A13E-1595
A Southern Ocean cyclone climatology based on high resolution NWP model output
Mesoscale cyclones over the Southern Ocean represent an important element in the global circulation of heat and moisture and the maintenance of Southern Hemisphere climate. We seek to advance the understanding of atmospheric processes responsible for the development and decay of Southern Ocean mesocyclones and to explore the interactions between these cyclones and the underlying surface conditions, including sea ice extent, thickness, concentration, motion, and temperature. In this presentation, we will describe a Southern Ocean mesocyclone climatology, created based on the Antarctic Mesoscale Prediction System (AMPS) output. The method applied to construct the climatology utilizes the Self-Organizing Map (SOM) technique. The seasonal variability between large-scale circulation patterns and their association to the cyclone formation over the Southern Ocean is of particular interest. In addition, variables that influence the formation of a cyclogenetic environment, such as sea surface temperatures and the sea-ice extent, are analysed.
A13E-1596
Relationship Between Extreme Precipitation Events in the Mackenzie and Yukon Watersheds and Synoptic Circulation Patterns
The relationships between synoptic circulation patterns and extreme precipitation events in northwestern North America are investigated. The method of self-organizing maps (SOMs) is used to objectively identify synoptic circulation patterns in sea level pressure data from the ERA40 Reanalysis over the time period of 1957-2002. Days with extreme basin-averaged precipitation values (defined as days in the top 10th and 1st percentiles) were identified for the Mackenzie and Yukon watersheds using precipitation from the ERA40 Reanalysis dataset. The synoptic patterns associated with the extreme events were identified for both annual and seasonal time scales to allow for analysis of the patterns responsible for extreme events throughout the year. Many of the extreme events identified were associated with synoptic patterns characterized by pronounced cyclonic systems.
A13E-1597
Arctic Cyclone Activity: Synoptic-scale Modeling Study and Upscaling Implication for Large- scale Climate Variability and Change
Observational study indicates that synoptic-scale cyclone activity has intensified in recent decades in the Arctic region, concurrently with a large-scale reduction of the mean sea level pressure, an amplification of the Arctic/North Atlantic Oscillation, and a decrease of sea ice extent. This intensification can be attributed to the more enormous and more intense cyclones both generated locally and traveling into the Arctic from mid-latitudes. The latter is manifested by a poleward shift of storm tracks. Not only do the intensified cyclones may bring high frequency weather extremes, but also they may result in an enhancement of atmosphere-sea ice-ocean interactions and, in turn, leave fingerprints on large-scale climate variability and change. In this paper, we will first introduce an Arctic regional coupled model with atmosphere-sea ice-ocean interactions taken into account. Then, we will show a model simulation result of a cyclone process over the Bering Sea and the Beaufort Sea area during the deployment period of the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment. The simulation results will be validated by the SHEBA measurements. The thermodynamic feedback between atmosphere, sea ice and ocean in the cyclone process will be presented. Finally, the upscaling implication of synoptic-scale cyclone activity for large-scale Arctic climate variability and change will be discussed.
A13E-1598
Can Polar Lows Lead to a Warming of the Ocean Surface?
The main objectives of this research has been to investigate the effect of intense polar lows on the upper ocean. Our hypothesis is that vertical mixing induced by surface stresses may lead to entrainment of waters from a warm and saline core beneath the sea-surface. If the time scale of the entrainment and subsequent surface warming is sufficiently short, a positive feedback on the cyclone intensity is possible. As will be demonstrated, in the seas where polar lows often developed, the North-Atlantic Current (NAC) sub- ducts under colder and less saline waters and Arctic water flows on top of warmer and more saline waters. The result of this is the frequent presence of a temperature inversion in winter-time hydrographic sections in areas influenced by the NAC. By temperature inversion, we will here refer to temperatures increasing with depth in upper ocean. For sufficiently strong wind events, turbulent entrainment of this sub-surface warm core may lead to a rapid surface warming. One of our main findings is that the surface warming of more than 1°C may take place within a few hours. The result is based on model runs with initial temperature and salinity profiles from CTD-observations. Observational evidence of surface temperatures that support the hypothesis are found from microwave satellite observations of polar low events. In the cases presented here, increased sea-surface temperatures between 1°C and 2°C were observed in the wake of polar lows. The impact of increased SST on the cyclone intensity has been investigated using Carnot theory and by performing simulations with an axis-symmetric cyclone model coupled to a model for turbulent mixing in the upper ocean. For realistic temperature differences between the atmosphere and ocean, a one degree increase in SST yields an additional pressure drop of about 2-3 hPa. Entraintment of waters from a sub-surface warm core due to mixing by storms leads to a cooling of the ocean. As warm waters from deeper layers become exposed to the atmosphere and gradually cools, heat is lost from the ocean. Hence, frequent storm events during winter may strengthen the thermohaline circulation. In the absence of storm mixing the fresher water masses overlaying the NAC act as insulation to heat exchange between the NAC and the atmosphere. Through the cooling and transformation of Atlantic waters, polar lows and intense storm in the Nordic Seas may thus have an impact on the climate.
A13E-1599
Statistics of the Arctic cyclone activity and tracks
The objective of this study is to analyse changes in frequency and parameters of the development of the cyclones formed within the Arctic Circle and outside the Arctic but moving into it during the period 1948-2002. The database of cyclones described by Gulev et al. (2001) was used in this study. The database consists of cyclone tracking output of the 6-hourly NCEP/NCAR reanalysis SLP fields using the software worked out by Grigoriev et al. (2000). Cyclones are presented by the geographical coordinates of their centres and SLP at these points. The Arctic basin is defined here as an area located north from 68°N. Arctic cyclones are defined as cyclones that have formed north from 68°N while entering cyclones have formed south from that borderline and have entered into the Arctic Circle north from 68°C. Also, deep cyclones with minimum sea-level pressure below 1000 hPa and shallow cyclones were selected. The following variables are determined and studied: total number of cyclones, duration of cyclones expressed in the number of tracking points with 6-hours interval, mean sea-level pressure of cyclones, sea-level pressure at the first, last, deepest and the northernmost points of cyclones. A longitude of the point on the parallel 68°N is calculated where the cyclone has entered the Arctic region. According to these points, the entering cyclones are grouped into sectors. The sectors are chosen by 40-degree longitude while the number of cyclones in the middle of the sectors is usually higher than near to the borders. All together eight 40-degree sectors were defined for the analysis and two 20-degree sectors were selected – in the south-eastern part of Greenland and in the Canadian Arctic Archipelago between 100–80°W. Time series of annual and seasonal mean values are formed. Seasons were defined by grouping three months – winter (DJF), spring (MAM), summer (JJA) and autumn (SON). Linear regression analysis is applied for detecting long-term changes. A linear trend is calculated for every time series and their significance level is found using the Student's t-test. Trends are considered statistically significant on P<0.05 level. The main conclusions concerning long-term changes in cyclone activity are the following: 1) The total number of the entering into the Arctic Circle cyclones has significantly increased during 1948-2002, but not of the Arctic cyclones formed within the Arctic Circle (north from 68°N). 2) Percentage of deep and shallow entering cyclones has not changed. The number of deep cyclones (mean SLP below 1000 hPa), deep entering as well as deep Arctic cyclones, has clearly increased while the number of shallow Arctic cyclones has decreased. The biggest changes in seasonal values revealed in winter. 3) Duration of the cyclones almost has not changed. 4) Mean SLP of deep cyclones has decreased significantly. This change is more substantial in case of deep Arctic cyclones. This trend exists during all seasons with the exception of autumn. The change is the highest in winter. 5) Generally, the frequencies of tracking points with deep pressure have increased and mean pressure has decreased in the first, last, deepest and northernmost tracking points. Increase in total number of entering into the Arctic basin cyclones in winter is much higher than during the rest of the three seasons all together. 6) An increase in the number of entering cyclones has taken place in the sectors of the Bering Strait, Alaska, Baffin Sea and East Siberia. References Grigoriev, S., Gulev, S. K., Zolina, O. (2000). Innovative software facilitates cyclone tracking and analysis. EOS 81. Gulev, S. K., Zolina, O., Grigoriev, S., (2001). Extratropical cyclone variability in the Northern Hemisphere winter from the NCEP/NCAR reanalysis data. Climate Dynamics 17, 795–809
A13E-1600
Decadal Trend in Atmospheric Wave-Activity
An apparent trend of increasing planetary wave amplitude has been detected in the upper troposphere using a novel circulation based diagnostic. Conventional wave-activity diagnostics, such as E-P flux, have provided insight about the propagation of atmospheric waves. However, computation of absolute eddy amplitude remains somewhat unclear, since it relies on the construction of a wave-free basic state. Here a new diagnostic is introduced, which utilizes the equivalent latitude transformation (the latitude given by an area preserving, axisymmetric rearrangement of a potential vorticity contour) to establish the basic state of the flow. Analysis of both UKMet and NCEP data illustrates some of the advantages of this diagnostic as well as some differences between these two reanalysis products.
A13E-1601 [WITHDRAWN]
Storm Track Variability and Interaction with the Mean Flow on Daily, Interannual and Climate Change Time Scales
"Pulsing" and "latitudinal shifting" modes of extratropical storm track variance are identified as fundamental modes of climate variability in the storm track basins of both the Northern and Southern Hemisphere. The secondary latitudinal shifting modes of storm track variability can be related to well-known modes of atmospheric flow variability, such as the Northern Annular Mode / North Atlantic Oscillation, Pacific North America pattern and the Southern Annular Mode. The dominant pulsing modes of storm track variability have received somewhat less attention in terms of their relationship to the mean atmospheric flow. At month-to-month and interannual time scales, pulsing and shifting modes of storm track variability are related to zonal wind anomalies that are consistent with anticipated forcing by the eddy-induced momentum fluxes. At day- to-day time scales, there is some indication that the pulsing mode of storm track variability may evolve into the shifting mode of storm track variability--resulting in wind anomalies consistent with the patterns of atmospheric flow variability previously described. At climate change time scales, extratropical storminess appears to be strongly suppressed in cold climates and moderately enhanced in warm climates, relative to the modern day in a fully-coupled general circulation model's output from Last Glacial Maximum, modern day and quadrupled carbon dioxide experiments. Observations from the Northern Hemisphere Pacific and especially throughout the Southern Hemisphere seem to indicate a recent intensification of the storm tracks geographically consistent with the general circulation model output.
A13E-1602
Cyclones of the Arctic: definitions, pattern and regional trends
Cyclones are essential players in the high latitude climate system, as they transfer heat and moisture into high latitudes. Through their impacts on precipitation patterns and sea ice, cyclones play a significant role in modulating the freshwater budget in the Arctic. To elucidate the mechanisms of synoptic impact on the high latitude climate we analyze cyclone activity trends and compare them with trends in major variables (temperature, precipitation, clouds) for the Lena river basin. We utilize two cyclone tracking algorithms in the Arctic region. One algorithm, developed by Mark Serreze, makes use of the sea level pressure (SLP) field, while the other, developed by Kevin Hodges, uses 850 hPa and 700 hPa vorticity fields to identify a synoptic system. We compare the results from these two algorithms and evaluate their performances in the Arctic. Overall, both algorithms reveal similar patterns. Winter is dominated by the Aleutian and Icelandic lows, while in summer these centers are accompanied by the "Arctic frontal zone" located along the coast of Siberia and propagating into the Arctic Ocean. Cyclogenesis patterns are even more similar between the two algorithms. We also perform correlation analysis for the Nordic Seas region (roughly corresponding to the Icelandic low) in winter and the Lena River basin (corresponding to the Arctic frontal zone) in summer. These two regions are prominent centers of synoptic activity in the Arctic and are expected to be the most significant in terms of cyclone activity and impact. As expected, the correlations are highest for the Nordic Seas in winter and the Lena River basin in summer, for both cyclone track counts and cyclone intensities. Cyclones identified in the reanalysis also appear to correlate with trends in the precipitation and temperature over northern Eurasia, particularly in winter. A decrease in the number of total summer cyclones since the middle of the 20th century is also consistent with the modest decline in precipitation at stations in the Lena Basin. An apparent shift over this time from short-lived cyclones to longer-lived storms may also be consistent with the observed shift in cloud distributions from low to high clouds types.
A13E-1603
Structures of moisture transport to Norway associated with winter cyclones in the North Atlantic
Understanding of the atmospheric processes leading to periods of extreme precipitation is still limited. Previous studies have shown that both, remote and local moisture transport contribute to heavy precipitation events in Norway and the American West coast via moisture conveyor belts or 'atmospheric rivers', which are linked to mid- latitude cyclones. In this work we want to gain more insight into the processes and the structures of moisture transport associated with mid-latitude cyclones. We studied a period of above-average precipitation in southern Norway during winter 2006/07, which was related to a series of mid-latitude cyclones moving into the Scandinavian land mass. A number of high-resolution simulations was conducted with a limited-area NWP model, using ECMWF's high-resolution operational data. The limited-area model has been fitted with water vapour tracers, which allow to tag water vapor at its area of evaporation, and to follow it's movement through the model's hydrological cycle. We determined the evaporation sources for precipitation in southern Norway for several of the mid-latitude cyclones. In addition, the spatial structure in the cyclones the and temporal evolution of the moisture from different evaporation sources are presented. A variety of latitude-longitude sectors provide substantial amounts of moisture to different sectors of the cyclones, including the sub-tropcial North Atlantic. Our results have implications for understanding the role of SST anomalies, and provide new insight into the mechanisms of moisture transport in and precipitation from mid-latitude cyclones.
A13E-1604
Recent Intensification of Northern High Latitude Hydrological Cycle: Perspective from synoptic-scale storm activity
It has been documented that hydrological cycle has been intensified in the northern high latitude in recent decades, which is strikingly evidenced by an increase of the Eurasian river discharges. The enhanced river freshwater inflow into the Arctic Ocean has an important implication for global climate variability and change, through impacting strength of the North Atlantic deep convection and meridional ocean circulation. Predominately, the Eurasian river water originates from precipitation less evapotranspiration, which is an integral representation of atmospheric meridional moisture transport. Synoptic-scale storms, as fundamental weather elements, are major contributors to this moisture transport. In this study, we first analyzed variability and long- term linear trends of atmospheric moisture transport at a high temporal resolution. We then examined the associations of variability and long-term changes between moisture transport and storm activity over the Eurasian and American continents. The results reveal how shift of storm track and changes in storm activity, which is measured by storm intensity, trajectory count and position, and duration, modulate and contribute to the recently observed changes in the Eurasian and American river discharge.
A13E-1605
Analysis of the Synoptic Forcing for Precipitation Over Greenland From 1961 to 1999
Studying the synoptic climatology and precipitation patterns over the North Atlantic region is necessary in order to better understand the atmospheric input to the mass balance of the Greenland ice sheet. The self-organizing map (SOM) technique is applied to ERA-40 average daily sea-level pressure (SLP) data from 1961 to 1999 to objectively identify synoptic SLP patterns over the North Atlantic region. A total of 35 different SLP patterns were identified. Analysis of precipitation patterns associated with each SLP pattern revealed the various forcings for precipitation over portions of Greenland. In most cases the largest precipitation events are associated with passing cyclones, which create onshore flow, allowing for the air to be lifted orographically by the ice sheet. The southern portion of Greenland received the largest magnitudes of precipitation due to its extreme topography and its location near open water and, therefore, moist air. Each portion of Greenland has its own preferred synoptic pattern for receiving precipitation. The frequency and efficiency of these patterns to bring precipitation to Greenland were the focus of a trend analysis revealing that a slight increasing trend in precipitation from 1961 to 1999 was due to both circulation changes and thermodynamic changes near Greenland. This analysis will be repeated for 15 climate model predictions for the reanalysis period in order to evaluate the ability of the various climate models to reproduce the synoptic climatology of the North Atlantic region. The models that perform the best over the reanalysis period will be used to create a synoptic climatology for the twenty-first century for studying future trends in synoptic patterns and precipitation over Greenland.
A13E-1606
Detection of North Atlantic Polar Lows in Climate Mode Simulations
Polar lows are not properly resolved in global re-analyses. Atmospheric limited area models (LAMs), which post- process re-analysis data, may be an appropriate tool for describing the year-to-year variability and decadal trends in the formation of Polar Lows. The merits and potential of this approach are examined in case studies of reproducing polar low occurrences with a LAM. A series of three week long ensemble simulations of weather situations over the NE Atlantic with a RCM/LAM (CLM) was conducted and its capability to reproduce polar lows was investigated. To keep the influence of the initial field low, the simulations were begun approximately two weeks prior to the polar low formation. It is shown that polar lows can be reproduced with the LAM. When "spectral nudging" is applied a polar low develops in all ensemble members and the simulations are very insensitive to the initial conditions. However there are differences in detail compared to observational data, e.g extent of pressure decline and polar low's location. In a scond step towards our goal of determining trends in changing frequencies and characteristics, an algorithm for automatical detection polar lows has been designed and tested. This algorithm was applied to the output fields of a long-term simulation and first results are shown.
A13E-1607
Examination of the Influence of the State of the Gulf Stream on Storms in the North Atlantic
A cyclone tracking algorithm is applied to 7 years of QuikSCAT data merged with re-analysis data from ECMWF to create a Lagrangian storm track data set. To characterize the storms we compare several metrics: central pressure, pressure difference, and vorticity. Good correspondence was found between all of the intensity measures. We confirm previous findings that the greatest intensification of storms coincides with the warm core of the Gulf Stream region. To investigate feedbacks between storm growth and the Gulf Stream, we composite the sensible and latent heat fluxes during and prior to storm events. Separately, the variance of the high-frequency meridional winds from the re-analysis data is calculated to create an Eulerian storm track climatology. The storm activity found from both the Eulerian and Lagrangian perspectives is compared with indices of the ocean state, including upper ocean heat content, current strength and the structure of the sea surface temperature, to assess the importance of the ocean conditions for storm track behavior.
A13E-1608
The Effect of Sea-Ice Extent on storm activity in the North Pacific/Western Arctic
High-latitude storm activity plays important roles at various time and space scales, ranging from the local scale, with for example, severe erosion suffered by coastal margins in Alaska and other arctic regions, to the continental scale, where for example storm corridor position and strength strongly affect the exchange of moisture and heat between the Arctic and the lower latitudes. Sea-ice, specifically the location of the ice edge, plays an important role in the location of storm tracks as well. Its presence can impede storm progression into the Arctic by creating a cold friction zone over which storms lose energy. At the ice edge is often found a strong baroclinic zone which can enhance storm activity by both strengthening storms and by acting to preferential guide their trajectory. The extent of sea-ice varies considerably from year to year and has exhibited distinct decreasing trends over time with attendant impacts on storm track location. This paper studies the effects of sea-ice edge position on the location of storm tracks in the western Arctic (Bering and Chukchi Seas) by examining the response of storms during the active months of October and November under the influence of three different sea-ice scenarios: two historical for the 1952 - 2002 period, and one projected for the year 2050. The studies are conducted using SPEEDY, an Atmospheric Global Circulation Model (Molteni, 2003), with a spectral resolution of T30 (approximately 3.75° horizontal resolution). The sea ice forcing fields for the following three scenarios are based on datasets from the Met Office (HadISST 1.1) and the Arctic Climate Impact Assessment: 1) the maximum ice extent; 2) the minimum ice extent, and 3) the minimum projected sea-ice edge for the next 50 years using the ACIA 5-model composite data. Storm track response is investigated using metrics that include variance of the 500 mb geopotential height field and potential vorticity maxima. The impact of these storm activity variations on poleward heat and moisture transfer are also examined.
A13E-1609
Using Normalized Anomalies to classify extratropical/tropical interactions
A climatology of extreme events within a normalized anomaly framework is presented using current atmospheric reanalysis products. Based upon a long-term mean (30 years), these normalized anomalies provide a meaningful analysis of phenomena that is unusual for a given space and time. For instance, tropical cyclones undergoing extratropical transition introduce considerable geopotential height perturbations on the midlatitude background flow. Likewise, cut-off lows formed from wave-breaking processes introduce poleward originating air into the tropics. Many of these highly anomalous events are associated with sensible weather and climate events on a variety of scales, which may have significant impact upon society. This normalized anomaly method is applied to a selection from the IPCC climate model ensemble to highlight the frequency and magnitude of extratropical/tropical interactions in a warming world. http://www.coaps.fsu.edu/~maue/extreme/
A13E-1610
Assimilating QuikSCAT SeaWinds With WRF Model for High Latitude Sea Breeze Simulation
Sea breezes along the Arctic coastal areas, especially in the Beaufort Sea coast, are associated with seasonal change of solar insolation, sea ice retreat, and complex terrain of the Brooks Range. Improvement of sea surface wind simulation with a high resolution mesoscale model is of particular interest in oil spill impact assessment and management, as well as in coastal erosion assessment. The objective of this study is to use the mesoscale weather research and forecast (WRF) model with satellite data assimilation to achieve a high quality simulation of Beaufort Sea regional weather patterns. The SeaWinds instrument onboard the polar-orbiting quick scatterometer (QuikSCAT) satellite is a specialized radar that measures ice-free ocean surface wind speed and direction. This data provides significant information over open water areas, where conventional observations are very sparse, for model validation and simulation improvement. The QuikSCAT SeaWinds level 2B data which have a horizontal resolution of 12.5 km are assimilated into the WRF model and its three-dimensional variational data assimilation system (WRF-Var) to study the impacts of QuikSCAT data on WRF simulations of surface wind fields and sea breezes. The modeling domain has been setup to cover the Beaufort Sea area with 235x136 grid points at 10 km resolution and is centered at (155W, 71.3N). In order to apply the WRF-Var system, a customized WRF model error statistics and length scales are created using a one-year simulation of WRF for this particular domain. Two 5-day periods, 10/01/2002-10/05/2000 and 9/20/2004-9/24/2004, are selected to conduct our case study. The years 2002 and 2004 had most open water area, i.e., the farthest ice retreat, during recent seven years; and thus, there are more QuikSCAT SeaWinds data available for our study. Preliminary results show that the WRF model is capable of simulating wind fields very well by verifying modeled winds with station observations. Further results on the impact of QuikSCAT data from validating model output with both surface station observations and QuikSCAT winds, and sea breezes analysis will be presented at the conference.
A13E-1611
Relationship Between the Northern Hemisphere Polar Vortext ,North Pacific Storm Track and the West Wind Drift
Abstract: For this study, we use the daily and the monthly data from the National Centers for Environmental Prediction- National Centers for Atmospheric Research(NCEP-NCAR) reanalysis. In addition, we employ the monthly sea surface temperature data and the area index of the polar vortex in the Pacific sector from Beijing Climate Center. The time of the data set covers from 1951 to 2002.We study the relationship between the polar area in the northern hemisphere and the sea surface temperature in the west wind drift( we also do the same work between the storm track and the polar area) in the spring, the result shows that: (1) In the Pacific sector (150° E - 120° W), there is obvious negative correlation between the area index of the polar vortex and the index of sea surface temperature in west wind drift area in spring. Moreover, we also find the same phenomenon between the Pacific sector polar vortex area index and the storm track in the spring. With respect to the definition of the storm track index and the index of the west wind drift area sea surface temperature, we will give some explanation at the end of the paper. (2)By using the M-K(Mann-Kendall) test method, we found that the spring polar area index in the Pacific sector and the spring storm track index all have an abrupt change in 1969. The value of the spring polar area index is smaller than that of the average throughout the 1950s and 1960's,but during the 1970s and 1980s it is on the contrary. Analysing the index variety we can conclude that the location of the Pacific storm track is more northward throughout the 1950's and 1960's but more southward during the 1970s and 1980s.Using the M-K(Mann-Kendall) test method ,we also found the spring index of the sea surface temperature in the west wind drift area have an abrupt change in 1973.It means that the sea surface temperature of the west wind drift is colder during the 1950s and 1960's,but during the 1970s and 1980s it is warmer. (3)During the 1950s and 1960s,there is a negative anomalies of geopotential height cincture zone in the subtropical area from India to north America in the Northern Hemisphere，but during the 1970s and 1980s the negative anomalies replaced by the positive anomalies in the same zone, what is more, this distribution of the anomalies is persistence from spring to summer not only throughout the 1950s and 1960s but also throughout the 1970s and 1980s. While in America the anomalies pattern show as northeast negative and southwest positive from Canada to America ,which cause the America continent climate shows different character, compared the 1950s and 1960s to the 1970s and 1980s.In the East Asian, during the 1970s and 1980s,the location of the subtropical high is more eastward and more southward ,so the summer monsoon is obviously weaker than the 1950s and 1960s.
A13E-1612
Synoptic Analysis of the Pacific-North American Teleconnection Pattern
In this study we investigate the synoptic characteristics of Pacific North America (PNA) teleconnection pattern events by examining the evolution of the potential temperature field on the nominal tropopause (the 2 Potential Vorticity Unit (PVU) surface). This quantity is obtained from the National Center for Environment Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data set. These data cover the period 1958 through 1997 for the months December through February. In this study we present observational evidence that both PNA phases undergo cyclonic wave breaking over the Pacific. The remnants undergo further linear growth and develop into the PNA pattern. The streamfunction anomalies resulting from the cyclonic wave breakings resemble the optimal structures of the PNA, suggesting that the optimal arises from wave breaking. An investigation of the Pacific jet suggests that the phase of the PNA is determined by the strength of the Pacific subtropical jet and is consistent with the theory of stationary waves. A stronger (weaker) jet leads to the positive (negative) phase of the PNA. An investigation of OLR suggests that tropical convection influences the strength of the jet. This is in contrast to the North Atlantic Oscillation (NAO).
A13E-1613
The influence of sea ice and topography on the wind regime of the Beaufort Sea coast
The North Slope region of Alaska exhibits an Arctic climate and is bounded to the south by the southwest to northeast oriented Brooks Range and to the north by the Beaufort Sea. Winds are consistently high and most often come from an easterly direction. As is typical for an Arctic region, precipitation is light and snow cover is present for much of the year. This area is of high economic significance because of resource extraction and development in the nearshore and offshore portions of the North Slope. A study has recently been funded to investigate the wind field throughout this region, specifically in relation to the documented sea breeze and topographic effects. Data from meteorological observing stations were compiled from the various networks available, including the National Weather Service, Federal Aviation Administration, University of Alaska Fairbanks, and the Minerals and Management Service. These data include inland, coastal, and offshore stations in this region and cover a time period of at least five years, with some more than 30 years. Wind speed and direction components were viewed in relation to distance from the coast to determine the magnitude and strength of the sea breeze effect, which is primarily confined to the ice and snow-free period. The variability of sea ice cover may also impact sea breeze frequency and magnitude. In addition, the wind field was shown to exhibit influences from the Brooks Range, which is dependent on the prevailing direction. These results will further be used to evaluate forecasts generated by mesoscale meteorological models, such as the Weather Research and Forecasting (WRF) model, with the ultimate goal of improved forecasts in this region.
A13E-1614
Sea Ice, High-Latitude Convection, and Equable Climates
It is argued that deep atmospheric convection might occur during winter in ice-free high-latitude oceans, and that the surface radiative warming effects of the clouds and water vapor associated with this winter convection could keep high-latitude oceans ice-free through polar night. In such an ice-free high-latitude ocean the annual-mean SST would be much higher and the seasonal cycle would be dramatically reduced - making potential implications for equable climates manifest. The constraints that atmospheric heat transport, ocean heat transport, and CO2 concentration place on this mechanism are established. These ideas are investigated using a column model with state-of-the-art atmospheric physics, high vertical resolution, a full seasonal cycle, a thermodynamic sea ice model, and a mixed layer ocean (the SCAM).
A13E-1615
Impact of the Projected Future Sea Ice Concentrations on the Atmospheric Circulation
The observed decline of Arctic sea ice cover and the projected future reduction raise the question of how reduced Artic sea ice will influence/feed back on the climate. Using the projected future sea ice concentration by the coupled atmosphere ocean sea ice model ECHAM5/MPI, we investigate the impact of the changed sea ice cover on the atmospheric circulation by conducting atmospheric general circulation model (AGCM) experiments. We force the AGCM ECHAM5 by the current seasonal cycle of arctic sea ice and the projected future arctic seasonal cycle. The goal of this experiment is to determine the impact of the Artic sea ice reduction - that is projected in the next 100 years - on the atmospheric circulation. We find a significant reduction in storminess. The response to sea ice concentration changes differs strongly even on intraseasonal time scales.