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[1] The warm and saline inflow of the North Atlantic Current to the Nordic Seas is highly relevant for the region and the global climate. North of the Greenland-Scotland Ridge, the Norwegian Atlantic Current consists of two 40–60 km wide branches, situated at the slope and 150–200 km offshore, respectively. To interpret changes in these branches in terms of climate variability in the northern North Atlantic, it is important to understand their spatiotemporal response to both atmospheric forcing and advection. We analyzed three decades of synoptic hydrographic observations of the branches' variability, with particular focus on the response to the North Atlantic Oscillation (NAO) and related wind stress curl changes in the Nordic Seas. To do so, we separated the effect of fluctuating position and thickness of the branches from the variability in temperature and salinity in the spatially fluctuating cores. As a rapid response to the NAO we find a deflection of both branches toward the coast which is consistent with an enhanced basin-wide cyclonic circulation. While the immediate correlation between hydrographic properties and the NAO is weak, we find a significant negative correlation when the NAO leads temperature and salinity by 4–6 years. We attribute the overall delayed response to the advection of anomalies generated in the northwestern North Atlantic through NAO induced air-sea interaction and changes in the position of the subpolar front.

[1] The sensitivity of the North Atlantic Ocean Circulation to an abrupt change in the Nordic Sea overflow is investigated for the first time using a high resolution eddy-permitting global coupled ocean-atmosphere model (GFDL CM2.5). The Nordic Sea overflow is perturbed through the change of the bathymetry in GFDL CM2.5. We analyze the Atlantic Meridional Overturning Circulation (AMOC) adjustment process and the downstream oceanic response to the perturbation. The results suggest that north of 34°N, AMOC changes induced by changes in the Nordic Sea overflow propagate on the slow tracer advection timescale, instead of the fast Kelvin wave timescale, resulting in a time lead of several years between subpolar and subtropical AMOC changes. The results also show that a stronger and deeper-penetrating Nordic Sea overflow leads to stronger and deeper AMOC, stronger northward ocean heat transport, reduced Labrador Sea deep convection, stronger cyclonic Northern Recirculation Gyre (NRG), westward shift of the North Atlantic Current (NAC) and southward shift of the Gulf Stream, warmer sea surface temperature (SST) east of Newfoundland and colder SST south of the Grand Banks, stronger and deeper NAC and Gulf Stream, and stronger oceanic eddy activities along the NAC and the Gulf Stream paths. A stronger/weaker Nordic Sea overflow also leads to a contracted/expanded subpolar gyre (SPG). This sensitivity study points to the important role of the Nordic Sea overflow in the large scale North Atlantic ocean circulation, and it is crucial for climate models to have a correct representation of the Nordic Sea overflow.

[1] Intrusions of Gulf Stream waters onto the South Atlantic Bight continental shelf are analyzed using historical observations spanning several decades. Intrusions of dense water near the bottom occur predominantly during summer, when they are observed 30% of the time at the middle and outer shelf, but less frequently (

[1] Recent work has shown that variability in the properties and/or transport of Mediterranean Seawaters spilling across the Strait of Gibraltar into the North Atlantic have had little impact on the variability of Mediterranean Outflow Water (MOW) in the that basin over the past fifty years. Here we investigate whether circulation changes are the dominant source of MOW variability in the North Atlantic between 1948 and 2006. Using a 1/3° North Atlantic configuration of the HYbrid Coordinate Ocean Model combined with the Marginal Sea Boundary Condition model, two simulations forced by either climatological or interannual atmospheric fields are performed. The interannual simulation reproduces the observed MOW variability without Mediterranean Seawater changes. Thus, we conclude that MOW variability in the last 60 years is a consequence of circulation changes in the North Atlantic. A series of simulations that separate the mechanical effect of the wind from the impact of buoyancy forcing show that MOW variability can be attributed to shifts between its dominant northward and westward pathways. The pathway shifts from predominantly northward between 1950 and 1975 to predominantly westward between 1975 and 1995 and finally back to northward after 1995. Though these pathway shifts appear to be wind-induced, the property changes are caused by the combined impact of wind and buoyancy forcing on the circulation of the North Atlantic.

[1] The Lagrangian separation distance between the endpoints of simulated and observed drifter trajectories is often used to assess the performance of numerical particle trajectory models. However, the separation distance fails to indicate relative model performance in weak and strong current regions, such as a continental shelf and its adjacent deep ocean. A new skill score is proposed based on the cumulative Lagrangian separation distances normalized by the associated cumulative trajectory lengths. This skill score is used to evaluate surface trajectories implied by Global HYCOM hindcast surface currents as gauged against actual satellite-tracked drifter trajectories in the eastern Gulf of Mexico during the 2010 Deepwater Horizon oil spill. It is found that the new skill score correctly indicates the relative performance of the Global HYCOM in modeling the strong currents of the Gulf of Mexico Loop Current and the Gulf Stream and the weaker currents of the West Florida Shelf. In contrast, the Lagrangian separation distance alone gives a misleading result. The proposed dimensionless skill score is particularly useful when the number of drifter trajectories is limited and neither a conventional Eulerian-based velocity nor a Lagrangian-based probability density function may be estimated.

[1] The gridded mean sea level pressure (MSLP) series developed by the European and North Atlantic Daily to Multidecadal Climate Variability (EMULATE) project is used in this paper to analyze changes in storm activity in the northeast Atlantic region over the period 1851–2003. Zonal and meridional geostrophic wind speed components were calculated for each grid square over the domain 65°W–45°E, 30°N–65°N, and seasonal percentiles from these data were used as a measure of storm activity in the region. Despite the relatively coarse temporal (daily) and spatial (5° × 5°) resolution of the data, the results indicate that the series are able to provide useful information about storm activity across the northeast Atlantic domain back to 1881 and in the data-rich area of the North Sea back to 1851. The variability of winter storm activity observed across the North Sea over the last 153 years, and particularly the increase to high values in the 1990s, is closely associated with the variability of the North Atlantic storm track. In contrast, high summer values in the late nineteenth century appear to have been related to a Greenland-blocking system, leading to high meridional flow in the region. This latter conclusion is limited, however, by poor data coverage in the northwest Atlantic region.

Ocean state estimates from 1958 to 2005 from the Simple Ocean Assimilation System (SODA) system are analyzed to understand circulation between subtropical and subpolar Atlantic and their connection with atmospheric forcing. This analysis shows three periods (1960s, around 1980, and 2000s) with enhanced warm, saline waters reaching high latitudes, alternating with freshwater events originating at high latitudes. It complements surface drifter and altimetry data showing the subtropical -subpolar exchange leading to a significant temperature and salinity increase in the northeast Atlantic after 2001. The warm water limb of the Atlantic meridional overturning cell represented by SODA expanded in density/salinity space during these warm events. Tracer simulations using SODA velocities also show decadal variation of the Gulf Stream waters reaching the subpolar gyre and Nordic seas. The negative phase of the North Atlantic Oscillation index, usually invoked in such variability, fails to predict the warming and salinization in the early 2000s, with salinities not seen since the 1960s. Wind stress curl variability provided a linkage to this subtropical/subpolar gyre exchange as illustrated using an idealized two ]layer circulation model. The ocean response to the modulation of the climatological wind stress curl pattern was found to be such that the northward penetration of subtropical tracers is enhanced when amplitude of the wind stress curl is weaker than normal. In this case both the subtropical and subpolar gyres weaken and the subpolar density surfaces relax; hence, the polar front moves westward, opening an enhanced northward access of the subtropical waters in the eastern boundary current.

[1] Over the past several decades, the minimum Northern Hemisphere summer sea ice extent has decreased substantially. We present an analysis of the influence of declining Arctic sea ice cover on the atmosphere, specifically during the autumn/early winter following an extreme summer minimum event. Using ensemble simulations from the Weather Research and Forecast model (v 3.0.1), we compare the atmospheric response for the case of the extreme sea ice minimum of 2007 to the corresponding response for the more typical ice conditions of 1984, the year with median ice extent for the 1979–2008 satellite era. Increased open water enhances heat and moisture flux from the Arctic Ocean to the atmosphere during autumn. We characterize the fluxes both horizontally and vertically and determine the spatial breadth of their influence. The atmospheric response is characterized by a strong increase in 2 m temperature and decrease in sea level pressure locally and by remote responses in the atmospheric circulation throughout the troposphere characterized by a quasi-barotropic ridge/trough signal in North America. The circulation anomalies drive remote anomalies of temperature and precipitation over eastern North America and the North Atlantic. Advectively driven temperature anomalies, in turn, cause surface flux anomalies over remote regions such as the Great Lakes and the Gulf Stream. The maximum response, as measured by difference in 2 m temperatures over the polar cap from 70°N, occurs between 10 September and approximately 15 November. The persistence of the signal over a 2 month period implies the potential for seasonal predictability of the stronger atmospheric response features. In addition, we determine the significance of prominent features, finding 95% significance in some remote features as far away as the North Atlantic.

[1] Predictability of sea surface temperature (SST) variations over the North Atlantic Ocean is studied on the basis of the first-mode baroclinic Rossby wave model. This simple linear wave model can explain about 10%∼46% of the wintertime SST variability in several regions of the North Atlantic basin, including the tropics between 5°N and 20°N, the eastern subtropics between 27°N and 34°N, and the east of Newfoundland. It is shown that the wintertime SST anomalies can be skillfully predicted up to 3 months ahead in the tropical Atlantic, 18 months in the eastern subtropical Atlantic, and 3 years east of Newfoundland on the basis of the first-mode baroclinic Rossby wave adjustment.

[1] High-resolution hydrographic measurements collected along the southern edge of Georges Bank during March and June–July 1997 focused on characterizing processes that drive fluxes of material between the slope and bank. Wintertime sampling characterized changes driven by a strong storm. A Scotian Shelf crossover event produced a ribbon of anomalously fresh water along the bank's southern flank that was diluted during the storm. Comparison of prestorm and poststorm sections shows that over the bank changes in heat and salt inventories are consistent with those expected solely from local surface fluxes. In deeper waters, advective effects, likely associated with frontal motion and eddies, are clearly important. Summertime surveys resolve the development of a massive intrusion of Gulf Stream-like waters onto the bank. East of the intrusion, a thin extrusion of bank water is drawn outward by the developing ring, exporting fresher water at a rate of about 7 × 104 m3/s. A large-amplitude Gulf Stream meander appears to initiate the extrusion, but it quickly evolves, near the bank edge, into a warm core ring. Ring water intrudes to approximately the 80 m isobath, 40 km inshore from the bank edge. The intrusion process seems analogous to the development of Gulf Stream shingles (a hydrodynamic instability) in the South Atlantic Bight. It appears that, once the intruded water is established on the bank, it remains there and dissipates in place. Although the intrusion is an extremely dramatic event, it is probably not actually a major contributor to shelf edge exchanges over a seasonal time scale.

[1] Geostrophic surface velocity anomalies are used to analyze the annual variations of the large-scale geostrophic currents and of the eddy kinetic energy (EKE) field of the ocean circulation. The underlying geostrophic currents were estimated from the Jason-1-TOPEX/Poseidon (JTP) tandem altimetric sea surface height data using the “parallel track approach” with a 10 km along-track resolution; however, because of the given separation of the tracks of the two satellites only large mesoscale eddies are resolved by the tandem measurements. The analysis covers the entire 3 year period of the tandem mission (109 repeat cycles) from September 2002 to September 2005. The analysis of the seasonal flow changes reveals annual changes of all major current systems, but especially of the zonal flow field in low latitudes, leading to zonal jets on the annual cycle in the southern Pacific, Atlantic, and Indian oceans. In middle and high latitudes, indications of a seasonally modulated strength of the Sverdrup circulation emerge from the analysis. The EKE field also shows changes in its amplitude on the annual period. In low latitudes, those can be rationalized as resulting from seasonally modulated currents. In middle and high latitudes, changes in the wind-driven barotropic circulation loom large, which are not represented in other altimetric velocity products. Results shown suggest that velocity time series of the JTP tandem mission should be continued through similar constellations, e.g., of Jason-1 and Jason-2.

[1] This study examines climate simulations with the National Center for Atmospheric Research Community Atmosphere Model version 3 (NCAR CAM3) using a new air-sea turbulent flux parameterization scheme. The current air-sea turbulent flux scheme in CAM3 consists of three basic bulk flux equations that are solved simultaneously by an iterative computational technique. We recently developed a new turbulent flux parameterization scheme where the Obukhov stability length is parameterized directly by using a bulk Richardson number, an aerodynamic roughness length, and a heat roughness length. Its advantages are that it (1) avoids the iterative process and thus increases the computational efficiency, (2) takes account of the difference between z0m and z0h and allows large z0m/z0h, and (3) preserves the accuracy of iteration. An offline test using Tropical Ocean–Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) data shows that the original scheme overestimates the surface fluxes under very weak winds but the new scheme gives better results. Under identical initial and boundary conditions, the original CAM3 and CAM3 coupled with the new turbulent flux scheme are used to simulate the global distribution of air-sea surface turbulent fluxes, and precipitation. Comparisons of model outputs against the European Remote Sensing Satellites (ERS), the Objectively Analyzed air-sea Fluxes (OAFlux), and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) show that: (1) the new scheme produces more realistic surface wind stress in the North Pacific and North Atlantic trade wind belts and wintertime extratropical storm track regions; (2) the latent heat flux in the Northern Hemisphere trade wind zones shows modest improvement in the new scheme, and the latent heat flux bias in the western boundary current region of the Gulf Stream is reduced; and (3) the simulated precipitation in the new scheme is closer to observation in the Asian monsoon region.

Using a robust diagnostic model, the steady circulation in the world ocean is determined from climatological annual mean fields of hydrographic data and wind stress. The upper circulation reflects the surface geopotential anomaly of the input hydrographic data. In addition, the “warm water route” is prominent: the water passing through the Indonesian seas flows westward across the Indian Ocean to return to the North Atlantic. In the deep Atlantic Ocean, the North Atlantic Deep Water is transported southward by the dominant deep western boundary current in the depth range from about 1500 m to the bottom, whereas the Weddell Sea Deep Water does not extend northward from the Argentine Basin. An important result is a westward current appearing near 25°N in the middepth Atlantic Ocean. Such a current has been inferred from tracer distributions but hardly detected with objective methods. The velocity maximum at middepth suggests that the current is not a lower part of a wind gyre but is a baroclinic flow caused by a thermohaline effect. In the Indian Ocean a swift middepth current appears along the western boundary, but it may be questionable because the resulting heat transport does not agree with earlier estimates. The deep water flowing into the southern basins from the Antarctic Circumpolar Current does not flow further northward even in the Madagascar Basin. The cause of these unsuccessful results is probably that the present model does not take into account large seasonal variations in the Indian Ocean.

[1] Satellite altimeter data are used to study the characteristics of the horizontal propagation of eddy variability of the global oceans. Decade-long time series of sea surface height is analyzed for finding the maximum cross correlation with neighboring time series within a window of space and time lags. The space and time lags corresponding to the maximum correlation allow an estimate of the propagation velocity of the eddy variability that dominates the variance of sea surface height anomalies. The method cannot distinguish the various forms of eddy variability: isolated eddies and fronts, the meandering of ocean currents, or planetary waves. However, the results provide, at a given location of the global oceans, a uniquely determined propagation velocity that represents a time-averaged description of the tendency of the movement of the local eddy variability. The propagation velocity is highly inhomogeneous in space. Outside the equatorial zone, the zonal propagation is intrinsically westward, modified by ocean currents, which could reverse the zonal propagation to eastward in regions like the Gulf Stream and the Antarctic Circumpolar Current. At midlatitudes and high latitudes, the propagation pattern is highly affected by the path of ocean currents and the shape of bottom topography. At tropical latitudes, the meridional propagation is convergent toward the equator in the western basins and divergent away from the equator in the eastern basins. Comparison with the simulations of an eddy-permitting ocean general circulation model shows overall agreement, especially in the latitudinal variation of the zonal propagation velocity. The result suggests that the model has captured the essence of the dynamics governing the propagation of ocean eddy variability.

[1] The trend and variability of global ocean heat transport for the period 1958-2004 are investigated using the Simple Ocean Data Assimilation (SODA) analysis. The ocean model is forced with the European Center for Medium Range Weather Forecast (ECMWF) ERA-40 atmospheric reanalysis winds from 1958 to 2001 and with QuikSCAT winds from 2002 to 2004. The assimilation is based on a sequential estimation algorithm, with observations from the historical archive of hydrographic profiles supplemented by ship intake measurements, moored hydrographic observations and remotely sensed sea surface temperature. Heat transport is calculated using temperature and velocity from the ocean analysis. Mean heat transport from the analysis generally agrees with previously published estimates from observational and modeling studies. Trends of heat transport show a range of behaviors. In the Atlantic and Pacific Oceans there is mostly increasing poleward heat transport with two important exceptions. In the Atlantic Ocean there is decreasing heat transport around 50°N and 60°N, and in both the Atlantic and Pacific Oceans there is decreasing heat transport near 10°S. There is also prominent interannual and decadal variability in all of the ocean basins. The results suggest that ocean heat transport variability is primarily determined by the strength of the meridional overturning circulation (MOC), which is controlled by complex processes governing fresh water flux in the northern North Atlantic and surface wind stress. However, the role of temperature variability increases at high latitude, particularly in the northern North Atlantic Ocean. Eddies play an important role in heat transport in the Gulf Stream and its extension in the Atlantic Ocean, and the Kuroshio and its extension in the Pacific Ocean and enhanced Subtropical cells (STCs) affect heat transport estimates in the tropics. In the northern North Atlantic Ocean, a small increase in meridional heat transport and a slight weakening of MOC are detected. Weakening in the northern North Atlantic MOC mainly arises from a freshening in the Labrador Sea and slowdown of the overflows from the Nordic Seas into the northern North Atlantic Ocean. Trends in North Atlantic surface momentum forcing are uniform across several atmospheric reanalyses, however there is less agreement in the role of precipitation in forcing trends of MOC and this exists as a primary source of uncertainty in our analysis.

The upper branch of the meridional overturning circulation in the North Atlantic is fed by cross‐equatorial transport of various water masses from the Southern Hemisphere. Here, we study the large‐scale spreading of South Atlantic Water (SAW) into the western tropical North Atlantic from the equator to 25°N. The fractions of SAW in the upper ocean water masses are quantified using a water mass analysis applied on a data set of conductivity‐temperature‐depth data from the Hydrobase project and the Argo float program. To fill gaps in the data coverage and to gain insight into the mechanisms involved, the observations are complemented with results from the high‐resolution Family of Linked Atlantic Model Experiments model (equation image°), which has been shown to realistically simulate the inflow of SAW into the Caribbean. The analysis reveals the mean SAW propagation pathways in the North Atlantic and identifies the regions of largest variability. High SAW fractions in the thermocline and central water layers are limited to the region south of 10°N, where the water body consists of 80%–90% SAW. Thus, the zonal currents in the equatorial gyre are mainly formed of SAW. The weaker currents in the intermediate layer combined with a northward excursion of the North Equatorial Current allow the SAW in this layer to intrude farther north compared to the layers above. The transition into North Atlantic Water occurs gradually from 12°N to 20°N in the intermediate layer.

[1] Analysis of surface drifter tracks in the North Atlantic Ocean from the time period 1990 to 2007 provides evidence that warm subtropical waters have recently increased their penetration toward the Nordic seas. Prior to 2000, the warm water branches of the North Atlantic Current fed by the Gulf Stream turned southeastward in the eastern North Atlantic. Since 2001, these paths have shifted toward the Rockall Trough, through which the most saline North Atlantic waters pass to the Nordic seas. These surface drifters are able to overcome the Ekman drift, which would force them southward under the westerly winds dominating the subpolar Atlantic, yet the changes in path cannot be accounted for by changes in Ekman drift. Eddy kinetic energy from satellite altimetry shows increased energy along the shifted drifter pathways across the Mid-Atlantic Ridge since 2001. These near-surface changes have occurred during continual weakening of the North Atlantic subpolar gyre, as seen by altimetry. They are also consistent with the observed increase in temperature and salinity of the waters flowing northward into the Nordic seas. These findings suggest the changes in the vertical structure of the northern North Atlantic Ocean, its dynamics, and exchanges with the higher latitudes. Wind stress and its curl changes are discussed as a possible forcing of the changes in the pathways of the subtropical waters.

[1] Shelfbreak and slope eddies have been implicated as important agents in the exchange of water between the shelf and slope domains of the Middle Atlantic Bight (MAB). Here we present temperature, salinity, and velocity data from a series of shipboard transects that intercepted a rich eddy field over the slope of the southern MAB. Attention is focused on a well-sampled cyclonic eddy, of roughly 60-km diameter and 300-m depth, that translated southward at 0.1 m s−1. The eddy was composed of a mix of water masses including MAB shelf and slope water, Gulf Stream water, and water from the MAB shelfbreak front. Gradient Richardson numbers suggest that these water masses were subject to vigorous turbulent vertical mixing. The transport of shelfbreak frontal water contained within the eddy was substantial. In the upper 100 m, shelfbreak frontal water comprised ∼75% of the eddy's volume. This frontal water fraction moved southward with a transport of ∼0.4 Sv, comparable with the volume transport within the shelfbreak frontal jet. A number of factors indicate that this highly energetic eddy, with maximum azimuthal velocity of 0.7 m s−1, was generated through instability of the shelfbreak frontal jet. The eddy had apparently developed rapidly (in

[1] The currently available theoretical and observational evidence for a latitudinal structure of thermocline vertical diffusivity is synthesized and included in a state of the art coupled climate model. Compared to the standard background value of 0.1 cm2 s−1, the simulations with the latitudinal structure show only little change in the meridional overturning circulation or northward heat transport. However, two regions are identified which are sensitive to the value of vertical diffusivity: the equatorial band, where only small changes in sea surface temperature lead to precipitation responses with basin-wide teleconnections, and the North Atlantic, where diffusivity affects the spiciness of Labrador Sea water and subsequently the Gulf Stream path.

[1] Direct current measurements were conducted from April 2003 to March 2004 to investigate the current variability of the Kuroshio near the separation point from the western boundary. During the study period, the main stream of the Kuroshio flowed northeastward in the upper layer slightly offshore from the array of four mooring systems, while a southwestward current was observed in the intermediate layer on the coastal side of the array. Analyses based on the velocity and temperature measurement of the mooring meter array revealed eddy-to-mean kinetic and mean-to-eddy potential energy conversion in both the upper and intermediate layers. Downstream-propagating waves detected as significant extended empirical orthogonal functions (EEOFs) are predominant over velocity fluctuations of periods shorter than 50 days, explaining 67% of the total variance. The five apparent wave groups have periods of 7–18 days, wavelengths of 220–380 km, and phase velocities of 22–30 cm s−1, respectively, similar to the values reported in previous studies of upstream regions. Although the phase velocity at a given wavelength in the Kuroshio is lower than that in the Gulf Stream, the dispersion tendency (i.e., that phase velocity increases with decreasing period and wavelength) is the same for both currents. The relatively low phase velocity of the Kuroshio is considered to reflect its relatively low background velocity. Data regarding growth rate, vertical phase lags, and energetics suggest that kinetic energy in this region is transferred from small to large scales mainly via eddies resulting from baroclinic instability, which is possibly related to synoptic-scale path variability and the penetration of areas of high kinetic energy into the Kuroshio Extension.

[1] Mixed layer temperature (MLT) and bulk sea surface temperature (SST) are frequently used interchangeably or assumed to be proportional in climate studies. This study examines historical analyses of bulk SST and MLT from contemporaneous ocean profile observations during 1960–2007, looking for systematic differences between these variables. The results show that globally and time-averaged MLT is cooler than SST by approximately 0.1°C. MLT is cooler than SST in upwelling zones where abundant net surface warming is compensated for by cooling across the base of the mixed layer. In the upwelling zone of the equatorial eastern Pacific this negative MLT-SST difference varies out of phase with seasonal SST, reaching a negative extreme of −0.8°C in boreal spring when SST is warm, solar radiation is high, and winds are weak. In contrast, on interannual timescales MLT-SST varies in phase with SST with small differences during El Ninos as a result of low solar heating and enhanced rainfall and with large differences, approaching −0.8°C, during La Ninas. On shorter diurnal timescales, during El Ninos, MLT-SST differences associated with temperature inversions occur in response to nocturnal cooling in the presence of near-surface freshening. Near-surface freshening produces persistent shallow (a few meters depth) warm layers in the northwestern Pacific during boreal summer when solar heating is strong. In contrast, shallow cool layers occur in the Gulf Stream area of the northwest Atlantic in boreal winter when fresh surface layers caused by lateral exchange are cooled by abundant turbulent heat loss. The different impacts of shallow barrier layers on near-surface temperature gradients in these different dynamical regimes are explored with a one-dimensional mixed layer model.

[1] The flow patterns of the Gulf Stream warm core rings and surrounding shelf break and slope water are constructed from shipboard acoustic Doppler current profiler (ADCP) transects of the Oleander Project. For each warm ring, consecutive ADCP transects are colocated to a common ring center and are mapped into a stream function. Methods to locate ring centers from the ADCP transect and advanced very high resolution radiometer image are developed and tested. Two warm rings in 1999, which have relatively complete data coverage, are examined to study the ring-induced warm and cold streamers. For cold streamers, the estimated volume flux, based on more than 10 independent ADCP transects, is at least 1 × 106 m3 s−1. This result agrees well with the previous estimate in the Warm Core Ring Experiment. For warm streamers, the result is new. On the basis of a large number of ADCP transects, the estimated volume flux is about 2.5 × 106 m3 s−1. By pulling large transports associated with the cold/warm streamers, the warm rings likely play a fundamental role in the water exchange in the Slope Sea.

[1] North Atlantic sea surface temperature (SST) distributions derived from observations and a coupled model from NOAA's Geophysical Fluid Dynamics Laboratory, CM2.1, are compared to evaluate the model's ability to simulate recent (1900 to the present) oceanic surface characteristics. The North Atlantic focus will limit our analyses to spatial scales less than gyre, scales usually not addressed in previous model-observation comparisons. Identifying model differences from observations at these scales will assist modelers in identifying problems to be considered and remedies to be applied. The properties compared are the mean annual SST, standard deviation, amplitude of the annual and semiannual harmonic, decadal meridional movements of the axis of the Gulf Stream, propagation of SST anomalies along the axis of the Gulf Stream, and 100-year trends in SST records. Because of the dependence of SST on surface currents, observed flow from surface drifters and simulated flow from 15 m fields are also compared. The model simulates the large-scale properties of all the variables compared. However, there are areas of differences in some variables that can be related to inadequacies in the simulated current fields. For example, the model Gulf Stream (GS) axis after separation from the western boundary is located some 100 km north of the observed axis, which contributes to an area of warmer simulated SSTs. The absence of a slope current in the same region that advects colder water from the Labrador Sea in the observations also contributes to this area of higher model SSTs. The model North Atlantic Current (NAC) is located to the east of the observed NAC contributing to a large area of SST discrepancy. The patterns of the amplitude of the annual harmonic are similar with maximum amplitude off the east coast of northern North America. The semiannual harmonic exhibits relatively large amplitudes (>1°C) north of about 55°N, a signal not found in the observations. In both the model and observations, a region of increased standard deviations encompasses the GS and NAC. The model simulates north-south migrations of the GS core but at a longer period (20 years) than observed. The model does not simulate the SST anomalies that propagate along the observed GS and NAC. The model captures both the spatial and temporal characteristics of the Atlantic Multidecadal Oscillation. Both model and observations exhibit a dipole in trends, with positive trends in the subtropical Atlantic and negative trends in the subpolar gyre. The modeled region of negative trends is limited to the western subpolar Atlantic. The observed trends extend farther to the east.

[1] Severe drought is arguably one of the greatest recurring natural disasters that strikes North America. A synthesis of multiproxy data shows that North America was in the grip of a severe centennial-scale drought during medieval times (800–1300 AD). In this study, the Community Atmospheric Model (CAM) is used to investigate the role of sea surface temperature (SST) anomalies from the North Atlantic and the tropical Pacific Ocean on this megadrought. These anomalies are obtained from proxy reconstructions of SST. Four model experiments with prescribed SST anomalies in the tropical Pacific and/or North Atlantic Ocean were made. The CAM results captured the major dry features that occurred during medieval times in North America. The cold tropical Pacific alone can simulate essentially the drought intensity, while the warm North Atlantic alone can simulate the drought areal extent. The two working together can explain the severity and longevity of the drought. During the spring season, the cool tropical Pacific, or the warm North Atlantic, or both, results in less moisture transport to the High Plains, with a 15–40% decrease in rainfall. The importance of the Atlantic Ocean on medieval drought in North America suggests that attention should be paid not only to the tropical Pacific Ocean but also to the North Atlantic Ocean in understanding the North America drought variability and predictability, both at present and during the past. This is especially true because the Pacific Ocean SST anomalies in medieval times as recorded by proxy data are somewhat controversial, while the North Atlantic anomalies seem more certain.

[1] The container motor vessel CMV Oleander, which operates between New Jersey and Bermuda, crosses the Gulf Stream and Sargasso Sea all year round on a semiweekly schedule. Using an acoustic Doppler current profiler, measurements of upper ocean currents have been made on a regular basis since the fall of 1992. In this paper we examine the database for evidence of axisymmetric coherent vortices including the distribution and intensity of cold core rings. To detect the existence of coherent vortices, the patterns of current vectors averaged between 40 and 80 m depth were fit to an axisymmetric Gaussian vortex model. The parameters of the model were axis location, maximum tangential, or swirl, speed, and radius at which the maximum swirl was measured. We were able to distinguish between the well-known cold core “rings” (CCRs) pinched from the Gulf Stream, and a population of cyclonic and anticyclonic “vortices” in the Sargasso Sea. Both the rings and the Sargasso Sea vortices showed radii of 64 ± 18 km, albeit with different swirl speeds. The rings, close to the Gulf Stream, exhibited a typical maximum swirl speed of 0.98 ± 0.40 m s−1 and a center relative vorticity of 0.64 ± 0.35 × 10−4 s−1, almost 80% of the planetary vorticity for the region. The more uniform population of Sargasso Sea vortices contained approximately equal numbers of cyclones and anticyclones, with mean speeds of +0.43 and −0.55 m s−1, and center relative vorticities of +0.24 × 10−4 s−1 and −0.29 × 10−4 s−1, respectively.

[1] Spatial and temporal variability of the impact of air-sea stratification on the differences between satellite-derived 10 m equivalent neutral wind speeds and stability-dependent (e.g., in situ) 10 m wind speeds are quantitatively examined over the global ocean. The influences of stability are compared with three air-sea flux algorithms, Coupled Ocean-Atmosphere Response Experiment (version 3.0), Bourassa-Vincent-Wood, and Liu-Katsaros-Businger. Analyses are first presented at many individual buoy locations and then are extended to the global ocean with the use of rain-free wind measurements from the SeaWinds scatterometer on the QuikSCAT satellite, gridded at a resolution of 0.25° × 0.25°. Overall, stability-dependent winds are found to be weaker than equivalent neutral winds by 0.2 m s−1 on the basis of 7619 monthly mean values from 208 buoys during 2000–2005. Differences based on hourly winds can be as large as ±0.5 m s−1. Results remain robust regardless of which air-sea flux algorithm is used. Monthly rain-free gridded QuikSCAT measurements, combined with atmospheric stability determined using near-surface variables from the European Centre for Medium-Range Weather Forecasts 40-year reanalysis, demonstrate the effects of stratification on the 10 m winds globally. Differences in stability-dependent and neutral winds are substantially nonsymmetrical and reveal locations where the former is stronger than the latter. These differences may cause physically significant biases in air-sea fluxes if they are not properly considered, especially near the Kuroshio and Gulf Stream current systems.

[1] Hydrography from the Nordic seas from 1951 to 2003 was analyzed focusing on the upper ocean conditions during known periods of deep mixing. The analysis used data from NOAA National Oceanographic Data Center and from International Council for the Exploration of the Sea Oceanographic Database. Because of the sparseness of the wintertime data, only summertime data are analyzed, primarily at 200 m, which retains memory of previous winter’s effects below summer mixed layer. It is found that the salinity variability in the central Greenland Gyre follows closely the sea level pressure (SLP) fluctuations found along the Greenland Coast, e.g., at Angmagssalik. Corresponding large-scale SLP field resembles North Atlantic Oscillation (NAO) in its negative index phase. The dissimilarity between the central gyre salinity fluctuations and the incoming Atlantic water salinity fluctuations suggests that mixing with other water masses (Arctic origin waters and recirculating Atlantic waters) present in the central gyre is important in modifying the incoming Atlantic water salinity at interannual timescales. However, the largest freshwater addition to the Atlantic waters takes place in the Norwegian Sea or even further upstream in the North Atlantic. The variability of this freshwater intake resembles the NAO index on multidecadal timescales.

[1] The deep variability in two areas of the Atlantic Western Boundary is studied. One site is located at 38°N, inshore of the Gulf Stream. The other is at 8°S, off the Brazilian coast. Analyses are centered on current time series estimated by a method that uses expendable bathythermograph derived dynamic heights to remove the near-surface signal from altimetric sea surface height. Both series are approximately 6 years long. Currents are compared to scatterometer derived alongshore wind stress and basin-wide wind stress curl. In both areas, current variability is correlated to basin-averaged wind stress curl and also to alongshore wind stress. The relationship between currents and wind curl is coherent with the western boundary currents response to interior Sverdrup flow. We propose that alongshore wind stress exerts control over the flow by divergence of the Ekman flow at the coast. In the north, the variability is dominated by interannual oscillations of the wind curl. The effects of the alongshore stress are secondary and have annual frequency. In the southern site, the alongshore effect appears to be the dominant forcing. The main observed results are confirmed by data from a numerical model with 1/6° horizontal resolution.

[1] Mesoscale eddies are an important contributor to subduction in the Gulf Stream region and the Antarctic Circumpolar Current, but is eddy subduction also important in the relatively quiescent interior of the world's subtropical gyres? Observations from the Subduction Experiment of the northeast Atlantic do not have the spatial resolution necessary to calculate eddy subduction and answer this question. Regional numerical models can diagnose subduction, but their representativeness is unknown. Furthermore, water mass budgets in an open-ocean domain show that the simulated properties of subducted water directly depend upon uncertain open-boundary conditions and surface fluxes. To remedy these problems, a state estimate of the ocean circulation is formed by constraining an eddy-permitting general circulation model to observations by adjusting the model parameters within their uncertainty. The resulting estimate is self-consistent with the equations of motion and has the necessary resolution for diagnosing subduction. In the northeast Atlantic during 1991–1993, the time-variable circulation contributes less than 1 Sv of net subduction, while the total subduction is 4 Sv. Eddy volume fluxes of 40 m/yr in the North Equatorial Current and the Azores Current, however, are significant and rival the subduction by Ekman pumping locally. Furthermore, a state estimate at 1/6° resolution has 2–3 Sv more subduction in the density bands centered around σ = 24.0 kg/m3 and σ = 26.0 kg/m3 than a 2° state estimate. This result implies that the inability to accurately simulate mesoscale phenomena and surface fluxes in climate models would lead to an accumulation of errors in water mass properties over 10–20 years, even in the interior of the subtropical gyre.

[1] This paper investigates the influence of atmospheric variables (net solar radiation, wind speed, precipitation and vapor mixing ratio, all of which are at or near the sea surface) on the annual and seasonal cycle of near surface air minus sea surface temperature (Tair–Tsst) over the global ocean. The importance order of these variables is discussed using several statistical methods and two global data sets. After demonstrating that neither Tair nor Tsst exhibit any skill in determining difference between the two, a regression tree model (the so-called Generalized, Unbiased, Interaction Detection and Estimation (GUIDE) algorithm) is used to investigate influences of the atmospheric variables mentioned above in regulating Tair–Tsst. Overall, net solar radiation (sum of net shortwave and longwave radiation) at the sea surface is found to be the most important variable in driving the seasonal cycle of Tair–Tsst over the global ocean when the nonlinear relationship between Tair–Tsst and atmospheric variables is taken into account. This is true for both annual and seasonal (May through August) or monthly (November and December) timescales. Similar to the GUIDE results, a simple linear regression analysis also confirms that the net solar radiation explains most of the variance in the seasonal cycle of Tair–Tsst over most (≈50%) of the global ocean. The importance of the net solar radiation in controlling Tair–Tsst is even more significant in the regions surrounding the Kuroshio and the Gulf Stream current systems. The results presented in this paper have various implications for air–sea interaction and ocean mixed layer studies.

The accuracy with which the Argo profiling float dataset can estimate the upper ocean temperature and heat storage in the North Atlantic is investigated. A hydrographic section across 36°N is used to assess uncertainty in Argo-based estimates of the temperature field. The root-mean-square (RMS) difference in the Argo-based temperature field relative to the section measurements is about 0.6°C. The RMS difference is smaller, less than 0.4°C, in the eastern basin and larger, up to 2.0°C, toward the western boundary. In comparison, the difference of the section with respect to the World Ocean Atlas (WOA) is 0.8°C. For the upper 100 m, the improvement with Argo is more dramatic, the RMS difference being 0.56°C, compared to 1.13°C with WOA. The Ocean Circulation and Climate Advanced Model (OCCAM) is used to determine the Argo sampling error in mixed layer heat storage estimates. Using OCCAM subsampled to typical Argo sampling density, it is found that outside of the western boundary, the mixed layer monthly heat storage in the subtropical North Atlantic has a sampling error of 10–20 Wm-2 when averaged over a 10° A~ 10° area. This error reduces to less than 10 Wm-2 when seasonal heat storage is considered. Errors of this magnitude suggest that the Argo dataset is of use for investigating variability in mixed layer heat storage on interannual timescales. However, the expected sampling error increases to more than 50 Wm-2 in the Gulf Stream region and north of 40°N, limiting the use of Argo in these areas.

This study investigates the Atlantic inflow to the North Sea; its variabil- ity, pathways and destination. Results from a numerical model show that the variability of Atlantic inflow is dependent on the inflow location. The inflow between Orkneys and Shetland and in the Shetland shelf area show a strong connection to the strength in westerly winds in winter and spring on a weekly time scale, while the inflow in the Norwegian Trench has a longer response time to the large scale wind pattern. About 50% of the Atlantic water that enters the North Sea is mixed with fresher water before it leaves the North Sea as the Norwegian Coastal Current. This illustrates the important role of estuarine processes within the North Sea and Skagerrak area, and their interaction with the Atlantic water. acceptedVersion

[1] The response of surface winds to ocean fronts characterized by sharp gradients in both sea surface temperature (SST) and ocean currents was analyzed using scatterometer (NSCAT and QuikSCAT) wind data and Gulf Stream path positions in conjunction with simulations made with the Pennsylvania State University (PSU)-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). All match-ups, between each scatterometer pass and the Gulf Stream path, were visually examined and only those for which the wind field was free of atmospheric fronts or large curvature over a reasonably straight segment of the Gulf Stream were selected. Ten match-ups met these criteria for the period studied from 16 September 1996 to 29 June 1997 for NSCAT and from 24 July 1999 to 31 December 2000 for QuikSCAT. Changes in the modeled surface wind field across the front in each of the ten cases agree well with changes in the observed winds. Our findings suggest that the perturbation pressure gradient resulting from the thermal forcing by the front accounts for the decrease in wind speed when moving from warm to cold water and the increase observed in the converse. In the cases examined, the adjustment of the surface wind to the front occurred as a result of the vertical motion induced by horizontal divergence/convergence and advection in the marine atmospheric boundary layer (MABL). The dynamical forcing associated with strong surface currents is also shown to modify scatterometer-derived winds. Finally the numerical simulations suggest that the dynamical and thermal effects are very nearly additive.

[1] Since 1992, upper ocean ADCP current data between New York and Bermuda have been gathered from the container ship Oleander to identify long-term changes in the shelf, slope, Gulf Stream and Sargasso Sea. Temperature and surface salinity data have been been collected along this route since 1978 by NOAA/NMFRC. The first ten years of ADCP data from which the effects of warm ring have been removed are used to describe processes within the shelfbreak frontal sub-region. The Eulerian mean velocity structure shows an along-isobath shelfbreak jet with maximum speeds of O(0.15 m s−1) offshore of which is a ∼30 km wide relatively quiescent region. There is also an offshore slope current 40 to 50 km wide extending vertically to 300 m, with similar velocities as those found in the shelfbreak jet. The mean shelfbreak jet transport is 0.4 Sv while the slope current adds another 2.5 Sv. Maximum shelfbreak transport occurs in the fall and winter while the slope current reaches its maximum during the spring. In stream coordinates, the shelfbreak jet has maximum speeds of 0.35 m s−1, a width of ∼30 km and a vertical decay scale of ∼50 m. The maximum Rossby number within the jet, defined by ∣dU/dy∣max/f, is about 0.2. Significant interannual fluctuations occur in upper ocean temperature, salinity and currents, some of which appear related to changes in the NAO index. Seasonal changes in the slope current appear to be related to seasonal changes in the wind stress curl over the slope sea.

Skewness of sea level variability for the world's oceans is calculated using gridded altimeter data for the period 1993-2001. Many well-known ocean features can be identified in the skewness map, including the Gulf Stream, Kuroshio Extension, Brazil-Malvinas Confluence, and the Agulhas Retroflection. It is shown, through an idealized example and results from a quasi-geostrophic model, that sea level skewness can be used to identify the mean path of unstable ocean jets and also regions dominated by eddies with a preferred sense of rotation. These ideas are confirmed with a more detailed analysis of the skewness fields for the northwest Atlantic and Agulhas Retroflection region. Finally, it is argued that sea level skewness, like variance, is a potentially powerful diagnostic for testing the realism of high-resolution ocean circulation models.

[1] Unusually cold seawater temperatures were observed along much of the U.S. eastern seaboard during the summer of 2003. In this study, hydrographic and atmospheric observations from spring through summer were analyzed to track the evolution of the cold water event in the South Atlantic Bight (SAB) and investigate links to various forcing mechanisms. The hydrographic observations included 13 cross-shelf transects over the central region of the SAB, surface temperature time series from several NDBC stations, and bottom temperatures from a mid shelf mooring. Atmospheric data were obtained from NDBC stations. Additional data included water level from NOS stations and river discharge from USGS stations. The conditions observed during spring and summer of 2003 were compared with climatological values. Record precipitation and increased river discharge during spring produced strong salinity stratification over the inner and mid shelf. Anomalously intense and persistent upwelling-favorable winds were present from May until August. On the mid and outer shelf the resulting upwelling and subsurface shoreward penetration of cold water acted as a feedback mechanism to preserve the stratified conditions through the summer. The characteristics of the upwelled water corresponded to water from the lower part of the Gulf Stream water column. On the shelf the resulting temperature values under the thermocline were significantly lower than climatological temperatures by 5°–7°C.

[1] Modifications to surface winds by currents and sea surface temperature (SST) gradients near frontal boundaries of Gulf Stream rings are analyzed using satellite SST and scatterometer (NASA's Quick Scatterometer (QuikSCAT), NASA scatterometer (NSCAT)) wind observations. A component of scatterometer wind approximately equal and opposite to the surface current vector is observed and attributed to the fact that scatterometers detect relative motion of water and air. Warm-core ring (WCR) SSTs act to destabilize the marine atmospheric boundary layer (MABL), increasing surface wind magnitude by 10–15% and decreasing veering angle by 5–15° relative to large-scale mean winds. Cold-core ring (CCR) SSTs cause impacts of similar magnitude and opposite sense. Magnitudes and directions of modifications are accounted for by MABL dynamics of a nonlinear planetary boundary layer model forced by air-sea temperature differences. Wind modifications occur within tens of kilometers of SST fronts, implying a wind response timescale of order 1 hour. By contrast, uniformity of modified winds across the larger area within rings suggests the response time for the MABL to return to equilibrium downstream from a front exceeds 10 hours. Over WCRs, strong divergence (convergence) occurs on the upwind (downwind) side; curl is strongly negative (positive) to the right (left) side facing downwind. Opposite patterns are generally seen over CCRs. Divergence (curl) peaks where winds blow perpendicular (parallel) to SST fronts. SST image analysis indicates enhanced cloudiness occurs with downwind convergence over WCRs. Wind stress curl due to ring modifications causes dipolar Ekman pumping sufficient to influence ring translation and decay processes.

A global database of approximately 1.7 million observations of the partial pressure of carbon dioxide in surface ocean waters (pCO2) collected between 1970 and 2003 is used to estimate its spatial autocorrelation structure. The patterns of the lag distance where the autocorrelation exceeds 0.8 is similar to patterns in the spatial distribution of the first baroclinic Rossby radius of deformation indicating that ocean circulation processes play a significant role in determining the spatial variability of pCO2. For example, the global maximum of the distance at which autocorrelations exceed 0.8 averages about 140 km in the equatorial Pacific. Also, the lag distance at which the autocorrelation exceed 0.8 is greater in the vicinity of the Gulf Stream than it is near the Kuroshio, approximately 50 km near the Gulf Stream as opposed to 20 km near the Kuroshio. Separate calculations for times when the sun is north and south of the equator revealed no obvious seasonal dependence of the spatial autocorrelation scales. The pCO2 measurements at Ocean Weather Station (OWS) 'P', in the eastern subarctic Pacific (50 N, 145 W) is the only fixed location where an uninterrupted time series of sufficient length exists to calculate a meaningful temporal autocorrelation function for lags greater than a few days. The estimated temporal autocorrelation function at OWS 'P', is highly variable. A spectral analysis of the longest four pCO2 time series indicates a high level of variability occurring over periods from the atmospheric synoptic to the maximum length of the time series, in this case 42 days. It is likely that a relative peak in variability with a period of 3-6 days is related to atmospheric synoptic period variability and ocean mixing events due to wind stirring. However, the short length of available time series makes identifying temporal relationships between pCO2 and atmospheric or ocean processes problematic.

[1] The general circulation of the subtropical western North Atlantic is described using hydrography and subsurface velocity measured simultaneously by 71 profiling floats during the period July 1997 to December 2002. Subsurface trajectories of the floats revealed a strong topographic influence and depth-dependence of the Gulf Stream, its recirculation gyres, and the North Atlantic Current. We discuss eddy-like and nonsteady flows in several subregions, especially near the major topographic features. The seasonal cycle of volume transport, estimated using the absolute geostrophic velocity in the upper 900 m, was shown to be a maximum in winter and minimum in summer. The eddy kinetic energy and eddy diffusivity, derived from the float data and mapped over the subtopical gyre, show seasonal variability in the vicinity of the Gulf Stream with a maximum in spring and minimum in winter for both quantities.

[1] Variabilities of temperature and salinity over Long Island Sound (LIS), New York, are examined using observations from 1991 to 2002. There is a strong seasonal variation in the temperature, and its interannual variability is characterized by a higher variance during winter than summer. The salinity exhibits regular seasonal patterns driven by freshwater input, but there is a long-term change throughout LIS. Anomaly maps of temperature and salinity indicate strong longitudinal gradients increasing in the westward direction. Empirical orthogonal function analyses indicate that the first modes of temperature and salinity anomalies can explain 87% and 89% of the total variances, respectively. The first mode principle components of the temperature and salinity anomaly contain quasi-biennial periodicities. The salinity anomaly also contains an additional signal at a decadal timescale. Seasonal variations in the temperature and salinity are primarily associated with heat flux and freshwater discharge. However, the surface heat flux anomaly only accounts for 17% of the total variance of the time rate change of the temperature anomaly, and the freshwater discharge anomaly explains 25% of the variance of the salinity anomaly. Contrary to traditional paradigm about estuaries, this result shows that forcings other than local processes control the interannual variabilities of the temperature and salinity in LIS, most probably through horizontal exchanges. Also, the significant correlation between the salinity anomaly and the Gulf Stream (GS) position suggests that the interannual variability of salinity in LIS is possibly connected to shelf slope water properties associated with changes in the GS position.

[1] Observations from autumn 2000 near the shelfbreak front in the Middle Atlantic Bight are used to describe the transition from stratified summer conditions to well-mixed winter conditions over the shelf. During the observational period, the front differed dramatically from climatological conditions, with buoyant Gulf Stream water found shoreward over the subsurface shelfbreak front. Water mass analysis shows a large number of separate water masses with shelf, slope, and Gulf Stream origins. The coolest shelf water was located at the shelfbreak and may be related to “cold pool” water masses observed to the north during summer. Shoreward of this shelfbreak water mass, a mid-shelf front was present which intersected the bottom at the 50-m isobath. High-volume transports were associated with both the shelfbreak and mid-shelf fronts. Transport estimates from the cross-shelf sections were approximately 1 Sverdrup, which is large relative to previous estimates of shelf transport. The foot of the front was near the 130-m isobath, much deeper than the climatological position near the 75-m isobath; however, this is consistent with a recent theory relating the magnitude of alongshelf transport to the depth at which the front intersects the bottom.

[1] The summers of 1997 and 1998 were characterized by large anomalous atmospheric circulations in the tropical western Pacific, which in turn were associated with anomalous atmospheric circulation and precipitation in the adjacent areas. We use an atmospheric general circulation model to investigate the role of sea surface temperatures (SSTs) on the anomalous atmospheric circulations and associated precipitation in the tropics in these two summers. It is found that the circulation and rainfall anomalies are well reproduced when the model is forced with the observed global SSTs. A further experiment is performed to investigate the role of the SST anomalies in the Atlantic. In summer 1997, the Atlantic SST anomalies, compared with the SST anomalies outside the Atlantic, play a secondary role in influencing the circulation and rainfall anomalies in the tropical Indian Ocean and Pacific. By contrast, it shows that the SST anomalies in the Atlantic play a comparable role to those outside the Atlantic in influencing the circulation and rainfall anomalies in the remote tropics in summer 1998. In this summer, equatorial stationary waves, similar to a Kelvin wave and a Rossby wave, are clearly illustrated by the results implied by the Atlantic SST anomalies. The eastward equatorial stationary wave extends into the tropical western Pacific, and suppresses the atmospheric convection in the intertropical convergence zone in the Indian Ocean and Pacific. The evidence presented here for the role of Atlantic Ocean conditions on summer climates suggests that attention should be paid to accurate simulation of the Atlantic Ocean state as well as the states of the Pacific and Indian oceans in developing a seasonal forecasting system.

The median Doppler shift of radar echoes is analyzed in measurements by ENVISAT's Advanced Synthetic Aperture Radar (ASAR) over the ocean. This Doppler centroid differs from a predicted signal based on the predicted motion of the satellite and Earth. This anomaly, converted to a surface Doppler velocity U D , appears to be of geophysical origin. Two wide-swath images over the Gulf Stream around Cape Hatteras suggest that U D contains high-resolution information on surface currents, while on a global scale, U D is found to vary with the wind speed in the range direction. A simple quantitative forward model is proposed, based on a practical two-scale decomposition of the surface geometry and kinematics. The model represents the effect of the wind through the wave spectrum, and gives U D ≈ γU 10 ∥ + U c ∥ , with U 10 ∥ and U c ∥ as the 10 m wind speed and quasi-Eulerian current in the line of sight of the radar projected on the sea surface, respectively, and γ as a coefficient function of the wind speed, wave development, and radar geometry. It is found that for an incidence angle of 23°, γ ≈ 0.3 for moderate winds and fully developed seas. This model is validated with a global data set of ASAR Wave Mode observations, with colocated model winds, acquired over the global ocean during the years 2003 and 2004. The Doppler signal therefore provides the signed parameter U D that can be used to reduce the wind direction ambiguity in the inversion of high-resolution wind fields from SAR imagery. A qualitative validation of current effects is shown for the English Channel where tidal currents dominate. Thus it should be possible to combine this previously ignored geophysical Doppler signal with traditional information on sea surface roughness, in order to provide very high resolution wind and current fields.

[1] In July–August 1997, a hydrographic/Acoustic Doppler Current Profiler (ADCP)/tracer section was occupied along 52°W in the North Atlantic as part of the World Ocean Circulation Experiment Hydrographic Program. Underway and lowered ADCP (LADCP) data have been used to reference geostrophic velocities calculated from the hydrographic data; additional (small) velocity adjustments provided by an inverse model, constraining mass and silicate transports in 17 neutral density layers, yield the absolute zonal velocity field for 52°W. We find a vigorous circulation throughout the entire section, with an unusually strong Gulf Stream (169 Sv) and southern Deep Western Boundary Current (DWBC; 64 Sv) at the time of the cruise. At the northern boundary, on the west side of the Grand Banks of Newfoundland, we find the westward flowing Labrador Current (8.6 Sv), whose continuity from the Labrador Sea, east of our section, has been disputed. Directly to the south we identify the slopewater current (12.5 Sv eastward) and northern DWBC (12.5 Sv westward). Strong departures from strictly zonal flow in the interior, which are found in the LADCP data, make it difficult to diagnose the circulation there. Isolated deep property extrema in the southern portion, associated with alternating bands of eastward and westward flow, are consistent with the idea that the rough topography of the Mid-Atlantic Ridge, directly east of our section, causes enhanced mixing of Antarctic Bottom Water properties into overlying waters with distinctly different properties. We calculate heat and freshwater fluxes crossing 52°W that exceed estimates based on air-sea exchanges by a factor of 1.7.

This paper describes the results from a comparative wavelet analysis of a 0.1 , 40-level Parallel Ocean Program model simulation of the North Atlantic with coincident in situ measurements of sea surface height (SSH) and temperature. The wavelet analysis is used to examine the realism of the surface’s variability and its high frequency signals (less than a year). Along the coast, it shows that the model’s simulated fields of SSH are realistic with the correlations to tide gauge measurements on the order of 0.8. The wavelet spectra show that the model replicates the observations’ frequency space. Comparisons of the model’s temperatures to temperatures from NOAA buoys north and south of the Gulf Stream show that, while not replicating the location of mesoscale features all the time, the model’s energy in the strong mesoscale regions compares favorably to data. Due to the TOPEX/POSEIDON’s (T/P) sampling, which requires large areal averages and because of the model’s spontaneous eddy field, the evaluation of the simulation in the open ocean is less conclusive. The model does show similarities to the T/P data at high latitudes where the sampling by the sensor is denser and also on the eastern side of the basin with its lower mesoscale activity. Spatially, there are similarities in the amplitude of the signals between the model and the observations in areas with a significant signal in a given spectral band.

[1] Monthly circulation of the South Atlantic Bight is diagnosed using a 3-D, shallow water, finite element model forced with monthly wind stress and hydrographic climatology. Temperature and salinity observations from the period 1950–1999 are objectively interpolated onto the model domain, and Comprehensive Ocean-Atmosphere Data Set (COADS) wind velocities from 1975–1999 are used to prescribe the model surface wind stress. The resulting monthly temperature and salinity fields compare favorably to existing shelf climatology. River discharge maxima are evident in the spring temperature and salinity fields, and the rapid heating and cooling of the shelf are captured. The diagnostic circulation is largely wind-driven in the inner and mid-shelf, and the Gulf Stream is apparent in the solutions on the outer shelf. We present the monthly fields, including the temporal and spatial distribution of available hydrographic data, the regional COADS data that provide surface wind stress forcing, the objective analysis, and the model response to these forcings. The hydrographic and velocity fields provide best-prior-estimates of the circulation for data assimilation studies in the region, as well as initial conditions for process-oriented prognostic model studies in the Georgia coastal region.

[1] Velocity records from two bottom-mounted acoustic Doppler current profilers, deployed at the shelf break south of New England are used to characterize the mesoscale variability of the shelf break frontal jet in the Middle Atlantic Bight. While on average the jet is equatorward, energetic current fluctuations dominate the 18-month record at a period near 13 days. The fluctuations are characterized by a deceleration in the equatorward flow over the full water column and are strong enough to reverse the jet near the bottom. The origin of the observed variability is explored with the help of local wind records, concurrent velocity measurements from three tall current meter moorings deployed over the continental slope, and Gulf Stream frontal position information. While we are unable to attribute definitively the observed variability to a single forcing mechanism, we are able to discount several of the traditional possibilities. These include local effects, such as wind driving and tidal rectification, as well as offshore forcing due to Gulf Stream rings, topographic Rossby waves, and the meandering of the Gulf Stream. It is most probable that the fluctuations are caused by baroclinic instability of the shelf break jet. However, we are not able to determine this unequivocally, as our records differ from historical observations of instabilities, and stability models tend to oversimplify the salient features of the jet.

[1] This paper presents a numerical study of the warm water pathways, transports, and water mass transformation in the Newfoundland Basin region and an investigation of the rectification effects of a series of cold air outbreaks (CAOs) on the above processes. An initial “mean state” simulation was compared with observations and showed good agreement. Then, repeated CAO events were explicitly included in the surface fluxes and illustrated the following rectification effects. The thermal regime of the entire baroclinic layer was impacted, as the thermocline deepened and the temperature anomaly was noticeable down to ∼500 m. Different mechanisms were responsible for the propagation of the temperature anomaly at different stages. During the onset of the CAO, vertical diffusion propagated the temperature anomaly downward near the surface, then vertical advection further propagated the anomaly downward between CAO events to about 500 m depth. With CAOs, the North Atlantic Current carried more warm water, and its pathway in the Northwest Corner shifted towards the southeast. The volume-averaged mean and eddy kinetic energy increased by 30% and 20%, respectively, and the water mass transformation rate in the Newfoundland basin was doubled. The Gulf Stream carried more heat, but heat transport to the eastern basin decreased owing to the increase in heat release to the atmosphere.

[1] Processes contributing to the heat anomaly budget of the upper ocean mixed layer are examined over the North Atlantic in summer season on the basis of the inversion of the sea temperature profiles. The net advection velocity, the subgrid heat diffusivity, and the surface heat flux feedback are estimated. Two inversion strategies are pursued, with and without the explicit parameterization of the entrainment heat flux into the upper mixed layer. The regression estimator is employed for the inversion. The data involved are the compilation of the Comprehensive Ocean-Atmosphere Data Set sea surface temperature (SST) and World Ocean Database 2001 temperature profiles over a 2° latitude × 3° longitude grid for sequential 10-day intervals in June–September during 1965–2000. The inversion is constrained by the autocorrelation of the National Centers for Environmental Prediction net air-sea heat flux. The regression estimator allows for prediction of up to 60–70% of the SST anomaly value 10 days in advance by using the regressions on the SST anomalies lagged by two time steps. The SST anomaly transport over the North Atlantic agrees broadly with the known features of the surface circulation, although the quantitative details of the SST anomaly propagation obtained in this inversion have not previously been available. The heat anomaly that originates in the Gulf Stream region near the western boundary either moves northeastward with speed of up to 0.12 m s−1 downstream of the North Atlantic Current and the North Atlantic Drift Current or it turns southeastward and then propagates southwestward along the broad subtropical recirculation; in the subtropics the westward propagation is seemingly reinforced by the effects of the planetary waves. The air-sea feedback is negative with the feedback factor having the average value of 39 W m−2 K−1 with substantial variations throughout the North Atlantic. The entrainment rate is generally about 0.15 m d−1; relatively rapid deepening of the mixed layer by more than 2 × 10−6 m s−1 occurs near the Gulf Stream.

[1] Close to 1800 surface drifters are used to investigate the 15 m circulation of the North Atlantic Ocean. The data are used to describe structures of the average Eulerian circulation and of the associated eddy variability. The data resolve scales on the order of 50 km, which have hitherto not been systematically described, in particular, near shelf breaks and near the most intense currents, the Gulf Stream, the North Atlantic Current (NAC), and the frontal currents of the subpolar gyre. This reveals a complex series of quasi-permanent eddies, meanders, and recirculation patterns. Gulf Stream intensity is portrayed as changing abruptly near 54°W, east of which, it is identified as two current branches centered near 39° and 41.5°N, the northern one connecting more directly with the NAC, and the southern one with the recirculation gyre and the Azores Current (AC). Many features of the currents are controlled by topography, in particular, currents are often intensified near shelf breaks or parallel to ridges in the subpolar gyre. However, the largest northward branch of the NAC in the Icelandic basin is located near the deepest bathymetry, not near steep bathymetry. Other currents, in particular, in the subtropical gyre, are less clearly related to topography: for instance, the AC is featured as a zonal eastward current extending far west of the Mid-Atlantic Ridge (MAR) to at least 55°W and possibly 63°W. In the interior and away from topographic features the eddy kinetic energy (EKE) is the largest where the mean currents are the largest. In the subpolar gyre, there are striking differences in EKE between southward flowing currents (the Labrador and east Greenland Current) and the northward flowing currents (west Greenland Current and branches of the NAC), which have higher EKE. The areas of weakest variability are located in the southwest part of the subpolar gyre, northeast of Iceland, and in the eastern Atlantic south of 45°N. The AC eddies and the mesoscales south of the Canary Islands transect this eastern eddy desert. Drifter trajectories are used as realizations of Lagrangian particles in the vicinity of current cores. These illustrate the variety of paths or connections between different current systems and demonstrate cross-stream dispersions.