Gulf Stream Sea Surface Temperature Research Articles

Resolving Weather Fronts Increases the Large‐Scale Circulation Response to Gulf Stream SST Anomalies in Variable‐Resolution CESM2 Simulations

AbstractCanonical understanding based on general circulation models (GCMs) is that the atmospheric circulation response to midlatitude sea‐surface temperature (SST) anomalies is weak compared to the larger influence of tropical SST anomalies. However, the ∼100‐km horizontal resolution of modern GCMs is too coarse to resolve strong updrafts within weather fronts, which could provide a pathway for surface anomalies to be communicated aloft. Here, we investigate the large‐scale atmospheric circulation response to idealized Gulf Stream SST anomalies in Community Atmosphere Model (CAM6) simulations with 14‐km regional grid refinement over the North Atlantic, and compare it to the responses in simulations with 28‐km regional refinement and uniform 111‐km resolution. The highest resolution simulations show a large positive response of the wintertime North Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream, a 0.4‐standard‐deviation anomaly in the seasonal‐mean NAO for 2°C SST anomalies. The lower‐resolution simulations show a weaker response with a different spatial structure. The enhanced large‐scale circulation response results from an increase in resolved vertical motions with resolution and an associated increase in the influence of SST anomalies on transient‐eddy heat and momentum fluxes in the free troposphere. In response to positive SST anomalies, these processes lead to a stronger and less variable North Atlantic jet, as is characteristic of positive NAO anomalies. Our results suggest that the atmosphere responds differently to midlatitude SST anomalies in higher‐resolution models and that regional refinement in key regions offers a potential pathway to improve multi‐year regional climate predictions based on midlatitude SSTs.

Nonstationarity in the NAO–Gulf Stream SST Front Interaction

Abstract The interaction between the North Atlantic Oscillation (NAO) and the latitudinal shifts of the Gulf Stream sea surface temperature front (GSF) has been the subject of extensive investigations. There are indications of nonstationarity in this interaction, but differences in the methodologies used in previous studies make it difficult to draw consistent conclusions. Furthermore, there is a lack of consensus on the key mechanisms underlying the response of the GSF to the NAO. This study assesses the possible nonstationarity in the NAO–GSF interaction and the mechanisms underlying this interaction during 1950–2020, using reanalysis data. Results show that the NAO and GSF indices covary on the decadal time scale but only during 1972–2018. A secondary peak in the NAO–GSF covariability emerges on multiannual time scales but only during 2005–15. The nonstationarity in the decadal NAO–GSF covariability is also manifested in variations in their lead–lag relationship. Indeed, the NAO tends to lead the GSF shifts by 3 years during 1972–90 and by 2 years during 1990–2018. The response of the GSF to the NAO at the decadal time scale can be interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the response of deep oceanic circulation, and the propagation of Rossby waves. However, there is evidence of Rossby wave propagation only during 1972–90. Here it is suggested that the nonstationarity of Rossby wave propagation caused the time lag between the NAO and the GSF shifts on the decadal time scale to differ between the two time periods.

Gulf Stream Sea Surface Temperature Anomalies Associated With the Extratropical Transition of North Atlantic Tropical Cyclones

AbstractThis study investigates Gulf Stream (GS) sea surface temperature (SST) anomalies associated with the extratropical transition (ET) of tropical cyclones (TCs) in the North Atlantic. Composites of western North Atlantic TCs indicate that GS SSTs are warmer, and both large‐ and fine‐scale SST gradients are weaker than average, for TCs that begin the ET process but do not complete it, compared with TCs that do. Further analysis suggests that the associated fine‐scale GS SST gradient anomalies are related to atmospheric processes but not the same as those that are typically associated with the onset of ET. As sensible heat flux gradients and surface diabatic frontogenesis are shown to generally scale with the local SST gradient strength, these results suggest that knowledge of the fine‐scale GS SST gradient in the weeks prior to the arrival of a TC might potentially provide additional information regarding the likelihood of that system completing ET.

The Atmospheric Response to Meridional Shifts of the Gulf Stream SST Front and Its Dependence on Model Resolution

Abstract The Gulf Stream (GS) plays a key role in shaping the North Atlantic climate. Moreover, the associated sea surface temperature (SST) front undergoes interannual-to-decadal variability that is thought to force significant atmospheric circulation anomalies. However, general circulation models do not accurately reproduce the atmospheric response to SST front variability as estimated from observations. In this work we analyze the atmospheric response to the GS SST front (GSF) shifts in a multimodel ensemble of atmosphere-only simulations forced with observed SSTs (1950–2014). The atmospheric response is found to be resolution dependent. Only the high-resolution simulations produce a wintertime response similar to observed anomalies. More specifically, (i) analysis of the atmospheric thermodynamic balance close to the GSF showed that the anomalous diabatic heating associated to the GSF displacement is mainly balanced by vertical motion and by meridional transient eddy heat transport (not the case for low-resolution models), while (ii) the large-scale response includes a meridional shift of the North Atlantic eddy-driven jet and storm track homodirectional to the GSF displacement. This atmospheric response is accompanied by changes in low-level baroclinicity close to and north of the GSF, resulting from the oceanic forcing and the zonal atmospheric circulation anomalies respectively. The low-level baroclinicity anomalies lead to changes in baroclinic eddy activity and, ultimately, in the jet via eddy–mean flow interaction. Considering the two-way nature of air–sea interactions, using historical atmosphere-only simulations is a powerful way to isolate the impact of realistic oceanic variability on the atmosphere. Our results suggest that interannual-to-decadal predictability may be higher than what low-resolution models currently indicate.

Barents-Kara sea-ice decline attributed to surface warming in the Gulf Stream

Decline in winter sea-ice concentration (SIC) in the Barents-Kara Sea significantly impacts climate through increased heat release to the atmosphere. However, the past Barents-Kara SIC decrease rate is underestimated in the majority of Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models. Here we show that climate model simulations can reproduce the Barents-Kara SIC trend for 1970–2017 when sea surface temperature (SST) variability in the Gulf Stream region is constrained by observations. The constrained warming of the Gulf Stream strengthens ocean heat transport to the Barents-Kara Sea that enhances the SIC decline. The linear trends between the SIC and SST are highly correlated in the CMIP6 ensemble, suggesting that the externally forced component of the Gulf Stream SST increase explains up to 56% of the forced Barents-Kara SIC trend. Therefore, future warming of the Gulf Stream can be an essential pacemaker of the SIC decline.

Contribution of the cold sector of extratropical cyclones to mean state features over the Gulf Stream in winter

Recent studies have shown that midlatitude air–sea interactions are strongly modulated by synoptic variability. The present study investigates how air–sea interactions over the Gulf Stream sea surface temperature (SST) front vary in different synoptic regimes. We focus more particularly on the variation of three atmospheric mean state features which are colocated with the SST front in winter: enhanced ascent and precipitation on the warm side of the SST front, and enhanced low‐level baroclinicity at the entrance of the storm track. These three fields are partitioned depending on whether they occur in the cold sector of extratropical cyclones or in any other synoptic features. The analysis is based on ERA‐Interim winter data covering the period 1979–2012. Results are twofold:(i) Cold sector precipitation is confined within a 5° latitude band south of the SST front and reaches 2 mm day−1, whereas precipitation occurring outside the cold sector forms a broader spatial pattern. The same partitioning applied to vertical wind shows that the ascent on the warm side of the Gulf Stream is not a feature of the cold sector. These results mean that a significant part of the anchoring effect of the SST front on precipitation occurs via the cold sector and that the observed colocation of ascent and precipitation is not causal, in contrast to what was suggested by previous studies, but is rather the result of two different mechanisms.(ii) The surface heat fluxes and convection occurring in the cold sector restore low‐level atmospheric temperature gradients within two days after a time maximum of meridional eddy heat flux, such that low‐level baroclinicity remains largely unchanged after the passage of an extratropical cyclone.This ‘cold path mechanism’ opens new avenues to understand how SST forces climate variability in the midlatitudes.

A “Cold Path” for the Gulf Stream–Troposphere Connection

Abstract The mechanism by which the Gulf Stream sea surface temperature (SST) front anchors a band of precipitation on its warm edge is still a matter of debate, and little is known about how synoptic activity contributes to the mean state. In the present study, the influence of the SST front on precipitation is investigated during the course of a single extratropical cyclone using a regional configuration of the Met Office Unified Model. The comparison of a control run with a simulation in which SST gradients were smoothed brought the following conclusions: a band of precipitation is reproduced for a single extratropical cyclone, and the response to the SST gradient is dominated by a change of convective precipitation in the cold sector of the storm. Several climatological features described by previous studies, such as surface wind convergence on the warm edge or a meridional circulation cell across the SST front, are also reproduced at synoptic time scales in the cold sector. Based on these results, a simple boundary layer model is proposed to explain the convective and dynamical response to the SST gradient in the cold sector. In this model, cold and dry air parcels acquire more buoyancy over a sharp SST gradient and become more convectively unstable. The convection sets a pressure anomaly over the entire depth of the boundary layer that drives wind convergence. This case study offers a new pathway by which the SST gradient can anchor a climatological band of precipitation.

Atlantic near‐term climate variability and the role of a resolved Gulf Stream

AbstractThere is a continually increasing demand for near‐term (i.e., lead times up to a couple of decades) climate information. This demand is partly driven by the need to have robust forecasts and is partly driven by the need to assess how much of the ongoing climate change is due to natural variability and how much is due to anthropogenic increases in greenhouse gases or other external factors. Here we discuss results from a set of state‐of‐the‐art climate model experiments in comparison with observational estimates that show that an assessment of predictability requires models that capture the variability of major oceanic fronts, which are, at best, poorly resolved and may even be absent in the near‐term prediction of Intergovernmental Panel on Climate Change class models. This is the first time that air‐sea interactions associated with resolved Gulf Stream sea surface temperature have been identified in the context of a state‐of‐the‐art global coupled climate model with inferred near‐term predictability.

The influence of the Gulf Stream on wintertime European blocking

Wintertime blocking is responsible for extended periods of anomalously cold and dry weather over Europe. In this study, the influence of the Gulf Stream sea surface temperature (SST) front on wintertime European blocking is investigated using a reanalysis dataset and a pair of atmospheric general circulation model (AGCM) simulations. The AGCM is forced with realistic and smoothed Gulf Stream SST, and blocking frequency over Europe is found to depend crucially on the Gulf Stream SST front. In the absence of the sharp SST gradient European blocking is significantly reduced and occurs further downstream. The Gulf Stream is found to significantly influence the surface temperature anomalies during blocking periods and the occurrence of associated cold spells. In particular the cold spell peak, located in central Europe, disappears in the absence of the Gulf Stream SST front. The nature of the Gulf Stream influence on European blocking development is then investigated using composite analysis. The presence of the Gulf Stream SST front is important in capturing the observed quasi-stationary development of European blocking. The development is characterised by increased lower-tropospheric meridional eddy heat transport in the Gulf Stream region and increased eddy kinetic energy at upper-levels, which acts to reinforce the quasi-stationary jet. When the Gulf Stream SST is smoothed the storm track activity is weaker, the development is less consistent and European blocking occurs less frequently.

Regional climate variability in the western subtropical North Atlantic during the past two millennia

[1] Western subtropical North Atlantic oceanic and atmospheric circulations connect tropical and subpolar climates. Variations in these circulations can generate regional climate anomalies that are not reflected in Northern Hemisphere averages. Assessing the significance of anthropogenic climate change at regional scales requires proxy records that allow recent trends to be interpreted in the context of long‐term regional variability. We present reconstructions of Gulf Stream sea surface temperature (SST) and hydrographic variability during the past two millennia based on the magnesium/calcium ratio and oxygen isotopic composition of planktic foraminifera preserved in two western subtropical North Atlantic sediment cores. Reconstructed SST suggests low‐frequency variability of ∼1°C during an interval that includes the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). A warm interval near 1250 A.D. is distinct from regional and hemispheric temperature, possibly reflecting regional variations in ocean‐atmosphere heat flux associated with changes in atmospheric circulation (e.g., the North Atlantic Oscillation) or the Atlantic Meridional Overturning Circulation. Seawater d 18 O, which is marked by a fresher MCA and a more saline LIA, covaries with meridional migrations of the Atlantic Intertropical Convergence Zone. The northward advection of tropical salinity anomalies by mean surface currents provides a plausible mechanism linking Carolina Slope and tropical Atlantic hydrology.

The Atmospheric Circulation over the North Atlantic as Induced by the SST Field

AbstractSpectral analyses of the sea surface temperature (SST) in the Simple Ocean Data Analysis (SODA) reanalysis for the past half-century identify prominent and statistically significant interannual oscillations in two regions along the Gulf Stream front over the North Atlantic. A model of the atmospheric marine boundary layer coupled to a baroclinic quasigeostrophic model of the free atmosphere is then forced with the SST history from the SODA reanalysis. Two extreme states are found in the atmospheric simulations: 1) an eastward extension of the westerly jet associated with the front, which occurs mainly during boreal winter, and 2) a quiescent state of very weak flow found predominantly in the summer. This vacillation of the oceanic-front-induced jet in the model is found to exhibit periodicities similar to those identified in the observed Gulf Stream SST front itself. In addition, a close correspondence is found between interannual spectral peaks in the observed North Atlantic Oscillation (NAO) index and the SODA-induced oscillations in the atmospheric model. In particular, significant oscillatory modes with periods of 8.5, 4.2, and 2.8 yr are found in both observed and simulated indices and are shown to be highly synchronized and of similar energy in both time series. These oscillatory modes in the simulations are shown to be suppressed when either (i) the Gulf Stream front or (ii) its interannual oscillations are omitted from the SST field. Moreover, these modes also disappear when (iii) the SST front is spatially smoothed, thus confirming that they are indeed induced by the oceanic front.

Storm track sensitivity to sea surface temperature resolution in a regional atmosphere model

A high resolution regional atmosphere model is used to investigate the sensitivity of the North Atlantic storm track to the spatial and temporal resolution of the sea surface temperature (SST) data used as a lower boundary condition. The model is run over an unusually large domain covering all of the North Atlantic and Europe, and is shown to produce a very good simulation of the observed storm track structure. The model is forced at the lateral boundaries with 15–20 years of data from the ERA-40 reanalysis, and at the lower boundary by SST data of differing resolution. The impacts of increasing spatial and temporal resolution are assessed separately, and in both cases increasing the resolution leads to subtle, but significant changes in the storm track. In some, but not all cases these changes act to reduce the small storm track biases seen in the model when it is forced with low-resolution SSTs. In addition there are several clear mesoscale responses to increased spatial SST resolution, with surface heat fluxes and convective precipitation increasing by 10–20% along the Gulf Stream SST gradient.

Radar backscatter across the Gulf Stream sea surface temperature front

Ku-band backscatter responses to frontal sea surface temperature (SST) changes are studied and applications of a high-resolution scatterometer to remote sensing of an SST front are suggested. Ocean backscatter signatures were measured with an airborne Ku-band scatterometer across the Gulf Stream SST front during the Surface Wave Dynamics Experiment. Oceanic and atmospheric parameters were measured by buoys, by the Scanning Radar Altimeter, and by the Advanced Very High Resolution Radiometer, and the surface current fields analyzed by the Fleet Numerical Meteorology and Oceanography Center were obtained. Vertical polarization backscatter difference across the SST front with an SST difference of 9/spl deg/C is more than 5 dB in several flight lines. Large horizontal polarization backscatter changes are also observed across the SST front with an SST difference approximately one-half of that in the vertical polarization backscatter case. Corresponding wind speed differences cannot account for the large backscatter changes in view of Seasat-A Satellite Scatterometer (SASS) geophysical model functions depending only on neutral wind velocity. For both polarizations, upwind backscatter on the cold side is smaller than or close to crosswind backscatter on the warm side for incidence angles larger than 20/spl deg/ and smaller than 60/spl deg/. This suggests that the SST front can be detected with backscatter measured by a spaceborne radar at a fixed azimuth angle even if wind directions are different over the cold and warm sides.

A numerical investigation of surface-induced mesocyclogenesis near the Gulf Stream

A series of numerical experiments designed to simulate the initial development stages of low-level coastal mesocyclogenesis near the Gulf Stream was recently conducted. Under initially quiescent conditions, surface cyclogenesis in the control simulation occurs along a Gulf Stream meander in a region where the gradients in sea surface temperature (SST) are maximized. A low-level mesovortex on the order of 140 km forms approximately 12 h into the simulation and continues to intensify through 42 h. During the 24–48 h time period, a mesoscale frontal feature develops in direct response to strong diabatic forcing associated with sustained surface latent and sensible heating near the Gulf Stream frontal zone south of the main circulation center. Due to the non-linear advection of the frontal feature during this time period, the previously quasi-stationary circulation center drifts eastward (and away) from the thermal forcing associated with the large SST gradients found to the west. This eastward frontal propagation acts to decrease the magnitude of the low level horizontal air temperature gradient near the center of circulation throughout the 24–42 h development period. During the 42–48-h period, the relatively quick moving frontal feature acts to severely shear the nearly stationary center of circulation in the east–west direction. As a result, the mesoscale system begins to fill during the final 6 h of integration. In addition to the control simulation, additional sensitivity experiments were conducted. These experiments were specifically designed to: (1) investigate how the magnitude of the Gulf Stream SST gradients affect the timing and degree of cyclonic development; (2) address the impact surface moisture fluxes and moist convection each have on the simulated low level mesocyclogenesis; (3) isolate the role surface sensible heating plays in the overall development of the simulated mesocyclone. Results from the SST gradient experiment indicate that a moderate enhancement of the SST distribution significantly affects the timing of the initial cyclogenesis and the maximum intensity of the simulated frontal circulation. For the “no turbulent heat flux” experiment, it appears that the elimination of surface sensible heating does not radically alter the overall structure of the simulated mesocyclone. However, the rate of development during the early stage of cyclogenesis, the absolute peak intensity of the system as well as the vertical depth of the simulated mesoscale frontal feature were all noticeably reduced when compared with the control simulation. The initial development of a closed low level circulation was delayed by nearly 18 h in the absence surface latent heat fluxes. Once formed, the system intensified throughout the 48-h period of integration, but unlike the control experiment, a mesoscale frontal feature south of the main circulation center was not simulated . Results from the “no surface moisture flux/no moist convection” simulation illustrate that moist convective processes play a dominant role in the overall development of the mesoscale cyclone. For this particular case, a weak and extremely shallow circulation was simulated after 24 h. This circulation quickly eroded however, and was virtually non-existent for integration times greater than 39 h. DOI: 10.1034/j.1600-0870.1995.00122.x

Gulf Stream warm surges: Thermal anomalies resulting from cold core ring interactions

Observations from thermal satellite imagery reveal a thermal anomaly pattern in the sea surface temperature (SST) field of the Gulf Stream downstream of Cape Hatteras which is a direct result of Gulf Stream‐eddy interactions upstream of Cape Hatteras. The general sequence of events observed in the images is as follows: (1) Detrainment of Gulf Stream surface water occurs as a result of an interaction with a cold core ring; (2) downstream of the interaction the Gulf Stream's SST decreases several degrees over a few days owing to the loss of the upstream surface flow; (3) a visible “jet” of warm surface water is advected down the major axis of the Gulf Stream after the stream‐ring interaction subsides and the detrainment diminishes. The jet maintains a 1–3°C temperature difference with the surrounding Gulf Stream water as it propagates downstream. This paired cooling/warming process is labeled a “warm surge event” because the sharp leading edge of the warm jet (i.e., the surge) is easily detectable in the satellite images, while the anomalous surface cooling preceding the passage of the warm jet is difficult to observe. Vertical profiles of temperature, downstream velocity, and cross‐stream velocity all indicate that the upper 100–150 m of the Gulf Stream are significantly altered during an event. Satellite and in situ data also indicate that just downstream of the interaction there is enhanced entrainment of Sargasso Sea surface waters into the Gulf Stream to replenish the detrained Gulf Stream water.

Marine Atmospheric Boundary Layer Circulations Forced by Gulf Stream Sea Surface Temperature Gradients

The Penn State/NCAR mesoscale model is initialized with calm winds, a barotropic temperature pattern, and a uniform surface pressure in studies of the response of the marine atmospheric boundary layer (MABL) to realistic differential fluxes of heal and moisture at the sea surface in the vicinity of the Gulf Stream. A maritime sounding from the GALE data region during Intensive Observation Period 2 (IOP 2) is used to define the initial vertical structure of the temperature and humidity fields. The sensitivity of the MABL to two sea surface temperature (SST) patterns is tested. One is a relatively smooth analysis that is typical of those used by research and operational models applied on the synoptic scale and mesoscale. The other is based on the experimental 14 km high-resolution analysis of NOAA. In addition, other simulations are used to determine the sensitivity of the MABL response to physical factors such as surface moisture fluxes, latent heating, and the sea-surface roughness. These studies have two purposes: one is to provide a better understanding of the three- dimensional MABL response to a realistic SST pattern; the other is to isolate the mesoscale circulations produced by this differential thermal forcing so that their interaction with other processes, such as cyclogenesis, can be inferred in real-data simulations. The results of simulations using the two SST analyses are quite different. For example, the MABL front that develops near the north wall of the Gulf Stream is much stronger with the high-resolution analysis. Horizontal temperature gradients below 950 mb are 2–3 times larger, horizontal velocities near the surface are in excess of 7 m s−1 instead of ∼2 m s−1, and the vertical velocity patterns showed significantly different spatial characteristics and amplitudes. In both simulations, responses to the surface forcing extended upward to about 800 mb. In the experiment with the high-resolution SST analysis, a moderately strong mesoscale circulation was produced in the MABL within 12 h. Additional factors found to be important contributors to the MABL response are latent heat release in the lower atmosphere and sea-surface fluxes of moisture. The enhancement of the heat and moisture fluxes associated with the higher winds in the vicinity of the MABL front also significantly contributes to the amplitude of the circulation.