An Enhanced Moisture Convergence–Evaporation Feedback Mechanism for MJO Air–Sea Interaction

Abstract Simulations using an atmospheric model forced with observed SST climatology and the same atmospheric model coupled to a slab-ocean model are used to investigate the role of air–sea interaction on the dynamics of the MJO. Slab-ocean coupling improved the MJO in Australia’s Bureau of Meteorology atmospheric model over the Indo-Pacific warm pool by reducing its period from 70–100 to 45–70 days, thereby showing better agreement with the 30–80-day observed oscillation. Air–sea coupling improves the MJO by increasing the moisture flux in the lower troposphere prior to the passage of active convection, which acts to promote convection and precipitation on the eastern flank of the main convective center. This process is triggered by an increase in surface evaporation over positive SST anomalies ahead of the MJO convection, which are driven by the enhanced shortwave radiation in the region of suppressed convection. This in turn generates enhanced convergence into the region, which supports evaporation–wind feedback in the presence of weak background westerly winds. A subsequent increase in low-level moisture convergence acts to further moisten the lower troposphere in advance of large-scale convection in a region of reduced atmospheric pressure. This destabilizing mechanism is referred to as enhanced moisture convergence–evaporation feedback (EMCEF) and is utilized to understand the role of air–sea coupling on the observed MJO. The EMCEF mechanism also reconciles traditionally opposing ideas on the roles of frictional wave–conditional instability of the second kind (CISK) and wind–evaporation feedback. These results support the idea that the MJO is primarily an atmospheric phenomenon, with air–sea interaction improving upon, but not critical for, its existence in the model.

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El Niño–Related Tropical Land Surface Water and Energy Response in MERRA-2

Journal of Climate ◽

10.1175/jcli-d-19-0231.1 ◽

2020 ◽

Vol 33 (3) ◽

pp. 1155-1176 ◽

Cited By ~ 1

Author(s):

Michael G. Bosilovich ◽

Franklin R. Robertson ◽

Paul W. Stackhouse

Keyword(s):

El Niño ◽

Land Surface ◽

Large Scale ◽

El Nino ◽

Lower Troposphere ◽

Atmospheric Model ◽

Forced Response ◽

Shortwave Radiation ◽

Tropical Regions ◽

Surface Warming

AbstractAlthough El Niño events each have distinct evolutionary character, they typically provide systematic large-scale forcing for warming and increased drought frequency across the tropical continents. We assess this response in the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), reanalysis and in a 10-member-model Atmospheric Model Intercomparison Project (AMIP) ensemble. The lagged response (3–4 months) of mean tropical land temperature to El Niño warming in the Pacific Ocean is well represented. MERRA-2 reproduces the patterns of precipitation in the tropical regions, and the AMIP ensemble reproduces some regional responses that are similar to those observed and some regions that are not simulating the response well. Model skill is dependent on event forcing strength and temporal proximity to the peak of the sea surface warming. A composite approach centered on maximum Niño-3.4 SSTs and lag relationships to energy fluxes and transports is used to identify mechanisms supporting tropical land warming. The composite necessarily moderates weather-scale variability of the individual events while retaining the systematic features across all events. We find that reduced continental upward motions lead to reduced cloudiness and more shortwave radiation at the surface, as well as reduced precipitation. The increased shortwave heating at the land surface, along with reduced soil moisture, leads to warmer surface temperature, more sensible heating, and warming of the lower troposphere. The composite provides a broad picture of the mechanisms governing the hydrologic response to El Niño forcing, but the regional and temporal responses can vary substantially for any given event. The 2015/16 El Niño, one of the strongest events, demonstrates some of the forced response noted in the composite, but with shifts in the evolution that depart from the composite, demonstrating the limitations of the composite and individuality of El Niño.

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Comparisons of atmosphere–ocean simulations of greenhouse gas-induced climate change for pre-industrial and hypothetical ‘no-anthropogenic’ radiative forcing, relative to present day

The Holocene ◽

10.1177/0959683611400200 ◽

2011 ◽

Vol 21 (5) ◽

pp. 793-801 ◽

Cited By ~ 13

Author(s):

J.E. Kutzbach ◽

S.J. Vavrus ◽

W.F. Ruddiman ◽

G. Philippon-Berthier

Keyword(s):

Climate Change ◽

Sea Ice ◽

Greenhouse Gas ◽

Ocean Circulation ◽

Radiative Forcing ◽

Deep Ocean ◽

Lower Surface ◽

Atmospheric Model ◽

Ocean Model ◽

Slab Ocean

We compare climate simulations for Present-Day (PD), Pre-Industrial (PI) time, and a hypothetical (inferred) state termed No-Anthropogenic (NA) based upon the low greenhouse gas (GHG) levels of the late stages of previous interglacials that are comparable in time (orbital configuration) to the present interglacial. We use a fully coupled dynamical atmosphere–ocean model, the CCSM3. We find a consistent trend toward colder climate (lower surface temperature, more snow and sea-ice cover, lower ocean temperature, and modified ocean circulation) as the net change in GHG radiative forcing trends more negative from PD to PI to NA. The climatic response of these variables becomes larger relative to the changed GHG forcing for each step toward a colder climate state (PD to PI to NA). This amplification is significantly enhanced using the dynamical atmosphere–ocean model compared with our previous results with an atmosphere–slab ocean model, a result that conforms to earlier idealized GHG forcing experiments. However, in our case this amplification is not an idealized result, but instead helps frame important questions concerning aspects of Holocene climate change. This enhanced amplification effect leads to an increase in our estimate of the climate’s response to inferred early anthropogenic CO2 increases (NA to PI) relative to the response to industrial-era CO2 increases (PI to PD). Although observations of the climate for the hypothetical NA (inferred from observations of previous interglacials) and for PI have significant uncertainties, our new results using CCSM3 are in better agreement with these observations than our previous results from an atmospheric model coupled to a static slab ocean. The results support more strongly inferences by Ruddiman concerning indirect effects of ocean solubility/sea-ice/deep ocean ventilation feedbacks that may have contributed to a further increase in late-Holocene atmospheric CO2 beyond that caused by early anthropogenic emissions alone.

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On the imprint of the mesoscale organization of tradewind clouds at cloud base and below

10.5194/egusphere-egu21-14901 ◽

2021 ◽

Author(s):

Sandrine Bony ◽

Pierre-Etienne Brilouet ◽

Patrick Chazette ◽

Pierre Coutris ◽

Julien Delanoë ◽

...

Keyword(s):

Large Scale ◽

Lower Troposphere ◽

Cloud Base ◽

Trade Wind ◽

Tropical Atlantic ◽

Lidar Measurements ◽

Research Aircraft ◽

Cloud Patterns

Trade-wind clouds can exhibit different patterns of mesoscale organization. These patterns were observed during the EUREC4A (Elucidating the role of cloud-circulation coupling in climate) field campaign that took place in Jan-Feb 2020 over the western tropical Atlantic near Barbados: while the HALO aircraft was observing clouds from above and was characterizing the large-scale environment with dropsondes, the ATR-42 research aircraft was flying in the lower troposphere, characterizing clouds and turbulence with horizontal radar-lidar measurements and in-situ probes and sensors. By analyzing these data for different cloud patterns, we investigate the extent to which the cloud organization is imprinted in cloud-base properties and subcloud-layer heterogeneities. The implications of our findings for understanding the roots of the mesoscale organization of tradewind clouds will be discussed.

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The Amplification of the ENSO Forcing over Equatorial Amazon

Journal of Hydrometeorology ◽

10.1175/2009jhm1108.1 ◽

2009 ◽

Vol 10 (6) ◽

pp. 1561-1568 ◽

Cited By ~ 7

Author(s):

Vasubandhu Misra

Keyword(s):

South America ◽

Diurnal Cycle ◽

Land Surface ◽

Large Scale ◽

French Guiana ◽

Southern Oscillation ◽

Moisture Flux ◽

Precipitation Anomalies ◽

Upper Level

Abstract The remote influence of the El Niño–Southern Oscillation (ENSO) strongly manifests over the equatorial Amazon (EA)—including parts of southern Venezuela, Guyana, French Guiana, and Suriname—when there is a large-scale anomalous upper-level divergence over continental tropical South America. Modeling studies conducted in this paper suggest that it is because of the modulation of the local diurnal cycle of the moisture flux convergence, which results in the local amplification of the ENSO signal over the EA. Further, it is shown that the local land surface feedback plays a relatively passive but important role of maintaining these interannual precipitation anomalies over the EA region.

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Role of Interactions between Aerosol Radiative Effect, Dynamics, and Cloud Microphysics on Transitions of Monsoon Intraseasonal Oscillations

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0179.1 ◽

2013 ◽

Vol 70 (7) ◽

pp. 2073-2087 ◽

Cited By ~ 44

Author(s):

Anupam Hazra ◽

B. N. Goswami ◽

Jen-Ping Chen

Keyword(s):

Radiative Forcing ◽

Large Scale ◽

Cloud Microphysics ◽

Relative Strength ◽

Moisture Convergence ◽

Freezing Level ◽

Intraseasonal Oscillations ◽

Active Condition ◽

Absorbing Aerosols

Abstract Extended-range prediction of monsoon intraseasonal oscillations (MISOs), crucial for agriculture and water management, is limited by their event-to-event variability. Here, the authors propose a hypothesis supported by a number of model simulations involving detailed cloud microphysical processes indicating that aerosols contribute significantly to the transitions from “break” to “active” phases of MISO. The role of aerosol indirect effect in the process of invigoration of precipitation is demonstrated with a high-resolution regional model for Indian summer monsoon breaks that are followed by an active condition (BFA) and contrasted with breaks that are not followed by an active condition (BNFA). The BFA are characterized by higher concentrations of absorbing aerosols that lead to a stronger north–south low-level temperature gradient and strong moisture convergence. Forced uplift beyond the freezing level initiates the cold-rain process involving mixed-phase microphysics and latent heat release at higher levels, thereby invigorating convection, enhancing precipitation, and resulting in an active condition. While more aerosols tend to reduce the cloud drop size and delay the warm rain, it is overcome by the higher moisture convergence during BFA and invigoration by cold-rain processes. The net production of rainfall is sensitive to cloud structure as it depends on the relative strength of the warm- and cold-rain initiation processes. The results indicate the importance of aerosols on transitions of MISO and a pathway by which they influence the transitions involving complex interactions between direct radiative forcing, large-scale dynamics, and cloud microphysics. Broader implications of these results in event-to-event variability of MISO and its predictability are also highlighted.

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Role of Convective Entrainment in Spatial Distributions of and Temporal Variations in Precipitation over Tropical Oceans

Journal of Climate ◽

10.1175/jcli-d-13-00701.1 ◽

2014 ◽

Vol 27 (23) ◽

pp. 8707-8723 ◽

Cited By ~ 18

Author(s):

Nagio Hirota ◽

Yukari N. Takayabu ◽

Masahiro Watanabe ◽

Masahide Kimoto ◽

Minoru Chikira

Keyword(s):

Large Scale ◽

Lower Troposphere ◽

Temporal Variations ◽

Spatial And Temporal Variations ◽

Spatial Distributions ◽

Atmospheric Models ◽

Tropical Oceans ◽

Appropriate Treatment ◽

Precipitation Events

Abstract The authors demonstrate that an appropriate treatment of convective entrainment is essential for determining spatial distributions of and temporal variations in precipitation. Four numerical experiments are performed using atmospheric models with different entrainment characteristics: a control experiment (Ctl), a no-entrainment experiment (NoEnt), an original Arakawa–Schubert experiment (AS), and an AS experiment with a simple empirical suppression of convection depending on cloud-layer humidity (ASRH). The fractional entrainment rates of AS and ASRH are constant for each cloud type and are very small in the lower troposphere compared with those in the Ctl, in which half of the buoyancy-generated energy is consumed by entrainment. Spatial and temporal variations in the observed precipitation are satisfactorily reproduced in the Ctl, but their amplitudes are underestimated with a so-called double intertropical convergence zone bias in the NoEnt and AS. The spatial variation is larger in the Ctl because convection is more active over humid ascending regions and more suppressed over dry subsidence regions. Feedback processes involving convection, the large-scale circulation, free tropospheric moistening by congestus, and radiation enhance the variations. The temporal evolution of precipitation events is also more realistic in the Ctl, because congestus moistens the midtroposphere, and large precipitation events occur once sufficient moisture is available. The large entrainment in the lower troposphere, increasing free tropospheric moistening by congestus and enhancing the coupling of convection to free tropospheric humidity, is suggested to be important for the realistic spatial and temporal variations.

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Large-scale connections between Fram Strait recirculation and warm water pathways towards Greenland fjords

10.5194/egusphere-egu21-8695 ◽

2021 ◽

Author(s):

Rebecca McPherson ◽

Torsten Kanzow ◽

Claudia Wekerle

Keyword(s):

Large Scale ◽

Atlantic Water ◽

Ocean Model ◽

Intermediate Water ◽

Fram Strait ◽

The Past ◽

Subsurface Waters ◽

West Spitsbergen ◽

Annual Time

In the last two decades, rising ocean temperatures have significantly contributed to the increased melting and retreat of marine-terminating glaciers along the coast of Greenland. Warming subsurface waters have also been shown to interact with the glaciers in Northeast Greenland, which until recently were considered stable, and caused their rapid retreat. The main source of these waters is the westward recirculation of subducted Atlantic Water (AW) in Fram Strait, which has shown a warming of up to 1&#176; C over the past few decades.In this study, the connection between the subsurface warm Atlantic Intermediate Water (AIW) found on the wide continental shelf of Northeast Greenland and in the fjords, and AW within the West Spitsbergen Current (WSC) is investigated using historical hydrographic observations and high-resolution numerical simulations with the Finite-Element Sea-ice Ocean Model (FESOM). We find that AW from the WSC takes between 10 &#8211; 14 months to recirculate across Fram Strait and reach the shelf break where it moves southwards. The pronounced inter-annual variability in the WSC is preserved as the water recirculates. However, the variability of temperature and AIW layer thickness on the shelf at seasonal or inter-annual time scales is at best weakly controlled by the AW temperature in the WSC. There is no significant correlation between AIW and the WSC anywhere on the shelf, suggesting advection from the WSC alone does not control AIW signals. The role of wind-driven, episodic upwelling is then investigated as a driver of transport of AIW from Fram Strait onto the shelf (following an approach by M&#252;nchow et al., 2020) where it then may follow the deep trough system towards the glaciers.

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Circulation Response to Eurasian versus North American Anomalous Snow Scenarios in the Northern Hemisphere with an AGCM Coupled to a Slab Ocean Model

Journal of Climate ◽

10.1175/jcli-d-11-00465.1 ◽

2013 ◽

Vol 26 (5) ◽

pp. 1502-1515 ◽

Cited By ~ 15

Author(s):

Gina R. Henderson ◽

Daniel J. Leathers ◽

Brian Hanson

Keyword(s):

North American ◽

Land Surface ◽

Lower Troposphere ◽

Ocean Model ◽

Atmospheric Response ◽

Early Winter ◽

Sea Ice Extent ◽

Eurasian Snow ◽

Slab Ocean ◽

Snow Conditions

Abstract The difference between snow-covered and snow-free conditions is the most climatically significant natural seasonal change the land surface can experience. Most GCM studies investigating snow–atmosphere interactions have focused on impacts of Eurasian snow anomalies caused by the magnitude of snow mass, while North American snow has been shown to have a weaker relationship with downstream climate. Experiment design of recent snow–atmosphere interactions studies has been limited to atmosphere-only models, with sea surface temperature (SST) and sea ice extent represented as boundary conditions. The authors explore the circulation response to anomalous snow scenarios, for both North America and Eurasia, using a slab ocean model. Surface response include significant SST cooling directly downstream of each individual forcing region in addition to upstream centers of remote cooling under maximum snow conditions. Atmospheric response to anomalous snow conditions is consistent through multiple levels in the lower troposphere under maximum snow conditions throughout much of the midlatitudes in both experiments during early winter. Areas of strengthened midtropospheric eddy kinetic energy correlate well with steep geopotential height gradient differences and increased zonal wind at 250 hPa over the western Pacific. Both experiments show similar atmospheric response pathways; however, circulation response to maximum Eurasian snow is focused downstream in early winter, whereas upstream response is particularly evident from the North American experiment. This paper focuses on differences as a result of Eurasian versus North American snow forcing in atmospheric circulation response using an AGCM with a slab ocean model.

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Convection Parameterization, Tropical Pacific Double ITCZ, and Upper-Ocean Biases in the NCAR CCSM3. Part II: Coupled Feedback and the Role of Ocean Heat Transport

Journal of Climate ◽

10.1175/2009jcli3109.1 ◽

2010 ◽

Vol 23 (3) ◽

pp. 800-812 ◽

Cited By ~ 27

Author(s):

Guang J. Zhang ◽

Xiaoliang Song

Keyword(s):

Surface Energy ◽

Heat Transport ◽

Energy Flux ◽

Ocean Model ◽

Tropical Pacific ◽

Ocean Heat Transport ◽

Surface Energy Flux ◽

Double Itcz ◽

Slab Ocean

Abstract This study investigates the coupled atmosphere–ocean feedback and the role of ocean dynamic heat transport in the formation of double ITCZ over the tropical Pacific in the NCAR Community Climate System Model, version 3 (CCSM3) and its alleviation when a revised Zhang–McFarlane (ZM) convection scheme is used. A hierarchy of coupling strategy is employed for this purpose. A slab ocean model is coupled with the atmospheric component of the Community Atmosphere Model, version 3 (CAM3) to investigate the local feedback between the atmosphere and the ocean. It is shown that the net surface energy flux differences in the southern ITCZ region between the revised and original ZM scheme seen in the stand-alone CAM3 simulations can cool the SST by up to 1.5°C. However, the simulated SST distribution is very sensitive to the prescribed ocean heat transport required in the slab ocean model. To understand the role of ocean heat transport, the fully coupled CCSM3 model is used. The analysis of CCSM3 simulations shows that the altered ocean dynamic heat transport when the revised ZM scheme is used is largely responsible for the reduction of SST bias in the southern ITCZ region, although surface energy flux also helps to cool the SST in the first few months of the year in seasonal variation. The results, together with those from Part I, suggest that the unrealistic simulation of convection over the southern ITCZ region in the standard CCSM3 leads to the double-ITCZ bias through complex coupled interactions between atmospheric convection, surface winds, latent heat flux, cloud radiative forcing, SST, and upper-ocean circulations. The mitigation of the double-ITCZ bias using the revised ZM scheme is achieved by altering this chain of interactions.

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A Simple Multicloud Parameterization for Convectively Coupled Tropical Waves. Part II: Nonlinear Simulations

Journal of the Atmospheric Sciences ◽

10.1175/jas3833.1 ◽

2007 ◽

Vol 64 (2) ◽

pp. 381-400 ◽

Cited By ~ 82

Author(s):

Boualem Khouider ◽

Andrew J. Majda

Keyword(s):

Large Scale ◽

Deep Convection ◽

Lower Troposphere ◽

Warm Pool ◽

Convective Precipitation ◽

Convective Heating ◽

Walker Cell ◽

Wave Trains ◽

Coupled Wave

Abstract Observations in the Tropics point to the important role of three cloud types, congestus, stratiform, and deep convective clouds, besides ubiquitous shallow boundary layer clouds for both the climatology and large-scale organized anomalies such as convectively coupled Kelvin waves, two-day waves, and the Madden–Julian oscillation. Recently, the authors have developed a systematic model convective parameterization highlighting the dynamic role of the three cloud types through two baroclinic modes of vertical structure: a deep convective heating mode and a second mode with lower troposphere heating and cooling corresponding respectively to congestus and stratiform clouds. The model includes both a systematic moisture equation where the lower troposphere moisture increases through detrainment of shallow cumulus clouds, evaporation of stratiform rain, and moisture convergence and decreases through deep convective precipitation and also a nonlinear switch that favors either deep or congestus convection depending on whether the lower middle troposphere is moist or dry. Here these model convective parameterizations are applied to a 40 000-km periodic equatorial ring without rotation, with a background sea surface temperature (SST) gradient and realistic radiative cooling mimicking a tropical warm pool. Both the emerging “Walker cell” climatology and the convectively coupled wave fluctuations are analyzed here while various parameters in the model are varied. The model exhibits weak congestus moisture coupled waves outside the warm pool in a turbulent bath that intermittently amplify in the warm pool generating convectively coupled moist gravity wave trains propagating at speeds ranging from 15 to 20 m s−1 over the warm pool, while retaining a classical Walker cell in the mean climatology. The envelope of the deep convective events in these convectively coupled wave trains often exhibits large-scale organization with a slower propagation speed of 3–5 m s−1 over the warm pool and adjacent region. Occasional much rarer intermittent deep convection also occurs outside the warm pool. The realistic parameter regimes in the multicloud model are identified as those with linearized growth rates for large scale instabilities roughly in the range of 0.5 K day−1.

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