RAPSODI: radiosonde atmospheric profiles from ship and island platforms during ORCESTRA, collected to Decipher the ITCZ

This study examines the large‐scale factors that govern global tropical cyclone (TC) formation and an upper bound on the annual number of TCs. Using idealized simulations for an aquaplanet tropical channel, it is shown that the tropical atmosphere has a maximum capacity in generating TCs, even under ideal environmental conditions. Regardless of how favorable the tropical environment is, the total number of TCs generated in the tropical channel possesses a consistent cap across experiments. Analyses of daily TC genesis events reveal further that global TC formation is intermittent throughout the year in a series of episodes at a roughly 2‐week frequency, with a cap of 8–10 genesis events per day. Examination of different large‐scale environmental factors shows that 600‐hPa moisture content, 850‐hPa absolute vorticity, and vertical wind shear are the most critical factors for this global episodic TC formation. Specifically, both the 850‐hPa absolute vorticity and the 600‐hPa moisture are relatively higher at the onset of TC formation episodes. Once TCs form and move to poleward, the total moisture content and the absolute vorticity in the main genesis region subside, thus reducing large‐scale instability and producing an unfavorable environment for TCs to form. It takes ∼ 2 weeks for the tropical atmosphere to remoisten and rebuild the large‐scale instability associated with the Intertropical Convergence Zone before a new TC formation episode can occur. These results offer new insight into the processes that control the upper bound on the global number of TCs in the range of 80–100 annually.

Clouds over the ocean, particularly throughout the tropics, are poorly understood and drive much of the uncertainty in model-based projections of climate change. In early 2010, the Max Planck Institute for Meteorology and the Caribbean Institute for Meteorology and Hydrology established the Barbados Cloud Observatory (BCO) on the windward edge of Barbados. At 13°N the BCO samples the seasonal migration of the intertropical convergence zone (ITCZ), from the well-developed winter trades dominated by shallow cumulus to the transition to deep convection as the ITCZ migrates northward during boreal summer. The BCO is also well situated to observe the remote meteorological impact of Saharan dust and biomass burning. In its first six years of operation, and through complementary intensive observing periods using the German High Altitude and Long Range Research Aircraft (HALO), the BCO has become a cornerstone of efforts to understand the relationship between cloudiness, circulation, and climate change.

Compared to the eastern Pacific and eastern Atlantic, tropical cyclogenesis associated with breakdown of the intertropical convergence zone (ITCZ) was less documented over the western North Pacific (WNP). A typical case in boreal summer 2006 over the WNP, in which tropical cyclogenesis is induced by ITCZ breakdown in association with synoptic‐scale wave train (SWT), is examined through observation analysis and numerical simulations. Observational analysis displays that a northwest–southeast‐oriented SWT developed and propagated northwestward several days prior to ITCZ breakdown. Furthermore, the comparisons of simulation results reveal the role of the SWT in ITCZ breakdown. On one hand, the SWT within the ITCZ region is conducive to break down the ITCZ by enhancing the potential vorticity (PV), strengthening the meridional PV gradient and producing an evident sign reversal of PV gradient. On the other hand, the anomalous anticyclonic circulation related to the SWT north of ITCZ can reduce the meridional scale of PV band and thus increase the meridional PV gradient to accelerate ITCZ breakdown.

Although tropical cyclogenesis occurs over all tropical warm ocean basins, the eastern Pacific appears to have the highest frequency of tropical cyclogenesis events per unit area. In this study, tropical cyclogenesis from merging mesoscale convective vortices (MCVs) associated with breakdowns of the intertropical convergence zone (ITCZ) is examined. This is achieved through a case study of the processes leading to the genesis of Tropical Storm Eugene (2005) over the eastern Pacific using the National Centers for Environmental Prediction reanalysis, satellite data, and 4-day multinested cloud-resolving simulations with the Weather Research and Forecast (WRF) model at the finest grid size of 1.33 km. Observational analyses reveal the initiations of two MCVs on the eastern ends of the ITCZ breakdowns that occurred more than 2 days and 1000 km apart. The WRF model reproduces their different movements, intensity and size changes, and vortex–vortex interaction at nearly the right timing and location at 39 h into the integration as well as the subsequent track and intensity of the merger in association with the poleward rollup of the ITCZ. Model results show that the two MCVs are merged in a coalescence and capture mode due to their different larger-scale steering flows and sizes. As the two MCVs are being merged, the low- to midlevel potential vorticity and tangential flows increase substantially; the latter occurs more rapidly in the lower troposphere, helping initiate the wind-induced surface heat exchange process leading to the genesis of Eugene with a diameter of 400 km. Subsequently, the merger moves poleward with characters of both MCVs. The simulated tropical storm exhibits many features that are similar to a hurricane, including the warm-cored “eye” and the rotating “eyewall.” It is also shown that vertical shear associated with a midlevel easterly jet leads to the downshear tilt and the wavenumber-1 rainfall structures during the genesis stage, and the upshear generation of moist downdrafts in the vicinity of the eyewall in the minimum equivalent potential temperature layer. Based on the above results, it is concluded that the ITCZ provides a favorable environment with dynamical instability, high humidity, and background vorticity, but it is the merger of the two MCVs that is critical for the genesis of Eugene. The storm decays as it moves northwestward into an environment with increasing vertical shear, dry intrusion, and colder sea surface temperatures. The results appear to have important implications for the high frequency of development of tropical cyclones in the eastern Pacific.

An objective method for the identification of the intertropical convergence zone (ITCZ) in gridded numerical weather prediction datasets is presented. This technique uses layer- and time-averaged winds in the lower troposphere to automatically detect the location of the ITCZ and is designed for use with datasets including operational forecasts and climate model output. The method is used to create a climatology of ITCZ properties from the Interim ECMWF Re-Analysis (ERA-Interim) dataset for the period 1979–2009 to serve as an indicator of the technique's ability and a benchmark for future comparisons. The automatically generated objective climatology closely matches the results from subjective studies, showing a seasonal cycle in which the oceanic ITCZ migrates meridionally and the land-based ITCZ features are predominantly summertime phenomena. Composites based on the phase of the El Niño–Southern Oscillation index show a major shift in the mean position and changes in intensity of the ITCZ in all ocean basins as the index varies. Under La Niña conditions, the ITCZ intensifies over the Maritime Continent and eastern Pacific, where the ITCZ weakens over the central and equatorial eastern Pacific. An analysis of changes in the ITCZ and its divergence during the period 1979–2009 indicates that the mean position of the ITCZ shifts southward in the western Pacific and a broad global intensification of the convergence into ITCZ regions. The relationship between tropical cyclogenesis and the ITCZ is also examined, finding that more than 50% of all tropical cyclones form within 600 km of the ITCZ.

Abstract. Shallow clouds in the trade-wind region over the North Atlantic contribute substantially to the global radiative budget. In the vicinity of the Caribbean island of Barbados, they appear in different mesoscale organization patterns with distinct net cloud radiative effects (CREs). Cloud formation processes in this region are typically controlled by the prevailing large-scale subsidence. However, occasionally weather systems from remote origin cause significant disturbances. This study investigates the complex cloud–circulation interactions during the field campaign EUREC4A (Elucidate the Couplings Between Clouds, Convection and Circulation) from 16 January to 20 February 2020, using a combination of Eulerian and Lagrangian diagnostics. Based on observations and ERA5 reanalyses, we identify the relevant processes and characterize the formation pathways of two moist anomalies above the Barbados Cloud Observatory (BCO), one in the lower troposphere (∼ 1000–650 hPa) and one in the middle troposphere (∼ 650–300 hPa). These moist anomalies are associated with strongly negative CRE values and with contrasting long-range transport processes from the extratropics and the tropics, respectively. The first case study about the low-level moist anomaly is characterized by an unusually thick cloud layer, high precipitation totals, and a strongly negative CRE. The formation of the low-level moist anomaly is connected to an extratropical dry intrusion (EDI) that interacts with a trailing cold front. A quasi-climatological (2010-2020) analysis reveals that EDIs lead to different conditions at the BCO depending on how they interact with the associated trailing cold front. Based on this climatology, we discuss the relevance of the strong large-scale forcing by EDIs for the low-cloud patterns near the BCO and the related CRE. The second case study about the mid-tropospheric moist anomaly is associated with an extended and persistent mixed-phase shelf cloud and the lowest daily CRE value observed during the campaign. The formation of the mid-level moist anomaly is linked to “tropical mid-level detrainment” (TMD), which refers to detrainment from tropical deep convection near the melting layer. The quasi-climatological analysis shows that TMDs consistently lead to mid-tropospheric moist anomalies over the BCO and that the detrainment height controls the magnitude of the anomaly. However, no systematic relationship was found between the amplitude of this mid-tropospheric moist anomaly and the CRE at the BCO. This is most likely due to the modulation of the CRE by above and below lying clouds and the fact that we used daily mean CREs, thereby ignoring the impact of the timing of the synoptic anomaly with respect to the daily cycle. Overall, this study reveals the important impact of the long-range moisture transport, driven by dynamical processes either in the extratropics or the tropics, on the variability of the vertical structure of moisture and clouds, and on the resulting CRE in the North Atlantic winter trades.

Recent advances in research on tropical cyclogenesis

Statistical evidence is presented to support the notion that tropical convection in the eastern Pacific and Atlantic intertropical convergence zone (ITCZ) during northern winter can be forced by disturbances originating in the extratropics. The synoptic-scale transients in these regions are characterized at upper levels by strong positive tilts in the horizontal and appear to induce vertical motions ahead of troughs as in midlatitude baroclinic systems. Two case studies of such interactions are examined, one for the eastern North Pacific ITCZ and another somewhat different type of interaction for the South Pacific convergence zone (SPCZ) over the western South Pacific. Both cases are associated with upper-level troughs, strong cold advection deep into the tropics, and the formation of a frontal boundary at low levels. The ITCZ case is characterized by the advection of anomalously high isentropic potential vorticity air southward, a strong poleward flux of heat and westerly momentum, and the devel...

Aerosols are one of the key factors influencing the hydrological cycle and radiation balance of the climate system. Although most aerosols deposit near their sources, the induced cooling effect is on a global scale and can influence the tropical atmosphere through slow processes, such as air–sea interactions. This study analyzes several simulations of fully coupled atmosphere–ocean climate models under the influence of anthropogenic aerosols, with the concentrations of greenhouse gases kept constant. In the cooling simulations, precipitation is reduced in deep convective areas but increased around the edges of convective areas, which is opposite to the “rich-get-richer” phenomenon in global warming scenarios in the first-order approximation. Tropical convection is intensified with a shallower depth, and tropical circulations are enhanced. The anomalous gross moist stability (M′) mechanism and the upped-ante mechanism can be used to explain the dynamic and thermodynamic processes in the changes in tropical precipitation and convection. There is a northward cross-equatorial energy transport due to the cooler Northern Hemisphere in most of the simulations, together with the southward shift of the intertropical convergence zone (ITCZ) and the enhancement of the Hadley circulation. The enhancement of the Hadley circulation is more consistent between models than the changes of the Walker circulation. The change in the Hadley circulation is not as negligible as in the warming cases in previous studies, which supports the consistency of the ITCZ shift in cooling simulations.

Here we investigate tropical cyclogenesis in warm climates, focusing on the effect of reduced equator-to-pole temperature gradient relevant to past equable climates and, potentially, to future climate change. Using a cloud-system resolving model that explicitly represents moist convection, we conduct idealized experiments on a zonally periodic equatorial β-plane stretching from nearly pole-to-pole and covering roughly one-fifth of Earth’s circumference. To improve the representation of tropical cyclogenesis and mean climate at a horizontal resolution that would otherwise be too coarse for a cloud-system resolving model (15 km), we use the hypohydrostatic rescaling of the equations of motion, also called reduced acceleration in the vertical. The simulations simultaneously represent the Hadley circulation and the intertropical convergence zone, baroclinic waves in mid-latitudes, and a realistic distribution of tropical cyclones (TCs), all without use of a convective parameterization. Using this model, we study the dependence of TCs on the meridional sea surface temperature gradient. When this gradient is significantly reduced, we find a substantial increase in the number of TCs, including a several-fold increase in the strongest storms of Saffir–Simpson categories 4 and 5. This increase occurs as the mid-latitudes become a new active region of TC formation and growth. When the climate warms we also see convergence between the physical properties and genesis locations of tropical and warm-core extra-tropical cyclones. While end-members of these types of storms remain very distinct, a large distribution of cyclones forming in the subtropics and mid-latitudes share properties of the two.

In this dissertation we research two aspects of the mountain gap wind northerlies of the Isthmus of Tehuantepec in Southern Mexico. The causes of extreme gap wind northerlies, emphasizing the causes by tropical weather systems, like easterly waves troughs or hurricanes, and the impacts of an extreme gap wind events on tropical cyclogenesis.Through the use of an index to diagnose the mountain gap wind strength, its evolution through the year and the mechanisms leading to extreme mountain gap winds was highlighted. The index revealed that the gap wind is strongly related to high surface pressure anomalies over the GoM throughout the year. We also observed that between June and August (summer), low surface pressure anomalies over the EPac also impact the gap wind, albeit to a lesser degree. Also, we detected that the gap wind has a significant diurnal cycle in summer. The index diagnosing the mountain gap also allows the identification of the extreme gap northerlies. We found that extreme northerlies are strongest between November and January (winter) and weaken between June and August (summer). Nevertheless, throughout the year, most of the extreme gap northerlies were associated with fluctuations with periods between 2 and 10 days and a surface pressure gradient across the mountain gap established in association with high surface pressures over the Gulf of Mexico. The high pressures in the Gulf of Mexico were related to surface ridges of midlatitude origin. In summer, when the midlatitude ridges retreat poleward and their pressure gradient diminishes over the Gulf of Mexico, other factors may trigger extreme gap northerlies. These factors consist of easterly wave troughs and tropical cyclones. When these tropical systems are located over the EPac, their low surface pressure establishes a pressure gradient that enhances the gap northerlies. When these systems are located over the GoM and their circulations are strong, their associated northerlies may enhance the gap northerlies as well. Additionally, we found that the gap outflow in the Eastern Pacific merges with the circulation of the easterly wave and the circulation intensifies. The case study of hurricane Patricia (2015) is used to explore the potential impacts of the gap northerlies on tropical cyclogenesis. Hurricane Patricia was formed from a low-level vortex, at around 950 hPa, underneath an easterly wave trough over the Eastern Pacific on 0600 20th of October, 2015. Leading up to cyclogenesis, three distinctive features were involved: an EW trough, the gap outflow vorticity and the Intertropical Convergence Zone (ITCZ) vorticity anomalies. The latter two were more evident at low levels. The EW trough stalled over the Eastern Pacific and the Yucatan peninsula for two days from the 18th until cyclogenesis occurred on the 20th of October. While stalled, the trough southerlies advected ITCZ vorticity anomalies and moisture underneath the trough. At the same time the trough northerlies as well as a midlatitude ridge enhanced the gap wind. Eventually, the two low-level vorticity anomalies merged underneath the trough and formed a vortex which leads to cyclogenesis. Once the vortex was formed, the ITCZ vorticity anomalies kept merging with the vortex for about 48 hours, between 20th and 22nd. These vorticity anomalies were associated with convection in the trough, also released latent heat by condensation helping to intensify the vortex. The vortex achieved hurricane strength winds on the 22nd of October. and kept intensifying for the next two days. Through a numerical experiment consisting of filling the mountain gap of the Isthmus of Tehuantepec the role of the gap outflow on the cyclogenesis of hurricane Patricia was studied. The experiment revealed that when the mountain gap is filled, the gap wind is suppressed, the vorticity underneath the EW trough is weaker, and cyclogenesis does not occur. We found that the gap outflow vorticity helps to form a low-level vortex underneath the trough. The main consequence of the absence of the gap wind and its outflow is that the vorticity in the EPac at low levels is decreased and only the ITCZ vorticity anomalies remain in the EPac. In this situation, the ITCZ vorticity anomalies remain scattered over a broad vortex underneath the trough failing to merge around a sole vorticity maximum characteristic of TC vortices. Thus, the additional vorticity associated with the gap outflow contributes to establish a vorticity maximum for the ITCZ vorticity anomalies to merge around, leading to a developing vortex. Therefore, the vorticity of the gap outflow acts as a catalyst to trigger cyclogenesis for hurricane Patricia.

The role of the Madden–Julian oscillation (MJO) in modulating the frequency and location of tropical cyclogenesis over the eastern Pacific and the Gulf of Mexico during August–September 1998 is examined. During the nonconvective phase of the MJO, convection and low-level cyclonic vorticity occurred primarily in conjunction with the intertropical convergence zone (ITCZ). During the convective phase, convection, low-level cyclonic vorticity, and convergence expanded into the northeastern Pacific and the Gulf of Mexico. This was accompanied by enhanced eddy kinetic energy and barotropic energy conversions as compared to the nonconvective phase, consistent with previous research. During the nonconvective phase of the MJO, vertical shear was relatively weaker but tropical cyclones tended to form mainly within the ITCZ. On the contrary, during the convective phase, vertical wind shear exceeded 10 m s−1 over much of this region and tropical cyclone development occurred north of the ITCZ, near the Mexican Pacific coast and the Gulf of Mexico. Idealized numerical experiments are conducted using a barotropic model with time-invariant basic states representative of the nonconvective and convective phases of the MJO. The simulations indicate that the propagation paths as well as the amplification of the eddies differ substantially between the two phases. During the nonconvective phase, the waves tend to propagate westward into the eastern Pacific. During the convective phase, stronger southerlies steer the waves into the Gulf of Mexico. The MJO-related modulation of tropical cyclogenesis in the eastern Pacific and Gulf of Mexico thus appears to involve anomalous convergence, cyclonic vorticity, vertical wind shear, and differing tracks of easterly waves.

Tropical cyclogenesis (TCG) is a multiscale process that involves interactions between large-scale circulation and small-scale convection. A near-global aquaplanet cloud-resolving model (NGAqua) with 4-km horizontal grid spacing that produces tropical cyclones (TCs) is used to investigate TCG and its predictability. This study analyzes an ensemble of three 20-day NGAqua simulations, with initial white-noise perturbations of low-level humidity. TCs develop spontaneously from the northern edge of the intertropical convergence zone (ITCZ), where large-scale flows and tropical convection provide necessary conditions for barotropic instability. Zonal bands of positive low-level absolute vorticity organize into cyclonic vortices, some of which develop into TCs. A new algorithm is developed to track the cyclonic vortices. A vortex-following framework analysis of the low-level vorticity budget shows that vertical stretching of absolute vorticity due to convective heating contributes positively to the vorticity spinup of the TCs. A case study and composite analyses suggest that sufficient humidity is key for convective development. TCG in these three NGAqua simulations undergoes the same series of interactions. The locations of cyclonic vortices are broadly predetermined by planetary-scale circulation and humidity patterns associated with ITCZ breakdown, which are predictable up to 10 days. Whether and when the cyclonic vortices become TCs depend on the somewhat more random feedback between convection and vorticity.

In this study, a wave-following Lagrangian framework was used to examine the evolution of tropical easterly wave structure, circulation, and convection in the days leading up to and including tropical cyclogenesis in the Atlantic and east Pacific basins. After easterly waves were separated into northerly, southerly, trough, and ridge phases using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis 700-hPa meridional wind, waves that developed a tropical cyclone [developing waves (DWs)] and waves that never developed a cyclone [nondeveloping waves (NDWs)] were identified. Day zero (D0) was defined as the day on which a tropical depression was identified for DWs or the day the waves achieved maximum 850-hPa vorticity for NDWs. Both waves types were then traced from five days prior to D0 (D − 5) through one day after D0. Results suggest that as genesis is approached for DWs, the coverage by convection and cold cloudiness (e.g., fractional coverage by infrared brightness temperatures ≤240 K) increases, while convective intensity (e.g., lightning flash rate) decreases. Therefore, the coverage by convection appears to be more important than the intensity of convection for tropical cyclogenesis. In contrast, convective coverage and intensity both increase from D − 5 to D0 for NDWs. Compared to NDWs, DWs are associated with significantly greater coverage by cold cloudiness, large-scale moisture throughout a deep layer, and large-scale, upper-level (~200 hPa) divergence, especially within the trough and southerly phases, suggesting that these parameters are most important for cyclogenesis and for distinguishing DWs from NDWs.

The role of Sumatra and adjacent topographic features in tropical cyclone (TC) formation over the Indian Ocean (IO) is investigated. Sumatra, as well as the Malay Peninsula and Java, have mountainous terrain that partially blocks low-level flow under typical environmental stratification. For easterly low-level flow, these terrain features often produce lee vortices, some of which subsequently shed and move westward from the northern and southern tips of Sumatra and thence downstream over the IO. Since Sumatra straddles the equator, extending in a northwest–southeast direction from approximately 6°N to 6°S, the lee vortices, while counter-rotating, are both cyclonic. Hence, they can serve as initial disturbances that eventually contribute to TC formation over the IO. In addition, low-level, equatorial westerly flow impinging on Sumatra is also typically blocked and diverges, at times contributing to cyclonic circulations over the IO, primarily near the southern end of the island.Data from two recent tropical campaigns, the 2008–10 Year of Tropical Convection (YOTC) and the 2011 Dynamics of the Madden–Julian Oscillation (DYNAMO), are used to study these phenomena. These datasets reveal the frequent occurrence of shed and nonshed terrain-induced cyclonic circulations over the IO, the majority of which occur during boreal fall and winter. During the 2.5 yr of the two campaigns, 13 wake vortices (13% of the shed circulations identified) were tracked and observed to subsequently develop into TCs over the northern and southern IO, accounting for 25% of the total TCs forming in the IO during that period.