Poleward Temperature Gradient Research Articles

Human influence on the recent weakening of storm tracks in boreal summer

Anthropogenic warming can alter large-scale circulation patterns in the atmosphere, which could have serious consequences for regional climate impacts and extreme weather. Observed thermodynamic changes in boreal extratropics have been attributed to human emissions with high confidence, but most circulation changes have not. In particular, not only that in the previous suite of climate models most models do not capture the recent boreal summer storm tracks weakening, but also a quantification of the role of human emissions in the recent storm tracks weakening has not been conducted to date. Here we use the latest suite of climate models, which are found to adequately capture the recent storm tracks weakening, and show that this weakening is attributable to anthropogenic emissions. Human emissions have resulted in more-rapid warming of the high latitudes, and the associated reduction in poleward temperature gradient has weakened the storms. The physical consistency between models and reanalyses increases our confidence in the projected weakening, which presents regional risks including hot-dry extremes in summer.

Influence of the Late-Winter North Atlantic Tripole Sea Surface Temperature Anomalies on Spring Land Surface Temperature in Mid-to-High Latitudes of Western Eurasia

Abstract The spring land surface temperature (LST) over western Eurasia, which is critical for ensuring food security, shows a clear interannual variability. Based on reanalysis data and numerical simulations, we investigated the potential influencing factors and the related mechanisms of spring LST variability in mid-to-high latitudes of western Eurasia (MHWEA). The results show that the North Atlantic tripole sea surface temperature anomalies (SSTAs) in February, which persist into spring, can significantly affect the spring LST variability over MHWEA. Analyses indicate that the positive phase of the North Atlantic tripole SSTAs pattern tends to increase the meridional SST gradient between positive SSTAs over the midlatitude North Atlantic and negative SSTAs over the south of Greenland, which strengthens the low-level atmospheric baroclinicity and thus induces more active transient eddy activities. Correspondingly, a Rossby wave train triggered by the eddy-mediated processes originates from the North Atlantic and propagates downstream, thereby causing anomalous anticyclonic circulation over MHWEA. Meanwhile, the westerly anomalies over the subpolar North Atlantic accelerate the polar front jet and provide a favorable thermodynamical condition for the tropospheric warming over the Barents–Kara Seas by bringing warm and moist oceanic air. The polar warming tends to weaken the poleward temperature gradient at mid-to-high latitudes and then decelerate the Eurasian midlatitude westerlies, thus dynamically contributing to the circulation changes that can affect spring LST over MHWEA. Model results suggest that the link can be generally reproduced. Therefore, the late-winter North Atlantic tripole SSTAs may act as a precursor for the prediction of spring LST over western Eurasia. Significance Statement The purpose of this study is to better understand the mechanisms underlying the interannual variability of the spring land surface temperature over western Eurasia, which is of great significance in ensuring food security. Here we show that the positive phase of the North Atlantic tripole sea surface temperature anomalies in February can modulate the tropospheric warming over the Barents–Kara Seas and further decelerate the Eurasian midlatitude westerlies in spring, which benefits local surface warming over western Eurasia. Our results provide a precursor for forecasting spring land surface temperature over western Eurasia.

Dipole response of early‐summer rainfall in eastern China to 1.5 and 2.0°C global warming

AbstractThe 2015 Paris Agreement set the goal of pursuing efforts to keep global warming to 1.5°C above the pre‐industrial level. It remains unclear how the early‐summer rainfall in China responds to moderate warming. Here, we investigate the changes in June rainfall in eastern China by 1.5 and 2.0°C warming using the ensemble projection from Half a degree Additional warming, Prognosis and Projected Impacts (HAPPI) dataset. Results reveal a dipole response in the June rainfall with the significant increase in southeastern China but a slight decrease in northeastern China at 1.5 and 2.0°C warming compared to the present day. The moisture budget analysis indicates that the dipole rainfall response is caused predominantly by the dynamic response in the anomalous circulation and in turn moisture transport. Compared to the present day, the upper‐tropospheric temperature is significantly enhanced in the tropics but decreased in middle latitudes at the 1.5 and 2.0°C warming. The enhanced poleward temperature gradient strengthens zonal wind on the south of the East Asian westerly jet, leading to a southward displacement of jet. As a result, an anomalous upward motion is reinforced over the north of 30°N with an anomalous downward motion over the south of 30°N, leading to a dipole rainfall anomaly in eastern China. In addition, the warming also induces a thermodynamic response with increased moisture and in turn rainfall across eastern China, further enhancing rainfall in southeastern China, but weakening the decreased rainfall in northeastern China.

First Detection of a Brief Mesoscale Elevated Stratopause in Very Early Winter

AbstractElevated stratopauses are typically associated with prolonged disturbed conditions in the Northern Hemisphere polar winter. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and the Microwave Limb Sounder (MLS) observed a short‐lived and highly zonally asymmetric stratopause at mesospheric altitudes in November 2009, the earliest in the season reported so far. The Arctic climatological winter stratopause vanished, and MIPAS and MLS measured temperatures of 260 K at 82 km and 250 K at 75 km, respectively, in a region smaller than in typical midwinter elevated stratopause events. Planetary wave activity was initially high. Zonal mean zonal winds and the poleward temperature gradient northward of 70°N stayed reversed during 7 days, but the mesosphere did not cool. Wave activity dropped until the eastward stratospheric winds resumed and a strong vortex restored in the mesosphere. The stratopause emerged at high altitudes, staying there for 2–5 days. It was accompanied by enhanced downward transport. It took the stratopause 9 days to move down to its typical winter altitudes.

Increased European heat waves in recent decades in response to shrinking Arctic sea ice and Eurasian snow cover

In recent decades, unprecedented extreme summer heat waves have occurred in Europe, and they have exhibited an increasing trend since 1970s. Although previous studies have suggested that these recent hot European summers could have been instigated by the underlying surface thermal conditions, the possible influence of shrinking Arctic sea ice and Eurasian snow cover on heat waves are not well understood. Herein, we present evidence obtained via observational analyses and numerical experiments indicating that the interdecadal increase in European heat waves is closely linked to the reductions in Arctic sea ice concentration (ASIC) and Eurasian snow cover fraction (EASC) across mid–high latitudes via the excitation of the anomalous Eurasian wave train. The combined effects of declined ASIC and EASC, accompanied by the drier soil and the stronger heat flux, tend to weaken the poleward temperature gradient at mid–high latitudes and affect the midlatitude jet stream and transient eddy activities. These dynamic and thermodynamic circulations increase the likelihood of more persistent European blocking events that favor frequent and strengthened heat waves. Further projection analysis of simulations from 13 CMIP5 climate models suggests that Europe may experience more hot summers as the ASIC and EASC continue to decline over the next century.

Evidence for a volcanic underpinning of the Atlantic multidecadal oscillation

The Atlantic multidecadal oscillation (AMO) is a 60–70 year pattern of sea-surface temperature (SST) variability in the North Atlantic commonly ascribed to internal ocean dynamics and changes in northward heat transport. Recent modeling studies, however, suggest that SSTs fluctuate primarily in response to major volcanic eruptions and changes in atmospheric circulation. Here, we utilize historical SST, atmospheric reanalysis, and stratospheric aerosol optical depth data to examine the basic evidence supporting a volcanic link. We find that cool intervals across the North Atlantic coincide with two distinct episodes of explosive volcanic activity (1880s–1920s and 1960s–1990s), where key eruptions include 1883 Krakatau, 1902 Santa María, 1912 Novarupta, 1963 Agung, 1982 El Chichón, and 1991 Pinatubo. Cool SST patterns develop in association with an increased prevalence of North Atlantic Oscillation (NAO)+ atmospheric patterns caused by stratospheric aerosol loading and a steepened poleward temperature gradient. NAO+ patterns promote wind-driven advection, evaporative cooling, and increased albedo from enhanced Saharan dust transport and anthropogenic aerosols. SSTs across the subpolar gyre are regulated by strength of low pressure near Iceland and the associated wind-driven advection of cold surface water from the Labrador Sea. This is contrary to an interpretation that subpolar SSTs are driven by changes in ocean overturning circulation. We also find that North Pacific and global mean SST declines can be readily associated with the same volcanic triggers that affect the North Atlantic. Thus, external forcing from volcanic aerosols appears to underpin multi-decade SST variability observed in the historical record.

Evidence linking rapid Arctic warming to mid-latitude weather patterns.

The effects of rapid Arctic warming and ice loss on weather patterns in the Northern Hemisphere is a topic of active research, lively scientific debate and high societal impact. The emergence of Arctic amplification—the enhanced sensitivity of high-latitude temperature to global warming—in only the last 10–20 years presents a challenge to identifying statistically robust atmospheric responses using observations. Several recent studies have proposed and demonstrated new mechanisms by which the changing Arctic may be affecting weather patterns in mid-latitudes, and these linkages differ fundamentally from tropics/jet-stream interactions through the transfer of wave energy. In this study, new metrics and evidence are presented that suggest disproportionate Arctic warming—and resulting weakening of the poleward temperature gradient—is causing the Northern Hemisphere circulation to assume a more meridional character (i.e. wavier), although not uniformly in space or by season, and that highly amplified jet-stream patterns are occurring more frequently. Further analysis based on self-organizing maps supports this finding. These changes in circulation are expected to lead to persistent weather patterns that are known to cause extreme weather events. As emissions of greenhouse gases continue unabated, therefore, the continued amplification of Arctic warming should favour an increased occurrence of extreme events caused by prolonged weather conditions.

Climatological onset date of summer monsoon in Vietnam

ABSTRACTIn this study, the climatological summer monsoon onset in Vietnam was investigated by using the 25‐year in situ observations and reanalysis gridded dataset recorded from 1979 to 2003. Results suggested that the onset date varies considerably among the sub‐regions. Rainfall criteria indicate that the earliest onset generally occurs in the northwestern mountainous area around April 25. That in the northern and southern plains is in mid‐May, along with the full establishment of monsoon circulation over the Indochina Peninsula. In the central coastal plain, however, the Foehn wind, an effect of the Asian summer monsoon, causes a dry summer rather than a rainy season. This peculiar characteristic differs substantially from the rainy season in the adjacent regions. Therefore, a new specific criterion for this region is proposed: The summer dry season onset date, which is subjectively indicated in late May, is defined as the appearance time of the dry, downslope Foehn wind.Monsoon onset in Vietnam and surrounding areas is preceded by the development of convective activity and circulation features over the equatorial Indian Ocean and the southern Bay of Bengal. It also coincides with the northward retreat of mid‐latitude westerlies and the eastward retreat of the easterly trade wind associated with the western Pacific sub‐tropical high. A relationship between the occurrence of the Meiyu front and the summer monsoon onset over the South China Sea (SCS) is identified. Particularly, cyclonic development concurrent with monsoon onset appears over the SCS and Taiwan area, and a large amount of moisture is observed to converge along the southern side of the front. Simultaneously, the northward extension of the tropical easterly jet (TEJ), which is driven by the reversed poleward temperature and pressure gradient as a consequence of warming over inland India relative to the surrounding ocean, also plays an important role in summer monsoon circulation.

Drivers of projected change in arctic moist static energy transport

AbstractWe explore annual and seasonal changes in moist static energy transport (MSE) into the Arctic over the 21st century as projected by the National Center for Atmospheric Research Community Climate System Model, version 3. Self‐organizing maps are used to assess changes in MSE and its components—the latent heat flux and dry static energy flux (DSE)—across 70°N. These are computed from multilevel fields of specific humidity, meridional wind, geopotential, and temperature spanning periods in the 20th century (1960 to 1999) and the 21st century (2070 to 2089). The 21st century simulation incorporates the Special Report on Emission Scenarios A2 scenario. A strong decrease in tropospheric DSE of about 9% by the end of 21st century offsets an increase in latent heat flux of about 20% relative to its 20th century average. The combined changes result in a total annual decrease in tropospheric MSE of about 3% by the late 21st century. The difference, while statistically not significant, represents a weak negative feedback on Arctic amplification. Self‐organizing maps also allow an attribution of changes in MSE to factors related to varying atmospheric dynamics and/or thermodynamics. A positive contribution to the MSE related to more frequent low pressure systems in high latitudes (dynamic factor) occurs in all seasons, particularly in the summer. The decrease in DSE is mainly due to a weakened poleward temperature gradient (thermodynamic factor) during all seasons except summer, which in turn is caused by amplified warming at high latitudes as a result of increased greenhouse gases.

Effects of global warming on wind energy availability

The use of wind energy reduces our greenhouse gas emissions into the atmosphere. In this study, we proposed a generic power-law relationship between global warming and the usable wind energy (Betz’s law). The power law index (∼4, region dependent) is then determined using simulated atmospheric parameters from eight global coupled ocean-atmosphere climate models (CGCMs). It is found that the power-law relationship holds across all eight climate models and also is time scale independent. Reduction of wind power scales with the degree of warming according to a generic power-law relationship. Thus, the earlier we switch to clean energy, and thereby decrease the global climate warming trend, the more cost-effective will be the harnessing of wind energy. This relationship is an area-averaged consequence of the reduced poleward temperature gradient as the climate warms during the 21st Century; it does not imply spatial uniformity over a region of interest.

Bjerknes Compensation at High Northern Latitudes: The Ocean Forcing the Atmosphere

Abstract The mechanisms for Bjerknes compensation of heat transport variations through the atmosphere and ocean on decadal time scales are investigated, using data output from a preindustrial control run of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It has recently been shown that Bjerknes compensation occurs on decadal time scales in a long preindustrial control run of HadCM3. This result is elaborated on by performing lead/lag correlations of the atmospheric and oceanic heat transports. By using statistical analysis, Bjerknes compensation is observed on decadal time scales at latitudes between 50° and 80°N. A maximum compensation rate of ∼55% occurs at 70°N. At this latitude, the correlation rate peaks when the ocean leads the atmosphere by one year. The mechanisms by which Bjerknes compensation occurs at this latitude are investigated. Anomalies in oceanic heat transport appear to be associated with variations in the strength of the Atlantic meridional overturning circulation (MOC). The associated sea surface temperature (SST) anomalies are in general too weak to assert a significant impact on the atmosphere. At 70°N, however, such SST anomalies are a prelude to the transition from sea ice coverage to open water after which the associated changes in heat exchange with the atmosphere are strong enough to force an atmospheric response. Because of the presence of a strong MOC component in the Atlantic Ocean, this interaction is confined to the region where the northeast Atlantic and Arctic Oceans connect. The atmospheric response to increased (decreased) heating from below is a decreased (increased) poleward temperature gradient, leading to a decreased (increased) heat transport by baroclinic eddies. The anomalous thermal low that is set up by heating from the ocean is associated with anomalous advection of cold air from the Greenland landmass.

A Generalized Momentum Framework for Looking at Baroclinic Circulations

Abstract This paper introduces the concept of potential momentum, which is a nonlocal measure of the thermal structure that has momentum units. Physically, it may be interpreted as the zonal momentum that the flow would realize through an adiabatic redistribution of mass that made the isentropic thickness uniform poleward of a reference latitude. At the surface, a poleward temperature gradient is equivalent to an easterly reservoir of potential momentum. Potential momentum gives a global picture of the thermal field that also takes into account the meridional structure. When the mean flow is redefined in terms of the total momentum (the standard zonal momentum plus this newly defined potential momentum), the mean-flow response to the eddy forcing can be formulated locally. This allows one to relate in equilibrium the eddy absorption and the restoration of the mean flow. Using the concept of potential momentum, vertical propagation can be related to thermal forcing by means of an equation analogous to the relation between meridional propagation and surface friction. Based on these ideas, it is argued that the time-mean eddy propagation is largely constrained by the strength and structure of the forcing. The new formalism is applied to a forced–dissipative two-layer model to study the dependence of meridional and vertical propagation and of the global circulation on changes in the diabatic and frictional forcing time scales. It is found that baroclinic adjustment, in the form of a fixed potential vorticity (PV) gradient, is a good approximation because the intensity of the circulation only depends weakly on the forcing time scale. Another remarkable result is that, owing to the robustness of this PV gradient, thermal homogenization is always enhanced with stronger friction.

Momentum and heat balances in steady coastal currents

The momentum balance for coastal currents forced by a longshore wind stress and a longshore density gradient is examined for coastlines of various orientations. We isolate the coastal circulation by requiring that the longshore wind stress and density gradient become uniform in the adjacent deep ocean in which the longshore bottom pressure gradient is assumed to be zero, and derive a vorticity equation for the longshore current for a prescribed cross-shelf variation of wind stress and density structure. Solutions of this equation are obtained for a uniform longshore wind stress along a coastline with an exponential shelf in two situations: (A) For a uniform cross-shelf density on the inner shelf and a zero longshore bottom pressure gradient on the outer shelf, the division between the two regions occurring approximately at the depth of the main thermocline, and (B) for a thermohaline structure which is independent of the cross-shelf co-ordinate such that the coastal current is barotropic. It is shown that on a zonal coast the coastal transports due to the wind stress forcing are of O(σ τ Wl ϱ 0β ) , and due to the density gradient forcing of O(− σ ϱ 0 gh T 2/ β∂δ 0/ ∂l) where τ Wl and − ϱ 0 ∂δ 0/ ∂l are respectively, the longshore wind stress and surface density gradient, ϱ 0 is a reference density for seawater, g is the acceleration of gravity, h T is the scale depth for the main thermocline, β= df dy where f=2Ω sinϕ in which Ω is the angular speed of rotation of Earth, and ϕ is the latitude, and σ= W T W is a shelf parameter in which W T is the exponential shelf scale depth and W is an eddy scale. On eastern boundaries the transports are less [especially in (A)] and are identically zero on western boundaries on which the transport is determined by external (non-local) processes. The longshore velocity structure on the western boundary depends strongly on σ and incorporates a system of currents and counter-currents. The heat balance for the coastal circulation is investigated by considering the solution of the heat conservation equation for the surface temperature with the cross-shelf boundary conditions of zero heat flux through the coast and into deep ocean. It is found that the sea surface temperature anomaly is zero for wind stress forcing and positive for density gradient forcing (at constant salinity). Mixed forcing however may lead to upwelling which is characterized by a negative sea surface anomaly. Upwelling occurs on non-western boundaries on which there is a longshore wind stress of sufficient intensity or on western boundaries where there is an external transport of sufficient magnitude to generate a net transport up the longshore surface density gradient. This occurs on eastern coasts where there is poleward advection of warm water and sufficiently strong equatorward meridional winds, and on western coasts in a polar gyre where there is a strong equatorward external transport and a poleward temperature gradient. The Ekman circulation maintains the upwelling anomalies.

The 2‐day wave in the middle atmosphere: Observations and theory

A recurrent, global‐scale, 2‐day oscillation, in the middle atmosphere is well documented in a variety of data sources: meteor wind, partial reflection, satellite, and rocketsonde. The observed period is almost invariably near 2.1 days, and the zonal structure has been established as a westward propagating, wave number 3 mode. Observations also indicate that the meridional wind component maximizes near the equator and dominates the wind field at low latitudes. These features suggest the disturbance's identification with the wave number 3 Rossby‐gravity normal mode of a windless, isothermal atmosphere. Such modes are found to exist largely independent of the mean fields. The eigenperiods are only modestly altered by mean wind variations on the lowest three scale heights, and the influence on the mode structure is local. The vertical growth of amplitude is enhanced in regions of weak westerly wind relative to the wave and equatorward temperature gradient, while the vertical growth rate is retarded and amplitudes may even decay in regions of strong westerlies or poleward temperature gradient. In realistic mean fields, the third Rossby‐gravity mode has magnified responses very near 2.25 days in both equinox and solstice conditions. The horizontal structure remains largely intact in the lowest levels. However, considerable asymmetry is introduced in the solstice upper stratosphere and mesosphere by the magnified vertical growth of amplitude in the summer hemisphere. The latter results from the moderate easterlies and equatorward temperature gradient characteristic of the region. As for the simple eigenstructure, the meridional wind component in this region maximizes near the equator and dominates the wind field up to mid‐latitudes.

Forced and Free Variations of Weather and Climate

Variations of weather and climate are termed “forced” or “free” according to whether or not they are produced by variations in external conditions. In many simple climate models, the poleward transport of sensible heat in the atmosphere has been treated as a diffusive process, and has been assumed to be proportional to the poleward temperature gradient. The validity of this assumption, for various space and time scales, is tested with 10 years of twice-daily upper level weather data. The space scales are defined by a spherical harmonic analysis, while the time scales are defined by a “poor man's spectral analysis.” The diffusive assumption is verified for the long-term average and the seasonal variations of the largest space scale, but it fails to hold for most of the remaining scales. It is shown that diffusive behavior can be expected only for forced scales. It is suggested that most of the scales resolved by the data are free.

Venezuelan Rain Systems and the General Circulation of the Summer Tropics II: Relations between Low and High Latitudes

Abstract The question, Why are there discrete synoptic weather systems in the tropics without a basic current that can become baroclinically unstable, is considered. Release of kinetic energy through vertical circulations and maintenance of the surface flow against friction by mean values indicate that existence of a simple general circulation cell should be possible. But, in the descending branch the requisite poleward temperature gradient cannot be maintained with radiation cooling. An additional mechanism for heat removal must be sought. Extratropical troughs extending deep into the tropics may provide such a mechanism. At first their role in setting off tropical rain areas is investigated dynamically. It is found that a large imbalance of forces develops in the northerly current on the western side of the troughs which causes the equator-bound air to turn sharply clockwise. Absolute vorticity approaches zero and intense high-tropospheric divergence is initiated acting as a siphon on the lower layers. ...

Winds and Wave Motions to 110 km at Mid-Latitudes. II. Mean Winds at 52°N, 1969–73

Abstract The results of radiowave partial reflection wind (drift) observations, 62–116 km, for the years 1969–73, at 52°N, 107°W (Saskatoon), are presented, and are compared with current empirical models. Agreement is satisfactory to 85 km; but at higher altitudes, differences exist; notably an annual variation of zonal flow above 100 km, whose direction is eastward in summer and westward in winter. The semi-annual variation of winds is shown to be limited to 85–100 km, and is considered to be due to two out-of-phase annual variations identifiable at higher and lower altitudes. A region of positive (poleward) temperature gradient (high-latitude warming) is identified in the range ∼80 to at least 107 km in winter, and another region (∼75 to ≳107 km) of negative temperature gradient is identified in summer. The relationships of these regions to circulation characteristics are discussed.