and response. This study focuses on such scenarios from potential oil spills in the Cuban Exclusive Economic Zone (EEZ), a vast area in the GoM and the Straits of Florida that has not received much attention in oil spill studies, even though this region has been under evaluation for oil exploration. The Cuban EEZ is also in the crossroads of heavy tanker traffic, from the areas of intense oil exploration in the Northern GoM to the adjacent Caribbean Sea and Atlantic Ocean. The study also evaluates how the oil transport and fate are influenced by the main circulation patterns of the GoM, such as the Loop Current (LC) system and the mesoscale dynamics inside the Straits of Florida, such as the Florida Current (FC) and the accompanying cyclonic (along the northern Straits) and anticyclonic (along the Cuban coasts) eddies. We used oil spill numerical simulations, in tandem with high resolution data-assimilative ocean simulations, to test the fate of potential oil spills originating from different release sites within the Cuban EEZ during a six-year period (2011–2016) to exhibit certain aspects of interannual variability of ocean dynamics. The LC extended and retracted phases in the GoM interior revealed different impacts on the oil fate depending on the release site. The meandering of the FC, which is strongly related to the mesoscale eddies that evolve inside the Straits of Florida, controlled oil pathways either towards the northern Straits or along the Cuban coast. The most likely scenario for oil stranding at southern Florida is from oil released at the deep central Straits of Florida. Oil release near the Yucatan Strait and in the deep Gulf interior showed the highest risk of overall oil beaching at the Gulf beaches. The regional (e.g., LC) and local (e.g., eddies in the Straits) dynamics are proven to be significant indicators to predict the oil fate and stranding along the Gulf coasts, which should lead to improving planning and preparedness in the case of a spill in the Cuban EEZ.

Biogeography of the Oceans: a Review of Development of Knowledge of Currents, Fronts and Regional Boundaries from Sailing Ships in the Sixteenth Century to Satellite Remote Sensing

The development of knowledge of global biogeography of the oceans from sixteenthcentury European voyages of exploration to present-day use of satellite remote sensing is reviewed in three parts; the pre-satellite era (1513–1977), the satellite era leading to a first global synthesis (1978–1998), and more recent studies since 1998. The Gulf Stream was first identified as a strong open-ocean feature in 1513 and by the eighteenth century, regular transatlantic voyages by sailing ships had established the general patterns of winds and circulation, enabling optimisation of passage times. Differences in water temperature, water colour and species of animals were recognised as important cues for navigation. Systematic collection of information from ships’ logs enabled Maury (The Physical Geography of the Sea Harper and Bros. New York 1855) to produce a chart of prevailing winds across the entire world’s oceans, and by the early twentieth century the global surface ocean circulation that defines the major biogeographic regions was well-known. This information was further supplemented by data from large-scale plankton surveys. The launch of the Coastal Zone Color Scanner, specifically designed to study living marine resources on board the Nimbus 7 polar orbiting satellite in 1978, marked the advent of the satellite era. Over subsequent decades, correlation of satellite-derived sea surface temperature and chlorophyll data with in situ measurements enabled Longhurst (Ecological Geography of the Sea. Academic Press, New York 1998) to divide the global ocean into 51 ecological provinces with Polar, Westerly Wind, Trade Wind and Coastal Biomes clearly recognisable from earlier subdivisions of the oceans. Satellite imagery with semi-synoptic images of large areas of the oceans greatly aided definition of boundaries between provinces. However, ocean boundaries are dynamic, varying from season to season and year to year. More recent work has focused on the study of variability of currents, fronts and eddies, which are often the focus of high biological productivity. Direct tracking of animals using satellite-based systems has helped resolve the biological function of such features and indeed animals instrumented in this way have helped the study of such features in three dimensions, including depths beyond the reach of conventional satellite remote sensing. Patterns of surface productivity detected by satellite remote sensing are reflected in deep sea life on the sea floor at abyssal depths >3,000 m. Satellite remote sensing has played a major role in overcoming the problems of large spatial scales and variability in ocean dynamics and is now an essential tool for monitoring global change.

Internal Variability of Indian Ocean SST

Abstract A 40-yr integration of an eddy-resolving numerical model of the tropical Indian Ocean is analyzed to quantify the interannual variability that is caused by the internal variability of ocean dynamics. It is found that along the equator in the western Indian Ocean internal variability contributes significantly to the observed interannual variability. This suggests that in this location the predictability of SST is limited to the persistence time of SST anomalies, which is approximately 100 days. Furthermore, a comparison with other sources of variability suggests that internal variability may play an important role in modifying the Indian monsoon or preconditioning the Indian Ocean dipole/zonal mode.

Internal variability of the tropical Pacific ocean

A 40 year integration of an eddy resolving numerical model of the tropical Pacific ocean is analyzed to quantify the interannual variability caused by internal variability of ocean dynamics. It is found that along the Pacific cold tongue internal variability contributes a significant amount to the observed interannual variability. This suggests that in this location the predictability of SST is limited to the persistence time of SST anomalies which is approximately 100 days. Furthermore, a comparison with other sources of variability suggests that internal variability may play an important role in modifying or setting up El Niño.

Predicting the response of Gulf of Mexico hypoxia to variations in Mississippi River nitrogen load

The effects of nutrient loading from the Mississippi River basin on the areal extent of hypoxia in the northern Gulf of Mexico were examined using a novel application of a dissolved oxygen model for a river. The model,driven by river nitrogen load and a simple parameterization of ocean dynamics, reproduced 17 yr of observed hypoxia location and extent, subpycnocline oxygen consumption, and cross‐pycnocline oxygen flux. With Monte Carlo analysis, we illustrate through hindcasts back to 1968 that extensive regions of low oxygen were not common before the mid‐1970s. The Mississippi River Watershed/Gulf of Mexico Hypoxia Task Force set a goal to reduce the 5‐yr running average size of the Gulf’s hypoxic zone to less than 5,000 km2 by 2015 and suggested that a 30% reduction from the 1980–1996 average nitrogen load is needed to reach that goal. Here we show that 30% might not be sufficient to reach that goal when year‐to‐year variability in ocean dynamics is considered.

Interannual variability of the oceanic sink of CO2 from 1979 through 1997

We have estimated the interannual variability in the oceanic sink of CO2with a three‐dimensional global‐scale model which includes ocean circulation and simple biogeochemistry. The model was forced from 1979 to 1997 by a combination of daily to weekly data from the European Centre for Medium‐Range Weather Forecast and the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis as well as European Remote Sensing satellite observations. For this period, the ocean sink of CO2 is estimated to vary between 1.4 and 2.2 Pg C yr−1, as a result of annually averaged interannual variability of ±0.4 Pg C yr−1 that fluctuates about a mean of 1.8 Pg C yr−1. Our interannual variability roughly agrees in amplitude with previous ocean‐based estimates but is 2 to 4 times less than estimates based on atmospheric observations. About 70% of the global variance in our modeled flux of CO2 originated in the equatorial Pacific. In that region, our modeled variability in the flux of CO2 generally agreed with that observed to ±0.1 Pg C yr−1. The predominance of the equatorial Pacific for interannual variability is caused by three factors: (1) interannual variability associated with El Niño events occurs in phase over the entire basin, whereas elsewhere positive and negative anomalies partly cancel each other out (e.g., for events such as Antarctic Circumpolar Wave and the North Atlantic Oscillation); (2) dynamic processes dominate in the equatorial Pacific, whereas dynamic, thermodynamic, and biological processes partly cancel one another at higher latitudes; and (3) our model underestimates the variability in ocean dynamics and biology at high latitudes.