Hadley Circulation Research Articles

Influence of the Interdecadal Pacific Oscillation on Super Cyclone Activities over the Bay of Bengal during the Primary Cyclone Season

An obvious interdecadal change can be measured in the super cyclones (SCs, categories 4 and 5) that occur from October to November over the Bay of Bengal (BoB). This change may be modulated by the interdecadal Pacific oscillation (IPO). A La Niña-like difference between the 1977–1998 (IP1) and 1999–2014 (IP2) periods forced a local Hadley circulation in the eastern tropical Indian Ocean by strengthening the Walker circulation, which caused plummeting upper-level temperatures and ultimately created favorable thermodynamic conditions to enhance the cyclone intensity. Meanwhile, an equatorial downwelling Kelvin wave caused by heating and westerly wind differences entered the BoB rim along the coast and aptly intensified the cyclone, such that the downwelling Kevin wave and Rossby wave generated by its reflection deepened the thermocline in the BoB. The favorable atmospheric and oceanic conditions in IP2 jointly and preferentially cause far more SC activities from October to November over the BoB compared to IP1.

Changes in Hadley circulation and intertropical convergence zone under strategic stratospheric aerosol geoengineering

Stratospheric aerosol geoengineering has been proposed as a potential solution to reduce climate change and its impacts. Here, we explore the responses of the Hadley circulation (HC) intensity and the intertropical convergence zone (ITCZ) using the strategic stratospheric aerosol geoengineering, in which sulfur dioxide was injected into the stratosphere at four different locations to maintain the global-mean surface temperature and the interhemispheric and equator-to-pole temperature gradients at present-day values (baseline). Simulations show that, relative to the baseline, strategic stratospheric aerosol geoengineering generally maintains northern winter December–January–February (DJF) HC intensity under RCP8.5, while it overcompensates for the greenhouse gas (GHG)-forced southern winter June–July–August (JJA) HC intensity increase, producing a 3.5 ± 0.4% weakening. The residual change of southern HC intensity in JJA is mainly associated with stratospheric heating and tropospheric temperature response due to enhanced stratospheric aerosol concentrations. Geoengineering overcompensates for the GHG-driven northward ITCZ shifts, producing 0.7° ± 0.1° and 0.2° ± 0.1° latitude southward migrations in JJA and DJF, respectively relative to the baseline. These migrations are affected by tropical interhemispheric temperature differences both at the surface and in the free troposphere. Further strategies for reducing the residual change of HC intensity and ITCZ shifts under stratospheric aerosol geoengineering could involve minimizing stratospheric heating and restoring and preserving the present-day tropical tropospheric interhemispheric temperature differences.

Sensitivity of regional monsoons to idealised equatorial volcanic eruption of different sulfur emission strengths

The impact of volcanic forcing on tropical precipitation is investigated in a new set of sensitivity experiments within the Max Planck Institute Grand Ensemble framework. Five ensembles are created, each containing 100 realizations for an idealized ‘Pinatubo-like’ equatorial volcanic eruption with emissions covering a range of 2.5-40 Tg sulfur (S). The ensembles provide an excellent database to disentangle the influence of volcanic forcing on monsoons and tropical hydroclimate over the wide spectrum of the climate’s internal variability. Monsoons are generally weaker for two years after volcanic eruption and their weakening is a function of emissions. However, only a stronger than Pinatubo-like eruption (10 Tg S) leads to significant and substantial monsoon changes, and some regions (such as North and South Africa, South America and South Asia) are much more sensitive to this kind of forcing than the others. The decreased monsoon precipitation is strongly tied to the weakening of the regional tropical overturning. The reduced atmospheric net energy input and increased gross moist stability at the Hadley circulation updraft due to the equatorial volcanic eruption, require a slowdown of the circulation as a consequence of less moist static energy exported away from the intertropical convergence zone.

Control of the oscillations of the martian Northern Hemisphere polar vortex by the Hadley cell and topographic forcing

The oscillations of the Martian Northern Hemisphere polar vortex are examined by using the Kida vortex model to interpret the dynamics of the vortex in reanalysis data. The vortex nutating around a fixed angle with little variability in its aspect ratio is shown to be controlled by the Hadley cell and wavenumber-2 planetary-scale waves that are characterised with a stationary component that is shown to be linked directly to Martian topography. The wave forcing and the Hadley cell structure are modelled as a strain and a rotational flow in Kida’s model. It is shown that the strain and rotational flow are strongly negatively correlated, acting against vortex elongation in Kida’s model. The result suggests that periods of high topographic forcing events are accompanied with a weakening in the Hadley cell winds and thus provides an explanation why vortex splits are rare on Mars.

Investigating Tropical Versus Extratropical Influences on the Southern Hemisphere Tropical Edge in the Unified Model

AbstractSince the late 1970s, observations have shown a widening of the tropical Hadley cell (HC) circulation. State‐of‐the‐art climate models reproduce the general trend along with a projected continuous expansion. Discrepancies in expansion rates of observation‐ and model‐based studies have been attributed to differences in applied methods, natural variability and model shortcomings. Furthermore, the driving influence of tropical or extratropical processes on these changes is not well understood. All of this highlights the dynamical mechanisms and the region of origin controlling the tropical width are still insufficiently understood. Here we examine the influence of systematic model biases of the atmosphere‐only Unified Model (UM) onto the simulation of the Southern Hemisphere (SH) tropical edge. We utilize nudged experiments with prescribed sea surface temperatures, where potential temperature and horizontal winds are relaxed back to ERA‐Interim reanalysis for a 20‐year period in selected regions. Correcting model biases in the tropics and extratropics separately allows us to dissect the dominant remote impacts of present model errors onto the SH tropical edge simulation. The experiments are applied to established tropical width metrics ranging from near‐surface to upper‐level metrics capturing the poleward flank of the HC. We find both regions work remotely to reduce errors in the UM fields and location of the tropical edge. Surprisingly, correcting the extratropical biases, south of 45°S, more consistently improves the tropical width across the metrics and seasons than nudging the tropics (10°N–10°S). These findings demonstrate the substantial role of extratropical influences in locating the SH tropical edge.

Response of atmospheric quasi‐stationary waves to La Niña conditions in Northern Hemisphere winter

AbstractLarge‐scale atmospheric quasi‐stationary waves (QSWs) are strongly linked to synoptic variability and extreme events such as blocking and heatwaves. It is therefore essential to gain a deeper insight into the drivers of QSW variability. Previous research highlighted the El Niño Southern Oscillation as one of the main drivers for global QSW variability with La Niña conditions of particular interest because of their link to anomalous strong QSW amplitudes. This connection between La Niña and QSW activity in the Northern Hemisphere is analysed in this study using a set of aquaplanet experiments with a slab ocean, mimicking La Niña conditions of varying strengths. The experiments demonstrate La Niña conditions are consistent with a weaker zonal mean Hadley cell and a corresponding northward shift in the midlatitude wave guide associated with strong meridional gradients in absolute vorticity. The QSW region generally follows this shift in the wave guide with an overall increase in QSW amplitudes along a slightly weaker mean vorticity gradient. For weak to moderate modifications of the ocean heat flux convergence, there is an approximately linear relationship with the associated QSW response, though for stronger slab heat fluxes the overall response becomes highly nonlinear. It is therefore concluded that the ENSO state may have significant consequences for QSW activity across the Northern Hemisphere.

Boreal Summer Negative Correlation Relationship Between Interannual SST and Precipitation Anomalies in the Tropical and Subtropical Western North Pacific

AbstractWestern North Pacific (WNP) is a key region for ENSO development and its effect on East Asian summer monsoon. Different from most other tropical oceanic areas, sea surface temperature (SST) and precipitation anomalies over the WNP are negatively correlated in boreal summer, indicating the ocean is mainly driven by the atmosphere rather than the opposite over there. Two interannual variability modes of such negative correlation relationship are revealed with SVD method, called tropical and subtropical WNP air‐sea coupled modes (WNP‐TCM and WNP‐STCM) in terms of their locations accordingly. Their core regions are both consistent with the near‐zero line of low‐level zonal wind, corresponding to the Northwest Pacific monsoon trough and the west ridge of WNP subtropical high, respectively. In WNP‐TCM, precipitation anomaly is caused by local vorticity anomaly, which is the result of Matsuno‐Gill response to SST anomalies with the reversed signs in central equatorial Pacific and eastern equatorial Indian Ocean. In WNP‐STCM, it is produced by local divergence anomaly, which is remotely forced by SST anomalies with the same signs in equatorial Pacific and Indian Ocean through anomalous Hadley circulation. SST anomalies in the two modes are both controlled by ocean thermal process, especially the solar radiation anomaly. The key factors determining such unique WNP air‐sea interaction are the weak latent heat flux and horizontal SST advection anomalies, which are both closely related to the near‐zero low‐level zonal wind. In addition, WNP‐TCM often happens in ENSO developing summer whereas WNP‐STCM usually appears in ENSO decaying summer.

Weak future Hadley cell intensity changes due to compensating effects of tropical and extratropical radiative forcing

Weak future Hadley cell intensity changes due to compensating effects of tropical and extratropical radiative forcing

The Effect of Explicit Convection on Climate Change in the West African Monsoon and Central West African Sahel Rainfall

AbstractThe West African monsoon (WAM) is the dominant feature of West African climate providing the majority of annual rainfall. Projections of future rainfall over the West African Sahel are deeply uncertain, with a key reason likely to be moist convection, which is typically parameterized in global climate models. Here, we use a pan-African convection-permitting simulation (CP4), alongside a parameterized convection simulation (P25), to determine the key processes that underpin the effect of explicit convection on the climate change of the central West African Sahel (12°–17°N, 8°W–2°E). In current climate, CP4 affects WAM processes on multiple scales compared to P25. There are differences in the diurnal cycles of rainfall, moisture convergence, and atmospheric humidity. There are upscale impacts: the WAM penetrates farther north, there is greater humidity over the northern Sahel and the Saharan heat low regions, the subtropical subsidence rate over the Sahara is weaker, and ascent within the tropical rain belt is deeper. Under climate change, the WAM shifts northward and Hadley circulation weakens in P25 and CP4. The differences between P25 and CP4 persist, however, underpinned by process differences at the diurnal scale and large scale. Mean rainfall increases 17.1% in CP4 compared to 6.7% in P25 and there is greater weakening in tropical ascent and subtropical subsidence in CP4. These findings show the limitations of parameterized convection and demonstrate the value that explicit convection simulations can provide to climate modelers and climate policy decision makers.

Weakening and Poleward Shifting of the North Pacific Subtropical Fronts from 1980 to 2018

Abstract Recent evidence shows that the North Pacific subtropical gyre and the Kuroshio Extension (KE) and Oyashio Extension (OE) fronts have moved poleward in the past few decades. However, changes of the North Pacific Subtropical Fronts (STFs), anchored by the North Pacific subtropical countercurrent in the southern subtropical gyre, remain to be quantified. By synthesizing observations, reanalysis, and eddy-resolving ocean hindcasts, we show that the STFs, especially their eastern part, weakened (20% ± 5%) and moved poleward (1.6° ± 0.4°) from 1980 to 2018. Changes of the STFs are modified by mode waters to the north. We find that the central mode water (CMW) (180°–160°W) shows most significant weakening (18% ± 7%) and poleward shifting (2.4° ± 0.9°) trends, while the eastern part of the subtropical mode water (STMW) (160°E–180°) has similar but moderate changes (10% ± 8%; 0.9° ± 0.4°). Trends of the western part of the STMW (140°–160°E) are not evident. The weakening and poleward shifting of mode waters and STFs are enhanced to the east and are mainly associated with changes of the northern deep mixed layers and outcrop lines—which have a growing northward shift as they elongate to the east. The eastern deep mixed layer shows the largest shallowing trend, where the subduction rate also decreases the most. The mixed layer and outcrop line changes are strongly coupled with the northward migration of the North Pacific subtropical gyre and the KE/OE jets as a result of the poleward expanded Hadley cell, indicating that the KE/OE fronts, mode waters, and STFs change as a whole system.

The Greater Mekong Subregion (GMS) and tropical expansion: A regional study of convection and precipitation

A series of data sets including brightness temperature from HIRS-UTWV, high cloud cover, precipitation and evaporation from ERA-Interim reanalysis data, and deep convective cloud from ISCCP were used to examine the latitudinal changes of the Hadley cell in the Greater Mekong Subregion (GMS) over the 1979–2018 period. In the study region, a pronounce poleward expansion was found during the winter season using three metrics of expansion: latitude of maximum brightness temperature associated with the anticyclonic center; latitude of zero crossing of precipitation minus evaporation, marking the extent of the tropical region; and latitude where high cloud cover reaches 50% of its maximum tropical value. However, no trends were discovered for the summer season. During the study period, there was an increase in equatorial convection as indicated by increased high cloud cover features. When precipitation minus evaporation was less than 0, there was diminishing dry zone evidence.

Modulation of Coupled Modes of Tibetan Plateau Heating and Indian Summer Monsoon on Summer Rainfall over Central Asia

Abstract It has been suggested that summer rainfall over central Asia (CA) is significantly correlated with the summer thermal distribution of the Tibetan Plateau (TP) and the Indian summer monsoon (ISM). However, relatively few studies have investigated their synergistic effects of different distribution. This study documents the significant correlations between precipitation in CA and the diabatic heating of TP and the ISM in summer based on the results of statistical analysis and numerical simulation. Summer precipitation in CA is dominated by two water vapor transport branches from the south that are related to the two primary modes of anomalous diabatic heating distribution contributed by the TP and ISM precipitation, that is, the “+−” dipole mode in the southeastern TP and the Indian subcontinent (IS), and the “+−+” tripole mode in the southeastern TP, the IS, and southern India. Both modes exhibit obvious midlatitude Silk Road pattern (SRP) wave trains with cyclone anomalies over CA, but with different transient and stationary eddies over South Asia. The different locations of anomalous anticyclones over India govern two water vapor transport branches to CA, which are from the Arabian Sea and the Bay of Bengal. The water vapor flux climbs while being transported northward and can be transported to CA with the cooperation of cyclonic circulation. The convergent water vapor and ascending motion caused by cyclonic anomalies favor the precipitation in CA. Further analysis corroborates the negative south Indian Ocean dipole in February could affect the tripole mode distribution of TP heating and ISM via the atmospheric circulation, water vapor transport, and anomalous Hadley cell circulation. The results indicate a reliable prediction reference for summer precipitation in CA.

Opposite responses of sea level variations to ENSO in the Northwestern Pacific: A transition latitude at 20°N

The interannual sea level variations in the Northwestern Pacific are significantly tied to the El Niño-Southern Oscillation (ENSO), with opposite responses and asymmetric strength to the south and north of 20°N. Using a 1.5-layer reduced gravity model, we show that the spatial pattern of sea level interannual variations is not sensitive to the existence of the Luzon Strait located at around 20°N, but is dominated by the basin-scale wind stress curl associated with ENSO. Dynamically, sea level variations tied to ENSO are attributed to long baroclinic Rossby wave adjustment driven by wind stress curl in the western/central Pacific, as previous studies have indicated. However, the responses of wind stress curl to ENSO are dynamically different to the south and north of 20°N. In the tropics, the wind stress curl anomaly results from the ENSO event directly, through the Matsuno-Gill response with the transition of zonal wind anomaly at approximately 20°N, while in the subtropics, the wind stress curl anomaly is mainly set up by atmospheric vertical motions through the Hadley Cell.

Coriolis effect recorded in Late Pleistocene Marine Isotope Stage 5e Bahamian aeolianites

Abstract The windward islands of the Lucayan Archipelago (Bahamas) form an Atlantic Ocean–facing transect spanning >950 km in length and 6° of latitude. The islands' topography is largely constructed from carbonate wind-blown dunes (i.e., aeolianites) deposited during the interglacial phases of the Late Pleistocene and Holocene. New digital elevation data from satellite radar interferometry (TanDEM-X German Earth observation satellite) enables a step change in the ability to map and quantify Bahamian aeolian landforms across the archipelago. A semi-automated mapping approach that leverages object-based image analysis yields a total aeolianite area of ~1674 km2 across Great Abaco, Eleuthera, Cat, San Salvador, Long, Crooked, Acklins, and Mayaguana islands (Bahamas) and the Turks and Caicos Islands. Longitudinal axis measurements from 747 Pleistocene parabolic dunes record increasing consistency of east-west orientation with decreasing latitude. Three U.S. National Data Buoy Center data buoys provided modern wind direction and velocity measurements (n = 730,933 of each) along this transect. Analysis of wind vectors (>P90 [90th percentile], n = 70,095) demonstrates increasing organization of easterlies at southern latitudes and an offset in directionality compared to formational winds of Pleistocene Marine Isotope Stage (MIS) 5e deposits. Southward trends of increasing wind strength and consistency reflect geostrophic flow driven by atmospheric circulation within the Hadley cell and right-hand deflection of the Coriolis effect in the Northern Hemisphere. We propose that the offset in directionality between dune axes and modern wind vectors is related to changes in latitudinal width of the Hadley cell from the Late Pleistocene (MIS 5e) to today. This data set is robust enough to serve as a benchmark against which future atmospheric circulation models can be compared.

Comprehensive assessment of RegCM4 towards interannual variability of Indian Summer Monsoon using multi-year simulations

In this study, the interannual variability (IAV) of Indian Summer Monsoon (ISM) is investigated using multi-year (1982‒2016) seasonal scale simulations (May‒September) of the regional climate model RegCM4. Model-simulated fields such as surface temperature, wind and rainfall are validated initially to testify the climatological behaviour of ISM. Subsequently, different aspects of IAV associated with ISM are discussed primarily focusing on model simulated rainfall and are verified against high-resolution rainfall analysis from India Meteorological Department (IMD). Analysis indicated that RegCM4 shows reasonable accuracy in simulating major large-scale features, however, has cold bias over entire India and wet (dry) bias over northwest and peninsular (central) India. Easterly (westerly) bias is noticed in the model simulated low (upper) level wind that affects regional Hadley circulation. The cold bias is found to be associated with the feedback cycle of land–atmosphere interaction. Surface evaporative cooling likely affects the static instability in the atmospheric column, thereby limiting the convection and thus reducing rainfall. While categorizing, it is noticed that the efficacy of the model is found to be better in simulating normal monsoon as compared to contrasting monsoon (deficit and excess) year, thereby reducing the simulation skill for the entire period. EOF analysis revealed that first two leading modes of IMD rainfall are linked with large-scale variabilities, viz. El-Nino Southern Oscillation and Indian Ocean Dipole, respectively, but RegCM4 could not well reproduce these relationships. The spectral analysis showed 2–7 year periodicity in the model. However, the associated spectral peaks are close to the red noise spectrum due to their weak power suggesting limited model skill to capture large-scale variability. Overall, this study advocates that the RegCM4 could capture the climatological features of ISM fairly well, but needs further improvement in representing the IAV more accurately.

Depositional timing and palaeoclimate interpretation of the Tamala Limestone aeolianites in Shark Bay, Western Australia

• AAR dating shows an aging of the Tamala Limestone aeolianites towards the east. • The Tamala Limestone aeolianites are landward-advancing (i.e., transverse) dunes that have migrated from west to east. • Dune accumulation occurred during dry/cold glacials of the Pleistocene period. • Dune activity in the region is strongly linked to a weak Leeuwin Current. • Dune accumulation occurred during a northward shift of both the Hadley and Ferrel cells. The carbonate units grouped under the name “Tamala Limestone” outcrop for a thousand kilometres along the coast of Western Australia. The extensive Zuytdorp Cliffs shaping the northern half of the coastline up to Shark Bay expose and offer an exceptional access to the stratigraphy of this formation. The regional survey of the Shark Bay region, which combines both stratigraphic and sedimentological analyses, reveals that the Tamala Limestone is a dry accumulating aeolian system composed of large transverse dunes migrating parallel to the prevailing winds. Accordingly, the amino acid-data show an aging of the units towards the east. Episodes of carbonate aeolian sedimentation correlate with the successive glacial intervals of the Pleistocene whilst paleosols are correlated with breaks in the sedimentation during interglacial intervals. Palaeoclimate reconstructions reveal that sea level and sea surface temperature of the Indo-Pacific Warm Pool were lower during glacial intervals. The weakened Leeuwin Current, which flows along the western coast of Australia and is the main source of precipitation, contributed to the aridification of the region. Consequently, and associated with a northward migration of the Hadley and Ferrel cells, periods of glaciation were drier. By contrast, paleosols developed through dissolution of the carbonate units during more humid interglacial intervals.

Correlation between the latitudinal profile of the 7Be air concentration and the Hadley cell extent in the Southern Hemisphere

The cosmogenic radionuclide 7Be is one of the best tracers for aerosol transport since its half-life of 53 days is in the time scale of many atmospheric circulation phenomena. In this work, we analyze a 12-years-long daily time-series for the airborne 7Be concentration for nine air filtering stations in the Southern Hemisphere or close to it. The observed latitudinal distribution of 7Be concentration, with its maximum at the southern subtropical high-pressure belt, is similar to the one in the Northern Hemisphere. A good time correlation was found between the 7°-shift of the 7Be concentration latitudinal distribution and the seasonal displacement of the extent of the Hadley cell. This is consistent with tropopause folding events, mostly occurring in spring, being the main contribution for the injection of stratospheric 7Be into the descending branch of the Hadley cell.

Synergistic effect of El Ni\xf1o and the North Pacific Oscillation on wintertime precipitation over Southeastern China and the East China Sea Kuroshio area

Wintertime precipitation in China is most pronounced over the southeastern area, and the Kuroshio in the East China Sea anchors a prominent precipitation band over the warm side of the sea surface temperature front. Previous studies have suggested that many factors contribute to the interannual variation of the precipitation over southeastern China (SC), whereas less attention has been paid to precipitation variability over the East China Sea Kuroshio (ECSK) area. This study focuses on the interannual variation of wintertime precipitation over the SC and ECSK areas. Empirical orthogonal function analysis reveals a spatially uniform pattern from SC to the ECSK area. Composite analysis shows that an El Niño event intensifies wintertime precipitation over our target region, and this effect is tripled when an El Niño follows a positive North Pacific Oscillation (NPO) event in the previous winter. The positive NPO event in the previous winter intensifies the El Niño event via the Victoria mode ocean bridge and the subsequent Bjerknes feedback. In comparison with single-factor El Niño events, a much weaker Walker cell induced by the joint event induces a much weaker regional Hadley cell through anomalous descending motion over the western tropical Pacific. The weakened regional Hadley circulation over the western Pacific directly enhances the precipitation over the SC and ECSK area. In this study, the synergistic effect of an El Niño event and a positive NPO event indicates that the influence of the El Niño event can be amplified by the positive NPO event in the previous winter.

Contributions of regional CO2 forcings to Hadley cell intensity

Contributions of regional CO2 forcings to Hadley cell intensity

Decadal variation of the precipitation relationship between June and August over South China and its mechanism

This paper investigates the causes for the interdecadal change in the relationships between early and late summer precipitation over South China (SC). It is found that the correlations of the precipitation over SC between June and August shift from weak positive in 1979–1995 to significantly negative in 1996–2019. Further analysis demonstrates that the distinct evolution of sea surface temperature (SST) pattern between the two periods accounts for the interdecadal variations of their relationships. Although the warming of the tropical western Indian Ocean (WIO) in June favors increasing precipitation over the SC via enhancing the Northwest Pacific Subtropical High (NWPSH) during the whole period, the associated SST anomalies in August are rather different between the two periods. Specifically, the WIO warming in June corresponds to slightly positive anomalies over the tropical central-eastern Pacific in August during the 1979–1995, which has weak impact on the NWPSH and results in a weak precipitation correlation between June and August. However, the WIO warming in June corresponds to La Niña’s rapid development in August during the 1996–2019, which favors the enhancement of the NWPSH via increasing the regional Hadley circulation. Due to the climatologically northward movement of NWPSH from June to August, the enhanced NWPSH in August acts to decrease the precipitation over SC, causing a significantly negative correlation between precipitation in June and August. Overall, the distinct evolution of tropical SST pattern is the key factor inducing the change of the relationships between June and August precipitation in the two periods.