Lejiang Yu

Observations of near‐surface wind and temperature structures and their variations with topography and latitude in East Antarctica

Received 10 December 2008; revised 30 March 2009; accepted 5 June 2009; published 15 September 2009. [1] The first multiyear surface meteorological observations over Dome A, the highest ice feature in the entire Antarctica continent, are analyzed to understand the surface wind, temperature, and stability climatology over Dome A and how it differs from the surface climatology at two lower-latitude/lower-elevation sites along similar longitude in East Antarctica. The climatology is also compared with that over Dome C. In contrast to the surface winds at lower sites, where moderate to strong northeasterly winds prevail with a distinct diurnal oscillation in wind speed in response to the diurnal change in katabatic forcing, summertime surface winds over Dome A are very weak, are variable in direction, and show little diurnal variation. Although both temperature and temperature gradient oscillate diurnally, the gradient over Dome A remained positive all day long, indicating a persistent surface inversion, while at the two lower sites, as well as over Dome C, sufficient insolation leads to the breakup of inversion and the development of a convective boundary layer in the afternoon. Wavelet analysis of near-surface stability revealed that besides the strong diurnal signal, the near-surface stability also exhibits annual, semiannual, and interseasonal (period 50 days) oscillations at all locations. These oscillations in near-surface stability are linked to the same peaks in the 500-hPa geopotential height spectra and therefore are believed to be caused by variations of synoptic conditions.

Temporal and spatial variability of wind resources in the United States as derived from the climate forecast system reanalysis

AbstractThis study examines the spatial and temporal variability of wind speed at 80 m above ground (the average hub height of most modern wind turbines) in the contiguous United States using Climate Forecast System Reanalysis (CFSR) data from 1979 to 2011. The mean 80-m wind exhibits strong seasonality and large spatial variability, with higher (lower) wind speeds in the winter (summer), and higher (lower) speeds over much of the Midwest and U.S. Northeast (U.S. West and Southeast). Trends are also variable spatially, with more upward trends in areas of the Great Plains and Intermountain West of the United States and more downward trends elsewhere. The leading EOF mode, which accounts for 20% (summer) to 33% (winter) of the total variance and represents in-phase variations across the United States, responds mainly to the North Atlantic Oscillation (NAO) in summer and El Nino–Southern Oscillation (ENSO) in the other seasons. The dominant variation pattern can be explained by a southerly/southwesterly (wes...

Contribution of large-scale circulation anomalies to changes in extreme precipitation frequency in the United States

The mean global climate has warmed as a result of the increasing emission of greenhouse gases induced by human activities. This warming is considered the main reason for the increasing number of extreme precipitation events in the US. While much attention has been given to extreme precipitation events occurring over several days, which are usually responsible for severe flooding over a large region, little is known about how extreme precipitation events that cause flash flooding and occur at sub-daily time scales have changed over time. Here we use the observed hourly precipitation from the North American Land Data Assimilation System Phase 2 forcing datasets to determine trends in the frequency of extreme precipitation events of short (1 h, 3 h, 6 h, 12 h and 24 h) duration for the period 1979-2013. The results indicate an increasing trend in the central and eastern US. Over most of the western US, especially the Southwest and the Intermountain West, the trends are generally negative. These trends can be largely explained by the interdecadal variability of the Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation (AMO), with theAMOmaking a greater contribution to the trends in both warm and cold seasons.

How does the Indian Ocean subtropical dipole trigger the tropical Indian Ocean dipole via the Mascarene high

The variation in the Indian Ocean is investigated using Hadley center sea surface temperature (SST) data during the period 1958–2010. All the first empirical orthogonal function (EOF) modes of the SST anomalies (SSTA) in different domains represent the basin-wide warming and are closely related to the Pacific El Niño-Southern Oscillation (ENSO) phenomenon. Further examination suggests that the impact of ENSO on the tropical Indian Ocean is stronger than that on the southern Indian Ocean. The second EOF modes in different domains show different features. It shows a clear east-west SSTA dipole pattern in the tropical Indian Ocean (Indian Ocean dipole, IOD), and a southwest-northeast SSTA dipole in the southern Indian Ocean (Indian Ocean subtropical dipole, IOSD). It is further revealed that the IOSD is also the main structure of the second EOF mode on the whole basin-scale, in which the IOD pattern does not appear. A correlation analysis indicates that an IOSD event observed during the austral summer is highly correlated to the IOD event peaking about 9 months later. One of the possible physical mechanisms underlying this highly significant statistical relationship is proposed. The IOSD and the IOD can occur in sequence with the help of the Mascarene high. The SSTA in the southwestern Indian Ocean persists for several seasons after the mature phase of the IOSD event, likely due to the positive wind-evaporation-SST feedback mechanism. The Mascarene high will be weakened or intensified by this SSTA, which can affect the atmosphere in the tropical region by teleconnection. The pressure gradient between the Mascarene high and the monsoon trough in the tropical Indian Ocean increases (decreases). Hence, an anticyclone (cyclone) circulation appears over the Arabian Sea-India continent. The easterly or westerly anomalies appear in the equatorial Indian Ocean, inducing the onset stage of the IOD. This study shows that the SSTA associated with the IOSD can lead to the onset of IOD with the aid of atmosphere circulation and also explains why some IOD events in the tropical tend to be followed by IOSD in the southern Indian Ocean.

Influence of the Antarctic Oscillation, the Pacific–South American modes and the El Niño–Southern Oscillation on the Antarctic surface temperature and pressure variations

Abstract In this study, the impacts of the Antarctic Oscillation (AAO), the Pacific–South American teleconnection (PSA) and the El Niño–Southern Oscillation (ENSO) on Antarctic sea level pressure and surface temperature are investigated using surface observational data, European Centre for Medium-Range Weather Forecasts (ECMWF) 40 Year Re-analysis (ERA-40) and the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) re-analysis data from 1958–2001. There is the most significant correlation between PSA and Antarctic sea level pressure and surface temperature in the northern Antarctic Peninsula during four seasons. But the correlation between Southern Oscillation Index and surface temperature and sea level pressure is significant at some stations only in spring. The three indices can explain a large portion of the trends found in sea level pressure and temperature at some stations, but not at all stations. Among the three indices the most important contribution to the trends in the two surface variables comes from AAO, followed by PSA, and finally by ENSO. The two re-analysis datasets show great similarity for the trends in surface temperature and sea level pressure in April–May and October–November, but not December–February. In summer the trends in surface temperature and sea level pressure in East Antarctica for ERA-40 re-analysis are opposite to those of NCEP re-analysis.

An inter-comparison of six latent and sensible heat flux products over the Southern Ocean

The latent heat fluxes (LHF) and sensible heat fluxes (SHF) over the Southern Ocean from six different data sets are inter-compared for the period 1988–2000. The six data sets include three satellite-based products, namely, the second version of the Goddard Satellite-Based Surface Turbulent Fluxes data set (GSSTF-2), the third version of the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data (HOAPS-3) and the Japanese Ocean Fluxes Data Sets with Use of Remote Sensing Observations (J-OFURO); two global reanalysis products, namely, the National Centers for Environmental Prediction–Department of Energy Reanalysis 2 data set (NCEP-2) and the European Centre for Medium-Range Weather Forecasts 40 Year Re-analysis data set (ERA-40); and the Objectively Analyzed Air–Sea Fluxes for the Global Oceans data set (OAFlux). All these products reveal a similar pattern in the averaged flux fields. The zonal mean LHF fields all exhibit a continuous increase equatorward. With an exception of HOAPS-3, the zonal mean SHF fields display a minimum value near 50°S, increasing both pole- and equatorward. The differences in the standard deviation for LHF are larger among the six data products than the differences for SHF. Over the regions where the surface fluxes are significantly influenced by the Antarctic Oscillation and the Pacific–South American teleconnection, the values and distributions of both LHF and SHF are consistent among the six products. It was found that the spatial patterns of the standard deviations and trends of LHF and SHF can be explained primarily by sea–air specific humidity and temperature differences; wind speed plays a minor role.

The intraseasonal variability of winter semester surface air temperature in Antarctica

This study investigates systematically the intraseasonal variability of surface air temperature over Antarctica by applying empirical orthogonal function (EOF) analysis to the National Centers for Environmental Prediction, US Department of Energy, Reanalysis 2 data set for the period of 1979 through 2007. The results reveal the existence of two major intraseasonal oscillations of surface temperature with periods of 26–30 days and 14 days during the Antarctic winter season in the region south of 60°S. The first EOF mode shows a nearly uniform spatial pattern in Antarctica and the Southern Ocean associated with the Antarctic Oscillation. The mode-1 intraseasonal variability of the surface temperature leads that of upper atmosphere by one day with the largest correlation at 300-hPa level geopotential heights. The intraseasonal variability of the mode-1 EOF is closely related to the variations of surface net longwave radiation the total cloud cover over Antarctica. The other major EOF modes reveal the existence of eastward propagating phases over the Southern Ocean and marginal region in Antarctica. The leading two propagating modes respond to Pacific–South American modes. Meridional winds induced by the wave train from the tropics have a direct influence on the surface air temperature over the Southern Ocean and the marginal region of the Antarctic continent.

Interpretation of recent trends in Antarctic sea ice concentration

We investigate seasonal trends in sea ice concentration and the relative contributions of the Antarctic Oscillation (AAO), the Pacific-South American two modes (PSA1 and PSA2), and the El Niño-Southern Oscillation (ENSO). The summer range of the trend in the Antarctic sea ice is the largest, from −83.8% to 59.6% per 29 yr over the period of 1979 through 2007, while the autumn range is the least, from −49.7% to 39.6% per 29 yr for the period of 1979 through 2007. In autumn, among the four indices the largest contribution to the trend in sea ice is the AAO; in winter the ENSO and the PSA1 are better than the other two indices; during spring and summer a change of more than 15% per 29 yr is associated with PSA1. No matter the season, the spatial pattern of the residual trend is similar to that of the total trend; moreover, the combined trends of the four indices only explains less than one-third of the total trend.

Future changes in the climatology of the Great Plains low-level jet derived from fine resolution multi-model simulations

The southerly Great Plains low-level jet (GPLLJ) is one of the most significant circulation features of the central U.S. linking large-scale atmospheric circulation with the regional climate. GPLLJs transport heat and moisture, contribute to thunderstorm and severe weather formation, provide a corridor for the springtime migration of birds and insects, enhance wind energy availability, and disperse air pollution. We assess future changes in GPLLJ frequency using an eight member ensemble of dynamically-downscaled climate simulations for the mid-21st century. Nocturnal GPLLJ frequency is projected to increase in the southern plains in spring and in the central plains in summer, whereas current climatological patterns persist into the future for daytime and cool season GPLLJs. The relationship between future GPLLJ frequency and the extent and strength of anticyclonic airflow over eastern North America varies with season. Most simulations project a westward shift of anticyclonic airflow in summer, but uncertainty is larger for spring with only half of the simulations suggesting a westward expansion. The choice of regional climate model and the driving lateral boundary conditions have a large influence on the projected future changes in GPLLJ frequency and highlight the importance of multi-model ensembles to estimate the uncertainty surrounding the future GPLLJ climatology.

A comparison of the effects of El Niño and El Niño Modoki on subdaily extreme precipitation occurrences across the contiguous United States

Intense precipitation over a short duration is a major cause of flash floods. Using hourly rainfall data from the North America Land Data Assimilation System Phase 2 (NLDAS-2) from 1979-2013, we compared the differences in the response of the sub-daily extreme precipitation occurrences across the contiguous U.S. to strong anomalous warming over the eastern equatorial Pacific known as El Nino, and over the central equatorial Pacific known as El Nino Modoki. For both types of anomalous equatorial Pacific warming, the teleconnection is much stronger in the cold season (November through April) than warm season (May through October). During the warm season, while both types correspond to an increase in sub-daily extreme precipitation in areas of Texas and a decrease in some areas of the northern Plains, El Nino is associated with a significant increase in the northern Rockies, and El Nino Modoki is associated with a decrease in the Intermountain West. During the cold season, the overall patterns are similar between the two types, with positive anomalies over much of the southern and negative anomalies over the northern U.S. However, large regional differences exist, with El Nino exerting a much stronger influence on sub-daily extreme precipitation in the Atlantic and Gulf tates as well as California, while the influence of El Nino Modoki is slightly broader over the Midwest. These differences can be largely explained by the vertical velocity, moisture convergence, and jet stream patterns induced by El Nino and El Nino Modoki episodes.