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Ngar-Cheung Lau and Mary Jo Nath

Abstract

The characteristics of summertime heat waves in North America are examined using reanalysis data and simulations by two general circulation models with horizontal resolution of 50 and 200 km. Several “key regions” with spatially coherent and high amplitude fluctuations in daily surface air temperature are identified. The typical synoptic features accompanying warm episodes in these regions are described. The averaged intensity, duration, and frequency of occurrence of the heat waves in various key regions, as simulated in the two models for twentieth-century climate, are in general agreement with the results based on reanalysis data.

The impact of climate change on the heat wave characteristics in various key regions is assessed by contrasting model runs based on a scenario for the twenty-first century with those for the twentieth century. Both models indicate considerable increases in the duration and frequency of heat wave episodes, and in number of heat wave days per year, during the twenty-first century. The duration and frequency statistics of the heat waves in the mid-twenty-first century, as generated by the model with 50-km resolution, can be reproduced by adding the projected warming trend to the daily temperature data for the late twentieth century, and then recomputing these statistics. The detailed evolution of the averaged intensity, duration, and frequency of the heat waves through individual decades of the twentieth and twenty-first centuries, as simulated and projected by the model with 200-km resolution, indicates that the upward trend in these heat wave measures should become apparent in the early decades of the twenty-first century.

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Ngar-Cheung Lau and Mary Jo Nath

Abstract

The structural characteristics and vorticity dynamics of westward-traveling patterns (WTP) in the troposphere are examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses based on observations for the 1973–95 period, as well as the output from a 100-yr integration of a general circulation model (GCM) with a rhomboidal truncation at 30 wavenumbers and 14 vertical levels. An identical set of diagnostic tools, including progressive/retrogressive variance analysis, cross-spectra, and complex empirical orthogonal functions (EOFs), are applied to the reanalysis and GCM datasets for 300-mb height. These diagnoses all indicate that the WTP are most prominent during the cold season in the high-latitude zone extending westward from northwestern Canada to northeastern Siberia, with a typical period of ∼22 days. Outstanding episodes are identified on the basis of the temporal coefficients of the leading complex EOF. Composite charts of the anomalous 300-mb height, sea level pressure, and 850-mb temperature fields at various phases of these events are constructed. The typical circulation changes accompanying the passage of the WTP are similar to those associated with well-known regional weather phenomena such as amplified pressure ridges over Alaska, cold air outbreaks over western North America and east Asia, and heavy snowfall over the Pacific Northwest. The occurrence of the WTP over the North Pacific is also characterized by notable changes in the spatial distribution and intensity of synoptic scale activity.

The contributions of relative vorticity advection, planetary vorticity advection (the “β effect”), and horizontal divergence to the vorticity tendency in various phases of the composite wave at 300, 500, and 850 mb are investigated. In the mid- and upper troposphere, the vorticity dynamics of the WTP is similar to that of free external Rossby waves, with the β effect (which leads to westward propagation) being the dominant term, whereas the eastward advection of relative vorticity is less important due to the weak mean zonal flow in the Alaska–Siberia sector.

Most of the essential characteristics of the observed WTP deduced from the NCEP–NCAR reanalyses are well reproduced by the GCM. The realism with which this phenomenon can be simulated in a model environment offers considerable promise for using the GCM as a tool for studying the impact of WTP on intraseasonal atmospheric variability in extended model experiments, and for assessing the dependence of the locality and activity level of the WTP on various states of the ambient circulation.

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Ngar-Cheung Lau and Mary Jo Nath

Abstract

The role of the atmospheric circulation as a “bridge” between sea surface temperature (SST) anomalies in the tropical Pacific and those in the midlatitude northern oceans is assessed. The key processes associated with this atmospheric bridge are described using output from four independent simulations with a general circulation model subjected to month to month SST variations observed in the tropical Pacific during the 1946–1988 period and to climatological SST conditions elsewhere (the “TOGA” runs). In episodes with prominent SST anomalies in the tropical Pacific, extratropical perturbations in the simulated atmospheric temperature, humidity, and wind fields induce changes in the latent and sensible heat fluxes across the air-sea interface of the midlatitude oceans. These anomalous fluxes in turn lead to extratropical SST changes.

The relevance of the atmospheric bridge mechanism is evaluated by driving a motionless, 50-m deep oceanic negative mixed layer model at individual grid points with the local surface fluxes generated in the TOGA runs. The negative feedback of the mixed layer temperature anomalies on the imposed flux forcing is taken into account by introducing a linear damping term with a 5-mouth dissipative time scale. This simple system reproduces the basic spatial and temporal characteristics of the observed SST variability in the North Pacific and western North Atlantic.

The two-way air-sea feedbacks associated with the atmospheric bridge are investigated by performing four additional 43-year runs of a modified version of the TOGA Experiment. These new “TOGA-ML” runs predict the ocean temperature outside the tropical Pacific by allowing the atmosphere to interact fully with the same mixed layer model mentioned above. The results support the notion that midlatitude ocean-atmosphere interaction can be modeled as a first-order Markov process, in which the red-noise response of mixed layer temperature is driven by white–noise atmospheric forcing in the presence of linear damping.

The amplitude of near-surface atmospheric anomalies appearing in the TOGA-ML runs is higher than that in the TOGA runs. This finding implies that, in the TOGA-ML scenario, the midlatitude oceanic responses to atmospheric driving could exert positive feedbacks on the atmosphere, thereby reinforcing the air-sea coupling. The enhanced atmosphere-ocean interactions operating in TOGA-ML prolong the duration of persistent meteorological episodes in that experiment. A comprehensive survey is conducted of the persistence characteristics simulated in TOGA, TOGA- ML, and several other experiments subjected to prescribed SST forcing at various sites. Model scenarios in which observed tropical Pacific SST anomalies act in conjunction with SST perturbations in midlatitudes (either prescribed or predicted) are seen to produce the highest frequency of persistent events.

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Ngar-Cheung Lau and Mary Jo Nath

Abstract

The climatological characteristics and interannual variations of the development of the South Asian summer monsoon (SASM) in early summer are studied using output from a 200-yr simulation of a coupled atmosphere–ocean general circulation model (CM2.1). Some of the model results are compared with corresponding observations. Climatological charts of the model and observational data at pentadal intervals indicate that both the precipitation and SST signals exhibit a tendency to migrate northward. Enhanced monsoonal precipitation at a given site is accompanied by a reduction in incoming shortwave radiation and intensification of upward latent heat flux, and by oceanic cooling.

An extended empirical orthogonal function analysis is used to identify the dates for initiation of the northward march of SASM in individual summers. It is noted that early monsoon development prevails after the mature phase of La Niña events, whereas delayed development occurs after El Niño.

Sensitivity experiments based on the atmospheric component of CM2.1 indicate that the effects of SST forcings in the tropical Pacific (TPAC) and Indian Ocean (IO) on monsoon development are opposite to each other. During El Niño events, the atmospheric response to remote TPAC forcing tends to suppress or postpone monsoon development over South Asia. Conversely, the warm SST anomalies in IO, which are generated by the “atmospheric bridge” mechanism in El Niño episodes, lead to accelerated monsoon development. The net result of these two competing effects is an evolution scenario with a timing that is intermediate between the response to TPAC forcing only and the response to IO forcing only.

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Ngar-Cheung Lau and Mary Jo Nath

Abstract

The three-dimensional structure and temporal evolution of tropospheric fluctuations appearing on various time scales in observed and model-simulated atmospheres are investigated using cross-spectral analyses. The datasets examined include NMC analyses of the 500 mb height and sea level pressure fields for 18 winters, as well as a 12-winter simulation of the same fields by a 15-wavenumber general circulation model at GFDL. Statistically significant phase differences between 500 mb height fluctuations at selected centers of action and the corresponding fluctuations at all other grid points are displayed for various frequency bands using a vectorial format. Similar plots are constructed to elucidate the vertical phase structure in the middle and lower troposphere at individual grid points, as well as the propagation characteristics of the sea level pressure field in the vicinity of sloping terrain. It is demonstrated that these phase/coherence diagrams offer a useful alternative for quantifying the lead/lag relationships between different anomaly centers associated with some of the well-known teleconnection patterns.

The spectral results presented here indicate that the spatial and temporal behavior of the Pacific/North American, Atlantic and Northern Asian Patterns, as documented in various recent studies, exhibit a notable frequency dependence in both real and model atmospheres. For periods of 27–80 days, the atmospheric variability over the Pacific and Atlantic Basins is organized in north–south oriented dipoles, with an almost 180° out-of-phase relationship between oscillations at the opposite poles. As attention is shifted to the 20- and 10-day time scales the north–south seesaw pattern gradually weakens, and all three teleconnection patterns mentioned above are characterized by successive downstream development from west to east of alternating troughs and ridges. Fluctuations with periods longer than 20 days acquire an equivalent barotropic structure over much of the northern oceans, with in-phase variations at 500 mb and at sea level.

The eddy behavior undergoes still further changes as one considers the 4-day period band. The high frequency disturbances tend to be elongated in the meridional direction. The corresponding horizontal phase variations are indicative of continuous eastward propagation across the midlatitude oceans and northern Siberia. The vertical phase variations suggest a systematic transition from a distinctly baroclinic structure at the starting points of such cyclone tracks, to a more barotropic structure in regions farther east.

The perturbations near the eastern and northern peripheries of the Tibetan Plateau are noted for their weak coherence in the vertical direction. Horizontal phase diagrams based on sea level pressure data reveal that the path of near-surface fluctuations tends to be aligned parallel to the local topographic contours in this region.

Comparison between model and observational results indicates that the GCM examined here is capable of reproducing the frequency and geographical dependence of the principal modes of variability in the Northern Hemisphere wintertime circulation.

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Ngar-Cheung Lau and Mary Jo Nath

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The summertime northeastward march of the climatological maritime monsoon over the South China Sea (SCS) and subtropical western North Pacific (WNP) is examined using the output from a 200-yr integration of a coupled atmosphere–ocean general circulation model (GCM). Increased cloud cover and surface wind speed during monsoon onset over the SCS in May–June reduce the incoming shortwave flux and enhance the upward latent heat flux at the ocean surface, thereby cooling the local sea surface temperature (SST). The resulting east–west gradient in the SST pattern, with lower temperature in the SCS and higher temperature in the WNP, is conducive to eastward migration of the monsoon precipitation over this region. Upon arrival of the precipitation center in the WNP in July–August, the local circulation changes lead to weakening of the mei-yu–baiu rainband near 30°N. The subsequent increases in local shortwave flux and SST impart a northward tendency to the evolution of the WNP monsoon. Many of these model inferences are supported by a parallel analysis of various observational datasets.

The modulation of the above climatological scenario by El Niño–Southern Oscillation (ENSO) events is investigated by diagnosing the output from the coupled GCM and from experiments based on the atmospheric component of this GCM with SST forcings being prescribed separately in the equatorial Pacific, Indian Ocean, and SCS/WNP domains. During the May period after the peak phase of ENSO, the simulated monsoon onset over the SCS occurs later (earlier) than normal in El Niño (La Niña) events. These changes are primarily remote responses to the anomalous SST forcing in the equatorial Pacific and Indian Ocean. The ENSO-related changes in the SCS/WNP are associated with above-normal (below normal) mei-yu–baiu activity during warm (cold) events. In the ensuing July period of the warm events, the simulated precipitation response over the SCS to the local warm SST anomaly tends to oppose the remote response to SST forcing in the northern Indian Ocean. In the July period of cold events, the equatorial Pacific SST anomaly retains its strength and moves still farther westward. This forcing cooperates with the cold SST anomaly in the SCS in influencing the precipitation pattern in the SCS/WNP sector.

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Ngar-Cheung Lau and Mary Jo Nath

Abstract

The influences of El Niño–Southern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 1950–95 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios.

The essential characteristics of the model climatology for the Asian–Australian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding warm and cold ENSO events reveal that a majority of the warm episodes are accompanied by below-normal summer rainfall in India and northern Australia, and above-normal winter rainfall in southeast Asia. The polarity of these anomalies is reversed in the cold events. These relationships are particularly evident in the TOGA experiment.

Composite charts of the simulated flow patterns at 850 and 200 mb indicate that the above-mentioned precipitation changes are associated with well-defined circulation features over the affected monsoon regions. Dry conditions are typically coincident with low-level anticyclonic anomalies, and vice versa. These circulation centers are situated to the northwest and southwest of a prominent precipitation anomaly situated near 120°–150°E at the equator, which corresponds to the western half of a dipolar heating pattern resulting from east–west displacements of the ascending branch of the Walker circulation during ENSO. The large-scale anomalous circulation over the monsoon regions is similar to that of a Rossby wave pattern associated with a condensational heat source or sink in the western equatorial Pacific.

Diagnosis of the output from the TOGA-ML experiment reveals that variations in the circulation and cloud cover accompanying ENSO-induced monsoon anomalies could modulate the latent heat and shortwave radiative fluxes at the air–sea interface in the Indian Ocean, thereby changing the SST conditions in that basin. These simulated SST anomalies compare well with observational results. The local atmospheric response to these SST anomalies opposes the remote response of the south Asian monsoon flow to SST anomalies in the tropical Pacific, thus leading to a negative feedback loop in the air–sea coupled system.

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Ngar-Cheung Lau and Mary Jo Nath

Abstract

The influences of El Niño–Southern Oscillation (ENSO) events in the tropical Pacific on interannual variability of the coupled ocean–atmosphere systems in the North Pacific and North Atlantic have been studied using a suite of experiments with a rhomboidal 30-wavenumber, 14-layer general circulation model (GCM). Observed month-to-month fluctuations of the sea surface temperature (SST) in the tropical Pacific during the 1950–95 period were prescribed as the lower boundary condition for the GCM. The SST conditions outside of the tropical Pacific were predicted by a simple ocean mixed layer model with a constant depth. Four independent integrations under this “Tropical Ocean–Global Atmosphere–Mixed Layer (TOGA-ML)” scenario were conducted.

Both observational and model results indicate that the imposed ENSO forcing during midwinter is accompanied by prominent atmospheric circulation changes over the North Pacific and Atlantic. These teleconnection patterns in turn alter the heat exchange across the local sea–air interface. The extratropical SST anomalies generated by this “atmospheric bridge” mechanism typically attain maximum amplitudes in late winter or early spring.

Detailed diagnoses of the monthly evolution of the surface heat budget during ENSO episodes at selected sites have been performed. The simulated SST response exhibits a 1–2 month delay relative to temperature changes in the overlying atmosphere. This lag relationship is associated with a polarity change of sea-to-air gradients in anomalous temperature and water vapor mixing ratio in late winter. From late autumn through midwinter, the action of the climatological wind on these gradients results in enhancement of the developing SST anomalies. In the months thereafter, the reversed gradients lead to attenuation of the SST signal. Shortwave radiative fluxes associated with variations in cloud cover play an important role in SST variability at some of the subtropical sites.

The nature of sea–air feedbacks in the extratropics has been studied by contrasting the output from the TOGA-ML experiment and from another “TOGA” experiment in which two-way interactions between the atmosphere and ocean outside of the tropical Pacific were eliminated. Incorporation of sea–air coupling in TOGA-ML is seen to enhance the persistence of the ENSO-related atmospheric anomalies in the extratropics through late winter and early spring. Comparison with results from previous studies on midlatitude sea–air interactions suggests that part of the atmospheric signal in TOGA-ML may be attributed to forcing from extratropical SST anomalies produced by the atmospheric bridge mechanism.

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Ngar-Cheung Lau and Mary Jo Nath

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The impacts of ENSO on the evolution of the East Asian monsoon have been studied using output from a general circulation model experiment. Observed monthly variations of the sea surface temperature (SST) field have been prescribed in the tropical eastern and central Pacific, whereas the atmosphere has been coupled to an oceanic mixed layer model beyond this forcing region. During the boreal summer of typical El Niño events, a low-level cyclonic anomaly is simulated over the North Pacific in response to enhanced condensational heating over the equatorial central Pacific. Advective processes associated with the cyclone anomaly lead to temperature tendencies that set the stage for the abrupt establishment of a strong Philippine Sea anticyclone (PSAC) anomaly in the autumn. The synoptic development during the onset of the PSAC anomaly is similar to that accompanying cold-air surges over East Asia.

The air–sea interactions accompanying the intraseasonal variations (ISV) in the model atmosphere exhibit a strong seasonal dependence. During the summer, the climatological monsoon trough over the subtropical western Pacific facilitates positive feedbacks between the atmospheric and oceanic fluctuations. Conversely, the prevalent northeasterly monsoon over this region in the winter leads to negative feedbacks. The onset of the PSAC anomaly is seen to be coincident with a prominent episode of the leading ISV mode. The ENSO events could influence the amplitude of the ISV by modulating the large-scale flow environment in which the ISV are embedded. Amplification of the summer monsoon trough over the western Pacific during El Niño enhances air–sea feedbacks on intraseasonal time scales, thereby raising the amplitudes of the ISV. A weakening of the northeasterly monsoon in El Niño winters suppresses the frequency and strength of the cold-air surges associated with the leading ISV mode in that season.

Many aspects of the model simulation of the relationships between ENSO and the East Asian monsoon are in agreement with observational findings.

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Ngar-Cheung Lau and Mary Jo Nath

Abstract

The heat and vorticity transports by synoptic-scale eddies at various levels between 1000 and 100 mb have been compiled for each winter month of the 1966–84 period using time-filtered daily analyses produced by the U.S. National Meteorological Center. These circulation statistics were used to compute the three-dimensional distributions of the quasigeostrophic geopotential tendency and vertical motion induced by baroclinic and barotropic eddy processes in individual months. The latter fields serve as the basis for describing the synoptic-scale eddy forcing associated with the leading modes of month-to-month variability of the storm tracks over the North Pacific and North Atlantic. These modes are associated with the meridional displacements of the storm-track axes from their climatological positions.

The placement of a storm track at a certain latitude ϕ in a certain month is accompanied by enhanced convergence of eddy heat fluxes poleward of ϕ. In the tropospheric column poleward of the storm track, this baroclinic eddy forcing leads to positive geopotential tendency near the tropopause and negative geopotential tendency near sea level, as well as strong positive temperature tendency and rising motion. In the same month, the convergence of eddy vorticity transport is also enhanced poleward of ϕ. This barotropic forcing results in negative geopotential tendency throughout the troposphere, as well as rising motion and weak negative temperature tendency poleward of ϕ. All of these features appear with reversed polarity in latitudes equatorward of ϕ.

In the upper troposphere, the geopotential tendency induced by vorticity fluxes is stronger than the opposing effects due to heat fluxes, so that the net eddy forcing retains most of the characteristics of the forcing associated with barotropic processes alone, but with considerably reduced amplitudes. Near sea level, the geopotential tendencies induced by heat and vorticity fluxes reinforce each other and are comparable in amplitude. Throughout the troposphere, the patterns of net geopotential tendency exhibit a positive spatial correlation with those of the concurrent monthly averaged height anomaly. The characteristic time scale associated with this constructive eddy forcing in the storm-track region ranges from several days at 1000 mb, to 1–2 months near the tropopause. On the other hand, the eddy-induced temperature tendency is negatively correlated with the local monthly mean temperature anomaly. The dissipative time scale for this thermal forcing in the storm-track region is ∼10 days at 850 mb.

The barotropic geopotential tendency and the baroclinic temperature tendency are essentially determined by the convergences of vorticity and heat fluxes, respectively. The eddy-induced secondary circulation plays a minor role in these tendencies.

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