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X. L. Wang

Abstract

In the tropical Pacific, the annual variation of sea surface temperature (SST) consists of two distinct components with respect to the equator, 1) an antisymmetric extratropical annual cycle and 2) a symmetric equatorial annual cycle (SEAC). The former, explaining about 70% of SST variance on average, is primarily a delayed response to the solar radiation, while the latter, accounting for about 15% of total SST variance, is a result of equatorial ocean-atmosphere interaction.

The antisymmetric extratropical annual cycle does not interact directly with the EI Niño-Southern Oscillation (ENSO). However, the symmetric equatorial annual cycle is mutually coupled with ENSO evolution, and the artificial separation between the two can result in a distorted description of some ENSO characteristics.

The SEAC-ENSO coupling involves at least two modes. One mode represents the equatorial eastern Pacific with an apparent phase evolution corresponding to the annual development of the cold tongue regime. In this mode, about half of the local interannual variance is modulated by the annual cycle, and the most preferable modulation occurs around later winter-early spring. The other mode is a standing SST pattern concentrated in the central Pacific. About one-third of the local interannual variance is directly projected on the mean annual cycle with a maximum strength in later northern summer to early fall.

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Kingtse C. Mo
and
X. L. Wang

Abstract

The sensitivity of the systematic error of extended-range forecasts to sea surface temperature (SST) anomalies is investigated. General circulation model (GCM) experiments were performed to quantify error patterns for warm, normal, and cold SST anomalies in the equatorial central Pacific. The model underestimates the strength of tropical convection during warm El Niño-Southern Oscillation (ENSO) episodes and has large zonal mean errors in midlatitudes. The model captures the negative Pacific-North American teleconnection (PNA) pattern during the cold ENSO episodes, but the simulated amplitude is too weak. The time-mean errors during warm and cold ENSO events bear little resemblance to the errors estimated from a 10-yr integration, which includes both warm and cold episodes. The time-mean error of a 10-yr integration is a good estimate of the systematic model error only for those years when SSTs are close to climatology.

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X. L. Wang
and
C. F. Ropelewski

Abstract

Secular changes in the spatial and temporal structures of the El Niño/Southern Oscillation (ENSO) cycle using four different versions of global sea surface temperature (SST) analysis are examined. The assessments were made for both multidecadal climate means and multidecadal measures of variability by separating the SST variations into low-frequency (periods longer than 30 years) and high-frequency components. The reliability of these estimates is also addressed.

This study substantiates a conceptual framework that views the multidecadal, low-frequency variations as a varying climate “base state” upon which ENSO-scale variability is superposed. The secular changes of the climate base state were quantified both in space and in time. The analysis suggests that multidecadal SST variability has been concentrated in the South Atlantic and Indian Ocean Basins. The Pacific is dominated by the ENSO-scale variability. The analyses reveal that variations in the climate base state and ENSO-scale variability were positively correlated; that is, ENSO-scale variability is higher (lower) when the climate mean SST is relatively warmer (colder). However, the quantification of secular changes of the ENSO-scale variability was found to be sensitive to the particular SST analysis used. Therefore, the conclusions from this study are subject to further verification by using more variables and longer records.

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Julian X. L. Wang
and
Dian J. Gaffen

Abstract

Climatological surface temperature and humidity variables for China are presented based on 6-hourly data from 196 stations for the period of 1961–90. Seasonal and annual means for daytime, nighttime, and the full day are shown. The seasonal cycle of moisture is primarily controlled by the east Asia monsoon system, with dominant factors of temperature change in northern and western China and of moisture advection associated with monsoon circulations in the southeast.

Trends during 1951–94 are estimated for each station and for four regions of the country, with attention paid to the effects of changes in instrumentation, observing time, and station locations. The data show evidence of increases in both temperature and atmospheric moisture content. Temperature and specific humidity trends are larger at nighttime than daytime and larger in winter than summer. Moisture increases are observed over most of China. The increases are several percent per decade for specific humidity, and several tenths of a degree per decade for temperature and dewpoint. Increasing trends in summertime temperature and humidity contribute to upward trends in apparent temperature, a measure of human comfort.

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B. Yu
,
X. L. Wang
,
X. B. Zhang
,
J. Cole
, and
Y. Feng

Abstract

The decadal covariability of northern wintertime land surface temperature and 500-hPa geopotential anomalies is examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research and the Twentieth-Century Reanalyses over the twentieth century and a 996-yr preindustrial climate simulation from the Canadian Earth System Model. Based on the reanalysis data, the covariability is dominated by two leading maximum covariance analysis (MCA) modes. MCA1 is characterized by temperature anomalies over most of Canada, the eastern United States, Mexico, and Eurasian mid- to high latitudes, accompanied by anomalies of opposite sign elsewhere over northern landmasses. MCA2 features temperature anomalies over most of North America, Eurasia, and Greenland with opposite anomalies elsewhere. In the upper troposphere the synoptic vorticity fluxes reinforce the anomalous circulation, while in the lower troposphere advection by the anomalous mean flow offsets the eddy forcing and maintains the decadal temperature perturbation. The MCA1-associated variability has a broad spectrum over decadal–interdecadal time scales, while the MCA2-related variability has a significant power peak around 20 yr. The change of temperature and geopotential trends around 1990 tends to be a decadal-scale shift in winter and has significant features of the leading mode of the decadal covariability. The climate model has broadly similar decadal covariability, including the leading MCA patterns as well as the temporal evolution of the patterns. The decadal temperature and geopotential anomalies primarily covary with the North Atlantic Oscillation but also with the variability of the North Pacific index, while the Southern Oscillation index variability tends to be the least important predictor for the northern decadal temperature and geopotential anomalies.

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Kingtse C. Mo
,
X. L. Wang
, and
M. S. Tracton

Abstract

The impact of the sea surface temperature (SST) anomalies on predictions in the extratropics has been studied by comparing circulation changes in general circulation model experiments generated with observed and climatological sea surface temperatures for warm and cold Southern Oscillation events. The small samples may be insufficient for drawing firm conclusions, but results suggest that the linkage between tropical and extratropical circulations in the model resembles observed relationships.

As the atmosphere responds to the warm (cold) tropical SSTs, the convection in the Pacific intensifies (diminishes). The enhanced (suppressed) convection in the tropics enhances (suppresses) the local Hadley circulation and changes the position and strength of the divergent outflow. This in turn changes the position, shape, and strength of the upper-level subtropical jet streams. After the jets move to their new positions, synoptic eddies organize themselves at the exit regions of the jets.

The response time for the upper-level streamfunction in the tropics is about 10 days, but the changes in the position of the subtropical jets occur after 15–20 days. The largest impact on predictions is located in the tropics and downstream in the Pacific-North America and the Pacific-South America regions.

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R. W. Higgins
,
Y. Yao
, and
X. L. Wang

Abstract

Key features of the U.S. summer precipitation regime are examined within the context of the evolving North American monsoon system. The focus is on the antecedent and subsequent atmospheric conditions over the conterminous United States relative to the onset of monsoon precipitation over the southwestern United States, which typically begins in early July. The onset of the monsoon in this region is determined using a precipitation index, based on daily observed precipitation for a 31-yr (1963–94) period. Lagged composites of the observed precipitation and various fields from the NCEP–NCAR reanalysis for the period 1979–94 provide a comprehensive picture of atmospheric conditions during the evolution of the U.S. warm season precipitation regime.

The summer precipitation regime is characterized by an out-of-phase relationship between precipitation over the Southwest and the Great Plains–northern tier and an in-phase relationship between precipitation over the Southwest and the East Coast. Changes in the upper-tropospheric wind and divergence fields (mean vertical motion) are broadly consistent with the evolution of this precipitation pattern. Enhanced upper-tropospheric divergence in the vicinity and south of the upper-tropospheric monsoon high coincides with enhanced upper-tropospheric easterlies and Mexican monsoon rainfall after onset. Over the Great Plains and along the northern tier, the middle- and upper-tropospheric flow is more convergent and rainfall diminishes after onset to the north and east of the monsoon high. The frequency of occurrence of the Great Plains low-level jet (LLJ) and southerly moisture transport change little during the evolution. However, LLJ-related precipitation is controlled by changes in the large-scale flow related to the North American monsoon system. There is increased upper-tropospheric divergence and precipitation after onset in the vicinity of an “induced” trough over the eastern United States. The pattern of evaporation minus precipitation from the NCEP–NCAR reanalysis shows broad consistency with the divergence of the vertically integrated flux of water vapor during the monsoon, although the resolution in the NCEP–NCAR reanalysis is inadequate to yield quantitatively accurate regional estimates of these fields. In agreement with earlier studies, the NCEP–NCAR reanalysis indicates that most of the moisture below 850 hPa over the desert Southwest comes from the northern Gulf of California, while most of the moisture at and above 850 hPa arrives from over the Gulf of Mexico.

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B. Yu
,
H. Lin
,
V. V. Kharin
, and
X. L. Wang

Abstract

The interannual variability of wintertime North American surface temperature extremes and its generation and maintenance are analyzed in this study. The leading mode of the temperature extreme anomalies, revealed by empirical orthogonal function (EOF) analyses of December–February mean temperature extreme indices over North America, is characterized by an anomalous center of action over western-central Canada. In association with the leading mode of temperature extreme variability, the large-scale atmospheric circulation features an anomalous Pacific–North American (PNA)-like pattern from the preceding fall to winter, which has important implications for seasonal prediction of North American temperature extremes. A positive PNA pattern leads to more warm and fewer cold extremes over western-central Canada. The anomalous circulation over the PNA sector drives thermal advection that contributes to temperature anomalies over North America, as well as a Pacific decadal oscillation (PDO)-like sea surface temperature (SST) anomaly pattern in the midlatitude North Pacific. The PNA-like circulation anomaly tends to be supported by SST warming in the tropical central-eastern Pacific and a positive synoptic-scale eddy vorticity forcing feedback on the large-scale circulation over the PNA sector. The leading extreme mode–associated atmospheric circulation patterns obtained from the observational and reanalysis data, together with the anomalous SST and synoptic eddy activities, are reasonably well simulated in most CMIP5 models and in the multimodel mean. For most models considered, the simulated patterns of atmospheric circulation, SST, and synoptic eddy activities have lower spatial variances than the corresponding observational and reanalysis patterns over the PNA sector, especially over the North Pacific.

Open access
L. A. Vincent
,
X. Zhang
,
R. D. Brown
,
Y. Feng
,
E. Mekis
,
E. J. Milewska
,
H. Wan
, and
X. L. Wang

Abstract

Trends in Canada’s climate are analyzed using recently updated data to provide a comprehensive view of climate variability and long-term changes over the period of instrumental record. Trends in surface air temperature, precipitation, snow cover, and streamflow indices are examined along with the potential impact of low-frequency variability related to large-scale atmospheric and oceanic oscillations on these trends. The results show that temperature has increased significantly in most regions of Canada over the period 1948–2012, with the largest warming occurring in winter and spring. Precipitation has also increased, especially in the north. Changes in other climate and hydroclimatic variables, including a decrease in the amount of precipitation falling as snow in the south, fewer days with snow cover, an earlier start of the spring high-flow season, and an increase in April streamflow, are consistent with the observed warming and precipitation trends. For the period 1900–2012, there are sufficient temperature and precipitation data for trend analysis for southern Canada (south of 60°N) only. During this period, temperature has increased significantly across the region, precipitation has increased, and the amount of precipitation falling as snow has decreased in many areas south of 55°N. The results also show that modes of low-frequency variability modulate the spatial distribution and strength of the trends; however, they alone cannot explain the observed long-term trends in these climate variables.

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M. L. Yu
,
F. X. Giraldo
,
M. Peng
, and
Z. J. Wang

Abstract

Gibbs oscillation can show up near flow regions with strong temperature gradients in the numerical simulation of nonhydrostatic mesoscale atmospheric flows when using the high-order discontinuous Galerkin (DG) method. The authors propose to incorporate flow-feature-based localized Laplacian artificial viscosity in the DG framework to suppress the spurious oscillation in the vicinity of sharp thermal fronts but not to contaminate the smooth flow features elsewhere. The parameters in the localized Laplacian artificial viscosity are modeled based on both physical criteria and numerical features of the DG discretization. The resulting numerical formulation is first validated on several shock-involved test cases, including a shock discontinuity problem with the one-dimensional Burger’s equation, shock–entropy wave interaction, and shock–vortex interaction. Then the efficacy of the developed numerical formulation on stabilizing thermal fronts in nonhydrostatic mesoscale atmospheric modeling is demonstrated by two benchmark test cases: the rising thermal bubble problem and the density current problem. The results indicate that the proposed flow-feature-based localized Laplacian artificial viscosity method can sharply detect the nonsmooth flow features, and stabilize the DG discretization nearby. Furthermore, the numerical stabilization method works robustly for a wide range of grid sizes and polynomial orders without parameter tuning in the localized Laplacian artificial viscosity.

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