Tropospheric Gradient Research Articles

Global Warming Patterns over the North Pacific: ENSO versus AO

The relationships between the natural variability and CO 2 -induced response over the Pacific region are investigated in terms of the spatial anomaly pattern of SST, sea level pressure and precipitation by a multi-model intercomparison analysis, based on the 18-model results contributing to the IPCC Fourth Assessment Report. The analysis indicates that the CO 2 -induced response pattern is related with the model natural variability modes, ENSO and AO. In the tropical Pacific, an ENSO-like global warming pattern is simulated by the majority of the models, with mostly El Nino-like change. In the Arctic region, an AO-like global warming pattern is simulated by many models, with the positive definite AO-phase change, if AO-like. It is suggested that the increase in meridional temperature gradient in the upper troposphere, and the lower stratosphere, provides a preferable condition for the positive AO-like change in the high latitudes by intensifying the subtropical jet, while the increase in the static stability provides a preferable condition for the El Nino-like change in the low latitudes, by reducing the large-scale ambient circulations. However, the sign of the mass (SLP) anomaly is incompatible over the North Pacific, between the positive AO-like change and the El Nino-like change. As a result, the present models cannot fully determine the relative importance between the mechanisms inducing the positive AO-like change and inducing the ENSO-like change, leading to scattering in global warming patterns in regional scales over the North Pacific.

The thermodynamic and dynamical features of double front structures during 21–31 July 1998 in China

The daily 1° × 1° data of the Aviation (AVN) model, the black body temperature (TBB) data of cloud top, and cloud images by geostationary meteorological satellite (GMS) are used to identify a dew-point front near the periphery of the western Pacific subtropical high (WPSH). The results clearly demonstrate the existence of the dew-point front, and its thermodynamic and dynamic structural characteristics are analyzed in detail. The dew-point front is a transitional belt between the moist southwest monsoon flow and the dry adiabatic sinking flow near the WPSH, manifested by a large horizontal moisture gradient in the mid-lower troposphere and conjugated with the mei-yu front to form a predominant double-front structure associated with intense rainfall in the mei-yu period. The mei-yu front is located between 30° and 35°N, vertically extends from the ground level to the upper level and shifts northward. The dew-point front is to the south of the mei-yu front and lies up against the periphery of the WPSH. Generally, it is located between 850 hPa and 500 hPa. On the dew-point front side, the southwesterly prevails at the lower level and the northeasterly at the upper level; this wind distribution is different from that on the mei-yu front side. Vertical ascending motion exists between the two fronts, and there are descending motions on the north side of the mei-yu front and on the south side of the dew-point front, which form a secondary circulation. The dynamics of the double fronts also have some interesting features. At the lower level, positive vertical vorticity and obvious convergence between the two fronts are clearly identified. At the mid-lower level, negative local change of the divergence (corresponding to increasing convergence) is often embedded in the two fronts or against the mei-yu front. Most cloud clusters occur between the two fronts and propagate down stream in a wave-like manner.

Characteristics of microwave propagation in vapourised troposphere

Microwave communication equipment design in temperate region may not be very suitable in the tropics because the characteristics of the troposphere as the medium of propagation differ appreciably. This paper present an investigation on effect of vertical gradient of water vapour on refractivity gradient and microwave propagation in a typical troposphere of the tropics. The meteorological vertical data taken from radiosonde confirms evidence of super-refraction and ducting of the microwave signal. KEY WORDS: Ducting, humidity, inversion, refraction, variation. Global Journal of Mathematical Sciences Vol.3(2) 2004: 123-136

Observation of the gravity waves from GPS/MET radio occultation data

We show that the amplitude of the Global Positioning System (GPS) signals in the radio occultation (RO) experiments is sensitive to the atmospheric wave structures. Earlier the phase of the GPS occultation signals have been used for statistical investigation of the gravity waves (GW) activity in the height interval 10–40 km on a global scale. Analysis of the RO amplitude data revealed wave clusters (quasi-regular structures) with the vertical size of about 10 km and interior vertical period ∼0.8–2 km in the tropopause and lower stratosphere. The amplitude RO data may be utilized to determine the temperature vertical profiles and its vertical gradient in the upper troposphere and stratosphere. In the considered RO events variations of the vertical temperature gradient dT(h)/dh corresponding to the amplitude clusters are in the range from −9 K/km to 6 K/km with vertical scales ∼1–3 km. We show that these variations can be linked to the GW propagation in the atmosphere. We use the polarization and dispersion relationships and Hilbert transform to find the 1-D GW image in the atmosphere by analyzing the vertical temperature gradient d T( h)/d h. The GW image consists of the phase and amplitude of the GW as functions of height. The GW amplitude is non-uniformly distributed with main contribution associated with the tropopause and the secondary maximums in height interval 18–35 km. Using our method we find vertical profiles of the horizontal wind perturbations and their vertical gradient associated with the GW influence. The horizontal wind perturbations are changing in the interval v∼2–12 m/s with vertical gradients d v/d h∼4–25 m s −1 km −1) in the tropopause area and v∼ 3–9 m/s, dv/dh ∼ 2–15 m/(s km) in the stratosphere for the considered events. For one RO event we compared the estimated values of the horizontal wind perturbations with aero-logical data and found fairly good agreement. The height dependence of the GW vertical wavelength was inferred through the differentiation of the GW phase. Analysis of this dependence using the dispersion relationship for the GW gives estimation of the GW intrinsic phase speed. For the considered events the magnitude of the intrinsic phase speed changes between 1.5–15 m/s at heights 10–40 km. We conclude that the amplitude of the GPS occultation signals contain important information about the GW propagation in the atmosphere on a global scale.

Generation and Development of a Polar Mesoscale Cyclone over the East Coast of Asia as Simulated in an AGCM

A polar mesoscale cyclone (PMC) developed over the east coast of Asia in an atmospheric general circulation model (AGCM) that used seasonally varying climatological sea surface temperatures (SSTs). The AGCM had 52 layers and triangular spectral truncation at wavenumber 106 (T106L52). This study compares the model PMC, which formed in February, 8 years after model spin-up, with observational studies. The simulated PMC formed over the western Sea of Japan under the influence of a short-wave trough propagating around a cold upper low. PMC genesis occurred as a surface trough over the Sea of Japan. The surface trough extended northward from a major cyclone that developed over the western North Pacific south of Japan. Large sensible heat fluxes from the sea surface facilitated PMC genesis by decreasing the vertical stability and maintaining a thermal gradient in the lower troposphere. Significant weakening of the PMC as it passed over Japan suggests that energy supplied from the surface played an important role in the evolution of the PMC. The PMC subsequently developed rapidly over the western North Pacific east of Japan, under the influence of the upper cold low and a short-wave trough. Precipitation, diabatic heating, and energy from the sea surface all increased as the PMC developed. The evolution of the simulated PMC matched the evolution of observed PMCs. Realistic formation of synoptic-scale circulation systems, such as the upper cold low, short-wave troughs, and a major cyclone in the model, are crucial if PMCs are to be simulated realistically in the AGCM.

A Diagnostic Study of Formation and Structures of the Meiyu Front System over East Asia

The concept of a dew-point front was introduced, and its existence was identified near the periphery of the west Pacific subtropical anticyclone by using the daily 1 � � 1 � data of the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The dew-point front was a transitional belt between the moist southwest monsoon flow and the dry adiabatic sinking flow, marked by a large horizontal moisture gradient in the mid-lower troposphere. The dew-point front and the Meiyu front, to its north side, formed the Meiyu front system (MYFS). The Meiyu front was the north branch of the MYFS, and extended northward in the mid-lower troposphere. The dew-point front was the south branch of the MYFS, located between 500 hPa and 700 hPa. Along the dew-point front (Meiyu), westerly (easterly) and easterly (westerly) winds prevailed in the lower (upper) and upper (lower) troposphere respectively. Strong ascending motion in the passsageway between the two fronts was surrounded by subsidence. Further, a frontogenesis function was used to diagnose the frontogenesis of the MYFS. The analysis indicated that the convergence and deformation of the horizontal wind were important factors responsible for both the formation and development of the Meiyu front and the dew-point front.

Tropospheric Gradient Estimation at CODE: Results from Global Solutions

Observations at low elevation angles are gathered and used for high accuracy GPS analyses. Introducing tropospheric gradient parameters in the estimation process allows it to take into account of azimuthal variations of tropospheric refraction. This measure significantly improves the repeatability of station coordinates. Most of the mean gradients for stations in the northern and the southern hemisphere show a similar behavior. By combining several years of data from a global solution results a consistent pattern.

Interannual Variability of the Western North Pacific Summer Monsoon: Differences between ENSO and Non-ENSO Years

Abstract The interannual variability of the western North Pacific (WNP) summer monsoon is examined for the non-ENSO, ENSO developing, and ENSO decaying years, respectively. The ENSO developing (decaying) year is defined as the year before (after) the mature phase of ENSO, and the non-ENSO year is defined as the year that is neither the ENSO developing year nor the ENSO decaying year. A strong (weak) WNP summer monsoon tends to occur during the El Niño (La Niña) developing year and a weak (strong) WNP summer monsoon tends to occur during the El Niño (La Niña) decaying year. In all non-ENSO, ENSO developing, and ENSO decaying years, the strong (weak) WNP summer monsoon is associated with the positive (negative) rainfall anomalies, cold (warm) sea surface temperature anomalies, warm (cold) upper-tropospheric temperature anomalies, low (high) surface pressure anomalies, and a low-level cyclonic (anticyclonic) circulation anomaly over the subtropical WNP. The 850-hPa wave train associated with the WNP and east Asian (EA) summer monsoons in the non-ENSO, ENSO developing, and ENSO decaying years extends northward and suggests a possible teleconnection between the WNP summer monsoon and the North American climate. The wave train extended into the Southern Hemisphere in the non-ENSO and ENSO developing years implies a teleconnection between the WNP summer monsoon and the Australian winter climate. The anomalous WNP monsoon in the non-ENSO and ENSO developing years exists only in summer, while the anomalous WNP monsoon in the ENSO decaying year persists from the beginning of the year to the summer season. The anomalous WNP summer monsoon exhibits a strong ocean–atmosphere interaction, especially in the ENSO decaying year. This study suggests that the anomalous WNP summer monsoon in the non-ENSO year is associated with the variation of the meridional temperature gradient in the upper troposphere, while the anomalous WNP summer monsoon in the ENSO developing and decaying years is associated with ENSO-related SST anomalies.

Arctic Oscillation response to the 1991 Mount Pinatubo eruption: Effects of volcanic aerosols and ozone depletion

Observations show that strong equatorial volcanic eruptions have been followed by a pronounced positive phase of the Arctic Oscillation (AO) for one or two Northern Hemisphere winters. It has been previously assumed that this effect is forced by strengthening of the equator‐to‐pole temperature gradient in the lower stratosphere, caused by aerosol radiative heating in the tropics. To understand atmospheric processes that cause the AO response, we studied the impact of the 1991 Mount Pinatubo eruption, which produced the largest global volcanic aerosol cloud in the twentieth century. A series of control and perturbation experiments were conducted with the GFDL SKYHI general circulation model to examine the evolution of the circulation in the 2 years following the Pinatubo eruption. In one set of perturbation experiments, the full radiative effects of the observed Pinatubo aerosol cloud were included, while in another only the effects of the aerosols in reducing the solar flux in the troposphere were included, and the aerosol heating effects in the stratosphere were suppressed. A third set of perturbation experiments imposed the stratospheric ozone losses observed in the post‐Pinatubo period. We conducted ensembles of four to eight realizations for each case. Forced by aerosols, SKYHI produces a statistically significant positive phase of the AO in winter, as observed. Ozone depletion causes a positive phase of the AO in late winter and early spring by cooling the lower stratosphere in high latitudes, strengthening the polar night jet, and delaying the final warming. A positive phase of the AO was also produced in the experiment with only the tropospheric effect of aerosols, showing that aerosol heating in the lower tropical stratosphere is not necessary to force positive AO response, as was previously assumed. Aerosol‐induced tropospheric cooling in the subtropics decreases the meridional temperature gradient in the winter troposphere between 30°N and 60°N. The corresponding reduction of mean zonal energy and amplitudes of planetary waves in the troposphere decreases wave activity flux into the lower stratosphere. The resulting strengthening of the polar vortex forces a positive phase of the AO. We suggest that this mechanism can also contribute to the observed long‐term AO trend being caused by greenhouse gas increases because they also weaken the tropospheric meridional temperature gradient due to polar amplification of warming.

Effects of changing baroclinicity on the southern hemisphere extratropical circulation

AbstractExtratropical cyclones of the southern hemisphere provide the primary mechanism of poleward horizontal heat and moisture transport. Their behaviour is influenced by the strength of the meridional temperature gradient in the mid troposphere. As the semi‐annual oscillation (SAO) in the annual cycle of this gradient has become less dominant in recent years, we investigate the impact of modifying the gradient in two simulations with the Arpège general‐circulation model. Model temperatures at 50°S near the 500 hPa level are increased (strengthening the gradient) or decreased (weakening the gradient) in two different simulations T50+ and T50−, respectively.The temperature nudging had a greater impact on the temperature gradient in T50+, but both simulations showed significant surface warming on the Antarctic coast in winter. In T50+, 500 hPa temperature and surface pressure gradients had weaker SAOs, as the circumpolar trough contracted and intensified and cyclone numbers increased along the Antarctic coast. The SAOs strengthened in T50−, and the trough became more disperse, but cyclone numbers increased north of 50°S.Baroclinicity increased (decreased) with a stronger (weaker) meridional temperature gradient, but the static stability of the atmosphere also changed with the opposite impact on baroclinicity. Thus, in T50+ the increase in cyclone numbers was greatest near the Antarctic coast but this was counteracted somewhat by the static stability increase north of 60°S. In T50− cyclone numbers increased north of 50°S because baroclinicity was increased by a stronger meridional temperature gradient and reduced static stability. Hence, the effect on the Antarctic surface climate was nonlinear, with a warmer coast in both cases in winter, and a windier East Antarctica in T50+ but calmer katabatic flow in T50−. Copyright © 2002 Royal Meteorological Society

Comparison of tropospheric gradients determined by VLBI and GPS

The estimation of geodetic parameters from GPS and VLBI requires the modeling of tropospheric refraction in order to account for the path delays of radio signals being emitted by extragalactic radio sources and satellites, respectively. Precise models consider the deviation from the azimuthal symmetry by introducing horizontal tropospheric gradients. While estimation of gradients had been included in the GPS software package GIPSY several years ago it was implemented in the BERNESE GPS software package (version 4.2) and in the VLBI software OCCAM (modified version 4.0) quite recently. We present a comparison of tropospheric gradient estimates derived with the three software packages mentioned above. The parameters have been estimated for the fundamental station Wettzell (Germany) for four VLBI experiments in the first half of 1999 (Euro48, Euro50, Iris 136, Iris 139) and for simultaneous GPS observations. The gradients obtained by the different software packages show a similar behaviour although offsets between the solutions can be as large as two millimeters. Astonishingly there is no better agreement between the two gradient estimates derived by GPS (BERNESE compared with GIPSY) than between gradients from the different techniques (OCCAM compared with GIPSY or BERNESE). This is probably due to the application of different estimation strategies by the GPS solutions.

First Transition of the Asian Summer Monsoon in 1998 and the Effect of the Tibet-Tropical Indian Ocean Thermal Contrast.

The Asian summer monsoon (ASM) in 1998 first broke out over the Bay of Bengal (BoB) on May 15, then the South China Sea (SCS) on May 25, and South Asia (the Indian summer monsoon) around June 10. However, the meridional temperature gradient in the upper troposphere (200-500 hPa) south of the Tibetan Plateau did not reverse its sign until the onset of the Indian summer monsoon. This suggests that some other mechanism must be responsible for the monsoon onset over the BoB, which is often referred to as the first transition of the ASM. The main objective of this study is to explore such a mechanism through an observational analysis. The thermal condition over the Tibetan Plateau was dominated by both sensible and latent heat fluxes, but the sensible heat flux (SHF) shows a substantial increase at the end of April, which is much earlier than the time of the first transition of the ASM. In contrast, changes in the thermal condition over the tropical Indian Ocean is mainly contributed by latent heat fluxes (LHF), the drastic increase of which in mid May coincided with the onset of the ASM. Such an increase occurred concomitantly with a migration of warm water from the southern Indian Ocean and across the Arabian Sea, as well as a strengthening of westerlies associated with the development of a tropical cyclone over the Bay of Bengal. The dynamics of the tropical cyclone allowed the latent heat to be realized through deep convection, which led to the onset of the ASM over the BoB.

Identification of Dynamical Processes at the Tropopause during the Decay of a Cutoff Low Using High-Resolution Airborne Lidar Ozone Measurements

In June 1996, an airborne ozone lidar was successfully used to observe the decay of a cutoff low over southern Europe. This weather system was tracked during several days and sampled with an 8-km horizontal resolution. Most of the measurements took place in the 4–12-km altitude range and ozone-rich layers with a vertical thickness often less than 500 m were seen. Ozone vertical cross sections were obtained at the edges of the cutoff low or in frontal regions next to it. Using complementary data it is shown that these bidimensional cross sections characterize the ozone spatial field well. Two main features of the ozone distribution are large variability of the ozone tropopause and the presence of numerous ozone-rich layers within the troposphere. This paper focuses on the first feature. Observed ozone tropopauses compare well with potential vorticity ones derived from ECMWF analyses. The magnitude of the ozone to potential vorticity ratio also indicates that no significant diabatic mechanism contributes to the ozone transfer from the stratosphere to the troposphere above 325 K. An analysis of the evolution of the ozone vertical gradient in the upper troposphere (80–120 ppb) and lowermost stratosphere (120–200 ppb) is used to illustrate its usefulness as a diagnostic tool of dynamical processes. Large differences are found between air masses near and within the cutoff low. Vertical stretching induced by the PV anomaly cannot completely account for them. Differential advection in frontal regions and convective erosion on the eastern edge of the cutoff low tend to sharpen the vertical ozone gradient. On the contrary, clear sky turbulent mixing tends to smooth it. Convective erosion is also likely to transfer ozone from the stratosphere to the troposphere. This is corroborated by ozone vertical cross sections sampling tropospheric air masses three days after the decay of the cutoff low.

An Energy Balance Model Based on Potential Vorticity Homogenization

It has long been suggested that the extratropical eddies originating in baroclinic instability act to neutralize the atmosphere with respect to baroclinic instability. These studies focused on the Charney–Stern condition for stability, and since the implication of this condition was the elimination of meridional temperature gradients at the surface, contrary to observations, there appeared little possibility that the hypothesis was correct. However, Lindzen found that potential vorticity (PV) mixing along isentropic surfaces accompanied by elevated tropopause height and/or reduced jet width could also lead to baroclinic neutralization. Since it is not obvious what implications such a neutral state would have for meridional structure of wind and especially temperature, the authors examine, as a first step, in this paper the implications of an assumed fixed PV gradient in the extratropical troposphere. It is shown that this assumption, combined with an assumption of a moist adiabatic temperature structure in the Tropics, a constraint on surface static stability, and overall radiative equilibrium, suffices to constrain a model earth’s zonal mean climate. Comparison of the model climate with the observed climate, and variation of certain of the model’s assumptions to resolve differences, allow the authors to consider the role of deep convection in the climate of the midlatitudes, to investigate the connection between surface turbulent heat fluxes and meridional energy fluxes carried by baroclinic eddies, and to deduce the role of the stratosphere’s overturning circulation in determining the height of the tropopause.

Pictorial 8-2 : An Improvememt of GPS Positioning in GEONET by the Analysis Including Tropospheric Gradient Model

Pictorial 8-2 : An Improvememt of GPS Positioning in GEONET by the Analysis Including Tropospheric Gradient Model

Transport into the northern hemisphere lowermost stratosphere revealed by in situ tracer measurements

The Lightweight Airborne Chromatograph Experiment (LACE) has made in situ measurements of several long‐lived trace gases in the upper troposphere and lower to middle stratosphere as part of the Observations of the Middle Stratosphere (OMS) balloon program. The tracers measured by LACE include several photolytic species (CFC‐11, CFC‐12, and halon‐1211) as well as SF6. LACE measurements of these long‐lived tracers as well as nearly simultaneous measurements of water vapor and CO2 are used to investigate transport into the lowermost stratosphere, a region where few in situ measurements exist. The measured photolytic species and water vapor are used in a simple mass balance calculation to estimate the mixture of tropospheric and overworld (θ>380 K) air in the lowermost stratosphere. In the northern midlatitudes during September 1996, most of the air in the lowermost stratosphere sampled at the flight location (34.5°N) was transported quasi‐isentropically from the troposphere. Measurements from both a May 1998 midlatitude flight and a June 1997 high‐latitude flight (64.5°N) revealed the air sampled in the lowermost stratosphere to be dominated by downward advection from the overworld. Atmospheric SF6 and CO2 can uniquely reveal timescales and spatial scales of transport due to these species' large growth rates and subsequent latitudinal surface and free tropospheric gradients. Measurements in the lowermost stratosphere from the September northern midlatitude flight coupled with surface measurements of these species revealed a transport timescale of no more than 1.5 months from the surface to the lowermost stratosphere. The SF6 and CO2 mixing ratios were also consistent with mostly Northern Hemisphere tropospheric air in the lowermost stratosphere. These results point out the usefulness of high‐resolution in situ measurements of long‐lived tracers to help determine timescales and spatial scales of transport in the region of the upper troposphere and lowermost stratosphere.

Estimating horizontal gradients of tropospheric path delay with a single GPS receiver

We present evidence that modeling troposphere delay gradients in precise Global Positioning System (GPS) geodesy improves the accuracy and precision of the estimated quantities, and that the estimated gradients resemble real atmospheric moisture gradients observed with a water vapor radiometer (WVR). Using a low elevation angle cutoff, combined with a model of the atmospheric delay gradient as a random walk process leads to 19.5% and 15% average improvement in radial and horizontal site position repeatabilities, respectively, relative to a current state‐of‐the‐art estimation strategy that does not model horizontal gradients and imposes high elevation angle cutoff. The agreement between estimated values of zenith wet delay from collocated GPS receivers and WVRs was improved by at least 25%. Merely lowering the elevation angle cutoff improves the repeatability of the radial component of the site's position vector but tends to degrade the repeatability of the horizontal components of the position vector if troposphere gradients are not properly modeled. The estimates of wet delay gradients from a collocated GPS receiver and a WVR at Onsala, Sweden, seem to be correlated over timescales as short as 15 min. The agreement in azimuth between the GPS‐based and the WVR‐based gradients was at the 10° level, for significant gradients. The GPS was found to under‐estimate the magnitude of the gradients by about 60% relative to the WVR‐based gradients. The ability to sense atmospheric moisture gradients from a single GPS receiver increases the useful information content from networks of GPS receivers by providing additional spatial information for weather forecasting applications.

A study of frost events in areas characterised by the absence of observations

Serious events of radiative frost were reported in an important agricultural area of central Macedonia, the valley of Lagadas, at the end of March 1994. Since the phenomenon has a strong local character and no surface observations are available, a simulation is performed using a three dimensional numerical model in order to investigate the horizontal and vertical thermal structure of the lower troposphere during the frost event. The synoptic analysis reveals that the background flow was weak, associated with anticyclonic circulation and insignificant pressure gradient in the lower troposphere. Under these synoptic conditions, it was demonstrated that the meteorological model represents successfully the temperature and wind fields over the valley of Lagadas during the frost event. It appears that the model can serve as an alternative method to analyse the characteristics of the radiative frost that occurs very frequently over the examined area associated with substantial economic repercussion.

Sensitivity of a multi‐layer quasi‐geostrophic β‐channel to the vertical structure of the equilibrium meridional temperature gradient

AbstractA multi‐layer, quasi‐geostrophic, Boussinesq model on a β‐channel is used to study the sensitivity of the mid‐latitude storm‐track to changes in the vertical structure of the zonal mean equilibrium meridional temperature gradient. Motivation for this study is taken from the observation that the doubled CO2 climatology of general‐circulation models presents a weaker gradient in the lower troposphere and a stronger gradient in the upper troposphere than in a ‘control’ climatology.It is observed that the dynamics of the quasi‐geostrophic model are generally more sensitive to the lower than to the upper tropospheric temperature gradient as a consequence of the importance of shallow eddies. In order to give a good representation of the eddy dynamics it is necessary to use a sufficiently high resolution, which, in the case chosen, proved to be between 10 and 20 tropospheric layers. If only three layers are used the general features of the response are still reproduced, but the model does not represent the sensitivities of the eddy meridional momentum‐flux convergence adequately. It is argued that this is connected with the particular choice taken when parametrizing the surface Ekman friction. When a higher‐resolution model is used, the eddy momentum‐flux convergence proves to be more sensitive to changes in the upper than in the lower meridional equilibrium temperature gradient but is generally much less responsive to these changes than other quantities like the surface eddy potential‐vorticity flux or eddy kinetic energy.For each variable the two sensitivities become close to each other as the baroclinicity of the equilibrium flow is increased because of the prevalence of deep eddies in the dynamics, sensitive only to the total radiative equilibrium wind vertical gradient. For very high values of the baroclinicity of the equilibrium flow the sensitivities approach a constant value in accordance with a simple scaling argument.

Global climate change and ocean upwelling

ABSTRACTThe influence of global greenhouse warming on the ocean's biological productivity may be more complicated and weaker than that proposed by Bakun (1990). A doubled carbon dioxide simulation made with the Canadian Climate Centre atmospheric general circulation model coupled to a simplified mixed layer ocean model suggests that the midlatitude continents do not all follow the Bakun scenario in developing anomalous low pressure in summer and enhancing coastal winds favorable to upwelling. In the open ocean the equatorial and subpolar zonal upwelling bands and the subtropical downwelling bands generally weaken as winds diminish owing to the weakening of the equator‐to‐pole temperature gradient in the lower troposphere under global warming. With a weakening of open ocean upwelling and an absence of enhanced coastal upwelling, the overall effect of global warming could be to decrease the global biological productivity.