Courtenay Strong

Tropospheric Rossby wave breaking and the NAO/NAM

Objective analysis of several hundred thousand anticyclonic and cyclonic breaking Rossby waves is performed for the Northern Hemisphere (NH) winters of 1958–2006. A winter climatology of both anticyclonic and cyclonic Rossby wave breaking (RWB) frequency and size (zonal extent) is presented for the 350-K isentropic surface over the NH, and the spatial distribution of RWB is shown to agree with theoretical ideas of RWB in shear flow. Composites of the two types of RWB reveal their characteristic sea level pressure anomalies, upper- and lower-tropospheric velocity fields, and forcing of the upper-tropospheric zonal flow. It is shown how these signatures project onto the centers of action and force the velocity patterns associated with the North Atlantic Oscillation (NAO) and Northern Hemisphere annular mode (NAM). Previous studies have presented evidence that anticyclonic (cyclonic) breaking leads to the positive (negative) polarity of the NAO, and this relationship is confirmed for RWB over the midlatitudes centered near 50°N. However, an opposite and statistically significant relationship, in which cyclonic RWB forces the positive NAO and anticyclonic RWB forces the negative NAO, is shown over regions 20° to the north and south, centered at 70° and 30°N, respectively. On a winter mean basis, the frequency of RWB over objectively defined regions covering 12% of the area of the NH accounts for 95% of the NAO index and 92% of the NAM index. A 6-hourly analysis of all the winters indicates that RWB over the objectively defined regions affects the NAO/NAM without a time lag. Details of the objective wave-breaking analysis method are provided in the appendix.

Observed Feedback between Winter Sea Ice and the North Atlantic Oscillation

Abstract Feedback between the North Atlantic Oscillation (NAO) and winter sea ice variability is detected and quantified using approximately 30 years of observations, a vector autoregressive model (VAR), and testable definitions of Granger causality and feedback. Sea ice variability is quantified based on the leading empirical orthogonal function of sea ice concentration over the North Atlantic [the Greenland Sea ice dipole (GSD)], which, in its positive polarity, has anomalously high sea ice concentrations in the Labrador Sea region to the southwest of Greenland and low sea ice concentrations in the Barents Sea region to the northeast of Greenland. In weekly data for December through April, the VAR indicates that NAO index (N) anomalies cause like-signed anomalies of the standardized GSD index (G), and that G anomalies in turn cause oppositely signed anomalies of N. This negative feedback process operates explicitly on lags of up to four weeks in the VAR but can generate more persistent effects because o...

Variability in the Position and Strength of Winter Jet Stream Cores Related to Northern Hemisphere Teleconnections

Abstract Numerous teleconnections have been identified based upon spatial variability in sea level pressure or lower-tropospheric geopotential height fields. These teleconnections, which are commonly strongest in winter when the mean meridional temperature gradient is large, typically are neither derived from nor linked to changes in the jet stream. Here, winter tropospheric jet stream cores over the Northern Hemisphere (NH) are recovered from 6-hourly gridded data and interannual variability in winter jet core position, speed, and pressure are investigated in the context of NH teleconnections. Common methods for researching jet stream speed and position variability may yield unrepresentative results because jet core pressure variability is ignored (only one isobaric surface is evaluated) or pressure variability effects are smoothed (values are vertically averaged across several isobaric surfaces). In this analysis, data are extracted at the surface of maximum wind, thus controlling for jet core pressure ...

The Winter Atmospheric Response to Sea Ice Anomalies in the Barents Sea

AbstractThe atmospheric response to sea ice anomalies over the Barents Sea during winter was determined by boundary forcing the Community Atmosphere Model (CAM) with daily varying high and low sea ice concentration (SIC) anomalies that decreased realistically from December to February. The high- and low-SIC anomalies produced localized opposite-signed responses of surface turbulent heat flux and wind stress that decreased in magnitude and extent as winter progressed. Responses of sea level pressure (SLP) and 500-mb height evolved from localized, opposite-signed features into remarkably similar large-scale patterns resembling the negative phase of the North Atlantic Oscillation (NAO). Hilbert empirical orthogonal function (HEOF) analysis of the composite high-SIC and low-SIC SLP responses uncovered how they differed. The hemispheric pattern in the leading HEOF was similar for the high-SIC and low-SIC responses, but the high-SIC response cycled through the pattern once per winter, whereas the low-SIC respon...

Thermodynamic attributes of Arctic boundary layer ozone depletion

During spring 2000, a major ozone depletion episode was observed in the boundary layer at Alert, Nunavut, Canada. The ozone in the atmospheric layer extending from the surface to 1500 m was completely removed. This event lasted more than 9 days and was associated with an abrupt transition to an air mass that emanated from the Arctic Ocean. This source region provides the substrates that participate in the chemical reactions leading to ozone removal from the lower atmosphere. After the onset of the ozone depletion episode, the ozone-depleted air mass remained stagnant over the Alert region. In this study, thermodynamic and atmospheric dynamic characteristics of the lower troposphere are examined before, during and after the ozone depletion episode. With these analyses, we evaluate the research hypothesis that extended and regional ozone depletion events require air masses to travel over extensive regions rich in halogens that chemically remove ozone from the lower atmosphere. We establish the links between the atmospheric dynamic and thermodynamic attributes of ozone depletion episodes, chemically active air mass source regions, and locally measured chemistry. Evidence is also provided to show that the principal mechanism to replenish the atmospheric boundary layer with ozone is vertical transport of ozone from aloft down to the lower atmosphere. This transport mechanism is supported by strong wind shear regimes observed aloft.

Incorporating Potential Severity into Vulnerability Assessment of Water Supply Systems under Climate Change Conditions

AbstractIn response to climate change, vulnerability assessment of water resources systems is typically performed based on quantifying the severity of the failure. This paper introduces an approach to assess vulnerability that incorporates a set of new factors. The method is demonstrated with a case study of a reservoir system in Salt Lake City using an integrated modeling framework composed of a hydrologic model and a systems model driven by temperature and precipitation data for a 30-year historical (1981–2010) period. The climate of the selected future (2036–2065) simulation periods were represented by five combinations of warm or hot, wet or dry, and central tendency projections derived from the World Climate Research Programme’s (WCRP’s) Coupled Model Intercomparison Project Phase 5. The results of the analysis illustrate that basing vulnerability on severity alone may lead to an incorrect quantification of the system vulnerability. In this study, a typical vulnerability metric (severity) incorrectly...

The Role of Tropospheric Rossby Wave Breaking in the Pacific Decadal Oscillation

Abstract The leading pattern of extratropical Pacific sea surface temperature variability [the Pacific decadal oscillation (PDO)] is shown to depend on observed variability in the spatiotemporal distribution of tropospheric Rossby wave breaking (RWB), where RWB is the irreversible overturning of potential vorticity on isentropic surfaces. Composite analyses based on hundreds of RWB cases show that anticyclonic (cyclonic) RWB is associated with a warm, moist (cool, dry) column that extends down to a surface anticyclonic (cyclonic) circulation, and that the moisture and temperature advection associated with the surface circulation patterns force turbulent heat flux anomalies that project onto the spatial pattern of the PDO. The RWB patterns that are relevant to the PDO are closely tied to El Nino–Southern Oscillation, the Pacific–North American pattern, and the northern annular mode. These results explain the free troposphere-to-surface segment of the atmospheric bridge concept wherein El Nino anomalies eme...

Daytime cycle of low-level clouds and the tropical convective boundary layer in southwestern Amazonia

During the wet season in the southwestern Amazon region, daytime water transport out of the atmospheric mixed layer into the deeper atmosphere is shown to depend upon cloud amounts and types and synoptic-scale velocity fields. Interactions among clouds, convective conditions, and subcloud-layer properties were estimated for two dominant flow regimes observed during the 1999 Tropical Rainfall Measuring Mission component of the Brazilian Large-Scale Biosphere–Atmosphere (TRMM-LBA) field campaign. During daytime the cloud and subcloud layers were coupled by radiative, convective, and precipitation processes. The properties of cloud and subcloud layers varied according to the different convective influences of easterly versus westerly lower-tropospheric flows. The most pronounced flow-regime effects on composite cloud cycles occurred under persistent lower-tropospheric flows, which produced strong convective cloud growth with a near absence of low-level stratiform clouds, minimal cumulative attenuation of incoming solar irradiance (25%), rapid daytime mixed-layer growth ( 100 mh 1 ), and boundary layer drying (0.22 g kg 1 h 1 ), high convective velocities (1.5 m s 1 ), high surface buoyancy flux ( 200 Wm 2 ), and high latent heat flux (600 W m 2 ) into cloud layer. In contrast, persistent westerly flows were less convective, showing a strong morning presence of low-level stratiform genera (0.9 cloud amount), greater cumulative attenuation of incoming solar irradiance (47%), slower mixed-layer growth ( 50 mh 1 ) with a slight tendency for mixed-layer moistening, and a delayed peak in the low-level cumuliform cloud cycle (2000 versus 1700 UTC). The results reported in this article indicate that numerical models need to account for cloud amounts and types when estimating water vapor transport to the cloud layer.

iSAW: Integrating Structure, Actors, and Water to Study Socio-Hydro-Ecological Systems

Urbanization, climate, and ecosystem change represent major challenges for managing water resources. Although water systems are complex, a need exists for a generalized representation of these systems to identify important components and linkages to guide scientific inquiry and aid water management. We developed an integrated Structure-Actor-Water framework (iSAW) to facilitate the understanding of and transitions to sustainable water systems. Our goal was to produce an interdisciplinary framework for water resources research that could address management challenges across scales (e.g., plot to region) and domains (e.g., water supply and quality, transitioning, and urban landscapes). The framework was designed to be generalizable across all human–environment systems, yet with sufficient detail and flexibility to be customized to specific cases. iSAW includes three major components: structure (natural, built, and social), actors (individual and organizational), and water (quality and quantity). Key linkages among these components include: (1) ecological/hydrologic processes, (2) ecosystem/geomorphic feedbacks, (3) planning, design, and policy, (4) perceptions, information, and experience, (5) resource access and risk, and (6) operational water use and management. We illustrate the flexibility and utility of the iSAW framework by applying it to two research and management problems: understanding urban water supply and demand in a changing climate and expanding use of green storm water infrastructure in a semi-arid environment. The applications demonstrate that a generalized conceptual model can identify important components and linkages in complex and diverse water systems and facilitate communication about those systems among researchers from diverse disciplines.

The surface of maximum wind as an alternative to the isobaric surface for wind climatology

] The surface of maximum wind (SMW) is introduced asan analysis frame for climate studies of fast uppertropospheric winds. As defined here, the vertical domainfor the SMW includes the troposphere above 500 hPa andupper tropospheric jet streams that may protrude into thelower stratosphere. We use NCEP-NCAR Reanalysis datafor summers 1958–2004 in the vicinity of the tropicaleasterly jet (TEJ) to show how the spatial and temporalvariability of the SMW relate to jet stream cores and thetropopause. We then compare the SMW climatology of theTEJ to an isobaric climatology of the TEJ, demonstratingthat the SMW climatology reveals descent and slowing ofthe TEJ over the period of record, whereas the isobaricclimatology provides only an overestimate of the TEJslowing trend.