Seasonal Variability of The Mixed Layer Depth in The Sulawesi Sea

The Sulawesi Sea is a semi-enclosed basin located in the Indonesian Seas and considered as the one of location in the west route of Indonesian Throughflow (ITF). There is less attention on the mixed layer depth investigation in the Sulawesi Sea. Concerning that the mixed layer plays an important role in influencing the ocean in air-sea interaction and affects biological activity, the estimation of mixed layer depth (MLD) in the Sulawesi Sea is important. Seasonal variation of the mixed layer in the Sulawesi Sea between 115°-125°E and 0°-8°N is estimated by using World Ocean Atlas 2013. Forcing elements on the mixed layer in terms of surface-forced turbulent mixing from mechanical forcing of wind stress and buoyancy forcing (from heat flux as well as freshwater flux) in the Sulawesi Sea is provided by using a reanalysis dataset. The MLD is estimated directly on grid profiles with interpolated levels based on chosen density fixed criterion of 0.03 kg.m^-3 and temperature criterion of 0.5°C difference from the surface. The results show that mixed layer depth in the Sulawesi Sea varies both spatially and temporally. Generally, the deepest MLD was occurred during the southwest monsoon (JJA), and the lowest MLD was occurred during the first transition (MAM) and second transition monsoon (SON). Strengthening and weakening MLD are influenced by mechanical forcing from wind stress and buoyancy flux. In the Sulawesi Sea, the mixed layer deepening coincides with the occurrence of a maximum in wind stress, and low buoyancy flux at the surface. This condition is the opposite when mixed layer shallowing occurs.

Semi-Permanent Zones of Radar Radial Shear within the Planetary Boundary Layer: Observations and Effects on High Intensity Precipitation in the Wider Auckland region, New Zealand

<p>This study investigates the role of mechanical forcing within the boundary layer in enhancing low-level precipitation and initiating/intensifying convective precipitation during cases of high intensity precipitation in the wider Auckland region, New Zealand. Eight cases, that occurred between 2001 and 2008 have been investigated. All cases were observed to be strongly dynamically forced, resulting from the passage of mid-latitude cyclones. These features were observed to be centred mainly to the north and west of the study area, with surface winds from the northeast quadrant over the wider Auckland region. Radar imagery is characterised by regions of both convective and stratiform precipitation for all the cases investigated; areas of convection are often observed to be embedded in areas of larger scale precipitation. These cases were subdivided into eleven heavy precipitation events. Nine of these events were subject to further investigation. Environmental conditions during these events were characterised by steady low-level winds from the northeast quadrant, weak to moderate convective instability, with 0-3km wind shear indicating a high level of directional shear in the lower atmosphere. To investigate mechanical forcing in the boundary layer, low-level Doppler velocity and reflectivity fields measured by the Mt Tamahunga radar, were examined. These data revealed mesoscale structures of the Doppler velocity field not previously documented in this region. Mechanical forcing was identified by the presence of mesoscale zones of radar radial shear, resulting from horizonal convergence and/or zones of horizontal shear. These features were observed to be semi-permanent on the windward side of Little Barrier and Great Barrier islands, the windward side of the Coromandel ranges, and along the west coast of the Auckland region. Further, zones of semi-permanent radar radial shear were observed to extend downstream (lee side) of Mt Moehau and Great Barrier, Little Barrier and Taranga islands in the Hauraki Gulf. These features have not been documented previously for this study area. The features, observed downstream of each obstacle, were characterised by a long thin low velocity zone present in PPI images of radar radial velocity and were bounded by the above mentioned shear zones. Further, these features were aligned parallel to the surface wind direction, with widths approximately equal to the diameter of the obstacle and extended up to 57km downstream of each obstacle. These features are consistent with characteristics of mountain wakes described in the literature. A partitioning algorithm was calibrated to identify the convective and stratiform components of the radar reflectivity field. This algorithm was applied to reflectivity data for each heavy precipitation event. Local maxima in the frequency of low-level enhanced precipitation were observed in the vicinity of topographic features such as the Coromandel Peninsula and Mt Tamahunga, in addition to the observed location of wakes in the lee of Great Barrier and Little Barrier Island. Finally, the relationship between mountain wakes observed in the Hauraki Gulf and low-level precipitation enhancement was examined. Investigations showed that when large scale areas of precipitation interacted with these wakes, in some cases convective precipitation was observed to be initiated or intensified. However, the observed areas of enhancement were observed to be short lived and shallow, reaching heights below the radar bright band at [approximately ]3.5 km.</p>

Semi-Permanent Zones of Radar Radial Shear within the Planetary Boundary Layer: Observations and Effects on High Intensity Precipitation in the Wider Auckland region, New Zealand

<p>This study investigates the role of mechanical forcing within the boundary layer in enhancing low-level precipitation and initiating/intensifying convective precipitation during cases of high intensity precipitation in the wider Auckland region, New Zealand. Eight cases, that occurred between 2001 and 2008 have been investigated. All cases were observed to be strongly dynamically forced, resulting from the passage of mid-latitude cyclones. These features were observed to be centred mainly to the north and west of the study area, with surface winds from the northeast quadrant over the wider Auckland region. Radar imagery is characterised by regions of both convective and stratiform precipitation for all the cases investigated; areas of convection are often observed to be embedded in areas of larger scale precipitation. These cases were subdivided into eleven heavy precipitation events. Nine of these events were subject to further investigation. Environmental conditions during these events were characterised by steady low-level winds from the northeast quadrant, weak to moderate convective instability, with 0-3km wind shear indicating a high level of directional shear in the lower atmosphere. To investigate mechanical forcing in the boundary layer, low-level Doppler velocity and reflectivity fields measured by the Mt Tamahunga radar, were examined. These data revealed mesoscale structures of the Doppler velocity field not previously documented in this region. Mechanical forcing was identified by the presence of mesoscale zones of radar radial shear, resulting from horizonal convergence and/or zones of horizontal shear. These features were observed to be semi-permanent on the windward side of Little Barrier and Great Barrier islands, the windward side of the Coromandel ranges, and along the west coast of the Auckland region. Further, zones of semi-permanent radar radial shear were observed to extend downstream (lee side) of Mt Moehau and Great Barrier, Little Barrier and Taranga islands in the Hauraki Gulf. These features have not been documented previously for this study area. The features, observed downstream of each obstacle, were characterised by a long thin low velocity zone present in PPI images of radar radial velocity and were bounded by the above mentioned shear zones. Further, these features were aligned parallel to the surface wind direction, with widths approximately equal to the diameter of the obstacle and extended up to 57km downstream of each obstacle. These features are consistent with characteristics of mountain wakes described in the literature. A partitioning algorithm was calibrated to identify the convective and stratiform components of the radar reflectivity field. This algorithm was applied to reflectivity data for each heavy precipitation event. Local maxima in the frequency of low-level enhanced precipitation were observed in the vicinity of topographic features such as the Coromandel Peninsula and Mt Tamahunga, in addition to the observed location of wakes in the lee of Great Barrier and Little Barrier Island. Finally, the relationship between mountain wakes observed in the Hauraki Gulf and low-level precipitation enhancement was examined. Investigations showed that when large scale areas of precipitation interacted with these wakes, in some cases convective precipitation was observed to be initiated or intensified. However, the observed areas of enhancement were observed to be short lived and shallow, reaching heights below the radar bright band at [approximately ]3.5 km.</p>

Radiative-dynamical Simulation of Jupiter’s Stratosphere and Upper Troposphere

We present a two-dimensional radiative-dynamical model of the combined stratosphere and upper troposphere of Jupiter to understand its temperature distribution and meridional circulation pattern. Our study highlights the importance of radiative and mechanical forcing for driving the middle atmospheric circulation on Jupiter. Our model adopts a state-of-the-art radiative transfer scheme with recent observations of Jovian gas abundances and haze distribution. Assuming local radiative equilibrium, latitudinal variation of hydrocarbon abundances is not able to explain the observed latitudinal temperature variations in the mid-latitudes. With mechanical forcing parameterized as a frictional drag on zonal wind, our model produces ∼2 K latitudinal temperature variations observed in low to mid-latitudes in the troposphere and lower stratosphere, but cannot reproduce the observed 5 K temperature variations in the middle stratosphere. In the high latitudes, temperature and meridional circulation depend strongly on polar haze radiation. The simulated residual mean circulation shows either two broad equator-to-pole cells or multi-cell patterns, depending on the frictional drag timescale and polar haze properties. A more realistic wave parameterization and a better observational characterization of haze distribution and optical properties are needed to better understand latitudinal temperature distributions and circulation patterns in the middle atmosphere of Jupiter.

Investigation of Mechanical and Thermal Wind Sensitivity on the Mesoscale Eddies in the Southern Ocean

&lt;p&gt;In this study, a high-resolution eddy resolving regional ocean + sea ice coupled model (MITgcm) is used to study the effects of increasing westerlies along the Southern Ocean. Previous studies only focused on increasing wind stress, thus not taking into account of atmosphere-to-ocean heat and freshwater fluxes. Here, we conduct two concurrent simulations; i) 1.5 times increased wind stress (i.e. increased only mechanical forcing) ii) 1.2247 times increased wind speed (i.e. both mechanical and thermal flux forcing). Model domain covers whole Southern Hemisphere with lateral open boundary conditions from ECCOv2 ocean reanalysis and surface boundary conditions from ECMWF ERA-5 atmospheric reanalysis. In both sensitivity scenarios, due to the increase in the wind stress, the Ekman transport towards Equator towards north is increased. This caused increased upwelling of warmer North Atlantic Deep Water (NADW) near the Antarctic ice sheet. Both scenarios show reduced sea ice formation with up to 2 million km&lt;sup&gt;2&lt;/sup&gt; in the austral summer and up to 4 million km&lt;sup&gt;2&lt;/sup&gt; during the austral winter. Sea ice extent is reduced more in the mechanical forcing simulation than the mechanical+thermal forcing one. This is a clear result that increased wind anomalies should be studied with increased wind rather than increased stress. The reduction in the sea ice coverage that is attributed to the warmer water mass can also be observed through the Sea Surface Temperature (SST) values. The first case shows up to 1 &amp;#8211; 1.5 &amp;#176;C very close to the Antarctica, whereas the second case shows a much limited SST change around 0.5 &amp;#176;C.&lt;/p&gt;&lt;p&gt;Both sensitivity scenarios show an increase of the transport along Drake Passage. However, the mechanical+thermal case shows larger increase in the Drake transport compared to the mechanical case. This indicates that a change in the Antarctic Circumpolar Circulation also modifies the meridional density gradient along with the upwelling characteristics. Finally, overturning transport in the density space shows that Subtropical Cell and ACC upper Cell strengthen in the mechanical+thermal case, while there are no significant changes in the thermal case. In both simulations, Subpolar Cell increases and Lower Cell decreases. We conclude that studying increased westerlies with two different approaches show significant changes in the surface and deep circulation. Previous studies which taken into only mechanical forcing part are missing thermal component of the wind effects.&lt;/p&gt;