The Use of Pre-Storm Boundary-Layer Baroclinicity in Determining and Operationally Implementing the Atlantic Surface Cyclone Intensification Index

The lateral motion of the Gulf Stream off the eastern seaboard of the United States during the winter season can act to dramatically enhance the low-level baroclinicity within the coastal zone during periods of offshore cold advection. The ralative close proximity of the Gulf Stream current off the mid-Atlantic coast can result in the rapid and intense destabilization of the marine atmospheric boundary layer directly above and shoreward of the Gulf Stream within this region. This airmass modification period often precedes either wintertime coastal cyclogenesis or the cyclonic re-development of existing mid-latitude cyclones. A climatological study investigating the relationship between the severity of the pre-storm, cold advection period and subsequent cyclogenic intensification was undertaken by Cione et al. in 1993. Findings from this study illustrate that the thermal structure of the continental airmass as well as the position of the Gulf Stream front relative to land during the pre-storm period (i.e., 24–48 h prior to the initial cyclonic intensification) are linked to the observed rate of surface cyclonic deepening for storms that either advected into or initially developed within the Carolina-southeast Virginia offshore coastal zone. It is a major objective of this research to test the potential operational utility of this pre-storm low level baroclinic linkage to subsequent cyclogenesis in an actual National Weather Service (NWS) coastal winter storm forecast setting.The ability to produce coastal surface cyclone intensity forecasts recently became available to North Carolina State University researchers and NWS forecasters. This statistical forecast guidance utilizes regression relationships derived from a nine-season (January 1982–April 1990), 116-storm study conducted previously. During the period between February 1994 and February 1996, the Atlantic Surface Cyclone Intensification Index (ASCII) was successfully implemented in an operational setting by the NWS at the Raleigh-Durham (RAH) forecast office for 10 winter storms. Analysis of these ASCII forecasts will be presented.     

Numerical simulation of January 28 cold air outbreak during GALE part II: The mesoscale circulation and marine boundary layer

A two-dimensional (2-D) mesoscale numerical model is applied to simulate the January 28 cold-air outbreak over the Gulf Stream region during the Intensive Observation Period-2 (IOP-2) of the 1986 Genesis of Atlantic Lows Experiment (GALE). The model utilizes a turbulence closure which involves the turbulent kinetic energy (TKE) and dissipation () equations and combines the level 2.5 formulations of Mellor and Yamada (1982) for better determination of the eddy Prandtl number.The modeled marine boundary layer (MBL) is in good agreement with the observations (Wayland and Raman, 1989) showing a low-level jet west of the Gulf Stream warm core and a constrained boundary layer due to the middle-level (2–4.5 km) stable layer. The MBL-induced single cloud and rain band first appears east of the Gulf Stream boundary, and then moves offshore at the speed of the circulation front. The front, however, moves slightly slower than the ambient flow. Removal of the tropopause does not influence the low-level circulation and the movement of the front. The speed of the front is slightly larger in the baroclinic downshear flow than in the barotropic flow. The results also indicate that the observed high cloud streets propagating downwind of the Gulf Stream may be related to upper-level baroclinic lee waves triggered by an elevated density mountain. The density mountain waves, however, become evanescent as the baroclinity (which gives a larger Scorer parameter) is removed.The modeled 2-D circulation systems are found to be sensitive to differing eddy Prandtl numbers, in contrast to the 1-D model results presented in Part I. Sensitivities become increasingly important as the clouds begin to interact with the MBL. A constant eddy Prandtl number of unity produces a more slantwise convection compared to that by the level 2.5 case. Cloud development is stronger in slantwise convection than in upright convection. The fastest development of clouds can be explained in terms of the conditional symmetric instability (CSI), which begins as the MBL baroclinity becomes sufficiently large.     

Mean and turbulent structure of a baroclinic marine boundary layer during the 28 January 1986 cold-air outbreak (GALE 86)

Aircraft (NCAR Electra), ship (R/V Cape Hatteras), buoy (NCSU Buoy 2) and satellite (NOAA-7 and 9) measurements have been used to observe the structure of the Marine Boundary Layer (MBL) offshore of Wilmington, North Carolina, during the intense cold-air outbreak of 28 January, 1986, as part of the Genesis of Atlantic Lows Experiment (GALE). Air mass modification processes, driven primarily by the surface turbulent latent and sensible heat fluxes, caused the overlying air mass to warm and moisten as it advected over the warmer waters of the eastern United States continental shelf. Maximum observed total (latent + sensible) heat flux was 1045W/m² (at a height of 49 m) over the core of the Gulf Stream. Heat flux values decreased both east and west of this region, primarily in response to changes in the air-sea temperature difference.MBL height increased steadily in the offshore direction in response to increasing convection. The turbulent structure showed a buoyancy-dominated MBL between 0.1 z/h and 0.8 z/h; whereas shear was important above and below this level, vertical transport of kinetic energy (KE) was dominant as a source term only above 0.8 z/h. The normalized turbulent kinetic energy (TKE) budgets observed at different offshore locations showed general agreement at different flight levels. Thus the findings support the validity of the similarity relations under intense convective conditions.     

Structure of the marine atmospheric boundary layer during two cold air outbreaks of varying intensities: GALE 86

Aircraft, surface, upper air and satellite measurements have been used to observe the evolution and growth of the convective Marine Atmospheric Boundary Layer (MABL) offshore of North Carolina in close proximity to the Gulf Stream, during the intense cold air outbreak of 28 January 1986 and the moderate event of 12 February 1986, as part of the Genesis of Atlantic Lows Experiment (GALE). Air mass modification processes, driven primarily by the ocean-atmosphere exchanges of surface turbulent sensible and latent heat fluxes, caused the overlying air mass to warm and moisten as it advected over the warmer waters of the eastern United States continental shelf. Maximum observed near-surface total heat fluxes were 1045 and 811 W·m^–2 over the core of the Gulf Stream, for 28 January and 12 February 1986, respectively. The observed changes in the overlying air mass occurred almost instantaneously as the ambient flow traversed different underlying SST conditions.The turbulent structure showed a buoyancy-dominated MABL below approximately 0.8z/h. However, shear was also observed to be an important production term above 0.8z/h and below 0.1z/h for the 28 January 1986 event. Dissipation of turbulent kinetic energy was the dominant destruction term in the budgets, but vertical transport of energy was a strong contributor below 0.5z/h, above which this term became a source of turbulent energy. Additionally, the normalized standard deviations of the horizontal velocity components showed a near-equal contribution to the turbulence, while the vertical velocity components displayed the characteristic mid-layer maximum profile observed for a convective, well-mixed boundary layer.     

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.     

Impact of cyclone Nilam on tropical lower atmospheric dynamics

A deep depression formed over the Bay of Bengal on 28 October 2012, and developed into a cyclonic storm. After landfall near the south coast of Chennai, cyclone Nilam moved north-northwestwards. Coordinated experiments were conducted from the Indian stations of Gadanki(13.5?N, 79.2?E) and Hyderabad(17.4?N, 78.5?E) to study the modification of gravity-wave activity and turbulence by cyclone Nilam, using GPS radiosonde and mesosphere–stratosphere–troposphere radar data. The horizontal velocities underwent large changes during the closest approach of the storm to the experimental sites. Hodograph analysis revealed that inertia gravity waves(IGWs) associated with the cyclone changed their directions from northeast(control time) to northwest following the path of the cyclone. The momentum flux of IGWs and short-period gravity waves(1–8 h) enhanced prior to, and during, the passage of the storm(±0.05 m2s-2and ±0.3 m2s-2, respectively), compared to the flux after its passage. The corresponding body forces underwent similar changes, with values ranging between ±2–4m s-1d-1and ±12–15 m s-1d-1. The turbulence refractivity structure constant(C2n) showed large values below 10 km before the passage of the cyclone when humidity in the region was very high. Turbulence and humidity reduced during the passage of the storm when a turbulent layer at ～17 km became more intense. Turbulence in the lower troposphere and near the tropopause became weak after the passage of the cyclone.     

Mesoscale Analysis of a Carolina Coastal Front

During the Intensive Observation Period (IOP) 7 (22 February 1986) of the Genesis of Atlantic Lows Experiment a persistent coastal front was observed along the Carolina coast in the eastern United States. An intensive baroclinic zone, associated with the cold air damming to the east of the Appalachian Mountains, and the warm marine atmospheric boundary layer over the Gulf Stream, resulted in a northeasterly low-level geostrophic wind maximum near the coast.Two convergence zones were observed, one near the shore and the other near the western edge of the Gulf Stream. The convergence zone near the coastline was relatively weaker than that near the Gulf Stream. The differential surface thermal forcing caused enhanced convergence associated with the frontogenesis, and a meso-low was observed over the offshore front. The terms in the frontogenesis equation are estimated, and the diabatic term is found to be larger than the frontogenetic confluence term along the shore.     

A Case Study of Satellite Synthetic Aperture Radar Signatures of Spatially Evolving Atmospheric Convection over the Western Atlantic Ocean

A case study of a particularly intense cold air outbreak over the northAtlantic Ocean extending from the northeast coast of the UnitedStates to the Gulf Stream is described. A RADARSAT satellite synthetic apertureradar (SAR) image of this outbreak dramatically illustrates the spatialevolution of convection. Nearly coincident images from the National Oceanic and Atmospheric Administration's Advanced Very HighResolution Radiometer are used to compare many interesting features.In addition, National Weather Service rawinsonde data, National Data Buoy Center buoy data, and Woods Hole Oceanographic Institute Coastal Mixing and Optics mooring data arepresented. We use these data to help describe the spatial evolution of the atmospheric boundary-layer processes involved in this outbreak.Rows of cellular convective clouds begin to appear some distance offshore and then slowly increase in horizontal diameter and wavelength in the downwind direction, with a subsequent jump in cloud diameter downwind of the Gulf Stream North Wall (GSNW). The SAR image shows a similar evolution of sea-surface footprints of these boundary-layer features. This change in boundary-layer structure is attributed to corresponding changes in static stability. About 300 km south of the GSNW in the SAR image, an even larger jump in cell diameter appears and the cells becomenon-uniform with bright crescents and filled semi-circles on thedownwind sides of the cells. These are believed to be surface effectsof gust fronts induced by the mesoscale cellular convection and enhanced by the overall northwesterly flow.     

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     

“0801南方雪灾”与同期蒙古高压中期活动的关系

用1951—2008年58 a 1月10日—2月2日1 000 hPa高度场逐日NCEP/NCAR再分析格点资料,求得逐日蒙古高压的强度P和中心位置λc、c指数,用其对＂0801南方雪灾＂期间蒙古高压的中期演变过程进行统计分析。结果表明,2008年1月10日—2月2日蒙古高压强度P和中心所在纬度φc出现了连续4次振荡,它们与我国南方降温、降水振荡过程准同步。由P、（λc,c）给出的综合动态图上蒙古高压4次活动过程也很明显地与降温、降水中期过程一一对应。分析表明2008年1月10日—2月2日蒙古高压的这一中期演变特征,是1951年以来仅有的一次。因此,可以认为,在充沛水汽供应条件下蒙古高压强而连续的爆发是导致＂0801南方雪灾＂产生的直接环流成因。     

Fine-Scale Characteristics of Temperature, Wind, and Turbulence in the Lower Atmosphere (0–1,300 m) Over the South Peruvian Coast

We report results of preliminary high-resolution in situ atmospheric measurements through the boundary layer and lower atmosphere over the southern coast of Perú. This region of the coast is of particular interest because it lies adjacent to the northern coastal edge of the sub-tropical south-eastern Pacific, a very large area of ocean having a persistent stratus deck located just below the marine boundary layer (MBL) inversion. Typically, the boundary layer in this region during winter is topped by a quasi-permanent, well-defined, and very large temperature gradient. The data presented herein examine fine-scale details of the coastal atmosphere at a point where the edge of this MBL extends over the coastline as a result of persistent onshore flow. Atmospheric data were gathered using a recently-developed in-house constructed, GPS-controlled, micro-autonomous-vehicle aircraft (the DataHawk). Measured quantities include high-resolution profiles of temperature, wind, and turbulence structure from the surface to 1,300 m.     

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.     

北京地区一次强沙尘暴过程的大气边界层结构和湍流通量输送特征

利用中国科学院大气物理研究所北京325 m气象塔的风速和温度平均场观测资料和湍流资料,以及北京市气象台地面常规气象资料和逐日08:00和20:00(北京时间)的探空资料,分析了2002年3月18～22日沙尘暴过境前后北京城市边界层结构特征和湍流输送特征,结果表明:1)在沙尘暴爆发前,边界层中水平风速一直较小;气温较高,大气层结稳定,在边界层上部有强大的逆温层.随着冷锋过境,沙尘暴爆发,边界层中水平风速和平均湍流速度急剧增强;温度也突然变化,先迅速增强后又持续下降,逆温层迅速被破坏.2)沙尘暴初期,280 m上为系统性上升气流,而47和120 m则为系统性的下沉气流.随着沙尘暴爆发,湍流动能、向下传输的动量以及向上传输的感热也迅速增大,并且120 m高度的湍能、动量通量以及感热通量明显高于47和280 m,这与北京的局地环流有关.3)本次沙尘暴过程中,120和47 m层的摩擦速度都明显超过了北京的临界摩擦速度,表明局地起沙也是本次沙尘暴过程中北京沙尘的一个重要沙源.     

Tropical–Mid-Latitude Interactions: Case Study of an Inland-Penetrating Atmospheric River During a Major Winter Storm Over North America

ABSTRACT

A detailed analysis is performed on an inland-penetrating atmospheric river (AR) driven by and coupled to a Colorado cyclone in the first week of February 2016. This winter weather system was initiated by a trough of low pressure moving across the Rocky Mountains from the California coast. The low-level jet ahead of the trough was capable of extracting water vapour from the Gulf of Mexico to feed a cyclone on the lee side of the Rocky Mountaains, and the jet stream eventually transformed into a powerful AR. The warm, moist flow from the south produced a narrow band of heavy precipitation along the major axis of the AR across the central and eastern United States and generated significant freezing rain in parts of the northeastern United States and eastern Canada as the AR flowed over the warm front. It is suggested that, in an operational weather forecasting and warning environment, ARs can be easily identified by using the vertically integrated horizontal water vapour transport, and the major AR contribution to heavy precipitation can be estimated from the horizontal moisture convergence. It is demonstrated that the AR analysis in this case can assist operational meteorologists in understanding and conceptualizing winter storm development and the associated high-impact weather pattern. The operational predictability of this winter storm and its possible teleconnection with the Madden–Julian Oscillation (MJO) are also investigated. Our lagged composite analysis reveals that a statistically significant increase in water vapour transport from the Gulf of Mexico over the North American continent could occur about 10–20 days after the MJO-related convection anomaly reaches the tropical Indian Ocean.     

The possible role of Brazilian promontory in Little Ice Age

The Gulf Stream, one of the strongest currents in the world, transports approximately 31 Sv of water (Kelly and Gille, 1990, Baringer and Larsen, 2001, Leaman et al., 1995) and 1.3 × 10¹⁵ W (Larsen, 1992) of heat into the Atlantic Ocean, and warms the vast European continent. Thus any change of the Gulf Stream could lead to the climate change in the European continent, and even worldwide (Bryden et al., 2005). Past studies have revealed a diminished Gulf Stream and oceanic heat transport that was possibly associated with a southward migration of intertropical convergence zone (ITCZ) and may have contributed to Little Ice Age (AD ∼1200 to 1850) in the North Atlantic (Lund et al., 2006). However, the causations of the Gulf Stream weakening due to the southward migration of the ITCZ remain uncertain. Here we use satellite observation data and employ a model (oceanic general circulation model – OGCM) to demonstrate that the Brazilian promontory in the east coast of South America may have played a crucial role in allocating the equatorial currents, while the mean position of the equatorial currents migrates between northern and southern hemisphere in the Atlantic Ocean. Northward migrations of the equatorial currents in the Atlantic Ocean have little influence on the Gulf Stream. Nevertheless, southward migrations, especially abrupt large southward migrations of the equatorial currents, can lead to the increase of the Brazil Current and the significant decrease of the North Brazil Current, in turn the weakening of the Gulf Stream. The results from the model simulations suggest the mean position of the equatorial currents in the Atlantic Ocean shifted at least 180–260 km southwards of its present-day position during the Little Ice Age based on the calculations of simple linear equations and the OGCM simulations.     

北太平洋风暴轴的三维空间结构

文中利用最新的0.5°×0.5°分辨率QuikSCAT(QuikBird Satellite Microwave Scatterometer Sea Winds Data)海面风场资料、NCEP(National Center for Environmental Prediction)的10 m高度风场资料和全球客观再分析资料,对1999-2005年冬季(1月)和夏季(7月)北太平洋风暴轴的三维空间结构进行了分析,发现冬季北太平洋风暴轴的强度较强,呈明显的纬向拉伸带状分布特征,位置偏南.夏季北太平洋风暴轴的强度较弱,位置偏北.根据不同高度上位势高度方差的水平分布特征,绘制了北太平洋风暴轴的三维结构示意图.利用高分辨率QuikSCAT资料对风暴轴特征的刻画更为细致,不但验证了Nakamu-ra在南大洋发现的双风暴轴现象,而且还发现在北太平洋和北大西洋下层分别存在"副热带风暴轴"和"副极地风暴轴"两个风暴轴.对1999-2005年冬季北太平洋气旋和反气旋的移动路径进行的统计分析,为北太平洋"双风暴轴"的存在提供了强有力的证据.     

Three-Dimensional Structure and Low-Level Features of the North Pacific Storm Track from 1999 to 2005

A storm track is a region in which synoptic eddy activities are statistically most prevalent and intense. At daily weather charts, it roughly corresponds to the mean trajectories of cyclones and anticyclones. In this paper, the recent QuikSCAT （Quick Scatterometer） satellite sea winds data with a 0.5°×0.5° horizontal resolution, and the NCEP （National Centers for Environmental Prediction） 10-m height Gaussian grid wind data and pressure-level reanalysis data, are employed to document the spatial structure of the North Pacific storm track in winter （January） and summer （July） from 1999 to 2005. The results show that in winter the North Pacific storm track is stronger, and is located in lower latitudes with a distinct zonal distribution. In summer, it is weaker, and is located in higher latitudes. Based on the horizontal distributions of geopotential height variance at various levels, three-dimensional schematic diagrams of the North Pacific storm track in winter and summer are extracted and presented. Analyses of the QuikSCAT wind data indicate that this dataset can depict the low-level storm track features in detail. The double storm tracks over the Southern Oceans found by Nakamura and Shimpo are confirmed. More significantly, two new pairs of low-level storm tracks over the North Pacific and the North Atlantic are identified by using this high-resolution dataset. The pair over the North Pacific is focused in this paper, and is named as the ＂subtropical storm track＂ and the ＂subpolar storm track＂, respectively. Moreover, statistical analyses of cyclone and anticyclone trajectories in the winters of 1999 to 2005 reveal as well the existence of the low-level double storm tracks over the North Pacific.     

The variable link between PNA and NAO in observations and in multi-century CGCM simulations

The link between the Pacific/North American pattern (PNA) and the North Atlantic Oscillation (NAO) is investigated in reanalysis data (NCEP, ERA40) and multi-century CGCM runs for present day climate using three versions of the ECHAM model. PNA and NAO patterns and indices are determined via rotated principal component analysis on monthly mean 500?hPa geopotential height fields using the varimax criteria. On average, the multi-century CGCM simulations show a significant anti-correlation between PNA and NAO. Further, multi-decadal periods with significantly enhanced (high anti-correlation, active phase) or weakened (low correlations, inactive phase) coupling are found in all CGCMs. In the simulated active phases, the storm track activity near Newfoundland has a stronger link with the PNA variability than during the inactive phases. On average, the reanalysis datasets show no significant anti-correlation between PNA and NAO indices, but during the sub-period 1973?C1994 a significant anti-correlation is detected, suggesting that the present climate could correspond to an inactive period as detected in the CGCMs. An analysis of possible physical mechanisms suggests that the link between the patterns is established by the baroclinic waves forming the North Atlantic storm track. The geopotential height anomalies associated with negative PNA phases induce an increased advection of warm and moist air from the Gulf of Mexico and cold air from Canada. Both types of advection contribute to increase baroclinicity over eastern North America and also to increase the low level latent heat content of the warm air masses. Thus, growth conditions for eddies at the entrance of the North Atlantic storm track are enhanced. Considering the average temporal development during winter for the CGCM, results show an enhanced Newfoundland storm track maximum in the early winter for negative PNA, followed by a downstream enhancement of the Atlantic storm track in the subsequent months. In active (passive) phases, this seasonal development is enhanced (suppressed). As the storm track over the central and eastern Atlantic is closely related to the NAO variability, this development can be explained by the shift of the NAO index to more positive values.     

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.     

Extratropical cyclone variability in the Northern Hemisphere winter from the NCEP/NCAR reanalysis data

 The winter climatology of Northern Hemisphere cyclone activity was derived from 6-hourly NCEP/NCAR reanalysis data for the period from 1958 to 1999, using software which provides improved accuracy in cyclone identification in comparison to numerical tracking schemes. Cyclone characteristics over the Kuroshio and Gulfstream are very different to those over continental North America and the Arctic. Analysis of Northern Hemisphere cyclones shows secular and decadal-scale changes in cyclone frequency, intensity, lifetime and deepening rates. The western Pacific and Atlantic are characterized by an increase in cyclone intensity and deepening during the 42-year period, although the eastern Pacific and continental North America demonstrate opposite tendencies in most cyclone characteristics. There is an increase of the number of cyclones in the Arctic and in the western Pacific and a downward tendency over the Gulf Stream and subpolar Pacific. Decadal scale variability in cyclone activity over the Atlantic and Pacific exhibits south-north dipole-like patterns. Atlantic and Pacific cyclone activity associated with the NAO and PNA is analyzed. Atlantic cyclone frequency demonstrates a high correlation with NAO and reflects the NAO shift in the mid 1970s, associated with considerable changes in European storm tracks. The PNA is largely linked to the eastern Pacific cyclone frequencies, and controls cyclone activity over the Gulf region and the North American coast during the last two decades. Assessment of the accuracy of the results and comparison with those derived using numerical algorithms, shows that biases inherent in numerical procedures are not negligible. Received: 7 July 2000 / Accepted: 30 November 2000