Research Interests
My main research focus is on investigating the large scale dynamics of the mid-latitude troposphere. The phenomena that I have investigated range from baroclinic waves/cyclones up to storm tracks and the large scale mean circulation. The distinctive characteristic of my research lies in the fact that I employ a wide range of tools in my research, ranging from analyses of gridded atmospheric analyses and GCM simulations to learn about the basic characteristics of the phenomena, examination of actual observations to validate what have been learnt from the gridded data, and dynamical studies using a suite of intermediate/mechanistic models to achieve better understanding of these observed phenomena. My major research interests and some recent publications are summarized below. For a full publication list, click here.
Variability and trends of storm tracks
Pacific and Atlantic storm track indices and 5-year running
mean (red dashed line)
based on ship observations (adapted from Chang, 2007).
We have analyzed reanalysis data and GCM simulations to examine the seasonal, interannual, and inter-decadal variability of the Northern Hemisphere storm tracks. In the seasonal time scale, our analyses of the energetics showed that the seasonally varying contributions of diabatic effects to EAPE generation can (at least partially) reconcile the observed apparent inconsistency that the Pacific storm track has a relative minimum during the time when the dry baroclinicity is strongest. In the inter-decadal timescale, our analyses suggested that based on reanalysis data, both the Pacific and Atlantic storm tracks have become significantly more active during the second part of the 20th century. Currently, we are examining how storm tracks are predicted to change under global warming in GCM simulations.
We have also examined observations, including aircraft, rawinsonde, and surface ship observations, to validate trends and variability found in reanalysis data. For example, our results suggested that the upward trend in Atlantic storm track activity found in observations is largely consistent with that found in the reanalysis data, but the trend in Pacific storm track activity found in the observations is much weaker than that found in both NCEP and ECMWF reanalyses (trend in observations is only about 20-60% of that found in the reanalysis).
Some recent publications (more publications):
Chang, E.K.M., and Y.F. Fu, 2002: Inter-decadal
variations in Northern Hemisphere winter storm track intensity. J. Climate, 15,
642-658.
Harnik, N., and E.K.M. Chang, 2003: Storm track variations as seen in radiosonde observations and reanalysis data. J. Climate, 16, 480-495.
Chang, E.K.M., 2007: Assessing the increasing trend
in Northern Hemisphere winter storm track activity using surface ship
observations and statistical storm track model. J. Climate, 20, 5607-5628.
Guo, Y, and E.K.M. Chang, 2008: Impacts of
assimilation of satellite and rawinsonde observations on Southern Hemisphere
baroclinic wave activity in the NCEP/NCAR reanalysis. J. Climate, 21, 3290-3309..
Storm track dynamics
Stationary wave (300 hPa streamfunction) and storm track (500 hPa 24-hr
filtered RMS z’) distributions
taken from NCEP/NCAR reanalysis and control idealized model simulation
(adapted from Chang, 2008)
In our research, we have employed numerical models of various degree of complexity, including statistical and dynamical models, as tools to improve our understanding of storm track variability. Using a statistical storm track model, we showed that storm track variability is strongly tied to that of the mean flow, and that the increase in Pacific storm track activity during the second half of the 20th century found in reanalysis data is inconsistent with the observed trend in mean flow quantities. Based on a nonlinear intermediate storm track model, we have shown that the mid-winter suppression of the Pacific storm track is unlikely to be explained by dry dynamics alone, and that Northern Hemisphere extratropical heating is the main reason why the Atlantic storm track is stronger than its Pacific counterpart.
Some recent publications (more publications):
Chang, E.K.M., S. Lee, and K.L. Swanson, 2002: Storm track dynamics. J. Climate, 15, 2163-2183.
Chang, E.K.M., and Y.F. Fu, 2003: Using Mean flow change as a proxy to infer interdecadal storm track variability. J. Climate, 16, 2178-2196.
Chang, E.K.M., 2006: An idealized nonlinear model of the Northern Hemisphere winter storm tracks. J. Atmos. Sci., 63, 1818-1839.
Chang, E.K.M., and P. Zurita-Gotor, 2007: Simulating the Seasonal Cycle of the Northern
Hemisphere Storm Tracks Using Idealized Nonlinear Storm Track Models. J. Atmos.
Sci., 64, 2309-2331.
Chang, E.K.M., 2008: Diabatic and orographic forcing of Northern winter stationary waves and storm tracks. J. Climate, accepted.
Dynamics and life cycle of baroclinic waves and
cyclones
Lagged composites, based on cyclone event over West Pacific
on day 0 (panel i), showing
development of cyclone over the west Pacific (panels f-j),
and evolution of upper
tropospheric wave train (panels a-e), propagating from Asia
across the Pacific,
approaching western US on day +1. Note the two branches of
wave activity over Asia
on days -3 and -2 (panels a-b). Adapted from Chang (2005).
Our results have shown that downstream development is a basic characteristic of observed mid-latitude baroclinic waves. Baroclinic waves are organized into wave packets that occur in both hemispheres during all seasons, and are most coherent over regions where the potential vorticity gradient is tightest. Our analyses also showed that cyclogenesis over the western Pacific is seeded by waves from two upstream sources – a northern branch across Siberia, and a southern branch along the subtropical jet across southern Asia.
Some recent publications (more publications):
Chang, E.K.M., 1993: Downstream Development of Baroclinic Waves as Inferred from Regression Analysis. J. Atmos. Sci., 50, 2038-2053.
Chang, E.K.M., and D.B. Yu, 1999: Characteristics of wave packets in the upper troposphere. Part I: Northern hemisphere winter. J. Atmos. Sci., 56, 1708-1728.
Chang, E.K.M., 2001: The structure of baroclinic wave packets. J. Atmos. Sci., 58, 1694-1713.
Chang, E.K.M., 2005: The impact of wave packets propagating across Asia on Pacific cyclone development. Mon. Wea. Rev., 133, 1998-2015.
Wave/Mean flow
interactions:
We have employed data analyses together with a suite of mechanistic and diagnostic models to understand the two-way feedback between mean flow and eddy anomalies. We have examined how changes in the intensity of the Hadley circulation may lead to shifts in the mid-latitude jet and storm tracks. The two-way feedback between baroclinic waves and mean flow anomalies that is involved in driving the two leading modes of Southern Hemisphere zonal flow variability has been examined by coupling a linear storm track model to a stationary wave model. Our results showed that the structures of the leading EOFs are characterized by mean flow anomalies that can drive eddy anomalies that provide positive eddy feedback to maintain the mean flow anomalies.
Some recent publications (more publications):
Chang, E.K.M., 1995: The Influence of Hadley Circulation Intensity Changes on Extratropical Climate in an idealized model. J. Atmos. Sci., 52, 2006-2024.
Chang, E.K.M., 2005: The role of wave packets in wave/mean flow interactions during Southern Hemisphere summer. J. Atmos. Sci., 62, 2467-2483.
Yang,
X., and E.K.M. Chang, 2007: Eddy-zonal flow feedback in the Southern Hemisphere
winter and summer. J. Atmos. Sci., 64, 3091-3112.
Tropical/extratropical
interactions, tropical meteorology:
Based on HURDAT, there is an apparent trend in the number of
tropical storms in the Atlantic during
the 20th century. Several studies have blamed
this trend on undetected storms prior to the satellite era
due to sparse observations over the ocean. We have used ship
observations to estimate whether this
could be the case, and our results suggested that the number
of underestimated storms is probably
insufficient to explain away the trend. For details, see
Chang and Guo (2007).
We have examined how changes in the intensity of the Hadley circulation may lead to shifts in the mid-latitude jet and storm tracks. We have also examined the possibility that the number of Atlantic tropical cyclones may have been significantly underestimated prior to the availability of satellite. Our results suggested that the frequency of ship observations is high enough such that it is very unlikely that the number of Atlantic tropical cyclones that did not hit land can be underestimated by more than 1 per year after World War I, suggesting that the characteristics of North Atlantic tropical cyclone statistics may have changed during the 20th century.
Some recent publications (more publications):
Chang, E.K.M., 1995: The Influence of Hadley Circulation Intensity Changes on Extratropical Climate in an idealized model. J. Atmos. Sci., 52, 2006-2024.
Chang, E.K.M., 1998: Poleward propagating angular momentum perturbations induced by zonally symmetric heat sources in the Tropics. J. Atmos. Sci., 55, 2229-2248.
Chang, E.K.M., and Y. Guo, 2007: Is the number of North Atlantic tropical cyclones significantly underestimated prior to the availability of satellite observations? Geophysical Research Letters, 34, L14801, doi: 10.1029/2007GL030169.