North Atlantic Wind Research Articles

Learning the spatiotemporal relationship between wind and significant wave height using deep learning

Abstract Ocean wave climate has a significant impact on near-shore and off-shore human activities, and its characterization can help in the design of ocean structures such as wave energy converters and sea dikes. Therefore, engineers need long time series of ocean wave parameters. Numerical models are a valuable source of ocean wave data; however, they are computationally expensive. Consequently, statistical and data-driven approaches have gained increasing interest in recent decades. This work investigates the spatiotemporal relationship between North Atlantic wind and significant wave height ( $ {H}_s $ ) at an off-shore location in the Bay of Biscay, using a two-stage deep learning model. The first step uses convolutional neural networks to extract the spatial features that contribute to $ {H}_s $ . Then, long short-term memory is used to learn the long-term temporal dependencies between wind and waves.

The influence of the Arctic Oscillation on North Atlantic wind and wave climate by the end of the 21st century

This work analyses the change in the Arctic Oscillation (AO) index by the end of the 21st century and its relationship with present and future met ocean conditions on the North Atlantic Ocean. The seasonal distribution of the AO index changes is analysed and an annual regression line is obtained. Pearson correlation is computed between AO index and wind and wave parameters. It is observed that for future climate, negative AO events become weaker but more frequent and positive index events remain stable in intensity, but their frequency is reduced. At the end of the 21st century, the correlation between significant wave height, peak period and zonal wind remains the same as in the present climate: positive (negative) correlation north (south) of 50°N. Results show that future climate correlation on Wave Watch III forced by EC-Earth (EC-Earth/WW3) projections using the high-emissions scenario (RCP 8.5) and AO index will remain similar to the one observed in the present climate.

Recent Decadal Trend in the North Atlantic Wind Energy Resources

This study presents the climatic trend of the North Atlantic wind energy using cross-calibrated, multiplatform (CCMP) wind data for the period 1988–2011. Results show the following. (1) The North Atlantic WPD exhibited a significant increasing trend of 4.45 (W/m2)/yr over the past 24 years. (2) The variation in the North Atlantic Ocean WPD shows a noticeable regional difference. More than half of the North Atlantic Ocean has a significantly increasing trend in WPD. The increasing trend in the mid-high latitudes is stronger than that in the low latitudes, and the trend is stronger in the west than in the east. The area with the strongest increasing trend is located along the southern coast of Greenland of 35 (W/m2)/yr. (3) There is a noticeable seasonal difference in the variation of WPD. The strongest increasing trend occurs in December-January-February (DJF), followed by September-October-November (SON) and March-April-May (MAM), and the weakest occurs in June-July-August (JJA). The increasing trend in different areas is dominated by different seasons. (4) There is no leading or lagging correlation between WPD and the North Atlantic Oscillation (NAO). However, there is a noticeable negative correlation between the Niño3 index and WPD in most of the North Atlantic.

Interdecadal Trichodesmium variability in cold North Atlantic waters

AbstractStudies of the nitrogen cycle in the ocean generally assume that the distribution of the marine diazotroph, Trichodesmium, is restricted to warm, tropical, and subtropical oligotrophic waters. Here we show evidence that Trichodesmium are widely distributed in the North Atlantic. We report an approximately fivefold increase during the 1980s and 1990s in Trichodesmium presence near the British Isles with respect to the average over the last 50 years. A potential explanation is an increase in the Saharan dust source starting in the 1980s, coupled with changes in North Atlantic winds that opened a pathway for dust transport. Results from a coarse‐resolution model in which winds vary but iron deposition is climatologically fixed suggest frequent nitrogen limitation in the region and reversals of the Portugal current, but it does not simulate the observed changes in Trichodesmium. Our results suggest that Trichodesmium may be capable of growth at temperatures below 20°C and challenge assumptions about their latitudinal distribution. Therefore, we need to reevaluate assumptions about the temperature limitations of Trichodesmium and the dinitrogen (N2) fixation capabilities of extratropical strains, which may have important implications for the global nitrogen budget.

Do the North Atlantic winds drive the nonseasonal variability of the Arctic Ocean sea level?

The Gravity Recovery and Climate Experiment (GRACE) satellites have observed coherent and nearly uniform nonseasonal fluctuations of bottom pressure throughout the Arctic Ocean and the Nordic Seas. Strong correlation between the nonseasonal GRACE and satellite altimetry data is found in the Nordic and Barents Seas, which suggests a possibility of using the longer altimetry records in these areas as a proxy for the nonseasonal sea level variability over the entire Arctic. This study identifies the dominant pattern of the nonseasonal atmospheric pressure variability that drives strong zonal wind anomalies over the northeastern North Atlantic associated with the nonseasonal sea level anomalies in the Nordic Seas. Our results show that wind-driven northward Ekman transport anomalies in the northeastern North Atlantic may induce coherent changes of sea level across the entire Arctic Ocean.

Stochastic Forcing of Ocean Variability by the North Atlantic Oscillation

Abstract At middle and high latitudes, the magnitude of stochastic wind stress forcing of the ocean by atmospheric variability on synoptic time scales (i.e., “weather” related variability) is comparable to that of the seasonal cycle. Stochastic forcing may therefore have a significant influence on the ocean circulation, climate, and ocean predictability. Here, the influence of stochastic forcing associated with the North Atlantic Oscillation on the subtropical gyre circulation of the North Atlantic is explored in an eddy-permitting quasigeostrophic framework. For the North Atlantic winds used in this study, the root-mean-square of the annual average Ekman pumping velocity of the seasonal cycle between 35° and 52°N is 1.3 × 10−7 m s−1, while the wintertime standard deviation of the stochastic component of the North Atlantic Oscillation over the same latitude band is 2.2 × 10−7 m s−1. Significant stochastically induced variability in the ocean circulation occurs near the western boundary region and along the western margins of the abyssal plains associated with vortex stretching, energy release from the mean flow, and the generation of topographic Rossby waves. Variability arises from a combination of two effects, depending on the measure of variance used: growth of unstable modes of the underlying circulation and modal interference resulting from their nonnormal nature, which dominates during the first 10 days or so of perturbation growth. Near the surface, most of the variability is associated with large-scale changes in the barotropic circulation, although more than 20% of the energy and enstrophy variability is associated with small-scale baroclinic waves. In the deep ocean, much of the stochastically induced variability is apparently due to topographic Rossby wave activity along the continental rise and ocean ridges. Previous studies have demonstrated that rectification of topographic Rossby wave–induced circulations in the western North Atlantic may contribute to the western boundary current recirculation zones. The authors suggest that a source of topographic Rossby wave energy, significant enough to rectify the mean ocean circulation, may arise from stochastic forcing by large-scale atmospheric forcing, such as the North Atlantic Oscillation and other atmospheric teleconnection patterns.

The Impact of North Atlantic Wind and Cyclone Trends on European Precipitation and Significant Wave Height in the Atlantic

An analysis of the frequency of cyclones and surface wind velocity for the Euro-Atlantic sector is performed by means of an objective methodology. Monthly and seasonal trends of cyclones and wind speed magnitude are computed and trends between 1960 and 2000 evaluated. Results reveal a significant frequency decrease (increase) in the western Mediterranean (Greenland and Scandinavia), particularly in December, February, and March. Seasonal and monthly analysis of wind magnitude trends shows similar spatial patterns. We show that these changes in the frequency of low-pressure centers and the associated wind patterns are partially responsible for trends in the significant height of waves. Throughout the extended winter months (October-March), regions with positive (negative) wind magnitude trends, of up to 5 cm/s/year, often correspond to regions of positive (negative) significant wave height trends. The cyclone and wind speed trends computed for January-March are well matched by the corresponding trends in significant wave height, with February being the month with the highest trends (negative south of lat 50 degrees N up to -3 cm/year, and positive up to 5 cm/year just north of Scotland). Trends in European precipitation are assessed using the Climatic Research Unit data set. The results of the assessment emphasize the link with the corresponding tendencies of cyclone frequencies. Finally, it is shown that these changes are associated, to a large extent, with the preferred phases of major large-scale atmospheric circulation modes, particularly with the North Atlantic Oscillation, the eastern Atlantic pattern, and the Scandinavian pattern.

Relation between the wind stress curl in the North Atlantic and the Atlantic inflow to the Nordic Seas

In this study an isopycnic coordinate ocean model has been used to investigate the relationships between the North Atlantic wind stress curl (WSC) and the inflow of Atlantic water to the Nordic Seas. For the period 1995–2001, there is a maximum in the correlation between the zonally averaged WSC at 55°N and the inflow with a 15‐month time lag, capturing a relation already found in observational data. In the model this relation is linked to the mixing along the western flank of the Rockall Bank (56°N, 15°W). For the period 1995–2001 the atmospheric forcing in the northeastern North Atlantic is relatively weak, and the depth of the mixed layer is shallower than the sill depths of the Greenland‐Scotland Ridge (GSR). Slowly moving, baroclinic disturbances caused by anomalies in the wind forcing will then be transmitted into the Nordic Seas where they are recorded as anomalous volume transports in the Norwegian Atlantic Current. In contrast, for the pentad prior to this period the atmospheric forcing is much more intense, and generates mixing well below sill depths of the GSR for all winters. Baroclinic disturbances forced by variations in the atmospheric forcing will then tend to follow f/H contours that do not enter the Nordic Seas, and the 15‐month lagged relations between the wind and the volume transports will vanish. Recent observational data support this view.

The impact of the wind stress curl in the North Atlantic on the Atlantic inflow to the Norwegian Sea toward the Arctic

The Norwegian Atlantic Current (NwAC) through the Norwegian Sea serves as a conduit of warm and saline Atlantic water from the North Atlantic to the Arctic Ocean, an important factor for climate and ecology. In this study, we concentrate on how the North Atlantic wind stress curl (NAWSC) affects interannual variability on the major branch of the NwAC—the Norwegian Atlantic Slope Current (NwASC). Based on wind stress data from the NCEP reanalysis and estimated volume transport of the NwASC from current records during 1995–2003 in the Svinøy section (62°N), our analysis shows that the volume transport in the NwASC exhibits a maximum correlation of 0.88 with the zonally integrated NAWSC at 55°N 15 months earlier. Our findings reveal the NAWSC to be a major forcing for interannual variability of the NwASC in the range of 3.0‐ to 5.3 Sv (Sv = 106 m3 s−1). The 15 month time lag appears to be in accordance with a forced baroclinic Rossby wave in response to the local Ekman pumping changing the baroclinicity and strength of the North Atlantic Current (NAC). After being converted to a nearly barotropic shelf edge current along the Irish‐Scottish shelf through interaction with bottom topography, it appears as a barotropic response downstream in the Svinøy section. This result suggests the possibility of predicting conditions influenced by the NwASC more than a year in advance, using the NAWSC as a proxy.

Air‐sea interaction and circulation changes in the northeast Atlantic

The upper ocean of the Rockall Trough exhibits coherent interannual variations in temperature and salinity over the past 26 years, with highs in the mid‐1980s and late 1990s and lows in the late 1970s and early 1990s, and with ranges of ±0.5°C and ±0.05 in salinity. The origins of the interannual changes are discussed, covering three potential influencing factors: the propagation of anomalies developed upstream of the basin, the effect of local air‐sea interaction, and the result of changes of regional circulation bringing different water masses into the region. The changes in heat and freshwater content of the upper ocean are directly compared to observed variations in air‐sea heat and freshwater fluxes over the period of the time series. It is shown that the role of the atmosphere in locally altering the oceanic properties, particularly salinity, is relatively small and insufficient to explain the changes. Two recent hydrographic surveys are analyzed to ascertain how the distribution of water masses to the south of the basin may influence the properties of the northern Rockall Trough upper ocean, and the results are reviewed in the context of historical analyses. It is found that the critical factor in determining the properties is the varying amount of relatively cool and fresh North Atlantic Current water mixing with the dominant water mass, the warm saline Eastern North Atlantic Central Water at the entrance to the basin. Variations of inflowing water masses are caused by east‐west changes in the location of the subpolar front, and the relationship of these changes in regional circulation to the North Atlantic wind stress field is discussed.

North Atlantic Wind Waves and Wind Stress Fields from Voluntary Observing Ship Data

On the basis of the collection of individual marine observations available from the Comprehensive Ocean–Atmosphere Data Set, major parameters of the sea state were evaluated. Climatological fields of wind waves and swell height and period, as well as significant wave height and resultant period are obtained for the North Atlantic Ocean for the period from 1964 to 1993. Validation of the results against instrumental records from National Data Buoy Center buoys and ocean weather station measurements indicate relatively good agreement for wave height and systematic biases in the visually estimated periods that were corrected. Wave age, which is important for wind stress estimates, was evaluated form wave and wind observations. The climatology of wave age indicates younger waves in winter in the North Atlantic midlatitudes and Tropics. Wave age estimates were applied to the calculations of the wind stress using parameterizations from field experiments. Differences between wave-age-based and traditional estimates are not negligible in wintertime in midlatitudes and Tropics where wave-induced stress contributes from 5% to 15% to the total stress estimates. Importance of the obtained effects for ocean circulation and climate variability is discussed.

On the influence of local and north Atlantic wind forcing on the seasonal variation of sea level on the Newfoundland and Labrador Shelf

Sea level data from St. John's, Newfoundland, and from Nain, Labrador, are compared with results from a 1/4°×1/4° resolution numerical model of the Newfoundland and Labrador shelf and the neighboring Labrador Sea. The model is barotropic, employs dynamics linearized about a state of rest, and uses linear bottom friction. The model is driven by the seasonal part of the Hellerman and Rosenstein wind stress field and by inflow from the rest of the North Atlantic specified along the eastern boundary. The latter is taken from a 1°×1° version of the model applied to the North Atlantic between 10°S and 80°N. The results indicate that our simple model can account for that part of the annual cycle of sea level on the shelf directly attributable to wind forcing. In particular, the model can account for the difference between observed monthly mean sea level at St. John's, corrected for atmospheric pressure variations, and the seasonal steric height anomalies above 150 dbar on the neighboring shelf. We also compare our model results with the linear regression analysis carried out by Thompson et al. on monthly mean sea level data from Nain for the period 1964 to 1972. The model confirms the gains calculated for both the local wind driven signal and also that part of the signal due to wind forcing over the North Atlantic beyond the Labrador Sea. Comparing St. John's with Nain, we find that at St. John's, the model response is accounted for primarily by wind set‐up on the shelf, whereas at Nain, the influence of North Atlantic wind forcing is a larger part of the total signal. These results suggest that data from the tide gauge at Nain could be useful for verifying model‐predicted changes in the large‐scale ocean circulation. The results also seem to be confirming the presence of the offshore transport variations predicted by the model.

Scales of North Atlantic wind stress curl determined from the comprehensive ocean‐atmosphere data set

Nineteen years of wind data over the North Atlantic are used to calculate a field of wind stress curl. An empirical orthogonal function (EOF) analysis is performed on this field, resulting in spatial patterns of wind stress curl and associated time series. A Monte Carlo technique is used to establish the statistical significance of each spatial pattern. The first four statistically significant EOF modes represent more than 50% of the curl variance. The spatial patterns of curl associated with these modes exhibit the major elements of North Atlantic climatology. The associated time series are spectrally analyzed. Most of the variance is contained in annual and semiannual frequencies. Features observed include the individual annual variation of the subtropical high and the subpolar low, the annual oscillation of intensity between the above pressure centers, the influence of localized strong sea surface temperature gradients and associated cyclogenesis regions, and the constant nature of the trades. The EOF curl patterns are in the form of basin‐sized standing waves.

Mapping of North Atlantic winds by HF radar sea backscatter interpretation

This work presents preliminary results on attempts to map winds of a storm at long range (500 to 1000 nmi) over a large area ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10^{5}</tex> mi <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) in the North Atlantic from the U. S. Naval Research Laboratory, HF radar research facilities at Chesapeake Beach, Md. [1] It appears that the short time response of the sea surface to local winds can be mapped by the analysis of a matrix of range-azimuth records containing frequency power spectra of HF radar signals backscattered from the sea surface via the ionosphere. This paper presents such a map based upon the ratio of the strengths of the first-order contributions to the backscatter spectrum (the approach-recede first-order Bragg lines) and is compared qualitatively with a U. S. Weather Bureau map of the area.