Northerly Component Research Articles

Relationship between rainfall and weather regimes in south‐eastern Queensland, Australia

AbstractThe present study aims to determine synoptic regimes associated with rain and no‐rain days in south‐eastern Queensland. A k‐means cluster analysis is used on upper‐air data from Brisbane Airport to identify dominant weather regimes for the region. Eight weather regimes appear to succinctly describe the main types of conditions experienced in south‐eastern Queensland. Using rainfall data from 307 sites across the region, the rainfall associated with each weather regime is then characterized. Four of the regimes are associated with moist conditions; each accounts for about 15–20% of the mean total annual rainfall. These regimes preferentially occur during the summer and are characterized by high onshore moisture flux. The four dry regimes are characterized by southerly moisture flux and generally occur throughout the year. These regimes combined account for less than 25% of the mean total annual rainfall but more than 60% of the days in a year. Back trajectories at five levels were computed for each of the regimes using the HYSPLIT model. Case studies were selected for the four wet regimes and examined in detail. The wet regimes exhibit geographically shorter back trajectories, particularly at lower levels, than the dry regimes. In addition, there is usually a northerly component to the 500 m level trajectories close to Brisbane for the wet regimes. Copyright © 2012 Royal Meteorological Society

Mesoscale processes for super heavy rainfall of Typhoon Morakot (2009) over Southern Taiwan

Abstract. Within 100 h, a record-breaking rainfall, 2855 mm, was brought to Taiwan by typhoon Morakot in August 2009 resulting in devastating landslides and casualties. Analyses and simulations show that under favorable large-scale situations, this unprecedented precipitation was caused first by the convergence of the southerly component of the pre-existing strong southwesterly monsoonal flow and the northerly component of the typhoon circulation. Then the westerly component of southwesterly flow pushed the highly moist air (mean specific humidity >16 g/kg between 950 and 700 hPa from NCEP GFS data set) eastward against the Central Mountain Range, and forced it to lift in the preferred area. From the fine-scale numerical simulation, not only did the convergence itself provide the source of the heavy rainfall when it interacted with the topography, but also convective cells existed within the typhoon's main rainband. The convective cells were in the form of small rainbands perpendicular to the main one, and propagated as wave trains downwind. As the main rainband moved northward and reached the southern CMR, convective cells inside the narrow convergence zone to the south and those to the north as wave trains, both rained heavily as they were lifted by the west-facing mountain slopes. Those mesoscale processes were responsible for the unprecedented heavy rainfall total that accompanied this typhoon.

Seasonal prediction of cyclonic disturbances over the Bay of Bengal during summer monsoon season : Identification of potential predictors

The cyclonic disturbances (CD) over the Bay of Bengal during monsoon season have significant impact on rainfall over India. On many occasions, they cause flood leading to loss of lives and properties. Hence, any early information about the frequency of occurrence of such disturbances will help immensely the disaster managers and planners. However, the studies are limited on the seasonal prediction of CD over the Bay of Bengal unlike other Ocean basins of the world. Hence, a study has been undertaken to find out the potential predictors during the months of April and May for prediction of frequency of cyclonic disturbances over the Bay of Bengal during monsoon season (June – September). For this purpose, best track data of India Meteorological Department and large scale field parameters based on NCEP/NCAR reanalysis data have been analyzed for the period of 1948 – 2007. The linear correlation analysis has been applied between frequency of CD and large scale field parameters based on NCEP/NCAR reanalysis data to find out the potential predictors. The large scale field parameters over the equatorial Indian Ocean, especially over west equatorial Indian Ocean and adjoining Arabian Sea (up to 15° N) should be favourable in April and May with lower mean sea level pressure (MSLP), lower geopotential heights and stronger southerlies in lower and middle levels, along with stronger northerly components at upper level for higher frequency of CD during subsequent monsoon season. Consequently, there should be increase in relative humidity (RH) and precipitable water content and decrease in outgoing longwave radiation (OLR) and temperature at lower levels over this region during April and May for higher frequency of CD during subsequent monsoon season. Comparing the area of significant correlation between frequency of CD and large scale field parameters and its stability from April to September, MSLP and geopotential heights are most influencing parameters followed by OLR, sea surface temperature, air temperature and RH at 850 hPa level.

Low-level jet development and the interaction of different scale physical processes

Two cases of low-level jet events and the interaction of the involved physical processes are investigated. The vertical distribution of the dominant atmospheric motions is studied, using data from a SODAR-RASS system and in situ instrumentation, at a coastal region of the Eastern Mediterranean (Messogia Plain in Attica—Greece). The first low-level jet (LLJ) case was observed during the cold period, after the passage of a cold front and could be characterized as purely synoptic. Coherent inertial motions were found to follow the frontal surface at both the wind components, with larger amplitudes at the alongfront wind component, a fact consistent with the deformation theory. The contribution of the diurnal cycle was found weak in both the components, due to the cloudiness and the small diurnal temperature range, while the synoptic scale dominated the northerly wind component. The second LLJ case represents two successive summertime large-scale jets. The absence of cloudiness and the increased solar radiation provided a favorable environment for the development of local flows which interacted with the synoptic wind field. This interaction led to a LLJ with an oscillating core and a variable depth. The diurnal cycle had a strong imprint on the wind component along the sea breeze direction (easterly), while the synoptic variations dominated the northerly wind component. Strong amplitudes of a coherent quasi 2-day variation were also found in both the cases.

Kinematic and Moisture Characteristics of a Nonprecipitating Cold Front Observed during IHOP. Part I: Across-Front Structures

Abstract A wide array of ground-based and airborne instrumentation is used to examine the kinematic and moisture characteristics of a nonprecipitating cold front observed in west-central Kansas on 10 June 2002 during the International H2O Project (IHOP). This study, the first of two parts, is focused on describing structures in the across-front dimension. Coarsely resolved observations from the operational network and dropsondes deployed over a 200-km distance centered on the front are combined with higher-resolution observations from in situ sensors, Doppler radars, a microwave radiometer, and a differential absorption lidar that were collected across a ∼40-km swath that straddled a ∼100-km segment of the front. The northeast–southwest-oriented cold front moved toward the southeast at ∼8–10 m s−1 during the morning hours, but its motion slowed to less than 1 m s−1 in the afternoon. In the early afternoon, the cold front separated cool air with a northerly component flow of 2–4 m s−1 from a 10-km-wide band of hot, dry air with 5 m s−1 winds out of the south-southwest. The average updraft at the frontal interface was ∼0.5 m s−1 and slightly tilted back toward the cool air. A dryline was located to the southeast of the front, separating the hot, dry air mass from a warm, moist air mass composed of 10 m s−1 southerly winds. Later in the afternoon, the warm, moister air moved farther to the northwest, approaching the cold front. The dryline was still well observed in the southwestern part of the observational domain while it vanished almost completely in the northeastern part. Low-level convergence (∼1 × 10−3 s−1), vertical vorticity (∼0.5 × 10−3 s−1), and vertical velocity (∼1 m s−1) increased. The strong stable layer located at ∼2.0–2.5 km MSL weakened in the course of the afternoon, providing a basis for the development of isolated thunderstorms. The applicability of gravity current theory to the cold front was studied. There was evidence of certain gravity current characteristics, such as Froude numbers between 0.7 and 1.4, a pronounced feeder flow toward the leading edge, and a rotor circulation. Other characteristics, such as a sharp change in pressure and lobe and cleft structures, remain uncertain due to the temporally and spatially variable nature of the phenomenon and the coarse resolution of the measurements.

Geological and biological evidence for regional drainage reversal during lateral tectonic transport, Marlborough, New Zealand

A 2500 km 2 mountainous river catchment in northern South Island, New Zealand, drained southwards until the late Quaternary. The catchment is being transported laterally at c . 36 mm a −1 relative to the southern South Island by dextral strike-slip motion along the Pacific–Australian Plate boundary (Alpine Fault). This lateral motion has moved the eastern half of the catchment from a zone of uplift into a zone of subsidence over the past million years, triggering drainage reversal. Sedimentation from the catchment switched from a sedimentary basin in the SE to a basin in the north. Profiles of river terrace remnants, formed from glacial outwash, show that drainage reversal occurred in the latter stages of deposition of the oldest recognizable terrace. Clast imbrication patterns and clast petrography in that terrace indicate southward flow, whereas flow had a northerly component in younger terraces and a nearby active river. In addition, 1.6% genetic divergence of fish on either side of the new drainage divide is consistent with late Quaternary separation by drainage reversal. The timing of drainage reversal is estimated from regional terrace correlation and the magnitude of the genetic divergence to be between 100 and 200 ka bp.

Extreme Anomalous Atmospheric Circulation in the West Antarctic Peninsula Region in Austral Spring and Summer 2001/02, and Its Profound Impact on Sea Ice and Biota*

AbstractExceptional sea ice conditions occurred in the West Antarctic Peninsula (WAP) region from September 2001 to February 2002, resulting from a strongly positive atmospheric pressure anomaly in the South Atlantic coupled with strong negative anomalies in the Bellingshausen–Amundsen and southwest Weddell Seas. This created a strong and persistent north-northwesterly flow of mild and moist air across the WAP. In situ, satellite, and NCEP–NCAR Reanalysis (NNR) data are used to examine the profound and complex impact on regional sea ice, oceanography, and biota. Extensive sea ice melt, leading to an ocean mixed layer freshening and widespread ice surface flooding, snow–ice formation, and phytoplankton growth, coincided with extreme ice deformation and dynamic thickening. Sea ice dynamics were crucial to the development of an unusually early and rapid (short) retreat season (negative ice extent anomaly). Strong winds with a dominant northerly component created an unusually compact marginal ice zone and a major increase in ice thickness by deformation and over-rafting. This led to the atypical persistence of highly compact coastal ice through summer. Ecological effects were both positive and negative, the latter including an impact on the growth rate of larval Antarctic krill and the largest recorded between-season breeding population decrease and lowest reproductive success in a 30-yr Adélie penguin demographic time series. The unusual sea ice and snow cover conditions also contributed to the formation of a major phytoplankton bloom. Unexpectedly, the initial bloom occurred within compact sea ice and could not be detected in Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) ocean color data. This analysis demonstrates that sea ice extent alone is an inadequate descriptor of the regional sea ice state/conditions, from both a climatic and ecological perspective; further information is required on thickness and dynamics/deformation.

Tropical Cyclone Motion in Response to Land Surface Friction

Abstract Numerical experiments are performed with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) to study the effects of surface-moisture flux and friction over land on the movement of tropical cyclones (TCs). On an f plane, the TCs are initially placed 150 km due east of a north–south-oriented coastline in an atmosphere at rest. It is found that a TC could drift toward land when the roughness length is 0.5 m over land, with an average drift speed of ∼1 m s−1. Friction, but not surface-moisture flux over land, is apparently essential for the movement toward land. The friction-induced asymmetry in the large-scale flow is the primary mechanism responsible for causing the TC drift. The mechanism responsible for the development of the large-scale asymmetric flow over the lower to midtroposphere (∼900–600 hPa) appears to be the creation of asymmetric vorticity by the divergence term in the vorticity equation. Horizontal advection then rotates the asymmetric vorticity to give a northeasterly flow in the TC periphery (∼500–1000 km from the TC center). The flow near the TC center has a more northerly component because of the stronger rotation by the tangential wind of the TC at inner radii. However, the TC does not move with the large-scale asymmetric flow. Potential vorticity budget calculations indicate that while the horizontal advection term is basically due to the effect of advection by the large-scale asymmetric flow, the diabatic heating and vertical advection terms have to be considered in determining the vortex landward drift, because of the strong asymmetry in vertical motion. Two mechanisms could induce the asymmetry in vertical motion and cause a deviation of the TC track from the horizontal asymmetric flow. First, the large-scale asymmetric flow in the upper troposphere differs from that in the lower troposphere, both in magnitude and direction, which results in a vertical shear that could force the asymmetry. A vertical tilt of the vortex axis is also found that is consistent with the direction of shear and also the asymmetry in rainfall and vertical motion. Second, asymmetric boundary layer convergence that results from the internal boundary layer could also force an asymmetry in vertical motion.

North foehn in the Austrian Inn Valley: A case study and idealized numerical simulations

This study presents high-resolution numerical simulations of north foehn in the Austrian Inn Valley which have been performed with the Penn State/NCAR mesoscale model MM5. As the Inn Valley is located north of the Alpine crest, north foehn occurs comparatively rarely in this valley, and there are only sparse observations available for this phenomenon. Simulations of the 24 January 1993 case as well as idealized simulations are performed to get a deeper insight into the dynamics of the north foehn. Moreover, the synoptic conditions leading to the occurrence of north foehn in the Inn Valley are investigated. The simulations indicate that there are at least four different paths for the foehn to penetrate into the valley. Two of them are running along side valleys entering the upper Inn Valley from the west. These flow paths appear to be most important when the large-scale flow has a significant westerly component. The other possible flow paths enter the Inn Valley from the northwest or north and require a strong northerly component of the large-scale flow. From a dynamical point of view, north foehn appears to be similar to the well researched south foehn in that vertically propagating gravity waves force the descent of the ambient flow into the valleys. However, there are also indications that trapped lee waves have a significant impact on the surface wind field, which has not been reported for south foehn so far. Moreover, the model results show that a precondition for the formation of north foehn in the Inn Valley is the absence of significant orographic precipitation. Evaporative cooling induced by precipitation falling into subsaturated air not only reduces the surface temperatures but also inhibits the formation of large-amplitude gravity waves, suppressing the development of stormy surface winds.

Establishment of the Low-Level Wind Anomalies over the Western North Pacific during ENSO Development*

The anomalous low-level anticyclone over the western North Pacific presents a link between El Nino and the western North Pacific-East Asian climate. During a La Nina event, however, the low-level wind anomalies over the western North Pacific reverse sign. The low-level wind anomalies move eastward from South Asia and are established over the western North Pacific in the fall of the El Nino (La Nina) developing year. The anomalous low-level anticyclone associated with El Nino is a response to suppressed convection. This suppressed convection is mainly induced by a cooling tendency associated with the vertical average of the anomalous horizontal advection of moist static energy that is defined as the mechanism of the horizontal advection of moist static energy. The El Nino-related sea surface temperature (SST) anomalies, over the eastern Pacific in particular, create negative meridional gradients of temperature and moisture over the western North Pacific. When the winter monsoon starts to dominate the Asian region, the northerly component of the winter monsoon transports low moist static energy air into South Asia that suppresses convection and induces the anomalous low-level anticyclone. Associated with the mean state of temperature and moisture, the meridional components of the anomalous low-level anticyclone transports cold and dry air to the southward branch and warm and moist air to the northward branch of the anomalous low-level anticyclone. This cooling-warming pattern of the anomalous horizontal advections of moist static energy tends to move the anomalous low-level anticyclone eastward. The mechanism of the horizontal advection of moist static energy implies a phase-locking behavior of the anomalous low-level anticyclone with the seasonal cycle of the mean state over the western North Pacific-East Asian region.

Spatial pattern of grain size in the Late Pliocene ‘Red Clay’ deposits (North China) indicates transport by low-level northerly winds

The confirmation of an eolian origin of the Tertiary ‘Red Clay’ from North China would make it a valuable terrestrial paleoclimatic record. However, it is still controversial as to which wind primarily transported the dust forming the ‘Red Clay’ onto the Loess Plateau. Careful consideration of this question is a prerequisite for better understanding the history of atmospheric circulation in central and East Asia during the Miocene and Pliocene epochs. In this paper, the grain-size distributions of bulk samples and chemically isolated quartz grains of the Late Tertiary ‘Red Clay’ were investigated to explore the potential transportation mechanisms of the dust as well as the atmospheric circulation at the time of deposition. The samples were taken from six localities spanning across 550 km on the Loess Plateau. The results show that the mean grain size of both bulk and quartz samples decreases southward, as does the coarse fraction content (>42 μm). These data not only give new evidence for the eolian origin of the ‘Red Clay’, but also indicate that the ‘Red Clay’ was probably transported mainly by northerly low-level winds, or the winds at least had a strong northerly component. Therefore, the spatial distribution of grain size of the ‘Red Clay’ is comparable to that of the overlying Quaternary eolian loess, which demonstrates a remarkable northwest–southeast decrease in grain size, sedimentation rate, and thickness during the Last Glacial, coinciding with the dominant wind direction of the northwesterly East Asian winter monsoon. However, the grain size of the ‘Red Clay’ is generally finer than the corresponding loess. This indicates that the ‘Red Clay’ may have been transported by a weaker winter monsoon associated with stronger pedogenic modification, and/or the dust source area was not extensive compared to that during the Quaternary. Moreover, the high content of grains coarser than 19 μm, transported mainly by energetic low-level winds over short distances, suggests that the dusts were not transported by upper-level westerly.

Systematic observation of westward propagating cloud bands over the Arabian Sea during Indian Ocean Experiment (INDOEX) from Meteosat‐5 data

During the field experiment phase of the Indian Ocean Experiment (INDOEX), linear cloud formations parallel to the West Indian coast and propagating westward have been observed. Meteosat‐5 images are used for the description of the life cycle of these events. Single cloud bands, or main cloud bands followed by (up to 10) secondary parallel cloud lines with a wavelike pattern, have been observed daily during four periods in the dry season of 1999 (15 January, 16–19 February, 27 February to 7 March, and 1–3 April 1999). During these four periods, one single cloud band or a packet of cloud bands is generated every day at nighttime. Their length reaches several hundreds of kilometers, their width lies below 40 km, and their duration in some cases exceeds 24 hours. The smaller cloud lines observed behind the leading cloud line are narrower and are separated by a distance of 2–10 km. Cloud tops are about 3–8°C colder than the Arabian Sea, corresponding to an altitude between 500 and 1200 m during the night and below 2500 m during daytime. Cloud bands travel westward over the Arabian Sea at a speed around 13 m/s, greater than the wind speed measured in the surrounding area. The motion of the cloud band presents similarities with the wavelike propagation of atmospheric phenomena such as the Australian morning glories. Common elements to the different cases are the following: weak low‐level winds close to the southwestern Indian coast when the cloud bands appear, winds with a northerly component in the vicinity of the northwestern Indian coast, the presence of a sea‐land breeze circulation along the West Indian coast, and the presence of high concentrations of pollutants over the northeastern Arabian Sea. However, existing physical links between these elements (if any) still have to be investigated.

A study on predictability of human physiological strain in Buenos Aires City

AbstractHigh atmospheric temperature and humidity can cause discomfort and even death among people in high‐risk groups. Forecasting such meteorological situations can be a useful tool for public health managers. The predictability of discomfort is analysed using a time series of the relative strain index in Buenos Aires. Other authors have found that index values exceeding the thresholds of both nocturnal and diurnal comfort can increase mortality. This study finds that such situations are associated with index values over 0.20 at 14:00 h. local time (RSI14>0.20). Consequently, the meteorological conditions associated with RSI14≥0.20 are studied. The anomalies of the geopotential monthly mean fields show the passage of a low‐pressure system south of 45°S. This system is related to a trough, possibly of frontal character, moving across the province of Buenos Aires, and a simultaneous intensification of the northwestern low in Argentina. The synoptic situation is associated with a general northerly flow that brings heat and moisture from the tropics. On 81% of days with RSI14≥0.20, winds with a northerly component were recorded. This increases to 84.4% when at least one day with northerly wind is previously observed. The synoptic characteristics are examined using a combination of circulation indices: meridional (R), zonal (Z) and curvature (C). The daily sequence of the weighted averages of the anomalies obtained are ΔR<0,ΔZ>0 and ΔC>0. Some 67% of the days when RSI14≥0.20 correspond to this daily index pattern estimated at locations near Buenos Aires. Copyright © 2003 Royal Meteorological Society

Application of Neural Networks to the Simulation of the Heat Island over Athens, Greece, Using Synoptic Types as a Predictor

The effect of the synoptic-scale atmospheric circulation on the urban heat island phenomenon over Athens, Greece, was investigated and quantified for a period of 2 yr, employing a neural network approach. A neural network model was appropriately designed and tested for the estimation of the heat island intensity at 23 stations during the examined period. The day-by-day synoptic-scale atmospheric circulation in the lower troposphere for the same period was classified into eight statistically distinct categories. The neural network model employed as an input the corresponding synoptic categories in conjunction with four meteorological parameters that are closely related to the urban heat island. It was found that the synoptic-scale circulation is a predominant input parameter, affecting considerably the heat island intensity. Also, it was demonstrated that the high pressure ridge mostly favors the heat island phenomenon and categories characterized by intense northerly component winds are responsible for its nonappearance or termination.

Chemical transport across the ITCZ in the central Pacific during an El Niño‐Southern Oscillation cold phase event in March‐April 1999

We examine interhemispheric transport processes that occurred over the central Pacific during the PEM‐Tropics B mission (PTB) in March‐April 1999 by correlating the observed distribution of chemical tracers with the prevailing and anomalous windfields. The Intertropical Convergence Zone (ITCZ) had a double structure during PTB, and interhemispheric mixing occurred in the equatorial region between ITCZ branches. The anomalously strong tropical easterly surface wind had a large northerly component across the equator in the central Pacific, causing transport of aged, polluted air into the Southern Hemisphere (SH) at altitudes below 4 km. Elevated concentrations of chemical tracers from the Northern Hemisphere (NH) measured south of the equator in the central Pacific during PTB may represent an upper limit because the coincidence of seasonal and cold phase ENSO conditions are optimum for this transport. Stronger and more consistent surface convergence between the northeasterly and southeasterly trade winds in the Southern Hemisphere (SH) resulted in more total convective activity in the SH branch of the ITCZ, at about 6°S. The middle troposphere between 4–7 km was a complex shear zone between prevailing northeasterly winds at low altitudes and southwesterly winds at higher altitudes. Persistent anomalous streamline patterns and the chemical tracer distribution show that during PTB most transport in the central Pacific was from SH to NH across the equator in the upper troposphere. Seasonal differences in source strength caused larger interhemispheric gradients of chemical tracers during PTB than during the complementary PEM‐Tropics A mission in September‐October 1996.

Links between Annual Variations of Peruvian Stratocumulus Clouds and of SST in the Eastern Equatorial Pacific

The hypothesis that Peruvian stratocumulus play an important role on both the annual mean and annual variations of sea surface temperature (SST) in the eastern equatorial Pacific is examined. The problem is addressed by performing sensitivity experiments using the University of California, Los Angeles, coupled atmosphere–ocean GCM with different idealized temporal variations of stratocumulus in a region along the coast of Peru. The results obtained are consistent with the notion that Peruvian stratocumulus are a key component of the interhemispherically asymmetric features that characterize the annual mean climate of the eastern equatorial Pacific, including the cold SSTs off Peru and the absence of a southern ITCZ. The principal new finding of this study is that the annual variations (i.e., deviations from the annual mean) of Peruvian stratocumulus are linked to the differences between the amplitude, duration, and westward propagation of the warm and cold phases of the equatorial cold tongue. In the model’s context, only if the prescribed annual variations of Peruvian stratocumulus have the same phase as the observed variations are those differences successfully captured. The impact of Peruvian stratocumulus on equatorial SST involves “dynamical” and “thermal” effects. The former develop through an enhancement of the northerly component of the surface wind from the Peruvian coast to the equator. The thermal effects develop through the special relationships between SST and surface evaporation over the equatorial cold tongue, which contributes to extend the cold phase until the end of the year. A successful portrayal of this behavior requires a realistic simulation of the annual variations of surface wind over the equatorial cold tongue.

Palaeomagnetism, rock magnetism and magnetic fabrics in Precambrian metamorphic terranes of the Huabei Shield, China

This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a 95=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a 95=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a 95=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.

The Early 19th Century Climate of the Bahamas and a Comparison with 20th Century Averages

Two weather records kept at Nassau, Bahamas, from 1811 to 1837, and from 1838 to 1845, respectively, are analyzed and compared to 20th century reference periods. The average annual temperature of the period is 24.2°C (±0.65°C), which is 0.4°C lower than 1961–1990 and 0.1°C lower than 1901–1920, the coolest period in the 20th century. Cold periods occurred from 1812–1819 and 1835–1839. A warmer phase prevailed between these two episodes and another warm episode occurred in 1840–1842. Temperature fell after the volcanic eruptions of Tambora (April, 1815) and Coseguina (January, 1835). The maximum cooling after Tambora is estimated at 1.0°C (±0.56°) and after Coseguina is estimated at 0.4°C (±0.56°). The post-Tambora cooling is in line with previous estimates (Robock, personal communication). The 1810s were a period of extreme drought at Nassau and are unequalled in later years. Rainfall frequency was below contemporary (1812–1837) averages from 1812–1820 and 1836–1837 but was above average from 1821–1835. Moist (dry) periods occurred almost simultaneously with warm (cool) periods. The months of October, November, and April show the greatest (negative) deviations in precipitation frequency. Gale force winds were 85% more frequent than from 1901–1960. Much of this increase took place in the months of September through November and represents an increase in tropical cyclone frequency in the Nassau area above that of 1901–1960. Resultant winds show a tendency towards greater northerly components than in the 20th century, especially during the winter months. The increase in northerly wind components, temperatures below the 20th-century average, and reduction in rainfall frequency in the winter half of the year indicates a synoptic situation in which high pressure was more frequent over the southeast North American continent.

Climate change in the western Antarctic Peninsula since 1945: observations and possible causes

Temperature records from slations on the west roast of the Antarctic Peninsula show a very high level of interannual variability and, over the last 50 years, larger warming trends than are seen elsewhere in Antarctica. in this paper we investigate the role of atmospheric circulation variability and sea-ice extent variations in driving these changes. Owing to a lack of independent data, the reliability of Antarctic atmospheric analyses produced in the 1950s and 1960s cannot be readily established, but examination of the available data suggests that there has been an increase in the northerly component of the circulation over the Peninsula since the late 1950s. Few observations of sea-ice extent are available prior to 1973, but the limited data available indicate that the ice edge to the west of the Peninsula lay to the north of recently observed extremes during the very cold conditions prevailing in the late 1950s. The ultimate cause of the atmospheric-circulation changes remains to be determined and may lie outside the Antarctic region.

Zonal gradient of the winter sea level atmospheric pressure at 50°N: An indicator of atmospheric forcing of North Pacific surface conditions

The Aleutian Low dominates the winter atmospheric circulation of the midlatitude North Pacific. The long‐term average of the lowest value of the winter (December, January, and February) mean sea level pressure, the AL, is located near 50°N, 177°E. Anomalies in the winter atmospheric circulation are primarily reflected in the magnitude and longitude of the lowest sea level pressure and also in the zonal gradient of sea level pressure at 50°N. West of the AL the zonal gradient is negative, indicating a northerly component of the geostrophic wind. East of the AL the zonal gradient is positive, indicating a southerly component of the geostrophic wind. Because of their pertinence to forcing of near‐surface ocean conditions, the zonal gradients at 50°N for the winters of 1947–1990 have been analyzed, and anomalous distributions have been identified. In this series, two distinct gradient patterns can be recognized. In the first pattern, proceeding eastward from the minimum near 140°E, the gradient rises monotonically to its maximum near 140°W. The magnitudes of the northerly and southerly components of the geostrophic winds are at their maximum at 140°E and 140°W longitudes, respectively. In the second group, again proceeding eastward from the minimum near 140°E, the gradient first rises to a secondary maximum, usually east of the AL and then declines to a secondary minimum before rising again to its maximum value near 130°W. In terms of the geostrophic wind, proceeding eastward from the longitude of the AL, the southerly component first increases then decreases and, in extreme cases, may reverse direction to a northerly component before increasing again to its maximum near the eastern boundary of the ocean. Results of model studies point to a possible linkage between patterns of the zonal gradient and tropical disturbances. To gain an appreciation of how an anomalous atmospheric circulation may affect near‐surface ocean conditions, the characteristic features of the zonal pressure gradient are examined and compared to previous studies. These include the effects on surface currents, on the turbulent flux of heat across the sea surface, and on the precipitation over the northwestern United States, British Columbia, and Alaska. The comparisons show that in the absence of direct observations, the zonal gradient of the mean winter (December, January, and February) sea level pressure at 50°N can be used to infer the consequences of an anomalous atmospheric circulation on ocean conditions in the high midlatitudes of the North Pacific.