Ngar-Cheung Lau

Variability of the Observed Midlatitude Storm Tracks in Relation to Low-Frequency Changes in the Circulation Pattern

Abstract The principal modes of month-to-month variability of the wintertime storm tracks over the North Pacific and North Atlantic are identified by empirical orthogonal function analysis of the root-mean-square statistics of bandpass (2.5–6 day) filtered geopotential height data for 19 yr. One of the two leading modes depicts fluctuations in the level of synoptic-scale activity without any noticeable spatial displacement of the storm track axes, whereas the other mode is associated with meridional shifts of the storm tracks from their time-averaged positions. Higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions. It is demonstrated that the leading storm track modes are linked to some of the best-known monthly averaged teleconnection patterns. The dipolar western Pacific and western Atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans, whereas the m...

Impact of ENSO on the Variability of the Asian–Australian Monsoons as Simulated in GCM Experiments

Abstract The influences of El Nino–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...

An Observational Study of the Northern Hemisphere Wintertime Circulation

Abstract Twice-daily synoptic analyses are statistically analyzed for a sample of nine individual winters. Temporal variance and covariance quantities at each individual grid point are partitioned into “low-pass” (approximately 10–90 day) and “band-pass” (approximately 2.5–6 day) components by means of conventional filtering procedures. The time-filtered variance and covariance fields are displayed in terms of hemisphere maps. Included in the analysis are sea level pressure, 300 mb height, 500 mb wind statistics, and 850 mb temperature and poleward heat flux. The most definitive results of the study involve the “band-pass” fluctuations which appear to be associated with developing baroclinic waves. The fields of band-pass 1000, 50 and 300 mb geopotential height, as well as the 500 mb meridional wind component and relative vorticity all exhibit elongated variance maxima coincident with the two major Northern Hemisphere storm tracks, which lie downstream and somewhat poleward of the cores of the Asian and N...

The Role of the “Atmospheric Bridge” in Linking Tropical Pacific ENSO Events to Extratropical SST Anomalies

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...

A modeling study of the relative roles of tropical and extratropical SST anomalies in the variability of the global atmosphere-ocean system

Abstract In three parallel experiments, an atmospheric general circulation model has been subjected to observed, monthly varying sea surface temperature (SSI) 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 5 1 5-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 value. The SST pattern of this mode resembles that associated with El Nino events. The accompanying 5 1 5-mb height anomaly i...

Atmosphere–Ocean Variations in the Indo-Pacific Sector during ENSO Episodes

Abstract The influences of El Nino–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 anom...

GFDL's CM2 Global Coupled Climate Models. Part III: Tropical Pacific Climate and ENSO

Abstract Multicentury integrations from two global coupled ocean–atmosphere–land–ice models [Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), developed at the Geophysical Fluid Dynamics Laboratory] are described in terms of their tropical Pacific climate and El Nino–Southern Oscillation (ENSO). The integrations are run without flux adjustments and provide generally realistic simulations of tropical Pacific climate. The observed annual-mean trade winds and precipitation, sea surface temperature, surface heat fluxes, surface currents, Equatorial Undercurrent, and subsurface thermal structure are well captured by the models. Some biases are evident, including a cold SST bias along the equator, a warm bias along the coast of South America, and a westward extension of the trade winds relative to observations. Along the equator, the models exhibit a robust, westward-propagating annual cycle of SST and zonal winds. During boreal spring, excessive rainfall south of the equator is linked to an unrealistic rever...

Interactions between Global SST Anomalies and the Midlatitude Atmospheric Circulation

Abstract A review is given of the processes contributing to variability of the atmosphere–ocean system on interannual timescales. Particular emphasis is placed on the relationships between midlatitude atmospheric fluctuations and sea surface temperature (SST) anomalies in various geographical sites. Various hypotheses are tested using output from a coordinated set of general circulation model experiments, which are subjected to time-varying SST forcing observed during 1946–88 in different parts of the worlds oceans. It is demonstrated that tropical Pacific SST fluctuations associated with El Nino–Southern Oscillation (ENSO) episodes produce a strong extratropical response in the model atmosphere, whereas the atmospheric signal associated with midlatitude SST anomalies is less robust. Analysis of a 100-yr control experiment, which is conducted in the absence of any interannual SST forcing, indicates that a substantial fraction of the simulated atmospheric variability may be attributed to internal dynamica...

Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances

Abstract The three-dimensional structure and propagation characteristics of tropical synoptic scale transients during the northern summer we studied with twice daily ECMWF global gridded analyses for the 1980–1987 period. Regions of enhanced variability in relative vorticity at 850 mb are identified in the western Pacific, eastern Pacific, Bay of Bengal/northern India and eastern Atlantic/western Africa sectors. Dominant spectral peaks with time scales ranging from 3 to 8 days are noted in the power spectra for these locations. The lag-correlation and regression statistics of tropical fluctuations with synoptic time scales are examined. Strong teleconnectivity and temporal coherence are found over all of the active sites with enhanced vorticity variance, as well as over the western Atlantic/Caribbean and the Indochinese Peninsula. These results indicate that a substantial amount of synoptic scale variability in the tropics is associated with propagating wavelike disturbances that remain coherent over seve...

Modeling the Seasonal Dependence of the Atmospheric Response to Observed El Niños in 1962–76

Abstract Two 15-year atmospheric GCM integrations are conducted with the lower boundary over the tropical Pacific being forced by observed month-to-month sea surface temperature (SST) changes during the period 1962–76. A descriptive account is given on selected aspects of the 30-year model climatology, as well as the anomalous model behavior through the life cycles of El Nino–Southern Osicillation (ENSO) episodes centered in the years 1965, 1969 and 1972. These model results are compared with available observations reported in the published literature. Particular attention is devoted to the timing of various simulated meteorological phenomena with respect to the spatially and temporally evolving SST forcing, and to the climatological seasonal cycle. An assessment is made of the capability of the model to simulate the seasonal dependence of various climatological features relevant to ENSO. The phenomena examined include the flow field and rainfall in different monsoon regions, the planetary scale waves in ...