55 results found
Shatwell P, Czaja A, Ferreira D, 2020, Ocean heat storage rate unaffected by MOC weakening in an idealized climate model, Geophysical Research Letters, Vol: 47, Pages: 1-9, ISSN: 0094-8276
To study the role of the Atlantic meridional overturning circulation (AMOC) in transient climate change, we perform an abrupt CO2‐doubling experiment using a coupled atmosphere‐ocean‐ice model with a simple geometry that separates the ocean into small and large basins. The small basin exhibits an overturning circulation akin to the AMOC. Over the simulated 200 years of change, it stores heat at a faster rate than the large basin by 0.6 ± 0.2 W m−2. We argue that this is due to the small basin MOC. However, we find that as the MOC weakens significantly, it has little impact on the small basin's heat storage rate. We suggest this is due to the effects of both compensating warming patterns and interbasin heat transports. Thus, although the presence of a MOC is important for enhanced heat storage, MOC weakening is surprisingly unimportant.
Czaja A, Frankignoul C, Minobe S, et al., 2019, Simulating the midlatitude atmospheric circulation: What might we gain from high-resolution modeling of air-sea interactions?, Current Climate Change Reports, Vol: 5, Pages: 390-406, ISSN: 2198-6061
Purpose of ReviewTo provide a snapshot of the current research on the oceanic forcing of the atmospheric circulation in midlatitudes and a concise update on previous review papers.Recent FindingsAtmospheric models used for seasonal and longer timescales predictions are starting to resolve motions so far only studied in conjunction with weather forecasts. These phenomena have horizontal scales of ~ 10–100 km which coincide with energetic scales in the ocean circulation. Evidence has been presented that, as a result of this matching of scale, oceanic forcing of the atmosphere was enhanced in models with 10–100 km grid size, especially at upper tropospheric levels. The robustness of these results and their underlying mechanisms are however unclear.SummaryDespite indications that higher resolution atmospheric models respond more strongly to sea surface temperature anomalies, their responses are still generally weaker than those estimated empirically from observations. Coarse atmospheric models (grid size greater than 100 km) will miss important signals arising from future changes in ocean circulation unless new parameterizations are developed.
Cobb A, Czaja A, 2019, Mesoscale Signature of the North Atlantic Oscillation and Its Interaction With the Ocean, GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 5575-5581, ISSN: 0094-8276
Wolf G, Brayshaw DJ, Klingaman NP, et al., 2018, Quasi-stationary waves and their impact on European weather and extreme events, Quarterly Journal of the Royal Meteorological Society, Vol: 144, Pages: 2431-2448, ISSN: 0035-9009
Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society. Large-scale, quasi-stationary atmospheric waves (QSWs) have long been known to be associated with weather extremes such as the European heatwave in 2003. There is much debate in the scientific literature as to whether QSW activity may increase under a changing climate, providing a strong motivation for developing a better understanding of the behaviour and drivers of QSWs. This paper presents the first steps in this regard: the development of a robust objective method for a simple identification and characterization of these waves. A clear connection between QSWs and European weather and extreme events is confirmed for all seasons, indicating that blocking anti-cyclones are often part of a broader-scale wave pattern. Investigation of the QSW climatology in the Northern Hemisphere reveals that wave activity is typically strongest in midlatitudes, particularly at the exit of the Atlantic and Pacific storm track, with weaker intensities in summer. In general, the structure of individual QSW events tends to follow the climatological pattern, except in winter where the strongest and most persistent QSWs are typically shifted polewards, indicating a distinct evolution of the “strongest” QSW events. Modes of interannual variability are calculated to better understand their importance and connection to European temperatures and to identify relevant QSW patterns. This analysis highlights that European winter temperatures are strongly associated with the meridional location of QSW activity whereas high European summer temperatures are associated with increases in the overall intensity of midlatitude QSW activity. QSWs are shown to be strongly connected to commonly used indices to describe the large-scale atmospheric circulation (NAO, AO, Niño 3.4, PNA) but offer a more direct link to understanding their impact on
Liang M, Czaja A, Graversen R, et al., 2018, Poleward energy transport: is the standard definition physically relevant at all time scales?, Climate Dynamics, Vol: 50, Pages: 1785-1797, ISSN: 0930-7575
Poleward energy transport in the atmosphere and oceans constitutes an important branch of the global energy budget, and its role in the climate system has been the subject of many studies. In the atmosphere, the transport is affected by “eddies” and large scale meridional cells, both with zero net mass transport across latitude circles, but also partly by processes associated with a net transport of mass across latitude circles. The latter must cease to operate in steady state, but they may be significant when time variability of the heat budget is considered. Indeed, examination of reanalysis data on short (daily to monthly) timescales shows that mass variations on these timescales result in surprisingly large fluctuations (in excess of 1015 W = 1 PW) in the poleward heat transport. These fluctuations are referred to as “extensive”, for they primarily alter the mass integrated energy of the region considered, but not its averaged value. It is suggested that extensive fluctuations mask more meaningful climate signals present in the heat transport variability on monthly and interannual timescales, and a new formulation is proposed to isolate the latter. This new formulation is applied successfully to reanalysis data and climate model simulations.
Hewitt HT, Bell MJ, Chassignet EP, et al., 2017, Will high-resolution global ocean models benefit coupled predictions on short-range to climate timescales?, OCEAN MODELLING, Vol: 120, Pages: 120-136, ISSN: 1463-5003
Messori G, Geen R, Czaja A, 2017, On the Spatial and Temporal Variability of Atmospheric Heat Transport in a Hierarchy of Models, JOURNAL OF THE ATMOSPHERIC SCIENCES, Vol: 74, Pages: 2163-2189, ISSN: 0022-4928
Parfitt R, Czaja A, Seo H, 2017, A simple diagnostic for the detection of atmospheric fronts, Geophysical Research Letters, Vol: 44, Pages: 4351-4358, ISSN: 1944-8007
In this article, a simple diagnostic to identify atmospheric fronts objectively from gridded data sets is presented. For this diagnostic, fronts are identified as regions where the normalized product of the isobaric relative vorticity and horizontal temperature gradient exceeds a threshold value. The purpose is to introduce a method that is both robust and particularly straightforward in calculation. A climatology of atmospheric fronts, as well as the identification of an individual frontal system, is computed using this diagnostic. These are subsequently compared to a more traditional frontal detection method and the similarities and differences discussed.
Vanniere B, Czaja A, Dacre HF, 2017, Contribution of the cold sector of extratropical cyclones to mean state features over the Gulf Stream in winter, QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol: 143, Pages: 1990-2000, ISSN: 0035-9009
Parfitt R, Czaja A, Kwon Y-O, 2017, The impact of SST resolution change in the ERA-Interim reanalysis on wintertime Gulf Stream frontal air-sea interaction, GEOPHYSICAL RESEARCH LETTERS, Vol: 44, Pages: 3246-3254, ISSN: 0094-8276
This paper examines the sensitivity to a change in sea surface temperature (SST) resolution of the interaction between atmospheric and oceanic fronts in the Gulf Stream region in the ERA-Interim reanalysis data set. Two periods are considered, January 1979 to December 2001 (SST resolution 1° × 1°) and December 2010 to February 2016 (SST resolution 0.05° × 0.05°). The winter season from the latter 6 years of high-resolution SST is compared against six random periods of six wintertime seasons from the low-resolution SST period, to assess the robustness of the result against natural climate variability. In all comparisons, a significant change in frontal air-sea sensible heat flux exchange is found that is highly correlated to the change in mean SST gradient. This leads to both increases and decreases in occurrence of atmospheric fronts and mean precipitation of up to 30%. These results reemphasize the importance of high SST resolution in resolving the influence of oceanic fronts on weather and climate.
Sheldon L, Czaja A, Vanniere B, et al., 2017, A warm path for Gulf Stream - troposphere interactions, Tellus Series A-Dynamic Meteorology and Oceanography, Vol: 69, ISSN: 1600-0870
Warm advection by the Gulf Stream creates a characteristic ‘tongue’ of warm water leaving a strong imprint on the sea surface temperature (SST) distribution in the western North Atlantic. This study aims at quantifying the climatological impact of this feature on cyclones travelling across this region in winter using a combination of reanalysis data and numerical experiments. It is suggested that the Gulf Stream ‘warm tongue’ is conducive to enhanced upward motion in cyclones because (i) it helps maintain a high equivalent potential temperature of air parcels at low levels which favors deep ascent in the warm conveyor belt of cyclones and (ii) because the large SST gradients to the north of the warm tongue drive a thermally direct circulation reinforcing and, possibly, destabilizing, the transverse circulation embedded in cyclones. This hypothesis is confirmed by comparing simulations at 12 km resolution from the Met Office Unified Model forced with realistic SST distribution to simulations with an SST distribution from which the Gulf Stream warm tongue was artificially removed or made colder by. It is also supported by a dynamical diagnostic applied to the ERA interim data-set over the wintertime period (1979–2012). The mechanism of oceanic forcing highlighted in this study is associated with near thermal equilibration of low level air masses with SST in the warm sector of cyclones passing over the Gulf Stream warm tongue, which is in sharp contrast to what occurs in their cold sector. It is suggested that this ‘warm path’ for the climatic impact of the Gulf Stream on the North Atlantic storm-track is not currently represented in climate models because of their coarse horizontal resolution.
Czaja A, Vanniere B, Dacre H, 2017, A "cold path" for Gulf Stream - troposphere connection, Journal of Climate, Vol: 30, Pages: 1363-1379, ISSN: 1520-0442
The mechanism by which the Gulf Stream sea surface temperature (SST)front anchors a band of precipitation on its warm edge is still a matter of debateand little is known about how synoptic activity contributes to the meanstate. In the present study, the influence of the SST front on precipitationis investigated during the course of a single extratropical cyclone using a regionalconfiguration of the Met Office Unified Model. The comparison of acontrol run with a simulation in which SST gradients were smoothed broughtthe following conclusions: a band of precipitation is reproduced for a singleextratropical cyclone and the response to the SST gradient is dominated bya change of convective precipitation in the cold sector of the storm. Severalclimatological features described by previous studies, such as surface windconvergence on the warm edge or a meridional circulation cell across the SSTfront, are also reproduced at synoptic time scales in the cold sector. Based onthese results, a simple boundary layer model is proposed to explain the convectiveand dynamical response to the SST gradient in the cold sector. In thismodel, cold and dry air parcels acquire more buoyancy over a sharp SST gradientand become more convectively unstable. The convection sets a pressureanomaly over the entire depth of the boundary layer which drives wind convergence.This case study offers a new pathway by which the SST gradientcan anchor a climatological band of precipitation.
Geen R, Czaja A, Haigh JD, 2016, The effects of increasing humidity on heat transport by extratropical waves, Geophysical Research Letters, Vol: 43, Pages: 8314-8321, ISSN: 1944-8007
This study emphasizes the separate contributions of the warm and cold sectors of extratropical cyclones to poleward heat transport. Aquaplanet simulations are performed with an intermediate complexity climate model in which the response of the atmosphere to a range of values of saturation vapor pressure is assessed. These simulations reveal stronger poleward transport of latent heat in the warm sector as saturation vapor pressure is increased and an unexpected increase in poleward sensible heat transport in the cold sector. The latter results nearly equally from changes in the background stability of the atmosphere at low levels and changes in the temporal correlation between wind and temperature fields throughout the troposphere. Increased stability at low level reduces the likelihood that movement of cooler air over warmer water results in an absolutely unstable temperature profile, leading to less asymmetric damping of temperature and meridional velocity anomalies in cold and warm sectors.
Hausmann U, Czaja A, Marshall J, 2016, Mechanisms controlling the SST air-sea heat flux feedback and its dependence on spatial scale, CLIMATE DYNAMICS, Vol: 48, Pages: 1297-1307, ISSN: 0930-7575
Parfitt R, czaja A, Minobe S, et al., 2016, The atmospheric frontal response to SST perturbations in the Gulf Stream region, Geophysical Research Letters, Vol: 43, Pages: 2299-2306, ISSN: 1944-8007
The link between sea surface temperature (SST) gradients and atmospheric fronts is explored in a general circulation model across the Gulf Stream (GS) region from December to February 1981–2000. Two model experiments are analyzed, one with a realistic control SST distribution and one with a spatially smoothed SST distribution. The analysis shows a noticeable change in regional atmospheric frontal frequency between the two experiments (up to 30%), with the distribution of change exhibiting a clear imprint of the GS SST front. Further analysis of the surface sensible heat flux gradient across cold fronts reveals the pattern of change to be mediated by a thermal interaction between the oceanic and atmospheric fronts (“thermal damping and strengthening”). These results not only emphasize the significance of the GS SST gradient for storm development in the North Atlantic but also highlight the importance of resolution in assessing the role of frontal air-sea interaction in midlatitude climate variability.
Vanniere B, Czaja A, Dacre H, et al., 2016, A Potential Vorticity Signature for the Cold Sector of Winter Extratropical Cyclones, Quarterly Journal of the Royal Meteorological Society, Vol: 142, Pages: 432-442, ISSN: 1477-870X
The cold sector of mid-latitude storms is characterised by distinctive features such as strong surface heat fluxes, shallow convection, convective precipitation and synoptic subsidence. In order to evaluate the contribution of processes occurring in the cold sector to the mean climate, an appropriate indicator is needed. This study describes the systematic presence of negative PV behind the cold front of extratropical storms in winter. The origin of this negative PV is analysed using ERA-Interim data, potential vorticity tendencies averaged over the depth of the boundary layer are evaluated. It is found that negative PV is generated by diabatic processes in the cold sector and by Ekman pumping at the low centre, whereas positive PV is generated by Ekman advection of potential temperature in the warm sector. We suggest here that the negative PV at low-levels can be used to identify the cold sector. A PV-based indicator is applied to estimate the respective contributions of the cold sector and the remainder of the storm to upward motion, and large scale and convective precipitation. We compare the PV-based indicator with other distinctive features that could be used as markers of the cold sector, and find that potential vorticity is the best criterion when taken alone, and the best when combined with any other.
Hausmann U, Czaja A, Marshall J, 2015, Estimates of air–sea feedbacks on sea surface temperature anomalies in the southern ocean, Journal of Climate, Vol: 29, Pages: 439-454, ISSN: 1520-0442
Sea surface temperature (SST) air–sea feedback strengths and associated decay time scales in the Southern Ocean (SO) are estimated from observations and reanalysis datasets of SST, air–sea heat fluxes, and ocean mixed layer depths. The spatial, seasonal, and scale dependence of the air–sea heat flux feedbacks is mapped in circumpolar bands and implications for SST persistence times are explored. It is found that the damping effect of turbulent heat fluxes dominates over that due to radiative heat fluxes. The turbulent heat flux feedback acts to damp SSTs in all bands and spatial scales and in all seasons, at rates varying between 5 and 25 W m−2 K−1, while the radiative heat flux feedback has a more uniform spatial distribution with a magnitude rarely exceeding 5 W m−2 K−1. In particular, the implied net air–sea feedback (turbulent + radiative) on SST south of the polar front, and in the region of seasonal sea ice, is as weak as 5–10 W m−2 K−1 in the summertime on large spatial scales. Air–sea interaction alone thus allows SST signals induced around Antarctica in the summertime to persist for several seasons. The damping effect of mixed layer entrainment on SST anomalies averages to approximately 20 W m−2 K−1 across the ACC bands in the summer-to-winter entraining season and thereby reduces summertime SST persistence to less than half of that predicted by air–sea interaction alone (i.e., 3–6 months).
Parfitt R, Czaja A, 2015, On the contribution of synoptic transients to the mean atmospheric state in the Gulf Stream region, Quarterly Journal of the Royal Meteorological Society, Vol: 142, Pages: 1554-1561, ISSN: 1477-870X
A new decomposition of the time mean sea level pressure, precipitation, meridional velocity (v) and pressure vertical velocity (ω) is applied to ERA-Interim reanalysis data over the North Atlantic ocean for the December-February 1979–2011 time period. The decomposition suggests that the atmosphere over the Gulf Stream is dominated by a continuous series of synoptic systems, or baroclinic waves, propagating across the region. The time mean value of precipitation, meridional velocity and ω (the latter being taken as a proxy for upward and downward motion) is accordingly set by the propagating waves. The result is particularly striking for ω (v) considering that ascent and descent (poleward and equatorward flow) could reasonably be expected to cancel out in such a series of waves.These results shed a new light on analyses of the storm track heat budget in which the residual between diabatic heating and “transient” eddy heat fluxes (singled out through band pass time filtering or spatial Fourier analysis) is interpreted as a Rossby wave source. This interpretation is questioned because, as a consequence of the filtering used, these studies prevent any direct contribution of the “transients” to the time mean ω or meridional velocity, attributing entirely both fields to the circulation associated with the thermally forced Rossby wave. The fact that “transients” directly contribute to the observed time mean ω over the Gulf Stream might also explain the discrepancy between the observed and predicted response of the vertical motion field to heating in midlatitudes.
Wang S, Toumi R, Czaja A, et al., 2015, An analytic model of tropical cyclone wind profiles, Quarterly Journal of the Royal Meteorological Society, Vol: 141, Pages: 3018-3029, ISSN: 1477-870X
A physically based analytic model (λ model) is presented to describe the wind profile of tropical cyclones in terms of the pressure deficit and a single shape parameter (λ). To test the λ model, an idealized full-physics numerical model is employed to provide wind-profile samples and also to show the influence of environmental temperature and the properties of initial vortices on tropical cyclone size. It is found that the λ model provides an accurate fit of the azimuthal wind profile at the top of the boundary layer. In the simulations, tropical cyclone size is sensitive to sea-surface temperature, upper tropospheric temperature and initial vortex structure. The numerical model confirms the assumed Gaussian distribution with width λ of the moist entropy in the boundary layer. A linear relationship between model cyclone size and inline image is found, in agreement with the λ model. The λ model predicts a weak relationship between tropical cyclone size and intensity, as is observed. In addition, the λ model suggests that change in tropical cyclone size should be closely related to angular momentum transport near the boundary layer, as has been found in observations. The good agreement of the λ model with the numerical model shows that the λ model could be a reasonable alternative for characterizing the wind structure of tropical cyclones with only one scaling parameter.
Messori G, Czaja A, 2015, On local and zonal pulses of atmospheric heat transport in reanalysis data, QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol: 141, Pages: 2376-2389, ISSN: 0035-9009
Czaja A, Marshall J, 2015, Why is there net surface heating over the Antarctic Circumpolar Current?, OCEAN DYNAMICS, Vol: 65, Pages: 751-760, ISSN: 1616-7341
O'Reilly CH, Czaja A, 2015, The response of the Pacific storm track and atmospheric circulation to Kuroshio Extension variability, Quarterly Journal of the Royal Meteorological Society, Vol: 141, Pages: 52-66, ISSN: 0035-9009
An index of the Kuroshio Extension front strength is produced using a maximum covariance analysis between sea‐surface temperature (SST) and sea‐surface height (SSH) gradient observations, and composites of the atmospheric state are presented during its positive and negative phases using reanalysis data (1992–2011).It is found that when the Kuroshio Extension is less (more) meandering, with a stronger (weaker) SST front, the atmospheric heat transport by transient eddies is increased in the western (eastern) Pacific region, consistent with an increase (decrease) in low‐level baroclinicity. Analysis of the eddy–mean flow interaction shows that this zonal shift in heat transport forces anomalous barotropic flow in the Eastern Pacific, where blocking frequency is strongly influenced.The above relationships cannot be reconciled with the known response of the North Pacific storm track to remote forcing from the Tropical Pacific, nor can they be explained by the response of the ocean to atmospheric forcing via surface heat fluxes or winds. Rather, the zonal shift in the storm track highlighted here, and the associated changes in the large‐scale circulation, are interpreted as a response to the interannual variability of the Kuroshio Extension front.
Messori G, Czaja A, 2014, Some considerations on the spectral features of meridional heat transport by transient eddies, QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol: 140, Pages: 1377-1386, ISSN: 0035-9009
Sheldon L, Czaja A, 2014, Seasonal and interannual variability of an index of deep atmospheric convection over western boundary currents, QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol: 140, Pages: 22-30, ISSN: 0035-9009
Heaviside C, Czaja A, 2013, Deconstructing the Hadley cell heat transport, QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol: 139, Pages: 2181-2189, ISSN: 0035-9009
Messori G, Czaja A, 2013, On the sporadic nature of meridional heat transport by transient eddies, QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol: 139, Pages: 999-1008, ISSN: 0035-9009
Hausmann U, Czaja A, 2012, The observed signature of mesoscale eddies in sea surface temperature and the associated heat transport, DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS, Vol: 70, Pages: 60-72, ISSN: 0967-0637
O'Reilly CH, Czaja A, LaCasce JH, 2012, The emergence of zonal ocean jets under large-scale stochastic wind forcing, GEOPHYSICAL RESEARCH LETTERS, Vol: 39, ISSN: 0094-8276
Czaja A, Blunt N, 2011, A new mechanism for ocean-atmosphere coupling in midlatitudes, QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Vol: 137, Pages: 1095-1101, ISSN: 0035-9009
Pauluis O, Czaja A, Korty R, 2010, The Global Atmospheric Circulation in Moist Isentropic Coordinates, JOURNAL OF CLIMATE, Vol: 23, Pages: 3077-3093, ISSN: 0894-8755
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