A24A-01 INVITED
Statistical Connections Between Extratropical Storminess Variability And Teleconnection Patterns
This study relates the extratropical storminess in the Northern Hemisphere during winter to the large scale flow using various regression models. Five teleconnection patterns were found to be statistically significant factors at the 5 percent level for storminess in the Euro-Atlantic region: the North Atlantic Oscillation (NAO), the East Atlantic pattern, the Scandinavian pattern, the East-Atlantic/Western-Russia pattern and the Polar/Eurasian pattern. In the North Pacific the dominant factor is found to be the Pacific North American (PNA) pattern. Furthermore, the relationship between teleconnection patterns and storminess can to a large extent be accounted for by a basic relation between storminess and the local mean SLP but with a few notable exceptions. In particular the East Atlantic pattern is identified as an important non-local factor for storminess over the Labrador Sea and the PNA pattern as an important non-local factor for storminess north of the Aleutian low. It will also be shown that the seriality of the synoptic lows in the northern midlatitudes is intimately linked with the teleconnection patterns. This is of particular relevance to regional climate and extreme events over Europe. Finally, we find that observed key relations between storm track seriality and several teleconnection patterns based are poorly represented in a GCM.
A24A-02 INVITED
Polar Lows: A Challenge for Experimental Studies and Numerical Weather Prediction
Polar low research has been a matter of interest of the scientific community for more than 30 years, since polar lows are affecting e.g. fishery, oil and gas exploration and logistical operations in polar regions. The term "polar meso-scale cyclone" or "polar mesocyclone" is now generally accepted for all polar cyclones poleward of the main polar front having scales smaller than 2000 km. The classical 'polar low' (PL) is included as a subtype, which is restricted to maritime systems with near surface winds exceeding 15 m/s. While in its early years polar low research was mainly focussed to the areas of European polar seas, polar MCs are now investigated over almost all oceans affected by cold air outbreaks in the Northern Hemisphere (NH) and also in the Southern Hemisphere (SH). For the NH, research has been conducted e.g. for the Japan Sea, the Labrador Sea, Hudson Bay and the Davis Street, and also for the areas of the Greenland Sea, the Norwegian Sea and the Barents Sea. For the SH, MCs have been studied e.g. for the areas of the Ross, Bellingshausen Sea, and Weddell Sea. Polar MCs are a challenge for experimental studies because of their short lifetime, their fast development and the lack of good numerical forecasts with respect to the location and time of their formation. Due to the sparsity of the observational network, only few data of polar low structures are available from conventional measurements. Valuable in-situ data have been obtained by a few aircraft campaigns. With the advent of more advanced satellite sensors comprehensive studies of e.g. the near-surface wind field and moisture structure have been possible. Despite the problems of forecasting PLs in operational weather prediction, numerical models have been a valuable tool in investigating the mechanisms of PL development. Process studies allow for the separation of individual mechanisms such as convection and baroclinic instability. Today's operational forecasts of PLs have improved, but it is still a challenging task and involves some empirical knowledge.
A24A-03 INVITED
Synoptic influences on Fram Strait sea ice flux
The recent decline of Arctic sea ice extent has attracted widespread scientific attention. Explanation of this decline is usually found in a combination of external (e.g. GHG) forcings and changes in atmospheric circulation patterns. Most efforts focus on large scale circulation patterns, such as the North Atlantic Oscillation and the Arctic Oscillation and their impact on sea ice. However, synoptic scale atmospheric features also play a significant role in modulating sea ice in the Arctic. I examine the influence of cyclones on sea ice export through Fram Strait - the primary region of sea ice export out of the Arctic, which accounts for about a quarter of the total freshwater export. Cyclones are identified using a tracking algorithm applied to 6-hourly NCEP/NCAR Reanalysis sea level pressure fields. Sea ice is examined using daily Polar Pathfinder Ease-Grid sea ice motion vectors and sea ice concentration fields derived from Nimbus-7, SMMR and DMSP SSM/I brightness temperatures distributed at NSIDC. A case study analysis reveals that depending on their position relative to the sea ice edge, cyclones can significantly increase or decrease (and even reverse the direction of) the sea ice flux out of Fram Strait. A statistical evaluation of the co- variability between synoptic activity and sea ice since 1979 is performed to evaluate recent trends and interannual variability in the Fram Strait region. The relationships between the synoptic-scale variability and large-scale circulation patterns such as the North Atlantic/Arctic Oscillation are also examined.
A24A-04
The last 50 and the next 100 years - extra-tropical storms and their coastal impacts in Northern Europe
A cascade of global and regional models combined with statistical modeling has been implemented - to describe in detail changing storminess, wave conditions and coastal surge statistics in the North Sea region during the past 50 years, and - to construct scenarios of possible future conditions of storminess, surge statistics and wave conditions for the coming 100 years. It turns out that storminess has undergone a significant intensification in 1960-1990, but that the trend has ceased; also the variations did not go beyond the range of variations estimated from proxy data since about 1800. For the future, marginal increases of strong wind incidence are expected, which lead to moderate increases of wind-induced surge heights and wave heights. When the expected wind-induced surge heights are combined with the expected mean sea level height, changes of extreme water levels of the order of 70 cm, or so, appear plausible for the end of this century. Data from these efforts are publicly available through the CoastDat web portal. http://www.coastdat.de/
A24A-05
Storm Track Analysis from CCSM3: Present and Future Climate
Climatology and interannual variations of wintertime extratropical cyclone frequency in CCSM3 20th-century simulation are compared with the NCEP/NCAR reanalysis during 1950-1999. CCSM3 can simulate the storm tracks reasonably well, although the model produces slightly less cyclones at the beginning of the Pacific and Atlantic storm tracks and weaker poleward deflection over the Pacific. As in the reanalysis, frequency of cyclones stronger than 980 hPa shows significant correlation with the Pacific/North America (PNA) teleconnection pattern over the Pacific region and with the North Atlantic Oscillation (NAO) in the Atlantic sector. Composite maps are constructed for opposite phases of El Nino-Southern Oscillation (ENSO) and the NAO and all anomalous patterns coincide with observed. One CCSM3 21st-century A1B scenario realization indicates there is significant increase in the extratropical cyclone frequency on the US west coast and decrease in Alaska. Meanwhile, cyclone frequency increases from the Great Lakes region to Quebec and decreases over the US east coast, suggesting a possible northward shift of the Atlantic storm tracks under the warmer climate. The cyclone frequency anomalies are closely linked to changes in seasonal mean states of the upper-troposphere zonal wind and baroclinicity in the lower troposphere. Due to lack of 6-hourly outputs, we cannot apply the cyclone tracking algorithm to the other 8 CCSM3 realizations. Based on the linkage between the mean state change and the cyclone frequency anomalies, it is likely a common feature among the other ensemble members that cyclone activity is reduced on the East Coast and in Alaska as a result of global warming.
A24A-06
Water Vapor, Cloud Liquid Water Paths, and Rain Rates over the Northern High Latitudes
Data from the Special Sensor Microwave Imager (years 1987-2006), Advanced Microwave Scanning Radiometer, and a surface-based radiometer at Barrow, Alaska are examined for insights into the behavior of water vapor, cloud liquid water and rainrates over the northern high latitudes. Screening for sea ice is accomplished by combining an independent dataset of monthly-mean sea ice fractions with the water vapor path (WVP), cloud liquid water path (LWP), and rainrate (RR) retrievals. The Wentz water vapor path retrieval shows no sensitivity to a proxy for sub-pixel sea ice presence, while the Wentz liquid water path retrievals are sensitive to sea ice presence during summertime when atmospheric variability is high but otherwise their sea-ice screening appears effective. The rainrate retrieval is highly sensitive to sea ice. The seasonal cycle and 1987-2006 time trends are examined. The WVP annual cycle has an amplitude of 1 cm with a July maximum phasing that is consistent with a continental influence. The springtime LWP increase usually occurs in tandem with the WVP increase and slightly lags the falltime WVP decrease. The maximum lag occurs over the northern Pacific, where the maximum LWP occurs in August, one month later than over the northern Atlantic, and is correlated to an August precipitation maximum. The strongest SSMI-derived trend is an increase in wintertime moisture south and southwest of Greenland, consistent with continental air outflow over warmer waters. An interesting feature is an autumnal increase in WVP and LWP north of the Bering Strait from 1989 to 2001 with a subsequent decrease in recent years. This cannot be explained by surface evaporation off of increased areas of open water and instead appears linked to recent decreases in cyclone activity. Winter and spring increases in LWP are noted in the surface-based dataset from Barrow, Alaska.
A24A-07
Storm impacts on the Arctic sea-ice export and melting observed by ice drifting buoys
Drifting buoys deployed near the North Pole in the 2000s revealed that sea-ice export from the Arctic Ocean through the Fram Strait was large in 2005. The relationship between ice drift and sea-level pressure (SLP) showed that the mean SLP from June to September is conducive to continuous sea-ice drift offshore from Siberia. A record minimum of the ice extent in 2005 is partly explained by the strongest SLP gradient across the Transpolar Drift Stream for the recent 27 years from 1979. The SLP pattern regressed on the linear trends of ice extent is characterized by cyclonic circulation anomalies along the Eurasian coast, which tend to enhance the SLP gradient across the Transpolar Drift Stream. The large export of freshwater and sea ice in 2005 could increase the salinity of the upper ocean and lead to development of the mixed layer and suppression of sea-ice growth during the subsequent winter. The role of wind conditions on sea-ice melting was also investigated in detail, one in 2002 under stormy conditions, one in 2003 under calm conditions. Although the ice concentration near the North Pole was the same in 2002 and 2003 during early summer, the heat used in bottom melting in 2003 was almost half of that in 2002. To obtain the total heat input into the upper ocean, heat used in lateral melting was additionally derived from a time series of ice concentration in 2002. Assuming the same heat input into the upper ocean, the heat used in lateral and bottom melting was estimated and compared between the years. It is thought that the warm fresh water embedded within the ice cover was mixed downward during the frequently stormy mid-summer of 2002, enhancing bottom melting. By contrast, the warm water in 2003 tended to be used for lateral melting due to the relatively calm conditions, suggesting that a continuously weak wind is favorable to decrease the ice cover during summer.
A24A-08
Seasonal Evolution of Aleutian Low-Pressure Systems: Implications for the North Pacific Sub- Polar Circulation
The development of Aleutian lows and how they evolve from early fall to early winter is analyzed using a combination of meteorological fields and sea surface temperature (SST) and wind data. The time period of the study is from September to December 2002, although results are shown to be representative of the longterm climatology. Characteristics of the storms were documented as they propagated across the North Pacific, including path, central pressure, deepening rate and speed of translation. Clear pattern emerged. Storms tended to spin up in two distinct geographical regions: the Gulf of Alaska in the early fall and the western North Pacific in late fall, seemingly related to the seasonal distribution of the SST field in both regions. In the Gulf of Alaska, a notch in the SST field seems to be of primary importance by optimizing differential heating in the storm and allowing air-sea fluxes to energize the storm more effectively. As winter approaches and the Sea of Okhotsk becomes partially ice covered and cold, the air emanating from the Asian continent leads to cyclogenesis in the region near Kamchatka. The impact of the oceanic circulation of the windstress curl pattern resulting from these two regions of storm development is investigated using historical hydrography data. It is argued that the seasonal bimodal input of cyclonic vorticity from the wind may be partly responsible for the two distinct North Pacific sub-Arctic gyres.
A24A-09
Cyclone variability and its impact on moisture transport into the Arctic
The Arctic is a region of moisture flux convergence, and the atmospheric water transported into the area, is a major part of the freshwater input to the Arctic Ocean, either directly through precipitation over the ocean/sea ice or as terrestrial precipitation that is then transported into the Arctic Ocean as river runoff. In addition variability or systematic changes in the moisture transport play an important role in the variability of Arctic radiation through cloud formation and water vapor. In this study characteristics of cyclones entering the Arctic (crossing 70°N) from 1949-2002 are examined. As both the cyclone number and their intensity are important for their impact on moisture transport, we have used the accumulated amount of positive relative vorticity at 70°N as a measure of cyclone activity. In terms of accumulated positive vorticity, cyclonic activity is by far most vigorous in the Greenland Sea during all seasons, except summer, when the Norwegian, Barents and Kara Sea have a comparable amount of cyclone activity. In terms of variability the Greenland Sea is also the main region. The number of cyclones traveling into the Arctic is approximately the same during all seasons, but with the winter cyclones being more intense, more variable and shorter lived than during summertime. The time series of total cyclone activity (CAI) cyclones entering the Arctic (defined as the accumulated vorticity at 70°N) indicates an upward trend (annually 3.1%/decade) that is statistical significant in three of four seasons, with the largest relative increase in summer (6.2%/decade) and spring (4.5%/decade). Further investigations should go into the causes of these strong spring and summertime trends that may have had a large impact on both sea ice dynamics and thermodynamics and the radiation characteristics of the Arctic. The cyclone activity variability at 70°N was statistically linked to the variability in moisture transport. In general the cyclone activity at 70°N is shown to be a good predictor for both seasonal and annual moisture transport variability. Annual total Arctic moisture transport variability is mainly driven by variability in cyclone activity over the Greenland, Kara and East Siberian Sea and the two regions together account for 55% of the total annual moisture transport variability. While the Greenland and East Siberian Sea cyclones are the main drivers of the Norwegian and Chukchi Sea moisture transport, respectively, the cyclones entering the Arctic from the Kara Sea is negatively correlated to the total moisture transport by reducing the Greenland Sea moisture transport as the Kara Sea cyclones will travel into the central Arctic and give northerly wind anomalies over the Greenland to Barents Sea region. Significant correlations were found between total cyclone activity and total moisture transport during winter (correlation of 0.67), autumn (0.49) and spring (0.69), while there was no connection between the two in summer (0.22). Variability and systematic changes in the Greenland and East Siberian Sea may be of particular importance, making the role of regional cyclone variability and changes an important topic in order to understand past, present and future Arctic hydrological variability. http://www.uib.no/People/gbsag/
A24A-10
An Eulerian View of storm events and their relationship to storm surge along the coast of Alaska
Automated, "objective" storm classification methods work with selected meteorological variables to identify cyclonic systems. Typically a Lagrangian approach is used, whereby a storm is identified and then tracked throughout its life cycle. For weather-dependent applications, however, the action of a storm at a given point is important, e.g., coastal engineering. An event algorithm that implements this Eulerian approach to storm identification has been developed by Atkinson (2005). Using this algorithm two storm event databases are built and related to surge events as recorded at NOAA water level stations for various sites around the coast of Alaska. The first event database is derived from wind records for select coastal weather stations in Alaska; the second using winds from the NCEP/NCAR reanalysis dataset.