Urban-Induced Changes on Local Circulation in Complex Terrain: Central Mexico Basin

Land use land cover (LULC) significantly impacts local circulation in the Mexico Basin, particularly wind field circulations such as gap winds, convergence lines, and thermally induced upslope/downslope wind. A case study with a high-pressure system over the Mexico Basin isolates the influence of large-scale synoptic forcing. Numerical simulations reveal a wind system composed of meridional circulation and a zonal component. Thermal pressure gradients between the Mexico basin and its colder surroundings create near-surface convergence lines as part of the meridional circulation. Experiments show that the intensity and organization of meridional circulations and downslope winds increase when LULC changes from natural and cultivated land to urban. Zonal circulation exhibits a typical circulation pattern with the upslope flow and descending motion in the middle of the basin. Large values of moist static energy are near the surface where air parcels pick up energy from the surface either as fluxes of enthalpy or latent heat. Surface heat fluxes and stored energy in the ground are drivers of local circulation, which is more evident in zonal circulation patterns.

Weakened seasonality of the African rainforest precipitation in boreal winter and spring driven by tropical SST variabilities

Precipitation in the equatorial African rainforest plays an important role in both the regional hydrological cycle and the global climate variability. Previous studies mostly focus on the trends of drought in recent decades or long-time scales. Using two observational datasets, we reveal a remarkable weakening of the seasonal precipitation cycle over this region from 1979 to 2015, with precipitation significantly increased in the boreal winter dry season (~ 0.13 mm/day/decade) and decreased in the boreal spring wet season (~ 0.21 mm/day/decade), which account for ~ 14% (the precipitation changes from 1979 to 2015) of their respective climatological means. We further use a state-of-the-art atmospheric model to isolate the impact of sea surface temperature change from different ocean basins on the precipitation changes in the dry and wet seasons. Results show that the strengthening precipitation in the dry season is mainly driven by the Atlantic warming, whereas the weakening precipitation in the wet season can be primarily attributed to the Indian Ocean. Warming Atlantic intensifies the zonal circulation over the African rainforest, strengthening moisture convergence and intensifying precipitation in the boreal winter dry season. Warming Indian Ocean contributes more to reducing the zonal circulation and suppressing the convection in the boreal spring wet season, leading to an opposite effect on precipitation. This result has important implication on local ecology as well as global climate system.

Invigoration of Indian Ocean zonal circulation drove Pleistocene eastern African aridification

<p>The coupled ocean-atmosphere circulation of the Indian Ocean Dipole (IOD) controls monsoon rainfall in eastern Africa and southeast Asia at seasonal to decadal time-scale. In years when the dipole is particularly active, it can lead to catastrophic floods and droughts. A growing body of evidence suggests that IOD variability influenced the continental hydroclimate also at longer timescales in the past and thus may have affected human evolution.  However, long-term continuous high-resolution well-dated records have so far been unavailable to test this hypothesis. In 2016, long-term continuous deep-sea sediment cores have been recovered from the Davie Ridge in the Mozambique Channel during Expedition 361 ‘Southern African Climates’ as part of the International Ocean Discovery Program (IODP).</p><p>Here, we present a more than seven million-year-long multi-proxy record of Mozambique Channel Throughflow (MCT), which is tightly coupled to IOD variability; defined here as the zonal sea surface temperature gradient (ΔSST) between the Indo-Pacific warm pool (IPWP) and the Arabian Sea. We show that the MCT was relatively weak and steady until 2.1 million years ago (Ma), when it started to significantly accelerate with progressively increasing glacial-interglacial amplitude, culminating in high flow speeds from 0.8 Ma onwards. The invigoration of MCT activity coincided with increasing zonal ΔSST, which fuels the atmospheric Walker Cell circulation along the tropical Indian Ocean.  Our results demonstrate that the overall intensification of the Indian Ocean Walker Cell amplified the coupled ocean-atmosphere Indian Ocean zonal circulation at orbital time-scales, which agrees with the heightened glacial continental aridity recorded in other eastern African climate proxy records. We argue that the corresponding progressively drier glacials alternated with relative humid interglacials, providing the climatic-environmental setting –varying at seasonal to orbital timescales- for speciation and global expansion of our genus <em>Homo</em> after 2.1 Ma.</p>

Equatorial atmospheric zonal circulation changes over India Ocean during recent decades

<p>The equatorial zonal asymmetric (Walker) circulation causes changes in the tropical rainfall pattern which induces devastation flood and drought that considerably impact the lives of millions of people. However, understanding of changes in zonal circulation is not yet certain. Here we examine the robustness of changes in Indian Walker Circulation (IWC) characteristics using different reanalysis and observation datasets in terms of the linear trends of IWC. The meridional (5<sup>o</sup>S:5<sup>o</sup>N)  averaged vertical velocity using different datasets are used to precisely locate the ascending (94<sup>o</sup>E:104<sup>o</sup>E, eastern) and descending (35<sup>o</sup>E:45<sup>o</sup>E, western) branch of IWC. We analyzed the zonal sea level pressure (SLP) gradient, velocity potential (VP) at 850 and 200 hPa, surface zonal wind (SZW) and zonal mass stream function (ZMSF) anomalies over the period of 1980–2017. We found that the magnitude of ZMSF representing anticlockwise circulation has an increasing trend in all the datasets. This kind of change is physically in agreement with the changes of SLP and SZW (an increasing trend in westerlies over the central IO) while the VP shows the decreasing trend which is in agreement with the strengthening of IWC during the recent decades. JRA55 is the most reliable which shows the significant and highest trend among all other datasets. The change point detection using the Pettitt method is applied to the normalized mean of all datasets which determines that in the post-1997-98 there is a significant strengthening of IWC as compared to the pre-1997-98 which demonstrates that IWC is highly sensitive by super El-Nino. The attribution of this strengthening can be examined using the CMIP5/6 datasets to determine the relative contribution of anthropogenic warming and natural variability.</p>

Depth of the dynamo region and zonal circulation of the molecular layer in Saturn inferred from its equatorially symmetric gravitational field

ABSTRACT The high-precision equatorially symmetric gravitational field of Saturn (the even gravitational coefficients J2, J4, …, J12) measured by the Cassini Grand Finale reflects its internal structure, its non-spherical shape caused by rotation and its strong zonal circulation whose location is controlled by the depth of its dynamo. We construct a four-layer, non-spheroidal (i.e. its shape is irregular) model of Saturn comprised of an inner core, a metallic dynamo region, an outer molecular envelope and a thin transition layer between the metallic and molecular regions. The model produces the even zonal gravitational coefficients that are in agreement with those measured by the Cassini Grand Finale within the error bars. Our Saturnian model reveals that (i) the observed cloud-top winds extending to any depth on cylinders cannot explain the measured coefficients J2, J4, …, J12; (ii) a deep zonal flow confined in the 20 000 km thick molecular layer is required to interpret them; (iii) the profile of the zonal flow – whose direction is sufficiently alternating with several retrograde peaks – significantly differs from that of the surface winds, implying that the observed winds are confined to a shallow layer, do not extend deeply into Saturn and do not contribute to the observed gravity; and (iv) the Saturnian dynamo can substantially affect the structure of its equatorially symmetric gravitational field by stopping the zonal-flow penetration and by changing, because of the boundary condition at the metallic and molecular interface, the distribution of the dynamic density anomalies.

Circulation epochs based on the Vangengeim-Girs large scale patterns (1891–2010)

This paper presents the results of an investigation of the variability in macro-circulation forms at the mid-tropospheric level distinguished in the Vangengeim-Girs (V-G) classification. The annual frequencies of circulation forms in the years 1891–2010 proved significant fluctuations, which provided the basis for distinguishing 7 circulation epochs. The epochs illustrate secular changes in the character of dominant forms – zonal circulation (W) prevailed at the turn of the 20th century; meridional forms E and C developed next, and zonal circulation began to dominate again after 1990.