A53E-01
Long-term temperature observations from the troposphere to upper mesosphere over Mauna Loa, HI (19.5N, 155.6W) and Table Mountain, CA (34.4N, 117.7W) by JPL Lidars and nearby Radiosondes
The JPL Rayleigh lidars at Mauna Loa Observatory (MLO), HI (19.5N, 155.6W) and Table Mountain Observatory (TMO), CA (34.4N, 117.7W) have been operated for the regular nighttime data acquisition of temperature since 1994 and 1989 respectively. Using the monthly mean temperature vertical profiles observed by the JPL lidars (35- 85km) and nearby radiosondes (5-30km), and with the linear regression analysis, we are able to extract the temperature trend, solar cycle, El Nino South Oscillation (ENSO), and Quasi-Biennial Oscillation (QBO) signals from the troposphere to upper mesosphere over MLO and TMO. The temperature trends show different behaviors at two sites, minor trend at MLO, but more negative trend at TMO. The solar cycle responses in temperature are generally positive above the middle stratosphere at both sites, but negative response at MLO and positive at TMO below. During the El Nino events, the warmer temperatures in the troposphere and upper mesosphere, and the colder temperatures in the stratosphere and lower mesosphere were observed at MLO and almost visa verse at TMO. The significant QBO oscillations were observed in the stratosphere with amplitudes of ~2-3K and with clearer downward phase progression at MLO than that at TMO. The mesospheric QBO near 75-85km is clearly present at both sites with amplitude of ~2K and with longer vertical wavelength than that in stratosphere. In addition, we calculated the GW variances using lidar temperature profiles with 30min and 1km resolutions in the upper stratosphere (38-50km) and lower mesosphere (50-62km), and nearby radiosondes in the lower stratosphere (18-30km). The monthly mean GW variances clearly show an annual oscillation with a maximum in the winter and minimum in the summer. The QBO signature could be clearly seen in the lower stratosphere. In the upper stratosphere, a longer period oscillation (~5-6 years) with maxima in 2000-2001 and 2006 was revealed to synchronize with the solar maximum and minimum. No clear signature of GW activity in the lower mesosphere could be associated to that in the upper stratosphere, suggesting that part of gravity waves may either dissipated or reflected when crossing the stratopause region.
A53E-02
Temporal and Spatial Changes in Northern Hemisphere Floating Climatological Seasons
Floating climatological seasons, for which onsets and durations vary temporally and spatially, are examined over Northern Hemisphere continents and oceans. Among the variables evaluated are surface air temperature, snow extent, vegetation greenness, and atmospheric carbon dioxide concentrations. Seasonal thresholds are defined for each variable (e.g. daily mean temperature exceeding 5°C (20°C) to mark the beginning of spring (summer)). The dates on which these thresholds are reached at a given location are determined for each year over the past three decades. These seasonal onsets and offsets "float" temporally and spatially from year to another. An analysis of floating dates finds that winter duration has shortened in Europe, eastern Asia, and western North America, primarily due to an earlier spring onset. The spatial pattern of this earlier onset is associated with a positive Arctic Oscillation (AO) regime in the previous winter months. The positive winter AO finds anomalously high pressure sitting in the middle latitudes in locations where spring arrives early. This is likely due to a combination of advective fluxes of warmth and moisture and the local enhancement of solar radiation reaching the surface under clear skies. This, in turn, promotes earlier snow melt that further enhances warming and an earlier green-up. Extended summer duration is observed over continents and oceans (except the Arctic Ocean, where summer does not exist). The oceanic zone along 30°N has experienced a particularly large increase in duration, suggesting Hadley cell expansion.
A53E-03
The Modulated Annual Cycle: An Alternative Reference Frame for Climate Anomalies
In climate science, an anomaly is the deviation of a quantity from its annual cycle (AC). There are many ways to define annual cycle. Traditionally, the annual cycle is taken to be an exact repetition of itself year after year. This stationary annual cycle may not reflect well the intrinsic nonlinearity of the climate system, especially under external forcing. In this study, we have reexamined the reference frame for anomalies by reexamining the annual cycle. We propose an alternative reference frame, the modulated annual cycle (MAC) that allows the annual cycle to change from year to year, for defining anomalies. In order for this alternative reference frame to be useful, we need to be able to define the instantaneous annual cycle. We therefore also introduce a new method to extract the MAC from climatic data. In the presence of an MAC, modulated in both amplitude and frequency, we can then define an alternative version of an anomaly, this time with respect to the instantaneous MAC rather than a permanent and unchanging AC. Based on this alternative definition of anomalies, we reexamine some familiar physical processes: in particular, the sea surface temperature (SST) reemergence and the ENSO phase locking to the annual cycle. We find that the re-emergence mechanism may be alternatively interpreted as an explanation of the change of the annual cycle instead of the interannual to interdecadal persistence of SST anomalies. We also find that the ENSO phase locking can largely be attributed to the residual annual cycle (the difference of the MAC and the corresponding traditional annual cycle) contained in the traditional anomaly, and, therefore, can be alternatively interpreted as a part of the annual cycle phase locked to the annual cycle itself. Two additional examples are also presented of the implications of using a MAC against which to define anomalies. We show that using MAC as a reference framework for anomaly can bypass the difficulty brought by concepts such as "decadal variability of summer (or winter) climate" for understanding the low-frequency variability of the climate system. We also point out the drawbacks related to the stationary assumption in previous studies of extreme weather and climate and propose instead the appropriateness of choosing a non-stationary framework to study extreme weather and climate events. The concept of an amplitude and frequency modulated annual cycle, a method to extract it, and its implications for the interpretation of physical processes, all may contribute potentially to a more consistent and fruitful way of examining past and future climate variability and change.
A53E-04
Examination of Discontinuities of Monthly Wind Speed and Measured Trends in Canada
Long-term wind speed observations are widely used in climate analysis and engineering applications. Wind speed data recorded at 122 stations in Canada for the period from 1953 - 2004 are analyzed in this study. Station metadata and logarithmic wind profile are used to adjust hourly wind speed to standard 10m level. Monthly means are then derived. Statistical homogeneity tests along with metadata are used to identify artificial mean- shifts (step-type changes) in the wind speed time series. Anemometer height change was found to be the main known source for discontinuities in wind speed time series. Station relocation, instrument changes, and site condition changes are other causes for wind data discontinuities. Homogenized Canadian hourly sea level pressure data (surface station observations) are used to derive hourly geostrophic winds, which are compared with the homogenized wind speed in terms of long-term trends. Homogenized wind speed series and geostrophic winds show consistent trends, which are notably different from trends estimated from unhomogenized (raw) wind speed series.
A53E-05
Mechanism of Increasing Indian Monsoon Rainfall Following a Winter El Nino
The mechanism of increasing Indian monsoon rainfall following major El Nino – Southern Oscillation (ENSO) events has been investigated using an atmospheric general circulation model (AGCM) coupled to a thermodynamic mixed layer. In our simulations, El Nino produces a warming of the troposphere and intensification of convection over the equatorial south Indian Ocean during the early spring. The anomalous convection progresses northwards up to the subcontinent of India during the early monsoon season (Jun-Jul). This progression occurs via anomalous moisture transport by the mean cross-equatorial flow, even though the intensity of the monsoon circulation decreases. Our simulations also suggest an intraseasonal evolution of the mechanisms associated with anomalous summer monsoon rainfall conditions over the north Indian Ocean and Indian subcontinent following a winter El Nino. During the early period of the monsoon (Jun-Jul), moisture transport by the mean cross-equatorial flow is the main source for increasing monsoon rainfall. As the moisture perturbation homogenizes during the late period (Aug-Sep), large-scale moisture transport weakens. Instead, anomalous surface latent heat fluxes associated with north Indian Ocean warming increase boundary layer moist static energy and support above normal rainfall. Additional idealized experiments demonstrate that the winter- early spring El Nino-induced Indian Ocean warming and subsequent local boundary layer moist processes are mostly responsible for increasing the monsoon rainfall. However, the strength of the monsoonal cross-equatorial flow is strongly modulated by the contemporaneous remote forcing from the eastern equatorial Pacific (Nino3 area), where El Nino's residual (decaying) signal remains. We discuss competing effects of the north Indian Ocean warming versus the anomalous subsidence on the Indian monsoon rainfall.
A53E-06
Developing the Local 3-Month Precipitation Outlook
In 2007, NOAA's National Weather Service (NWS) introduced the Local 3 Month Temperature Outlook (L3MTO), which downscales the NWS's probabilistic outlooks for the average 3-month temperature to a local station. Creating a local precipitation outlook is more complex than for temperature because of the higher spatial and temporal variability of precipitation. These characteristics of the observation data (1) make it difficult to fit a single probability distribution to data over a large spatial area and (2) may lead to questionable predictability even in the case of an adequate distribution fit. Different statistical downscaling techniques are tested for a forecasting procedure of the Local 3 Month Precipitation Outlook (L3MPO). The first technique being tested is the methodology used for the L3MTO that (1) applies a linear regression to identify the statistical relationship between a station parameter and its corresponding forecast region and (2) adjusts the regression parameters to the most recent trends at the station. We modified the original L3MTO linear regression methodology by setting the intercept to zero to account for the fact that precipitation is a discrete variable and is bounded at zero. Such modification, in theory, should increase the standard error of predictions, because as fewer parameters are estimated the degrees of freedom increase. Therefore, the L3MPO methodology has been evaluated by a verification analysis that utilizes L3MPO hind-casts, which are created using the archived forecast data from CPC's national outlooks for 232 sites in the western U.S. The Modified Heidke Skill Score (MHSS) at 75 percent confidence level is used as verification in this assessment of long-term forecast goodness. The MHSS was computed for each station using 11 years (1994-2005) and all leads for individual target 3-month periods: e.g. Jan-Mar, Feb-Apr, etc. The stations were then stratified by two criteria: 1) existence of potential predictability and 2) conditions favoring assumption of Normal distribution. Overall, there are about 60 percent of stations that show forecast improvement over the use of the 1971-2000 climatology. Forecasts were poor for only 10 percent of stations within the areas with existing potential predictability, whose data did not allow for the assumption of a Normal distribution. The forecast for such stations might improve if an alternative method to linear regression will be used. The alternative methodology makes use of a regression model with a normal-quantile transformation of the data. The transformation includes the use of the 1971-2000 climatological underlying distribution (Normal, Lognormal or Gamma) expressed as normal quantiles. The prediction is made in the units of the normal quantiles that are translated to metric units using the climatology distribution. The advantage of using this method avoids the problems associated with the asymmetric properties of precipitation distribution. However, this methodology also has a disadvantage because of its inability to adjust for the most recent trends, which might play an important role in forecasting precipitation in a changing climate.
A53E-07
Forecast Performance of the New Local Three Month Temperature Outlook
NOAA's National Weather Service (NWS) introduced a new operational Local 3-Month Temperature Outlook (L3MTO) in January 2007. The product is available for 1170 locations nationwide and can be accessed via any NWS Weather Forecast Office (WFO) climate website (under the "Climate Prediction" tab, or the NWS Climate website http://www.nws.noaa.gov/climate/l3mto.php ). L3MTO methodology (1) applies linear regression to identify the statistical relationship between a station parameter and the corresponding forecast region and (2) adjusts the regression parameters to the most recent temperature trends at the station. Long-term forecast performance evaluation plays an important role in the product development process because it is essential to guide the ongoing improvement of the forecasting procedures. The CPC 3-month temperature forecasts from 1994 to 2005 were used to create a L3MTO hind cast for the same time period. To avoid possible contamination of the verification results by dependent observations, the L3MTO hindcast computations train each year of 1994 to 2005 using the present forecast methodology and data for regression and trend adjustment of appropriate time period. For example, for the 1994 (2005) forecast, the regression is based on 1961-1990 (1971-2000) data, and the 1984 -1993 (1995-2004) 10-year trend is adjusted for significant changes in the difference between the station and CPC forecast region temperature. As the L3MTO presentation includes different formats (e.g., three categorical forecast, probability of exceedance, etc.) the verification uses different verification statistics: modified Heidke Skill Scores, Continuous Ranked Probability Skill Scores (CRPSS), and reliability diagrams. To avoid possible issues of small sampling, a combination of all stations is used to analyze the forecast skill versus the lead relationships. This analysis concludes that the 1994 -2005 forecasts overall did not indicate a large difference between skills for short and long leads. Therefore, to identify each individual station's performance, all leads have been used. This study identifies locations and 3-month periods with satisfactory forecast performance, and their spatial and temporal variability. The most skillful L3MTO performance is during late fall and early winter seasons in the Southwestern US.
A53E-08
Four years of gravity waves monitoring in Antarctica : Impact for global atmospheric studies
The development of the Infrasound International Monitoring System, used for the verification of the Comprehensive Test Ban Treaty, offers a powerful way to measure, permanently and at a global scale, the atmospheric waves at different latitudes. Infrasound stations using several microbarometers are very sensitive acoustic antennas, measuring the main characteristics of infrasound waves including velocity and direction of the wave front. Associated with new data processing methods, a global analysis of the atmospheric disturbances is now possible in a large frequency range. The networks if mostly sensitive to infrasound in the range 0.01 to 10 Hz, but most of gravity waves, which are characterized by very large amplitudes, are also detected by the network. The Antarctic stations are especially interesting for the study of gravity waves, because they are controlled by the polar vortex, and because they are rarely disturbed by the low latitude mountain gravity waves activity which is less important than in the Northern hemisphere. The monitoring of the gravity wave activity in the Antarctica station I27DE from 2003 up to 2007 reveals two active gravity wave systems. The first one, characterized by an azimuth from East, is produced in the troposphere by the wind blowing over mountains. The second system, characterized by an azimuth from West, is correlated with the wind and the temperature gradients in the lower stratosphere and is related with the polar stratospheric vortex. During magnetic storms infrasound waves are generally observed with a North azimuth at frequencies from 0.5 Hz to few Hz, however, gravity waves are generally not observed. A strong wave system has been observed coming from North only once in January 2005 several days after a major magnetic storm. The origin of this wave system in terms of magnetic storm or other processes related with the global dynamics of the stratosphere is discussed.
A53E-09
The Influence of Antarctic Sea-ice Extremes on Large Scale Atmospheric Variability in the Southern Hemisphere
The response of the extra-tropical Southern Hemisphere atmosphere to extremes of Antarctic sea ice concentration is investigated using a fully-coupled climate model - the NCAR Community Climate System Model-version 3. Average, maximum, and minimum sea ice concentration climatologies were extracted from satellite-observed sea ice concentrations and used to force the model for three 150-year simulations. Initial results for the atmospheric pressure and temperature fields for the summer season are discussed. The large scale modes of variability such as the Southern Hemisphere Annular Mode are seen to respond to the different sea ice forcings. The way in which the sea ice distribution can initiate these responses and the roles of the atmosphere and ocean in modulating them are discussed.
A53E-10
Antarctic Peninsula Climate Change and the Role of Atmospheric Quasi-Stationary Waves
During the late 20th century, the Antarctic Peninsula (AP) region has shown one of the largest observed regional surface warming trends globally, in contrast to much of the Antarctic region. Previous studies have attributed this warming to trends in the Southern Annular Mode (SAM), trends in west Antarctic sea ice cover and ocean warming in continental shelf west of the AP. In this study, the possible influence of the SH zonal wave one (ZW1) on AP surface temperature is explored using local station observations, satellite and reanalysis data. ZW1 is the dominant pattern of the SH zonally-asymmetric circulation, and there is evidence that it is linked to large-scale atmosphere-ocean interactions in the South Pacific basin, as well as airflow over the west Antarctic region. Evidence is presented of a seasonally-dependent relationship between AP surface temperature and ZW1 amplitude and location that is concentrated in the austral fall-early winter. The magnitude of ZW1 influence on AP temperature is compared to that of the SAM, the major mode of SH atmospheric variability. Possible physical mechanisms for the observed response are discussed.
A53E-11
Planetary Waves, Ozone Distribution And Tropopause Height Asymmetries In Connection To Antarctic Peninsula Warming
Decadal variations of the quasi-stationary wave amplitude and zonal structure are analyzed using the TOMS data. Seasonal dependence of total ozone content (TOC) is considered. The amplitudes of quasi-stationary planetary waves in TOC zonal distribution at high latitudes of Southern Hemisphere are calculated for 1979-2005. The highest values of the quasi-stationary wave amplitude at latitude 65S in October are observed. The asymmetry of total ozone distribution over Antarctic region during Austral spring is discussed. The amplitude and longitudinal position of zonal anomalies are calculated for total ozone content distribution along seven individual latitude bands at 5-degree intervals between 50S and 80S. The mid-latitude ozone-rich collar has a mid-latitude maximum with mean position between 90E-180E longitudes and with up to 390 DU. The significant planetary wave TOC minimum eastward shift about 40 degrees in longitude is observed over Weddell Sea during 1979-2005, whereas the zonal maximum is relatively stable in position. This displacement is discussed in connections to latest findings of the strengthening circumpolar westerlies and regional climate warming in Antarctic Peninsula. Tropopause height anomalies over Antarctic region show seasonal change associated with total ozone losses during spring months. The tropopause height anomaly in West Antarctica coupled with its increasing decadal trend could be involved in climate change in this region. The research was made in the framework of the SCAR ICESTAR Program, and ORACLE-O3 IPY Project, and partly supported by project 06BF051-12, Grant Greece-Ukraine M/86-2006, and Australian Antarctic Science project 737.