Impact of Rossby and Kelvin Wave Components on MJO Eastward Propagation

2018 ◽  
Vol 31 (17) ◽  
pp. 6913-6931 ◽  
Author(s):  
Lu Wang ◽  
Tim Li ◽  
Tomoe Nasuno
Keyword(s):  
Rossby Wave ◽  
General Circulation ◽  
Task Force ◽  
Phase Speed ◽  
General Circulation Models ◽  
Wave Component ◽  
Kelvin Wave ◽  
Free Atmosphere ◽  
The Impact ◽  
East West

There are contrasting views concerning the impact of Rossby wave component of MJO flow on its eastward propagation. One view (called “drag effect”) argues that because Rossby waves propagate westward, a stronger Rossby wave component slows down the eastward propagation. The other view (called “acceleration effect”) argues that a stronger Rossby wave enhances east–west asymmetry of moist static energy (MSE) tendency and thus favors the eastward propagation. This study aims to resolve this issue through diagnosis of both idealized aquaplanet simulations and 26 models from the MJO Task Force/GEWEX Atmospheric System Studies (MJOTF/GASS). In the aquaplanet experiments, three sets of zonally uniform, equatorially symmetric SST distributions are specified. The MJO phase speed is faster in the presence of a narrower SST meridional profile, in which both the Rossby and the Kelvin wave components are stronger and the east–west asymmetry of MSE tendency is larger. A further analysis of the 26 general circulation models reveals that the MJO propagation skill and phase speed are positively correlated to both the Rossby wave and the Kelvin wave strength in the lower free atmosphere (above 800 hPa). Models that have a stronger Rossby and Kelvin wave component tend to simulate realistic and faster eastward propagation. Therefore, both the aquaplanet and the multimodel simulations support the Rossby wave acceleration effect hypothesis.

Basin Resonances in the Equatorial Indian Ocean

2011 ◽  
Vol 41 (6) ◽  
pp. 1252-1270 ◽  
Author(s):  
Weiqing Han ◽  
Julian P. McCreary ◽  
Yukio Masumoto ◽  
Jérôme Vialard ◽  
Benét Duncan
Keyword(s):  
Indian Ocean ◽  
Sea Level ◽  
Rossby Wave ◽  
Time Scale ◽  
General Circulation ◽  
Equatorial Indian Ocean ◽  
Western Boundary ◽  
Kelvin Wave ◽  
Resonance Amplitude ◽  
The Impact

Abstract Previous studies have investigated how second-baroclinic-mode (n = 2) Kelvin and Rossby waves in the equatorial Indian Ocean (IO) interact to form basin resonances at the semiannual (180 day) and 90-day periods. This paper examines unresolved issues about these resonances, including the reason the 90-day resonance is concentrated in the eastern ocean, the time scale for their establishment, and the impact of complex basin geometry. A hierarchy of ocean models is used: an idealized one-dimensional (1D) model, a linear continuously stratified ocean model (LCSM), and an ocean general circulation model (OGCM) forced by Quick Scatterometer (QuikSCAT) wind during 2000–08. Results indicate that the eastern-basin concentration of the 90-day resonance happens because the westward-propagating Rossby wave is slower, and thus is damped more than the eastward-propagating Kelvin wave. Results also indicate that superposition with other baroclinic modes further enhances the eastern maximum and weakens sea level variability near the western boundary. Without resonance, although there is still significant power at 90 and 180 days, solutions have no spectral peaks at these periods. The key time scale for the establishment of all resonances is the time it takes a Kelvin wave to cross the basin and a first-meridional-mode (ℓ = 1) Rossby wave to return; thus, even though the amplitude of the 90-day winds vary significantly, the 90-day resonance can be frequently excited in the real IO, as evidenced by satellite-observed and OGCM-simulated sea level. The presence of the Indian subcontinent enhances the influence of equatorial variability in the north IO, especially along the west coast of India. The Maldives Islands weaken the 180-day resonance amplitude but have little effect on the 90-day resonance, because they fall in its “node” region. Additionally, resonance at the 120-day period for the n = 1 mode is noted.

A spectral perspective on the existence of hemispheric circulation regimes

2021 ◽  
Author(s):  
Jacopo Riboldi ◽  
Efi Rousi ◽  
Fabio D'Andrea ◽  
Gwendal Rivière ◽  
François Lott
Keyword(s):  
Rossby Wave ◽  
General Circulation ◽  
Large Scale ◽  
Phase Speed ◽  
General Circulation Models ◽  
Wave Pattern ◽  
Spectral Space ◽  
Compact Representation ◽  
Data Set ◽  
Spectral Patterns

<p>While the existence of regional weather regimes (e.g., over the North Atlantic) is a known result, the presence of preferred circulation patterns at the hemispheric scale is still disputed. Space-time spectral analysis can offer a different perspective to tackle this problem, as it provides a compact representation of the large-scale flow evolution. It can objectively extract the most relevant harmonics, in terms of spatial wavenumbers and temporal frequencies, that dominate the hemispheric Rossby wave pattern at a given time and is easily applicable to gridded data sets as Reanalysis or the output of general circulation models.</p><p>With the aim to highlight the existence of clusters in the spectral space, we build a data set of spectra of upper-level meridional wind over midlatitudes (35°N-75°N) in the wavenumber/phase-speed domain for the 1979-2019 Reanalysis period. A spectrum is assigned to each day being located in the center of a sliding 61-days time window. This data set contains interesting information about the stationarity and the persistence of the hemispheric Rossby wave pattern. The most persistent harmonics are the ones related to quasi-stationary or westward propagating waves, as confirmed by an analysis of the dominant harmonics during atmospheric blocking events.</p><p>Cluster analysis is performed using self-organizing maps (SOMs) on this data set. To assess its significance, the same procedure is applied to an artificially generated red noise with the same mean, variance and lag-1 covariance as the real data. This cross-check does not highlight a preferred number of circulation regimes in the spectral space. However, a subjective classification of the spectral patterns highlighted by the SOM analysis in four different groups can be attempted: 1) a ground state, with no particular deviation from climatology; 2) a state characterized by rapidly propagating, high wavenumber waves; 3) a state characterized by slowly propagating, low wavenumber waves; and 4) a state with a clear, dominant wavenumber. Spectral patterns corresponding to each of these groups are present regardless of the chosen number of SOMs.</p>

scholarly journals Kelvin/Rossby Wave Partition of Madden-Julian Oscillation Circulations

Climate ◽  
2020 ◽  
Vol 9 (1) ◽  
pp. 2
Author(s):  
Patrick Haertel
Keyword(s):  
Rossby Wave ◽  
Large Scale ◽  
Atmospheric Model ◽  
Circulation System ◽  
System Of Equations ◽  
Wave Component ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Realistic View ◽  
State Of Rest

The Madden Julian Oscillation (MJO) is a large-scale convective and circulation system that propagates slowly eastward over the equatorial Indian and Western Pacific Oceans. Multiple, conflicting theories describe its growth and propagation, most involving equatorial Kelvin and/or Rossby waves. This study partitions MJO circulations into Kelvin and Rossby wave components for three sets of data: (1) a modeled linear response to an MJO-like heating; (2) a composite MJO based on atmospheric sounding data; and (3) a composite MJO based on data from a Lagrangian atmospheric model. The first dataset has a simple dynamical interpretation, the second provides a realistic view of MJO circulations, and the third occurs in a laboratory supporting controlled experiments. In all three of the datasets, the propagation of Kelvin waves is similar, suggesting that the dynamics of Kelvin wave circulations in the MJO can be captured by a system of equations linearized about a basic state of rest. In contrast, the Rossby wave component of the observed MJO’s circulation differs substantially from that in our linear model, with Rossby gyres moving eastward along with the heating and migrating poleward relative to their linear counterparts. These results support the use of a system of equations linearized about a basic state of rest for the Kelvin wave component of MJO circulation, but they question its use for the Rossby wave component.

Sensitivity of simulated oxygen isotopes in ice cores and speleothems to Last Glacial Maximum surface conditions

2021 ◽  
Author(s):  
André Paul ◽  
Alexandre Cauquoin ◽  
Stefan Mulitza ◽  
Thejna Tharammal ◽  
Martin Werner
Keyword(s):  
Last Glacial Maximum ◽  
General Circulation ◽  
Ice Cores ◽  
General Circulation Models ◽  
Glacial Maximum ◽  
Sea Surface ◽  
Surface Conditions ◽  
Last Glacial ◽  
The Impact ◽  
Mean Square Errors

<p>In simulations of the climate during the Last Glacial Maximum (LGM), we employ two different isotope-enabled atmospheric general circulation models (NCAR iCAM3 and MPI ECHAM6-wiso) and use simulated (by coupled climate models) as well as reconstructed (from a new global climatology of the ocean surface duing the LGM, GLOMAP) surface conditions.</p><p>The resulting atmospheric fields reflect the more pronounced structure and gradients in the reconstructions, for example, the precipitation is more depleted in oxygen-18 in the high latitudes and more enriched in low latitudes, especially in the tropical convective regions over the maritime continent in the equatorial Pacific and Indian Oceans and over the equatorial Atlantic Ocean. Furthermore, at the sites of ice cores and speleothems, the model-data fit improves in terms of the coefficients of determination and root-mean square errors.</p><p>In additional sensitivity experiments, we also use the climatologies by Annan and Hargreaves (2013) and Tierney et al. (2020) and consider the impact of changes in reconstructed sea-ice extent and the global-mean sea-surface temperature.</p><p>Our findings imply that the correct simulation or reconstruction of patterns and gradients in sea-surface conditions are crucial for a successful comparison to oxygen-isotope data from ice cores and speleothems.</p>

scholarly journals A Simplistic Approach for Assessing Hydroclimatic Vulnerability of Lakes and Reservoirs with Regulated Superficial Outflow

Hydrology ◽  
2019 ◽  
Vol 6 (3) ◽  
pp. 61 ◽  
Author(s):  
Kleoniki Demertzi ◽  
Dimitris Papadimos ◽  
Vassilis Aschonitis ◽  
Dimitris Papamichail
Keyword(s):  
Climate Change ◽  
General Circulation ◽  
Drainage Basin ◽  
Management Strategies ◽  
General Circulation Models ◽  
Climatic Conditions ◽  
Modified Model ◽  
Natural Lakes ◽  
The Future ◽  
The Impact

This study proposes a simplistic model for assessing the hydroclimatic vulnerability of lakes/reservoirs (LRs) that preserve their steady-state conditions based on regulated superficial discharge (Qd) out of the LR drainage basin. The model is a modification of the Bracht-Flyr et al. method that was initially proposed for natural lakes in closed basins with no superficial discharge outside the basin (Qd = 0) and under water-limited environmental conditions {mean annual ratio of potential/reference evapotranspiration (ETo) versus rainfall (P) greater than 1}. In the proposed modified approach, an additional Qd function is included. The modified model is applied using as a case study the Oreastiada Lake, which is located inside the Kastoria basin in Greece. Six years of observed data of P, ETo, Qd, and lake topography were used to calibrate the modified model based on the current conditions. The calibrated model was also used to assess the future lake conditions based on the future climatic projections (mean conditions of 2061-2080) derived by 19 general circulation models (GCMs) for three cases of climate change (three cases of Representative Concentration Pathways: RCP2.6, RCP4.5 and RCP8.5). The modified method can be used as a diagnostic tool in water-limited environments for analyzing the superficial discharge changes of LRs under different climatic conditions and to support the design of new management strategies for mitigating the impact of climate change on (a) flooding conditions, (b) hydroelectric production, (c) irrigation/industrial/domestic use and (d) minimum ecological flows to downstream rivers.

scholarly journals Weights for general circulation models from CMIP3/CMIP5 in a statistical downscaling framework and the impact on future Mediterranean precipitation

2019 ◽  
Vol 39 (8) ◽  
pp. 3639-3654 ◽  
Author(s):  
Irena Kaspar‐Ott ◽  
Elke Hertig ◽  
Severin Kaspar ◽  
Felix Pollinger ◽  
Christoph Ring ◽  
...  
Keyword(s):  
Statistical Downscaling ◽  
General Circulation ◽  
General Circulation Models ◽  
The Impact

Hydrologic response to climate change: a case study for the Be River Catchment, Vietnam

2012 ◽  
Vol 3 (3) ◽  
pp. 207-224 ◽  
Author(s):  
Dao Nguyen Khoi ◽  
Tadashi Suetsugi
Keyword(s):  
Climate Change ◽  
General Circulation ◽  
Assessment Tool ◽  
Swat Model ◽  
General Circulation Models ◽  
Climate Change Scenarios ◽  
River Catchment ◽  
Calibration And Validation ◽  
Daily Streamflow ◽  
The Impact

The Be River Catchment was studied to quantify the potential impact of climate change on the streamflow using a multi-model ensemble approach. Climate change scenarios (A1B and B1) were developed from an ensemble of four GCMs (general circulation models) (CGCM3.1 (T63), CM2.0, CM2.1 and HadCM3) that showed good performance for the Be River Catchment through statistical evaluations between 15 GCM control simulations and the corresponding time series of observations at annual and monthly levels. The Soil and Water Assessment Tool (SWAT) was used to investigate the impact on streamflow under climate change scenarios. The model was calibrated and validated using daily streamflow records. The calibration and validation results indicated that the SWAT model was able to simulate the streamflow well, with Nash–Sutcliffe efficiency exceeding 0.78 for the Phuoc Long station and 0.65 for the Phuoc Hoa station, for both calibration and validation at daily and monthly steps. Their differences in simulating the streamflow under future climate scenarios were also investigated. The results indicate a 1.0–2.9 °C increase in annual temperature and a −4.0 to 0.7% change in annual precipitation corresponding to a change in streamflow of −6.0 to −0.4%. Large decreases in precipitation and runoff are observed in the dry season.

scholarly journals Weak Land–Atmosphere Coupling Strength in HadAM3: The Role of Soil Moisture Variability

2005 ◽  
Vol 6 (5) ◽  
pp. 670-680 ◽  
Author(s):  
David M. Lawrence ◽  
Julia M. Slingo
Keyword(s):  
Boundary Layer ◽  
Soil Moisture ◽  
Coupling Strength ◽  
General Circulation ◽  
General Circulation Models ◽  
Soil Conditions ◽  
Wide Range ◽  
The Impact ◽  
Soil Moisture Variability ◽  
Moisture Variability

Abstract A recent model intercomparison, the Global Land–Atmosphere Coupling Experiment (GLACE), showed that there is a wide range of land–atmosphere coupling strengths, or the degree that soil moisture affects the generation of precipitation, amongst current atmospheric general circulation models (AGCMs). Coupling strength in the Hadley Centre atmosphere model (HadAM3) is among the weakest of all AGCMs considered in GLACE. Reasons for the weak HadAM3 coupling strength are sought here. In particular, the impact of pervasive saturated soil conditions and low soil moisture variability on coupling strength is assessed. It is found that when the soil model is modified to reduce the occurrence of soil moisture saturation and to encourage soil moisture variability, the soil moisture–precipitation feedback remains weak, even though the relationship between soil moisture and evaporation is strengthened. Composites of the diurnal cycle, constructed relative to soil moisture, indicate that the model can simulate key differences in boundary layer development over wet versus dry soils. In particular, the influence of wet or dry soil on the diurnal cycles of Bowen ratio, boundary layer height, and total heat flux are largely consistent with the observed influence of soil moisture on these properties. However, despite what appears to be successful simulation of these key aspects of the indirect soil moisture–precipitation feedback, the model does not capture observed differences for wet and dry soils in the daily accumulation of boundary layer moist static energy, a crucial feature of the feedback mechanism.

Subtropical clouds stabilize near-Snowball Earth states

2020 ◽  
Author(s):  
Christoph Braun ◽  
Aiko Voigt ◽  
Johannes Hörner ◽  
Joaquim G. Pinto
Keyword(s):  
Sea Ice ◽  
General Circulation ◽  
General Circulation Models ◽  
Snowball Earth ◽  
Radiative Effects ◽  
Albedo Feedback ◽  
The Earth ◽  
Cloud Radiative Effects ◽  
Planetary Albedo ◽  
The Impact

<p>Atmospheric general circulation models developed for the Earth system include comprehensive parameterizations of clouds. Applying them to exoplanet atmospheres provides an opportunity to advance understanding of clouds, atmosphere dynamics, and their coupling in the context of planetary climate dynamics and habitability.</p><p>Here, we study a deep-time extreme climate of Earth as an example of the cold limit of the habitable zone. Geological evidence indicates near-global ice cover during the Neoproterozoic (1000 – 541 Million years ago) associated with considerable hysteresis of atmospheric CO<sub>2</sub>. The Snowball Earth hypothesis provides a straightforward interpretation of Neoproterozoic proxies based on a runaway of the sea-ice albedo feedback. However, the Snowball Earth hypothesis relies on the existence of local habitats to explain the survival of photosynthetic marine species on an entirely ice-covered planet. The Jormungand hypothesis may resolve this issue by considering a weakening of the sea-ice albedo feedback by exposure of dark bare sea ice when sea ice enters the subtropics. This potentially allows the Earth system to stabilize in a climate state - the Jormungand state - with near-global ice cover. Around the equator, a narrow strip of ocean remains ice-free, where life would have easily survived during the pan-glaciations.</p><p>The weakening of the sea-ice albedo feedback is based on the change of the meridional structure of planetary albedo with a moving sea-ice edge. While previous work focused on the contribution of surface albedo to planetary albedo, we here focus on the impact of subtropical and tropical cloudiness on planetary albedo. Enhanced cloudiness generally weakens the sea-ice albedo feedback and thus decreases the climate sensitivity of the Jormungand state, i.e. it stabilizes the Jormungand state. We analyze the impact of cloudiness on the stability of the Jormungand state in the general circulation models CAM3 and ICON-AES with idealized aquaplanet setups. While CAM3 shows significant CO<sub>2</sub>-hysteresis of the Jormungand state, ICON-AES exhibits no stable Jormungand state. Consistently, CAM3 exhibits stronger cloudiness than ICON-AES, especially in the subtropics. An analysis with a one-dimensional energy balance model shows that the Jormungand hysteresis strongly depends on the sensitivity of the planetary albedo to an advance of sea ice into the subtropics. Accordingly, we demonstrate that the absence of cloud-radiative effects within vertical columns in the subtropics drastically decreases the Jormungand hysteresis in CAM3.</p><p>Overall, the magnitude of the Jormungand hysteresis is tightly linked to the representation of cloud-radiative effects in general circulation models. Our results highlight the important role of uncertainties associated with cloud-radiative effects for climate feedbacks on planet Earth in the context of extreme climates, such as they have occurred in Earth’s deep past or might be found on Earth-like planets. In consequence, this also stresses the need and challenges of accounting for adequate cloud modeling for planetary climates.</p>

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