One-dimensional baroclinically unstable waves on the Gulf Stream potential vorticity gradient near Cape Hatteras

Application of linear baroclinic instability theory to the observed distributions of velocity, stratification, and potential vorticity in the Gulf Stream near 74° W is successful in predicting the time and length scales of the most rapidly growing disturbances. A continuously-stratified, one-dimensional model with realistic bottom slope predicts propagation speeds of 10–50 cm s⁻¹ associated with two regimes of rapid temporal growth centered at periods of 28 days and 5–7 days. This prediction is consistent with observations of the propagation and growth of Gulf Stream meanders derived from inverted echo sounder measurements in this region. The instability model also predicts that for realistic bottom slopes the baroclinic energy transfer should be weakly negative (eddy-to-mean) in deep water, but for low-frequency waves should change to significant positive (mean-to-eddy) transfer above depths of 1500 m, consistent with observations.     

A numerical modeling study of the marine boundary layer over the Gulf Stream during cold air advection

A two-dimensional mesoscale model has been developed to simulate the air flow over the Gulf Stream area where typically large gradients in surface temperature exist in the winter. Numerical simulations show that the magnitude and the maximum height of the mesoscale circulation that develops downwind of the Gulf Stream depends on both the initial geostrophic wind and the large-scale moisture. As expected, a highly convective Planetary Boundary Layer (PBL) develops over this area and it was found that the Gulf Stream plays an important role in generating the strong upward heat fluxes causing a farther seaward penetration as cold air advection takes place. Numerical results agree well with the observed surface fluxes of momentum and heat and the mesoscale variation of vertical velocities obtained using Doppler Radars for a typical cold air outbreak. Precipitation pattern predicted by the numerical model is also in agreement with the observations during the Genesis of Atlantic Lows Experiment (GALE).List of Symbols u east-west velocity [m s^–1] - v north-south velocity [m s^–1] - vertical velocity in coordinate [m s^–1] - w vertical velocity inz coordinate [m s^–1] - gq potential temperature [K] - q moisture [kg kg^–1] - scaled pressure [J kg^–1 K^–1] - U _g the east-south component of geostrophic wind [m s^–1] - V _g the north-south component of geostrophic wind [m s^–1] - vertical coordinate following terrain - x east-west spatial coordinate [m] - y north-south spatial coordinate [m] - z vertical spatial coordinate [m] - t time coordinate [s] - g gravity [m² s^–1] - E terrain height [m] - H total height considered in the model [m] - q _s saturated moisture [kg kg^–1] - p pressure [mb] - p ₀₀ reference pressure [mb] - P precipitation [kg m^–2] - vertical lapse rate for potential temperature [K km^–1] - L latent heat of condensation [J kg^–1] - C _p specific heat at constant pressure [J kg^–1 K^–1] - R gas constant for dry air [J kg^–1 K^–1] - R _v gas constant for water vapor [J kg^–1 K^–1] - f Coriolis parameter (2 sin ) [s^–1] - angular velocity of the earth [s^–1] - latitude [^o] - K _H horizontal eddy exchange coefficient [m² s^–1] - t integration time interval [s] - x grid interval distance inx coordinate [m] - y grid interval distance iny coordinate [m] - adjustable coefficient inK _H - subgrid momentum flux [m² s^–2] - subgrid potential temperature flux [m K s^–1] - subgrid moisture flux [m kg kg^–1 s^–1] - u _* friction velocity [m s^–1] - _* subgrid flux temperature [K] - q _* subgrid flux moisture [kg kg^–1] - w _* subgrid convective velocity [m s^–1] - z ₀ surface roughness [m] - L Monin stability length [m] - _s surface potential temperature [K] - k von Karman's constant (0.4) - v air kinematic viscosity coefficient [m² s^–1] - K _M subgrid vertical eddy exchange coefficient for momentum [m² s^–1] - K subgrid vertical eddy exchange coefficient for heat [m² s^–1] - K _q subgrid vertical eddy exchange coefficient for moisture [m² s^–1] - z _i the height of PBL [m] - h _s the height of surface layer [m]     

Simulated response to inter-annual SST variations in the Gulf Stream region

Recent studies show that mid-latitude SST variations over the Kuroshio-Oyashio Extension influence the atmospheric circulation. However, the impact of variations in SST in the Gulf Stream region on the atmosphere has been less studied. Understanding the atmospheric response to such variability can improve the climate predictability in the North Atlantic Sector. Here we use a relatively high resolution (～1°) Atmospheric General Circulation Model to investigate the mechanisms linking observed 5-year low-pass filtered SST variability in the Gulf Stream region and atmospheric variability, with focus on precipitation. Our results indicate that up to 70 % of local convective precipitation variability on these timescales can be explained by Gulf Stream SST variations. In this region, SST and convective precipitation are strongly correlated in both summer (r = 0.73) and winter (r = 0.55). A sensitivity experiment with a prescribed local warm SST anomaly in the Gulf Stream region confirms that local SST drives most of the precipitation variability over the Gulf Stream. Increased evaporation connected to the anomalous warm SST plays a crucial role in both seasons. In summer there is an enhanced local SLP minimum, a concentrated band of low level convergence, deep upward motion and enhanced precipitation. In winter we also get enhanced precipitation, but a direct connection to deep vertical upward motion is not found. Nearly all of the anomalous precipitation in winter is connected to passing atmospheric fronts. In summer the connection between precipitation and atmospheric fronts is weaker, but still important.     

Fresh water balance of the Gulf Stream system in a regional model study

 We investigate the dependence of surface fresh water fluxes in the Gulf Stream and North Atlantic Current (NAC) area on the position of the stream axis which is not well represented in most ocean models. To correct this shortcoming, strong unrealistic surface fresh water fluxes have to be applied that lead to an incorrect salt balance of the current system. The unrealistic surface fluxes required by the oceanic component may force flux adjustments and may cause fictitious long-term variability in coupled climate models. To identify the important points in the correct representation of the salt balance of the Gulf Stream a regional model of the northwestern part of the subtropical gyre has been set up. Sensitivity studies are made where the westward flow north of the Gulf Stream and its properties are varied. Increasing westward volume transport leads to a southward migration of the Gulf Stream separation point along the American coast. The salinity of the inflow is essential for realistic surface fresh water fluxes and the water mass distribution. The subpolar–subtropical connection is important in two ways: The deep dense flow from the deep water mass formation areas sets up the cyclonic circulation cell north of the Gulf Stream. The surface and mid depth flow of fresh water collected at high northern latitudes is mixed into the Gulf Stream and compensates for the net evaporation at the surface. Received: 19 September 2000 / Accepted: 5 February 2001     

Observations of atmospheric boundary layer evolution above the Gulf Stream frontal zone

Ship borne measurements of atmospheric boundary layer (ABL) parameters, sea-surface temperature and radar signals are analyzed to reveal the effects of the ABL transformation above the Gulf Stream temperature frontal zone. It was found that local changes in vertical gradients of wind speed and air temperature are well correlated with sub-mesoscale (~ 10 km) sea surface temperature variations. These effects are accompanied by appropriate variations in surface wind stresses that were identified from microwave backscatter.For steady atmospheric conditions the same effects were observed on spatial scales of 100 km, demonstrating positive radar signal contrast of the Gulf Stream warm waters with respect to surrounding Sargasso sea and shelf water areas. A simplified model of the ABL, accounting for an effect of spatial inhomogeneity by introducing an internal boundary layer, is used to analyze field observations. The model is able to reproduce both sub-mesoscale and mesoscale ABL evolution.     

An analysis of ozone measurements at Cerro Tololo (30°S, 70°W, 2200 m.a.s.l.) in Chile

Increases in tropospheric ozone (O₃) abundance are likely to take place in the near future in the populous and rapidly developing countries in the tropics and subtropics. An accurate evaluation of the future impact of increasing industrial activities in tropical and subtropical areas requires knowledge of the background levels of ozone. New ozone monitoring stations have been installed at several sites by the World Meteorological Organization (WMO) since the mid‐90s. We analyze ozone data collected during two years since April 1996 at Cerro Tololo (30°S, 70°W, 2200 m.a.s.l.) some 50 km east from the city of La Serena. In this paper, we describe some of the atmospheric chemistry and meteorology that characterizes the Tololo site. The data show a seasonal variation with maximum mixing ratios in late winter and spring and minimum mixing ratios in late summer and early fall. These variations are most likely associated with the large‐scale subsidence of the Hadley circulation and the location of the subtropical jet stream (STJ). Also, there is a diurnal variation that is probably partly associated with a mountain wind flow which is strongest in late spring and summer months. No significant mixing with marine boundary layer air perturbed by anthropogenic activities is apparent from the data. We find the Cerro Tololo site to be generally representative for background conditions of free‐tropospheric air in the subtropics of the Southern Hemisphere. This work is done within the framework of a larger effort recently started by several Chilean institutions in cooperation with research centers abroad.     

大气环流对北大西洋湾流区海温“再现”的响应

利用全球海洋—大气快速耦合模式（Fast Ocean-Atmosphere Model,FOAM）,采用模式中的初值方法,研究了湾流区海温再现过程及其对北半球大气环流和气候的影响。FOAM模式很好地模拟了北大西洋湾流区的海温＂再现＂过程,模式中海面热通量异常与SST异常表现出不同步的响应特征。海面热通量异常在初冬季节达到最大值,而SST异常滞后,在冬季晚期达到最大值,从而在初冬和晚冬对北半球大气环流造成不同的影响。初冬季节北半球大气环流主要受海洋热通量异常的强迫,在北大西洋和北太平洋上空呈现相当正压的异常低压槽响应,北极地区为异常高压脊,类似北极涛动的负位相,可能造成欧洲南部和北非大陆气温偏高,亚洲大陆气温偏低。而晚冬季节北半球大气环流主要受SST异常的驱动,在北大西洋和北太平洋上空表现为相当正压的异常高压脊响应,北极地区为异常低压槽,类似北极涛动的正位相,可能造成欧洲南部和北非大陆气温偏低,亚洲大陆气温偏高,中国东部降水异常偏多30%左右。北太平洋大气环流的异常由北大西洋湾流区海洋热通量和SST异常强迫下游大气环流所激发,进一步通过Rossby驻波的能量频散东传至北太平洋而造成的。     

Interannual variation of Gulf Stream heat transport in a high-resolution model forced by reanalysis data

The variability present in a 1/6th degree Atlantic ocean simulation forced by analysed wind stress and heat flux over a 20-year period is investigated by means of heat transport diagnostics. A section is defined which follows the Gulf Stream and its seaward extension, and transport of heat across this section is analysed to reveal the physical mechanisms responsible for intergyre heat exchanges on a variety of time scales. Heat transport across another section that crosses the Gulf Stream is also diagnosed to reveal the temporal behaviour of the gyre circulation. The Ekman response to wind stress variations accounts for the annual cycle and much of the interannual variability in both measures. For the intergyre heat transports, cancellation by transient-mean flow terms leads to a very weak annual cycle. Transient eddies account for approximately half the total intergyre transport of 0.7 Petawatts. They also account for a significant fraction of the interannual variability, but separate experiments with repeated-annual-cycle forcing indicate that the transient eddy component of the heat transport variability is internally generated. Links between the intergyre transport, the wind-driven gyre circulation, the surface heat budget and the atmospheric North Atlantic Oscillation are discussed.     

Second order modeling of turbulent transport in the surface mixed layer

A model for the simultaneous transport of heat and water vapor is presented. In an effort to resolve the structure of the entrainment region at the inversion base, models are constructed so as to satisfy realizability as far as possible. Density anomaly and water vapor mixture fraction (specific humidity) are taken as the basic variables. Algebraic expressions for the third moments are derived from first principles, and contain no adjustable constants. Separate equations are carried for the dissipation of each variance, constructed to give rational behavior of all time scale ratios. New forms for relaxation and cross-dissipation terms are constructed in such a way as to guarantee realizability. We describe how realizability was used as a tool to construct these models. We present preliminary results without mean velocity gradients for a dry surface mixed layer leaving the land and starting over water, producing a stable internal humidity boundary layer, but with large fluxes of sensible heat and water vapor (local advection).Prepared for presentation at the 29th OHOLO Conference on Boundary Layer Structure — Modelling and Application to Air Pollution and Wind Energy, Zichron Ya'acov, Israel, March 25–28, 1984. Supported in part by the U.S. Office of Naval Research under the following programs: Physical Oceanography (Code 422PO), Power (Code 473); in part by the U.S. National Science Foundation under grant no. ATM 79-22006; and in part by the U.S. Air Force Geophysics Laboratory.     

The Gulf Stream pathway and the impacts of the eddy-driven abyssal circulation and the Deep Western Boundary Current

A hydrodynamic model of the subtropical Atlantic basin and the Intra-Americas Sea (9–47°N) is used to investigate the dynamics of Gulf Stream separation from the western boundary at Cape Hatteras and its mean pathway to the Grand Banks. The model has five isopycnal Lagrangian layers in the vertical and allows realistic boundary geometry, bathymetry, wind forcing, and a meridional overturning circulation (MOC), the latter specified via ports in the northern and southern boundaries. The northward upper ocean branch of the MOC (14 Sv) was always included but the southward Deep Western Boundary Current (DWBC) was excluded in some simulations, allowing investigation of the impacts of the DWBC and the eddy-driven mean abyssal circulation on Gulf Stream separation from the western boundary. The result is resolution dependent with the DWBC playing a crucial role in Gulf Stream separation at 1/16° resolution but with the eddy-driven abyssal circulation alone sufficient to obtain accurate separation at 1/32° resolution and a realistic pathway from Cape Hatteras to the Grand Banks with minimal DWBC impact except southeast of the Grand Banks. The separation from the western boundary is particularly sensitive to the strength of the eddy-driven abyssal circulation. Farther to the east, between 68°W and the Grand Banks, all of the 1/16° and 1/32° simulations with realistic topography (with or without a DWBC) gave similar generally realistic mean pathways with clear impacts of the topographically constrained eddy-driven abyssal circulation versus very unrealistic Gulf Stream pathways between Cape Hatteras and the Grand Banks from otherwise identical simulations run with a flat bottom, in reduced-gravity mode, or with 1/8° resolution and realistic topography. The model is realistic enough to allow detailed model-data comparisons and a detailed investigation of Gulf Stream dynamics. The corresponding linear solution with a Sverdrup interior and Munk viscous western boundary layers, including one from the northward branch of the MOC, yielded two unrealistic Gulf Stream pathways, a broad eastward pathway centered at the latitude of Cape Hatteras and a second wind plus MOC-driven pathway hugging the western boundary to the north. Thus, a high resolution model capable of simulating an inertial jet is required to obtain a single nonlinear Gulf Stream pathway as it separates from the coast. None of the simulations were sufficiently inertial to overcome the linear solution need for a boundary current north of Cape Hatteras without assistance from pathway advection by the abyssal circulation, even though the core speeds of the simulated currents were consistent with observations near separation. In the 1/16° simulation with no DWBC and a 1/32° simulation with high bottom friction and no DWBC the model Gulf Stream overshot the observed separation latitude. With abyssal current assistance the simulated (and the observed) mean Gulf Stream pathway between separation from the western boundary and 70°W agreed closely with a constant absolute vorticity (CAV) trajectory influenced by the angle of the coastline prior to separation. The key abyssal current crosses under the Gulf Stream at 68.5–69°W and advects the Gulf Stream pathway southward to the terminus of an escarpment in the continental slope. There the abyssal current crosses to deeper depths to conserve potential vorticity while passing under the downward-sloping thermocline of the stream and then immediately retroflects eastward onto the abyssal plain, preventing further southward pathway advection. Thus specific topographic features and feedback from the impact of the Gulf Stream on the abyssal current pathway determined the latitude of the stream at 68.5–69°W, a latitude verified by observations. The associated abyssal current was also verified by observations.     

The Gulf Stream pathway and the impacts of the eddy-driven abyssal circulation and the Deep Western Boundary Current

A hydrodynamic model of the subtropical Atlantic basin and the Intra-Americas Sea (9–47°N) is used to investigate the dynamics of Gulf Stream separation from the western boundary at Cape Hatteras and its mean pathway to the Grand Banks. The model has five isopycnal Lagrangian layers in the vertical and allows realistic boundary geometry, bathymetry, wind forcing, and a meridional overturning circulation (MOC), the latter specified via ports in the northern and southern boundaries. The northward upper ocean branch of the MOC (14 Sv) was always included but the southward Deep Western Boundary Current (DWBC) was excluded in some simulations, allowing investigation of the impacts of the DWBC and the eddy-driven mean abyssal circulation on Gulf Stream separation from the western boundary. The result is resolution dependent with the DWBC playing a crucial role in Gulf Stream separation at 1/16° resolution but with the eddy-driven abyssal circulation alone sufficient to obtain accurate separation at 1/32° resolution and a realistic pathway from Cape Hatteras to the Grand Banks with minimal DWBC impact except southeast of the Grand Banks. The separation from the western boundary is particularly sensitive to the strength of the eddy-driven abyssal circulation. Farther to the east, between 68°W and the Grand Banks, all of the 1/16° and 1/32° simulations with realistic topography (with or without a DWBC) gave similar generally realistic mean pathways with clear impacts of the topographically constrained eddy-driven abyssal circulation versus very unrealistic Gulf Stream pathways between Cape Hatteras and the Grand Banks from otherwise identical simulations run with a flat bottom, in reduced-gravity mode, or with 1/8° resolution and realistic topography. The model is realistic enough to allow detailed model-data comparisons and a detailed investigation of Gulf Stream dynamics. The corresponding linear solution with a Sverdrup interior and Munk viscous western boundary layers, including one from the northward branch of the MOC, yielded two unrealistic Gulf Stream pathways, a broad eastward pathway centered at the latitude of Cape Hatteras and a second wind plus MOC-driven pathway hugging the western boundary to the north. Thus, a high resolution model capable of simulating an inertial jet is required to obtain a single nonlinear Gulf Stream pathway as it separates from the coast. None of the simulations were sufficiently inertial to overcome the linear solution need for a boundary current north of Cape Hatteras without assistance from pathway advection by the abyssal circulation, even though the core speeds of the simulated currents were consistent with observations near separation. In the 1/16° simulation with no DWBC and a 1/32° simulation with high bottom friction and no DWBC the model Gulf Stream overshot the observed separation latitude. With abyssal current assistance the simulated (and the observed) mean Gulf Stream pathway between separation from the western boundary and ∼70°W agreed closely with a constant absolute vorticity (CAV) trajectory influenced by the angle of the coastline prior to separation. The key abyssal current crosses under the Gulf Stream at 68.5–69°W and advects the Gulf Stream pathway southward to the terminus of an escarpment in the continental slope. There the abyssal current crosses to deeper depths to conserve potential vorticity while passing under the downward-sloping thermocline of the stream and then immediately retroflects eastward onto the abyssal plain, preventing further southward pathway advection. Thus specific topographic features and feedback from the impact of the Gulf Stream on the abyssal current pathway determined the latitude of the stream at 68.5–69°W, a latitude verified by observations. The associated abyssal current was also verified by observations.     

Impact of 1/8° to 1/64° resolution on Gulf Stream model–data comparisons in basin-scale subtropical Atlantic Ocean models

We investigate the impact of 1/8°, 1/16°, 1/32°, and 1/64° ocean model resolution on model–data comparisons for the Gulf Stream system mainly between the Florida Straits and the Grand Banks. This includes mean flow and variability, the Gulf Stream pathway, the associated nonlinear recirculation gyres, the large-scale C-shape of the subtropical gyre and the abyssal circulation. A nonlinear isopycnal, free surface model covering the Atlantic from 9°N to 47°N or 51°N, including the Caribbean and Gulf of Mexico, and a similar 1/16° global model are used. The models are forced by winds and by a global thermohaline component via ports in the model boundaries. When calculated using realistic wind forcing and Atlantic model boundaries, linear simulations with Munk western boundary layers and a Sverdrup interior show two unrealistic mean Gulf Stream pathways between Cape Hatteras and the Grand Banks, one proceeding due east from Cape Hatteras and a second one continuing northward along the western boundary until forced eastward by the regional northern boundary. The northern pathway is augmented when a linear version of the upper ocean global thermohaline contribution to the Gulf Stream is added as a Munk western boundary layer. A major change is required to obtain a realistic pathway in nonlinear models. Resolution of 1/8° is eddy-resolving but mainly gives a wiggly version of the linear model Gulf Stream pathway and weak abyssal flows except for the deep western boundary current (DWBC) forced by ports in the model boundaries. All of the higher resolution simulations show major improvement over the linear and 1/8° nonlinear simulations. Additional major improvement is seen with the increase from 1/16° to 1/32° resolution and modest improvement with a further increase to 1/64°. The improvements include (1) realistic separation of the Gulf Stream from the coast at Cape Hatteras and a realistic Gulf Stream pathway between Cape Hatteras and the Grand Banks based on comparisons with Gulf Stream pathways from satellite IR and from GEOSAT and TOPEX/Poseidon altimetry (but 1/32° resolution was required for robust results), (2) realistic eastern and western nonlinear recirculation gyres (which contribute to the large-scale C-shape of the subtropical gyre) based on comparisons with mean surface dynamic height from the generalized digital environmental model (GDEM) oceanic climatology and from the pattern and amplitude of sea surface height (SSH) variability surrounding the eastern gyre as seen in TOPEX/Poseidon altimetry, (3) realistic upper ocean and DWBC transports based on several types of measurements, (4) patterns and amplitude of SSH variability which are generally realistic compared to TOPEX/Poseidon altimetry, but which vary from simulation to simulation for specific features and which are most realistic overall in the 1/64° simulation, (5) a basin wide explosion in the number and strength of mesoscale eddies (with warm core rings (WCRs) north of the Gulf Stream, the regional eddy features best observed by satellite IR), (6) realistic statistics for WCRs north of the Gulf Stream based on comparison to IR analyses (low at 1/16° resolution and most realistic at 1/64° resolution for mean population and rings generated/year; realistic ring diameters at all resolutions), and (7) realistic patterns and amplitude of abyssal eddy kinetic energy (EKE) in comparison to historical measurements from current meters.     

准地转Q矢量在2008年夏季内蒙古一次大雨过程中的应用

文章使用T213风场及温度场资料、6小时地面观测资料,对2008年7月11日前后内蒙古地区的一次降水过程中的Q矢量散度场进行了分析。结果发现:Q矢量散度的负值区对内蒙古地区的降水落区有着较好的对应关系。     

增温停滞背景下黑潮与湾流区域潜热通量年代际趋势变化差异及其成因分析

本文使用美国伍兹霍尔海洋研究所发布的客观分析海气通量项目数据集及日本海洋科学技术中心的Ishii次表层温盐数据,利用经验正交函数分析方法、小扰动展开、线性回归、海水热力学方程2010等方法,主要研究在增温停滞背景（1979～2000年,升温阶段;2001～2013年,停滞阶段）下,北半球两支西边界流区域即黑潮及其延伸区域（简称黑潮区域）和墨西哥湾流区域（简称湾流区域）海表潜热通量的年代际趋势转变和影响因子,以及内部热含量的年代际变化。结果表明,两支西边界流在增温停滞背景下都发生了年代际尺度的趋势反转,而反转的时间节点以及前后的反转趋势都不相同:黑潮区域潜热通量年代际趋势于2001年左右由正转负;而湾流区域潜热通量年代际趋势于1993年左右由负转正。其影响因子在前后阶段也有不同:通过影响海表饱和比湿进而影响海气比湿差,海表温度是影响黑潮区域全时间段以及湾流区域1993～2013年时间段潜热通量变化的主要因素;而风速通过直接的影响以及对空气湿度的影响也会对潜热通量变化产生间接影响,主要在湾流区域的1979～1992年时间段体现。黑潮及湾流区域0～1000 m海水热含量的年代际变化同样存在差异:黑潮区域表层热含量年代际变化同混合层一致;湾流区域表层热含量年代际变化同深层相异,而表层以下的变化较为一致;两个区域的深层热含量变化都体现了增温停滞的现象,黑潮区域可能存在下层至上层的影响;而湾流区域可能存在上层至下层的影响。黑潮与湾流区域表面的差异可以归结为海洋与大气因素的影响差异,而内部热含量年代际变化的垂直差异可能归结为两区域的结构差异。增温停滞对两区域的变化影响显著,而区域的变化可能存在对增温停滞的反馈。