Pacific Intertropical Convergence Zone Research Articles

Proposal and application of a convective wind gustiness parameterization based on convective available potential energy

Wind gustiness is an effective approach for addressing mesoscale enhancement in estimating air-sea interface fluxes. In this study, an improved convective gustiness parameterization scheme based on convective available potential energy (CAPE) is formulated utilizing reanalysis data and wind observations from moored buoys. This new parameterization was examined by implementing it in the air-sea flux transfer scheme within the National Center for Atmospheric Research Community Earth System Model (NCAR CESM) version 2.1.3 and contrasting it with a previously proposed scheme based on convective precipitation rate. The results indicate a significant reduction in negative biases of surface winds and latent heat fluxes over the tropical Indian Ocean and western Pacific during summer with the implementation of either gustiness scheme, resulting in an average increase in latent heat flux of approximately 9 W·m− 2. Specifically, the CAPE-based scheme shows a more favorable impact in the Indo-Pacific Warm Pool region, whereas the precipitation-based scheme exhibits inferior performance in the western Pacific Intertropical Convergence Zone. Further analysis reveals that the inclusion of the gustiness scheme distinctly alters surface evaporation and latent heat flux, influencing vertical motion in the area with active convective activity and effectively improving precipitation simulations by up to 18%. Moreover, the CAPE-based scheme reduces the bias in summer precipitation simulations by approximately 5% more than the precipitation-based scheme.

The Relationship between Convectively Coupled Waves and the East Pacific ITCZ

Abstract Longstanding climate model biases in tropical precipitation exist over the east Pacific (EP) Ocean, especially during boreal winter and spring when models have excessive Southern Hemisphere (SH) precipitation near the intertropical convergence zone (ITCZ). In this study, we document the impact of convectively coupled waves (CCWs) on EP precipitation and the ITCZ using observations and reanalyses. We focus on the months when SH precipitation peaks in observations: February–April (FMA). CCWs explain 93% of total precipitation variance in the SH, nearly double the percent (48%) of the NH during FMA. However, we note that these percentages are inflated as they inevitably include the background variance. We further investigate the three leading high-frequency wave bands: mixed Rossby–gravity waves and tropical depression–type disturbances (MRG–TD type), Kelvin waves, and n = 0 eastward inertia–gravity waves (IG0). Compared to their warm pool counterparts, these three CCWs have a more zonally elongated and meridionally narrower precipitation structure with circulations that resemble past observational studies and/or shallow water theory. We quantify the contribution of all CCWs to four different daily ITCZ “states”: Northern Hemisphere (NH) (nITCZ), SH (sITCZ), double (dITCZ), and equatorial (eITCZ) using a new precipitation-based ITCZ-state algorithm. We find that the percent of total precipitation variance explained by each of the CCWs is heightened for sITCZs and eITCZs and diminished for nITCZs. Last, we find that nITCZs are most prevalent weeks after strong CCW activity happens in the NH, whereas CCWs and sITCZs peak simultaneously in the SH. Significance Statement Convectively coupled atmospheric waves (CCWs) are a critical feature of tropical weather and are an important source of precipitation near the region of highest precipitation on Earth called the intertropical convergence zone (ITCZ). Given three decades of climate model biases in CCWs and ITCZ precipitation over the east Pacific (EP) Ocean during spring, few studies have examined the relationship between CCWs and the springtime EP ITCZ. We explored the CCWs and EP ITCZ relationship through calculations of the percent of precipitation that comes from CCWs. A significant portion of the tropical precipitation is associated with CCWs during spring. CCWs are even more impactful when the ITCZ is in the SH or on the equator, which are both problematic in climate models.

Dynamical Importance of the Trade Wind Inversion in Suppressing the Southeast Pacific ITCZ

AbstractSea surface temperature (SST) gradients are a primary driver of low‐level wind convergence in the east Pacific Inter‐Tropical Convergence Zone (ITCZ) through their hydrostatic relationship to the surface pressure gradient force (PGF). However, the surface PGF may not always align with SST gradients due to variations in boundary layer temperature gradients with height, that is, the boundary layer contribution to the surface PGF. In this study, we investigate the observed northern hemisphere position of the east Pacific ITCZ using a slab boundary layer model (SBLM) driven by different approximations of the boundary layer virtual temperature field. SBLM simulations using the entire boundary layer virtual temperature profile produce a realistic northern hemisphere ITCZ. However, SST‐only simulations produce excessive equatorial divergence and southern hemisphere convergence, resulting in a latitudinally confined double ITCZ‐like structure. Observed virtual temperature gradients highlight the importance of northward temperature gradients strengthening with height from the equator to 15°S below the trade wind inversion (TWI). Our interpretation is that the equatorial cold tongue induces relatively weak high surface pressure and double ITCZ‐like convergence because the resulting layer of cold air is shallow. Concurrently, relatively strong high surface pressure spreads out in the southern hemisphere due to interactions between stratocumulus clouds and the ocean surface. Together, the equatorial cold tongue and the TWI/stratocumulus clouds enable a more northern hemisphere dominant ITCZ. Thus, we provide evidence of a dynamical link between the equatorial cold tongue, low clouds, and double ITCZs, which continue to be problematic in Earth system models.

Response of the Central Pacific Intertropical Convergence Zone to Northern Hemisphere Cooling During the Last Glacial Maximum and Heinrich Stadial 1

AbstractThe latitudinal position of the Intertropical Convergence Zone (ITCZ) reflects the energy imbalance between the hemispheres. Southward displacements of the ITCZ during the Last Glacial Maximum (LGM; 19–26.5 ka) and Heinrich Stadial 1 (HS1; 14.6–17.5 ka), are widely accepted, but their magnitude is controversial. Geochemistry of detrital fractions in down‐core sediments collected from 6°N to equator along the 131.5°W transect reveal a distinct shift in εNd, La/Yb, and La–Sc–Th composition from predominantly northern hemisphere‐sourced to mixed northern and southern hemisphere‐sourced signal at 3°N–4°N during the LGM and 3°N–6°N during HS1. These contrasting provenance signals point to the past ITCZ functioning as a dust barrier. Given that a comparable geochemical demarcation currently occurs at 6°N–7°N, our data suggest that the ITCZ migrated southward by ∼3° during the LGM and ∼1°–3° during HS1 relative to its modern position in the central Pacific.

On the Role of Wind–Evaporation–SST Feedbacks in the Subseasonal Variability of the East Pacific ITCZ

Abstract The intertropical convergence zone (ITCZ) is a zonally elongated band of near-surface convergence and precipitation near the equator. During boreal spring, the eastern Pacific ITCZ migrates latitudinally on daily to subseasonal time scales, and climate models exhibit the greatest ITCZ biases during this time of the year. In this work, we investigate the air–sea interactions associated with the variability in the eastern Pacific ITCZ’s latitudinal location for consecutive days when the ITCZ is only located north of the equator (nITCZ events) compared to when the ITCZ is on both sides of the equator or south of the equator (dsITCZ events) during February–April. The distribution of sea surface temperature (SST) anomalies and surface latent heat flux (SLHF) anomalies during the nITCZ and dsITCZ events follow the classic wind–evaporation–SST (WES) positive feedback mechanism. However, an alternative mechanism, embracing the effect of SST anomalies on vertical stratification and momentum mixing, gives rise to a negative WES feedback. Our results show that in the surface layer, there is a general progression of positive WES feedbacks happening in the weeks leading to the events followed by negative WES feedbacks occurring after the ITCZ events, with an alternate mechanism involving air–sea humidity differences limiting evaporation occurring in between. Additionally, the spatial structures of the components of the feedbacks are nearly mirror images for these opposite ITCZ events over the east Pacific during boreal spring. In closing, we find that understanding the air–sea interactions during daily to weekly varying ITCZ events (nITCZ and dsITCZ) helps to pinpoint how fundamental processes differ for ITCZs in different hemispheres.

Air‐Sea Heat Fluxes Associated With Convective Cold Pools

AbstractHigh‐resolution air‐sea observations collected by a suite of remotely‐piloted uncrewed surface vehicles (USV) provide new insight on surface latent and sensible heat fluxes associated with convective cold pools over the eastern Pacific Intertropical Convergence Zone (ITCZ). Convective cold pools are linked to isolated, heavily precipitating cells over the ocean that yield downdrafts of evaporatively‐cooled air that spreads out horizontally in all directions and are characterized by strong air temperature gradients and horizontal wind speeds. Cold pool frontal signatures are identified based on air temperature depression criteria of −1.5°C in 10 min or less, resulting in a total of 276 events observed within the eastern Pacific ITCZ across 590 total seadays of data from 10 drones over the course of 3 years. Composite analysis reveals enhanced latent and sensible heat fluxes from the ocean to the atmosphere immediately following the frontal passage, and a decomposition of the bulk flux formulas indicates that variations in latent heat flux are largely influenced by the anomalous wind field (i.e., the cold pool gust front) acting on the mean background air‐sea moisture gradient, while variations in sensible heat flux are largely influenced by the mean background wind acting on the anomalous air‐sea temperature gradient (i.e., the cold pool temperature front). High‐frequency variations in wind speed and velocity are further explored in the context of “gustiness” and averaging intervals used in bulk flux algorithms. A case study is also presented for a convective cold pool event captured by a traveling mesoscale network, or mesonet, of USVs.

Hysteresis of the El Niño-Southern Oscillation to CO2 forcing.

El Niño-Southern Oscillation (ENSO) is the strongest interannual climate variability with far-reaching socioeconomic consequences. Many studies have investigated ENSO-projected changes under future greenhouse warming, but its responses to plausible mitigation behaviors remain unknown. We show that ENSO sea surface temperature (SST) variability and associated global teleconnection patterns exhibit strong hysteretic responses to carbon dioxide (CO2) reduction based on the 28-member ensemble simulations of the CESM1.2 model under an idealized CO2 ramp-up and ramp-down scenario. There is a substantial increase in the ensemble-averaged eastern Pacific SST anomaly variance during the ramp-down period compared to the ramp-up period. Such ENSO hysteresis is mainly attributed to the hysteretic response of the tropical Pacific Intertropical Convergence Zone meridional position to CO2 removal and is further supported by several selected single-member Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulations. The presence of ENSO hysteresis leads to its amplified and prolonged impact in a warming climate, depending on the details of future mitigation pathways.

Seasonally Alternate Roles of the North Pacific Oscillation and the South Pacific Oscillation in Tropical Pacific Zonal Wind and ENSO

Abstract The western-central equatorial Pacific (WCEP) zonal wind affects El Niño–Southern Oscillation (ENSO) by involving a series of multiscale air–sea interactions. Its interannual variation contributes the most to ENSO amplitude. Thus, understanding the predictability of the WCEP interannual wind is of great importance for better predictions of ENSO. Here, we show that the North Pacific Oscillation (NPO) and the South Pacific Oscillation (SPO) alternate in fueling this interannual wind from late boreal winter to austral winter in the presence of background trade winds in different hemispheres. During the boreal winter–spring, the NPO registers footprints in the tropics by benefiting from the Pacific meridional mode and modulating the northwestern Pacific intertropical convergence zone (NITCZ). However, as austral winter approaches, the SPO takes over the role of the NPO in maintaining the anomalous NITCZ. Moreover, the interannual wind is further driven to the east in the positive phase of the SPO, by intensified central-eastern equatorial Pacific convection resulting from tropical–extratropical heat flux adjustments. A reconstructed WCEP interannual wind index involving only the NPO and the SPO possesses a long lead time for ENSO prediction of nearly one year. These two extratropical boosters enhance the viability of equatorial Pacific zonal wind anomalies associated with the large growth rate of ENSO, and the one in the winter hemisphere seems to be more efficient in forcing the tropics. Our result further indicates that the NPO benefits a long-lead prediction of the WCEP interannual wind and ENSO, while the SPO is the dominant extratropical predictor of ENSO amplitude. Significance Statement ENSO is closely linked to the interannual variability of equatorial Pacific zonal wind, and ENSO prediction is impeded by the weak predictability of the wind. We have found that the North Pacific Oscillation and the South Pacific Oscillation take turns in affecting the interannual variability of the zonal wind from the late boreal winter to austral winter, and the winter hemisphere extratropical booster is more efficient in modulating tropical convection and the associated surface winds. An estimated zonal wind index constructed by the two extratropical precursors possesses a long lead time for ENSO prediction. Our result provides useful information for better predicting ENSO by further considering winter hemisphere extratropical climate variability.

The Edge Intensification of Eastern Pacific ITCZ Convection

Abstract Tropical precipitation is climatologically most intense at the heart of the intertropical convergence zone (ITCZ), but this is not always true in instantaneous snapshots. Precipitation is amplified along the ITCZ edge rather than at its center from time to time. In this study, satellite observations of column water vapor, precipitation, and radiation as well as the thermodynamic field from reanalysis data are analyzed to investigate the behavior of ITCZ convection in light of the local atmospheric energy imbalance. The analysis is focused on the eastern Pacific ITCZ, defined as the areas where column water vapor exceeds 50 mm over a specified width (typically 400–600 km) in the domain of 20°S–20°N, 180°–90°W. The events with a precipitation maximum at the southern and northern edges of the ITCZ are each averaged into composite statistics and are contrasted against the reference case with peak precipitation at the ITCZ center. The key findings are as follows. When precipitation peaks at the ITCZ center, suppressed radiative cooling forms a prominent positive peak in the diabatic forcing to the atmosphere, counteracted by an export of moist static energy (MSE) owing to a deep vertical advection and a large horizontal export of MSE. When convection develops at the ITCZ edges, to the contrary, a positive peak of the diabatic forcing is only barely present. An import of MSE owing to a shallow ascent on the ITCZ edges presumably allows an edge intensification to occur despite the weak diabatic forcing.

Climate impacts of 2009/2010 mixed-type El Niño on double ITCZs over the eastern Pacific Ocean

A double intertropical convergence zone (ITCZ) pattern emerges over the eastern Pacific Ocean in March-April. Their inter-annual variabilities may exert major climatic and ecological effects. El Niño-Southern Oscillation (ENSO) can significantly regulate the inter-annual variability of double ITCZs. ENSO diversity has increased in recent years. A mixed-type El Niño occurred in 2009/2010. Our results reveal that the meridional SSTA pattern in mixed-type El Niño is a combination of “SST warming with a maximum at the equator” (i.e., EP-type delta SST) and “north-warm and south-cold” (i.e., CP-type delta SST) patterns. In a mixed-type El Niño, both meridional EP-type and CP-type delta SST played a role and had corresponding climate impacts. Under the coordinated modulation of EP-type and CP-type delta SST in a mixed-type El Niño, the climate effect of EP-type delta SST (increasing equatorial precipitation) is weakened; while the climate effect of CP-type delta SST (enhancing the northern ITCZ and weakening the southern ITCZ) is amplified. The climate effect of mixed-type El Niño finally resembles that of typical CP-type El Niños, intensifying the north-south asymmetry of the eastern Pacific double ITCZs. This study will contribute to a better understanding of the impacts of ENSO diversity on double ITCZs in the eastern Pacific region.

Did hydroclimate conditions contribute to the political dynamics of Majapahit?: A preliminary analysis

Majapahit was the largest Hindu-Buddhist empire that ruled the Indonesian archipelago from the late 13th to mid-16th centuries CE. Only now there is still a lot of history surrounding the Majapahit era that has yet to be revealed. One is about how environmental factors influenced the political dynamics at that time. This study tries to discuss the influence of hydroclimate regimes using the Paleo Hydrodynamics Data Assimilation (PHYDA) product on the political history of Majapahit during the dry season, which occurs during the boreal summer. We conducted a spatial analysis of the area of drought by taking data from the Palmer Drought Severity Index (PDSI) in the Maritime Continent (MC) for six crucial episodes in the history of Majapahit, namely during the reign of Jayanegara (1309 - 1328 CE), which was marked by various political instability, the golden age of Majapahit (1309 - 1328 CE). 1350 - 1389 CE), the time of the Paregreg civil war (1405 - 1406 CE), the great famine event (ca. 1426 CE), the candrasengkala event (1478 CE), and in 1527 CE, which was marked by the complete conquest of Majapahit by the Demak sultanate. The results show statistically significant dif ferences in most of these six episodes (except during the heyday of Majapahit) against the reference period, which is the average PDSI over the entire Majapahit era (1293 - 1527 CE). In addition, we also conducted a temporal analysis linking PDSI with shif ts in the West Pacific Inter Tropical Convergence Zone (WP ITCZ) and El Niño Southern Oscillation (ENSO) represented by Niño 3.4 Sea Surface Temperature (SST). This temporal analysis results show a positive correlation between WP ITCZ - PDSI, a negative correlation between Niño 3.4 SST - PDSI and a negative correlation between ITCZ - Niño 3.4 SST. All of these correlations are statistically significant. So the probable cause of dry/wet conditions in MC during the Majapahit era was triggered by a meridional ITCZ shif t which triggered dif ferent ENSO phases through Bjerknes feedback. This preliminary study has implications as opening the way to understand the influence of environmental factors on political conditions in the Majapahit era in more detail.

Inter ocean basin moisture fluxes and the onset of the summer rainy season over southern Mexico

The onset of the rainy season in monsoonal climates is a key element for the development of water related activities. In southern Mexico, it starts in late May or early June, while in western and northwestern part of the country in begins between early and mid-July. Previous studies indicate that it depends on the meridional position of the eastern Pacific Inter Tropical Convergence Zone (ITCZ). By mean of analyses of the Vertically Integrated Moisture Flux, the low level atmospheric dynamics in the tropical Americas is analyzed to establish the temporal evolution of inter ocean moisture fluxes that lead to the onset of the summer rainy season in southern Mexico. The onset of the rainy season depends on the longitudinal position of the center of maximum convective activity in the ITCZ, which induces a quasi-stationary low-level cyclonic circulation that results in moisture fluxes to the Mexican Pacific coast and rains. The Caribbean Low-Level Jet (CLLJ) is a mechanism that modulates inter ocean basin moisture fluxes into the eastern Pacific ITCZ and the location of the maximum convective activity in this system. We propose that the sequence of events that leads to the onset of the rainy season in southern Mexico is: (i) an episode of strong moisture flux from the Caribbean Sea into the eastern Pacific ITCZ, (ii) a region of intense convective activity in the ITCZ that triggers a quasi-stationary low-level cyclonic circulation which in turn, induces moisture flux into southern Mexico, and (iii) the development of convective activity in southern Mexico that determines the onset of the rainy season. The intensification of the CLLJ from June to July also results in a westward displacement of the region of maximum convection within the ITCZ that leads to changes in the atmospheric circulations and moisture fluxes that determine the onset of the rainy season at higher latitudes in the Mexican Pacific coast.

Role of the Climatological North Pacific High in the North Tropical Atlantic–ENSO Connection

Abstract Observational and climate model analysis showed that the anomalous sea surface temperature in the north tropical Atlantic (NTA) in boreal spring can trigger El Niño–Southern Oscillation (ENSO) in the subsequent winter. Similarly, the climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) are known to reasonably simulate the NTA effect. Nevertheless, the strengths of the NTA effect on ENSO among the climate models are also diverse. In this light, we revisited the possible causes that contributed to the different NTA effects on ENSO in the CMIP5 climate models. We found that the strength of the NTA triggering ENSO in the climate model tended to be proportional to the intensity of the climatological subtropical North Pacific high system in boreal spring. The stronger climatological subtropical North Pacific high accompanied enhanced trade wind, precipitation reduction, and cold sea surface temperature over the subtropics. Under these conditions, the moist static energy feedback process, also known as the moist enthalpy advection mechanism, effectively operated around the Pacific intertropical convergence zone. That is, the NTA-induced signals in the subtropical North Pacific readily intruded into the deep tropical Pacific with the aid of the feedback processes, leading to an ENSO event. Consistent with the CMIP5 analysis results, the observed NTA effect on ENSO became stronger during the decades when the climatological North Pacific subtropical high intensified, underpinning the importance of climatology in the subtropical North Pacific in the NTA–ENSO connection.

Mid-Pliocene El Niño/Southern Oscillation suppressed by Pacific intertropical convergence zone shift

The El Niño/Southern Oscillation (ENSO), the dominant driver of year-to-year climate variability in the equatorial Pacific Ocean, impacts climate pattern across the globe. However, the response of the ENSO system to past and potential future temperature increases is not fully understood. Here we investigate ENSO variability in the warmer climate of the mid-Pliocene (~3.0–3.3 Ma), when surface temperatures were ~2–3 °C above modern values, in a large ensemble of climate models—the Pliocene Model Intercomparison Project. We show that the ensemble consistently suggests a weakening of ENSO variability, with a mean reduction of 25% (±16%). We further show that shifts in the equatorial Pacific mean state cannot fully explain these changes. Instead, ENSO was suppressed by a series of off-equatorial processes triggered by a northward displacement of the Pacific intertropical convergence zone: weakened convective feedback and intensified Southern Hemisphere circulation, which inhibit various processes that initiate ENSO. The connection between the climatological intertropical convergence zone position and ENSO we find in the past is expected to operate in our warming world with important ramifications for ENSO variability. Suppressed El Niño/Southern Oscillation variability during the mid-Pliocene Warm Period was caused mainly by a northward displacement of the intertropical convergence zone, according to an analysis of a large ensemble of climate model simulations.

Varying contributions of fast and slow responses cause asymmetric tropical rainfall change between CO2 ramp-up and ramp-down

Tropical rainfall is important for regional climate around the globe. In a warming climate forced by rising CO2, the tropical rainfall will increase over the equatorial Pacific where sea surface warming is locally enhanced. Here, we analyze an idealized CO2 removal experiment from the Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) and show that the tropical rainfall change features a stronger pattern during CO2 ramp-down than ramp-up, even under the same global mean temperature increase, such as the 2 °C goal of the Paris Agreement. The tropical rainfall during CO2 ramp-down increases over the equatorial Pacific with a southward extension, and decreases over the Pacific intertropical convergence zone and South Pacific convergence zone. The asymmetric rainfall changes between CO2 ramp-down and ramp-up result from time-varying contributions of the fast and slow oceanic responses to CO2 forcing, defined as the responses to abrupt CO2 forcing in the first 10 years and thereafter, respectively, in the abrupt-4xCO2 experiment. The fast response follows the CO2 evolution, but the slow response does not peak until 60 years after the CO2 peak. The slow response features a stronger El Niño-like pattern, as the ocean dynamical thermostat effect is suppressed under stronger subsurface warming. The delayed and stronger slow response leads to stronger tropical rainfall changes during CO2 ramp-down. Our results indicate that returning the global mean temperature increase to below a certain goal, such as 2 °C, by removing CO2, may fail to restore tropical convection distribution, with potentially devastating effects on climate worldwide.

Disentangling the effect of regional SST bias on the double-ITCZ problem

This study investigates the causes of the double intertropical convergence zone (ITCZ) bias, characterized by too northward northern Pacific ITCZ, too dry equatorial Pacific, and too zonally elongated southern Pacific rainband. While the biases within one fully coupled model GFDL CM2.1 are examined, the large-scale bias patterns are broadly common to CMIP5/6 models. We disentangle the individual contribution of regional sea surface temperature (SST) biases to the double-ITCZ bias pattern using a series of slab ocean model experiments. A previously suggested Southern Ocean warm bias effect in displacing the zonal-mean ITCZ southward is manifested in the northern Pacific ITCZ while having little contribution to the zonally elongated wet bias south of the equatorial Pacific. The excessive southern Pacific precipitation is instead induced by the warm bias along the west coast of South America. The Southern Ocean bias effect on the zonal-mean ITCZ position is diminished by the neighboring midlatitude bias of opposite sign in GFDL CM2.1. As a result, the northern extratropical cold bias turns out to be most responsible for a southward-displaced zonal-mean ITCZ. However, this southward ITCZ displacement results from the northern Pacific branch, so ironically fixing the extratropical biases only deteriorates the northern Pacific precipitation bias. Thus, we emphasize that the zonal-mean diagnostics poorly represent the spatial pattern of the tropical Pacific response. Examination of longitude-latitude structure indicates that the overall tropical precipitation bias is mostly locally driven from the tropical SST bias. While our model experiments are idealized with no ocean dynamics, the results shed light on where preferential foci should be applied in model development to improve particular features of tropical precipitation bias.

Assessing the Vertical Velocity of the East Pacific ITCZ

AbstractReanalysis and satellite data sets disagree on the vertical structure of the meridional overturning circulation over the East Pacific (EP) intertropical convergence zone (ITCZ). Recent observations from the Organization of Tropical East Pacific Convection (OTREC) 2019 field campaign show a meridional slope of vertical motion with a shallow mode from 3° to 7°N, over a strong meridional sea surface temperature (SST) gradient, and a deeper mode from 7° to 10.5°N, where the SST is predominantly warm. However, reanalysis fields from the OTREC period show a predominant shallow mode from 3° to 9°N unless OTREC dropsondes are assimilated; then a weak deep mode is found at 8°N. Vertical motion over the OTREC region derived from Global Precipitation Measurement (GPM) precipitation radar latent heating retrievals has the opposite problem in that the deep mode is predominant from 3° to 10°N. A shallow mode is present with the addition of cloud radar data.

Evaluating the large-scale hydrological cycle response within the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) ensemble

Abstract. The mid-Pliocene (∼3 Ma) is one of the most recent warm periods with high CO2 concentrations in the atmosphere and resulting high temperatures, and it is often cited as an analog for near-term future climate change. Here, we apply a moisture budget analysis to investigate the response of the large-scale hydrological cycle at low latitudes within a 13-model ensemble from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The results show that increased atmospheric moisture content within the mid-Pliocene ensemble (due to the thermodynamic effect) results in wetter conditions over the deep tropics, i.e., the Pacific intertropical convergence zone (ITCZ) and the Maritime Continent, and drier conditions over the subtropics. Note that the dynamic effect plays a more important role than the thermodynamic effect in regional precipitation minus evaporation (PmE) changes (i.e., northward ITCZ shift and wetter northern Indian Ocean). The thermodynamic effect is offset to some extent by a dynamic effect involving a northward shift of the Hadley circulation that dries the deep tropics and moistens the subtropics in the Northern Hemisphere (i.e., the subtropical Pacific). From the perspective of Earth's energy budget, the enhanced southward cross-equatorial atmospheric transport (0.22 PW), induced by the hemispheric asymmetries of the atmospheric energy, favors an approximately 1∘ northward shift of the ITCZ. The shift of the ITCZ reorganizes atmospheric circulation, favoring a northward shift of the Hadley circulation. In addition, the Walker circulation consistently shifts westward within PlioMIP2 models, leading to wetter conditions over the northern Indian Ocean. The PlioMIP2 ensemble highlights that an imbalance of interhemispheric atmospheric energy during the mid-Pliocene could have led to changes in the dynamic effect, offsetting the thermodynamic effect and, hence, altering mid-Pliocene hydroclimate.

Comparison of Reanalysis and Observational Precipitation Datasets Including ERA5 and WFDE5

Precipitation is a key component of the hydrological cycle and one of the most important variables in weather and climate studies. Accurate and reliable precipitation data are crucial for determining climate trends and variability. In this study, eleven different precipitation datasets are compared, six reanalysis and five observational datasets, including the reanalysis datasets ERA5 and WFDE5 from the ECMWF family, to quantify the differences between the widely used precipitation datasets and to identify their particular strengths and shortcomings. The comparisons are focused on the common time period 1983 through 2016 and on monthly, seasonal, and inter-annual times scales in regions representing different precipitation regimes, i.e., the Tropics, the Pacific Inter Tropical Convergence Zone (ITCZ), Central Europe, and the South Asian Monsoon region. For the analysis, satellite-gauge precipitation data from the Global Precipitation Climatology Project (GPCP-SG) are used as a reference. The comparison shows that ERA5 and ERA5-Land are a clear improvement over ERA-Interim and show in most cases smaller biases than the other reanalysis datasets (e.g., around 13% high bias in the Tropics compared to 17% for MERRA-2 and 36% for JRA-55). ERA5 agrees well with observations for Central Europe and the South Asian Monsoon region but underestimates very low precipitation rates in the Tropics. In particular, the tropical ocean remains challenging for reanalyses with three out of four products overestimating precipitation rates over the Atlantic and Indian Ocean.

Easterly Waves in the East Pacific during the OTREC 2019 Field Campaign

Abstract Easterly waves (EWs) are off-equatorial tropical synoptic disturbances with a westward phase speed between 11 and 14 m s−1. Over the east Pacific in boreal summer, the combination of EWs and other synoptic disturbances, plus local mechanisms associated with sea surface temperature (SST) gradients, define the climatological structure of the intertropical convergence zone (ITCZ). The east Pacific ITCZ has both deep and shallow convection that is linked to deep and shallow meridional circulations, respectively. The deep convection is located around 9°N over warm SSTs. The shallow convection is located around 6°N and is driven by the meridional SST gradient south of the ITCZ. This study aims to document the interaction between east Pacific EWs and the deep and shallow meridional circulations during the Organization of Tropical East Pacific Convection (OTREC) field campaign in 2019 using field campaign observations, ERA5, and satellite precipitation. We identified three EWs during the OTREC period using precipitation and dynamical fields. Composite analysis shows that the convectively active part of the EW enhances ITCZ deep convection and is associated with an export of column-integrated moist static energy (MSE) by vertical advection. The subsequent convectively suppressed, anticyclonic part of the EW produces an increase of moisture and column-integrated MSE by horizontal advection that likely enhances shallow convection and the shallow overturning flow at 850 hPa over the southern part of the ITCZ. Therefore, EWs appear to strongly modulate shallow and deep circulations in the east Pacific ITCZ.