scholarly journals Winter Atmospheric Buoyancy Forcing and Oceanic Response during Strong Wind Events around Southeastern Greenland in the Regional Arctic System Model (RASM) for 1990–2010*

2016 ◽  
Vol 29 (3) ◽  
pp. 975-994 ◽  
Author(s):  
Alice K. DuVivier ◽  
John J. Cassano ◽  
Anthony Craig ◽  
Joseph Hamman ◽  
Wieslaw Maslowski ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Deep Convection ◽  
Heat Fluxes ◽  
System Model ◽  
Strong Wind ◽  
Self Organizing Map ◽  
Ocean Mixed Layer ◽  
Irminger Sea ◽  
Wind Events ◽  
Wind Patterns

Abstract Strong, mesoscale tip jets and barrier winds that occur along the southeastern Greenland coast have the potential to impact deep convection in the Irminger Sea. The self-organizing map (SOM) training algorithm was used to identify 12 wind patterns that represent the range of winter [November–March (NDJFM)] wind regimes identified in the fully coupled Regional Arctic System Model (RASM) during 1990–2010. For all wind patterns, the ocean loses buoyancy, primarily through the turbulent sensible and latent heat fluxes; haline contributions to buoyancy change were found to be insignificant compared to the thermal contributions. Patterns with westerly winds at the Cape Farewell area had the largest buoyancy loss over the Irminger and Labrador Seas due to large turbulent fluxes from strong winds and the advection of anomalously cold, dry air over the warmer ocean. Similar to observations, RASM simulated typical ocean mixed layer depths (MLD) of approximately 400 m throughout the Irminger basin, with individual years experiencing MLDs of 800 m or greater. The ocean mixed layer deepens over most of the Irminger Sea following wind events with northerly flow, and the deepening is greater for patterns of longer duration. Seasonal deepest MLD is strongly and positively correlated to the frequency of westerly tip jets with northerly flow.

scholarly journals Modelled Variations of Deep Convection in the Irminger Sea during 2003–10

2016 ◽  
Vol 46 (1) ◽  
pp. 179-196 ◽  
Author(s):  
Jean-Philippe Paquin ◽  
Youyu Lu ◽  
Simon Higginson ◽  
Frédéric Dupont ◽  
Gilles Garric
Keyword(s):  
Heat Loss ◽  
Sea Water ◽  
Deep Convection ◽  
Ocean Model ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Strong Wind ◽  
Irminger Sea ◽  
Wind Events ◽  
Active Convection

AbstractResults from a high-resolution ice–ocean model are analyzed to understand the physical processes responsible for the interannual variability of ocean convection over the Irminger Sea. The modeled convection in the open Irminger Sea for the winters of 2007/08 and 2008/09 is in good agreement with observations. Deep convection is caused by strong atmospheric forcing that increases the ocean heat loss through latent and sensible heat fluxes. Greenland tip jets are found to be the only strong wind events that directly affect the deep convection area and explain up to 53% of the total turbulent heat loss during active convection years. Deep convection is modeled where there is favorable preconditioning of the water column due to isopycnal doming inside the semienclosed Irminger Gyre. The region of deep convection is also characterized by weak eddy kinetic energy. Finally, an estimation of the surface-forced water mass transformation confirms the Irminger Sea as a region of intermittent production of Labrador Sea Water, with annual averages between 0.9 and 1.9 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of water denser than 27.7 kg m−3 for years of active convection.

The Southeast Pacific Warm Band and Double ITCZ

2010 ◽  
Vol 23 (5) ◽  
pp. 1189-1208 ◽  
Author(s):  
Hirohiko Masunaga ◽  
Tristan S. L’Ecuyer
Keyword(s):  
Mixed Layer ◽  
Deep Convection ◽  
Tropical Rainfall Measuring Mission ◽  
Heat Fluxes ◽  
Shortwave Radiation ◽  
Ocean Mixed Layer ◽  
The South ◽  
Surface Heat Fluxes ◽  
Double Itcz ◽  
Southeast Pacific

Abstract The east Pacific double intertropical convergence zone (ITCZ) in austral fall is investigated with particular focus on the growing processes of its Southern Hemisphere branch. Satellite measurements from the Tropical Rainfall Measuring Mission (TRMM) and Quick Scatterometer (QuikSCAT) are analyzed to derive 8-yr climatology from 2000 to 2007. The earliest sign of the south ITCZ emerges in sea surface temperature (SST) by January, followed by the gradual development of surface convergence and water vapor. The shallow cumulus population starts growing to form the south ITCZ in February, a month earlier than vigorous deep convection is organized into the south ITCZ. The key factors that give rise to the initial SST enhancement or the southeast Pacific warm band are diagnosed by simple experiments. The experiments are designed to calculate SST, making use of an ocean mixed layer “model” forced by surface heat fluxes, all of which are derived from satellite observations. It is found that the shortwave flux absorbed into the ocean mixed layer is the primary driver of the southeast Pacific warm band. The warm band does not develop in boreal fall because the shortwave flux is seasonally so small that it is overwhelmed by other negative fluxes, including the latent heat and longwave fluxes. Clouds offset the net radiative flux by 10–15 W m−2, which is large enough for the warm band to develop in boreal fall if it were not for clouds reflecting shortwave radiation. Interannual variability of the double ITCZ is also discussed in brief.

scholarly journals Winter Mixed Layer Development in the Central Irminger Sea: The Effect of Strong, Intermittent Wind Events

2008 ◽  
Vol 38 (3) ◽  
pp. 541-565 ◽  
Author(s):  
Kjetil Våge ◽  
Robert S. Pickart ◽  
G. W. K. Moore ◽  
Mads Hvid Ribergaard
Keyword(s):  
Mixed Layer ◽  
Deep Convection ◽  
Layer Model ◽  
Pressure System ◽  
Nao Index ◽  
Irminger Sea ◽  
Wind Events ◽  
Upper Level ◽  
The Impact ◽  
Mixed Layer Model

Abstract The impact of the Greenland tip jet on the wintertime mixed layer of the southwest Irminger Sea is investigated using in situ moored profiler data and a variety of atmospheric datasets. The mixed layer was observed to reach 400 m in the spring of 2003 and 300 m in the spring of 2004. Both of these winters were mild and characterized by a low North Atlantic Oscillation (NAO) index. A typical tip jet event is associated with a low pressure system that is advected by upper-level steering currents into the region east of Cape Farewell and interacts with the high topography of southern Greenland. Heat flux time series for the mooring site were constructed that include the enhancing influence of the tip jet events. This was used to force a one-dimensional mixed layer model, which was able to reproduce the observed envelope of mixed layer deepening in both winters. The deeper mixed layer of the first winter was largely due to a higher number of robust tip jet events, which in turn was caused by the steering currents focusing more storms adjacent to southern Greenland. Application of the mixed layer model to the winter of 1994–95, a period characterized by a high-NAO index, resulted in convection exceeding 1700 m. This prediction is consistent with hydrographic data collected in summer 1995, supporting the notion that deep convection can occur in the Irminger Sea during strong winters.

scholarly journals The Role of Wave Dynamics and Small-Scale Topography for Downslope Wind Events in Southeast Greenland

2015 ◽  
Vol 72 (7) ◽  
pp. 2786-2805 ◽  
Author(s):  
M. Oltmanns ◽  
F. Straneo ◽  
H. Seo ◽  
G. W. K. Moore
Keyword(s):  
Local Population ◽  
Wrf Model ◽  
Heat Fluxes ◽  
Small Scale ◽  
Strong Wind ◽  
Wave Dynamics ◽  
Irminger Sea ◽  
Downslope Wind ◽  
Wind Events ◽  
Ocean Convection

In Ammassalik, in southeast Greenland, downslope winds can reach hurricane intensity and represent a hazard for the local population and environment. They advect cold air down the ice sheet and over the Irminger Sea, where they drive large ocean–atmosphere heat fluxes over an important ocean convection region. Earlier studies have found them to be associated with a strong katabatic acceleration over the steep coastal slopes, flow convergence inside the valley of Ammassalik, and—in one instance—mountain wave breaking. Yet, for the general occurrence of strong downslope wind events, the importance of mesoscale processes is largely unknown. Here, two wind events—one weak and one strong—are simulated with the atmospheric Weather Research and Forecasting (WRF) Model with different model and topography resolutions, ranging from 1.67 to 60 km. For both events, but especially for the strong one, it is found that lower resolutions underestimate the wind speed because they misrepresent the steepness of the topography and do not account for the underlying wave dynamics. If a 5-km model instead of a 60-km model resolution in Ammassalik is used, the flow associated with the strong wind event is faster by up to 20 m s−1. The effects extend far downstream over the Irminger Sea, resulting in a diverging spatial distribution and temporal evolution of the heat fluxes. Local differences in the heat fluxes amount to 20%, with potential implications for ocean convection.

scholarly journals The Effect of Atmosphere–Ocean Coupling on the Structure and Intensity of Tropical Cyclone Bejisa in the Southwest Indian Ocean

Atmosphere ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 688
Author(s):  
Soline Bielli ◽  
Christelle Barthe ◽  
Olivier Bousquet ◽  
Pierre Tulet ◽  
Joris Pianezze
Keyword(s):  
Tropical Cyclone ◽  
Mixed Layer ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Ocean Mixed Layer ◽  
Surface Cooling ◽  
Southwest Indian Ocean ◽  
Left Hand ◽  
Sea Surface Cooling ◽  
The Impact

A set of numerical simulations is relied upon to evaluate the impact of air-sea interactions on the behaviour of tropical cyclone (TC) Bejisa (2014), using various configurations of the coupled ocean-atmosphere numerical system Meso-NH-NEMO. Uncoupled (SST constant) as well as 1D (use of a 1D ocean mixed layer) and 3D (full 3D ocean) coupled experiments are conducted to evaluate the impact of the oceanic response and dynamic processes, with emphasis on the simulated structure and intensity of TC Bejisa. Although the three experiments are shown to properly capture the track of the tropical cyclone, the intensity and the spatial distribution of the sea surface cooling show strong differences from one coupled experiment to another. In the 1D experiment, sea surface cooling (∼1 ∘C) is reduced by a factor 2 with respect to observations and appears restricted to the depth of the ocean mixed layer. Cooling is maximized along the right-hand side of the TC track, in apparent disagreement with satellite-derived sea surface temperature observations. In the 3D experiment, surface cooling of up to 2.5 ∘C is simulated along the left hand side of the TC track, which shows more consistency with observations both in terms of intensity and spatial structure. In-depth cooling is also shown to extend to a much deeper depth, with a secondary maximum of nearly 1.5 ∘C simulated near 250 m. With respect to the uncoupled experiment, heat fluxes are reduced from about 20% in both 1D and 3D coupling configurations. The tropical cyclone intensity in terms of occurrence of 10-m TC wind is globally reduced in both cases by about 10%. 3D-coupling tends to asymmetrize winds aloft with little impact on intensity but rather a modification of the secondary circulation, resulting in a slight change in structure.

Wind-forced submesoscale symmetric instability around deep convection in the NW Mediterranean Sea

2021 ◽  
Author(s):  
Anthony Bosse ◽  
Pierre Testor ◽  
Pierre Damien ◽  
Claude Estournel ◽  
Patrick Marsaleix ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Deep Convection ◽  
Realistic Model ◽  
Carbon Export ◽  
Ligurian Sea ◽  
Vertical Movements ◽  
Intermediate Layers ◽  
Nw Mediterranean ◽  
Wind Events ◽  
Symmetric Instability

<p>During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500m in the NW Mediterranean. Intense fronts around the deep convection area were repeatedly sampled by autonomous gliders, mainly as part of the MOOSE observatory of the NW Mediterrnean Sea (https://www.moose-network.fr/). Subduction down to 200-300m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution (SYMPHONIE model, https://sirocco.obs-mip.fr/ocean-models/s-model/).</p><p>During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1-5km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open-sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.</p>

On the Ocean Mixed Layer influence on the genesis of Mediterranean Tropical-Like cyclones

2020 ◽  
Author(s):  
Antonio Ricchi ◽  
Davide Bonaldo ◽  
Mario Marcello Miglietta ◽  
Sandro Carniel
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Case Studies ◽  
Mixed Layer ◽  
Heat Content ◽  
Heat Fluxes ◽  
Ocean Heat Content ◽  
Sea Surface ◽  
Lapse Rate ◽  
Ocean Mixed Layer

<p>The Mediterranean basin is the formation site of a vast number and type of cyclones. Among these, we can occasionally identify intense vortices showing tropical characteristics, called Tropical-Like Cyclones (TLC) or MEDIcanes (Mediterranean Hurricane). Their development has been studied in several case studies, showing the influence of synoptic scale upper level forcings and mesoscale features, such as the sea surface temperature and the characteristics of the air masses on the formation area. The importance of Sea Surface Temperature (SST) consists in modulating the intense latent and sensible heat fluxes, which control the development of the TLC. For tropical cyclones, one of the most studied factors in recent years is the ocean heat content in the formation basin of these storms. We plan here to extend this analysis to TLC. Besides innovative studies with coupled atmosphere-waves-ocean numerical models, a simpler approach for investigating the sole effect of the ocean heat content consists of adopting a simplified ocean (1-Dimensional) description by varying the local characteristics of the Ocean Mixed Layer (OML). In this work we use the WRF (Weather Research and Forecasting system) model, in standalone (atmospheric) mode, with 3 km grid spacing, forced with GFS-GDAL (0.25°x0.25° horizontal resolution) and SST initialization provided by the MFS-CMEMs Copernicus dataset. Three case studies of TLC are examined here, namely ROLF (06-09/11/2011), ILONA (19-21/01/2014) and NUMA (11-20/11/2017). The ocean is simulated with an OML approach, with SST updated at each iteration as a function of the atmospheric heat fluxes and with an average mixed layer deph (MDL) provided by the MFS-CMEMS dataset. For each TLC studied, the MDL is modified by increasing and decreasing its depth by 50% and increasing and decreasing its lapse rate by 50%. The results show how the structure of the MDL influences not only the intensity of the cyclone but also the structure and precipitation both in terms of quantity and location. These outcomes suggest that, as for hurricanes, also for MEDICANES the heat content of the mass of seawater plays a fundamental role in their intensification, suggesting further studies also in a climate change perspective.</p>

scholarly journals A Simplified Model of the Walker Circulation with an Interactive Ocean Mixed Layer and Cloud-Radiative Feedbacks

2005 ◽  
Vol 18 (20) ◽  
pp. 4216-4234 ◽  
Author(s):  
Matthew E. Peters ◽  
Christopher S. Bretherton
Keyword(s):  
Mixed Layer ◽  
Large Scale ◽  
Deep Convection ◽  
Walker Circulation ◽  
Surface Energy Budget ◽  
Ocean Mixed Layer ◽  
Horizontal Structure ◽  
Low Clouds ◽  
The Walker Circulation ◽  
Radiation Scheme

Abstract Cloud–climate feedbacks between precipitation, radiation, circulation strength, atmospheric temperature and moisture, and ocean temperature are studied with an idealized model of the Walker circulation in a nonrotating atmosphere coupled to an ocean mixed layer. This study has two main purposes: 1) to formulate a conceptual framework that includes the dominant feedbacks between clouds and a large-scale divergent circulation; and 2) to use this framework to investigate the sensitivity of the climate system to these interactions. Two cloud types—high, convective anvils and low, nonprecipitating stratus—are included and coupled to the large-scale dynamics. The atmosphere is coupled to an ocean mixed layer via a consistent surface energy budget. Analytic approximations with a simplified radiation scheme are derived and used to explain numerical results with a more realistic radiation scheme. The model simplicity allows interactions between different parts of the ocean–atmosphere system to be cleanly elucidated, yet also allows the areal extent of deep convection and the horizontal structure of the Walker circulation to be internally determined by the model. Because of their strong top-of-atmosphere radiative cancellation, high clouds are found to have little overall effect on the circulation strength and convective area fraction. Instead, to leading order, these are set by the horizontally varying ocean heat transport and clear-sky radiative fluxes. Low clouds are found to cool both the ocean and atmosphere, to slightly increase the circulation strength, and to shrink the convective area significantly. The climate is found to be less sensitive to doubled greenhouse gas experiments with low clouds than without.

scholarly journals On the Ocean Mixed Layer influence on the genesis of Mediterranean Tropical-Like cyclones

2021 ◽  
Author(s):  
Antonio Ricchi ◽  
Giovanni Liguori ◽  
Leone Cavicchia ◽  
Mario Marcello Miglietta ◽  
Davide Bonaldo ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Case Studies ◽  
Mixed Layer ◽  
Heat Content ◽  
Heat Fluxes ◽  
Ocean Heat Content ◽  
Sea Surface ◽  
Ocean Mixed Layer ◽  
Vast Number

<p>The Mediterranean basin is the formation site of a vast number and type of cyclones. Among these, we can occasionally identify intense vortices showing tropical characteristics, called Tropical-Like Cyclones (TLC). Their development has been studied in several case studies, showing the influence of synoptic scale upper level forcings and mesoscale features, such as the sea surface temperature and the characteristics of the air masses on the formation area. The importance of Sea Surface Temperature (SST) consists in modulating the intense latent and sensible heat fluxes, which control the development of the TLC. For tropical cyclones, one of the most studied factors in recent years is the ocean heat content in the formation basin of these storms. We plan here to extend this analysis to TLC. Besides innovative studies with coupled atmosphere-waves-ocean numerical models, a simpler approach for investigating the sole effect of the ocean heat content consists of adopting a simplified ocean description by varying the local characteristics of the Ocean Mixed Layer (OML). In this work we use the WRF (Weather Research and Forecasting system) model, in standalone (atmospheric) mode, with 3 km grid spacing, forced with GFS-GDAL (0.25°x0.25° horizontal resolution) and SST initialization provided by the MFS-CMEMs Copernicus dataset. Two case studies of TLC are examined here, namely ROLF (06-09/11/2011) and IANOS (14-19/09/2020). The ocean is simulated with an OML approach, with SST updated at each iteration as a function of the atmospheric heat fluxes and with an average mixed layer deph (MDL) provided by the MFS-CMEMS dataset. For each TLC studied, the MDL is modified by increasing and decreasing its depth by 10 mt, 30 mt, 50 mt . The preliminary results show how the structure of the MDL influences  the intensity of the cyclone but also the structure and precipitation both in terms of quantity and location. </p>

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