Bibliography - Mary Jo Nath

1. Lau, Ngar-Cheung, Ants Leetma, and Mary Jo Nath, February 2008: Interactions between the responses of North American climate to El Niño–La Niña and to the secular warming trend in the Indian–Western Pacific Oceans. Journal of Climate, 21(3), 476-494.
   [ Abstract PDF ]

   The modulation of El Niño and La Niña responses by the long-term sea surface temperature (SST) warming trend in the Indian–Western Pacific (IWP) Oceans has been investigated using a large suite of sensitivity integrations with an atmospheric general circulation model. These model runs entail the prescription of anomalous SST conditions corresponding to composite El Niño or La Niña episodes, to SST increases associated with secular warming in IWP, and to combinations of IWP warming and El Niño/La Niña. These SST forcings are derived from the output of coupled model experiments for climate settings of the 1951–2000 and 2001–50 epochs. Emphasis is placed on the wintertime responses in 200-mb height and various indicators of surface climate in the North American sector. The model responses to El Niño and La Niña forcings are in agreement with the observed interannual anomalies associated with warm and cold episodes. The wintertime model responses in North America to IWP warming bear a distinct positive (negative) spatial correlation with the corresponding responses to La Niña (El Niño). Hence, the amplitude of the combined responses to IWP warming and La Niña is notably higher than that to IWP warming and El Niño. The model projections indicate that, as the SST continues to rise in the IWP sector during the twenty-first century, the strength of various meteorological anomalies accompanying La Niña (El Niño) will increase (decrease) with time. The response of the North American climate and the zonal mean circulation to the combined effects of IWP forcing and La Niña (El Niño) is approximately equal to the linear sum of the separate effects of IWP warming and La Niña (El Niño). The summertime responses to IWP warming bear some similarity to the meteorological anomalies accompanying extended droughts and heat waves over the continental United States.

2. Lau, Ngar-Cheung, and Mary Jo Nath, in press: A Model Investigation of the Role of Air-Sea Interaction in the climatological evolution and ENSO-related variability of the Summer monsoon over South China Sea and western North Pacific. Journal of Climate. 7/08.
   [ Abstract ]

   The processes contributing to the summertime northeastward march of the climatological maritime monsoon over the South China Sea (SCS) and subtropical western North Pacific (WNP) are examined using the output from a 200-year integration of a coupled atmosphere-ocean general circulation model (GCM). Increased cloud cover and surface wind speed during monsoon onset over 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 SCS and higher temperature in WNP, is conducive to eastward migration of the monsoon precipitation over this region. Upon arrival of the precipitation center at WNP in July-August, the local circulation changes lead to weakening of the Meiyu-Baiu rainband near 30°N. The subsequent increases in the 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 SCS/WNP and equatorial Pacific domains. During the May-June period after the peak phase of ENSO, the simulated monsoon onset over 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. The above-normal precipitation in SCS in the ensuing July-August period of the warm events is mainly forced by the local warm SST anomaly, which is linked to ENSO forcing in the tropical Pacific through an ‘atmospheric bridge’. In the July-August period of cold events, the equatorial SST anomaly retains its strength and moves still closer to SCS/WNP. This forcing cooperates with the cold SST anomaly in SCS, and brings about dry conditions in SCS/WNP. The ENSO-related changes in SCS/WNP are associated with above-normal (below-normal) Meiyu-Baiu activity during warm (cold) events.

3. Lau, Ngar-Cheung, Ants Leetma, and Mary Jo Nath, 2006: Attribution of Atmospheric Variations in the 1997–2003 Period to SST Anomalies in the Pacific and Indian Ocean Basins. Journal of Climate, 19(15), 3507-3628.
   [ Abstract PDF ]

   The individual impacts of sea surface temperature (SST) anomalies in the deep tropical eastern–central Pacific (DTEP) and Indo-western–central Pacific (IWP) on the evolution of the observed global atmospheric circulation during the 1997–2003 period have been investigated using a new general circulation model. Ensemble integrations were conducted with monthly varying SST conditions being prescribed separately in the DTEP sector, the IWP sector, and throughout the World Ocean. During the 1998–2002 subperiod, when prolonged La Niña conditions occurred in DTEP and the SST in IWP was above normal, the simulated midlatitude atmospheric responses to SST forcing in the DTEP and IWP sectors reinforced each other. The anomalous geopotential height ridges at 200 mb in the extratropics of both hemispheres exhibited a distinct zonal symmetry. This circulation change was accompanied by extensive dry and warm anomalies in many regions, including North America. During the 1997–98 and 2002–03 El Niño events, the SST conditions in both DTEP and IWP were above normal, and considerable cancellations were simulated between the midlatitude responses to the oceanic forcing from these two sectors. The above findings are contrasted with those for the 1953–58 and 1972–77 periods, which were characterized by analogous SST developments in DTEP, but by cold conditions in IWP. It is concluded that a warm anomaly in IWP and a cold anomaly in DTEP constitute the optimal SST configuration for generating zonally elongated ridges in the midlatitudes. Local diagnoses indicate that the imposed SST anomaly alters the strength of the zonal flow in certain longitudinal sectors, which influences the behavior of synoptic-scale transient eddies farther downstream. The modified eddy momentum transports in the regions of eddy activity in turn feed back on the local mean flow, thus contributing to its zonal elongation. These results are consistent with the inferences drawn from zonal mean analyses, which accentuate the role of the eddy-induced circulation on the meridional plane.

4. Lau, Ngar-Cheung, and Mary Jo Nath, 2006: ENSO Modulation of the Interannual and Intraseasonal Variability of the East Asian Monsoon—A Model Study. Journal of Climate, 19(18), 4508-4530.
   [ Abstract PDF ]

   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.

5. Lau, Ngar-Cheung, Ants Leetma, Mary Jo Nath, and H L Wang, 2005: Influences of ENSO-induced Indo-Western Pacific SST anomalies on extratropical atmospheric variability during the Boreal summer. Journal of Climate, 18(15), doi:10.1175/JCLI3445.1.
   [ Abstract ]

   The causes for the observed occurrence of anomalous zonally symmetric upper-level pressure ridges in the midlatitude belts of both hemispheres during the year after warm El Niño-Southern Oscillation (ENSO) events have been investigated. Sea surface temperature (SST) anomalies in the Indo-western Pacific (IWP) sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed ENSO episodes in the eastern/central tropical Pacific. The spatiotemporal evolution of these SST conditions through a composite ENSO cycle was then inserted as lower boundary conditions within the IWP domain in an ensemble of atmospheric general circulation model (GCM) integrations. This experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of ENSO. The model evidence hence supports the notion that these global-scale atmospheric changes are primarily responses to SST perturbations in IWP, which are in turn linked to ENSO variability in the equatorial Pacific by the "atmospheric bridge" mechanism. Experimentation with a stationary wave model indicates that the Eastern Hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated with precipitation anomalies in the IWP region, which are closely related to the underlying SST changes. Diagnosis of the output from the GCM integrations reveals that these circulation changes due to diabatic heating are accompanied by alterations of the propagation path and intensity of the high-frequency eddies at locations farther downstream. The geopotential tendencies associated with the latter disturbances bear some resemblance to the anomalous height pattern in the Western Hemisphere. Such local eddy–mean flow feedbacks hence contribute to the zonal symmetry of the atmospheric response pattern to forcing in the IWP region. Analysis of zonally averaged circulation statistics indicates that the mean meridional circulation induced by divergence of anomalous transient eddy momentum fluxes in ENSO events could also generate zonally symmetric perturbations in midlatitudes. The model-simulated precipitation and surface temperature anomalies in the North American sector in response to SST changes in IWP suggest an increased frequency of droughts and heat waves in that region during the summer season after warm ENSO events.

6. Lau, Ngar-Cheung, and Mary Jo Nath, 2004: Coupled GCM simulation of atmosphere-ocean variability associated with zonally asymmetric SST changes in the tropical Indian Ocean. Journal of Climate, 17(2), 245-265.
   [ Abstract PDF ]

   The nature of a recurrent pattern of variability in the tropical Indian Ocean (IO) during the boreal autumn has been investigated using a 900-yr experiment with a coupled atmosphere-ocean general circulation model. This Indian Ocean Pattern (IOP) is characterized by zonal surface wind perturbations along the equator, as well as east-west contrasts in the anomalous sea surface temperature (SST), surface pressure, and precipitation fields. The IOP is seen to be linked to the El Niño-Southern Oscillation (ENSO) phenomenon in the tropical Pacific. By constructing composite charts and analyzing the heat budget for the top ocean layer, it is illustrated that the ENSO-related changes in the surface wind modify the intensity of oceanic upwelling, horizontal temperature advection, and surface heat fluxes in various parts of the IO basin. These processes lead to SST perturbations with opposite signs in the eastern and western equatorial IO. Further diagnosis of the model output reveals that some strong IOP episodes occur even in the near absence of ENSO influences. In such IOP events that do not coincide with prominent ENSO development, the most noteworthy signal is a zonally elongated sea level pressure anomaly situated south of Australia during the southern winter. The anomalous atmospheric circulation on the equatorward flank of this feature contributes to the initiation of IOP-like events when the ENSO forcing is weak. Both simulated and observational data show that the pressure anomaly south of Australia is part of a hemisphere-wide pattern bearing a considerable resemblance to the Antarctic Oscillation. This annular mode of variability is characterized by opposite pressure changes in the midlatitude and polar zones, and is only weakly correlated with ENSO. The findings reported here indicate that the IOP is attributable to multiple factors, including remote influences due to ENSO and extratropical changes, as well as internal air-sea feedbacks occurring within the IO basin.

7. Lau, Ngar-Cheung, Mary Jo Nath, and H Wang, 2004: Simulations by a GFDL GCM of ENSO-related variability of the coupled atmosphere-ocean system in the East Asian monsoon region In East Asian Monsoon, C.-P. Chang, Ed., World Scientific Series on Meteorology of East Asia, 271-300.
   [ Abstract ]

   The impact of El Niño-Southern Oscillation (ENSO) on the East Asian Monsoon (EAM) has been examined using a general circulation model (GCM). The observed monthly changes in sea surface temperature (SST) in the equatorial Pacific east of 172°E during 1950-99 were inserted as the lower boundary condition of the model. For all oceanic grid points lying outside of the region of SST prescription, the atmosphere was coupled to an oceanic mixed layer model. The typical evolution of the atmosphere-ocean system during ENSO was analyzed using composite charts. These patterns show that the key changes in the EAM sector are related to a prominent sea level pressure anomaly simulated over the South China Sea and subtropical northwestern Pacific. During warm ENSO events, the circulation anomalies associated with this anomalous anticyclone correspond to weaker winter monsoon flows along the East Asian coast, as well as above-normal precipitation over southern China. These signals move systematically eastward during the following spring and summer. Stationary wave modeling indicates that the atmospheric anomaly in the EAM region is essentially a Rossby-wave response to the ENSO-related diabatic heating pattern over the equatorial western Pacific. The wintertime atmospheric circulation anomalies over the western Pacific generate strong SST anomalies in the following spring. Further model diagnoses indicate that the feedback of these SST changes on the atmosphere leads to eastward propagation of the pressure anomaly in the EAM region, and to amplification of rainfall anomalies along the Meiyu-Baiu front. The atmospheric and oceanic changes in the EAM sector described in this chapter are discussed in the broader context of ENSO influences on the entire Asian-Australian monsoon system.

8. Lau, Ngar-Cheung, and Mary Jo Nath, 2003: Atmosphere-Ocean variations in the Indo-Pacific sector during ENSO episodes. Journal of Climate, 16(1), 3-20.
   [ Abstract PDF ]

   The influences of El Niño–Southern Oscillation (ENSO) events on air–sea interaction in the Indian–western Pacific (IWP) Oceans have been investigated using a general circulation model. Observed monthly sea surface temperature (SST) variations in the deep tropical eastern/central Pacific (DTEP) have been inserted in the lower boundary of this model through the 1950–99 period. At all maritime grid points outside of DTEP, the model atmosphere has been coupled with an oceanic mixed layer model with variable depth. Altogether 16 independent model runs have been conducted. #Composite analysis of selected ENSO episodes illustrates that the prescribed SST anomalies in DTEP affect the surface atmospheric circulation and precipitation patterns in IWP through displacements of the near-equatorial Walker circulation and generation of Rossby wave modes in the subtropics. Such atmospheric responses modulate the surface fluxes as well as the oceanic mixed layer depth, and thereby establish a well-defined SST anomaly pattern in the IWP sector several months after the peak in ENSO forcing in DTEP. In most parts of the IWP region, the net SST tendency induced by atmospheric changes has the same polarity as the local composite SST anomaly, thus indicating that the atmospheric forcing acts to reinforce the underlying SST signal. #By analyzing the output from a suite of auxilliary experiments, it is demonstrated that the SST perturbations in IWP (which are primarily generated by ENSO-related atmospheric changes) can, in turn, exert notable influences on the atmospheric conditions over that region. This feedback mechanism also plays an important role in the eastward migration of the subtropical anticyclones over the western Pacific in both hemispheres.

9. Lau, Ngar-Cheung, and Mary Jo Nath, 2000: Impact of ENSO on the variability of the Asian-Australian monsoons as simulated in GCM experiments. Journal of Climate, 13(24), 4287-4309.
   [ Abstract PDF ]

   The influences of El NiñoSouthern 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 195095 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 AsianAustralian 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 eastwest 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 airsea 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 airsea coupled system.

10. Lau, Ngar-Cheung, and Mary Jo Nath, 1999: Observed and GCM-simulated westward-propagating, planetary-scale fluctuations with approximately three-week periods. Monthly Weather Review, 127(10), 2324-2345.
    [ Abstract PDF ]

    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 accompanyi ng 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 "ß 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 ß 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.

11. Broccoli, Anthony J., Ngar-Cheung Lau, and Mary Jo Nath, 1998: The cold ocean-warm land pattern: Model simulation and relevance to climate change detection. Journal of Climate, 11(11), 2743-2763.
    [ Abstract PDF ]

    Surface air temperatures from a 1000-yr integration of a coupled atmosphere-ocean model with constant forcing are analyzed by using a method that decomposes temperature variations into a component associated with a characteristic spatial structure and a residual. The structure function obtained from the coupled model output is almost identical to the so-called cold ocean-warm land (COWL) pattern based on observations, in which above-average spatial mean temperature is associated with anomalously cold oceans and anomalously warm land. This pattern features maxima over the high-latitude interiors of Eurasia and North America. The temperature fluctuations at the two continental centers exhibit almost no temporal correlation with each other. The temperature variations at the individual centers are related to telecommunication patterns in sea level pressure and 500-mb height that are similar to those identified in previous observational and modeling studies. As in observations, variations in the polarity and amplitude of this structure function are an important source of spatially averaged surface air temperature variability. Results from parallel integrations of models with more simplified treatments of the ocean confirm that the contrast in thermal inertia between land and ocean is the primary factor for the existence of the COWL pattern, whereas dynamical air-sea interactions do not play a significant role. The internally generated variability in structure function amplitude in the coupled model integration is used to assess the importance of the upward trend in the amplitude of the observed structure function over the last 25 years. This trend, which has contributed to the accelerated warming of Northern Hemisphere temperature over recent decades, is unusually large compared with the trends generated internally by the coupled model. If the coupled model adequately estimates the internal variability of the real climate system, this would imply that the recent upturn in the observed structure function may not be purely a manifestation of unforced variability. A similar monotonic trend occurs when the same methodology is applied to a model integration with time-varying radiative forcing based on past and future CO2 and sulfate aerosol increases. This finding illustrates that this decomposition methodology yields ambiguous results when two distinct spatial patterns, the "natural" COWL pattern (i.e., that associated with internally generated variability) and the anthropogenic fingerprint, are present in the simulated climate record.

12. Lau, Ngar-Cheung, and Mary Jo Nath, 1996: The Role of the "Atmospheric Bridge" in Linking Tropical Pacific ENSO Events to Extratropical SST Anomalies. Journal of Climate, 9(9), 2036-2057.
    [ Abstract PDF ]

    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 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-month 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.

13. Lau, Ngar-Cheung, and Mary Jo Nath, 1995: A modeling study of the relative roles of tropical and extratropical SST anomalies in the variability of the global atmosphere-ocean system In Proceedings of the International Scientific Conference on Tropical Ocean Global Atmosphere [TOGA] Programme, WCRP-91, WMO/TD No. 717, Geneva, Switzerland, World Meteorological Organization, 578-582.
14. Lau, K M., and Mary Jo Nath, 1995: A modeling study of the relative roles of tropical and extratropical SST anomalies in the variability of the global atmosphere-ocean system In Proceedings of the International Scientific Conference on Tropical Ocean Global Atmosphere [TOGA] Programme, WCRP-91, WMO/TD No. 717, Geneva, Switzerland, World Meteorological Organization, 578-582.
15. Lau, Ngar-Cheung, and Mary Jo Nath, 1994: A modeling study of the relative roles of tropical and extratropical SST anomalies in the variability of the global atmosphere-ocean system. Journal of Climate, 7(8), 1184-1207.
    [ Abstract PDF ]

    In three parallel experiments, an atmospheric general circulation model has been subjected to observed, monthly varying sea surface temperature (SST) conditions in each of the following domains: near-global ocean (GOGA run), tropical Pacific (TOGA run), and midlatitude North Pacific (MOGA run). Four independent realizations were obtained for the model response to the sequence of SST anomalies during the 1946-88 period in each of the above regions. The principal modes of coupling between the imposed SST forcing and the simulated Northern Hemisphere wintertime 515-mb height field in various experiments have been identified using a singular value decomposition (SVD) procedure. The leading SVD mode for the GOGA experiment is qualitatively similar to that based on observational data, although the amplitudes of the simulated height anomalies are notably lower than the observed values. The SST pattern of this mode resembles that associated with El Niño events. The accompanying 515-mb height anomaly is dominated by a wavelike pattern in the North Pacific/North American sector. TOGA experiment reproduces many of the atmosphere-ocean relationships discerned from the GOGA output. Conversely, the MOGA run yields a much weaker and less reproducible response. The contrast between the TOGA and MOGA runs is indicative of the primacy of tropical Pacific SST anomalies in forcing the midlatitude atmospheric circulation. In the TOGA experiment, the remote atmospheric response to tropical Pacific SST anomalies influence the energy exchange across the local air-sea interface, and could thereby perturb the SST field outside of the tropical Pacific. Through this "atmospheric bridge", the tropical Pacific could set the pace for variability of the global ocean. Analysis of the TOGA output indicates that, over the North Pacific, changes in the surface energy fluxes are mainly determined by the surface wind speed and by the strength of temperature and moisture advection. Over the Indian Ocean, variations in the incident solar radiation due to changes in cloud cover also affect the surface fluxes. The worldwide SST tendencies inferred from the variations in surface fluxes simulated in the TOGA experiment are in good agreement with the local observed SST anomalies.

16. Lau, Ngar-Cheung, S G H Philander, and Mary Jo Nath, 1992: Simulation of ENSO-like phenomena with a low-resolution coupled GCM of the global ocean and atmosphere. Journal of Climate, 5(4), 284-307.
    [ Abstract PDF ]

    A 140-year simulation of the ocean-atmosphere climate system has been performed by the GFDL Climate Dynamics Project using a low-resolution coupled general circulation model (GCM). The model was subjected to annually averaged insolation throughout the integration. This coupled system exhibits well-defined fluctuations in the tropical Pacific, with a preferred time scale of 3-4 years. The characteristics of these recurrent anomalies were examined by applying an extended empirical orthogonal function (EEOF) analysis to selected model variables. These results indicate that the simulated oscillations are accompanied by coherent changes in the atmospheric and oceanic circulation. The spatial patterns associated with the leading EEOF mode indicate that SST anomalies make their first appearance off the Peru-Ecuador coast and then migrate steadily westward, with an average transit time of 12-15 months. The arrival and eventual decay of SST fluctuations in the western Pacific is typically followed by the initiation of anomalies of the opposite polarity along the American coasts. The space-time evolution of various meteorological and oceanographic signals exhibits well-defined phase relationships with the SST perturbations. Some aspects of the model behavior during these warm and cold episodes are reminiscent of observed phenomena associated with the El Niño-Southern Oscillation (ENSO). Analysis of the climatological heat budget for the top ocean layer indicates a near balance between horizontal and vertical temperature advection by the time-mean flow, vertical diffusion, and heat input from the overlying atmosphere. Contributions of transient effects to this balance are negligible. The principal mechanisms associated with the simulated ENSO-like cycles were then studied by examining the local heat budget for the SST perturbations. It is shown that the relative importance of various linear advective processes in the heat budget exhibits a notable dependence on geographical location and on the specific phase of the ENSO-like cycle.

17. Philander, S G., Ronald C Pacanowski, Ngar-Cheung Lau, and Mary Jo Nath, 1992: Simulation of ENSO with a global atmospheric GCM coupled to a high-resolution, tropical Pacific Ocean GCM. Journal of Climate, 5(4), 308-329.
    [ Abstract PDF ]

    A global atmospheric general circulation model (GCM) coupled to an oceanic GCM that is dynamically active only in the tropical Pacific simulates variability over a broad spectrum of frequencies even though the forcing, the annual mean incoming solar radiation, is steady. Of special interest is the simulation of a realistically irregular Southern Oscillation between warm El Niño and cold La Niña states. Its time scale is on the order of 5 years. The spatial structure is strikingly different in the eastern and western halves of the ocean basin. Sea surface temperature changes have their largest amplitude in the central and eastern tropical Pacific, but the low-frequency zonal wind fluctuations are displaced westward and are large over the western half of the basin. These zonal wind anomalies are essentially confined to the band of latitudes 10°N to 10°S so that they form a jet and have considerable latitudinal shear. During El Niño the associated curl contributes to a pair of pronounced minima in thermocline depth, symmetrically about the equator in the west, near 8°N and 8°S. In the east, where the low-frequency wind forcing is at a minimum, the deepening of the thermocline in response to the winds in the west have a very different shape-an approximate Gaussian shape centered on the equator. The low-frequency sea surface temperature and zonal wind anomalies wax and wane practically in place and in phase without significant zonal phase propagation. Thermocline depth variations have phase propagation; it is eastward at a speed near 15 cm s-1 along the equator in the western half of the basin and is westward off the equator. This phase propagation, a property of the oceanic response to the quasi-periodic winds that force currents and excite a host of waves with periods near 5 years, indicates that the ocean is not in equilibrium with the forcing. In other words, the ocean-atmosphere interactions that cause El Niño to develop at a certain time are countered and, in due course, reversed by the delayed response of the ocean to earlier winds. This "delayed oscillator" mechanism that sustains interannual oscillations in the model differs in its details from that previously discussed by Schopf and Suarez and others. The latter investigators invoke an explicit role for Kelvin and Rossby waves. These waves cannot be identified in the low-frequency fluctuations of this model, but they are energetic at relatively short periods and are of vital importance to a quasi-resonant oceanic mode with a period near 7 months that is excited in the model. The similarities and differences between the results of this simulation and those with other models, especially the one described in a companion paper, are discussed.

18. Lau, Ngar-Cheung, and Mary Jo Nath, 1991: Variability of the baroclinic and barotropic transient eddy forcing associated with monthly changes in the midlatitude storm tracks. Journal of the Atmospheric Sciences, 48(24), 2589-2613.
    [ Abstract PDF ]

    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 phi in a certain month is accompanied by enhanced convergence of eddy heat fluxes poleward of phi. 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 phi. This barotropic forcing results in negative geopotential tendency throughout the troposphere, as well as rising motion and weak negative temperature tendency poleward of phi. All of these features appear with reversed polarity in latitudes equatorward of phi. 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 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.

19. Lau, Ngar-Cheung, and Mary Jo Nath, 1990: A general circulation model study of the atmospheric response to extratropical SST anomalies observed in 1950-79. Journal of Climate, 3(9), 965-989.
    [ Abstract PDF ]

    The principal modes of variability of the seasonally averaged 515 mb height and SST fields have been identified using rotated principal component (RPC) analysis. The extrema of the first atmospheric mode reside over the North Atlantic and Eurasia, whereas the second mode is associated with height anomalies in the North Pacific/North American sector. Cross-correlation analysis reveals that these two atmospheric modes are linked to leading patterns of the SST field in the North Atlantic and North Pacific, respectively. It is also demonstrated that the extrema in leading RPC patterns of the SST field in the northern oceans are almost coincident with the sites of maximal covariability between the SST and 515 mb height fields. Regression charts of selected model parameters versus the SST variations off the Newfoundland coast and northwest of Hawaii have been constructed. These two reference maritime sites have been identified by the RPC and cross-correlation analyses as being correlated with the strongest atmospheric signals. The model fields examined in this manner include the geopotential height at various pressure levels, precipitation, heat flux across the air-sea interface, as well as temporal variance and covariance statistics. These regression maps indicate that the atmospheric response to midlatitude SST anomalies has an equivalent barotropic structure. The presence of SST perturbations in the extratropics are associated with displacements of the storm track axes, and with relocations of the midlatitude rainbelts and preferred sites of heat transfer from the underlying ocean. The changes in the locality of synoptic scale eddy activity are accompanied by alterations in the transient eddy forcing of the quasi-stationary flow. The geopotential height tendencies associated with these anomalous eddy effects exhibit a positive spatial correlation with the seasonally averaged, downstream atmospheric response. The time scale for the eddy induced tendencies to produce such seasonal height anomalies is on the order of several days. These findings suggest that the transient disturbances act as an essential intermediary between the extratropical SST forcing and the time-mean atmospheric response. The principal atmospheric anomaly pattern in the North Atlantic/Eurasian sector exhibits substantial correlations with SST fluctuations in the tropical South Atlantic; whereas oceanic anomalies in the equatorial Pacific are only weakly associated with atmospheric circulation changes in the North Pacific/North American region. The temporal lead/lag relationships between the simulated atmospheric anomalies and the prescribed SST changes have been explored.

20. Philander, S G., Ngar-Cheung Lau, Ronald C Pacanowski, and Mary Jo Nath, 1989: Two different simulations of the Southern Oscillation and El Niño with coupled ocean-atmosphere general circulation models. Philosophical Transactions of the Royal Society of London, A, 329, 167-178.
    [ Abstract PDF ]

    Two different coupled ocean-atmosphere models simulate irregular interannual fluctuations that in many respects resemble El Niño Southern Oscillation phenomena. For example, the spatial structure of various fields at the peaks of the warm El Niño and cold La Niña phases of the oscillation are realistic. This success indicates that the models capture certain aspects of the interactions between the ocean and atmosphere that cause the Southern Oscillation. The principal difference between the models, namely the prominence of oceanic Kelvin waves in one but not the other, causes the two models to differ significantly in the way El Niño episodes evolve, and in the mechanisms that cause a turnabout from El Niño to La Niña and vice versa. It is possible that the different processes that determine the properties of the simulated oscillations all play a role in reality, at different times and in different regions. Each of the models captures some aspects of what is possible. However, reality is far more complex than any model developed thus far and additional processes not yet included are also likely to have a significant influence on the observed Southern Oscillation.

21. Lau, Ngar-Cheung, and Mary Jo Nath, 1987: Frequency dependence of the structure and temporal development of wintertime tropospheric fluctuations- Comparison of a GCM simulation with observations. Monthly Weather Review, 115(1), 251-271.
    [ Abstract PDF ]

    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 vairability over the Pacific and Atlantic Basins is organized in north-south oriented dipoles, with an almost 180 degree out-of-phase relationship between oscillations at the oppposite 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.

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