Changes In Surface Winds Research Articles

The influence of tropical cyclone Melor on PM 10 concentrations during an aerosol episode over the Pearl River Delta region of China: Numerical modeling versus observational analysis

In this article, numerical simulations and observational analyses have been made for the aerosol episode that occurred over the Pearl River Delta (PRD) region in China during 1–3 November 2003. An air quality modeling system that consisted of the mesoscale model MM5, chemical transport model MODELS-3/CMAQ, and air pollutant emission model SMOKE, was employed. Studies have shown that this particulate matter (PM) pollution episode was apparently associated with the activity of tropical cyclone (TC) Melor. Model simulations revealed that Melor spawned this PM episode through dynamic and thermodynamic processes. The strong compensating subsidence induced by Melor's peripheral circulations created favorable meteorological conditions that enhanced local aerosol pollution. This strong downward motion produced significant adiabatic warming (2–4 °C daily) and dramatic drying in the low-level troposphere over the PRD. As a result, the PRD region was blanketed with a dry and warm air layer that strengthened the static stability of the lower troposphere. The descending motion also tended to dramatically lower the heights of the planetary boundary layer (PBL) through its dynamic effect. The fair weather created by this synoptic pattern further intensified the nocturnal temperature inversions through enhanced radiative cooling. All of these factors promoted a stagnant local atmosphere with very light winds near the surface. The horizontal and vertical dispersions of locally emitted aerosol particles were largely suppressed, leading to the accumulation of large amounts of PMs near local emission sources in the PRD region. As Melor drew near, changes in surface winds strengthened the horizontal transport of aerosol particles from inland sources to the area of Hong Kong downstream. This horizontal advection greatly contributed to the high PM 10 (particulate matters less than 10 μm in diameters) concentrations in Hong Kong.

Some Overlooked Features of Tropical Atlantic Climate Leading to a New Niño-Like Phenomenon*

Abstract The Atlantic Niño, an equatorial zonal mode akin to the Pacific El Niño–Southern Oscillation (ENSO), is phase-locked to boreal summer when the equatorial easterly winds intensify and the thermocline shoals in the Gulf of Guinea. A suite of satellite and in situ observations reveals a new mode of tropical Atlantic variability that displays many characteristics of the zonal mode but instead peaks in November–December (ND). This new mode is found to be statistically independent from both the Atlantic Niño in the preceding summer and the Pacific ENSO. The origin of this ND zonal mode lies in an overlooked aspect of the seasonal cycle in the equatorial Atlantic. In November the equatorial easterly winds intensify for the second time, increasing upwelling and lifting the thermocline in the Gulf of Guinea. An analysis of high-resolution climatological data shows that these dynamical changes induce a noticeable SST cooling in the central equatorial Atlantic. The shoaling thermocline and increased upwelling enhance the SST sensitivity to surface wind changes, reinvigorating equatorial ocean–atmosphere interaction. The resultant ocean–atmospheric anomalies are organized into patterns that give rise to positive mutual feedback as Bjerknes envisioned for the Pacific ENSO. This ND zonal mode significantly affects interannual rainfall variability in coastal Congo–Angola during its early rainy season. It tends to further evolve into a meridional mode in the following March–April, affecting precipitation in northeast Brazil. Thus it offers potential predictability for climate over the Atlantic sector in early boreal winter, a season for which local ocean–atmosphere variability was otherwise poorly understood.

Influence of the tropics on the climate of the South Atlantic

The climate of the tropical Atlantic is shown to undergo slow basinwide changes with time‐scales of five years that include changes in surface winds, SST, and sea level. Further south in the Agulhas eddy corridor 35°S–25°S there are sea level fluctuations (superimposed on the dominant mesoscale eddies) on a similar timescale with amplitudes of 10 cm and with westward phase propagation of 4 cm s−1, giving rise to phase variations between the western and eastern sides of the basin. In the eastern basin sea level anomalies are accompanied by 0.5°C SST variations, while in the west the relationship is more complex. Here we explore the possibility that these subtropical sea level fluctuations are produced by the poleward propagation of the tropical sea level anomalies.

Antarctic ozone depletion causes an intensification of the Southern Ocean super‐gyre circulation

Recent climate trends over the Southern Hemisphere (SH) summer feature a strengthening of the circumpolar westerly and a weakening of the midlatitude westerly extending from the stratosphere to Earth's surface. Much of the change is attributable to Antarctic ozone depletion. However, the consequential ocean circulation changes are unknown. Here I demonstrate that the observed surface wind changes have forced a southward shift and spin‐up of the super gyre, which links the subtropical South Pacific, Indian and Atlantic Ocean circulation, advecting more warm water southward. The circulation change includes a strengthening of the East Australian Current (EAC) flow passing through the Tasman Sea. The southward shift may be responsible for the observed unusually large warming in the SH midlatitude ocean and may contribute to the reported range extension to the south of many marine species in the South West Pacific.

On Equatorial Pacific Surface Wind Changes around 1977: NCEP–NCAR Reanalysis versus COADS Observations*

Abstract This note compares equatorial Pacific surface wind changes around 1977 in the NCEP–NCAR reanalysis and the Comprehensive Ocean–Atmosphere Data Set (COADS) observations. Significant discrepancies are found in wind changes over the equatorial central and eastern Pacific. In the NCEP–NCAR reanalysis, the easterlies weakened over the eastern equatorial Pacific, while the southerlies strengthened over the north equatorial central Pacific. As a result, the low-level convergence and precipitation decreased over the equatorial central Pacific. These wind and precipitation anomalies are opposite to those derived from the COADS observations. Independent observations of ocean heat content are used to validate the changes in equatorial zonal wind, and it is found that the zonal slope of the thermocline in an ocean model forced by the COADS wind is more consistent with ocean observations than forced by the reanalysis wind. The equatorial wind biases are also identified in the NCEP–NCAR reanalysis climatology,...

Long-term Surface Wind Changes in the Tokyo Metropolitan Area in the Afternoon of Sunny Days in the Warm Season.

The urban heat-island effect of the Tokyo metropolitan area is believed to cause widespread warming in the daytime of the warm season, resulting in intensified convergence over the central Kanto plain. In order to detect this change, a comparative analysis of surface wind fields was made for the periods 1923-1928 and 1991-2001 for sunny days (at least 8 hours’ sunshine) from May to August. It was found that the magnitude of daytime pressure fall (at 1400 LT relative to 0600 and 2200 LT) had increased by 0.2-0.3 hPa in the central and northwestern part of the plain, with a slight change in surface winds in such a way as to converge toward the central Kanto plain.

A Diagnostic Study of the Role of Remote Forcing in Tropical Atlantic Variability

This observational study focuses on remote forcing of the dominant pattern of north tropical Atlantic sea surface temperature (SST) anomalies by ENSO and NAO. Based on a spring SST index of the north tropical Atlantic (NTA) SST (5°–25°N), it is shown that almost all NTA–SST extreme events from 1950 to the present can be related to either ENSO or NAO. Since the SST NTA events lag NAO and ENSO events, NTA variability is interpreted as being largely a response to remote NAO or ENSO forcing. The local response of the tropical Atlantic to these external sources—whether it be ENSO or the NAO—is observed to be rather similar: changes in surface winds induce changes in latent heating that, in turn, generate SST anomalies. Once generated, the latter are damped through local air–sea interaction, at a rate estimated to be 10 W m−2 K−1. Experiments with simple models, but driven by observations, strongly suggests that variability on interannual to interdecadal timescales—both time series and spectral signatures—can be largely explained as a result of direct atmospheric forcing, without the need to invoke a significant role for local unstable air–sea interactions or ocean circulation.

Processes for the Northeastward Advance of the Summer Monsoon over the Western North Pacific.

In the South China Sea (SCS) and western North Pacific (WNP), climatological summer monsoon sets in first in the SCS in mid-May, extends into the southwestern Philippine Sea in mid-June, and jumps suddenly northeastward to around 20°N, 150°E in late-July. The processes leading to this distinct northeastward advance of the summer monsoon are investigated using NCEP/NCAR reanalysis data for the period of 1979-1995. It is found that the cloud-radiation and wind-evaporation feedback plays an important role for the seasonal change in sea surface temperature (SST) in the WNP. Analysis shows that monsoon-induced changes in cloudiness and surface wind produce contrasting changes in surface short wave radiation, and latent heat flux between the convection and pre-convection region. The resultant SST tendency difference turns around the SST gradient east of the convection region in about one month, and induces the northeastward shift of the highest SST center. Following the ocean surface warming in the pre-convection region, the convective instability and low-level moisture convergence increase. East of the convection region, the reversal of the SST gradient tends to be associated with the transition of low-level winds from easterly to westerly. The results suggest that the SST change induced by the monsoon onset facilitates the northeastward extension of convection. It is speculated that the summer monsoon advance over the WNP may result from air-sea interactions.

Modeling the high‐frequency barotropic response of the ocean to atmospheric disturbances: Sensitivity to forcing, topography, and friction

This study examines high‐frequency sea level variations forced by changes in surface atmospheric pressure and wind and their sensitivity to different forcing mechanisms, bottom topography resolution, and amount of friction in a barotropic ocean model. Optimal model performance, defined in terms of the explained variance in satellite altimeter and bottom pressure data, is found when using relatively strong friction, equivalent to a damping timescale of only a few days over the deep ocean, and topography with minimal smoothing. Spatial variations of the optimal friction parameter seem to reflect the roughness of bottom topography. The model demonstrates skill in simulating the wind‐driven response as well as the nonequilibrium response to atmospheric pressure variations.

Martian surface winds: Insensitivity to orbital changes and implications for aeolian processes

Aeolian features observed on the surface of Mars provide insight into current, and potentially past, surface wind systems. In some cases the features are clearly transient and related to the lifting and settling of atmospheric dust. Other features, like dunefields, yardangs, and ventifacts, are more persistent and likely require significant time to form. In this study we analyze the observed directions of selected aeolian features with the aid of the Geophysical Fluid Dynamics Laboratory Mars general circulation model (GCM). Initially, we examine bright and dark streaks which have been observed to form in association with global dust storms. The ability to match these features with Mars GCM wind directions provides an important validation of the model. More important, we are able to define best fit seasons and local times for both types of features which provide the basis for extension and modification of the Veverka et al. [1981] model of bright and dark dust streak formation. In addition these best fit times correspond well with the dark streak “wind storm” model of Magalhães and Young [1995]. The primary focus of this paper is to provide constraints on the range of mechanisms proposed to explain inconsistencies between current wind direction patterns and long‐term wind indicators (for example, the misalignment of rock tail and ventifact orientations at the Mars Pathfinder landing site). Specifically, we assess whether changes in planetary obliquity, precession, or global dust opacity could significantly alter patterns of surface wind directions. In all cases we find the seasonal and annual average wind direction patterns to be highly invariable. While changes in the dust loading (hence the partitioning of solar absorption between the surface and atmosphere) and in the surface latitude of maximum solstitial insolation cause the vigor of the large‐scale circulation to increase (especially the Hadley cell), the spatial patterns of the surface wind orientations remain essentially unchanged. In the case of perihelion during northern summer (opposite of the current perihelion position), the northern summer Hadley cell remains weaker than the southern summer cell despite the strengthened heating in the northern hemisphere. Taken together, these results cast significant doubt on orbital explanations for surface wind changes. It is thus suggested that significant changes in topography (e.g., Tharsis uplift, true polar wander) or climate (e.g., the existence of a significantly thicker atmosphere or an ocean at some point in the past) are more likely explanations for long‐term wind indicators such as the ventifact orientations at the Mars Pathfinder landing site.

Observations of Warm Water Volume Changes in the Equatorial Pacific and Their Relationship to El Niño and La Niña

This paper describes observed changes in surface winds, sea surface temperature (SST), and the volume of water warmer than 20°C (WWV) in the equatorial Pacific Ocean for the period 1980–99. The purpose is to test recent hypotheses about the relationship between variations in WWV and the El Niño–Southern Oscillation (ENSO) cycle. The results confirm inferences based on theory, models, and previous empirical analyses using proxy data (namely sea level) that ENSO involves a recharge and discharge of WWV along the equator and that the cyclic nature of ENSO results from a disequilibrium between zonal winds and zonal mean thermocline depth. The authors also find that the magnitude of ENSO SST anomalies is directly related to the magnitude of zonal mean WWV anomalies. Furthermore, for a given change in equatorial WWV, the corresponding warm El Niño SST anomalies are larger than the corresponding cold La Niña anomalies. This asymmetry between the warm and cold phases of the ENSO cycle implies differences in the relative importance of physical processes controlling SST during El Niño and La Niña events.

Mechanisms for the Indian Ocean warming during the 1997–98 El Niño

This study examines primary mechanisms that gave rise to the basin‐wide variations of the sea surface temperature (SST) in the Indian Ocean during the 1997–98 El Niño by using multi‐source data sets. The evolution of some key atmosphere‐ocean variables indicated that the SST variability in the Indian Ocean was largely attributable to the ENSO impact on the large‐scale atmospheric circulation. During June‐December 1997, when the El Niño in the Pacific was maturing, the Indian Ocean experienced the reversal of the Walker Circulation and the prolonged equatorward displacement of the southeast trades. The resultant changes in surface wind influenced the SST through the following means. In the equatorial region, the easterly winds associated with the reversed Walker Circulation forced equatorial Kelvin/Rossby waves, which then affected the equatorial ocean heat balance (mainly through upwelling/downwelling) and led to the reversal of the zonal SST gradient in the fall of 1997. The negative SST anomalies in the east and positive anomalies in the west in turn helped maintain and prolong the equatorial easterlies, a clear indication of coupled atmosphere‐ocean interactions in operation. Outside of the equatorial waveguide, changes of latent heat flux induced by wind speed variations played a major role in the broad‐scale warming. The effect was most significant during the summer/fall of 1997 when the southeasterly trade winds weakened considerably, leading to a dramatic reduction of latent heat release and subsequently a rapid surface warming in the southern ocean.

El Niño‐Southern Oscillation sea surface temperature and wind anomalies, 1946–1993

El Niño‐Southern Oscillation (ENSO) periods, which occur irregularly every few years, are a major perturbation of the Earth's climate system that involves large‐scale changes in winds, rainfall, sea surface temperature (SST), and surface pressure. In some areas of the world there are disastrous droughts, and in others there is serious flooding. North American weather patterns are also affected. It is important to develop skillful forecasts for ENSO periods. Our goal here is to provide an overview of the global ocean and atmosphere surface changes that typically occur during ENSO periods. Knowledge of these anomaly patterns is needed in order to improve our understanding and forecasts of ENSO. With a global surface data set we describe the statistically significant patterns of SST, surface wind, and surface pressure changes that on average are associated with the 10 post‐World War II ENSO periods. We present these average anomaly results as an “ENSO composite.” It is useful to identify phases of the typical ENSO and examine the statistically significant elements phase by phase. An ENSO by ENSO period search indicates that about two thirds of these elements occur in 90% or more of the ENSO periods: we define a “Robust ENSO Composite” from these frequently occurring elements and find it to be an Indo‐Pacific phenomenon. Limitations in the surface data set make it possible that this study has not identified all the important aspects of ENSO periods; data are very sparse in both space and time over much of the tropics and the southern hemisphere. However, we suggest that any theory or model of ENSO should at least exhibit the features of this robust composite, and is unlikely to able to represent adequately the large‐scale environmental impacts of ENSO unless it does so.

Precambrian climate: The effects of land area and Earth's rotation rate

Rotation rate and land area have changed significantly over the Earth's history. The associated climate changes are examined with a general circulation model (GCM) and compared with the current climate state. Unlike previous GCM studies, clouds in the model interact with the radiation field. We show that rotation rate changes can affect mean air temperatures by strongly affecting the cloud field. For example, an Earth day length of 14 hours causes the global mean cloud fraction to decrease 20%. There are also large changes in the wind field; indeed, at some mid‐latitudes the time‐averaged surface wind changes from westerlies to easterlies. If the land masses are removed from the model and with present‐day rotation, the global mean air temperature is 4 K higher than for the present‐day simulation; if in addition the day length is 14 hours, the global mean air temperature is 5.5 K. Thus the early atmosphere may not have contained the very large concentrations of CO2 thought to be necessary to prevent a frozen planet higher.

Isotopic responses to interannual climate variability simulated by an atmospheric general circulation model

High-resolution isotopic records from remote tropical and temperate sites may provide new insights into recent climatic varyibility, but the interpretation of these records requires a thorough understanding of the controls on internanual variability in the isotopic content of precipitation. To investigate the relationship between isotopic variability and interanual climate change, we use the isotopic tracer version of the Goddard Institute for Space Studies (GISS) general circulation model (GCM). We generate interannual-type climate variability by specifying sea surface temperature (SST) anomalies observed during extreme cool (1955–1956) and warm (1982–1983) phases of the El Nino/Southern Oscillation (ENSO) system during January and July. In response to SST anomalies, the GCM simulates many tropical and extratropical climatic features typical of ENSO variability. Across the tropics, simulated isotopic anomalies correlate significantly with rainfall amount. At higher latitudes, altered pressure distributions set up surface wind changes that generate significant regional temperature anomalies. Extratropical δ18 anomalies reach±4–5% but in most regions do not correlate with temperature anomalies. We examine four regions from which interannual paleoclimatic and/or modern isotopic records are available: the tropical western Pacific, tropical South America, western-central Europe and the Tibetan Plateau. In the Pacific warm pool, the GCM quantitatively reproduces the observed inverse relationship between the amount and isotopic content of rainfall, which provides the basis for coral paleoclimatic reconstruction of ENSO-related rainfall anomalies. In tropical South America, isotopic anomalies also correlate strongly with precipitation amount, consistent with the hypothesis that air mass stability over the Amazon basin controls δ18O variability downstream at the Quelccaya ice core site. In Europe, internanual temperature changes correlate positively with δ18O anomalies, quantitatively consistent with long-term observations and supporting temperature-based interpretation of paleoclimatic records. On the Tibetan Plateau, isotopic variations do not correlate with local climatic anomalies nor with changes in oceanic vapor sources, suggesting a complex climatic signal in Tibetan ice cores possibly mediated by continental processes. In contrast to our interannual results, last glacial maximum (18 ka BP) simulation yield significant correlaton between temperature and isotopic anomalies. The model thus suggests distinct modes of isotopic response to simulated climate variability that depend on the scale and processes of climate change. Internanual temperature changes often result from time-varying advective changes that can confound the relationship between temperature and isotopic variations, via changes in vapor sources and transport. The near-global temperature change associated with glacial-interglacial variation yiels a more consistent pattern of isotopic variability because (1) the effects of large-scale cooling are integrated over the entire pathway of vapor transport and (2) the degree of cooling causes an isotopic response that exceeds the level of variability produced by advective changes.

Possible connection between surface winds, solar activity and the Earth's magnetic field

RECORDS of 14C in tree rings provide a proxy for changes in solar activity on timescales of decades to centuries1,2. Here I report an association between Maunder-type solar cycles recorded in tree rings and fluctuations in surface wind intensity on a roughly 200-year timescale over a period of about 2,000 years in the mid-Holocene. The wind record is preserved as changes in the thickness of varved sediments in Elk Lake, Minnesota, which contain a large aeolian component. Changes in cyclonic activity and tropospheric winds have been reported1–5 to occur in this region a few days after strong coronal mass ejections (CMEs); increases of up to 7% in zonal flow have been reported5 at an altitude of ~ 300 m after strong CMEs in winter. The implication of this association is that century-scale changes in surface winds over Minnesota were the long-term (Maunder-scale) counterpart of short-term (daily) changes in winds after CMEs. The weak magnetic field strength of the Earth at this time6,7 might be relevant to a possible mechanism for the association.

Southerly Changes on the East Coast of New Zealand

Abstract A combined observational and climatological study of orographically influenced cold fronts over New Zealand, known locally as “southerly changes,” is presented. Four southerly changes that occurred along the east coast of New Zealand during the Southerly Change Experiment (SOUCHEX) in January and February of 1988 are analysed in detail using the higher spatial and temporal density of data established for the experimental period. Three of the southerly changes were associated with fronts originating over the Tasman Sea, while the other was not. A common feature in all four cases was the shallowness of the southerly flow for sonic hours after the surface wind change. The top of the southerly flow layer was less than the typical height of the Southern Alps (2000). Above this there was usually a maximum of the northerly component of the flow at or just above mountain- top levels and in three casts the prefrontal low-level flow was dominated by a warm northwesterly foehn. In the central South Island t...

Meso-scale surface wind changes associated with the passage of cold fronts along the eastern side of the Southern Alps, New Zealand

The movement of cold fronts along the eastern side of the Southern Alps has been studied in detail during the Southerly Change Experiment (SOUCHEX). An enhanced network of surface wind monitoring stations was established in the Canterbury region of the South Island to allow detailed study of meso-scale wind fields during the passage of fronts. Fivesoutherly changes occurred during the experiment, one of which failed to produce a clear wind change over much of the area. The often erratic movement of these fronts along the eastern side of the mountains is illustrated by isochrone maps of the onset of the wind change. Other general characteristics of these events include their shallowness (1000–1500 m deep) and in most cases their abrupt onset. Detailed analysis of anemograph data collected during SOUCHEX indicates great variability in the surface wind field associated with passage of the fronts. Maps of the meso-scale wind field plotted for the first southerly change of the 14 January 1988 illustrate the complexity of the wind changes experienced over the eastern South Island in particular. The arrival of the cold fronts at individual sites provided features of air mass interaction which appear to relate to time of day, and regional and local site factors. Variations, in wind, temperature and relative humidity provide clear examples of both single and double air mass changes. However, nocturnal changes seem to be less distinct due to lower ambient temperatures and increased boundary layer stability.

Practical dispersion modelling for buoyant elevated sources in a tropical region

The worst-case meteorological scenarios for the air pollution impact of coal-fired power stations located in tropical Australia are usually those of convective or seabreeze states. Under such conditions maximum hourly ground-level concentrations occur within 5 km of typical elevated sources and are potentially important unless either low sulphur coal is burnt (the usual situation for Australian coals) or restrictions are placed upon neighbouring land use. In such conditions even slightly complex surrounding terrain can cause major changes in surface and lower-level wind and turbulence characteristics, although the resultant effects on ground-level concentrations are probably relatively small. The monitoring data bases of an inland and a coastal power station in tropical Queensland give similar qualitative results to recent EPRI studies. Concurrent detailed meteorological measurements show that convective scaling techniques order both data sets in a satisfactory manner. The convective dispersion model of Spillane (1985, CSIRO internal report) has also shown more predictive skill than conventional approaches and emphasizes the importance of gathering vertical velocity statistics for most sites. Some of the forthcoming developments in related Australian dispersion work are outlined.

Monthly Mean Island Surface Winds in the Central Tropical Pacific and El Niño Events

Abstract The monthly mean surface wind changes during recent ENSO events, as observed from 11 islands in the tropical Pacific, are described. Two different composite ENSO wind fields are evaluated and compared. The month-to-month wind changes during each event are also discussed. The wind changes for each event between 1953 and 1980 except 1969 show several common features: (i) Westerly anomalies appear first west of the date line and then at the date line sometime in summer (0) to fall (0), then intensify over the following several months. The anomalies are confined to within ±3° of the equator during this stage. (ii) In either November (0), December (0), or January (+1) there is an abrupt southward shift of the narrow band of westerly anomalies, so that the maximum anomaly is then at ∼5°S latitude at the date line, and nearly normal conditions prevail north of the equator. (iii) Westerly anomalies are gone or greatly reduced one to two months after the southward shift. The event-to-event variations are ...