Yanjuan Guo

Storm-Track Activity in IPCC AR4/CMIP3 Model Simulations

AbstractThe climatological storm-track activity simulated by 17 Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/phase 3 of the Coupled Model Intercomparison Project (CMIP3) models is compared to that in the interim ECMWF Re-Analysis (ERA-Interim). Nearly half of the models show significant biases in storm-track amplitude: four models simulate storm tracks that are either significantly (>20%) too strong or too weak in both hemispheres, while four other models have interhemispheric storm-track ratios that are biased by over 10%. Consistent with previous studies, storm-track amplitude is found to be negatively correlated with grid spacing. The interhemispheric ratio of storm-track activity is highly correlated with the interhemispheric ratio of mean available potential energy, and this ratio is biased in some model simulations due to biases in the midlatitude temperature gradients. In terms of geographical pattern, the storm tracks in most CMIP3 models exhibit an equatorward bia...

A Systematic Relationship between the Representations of Convectively Coupled Equatorial Wave Activity and the Madden–Julian Oscillation in Climate Model Simulations

AbstractThe relationship between a model’s performance in simulating the Madden–Julian oscillation (MJO) and convectively coupled equatorial wave (CCEW) activity during wintertime is examined by analyzing precipitation from 26 general circulation models (GCMs) participating in the MJO Task Force/Global Energy and Water Cycle Experiment (GEWEX) Atmospheric System Study (GASS) MJO model intercomparison project as well as observations based on the Tropical Rainfall Measuring Mission (TRMM). A model’s performance in simulating the MJO is determined by how faithfully it reproduces the eastward propagation of the large-scale intraseasonal variability (ISV) compared to TRMM observations. Results suggest that models that simulate a better MJO tend to 1) have higher fractional variances for various high-frequency wave modes (Kelvin, mixed Rossby–gravity, and westward and eastward inertio-gravity waves), which are defined by the ratios of wave variances of specific wave modes to the “total” variance, and 2) exhibit...

Dynamics of the Stationary Anomalies Associated with the Interannual Variability of the Midwinter Pacific Storm Track—The Roles of Tropical Heating and Remote Eddy Forcing

Abstract The leading mode of interannual variability of the midwinter Pacific storm track is such that the storm track is weaker during the winters when the Pacific jet is strong, and stronger when the jet is weak. In this paper, experiments are conducted using a stationary wave model as well as an idealized global circulation model to explore the roles of anomalous tropical heating and eddy fluxes in forcing the observed Pacific jet anomalies. It is found that enhanced tropical heating over the region 60°E to the date line, 25°S to 25°N, acts to force a stronger and narrower Pacific jet. On average, tropical heating may account for about one-third of the strong jet anomaly, but there is significant year-to-year variability. Moreover, tropical heating does not appear to contribute to the weak jet anomaly. Much of the Pacific jet anomalies are forced by anomalous eddy fluxes. By examining the regional contributions from the Pacific, the Atlantic, and Asia, it is found that local eddy feedback over the Paci...

Modulation of the Convectively Coupled Kelvin Waves over South America and the Tropical Atlantic Ocean in Association with the Madden-Julian Oscillation

AbstractIn this study, evidence of the strong modulation of the convectively coupled Kelvin wave (CCKW) activity by the Madden–Julian oscillation (MJO) is presented, with a particular focus over the South America and tropical Atlantic region. The MJO impacts on CCKWs over this region, as noted in anomalous fields of rainfall as well as vertical profiles of wind, moisture, and temperature, are primarily through the modulation of Kelvin wave amplitude, with secondary effects on vertical structure, and little impact on wavenumber. CCKW activity is enhanced during MJO phases 8, 1, and 2 and damped during MJO phases 4, 5, and 6.Further analyses reveal that the strong modulation of the MJO on the CCKW activity could be largely through two factors: namely, the vertical zonal wind shear and the lower- to middle-tropospheric specific humidity. The CCKW activity tends to be enhanced during MJO phases when the easterly vertical wind shear and positive low- to midtroposphere moisture anomalies are present and vice ve...

Variations of Northern Hemisphere Storm Track and Extratropical Cyclone Activity Associated with the Madden–Julian Oscillation

AbstractThis study investigates the intraseasonal variations of the Northern Hemispheric storm track associated with the Madden–Julian oscillation (MJO) during the extended boreal winter (November–April) using 36 yr (1979–2014) of reanalysis data from ERA-Interim. Two methods have been used to diagnose storm-track variations. In the first method, the storm track is quantified by the temporal-filtered variance of 250-hPa meridional wind (vv250) or mean sea level pressure (pp). The intraseasonal anomalies of vv250 composited for eight MJO phases are characterized by a zonal band of strong positive (or negative) anomalies meandering from the Pacific all the way across North America and the Atlantic into northern Europe, with weaker anomalies of opposite sign at one or both flanks. The results based on pp are consistent with those based on vv250 except for larger zonal variations, which may be induced by surface topography. In the second method, an objective cyclone-tracking scheme has been used to track the ...

Comments on “The Source of the Midwinter Suppression in Storminess over the North Pacific”

AbstractIn a recent paper, Penny et al. employed feature tracking to investigate why there is a relative minimum in storminess during winter within the Pacific storm track. They concluded that reduced upstream seeding, especially seeding from northern Asia, is the main “source” of the midwinter suppression of the Pacific storm track. Results presented here show that during midwinter months when upstream seeding is as strong as that in spring/fall, the Pacific storm track is not significantly stronger than average and is still much weaker than that in spring/fall, suggesting that the strength of upstream seeding cannot be the primary cause of the midwinter suppression of Pacific storm-track activity.

Impacts of Assimilation of Satellite and Rawinsonde Observations on Southern Hemisphere Baroclinic Wave Activity in the NCEP–NCAR Reanalysis

Abstract In this study, the impacts of the assimilation of satellite and rawinsonde observations on Southern Hemisphere (SH) baroclinic wave activity in the NCEP–NCAR reanalysis are examined by comparing analyses made with and without the assimilation of satellite data (SAT and NOSAT, respectively) for the year 1979, as well as by comparing analyses to the corresponding first guesses from 1958 to 1999. Comparing the eddy kinetic energy (EKE) computed based on the SAT and NOSAT analyses, it is found that the assimilation of satellite data generally decreases the EKE in the SH, which is the opposite of the findings for the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) in previous studies. The decrease of EKE by satellite data in the NCEP–NCAR reanalysis can be traced back to a low bias in retrieved satellite temperature (SATEMP) variance. The eddy available potential energy (EPE) is decreased even more than the EKE with the assimilation of SATEMP, making the waves mor...

Is Pacific Storm-Track Activity Correlated with the Strength of Upstream Wave Seeding?

AbstractIn this paper, the relationship between upstream seeding over north Asia and downstream storm-track activity over the North Pacific in midwinter and spring/fall has been analyzed using 45 years of variance and feature-tracking statistics. It is shown that for each season, interannual variations in upstream seeding and downstream storm-track activity are largely uncorrelated. Moreover, during midwinter months in which the upstream seeding from north Asia is about as strong as that during a typical spring/fall month, the downstream storm track in central Pacific is still significantly weaker during midwinter than that during spring/fall. In addition, during cool seasons in which the midwinter suppression is more pronounced in the upstream seeding region, the suppression is not significantly enhanced in the downstream Pacific storm track.A recent study suggested that reduced upstream seeding from north Asia is the main “source” of the midwinter suppression of the Pacific storm track. Results presente...