North-south Pressure Gradient Research Articles

Characteristics of Coastal Low-Level Jets in the Boundary Layer of the Pearl River Estuary

It is well-known that coastal low-level jets (CLLJs) play an essential role in transporting heat, water vapor and pollutants. However, the CLLJ characteristics in the Pearl River Estuary have not been deeply revealed due to the lack of long-term observations. Based on the long-term observations from a wind lidar, we analyze the primary climatic characteristics of the CLLJs in the Pearl River Estuary and investigate their relationships with large-scale and local-scale synoptic systems. The results show that the CLLJs mainly appear during the flood season, with the most occurrence in May. The CLLJ occurrence during the flood season is mainly influenced by the large-scale north–south pressure gradient driven by the western Pacific subtropical high and terrestrial low-pressure systems. The occurrence of the CLLJs exhibits a distinct diurnal cycle with two different peaks in different seasons. One peak appears at nighttime, mainly during non-flood seasons. The other appears in the afternoon, mainly during the flood season. In the non-flood seasons, under the influence of cold air, the inertial oscillations triggered by the land–sea thermal contrast lead to CLLJ onset at nighttime in the Pearl River Estuary. During the flood season, the strong near-surface pressure gradient contributes to CLLJ onset in the afternoon, while the topography (blocking and passing) is more conducive to the occurrence of the CLLJs in the Pearl River Estuary. These findings reveal the formation mechanisms of the CLLJs over the Pearl River Estuary, thus providing a basis for further understanding the precipitation in the Pearl River Estuary and the occurrence of the CLLJs in other coastal areas with complex mountain ranges.

Modulation of daily PM2.5 concentrations over China in winter by large-scale circulation and climate change

Abstract. We use the United Kingdom Earth System Model, UKESM1, to investigate the influence of the winter large-scale circulation on daily concentrations of PM2.5 (particulate matter with an aerodynamic diameter of 2.5 µm or less) and their sensitivity to emissions over major populated regions of China over the period 1999–2019. We focus on the Yangtze River delta (YRD), where weak flow of cold, dry air from the north and weak inflow of maritime air are particularly conducive to air pollution. These provide favourable conditions for the accumulation of local pollution but limit the transport of air pollutants into the region from the north. Based on the dominant large-scale circulation, we construct a new index using the north–south pressure gradient and apply it to characterise PM2.5 concentrations over the region. We show that this index can effectively distinguish different levels of pollution over YRD and explain changes in PM2.5 sensitivity to emissions from local and surrounding regions. We then project future changes in PM2.5 concentrations using this index and find an increase in PM2.5 concentrations over the region due to climate change that is likely to partially offset the effect of emission control measures in the near-term future. To benefit from future emission reductions, more stringent emission controls are required to offset the effects of climate change.

Explaining Dynamics and Rapid Onset of the Somali Jet through Its Kinetic Energy Budget

Abstract The low-level Somali jet is the primary mechanism of moisture transport for the South Asian monsoon. It precedes monsoon onset over India and shares its key characteristic features such as rapid intensification and slower retreat during seasonal evolution. This study analyzes the kinetic energy (KE) budget of Somali jet region using high-spatiotemporal-resolution reanalysis (ERA5) to explain these key features. The KE budget reveals that in the Southern Hemisphere, the easterly flow that ultimately feeds the jet exhibits a conventional Ekman balance, with KE generation balanced by frictional dissipation. A unique “advective balance”—balance between KE generation in the northward flow and its advection emerges as the jet begins to form near the equatorial region. The fully formed Somali jet exhibits a three-way balance between KE generation, its advection, and dissipation. A nondimensional parameter–boundary layer local Rossby number (Ro) characterizes the transitions across these regimes. The large-Ro regime describes an advective balance under which KE-generating meridional winds become proportional to meridional pressure gradients yielding a nonlinear (quadratic) dependence of KE generation on pressure gradients. This nonlinear relation explains rapid onset of the jet as well as the asymmetry between rapid onset and slower retreat, leading us to propose a simple model for approximating the seasonal evolution of kinetic energy in the Somali jet given the evolution of pressure gradients. In summary, this work shows that Somali jet onset is closely tied to the seasonal evolution of Ro in the region where the advective boundary layer appears. Significance Statement The Somali jet is an important feature of the South Asian monsoon, contributing significantly to enhanced rainfall over the region. We study the dynamics of this jet by focusing on its kinetic energy (KE). Maintenance of the jet at different regions corresponds to different kinds of balances in the kinetic energy budget. A dimensionless parameter characterizes these different regimes of KE balance, which are captured at sufficiently high resolution. The rapid intensification of the jet can be explained as a nonlinear response of KE generation to the seasonal evolution of the north–south pressure gradient. These findings contribute to understanding of the jet and its nonlinear evolution, which is important for accurate representation and simulation of the Somali jet in climate models.

The Panama Low Level Jet: extension, annual cycle and modes of variation

The Panama Low Level Jet (PLLJ) is the inter-isthmic jet closest to the equator, whose activation modifies circulation, sea surface temperature and climate in most of the Eastern Tropical Pacific off Colombia. Through the analysis of 26 years (1992-2018) of sea wind data, the extent of the jet and its main frequencies of variability were determined, in addition to its relationship with phenomena of climatic variability. The area of direct influence of the jet was estimated to be 59295.34 km², being located between 4.63° and 9oN. The PLLJ is activated annually during the boreal summer (December-April) with intense intra-seasonal variability (with frequencies between 1 to 5 months) and inter annual perturbations associated with El Nino Southern Oscillation events. During warm ENSO events, the PLLJ activity last ~ 2 months more (from mid-November to the end of May) and its spatial extent increases southerly reaching ~ 2 ° N. Additionally, during warm ENSO years, there is an extemporaneous activation of PLLJ in the boreal summer, possibly linked to the intensification of the North Trade Winds over the Caribbean basin in the middle of the year and the increase in the north-south pressure gradient through the Panama isthmus during intense El Nino events. The entry of Caribbean winds in unusual seasons changes the climatology of the PLLJ and increasing its influence in the Eastern Tropical Pacific during the ENSO warm phases.

Influence of subtropical circulation systems on the changing El Niño-Indian summer monsoon relationship

Remote factors have a significant influence on the variability of Indian summer monsoon rainfall. The study investigates atmospheric conditions influenced by the El Niño episodes on the variable summer monsoon over India. Four composites of years were selected based on the occurrence / non-occurrence of El Niño and extreme monsoons over India. This study examines the upper tropospheric and lower tropospheric wind patterns, tropospheric temperature, and velocity potential for 1958 to 2019 using Japanese Reanalysis 55 (JRA 55) data. The regional meridional circulation established by the north-south pressure gradient is more susceptible to the monsoon condition over the Indian Subcontinent than the external factors. The spatial variability and intensity of Australian High in the southern hemisphere play a prominent role in the Indian summer monsoon characteristics. Subtropical circulations in both hemispheres influence monsoon conditions over India irrespective of El Niño condition. The tropical zonal circulation has appeared to be modulated by the El Niño Southern Oscillation (ENSO) condition over the tropical Pacific Ocean, thereby modify the monsoon circulation.

Climatology, variability, and trends in near‐surface wind speeds over the North Atlantic and Europe during 1979–2018 based on ERA5

AbstractThis study presents the monthly 10‐m wind speed climatology, decadal variability and possible trends in the North Atlantic and Europe from ERA5 reanalysis from 1979 to 2018 and investigates the physical reasons for the decadal variability. Additionally, temporal time series are examined in three locations: the central North Atlantic, Finland and Iberian Peninsula. The 40‐year mean and the 98th percentile wind speeds emphasize a distinct land‐sea contrast and a seasonal variation with the strongest winds over the ocean and during winter. The strongest winds and the highest variability are associated with the storm tracks and local wind phenomena such as the mistral. The extremeness of the winds is examined with an extreme wind factor (the 98th percentile divided by mean wind speeds) which in all months is higher in southern Europe than in northern Europe. Mostly no linear trends in 10‐m wind speeds are identified in the three locations but large annual and decadal variability is evident. The decadal 10‐m wind speeds were stronger than average in the 1990s in northern Europe and in the 1980s and 2010s in southern Europe. These decadal changes were largely explained by the positioning of the jet stream and storm tracks and the strength of the north–south pressure gradient in the North Atlantic. The 10‐m winds have a positive correlation with the North Atlantic Oscillation in the central North Atlantic and Finland on annual scales and during cold season months and a negative correlation in Iberian Peninsula mostly from July to March. The Atlantic Multi‐decadal Oscillation has a moderate negative correlation with the winds in the central North Atlantic but no correlation in Finland and Iberian Peninsula. Overall, our results emphasize that while linear trends in wind speeds may show a general long‐term trend, more information on the changes is obtained by analysing long‐term variability.

Shallow foehn on the northern leeside of Tianshan Mountains and its influence on atmospheric boundary layer over Urumqi, China — A climatological study

Abstract The climatology of shallow foehn, also known locally as elevated south-easterly gales (ESEGs), on the northern lee side of the central Tianshan Mountains is analysed using radiosonde data recorded over a period of 10 years at Urumqi, the capital of Xinjiang Uygur Autonomous Region in north-western China. ESEGs are a frequent weather phenomenon in the region, occurring on average 128.5 days/yr and most often in winter (47.0 days), particularly January (17.5 days), and least in summer (18 days). The vertical ESEG structures also undergo seasonal variations, with the height of the maximum ESEG wind and the base and top of the ESEG layer more elevated in the warm half (late spring through early fall) of the year than the cold half. However, the monthly mean maximum wind speeds, which vary around 10 m/s, appear to be independent of season. A typical synoptic pattern accompanying ESEGs is characterized by opposite air masses to the east (cold) and west (warm) of Xinjiang. A strong low-level west-east pressure gradient helps push cold air to enter Xinjiang from the east, which subsequently induces a north-south pressure gradient across the Tianshan Mountains. This regional pressure gradient combined with terrain channelling leads to the development of ESEGs. A comparison of boundary layer structure between days with/without ESEGs in winter shows that the ESEG days are accompanied by deeper (average of 900 m) and more intense (average 6 °C) inversion, but there is little difference in the cold air pool Froude Number between the two types of days.

Analyses of the winter low‐level jet over the southern Red Sea using the Weather Research and Forecasting model

AbstractThe three‐dimensional structure of the Red Sea Low‐Level Jet (RSLLJ) that occurs over the southern Red Sea during the winter season is examined using the Weather Research and Forecasting (WRF) model. The causes of the jet formation are investigated, and the simulations confirm that the jet is a terrain‐induced phenomenon. It initiates as a gap flow as the jet forms north of the strait of Bab el‐Mandab as a result of a hydraulic effect when a stably stratified cool layer is channeled through the mountainous gap at low levels over the strait into the southern Red Sea. The jet intensity is enhanced by the Red Sea trough over the southern Red Sea. The cool dense air mass south of the strait is supported by a generally persistent synoptic regime that includes a high‐pressure system over most of the eastern Arabian Peninsula and the Sudan low‐pressure system over Sudan. The jet extends more than 500 km northward to slightly north of 17°N from the strait around 12.5°N, with a width of 100–150 km over the southern Red Sea. The jet core lies at 500–600 m above sea level between 14°N and 16°N, with wind speeds exceeding 25 m/s. The north–south pressure gradient along the strait, as well as the depth of the stratified cool layer south of the strait, are directly responsible for the jet length and speed over the southern Red Sea, with stronger pressure gradients and deeper cool layers resulting in longer and stronger jets. Sea/land breezes also directly influence the spatial scale, intensity, and diurnal variation of the jet, with land breezes causing faster and more constricted jets at night and divergence due to sea breezes leading to relatively slower jets during the daytime.

Geostrophic Closure of the Zonally Averaged Atlantic Meridional Overturning Circulation

AbstractIt is typically assumed that the meridional density gradient in the North Atlantic is well and positively correlated with the Atlantic meridional overturning circulation (AMOC). In numerical “water-hosing” experiments, for example, imposing an anomalous freshwater flux in the Northern Hemisphere leads to a slowdown of the AMOC. However, on planetary scale, the first-order dynamics are linked to the geostrophic balance, relating the north–south pressure gradient to the zonal circulation. In this study, these two approaches are reconciled. At steady state and under geostrophic dynamics, an analytical expression is derived to relate the zonal and meridional pressure gradient. This solution is only valid where the meridional density gradient length scale is shorter than Earth’s curvature length scale, that is, north of 35°N. This theoretical expression links the north–south density gradient to the AMOC and can be used as a closure for zonally averaged ocean models. Assumptions and shortcomings of the approach are presented. Implications of these results for paleoclimate problems such as AMOC collapse and asymmetry in the meridional overturning circulation of the Atlantic and of the Pacific are discussed.

CO2-induced climate change in northern Europe: CMIP2 versus CMIP3 versus CMIP5

The simulation of near-present-day climate and climate change in northern Europe is compared between the CMIP2, CMIP3 and CMIP5 ensembles of the Coupled Model Intercomparison Project, focusing on an idealized scenario with a doubling of CO2 in 70 years. The control simulations in the three ensembles share similar biases, including generally colder than observed temperatures and an underestimated north–south pressure gradient across northern Europe. The CO2-induced climate changes are also similar, despite a slight increasing (decreasing) trend in the simulated warming (precipitation increase) from the older towards the newer models. About half of the intermodel variance in the simulated annual mean warming in northern Europe is explained by differences in the global mean warming, whereas differences in the regional mean sea level pressure change contribute nearly 40 % of the variance of the annual precipitation change. An energy and water balance analysis for a subset of the models additionally suggests that the spread in the simulated warming is strongly affected by the snow-albedo feedback in spring and changes in cloudiness in summer, while both increased evaporation and changes in atmospheric water vapour convergence are important for precipitation changes. A comparison for the CMIP5 ensemble reveals a close similarity in temperature change in northern Europe between the idealized CO2 simulations and the Representative Concentration Pathways RCP4.5 and RCP8.5 simulations for the twenty-first century, once differences in the rate of the ensemble-averaged global mean warming are accounted for. Precipitation tends to increase slightly more under the RCP scenarios, possibly due to the assumed decrease in anthropogenic aerosols.

Simulation of High Impact Rainfall Events Over Southeastern Hilly Region of Bangladesh Using MM5 Model

Simulation of high impact rainfall events over southeastern hilly region of Bangladesh has been carried out using Fifth-Generation PSU/NCAR Mesoscale Model (MM5) conducting two historical rainfall events, namely, 21 June, 2004 and 11 July, 2004. These extraordinary rainfall events were localized over the Rangamati region and recorded 304 mm and 337 mm rainfall on 21 June, 2004 and 11 July, 2004, respectively, over Rangamati within a span of 24 h. The model performance was evaluated by examining the different predicted and derived parameters. It is found that the seasonal monsoon trough has northerly position compared to normal and pass through Bangladesh extending up to northeast India for both cases. The heat low was found to be intense (996 hPa) with strong north-south pressure gradient (12–15 hPa). The analysis of the geopotential height field at 200 hPa shows that the Tibetan high is shifted towards south by 7-8° latitudes with axis along 22–25°N for both cases. The analysis of the wind field shows that the areas of high impact rainfall exhibit strong convergence of low level monsoon circulation (~19–58 knots). The strong southwesterlies were found to exist up to 500 hPa level in both cases. The lower troposphere (925–500 hPa) was characterized by the strong vertical wind shear (~9–18 ms−1) and high relative vorticity (~20–40 × 10−5 s−1). The analysis also shows that the areas of high impact rainfall events and neighbourhoods are characterized by strong low level convergence and upper level divergence. The strong southwesterly flow causes transportation of large amount of moisture from the Bay of Bengal towards Bangladesh, especially over the areas of Rangamati and neighbourhoods. The high percentage of relative humidity extends up to the upper troposphere along a narrow vertical column. Model produced details structure of the spatial patterns of rainfall over Bangladesh reasonably well though there are some biases in the rainfall pattern. The model suggests that the highly localized high impact rainfall was the result of an interaction of the mesoscale severe convective processes with the large scale active monsoon system.

Evaluating meteorological comparability in air quality studies: Classification and regression trees for primary pollutants in California's South Coast Air Basin

Meteorology confounds the comparison of air quality data across time and space. This presents challenges, for example, to comparisons of pollutant concentration data obtained with mobile monitoring platforms on different days and/or locations within the same airshed. In part to address this challenge, we employed a classification and regression tree (CART) modeling approach that can serve as a useful and straightforward tool in such air quality studies, to determine the comparability of meteorological conditions between measurement days and locations as well as to compare primary pollutant concentrations corrected by meteorological conditions. Specifically, regression trees were developed to obtain representative concentrations of traffic-related primary air pollutants such as NOx and CO, based on meteorological conditions for 2007–2009 in the California South Coast Air Basin (SoCAB). The resulting regression trees showed strong correlations between the regression classifications developed for different pollutant metrics, such as daily CO and NOx maxima, as well as between monitoring sites. For the SoCAB, the most important meteorological parameters controlling primary pollutant concentrations were the mean surface wind speed, geopotential heights at 925 mbar, the upper air north–south pressure gradient, the daily minimum temperature, relative humidity at 1000 mbar, and vertical stability, in approximate order of importance. The value of developing a regression tree for a single season was also explored by performing CART analysis separately on summer data. Although seasonal classifications were similar to those developed from annual data, the standard deviations of the classification groups were somewhat reduced.

Data Clustering Reveals Climate Impacts on Local Wind Phenomena

AbstractThe authors demonstrate the utility of k-means clustering for identifying relationships between winds at turbine heights and climate oscillations, thereby developing a method suited for predicting the impacts of climate change on wind resources. Fourteen years of data from an 80-m tower at the National Wind Technology Center (NWTC) in Colorado have been reduced to four dominant flow phenomena using k-means clustering. At this location, this method identifies two clusters of westerly inflow (strong and weak), another cluster of flow from the north, and one of flow from the south. Similar clusters are found for the data at all heights on the tower, and each follow distinct seasonal cycles. Time series of each cluster, as well as the mean wind speed at the NWTC, are retained for comparison with climate oscillations along with the local 500-hPa pressure gradient. The mean wind speed in the surface layer is strongly correlated with the local north–south pressure gradient. The frequency of strong westerly flow is also negatively correlated with the Niño-3.4 index, whereas weaker westerly winds are negatively correlated with the Pacific–North American pattern (PNA) and Arctic Oscillation (AO). Northerly winds at the NWTC did not strongly correlate with any of the investigated climate indices (AO, PNA, and Niño-3.4). These northerly winds occur more frequently in the summer months, suggesting that these winds are more influenced by local conditions than by mesoscale forcing. This method of identifying clusters in wind data allows objective identification of wind phenomena that may benefit the deployment of wind turbines, for example, in choosing combinations of wind speed and direction to investigate for turbine siting.

Characteristics of nocturnal low-level jets over Ulsan airport

We analyzed wind profiler data collected over Ulsan airport during the period from 2008 to 2009 to examine the characteristics of low level jets (LLJs). The Ulsan airport is located within the narrow valley with north-south axis. The frequency analysis results indicates that the nearly 19% of the total nocturnal periods have the presence of jets and LLJ occurrence rate is high in winter (32%) and low in summer (10%). The mode in the wind speed histogram is 4–6 m s−1. A majority of jet occurs below 100 m (about 77.8 m) above ground. The predominant wind direction of jet is northerly. In order to examine the favorable conditions for LLJ formation of Ulsan airport, we investigated temperature difference between valley and plain at the surface and synoptic wind direction and speed at 850 hPa. Our results show that air temperature in the valley is lower than over the plain during the nighttime, indicating the existence of thermal forcing for along-valley wind. Under a significant temperature difference along the valley, westerly wind speed at 850 hPa is slightly weaker on LLJs event night than no event night, indicating weaker north-south large-scale pressure gradient on LLJ event night. The magnitude of northerly wind at 850 hPa is much stronger on event night than no event night, implying higher downward transfer of northerly wind on event night. Our findings suggest that jet formation over Ulsan airport is related to the strong northerly wind at 850 hPa in the presence of thermal forcing due to temperature contrast between valley and plain.

The Key Role of the Western Boundary in Linking the AMOC Strength to the North–South Pressure Gradient

Abstract A key idea in the study of the Atlantic meridional overturning circulation (AMOC) is that its strength is proportional to the meridional density gradient or, more precisely, to the strength of the meridional pressure gradient. A physical basis that would indicate how to estimate the relevant meridional pressure gradient locally from the density distribution in numerical ocean models to test such an idea has been lacking however. Recently, studies of ocean energetics have suggested that the AMOC is driven by the release of available potential energy (APE) into kinetic energy (KE) and that such a conversion takes place primarily in the deep western boundary currents. In this paper, the authors develop an analytical description linking the western boundary current circulation below the interface separating the North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW) to the shape of this interface. The simple analytical model also shows how available potential energy is converted into kinetic energy at each location and that the strength of the transport within the western boundary current is proportional to the local meridional pressure gradient at low latitudes. The present results suggest, therefore, that the conversion rate of potential energy may provide the necessary physical basis for linking the strength of the AMOC to the meridional pressure gradient and that this could be achieved by a detailed study of the APE to KE conversion in the western boundary current.

Using synoptic type analysis to understand New Zealand climate during the Mid-Holocene

Abstract. Diagnosing the climate of New Zealand from low-resolution General Circulation Models (GCMs) is notoriously difficult due to the interaction of the complex topography and the Southern Hemisphere (SH) mid-latitude westerly winds. Therefore, methods of downscaling synoptic scale model data for New Zealand are useful to help understand past climate. New Zealand also has a wealth of palaeoclimate-proxy data to which the downscaled model output can be compared, and to provide a qualitative method of assessing the capability of GCMs to represent, in this case, the climate 6000 yr ago in the Mid-Holocene. In this paper, a synoptic weather and climate regime classification system using Empirical Orthogonal Function (EOF) analysis of GCM and reanalysis data was used. The climate regimes are associated with surface air temperature and precipitation anomalies over New Zealand. From the analysis in this study, we find at 6000 BP that increased trough activity in summer and autumn led to increased precipitation, with an increased north-south pressure gradient ("zonal events") in winter and spring leading to drier conditions. Opposing effects of increased (decreased) temperature are also seen in spring (autumn) in the South Island, which are associated with the increased zonal (trough) events; however, the circulation induced changes in temperature are likely to have been of secondary importance to the insolation induced changes. Evidence from the palaeoclimate-proxy data suggests that the Mid-Holocene was characterized by increased westerly wind events in New Zealand, which agrees with the preference for trough and zonal regimes in the models.

Circumglobal wave train and the summer monsoon over northwestern India and Pakistan: the explicit role of the surface heat low

This study examines the influence of the mid-latitude circulation on the surface heat low (HL) and associated monsoon rainfall over northwestern India and Pakistan using the ERA40 data and high resolution (T106L31) climate model ECHAM5 simulation. Special emphasis is given to the surface HL which forms over Pakistan and adjoining areas of India, Iran and Afghanistan during the summer season. A heat low index (HLI) is defined to depict the surface HL. The HLI displays significant correlations with the upper level mid-latitude circulation over western central Asia and low level monsoon circulation over Arabian Sea and acts as a bridge connecting the mid-latitude wave train to the Indian summer monsoon. A time-lagged singular value decomposition analysis reveals that the eastward propagation of the mid-latitude circumglobal wave train (CGT) influences the surface pressure anomalies over the Indian domain. The largest low (negative) pressure anomalies over the western parts of the HL region (i.e., Iran and Afghanistan) occur in conjunction with the upper level anomalous high that develops over western-central Asia during the positive phase of the CGT. The composite analysis also reveals a significant increase in the low pressure anomalies over Iran and Afghanistan during the positive phase of CGT. The westward increasing low pressure anomalies with its north–south orientation provokes enormous north–south pressure gradient (lower pressure over land than over sea). This in turn enables the moist southerly flow from the Arabian Sea to penetrate farther northward over northwestern India and Pakistan. A monsoon trough like conditions develops over northwestern India and Pakistan where the moist southwesterly flow from the Arabian Sea and the Persian Gulf converge. The convergence in association with the orographic uplifting expedites convection and associated precipitation over northwestern India and Pakistan. The high resolution climate model ECHAM5 simulation also underlines the proposed findings and mechanism.

Atmospheric Circulation Effects on Wind Speed Variability at Turbine Height

Abstract Mean monthly wind speed at 70 m above ground level is investigated for 11 sites in Minnesota for the period 1995–2003. Wind speeds at these sites show significant spatial and temporal coherence, with prolonged periods of above- and below-normal values that can persist for as long as 12 months. Monthly variation in wind speed primarily is determined by the north–south pressure gradient, which captures between 22% and 47% of the variability (depending on the site). Regression on wind speed residuals (pressure gradient effects removed) shows that an additional 6%–15% of the variation can be related to the Arctic Oscillation (AO) and Niño-3.4 sea surface temperature (SST) anomalies. Wind speeds showed little correspondence with variation in the Pacific–North American (PNA) circulation index. The effect of the strong El Niño of 1997/98 on the wind speed time series was investigated by recomputing the regression equations with this period excluded. The north–south pressure gradient remains the primary determinant of mean monthly 70-m wind speeds, but with 1997/98 removed the influence of the AO increases at nearly all stations while the importance of the Niño-3.4 SSTs generally decreases. Relationships with the PNA remain small. These results suggest that long-term patterns of low-frequency wind speed (and thus wind power) variability can be estimated using large-scale circulation features as represented by large-scale climatic datasets and by climate-change models.

Variations in typhoon landfalls over China

The interannual variability in typhoon landfalls over China is investigated using historical and modern records. For the purpose of substantiating and elaborating upon the claim of north to south variation in tropical cyclone activity over China, a north-to-south anti-correlation in yearly activity is confirmed in the historical cyclone records. When cyclone activity over the province of Guangdong is high (low), it tends to be low (high) over the province of Fujian. A similar spatial variation is identified in the modern records using a factor analysis model, which delineates typhoon activity over the southern provinces of Guangdong and Hainan from the activity over the northern provinces of Fujian, Taiwan, Zhejiang, Shanghai, Jiangsu, and Shandong. A landfall index of typhoon activity representing the degree to which each year follows this pattern of activity is used to identify correlated climate variables. A useful statistical regression model that includes sea-level pressure differences between Mongolia and western China and sea-surface temperature (SST) over the northwestern Pacific Ocean during the summer explains 26% of the interannual variability of the landfall index. It is suggested that a stronger than normal north-south pressure gradient increases the surface easterly wind flow over northern China; this, coupled with lower SST over the Pacific, favors typhoons taking a more westerly track toward southern China.

Nocturnal Low-Level Jet in a Mountain Basin Complex. Part I: Evolution and Effects on Local Flows

Abstract A Doppler lidar deployed to the center of the Great Salt Lake (GSL) basin during the Vertical Transport and Mixing (VTMX) field campaign in October 2000 found a diurnal cycle of the along-basin winds with northerly up-basin flow during the day and a southerly down-basin low-level jet at night. The emphasis of VTMX was on stable atmospheric processes in the cold-air pool that formed in the basin at night. During the night the jet was fully formed as it entered the GSL basin from the south. Thus, it was a feature of the complex string of basins draining toward the Great Salt Lake, which included at least the Utah Lake basin to the south. The timing of the evening reversal to down-basin flow was sensitive to the larger-scale north–south pressure gradient imposed on the basin complex. On nights when the pressure gradient was not too strong, local drainage flow (slope flows and canyon outflow) was well developed along the Wasatch Range to the east and coexisted with the basin jet. The coexistence of these two types of flow generated localized regions of convergence and divergence, in which regions of vertical motion and transport were focused. Mesoscale numerical simulations captured these features and indicated that updrafts on the order of 5 cm s−1 could persist in these localized convergence zones, contributing to vertical displacement of air masses within the basin cold pool.