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The Gulf Stream (GS) ocean front exhibits intense ocean–atmosphere interaction in winter, which has a significant impact on the genesis and development of extratropical cyclones in the North Atlantic. The atmospheric rivers (ARs), closely related with the cyclones, transport substantial moisture from the North Atlantic towards the Western European coast. While the influence of the GS front on extratropical cyclones has been extensively studied, its effect on ARs remains unclear. In this study, two sets of ensemble experiments are conducted using a high-resolution global Community Atmosphere Model forced with or without the GS sea surface temperature front. Our findings reveal that the inclusion of the GS front leads to approximately 25% enhancement of water vapor transport and precipitation associated with ARs in the GS region, attributed to changes in both AR frequency and intensity. Furthermore, this leads to a more pronounced downstream response in Western Europe, characterized by up to 60% (40%) precipitation increases (reductions) around Spain (Norway) for the most extreme events (exceeding 90 mm/day). The influence of the GS front on ARs is mediated by both thermodynamic and dynamic factors. The thermodynamic aspect involves an overall increase of water vapor in both the GS region and Western Europe, promoting AR genesis. The dynamic aspect encompasses changes in storm tracks and Rossby wave train, contributing to downstream AR shift. Importantly, we find the co-occurrence of ARs and the GS front is crucial for inducing deep ascending motion and heating above the GS front, which perturbs the deep troposphere and triggers upper-level Rossby wave response. These findings provide a further understanding of the complex interaction between the oceanic front in the western boundary current regions and extratropical weather systems and the associated dynamics behind them.

The Gulf Stream anchors a notable time-mean convergence zone during warm seasons, which produces strong precipitation and huge latent heat release in the free atmosphere. How the Gulf Stream front drives transient winds resulting in the convergence zone remains intensely debated. The oceanic-front-induced convergences are masked by strong synoptic disturbances. By synthesizing the 10-years high-frequency samplings by QuikSCAT, this study develops a novel procedure to isolate the subtle but climatically important convergence response to the oceanic front from the energetic synoptic disturbances. The binned convergence exhibits an evident cosine dependence on the angles between sea surface temperature (SST) gradients and transient winds and reaches its maximum when the winds deceleratingly blow down SST gradients, consistent with the so-called vertical mixing mechanism. The transient surface winds accelerate (decelerate) due to the enhanced (reduced) downward momentum transfer over the mesoscale warm (cold) waters. Thus, stronger down-SST-gradient winds cause stronger convergences over shaper SST gradients. The linear relationship between convergence and wind advection over SST gradients significantly contributes to the time-mean Gulf Stream convergence zone. The increased boundary-layer moisture acts to strengthen the SST-front-induced convergence and shifts convergence towards the Gulf Stream axis. As the synoptic disturbances frequent over the Gulf Stream, this study provides a potential pathway that the mesoscale SST gradients modulate synoptic disturbances through driving the surface wind convergences.

The stability of a horizontally and vertically sheared surface jet is examined, with a focus on the vertical structure of the resultant eddies. Over a flat bottom, the instability is mixed baroclinic/barotropic, producing strong eddies at depth that are characteristically shifted downstream relative to the surface eddies. Baroclinic instability is suppressed over a large slope for retrograde jets (with a flow antiparallel to topographic wave propagation) and to a lesser extent for prograde jets (with flow parallel to topographic wave propagation), as seen previously. In such cases, barotropic (lateral) instability dominates if the jet is sufficiently narrow. This yields surface eddies whose size is independent of the slope but proportional to the jet width. Deep eddies still form, forced by interfacial motion associated with the surface eddies, but they are weaker than under baroclinic instability and are vertically aligned with the surface eddies. A sinusoidal ridge acts similarly, suppressing baroclinic instability and favoring lateral instability in the upper layer. A ridge with a 1-km wavelength and an amplitude of roughly 10 m is sufficient to suppress baroclinic instability. Surveys of bottom roughness from bathymetry acquired with shipboard multibeam echo sounding reveal that such heights are common beneath the Kuroshio, the Antarctic Circumpolar Current, and, to a lesser extent, the Gulf Stream. Consistent with this, vorticity and velocity cross sections from a 1/50° HYCOM simulation suggest that Gulf Stream eddies are vertically aligned, as in the linear stability calculations with strong topography. Thus, lateral instability may be more common than previously thought, owing to topography hindering vertical energy transfer.

Substantial amounts of nitrogen fixation occur in the North Atlantic subtropical gyre, due to the activity of cyanobacteria with high iron requirements. Iron is delivered to this region by dust from the Sahara Desert. However, this dust deposition is typically localized and episodic. Therefore, other sources of iron may also be important. Here, we report observations of dissolved iron concentrations in a Gulf Stream cold-core ring, which transported iron-rich water from near the continental slope into the subtropical gyre. We find that iron concentrations were elevated in the ring compared with subtropical waters, reflecting its source waters. Using iron data from these source waters and the identification of ring activity in satellite data, we estimate that cold-core rings provide a net flux of 0.3 ± 0.17 × 108 mol Fe yr−1 across the northwestern gyre edge, on the order of 15% of our median estimates of gyre-wide supply of iron by dust deposition. We suggest that iron supply from cold-core rings is an important source of iron to the northwestern gyre edge. We conclude that mesoscale ocean circulation features may play an important role in subtropical nutrient and carbon cycling.

Rapid sea ice retreat and increases in temperature have characterised the Arctic basin in the last few decades. A number of studies have suggested that these changes have had a direct impact on extremes of weather and climate in the midlatitudes, while others have submitted that the evidence for this may not be robust. Sato et al (2014 Environ. Res. Lett. 9 084009) cast considerable light on this divergence of perspectives by revealing that apparent links between Barents Sea ice coverage and cold Eurasian winters form just a sector of a teleconnection pattern which originates remotely in the North Atlantic Gulf Stream region.

A high-resolution global atmospheric model is used to investigate the influence of the representation of small-scale North Atlantic sea surface temperature (SST) patterns on the atmosphere during boreal winter. Two ensembles of forced simulations are performed and compared. In the first ensemble (HRES), the full spatial resolution of the SST is maintained while small-scale features are smoothed out in the Gulf Stream region for the second ensemble (SMTH). The model shows a reasonable climatology in term of large-scale circulation and air–sea interaction coefficient when compared to reanalyses and satellite observations, respectively. The impact of small-scale SST patterns as depicted by differences between HRES and SMTH shows a strong meso-scale local mean response in terms of surface heat fluxes, convective precipitation, and to a lesser extent cloudiness. The main mechanism behind these statistical differences is that of a simple hydrostatic pressure adjustment related to increased SST and marine atmospheric boundary layer temperature gradient along the North Atlantic SST front. The model response to small-scale SST patterns also includes remote large-scale effects: upper tropospheric winds show a decrease downstream of the eddy-driven jet maxima over the central North Atlantic, while the subtropical jet exhibits a significant northward shift in particular over the eastern Mediterranean region. Significant changes are simulated in regard to the North Atlantic storm track, such as a southward shift of the storm density off the coast of North America towards the maximum SST gradient. A storm density decrease is also depicted over Greenland and the Nordic seas while a significant increase is seen over the northern part of the Mediterranean basin. Changes in Rossby wave breaking frequencies and weather regimes spatial patterns are shown to be associated to the jets and storm track changes.

This study investigated how warm‐core eddies (WCEs) in the Gulf Stream (GS) modulated the intensity of a distant tropical cyclone (TC) approaching the current in October. We performed cloud‐resolving regional simulations including a control run with the observed WCEs and a sensitivity run excluding the WCEs. These simulations found that the WCEs played a favorable role in the development of the distant TC. The WCEs affected the synoptic‐scale thermodynamic environments over the North Atlantic through the enhanced heat and moisture supply from the GS, increasing the moisture imports toward the distant TC. The WCEs‐enhanced moisture influx created very moist environments in the inner core. This inner‐core moistening was favorable for deep eyewall convection and an associated TC secondary circulation, leading to TC development. This result indicates the importance of WCEs in facilitating the remote process leading to TC development that previous studies have proposed. (a–c) Horizontal distributions of the daily‐mean surface enthalpy heat flux (EHF; shading), the sea level pressure (SLP; blue contours with 3 hPa intervals), and the 10 m wind (vectors) on (a) 10, (b) 11, and (c) 12 October 2016. The EHF is the sum of latent heat flux (LHF) and sensible heat flux (SHF). Positive values in EHF are for sea‐to‐air moisture and heat supply. Gray dotted areas show the missing value of EHF. Vectors of <5 m·s−1 are not shown. The LHF, SHF, SST and 10 m wind are derived from the satellite‐based observation (J‐OFURO3; Tomita et al., 2019, 2021). The SLP is obtained from the Japanese 55‐year reanalysis dataset (JRA‐55; Kobayashi et al., 2015). (d–f) Same as in (a–c), but for the magnified views of EHF anomaly (shading) and SST anomaly (contours with 1°C intervals). Contours of 0°C and ±1°C SST anomalies are not shown. The EHF and SST anomalies are deviation with respect to a 30‐year mean period (1988–2017). (g–i) Same as in (d–f), but for the sea level anomaly (SLA; shading) and absolute geostrophic velocity (vectors) derived from Data Unification and Altimeter Combination System (DUACS; Taburet et al., 2019). The SLA is calculated with respect to a 20‐year mean reference period (1993–2012). Vectors of <0.25 m·s−1 are not shown. The symbols (A–C) denote the locations of warm core eddies (WCEs) mentioned in this study.

Meandering of river systems has been attributed to erosion and deposition of sediments along river banks, yet a universal cause of the instability has not been identified that can be applied to other meandering systems. Here, we address the conditions that lead to the meander instability, in effect “upstream” of the many previous and thorough analyses of hydraulics and morphological patterns that ensue when such conditions exist. Rivers are only one of many fluid systems that exhibit meandering behavior, and no other involves sediment. Meandering is observed in the Gulf Stream, free-falling streams of viscous fluid, derailed trains, jackknifed trucks, and other systems in one, two or three dimensions. As such, a universal criterion is needed to explain meandering in general. We propose that meandering in all systems is driven by development of an adverse pressure gradient that results from an imbalance between driving and resistive forces. A universal framework will make it possible to determine under what conditions the meandering instability will manifest in altered flow and channel morphology. Toward that end, we conducted laboratory experiments in 2- and 3-dimensions and analyzed systems with either reduction in driving forces or increase in resistive forces. We also examine the ubiquitous example of natural rivers to identify the conditions under which rivers meander or ply a more direct course to base level. Further, we define a dimensionless “Meander Number” as (driving-resistive)/resistive forces. The observations presented here support a common cause that leads to instability of a wide range of meandering systems and with further exploration, may ultimately lead to a universal theory of meandering systems.

The Gulf Stream (GS) ocean front releases intense moisture and heat to the atmosphere and regulates storm tracks and zonal jets in winter. The large-scale sea surface temperature (SST) anomaly in the central North Atlantic provides important feedback to the atmosphere in winter, but the role played in this feedback by the GS front inside the SST anomaly has not been extensively studied. In this study, two sets of ensemble experiments were conducted using a global community atmosphere model forced by SST in boreal winters from 2000 to 2013. The regional averaged SST and its variation in the experiments were identical, with the only difference being the strength of the SST front in the GS region. The large-scale SST anomaly in the central North Atlantic in our model provides feedback to the atmosphere and excites a wave train that extends across Eurasia. With the inclusion of the strong GS front, the first center of the wave train in the North Atlantic is strengthened by approximately 40%, and the wave activity flux toward downstream is highly intensified. When the large-scale SST anomaly is combined with a strong GS front, greatly increased water vapor is released from the GS region, resulting in a 50% increase in moisture transport toward Western Europe. In this scenario, precipitation and diabatic heating both increase greatly on the western Scandinavian Peninsula. With the release of deep diabatic heating, a strong upward wave activity flux is triggered, and the wave train excited by the large-scale SST variation is significantly intensified. These findings suggest that the strong SST front in the large-scale SST anomaly in the central North Atlantic significantly amplifies its feedback to the atmosphere in winter.

Trajectory encounter volume – the volume of fluid that passes close to a reference fluid parcel over some time interval – has been recently introduced as a measure of mixing potential of a flow. Diffusivity is the most commonly used characteristic of turbulent diffusion. We derive the analytical relationship between the encounter volume and diffusivity under the assumption of an isotropic random walk, i.e., diffusive motion, in one and two dimensions. We apply the derived formulas to produce maps of encounter volume and the corresponding diffusivity in the Gulf Stream region of the North Atlantic based on satellite altimetry, and discuss the mixing properties of Gulf Stream rings. Advantages offered by the derived formula for estimating diffusivity from oceanographic data are discussed, as well as applications to other disciplines.