Tropical Heat Research Articles

Historical changes in wind-driven ocean circulation drive pattern of Pacific warming

The tropical Pacific warming pattern since the 1950s exhibits two warming centers in the western Pacific (WP) and eastern Pacific (EP), encompassing an equatorial central Pacific (CP) cooling and a hemispheric asymmetry in the subtropical EP. The underlying mechanisms of this warming pattern remain debated. Here, we conduct ocean heat decompositions of two coupled model large ensembles to unfold the role of wind-driven ocean circulation. When wind changes are suppressed, historical radiative forcing induces a subtropical northeastern Pacific warming, thus causing a hemispheric asymmetry that extends toward the tropical WP. The tropical EP warming is instead induced by the cross-equatorial winds associated with the hemispheric asymmetry, and its driving mechanism is southward warm Ekman advection due to the off-equatorial westerly wind anomalies around 5°N, not vertical thermocline adjustment. Climate models fail to capture the observed CP cooling, suggesting an urgent need to better simulate equatorial oceanic processes and thermal structures.

Drivers of Caribbean precipitation change due to global warming: analyses and emergent constraint of CMIP6 simulations

Using 31 CMIP6 models we have analyzed projected future Caribbean precipitation. The model mean projection results in a 0.5 mm/day (20%) drying under a SSP5-8.5 scenario for the end of this century over the Caribbean basin. The multi-model spread is large ranging from no drying to a 1 mm/day (40%) reduction in mean precipitation. Eastern and central Pacific warming, resembling an El Niño / positive phase of Pacific Decadal Oscillation (PDO), appears to be the main driver by shifting and weakening the Walker circulation and inducing subsidence over the Caribbean, especially during the wet season (May–November). This applies for the model mean as well as for the inter-model spread. During the dry season (December–April) the southward migration of the Intertropical Convergence Zone (ITCZ) and the advection of dry air from outside the Caribbean seem to be the dominant drivers of the projected drying. Another mechanism that contributes to the drying is the land-sea contrasts that induce divergence/convergence over the Caribbean. The incapability of CMIP6 models to simulate the current tropical Pacific warming and Walker circulation trends questions the reliability of precipitation projections in the Caribbean. Based on our understanding of the physical processes affecting Caribbean drying and on an emergent constraint analysis we state that the future drying in the Caribbean is likely to be weaker than the one projected by CMIP6 models.

Projected Changes in the Onset of the Summer Monsoon over the South Asian Marginal Seas Modulated by Intraseasonal Oscillation

Abstract The onset of the summer monsoon associated with global warming is of great concern to the scientific community. While observational data diagnosis has shown the impact of intraseasonal oscillation (ISO) on the monsoon onset, how the ISO may affect the onset of monsoon under global warming remains unknown. Here, by analyzing the onset of the summer monsoon over the South Asian marginal seas projected by models in phase 6 of the Coupled Model Intercomparison Project (CMIP6) under the SSP5–8.5 scenario, we show evidence that the majority of models (>70%) project an earlier onset over the Arabian Sea (ArS) but a delayed onset over the Bay of Bengal (BoB) and the South China Sea (SCS). The temporal shifts of the monsoon onset are attributed to the changes in the premonsoon northward migration of equatorial ISO (NMISO), which is a trigger of monsoon onset and will be advanced (postponed) over the ArS (BoB and SCS). The subtropical upper-level westerly anomaly, inducing delayed occurrence of easterly shear, acts to delay the NMISO over the entire Indian Ocean. However, the intensified low-level southerly wind over the ArS, as well as its induced asymmetric pattern of boundary layer moisture work together to advance the premonsoon NMISO in the area, outweighing the delayed impact from vertical shear. These large-scale circulation changes are driven by tropical warming in the upper troposphere, land warming over the Arabian Peninsula, and ocean warming over the eastern Pacific. This analysis enriches monsoon onset projections by highlighting the role of ISO in influencing the future changes in monsoon onset.

Upper Colorado River Streamflow Dependencies on Summertime Synoptic Circulations and Hydroclimate Variability

Abstract The southwestern United States is highly sensitive to drought, prompting efforts to understand and predict its hydroclimate. Oftentimes, the emphasis is on wintertime precipitation variability, yet the southwestern United States exhibits a summertime monsoon where a significant portion of annual precipitation falls through daily convection activities manifested by a midtropospheric ridge of high pressure. Here, we examine synoptic patterns of the southwestern ridge through a k-means clustering analysis and assess how these synoptic patterns translate into streamflow changes in the upper Colorado River basin. A linear perspective suggests ∼17% of upper Colorado River discharge at the Lee’s Ferry, Arizona, gauge comes from summertime monsoon rains. The ridge of high pressure exhibits diversity in its intensity, structure, and position, inducing changes in moisture advection and precipitation. A ridge shifted north or east of its climatological center increases moisture and precipitation over the southwestern United States, while a ridge toward the south or northwest inhibits precipitation. A ridge east of its climatological center contributes to increased streamflow, whereas a ridge west or northwest of its climatological center decreases streamflow. Cooling in the central tropical Pacific and the Pacific meridional mode region favors an eastward shift of the ridge of high pressure corresponding to wet days. Eastern tropical Pacific warming favors a southward shift of the ridge corresponding to dry days. These results support an intermediate scale between climate forcing and summertime Colorado River discharge through changes in the intensity, structure, and position of the southwestern ridge of high pressure, integral to the U.S. Southwest hydroclimate.

2023 Record marine heat waves: coral reef bleaching HotSpot maps reveal global sea surface temperature extremes, coral mortality, and ocean circulation changes

Abstract Coral reefs, the most sensitive ecosystem to high temperature, are on the precipice of mass extinction from global warming [1, 2]. 2023 was the hottest year in recorded history on land and in the sea, with dramatic and unexpected temperature increases [3, 4]. Coral Reef Bleaching HotSpot maps provide unique insight into global ocean circulation changes in response to greenhouse gas (GHG) forcing that caused dramatic global temperature rises [1, 2]. The highest excess daily air temperatures recorded in 175 countries, as well as the most prolonged excessive sea surface temperatures, were centered around Jamaica. 2023 marked the worst coral reef bleaching yet in the Northern Hemisphere, with the Southern Hemisphere poised to follow in early 2024. The HotSpot maps strongly suggest accelerated ocean poleward heat transport, slowdown in upwelling, and decreased deep water formation linked to sharply increased 2023 anomalous sea surface and air temperatures. The 2023 distribution of severe heat and bleaching follows both spatial patterns and temporal trends first shown from a baseline 1982–2001 global SST trend analysis [5]. Increased warming of both hot and cold ocean currents shows that horizontal mixing of tropical heat to the poles is accelerating, and that vertical mixing with cold deep water is slowing down, leading to increased ocean stratification, which will cause sea temperature to increase more rapidly and CO2 mixing with the deep ocean to decrease.

The Impact of Climate Change on Aeromedical Retrieval Services in Remote Northern Australia: Planning for a Hotter Future.

It is known that environmental heat is associated with increased morbidity manifesting as increasing demand on acute care health services including pre-hospital transport and emergency departments. These services play a vital role in emergency care, and in rural and remote locations, where resource capacity is limited, aeromedical and other retrieval services are a vital part of healthcare delivery. There is no research examining how heat impacts remote retrieval service delivery. The Northern Territory (NT) of Australia is characterised by very remote communities with limited acute healthcare capacities and is a region subject to regular extreme tropical heat. In this study, we examine the relationship between aeromedical retrievals and hot weather for all NT retrievals between February 2018 and December 2019. A regression analysis was performed on the number of retrievals by clinical reason for retrieval matched to the temperature on the day of retrieval. There was a statistically significant exposure response relationship with increasing retrievals of obstetric emergencies in hotter weather in the humid climate zone and surgical retrievals in the arid zone. Retrieval services appeared to be at capacity at all times of the year. Given that there are no obstetric services in remote communities and that obstetric emergencies are a higher triage category than other emergencies (i.e., more urgent), such an increase will impede overall retrieval service delivery in hot weather. Increasing surgical retrievals in the arid zone may reflect an increase in soft tissue infections occurring in overcrowded houses in the hotter months of the year. Given that retrieval services are at capacity throughout the year, any increase in demand caused by increasing environmental heat will have broad implications for service delivery as the climate warms. Planning for a hotter future must include building resilient communities by optimising local healthcare capacity and addressing housing and other socioeconomic inequities that amplify heat-related illness.

Multiethnic elites and the management of tropical heat in colonial Malaya, circa 1870s–1930s

Multiethnic elites and the management of tropical heat in colonial Malaya, circa 1870s–1930s

Coherent amplification of the Okhotsk high, Korean trough, and northwestern Pacific subtropical high during heavy rainfall over Japan in August 2021

In August 2021, rain front stagnation in Japan resulted in prolonged and disastrous rainfall across the entire country. During the heavy rainfall period, the large-scale atmospheric field over the East Asian–western North Pacific region was characterized by meridional tripolar circulation anomalies: the Okhotsk high (OH), the trough over the Korean Peninsula (Korean trough), and the northwestern Pacific subtropical high (NWPSH). Simultaneously, tropical convective activity was enhanced over the eastern Indian Ocean and suppressed over the tropical western–central Pacific. This study investigates the dynamic mechanism of linkage of the extratropical tripolar anomalies and the effects of tropical convective modulation using a reanalysis dataset, a cutoff low detection scheme, the potential vorticity inversion method, and numerical experiments. Upper-tropospheric blocking over eastern Siberia connected to the surface OH is conducive to the stagnation of synoptic depressions, including cutoff lows and troughs, over the Korean Peninsula, contributing to the development and maintenance of the quasi-stationary Korean trough. Rossby waves emanating from the Korean trough excite an anticyclonic anomaly over the northwestern Pacific. This upper-level anomalous anticyclone acts to enhance the surface NWPSH through zonal heat transport, accompanied by a northward tilting structure with height. Simultaneously, the tropical intraseasonal oscillation is amplified over the Indo–western Pacific Ocean sector under the negative-phase Indian Ocean dipole and multi-year La Niña conditions. The combination of enhanced convection over the eastern Indian Ocean and suppressed convection across the tropical western–central Pacific reinforces the NWPSH. The anomalous circulation associated with the extratropical tripolar pattern and concurrent tropical heat forcing causes more moisture transport, convergence, and anomalous ascent, which contribute to heavy rainfall in Japan. These results suggest that the dynamically correlated amplification of tropical and extratropical circulation anomalies plays a crucial role in precipitation variability in East Asia.

Step-wise shifted rainbelt throughout the early last deglaciation

Millennial climate change since the last deglaciation have drawn much attention. Even though the role of northern hemisphere is much focused on, how the tropic performed remains disputed. Here based on Mg/Ca and δ18Osw local from planktonic foraminifera Globigerinoides ruber, sub-millennial scale hydroclimate record throughout the early last deglaciation is presented from the northern South China Sea, near the northern boundary of Intertropical Convergence Zone (ITCZ). Coupled with previous research, we speculate that rainbelt/ITCZ was located northerly from 19 to 18 ka, and migrated southward within two-steps during Heinrich 1 event. Northerly ITCZ during 19–18 ka was rendered by tropical warming since 19 ka through the Gill-Matsuno response, which strengthened Atlantic meridional overturning circulation, contributing to intense monsoon precipitation over east Asia, and dry conditions around the western Pacific. The subsequent two-steps ITCZ southward shift during Heinrich 1 was driven by weakened Atlantic meridional overturning circulation, which was produced by the lowering northern hemisphere ice-sheet at ∼16.2 ka. Step-wise shifted rainbelt/ITCZ documented from our record indicates multiple factors dominated sub-millennial scale climate change during the whole last deglaciation. We highlight the important roles of tropical warming and ocean circulation in initiating the last deglaciation and the subsequent climate change, respectively.

Intrinsic predictability limits arising from Indian Ocean Madden–Julian oscillation (MJO) heating: effects on tropical and extratropical teleconnections

Abstract. Since the Madden–Julian oscillation (MJO) is a major source for tropical and extratropical variability on weekly to monthly timescales, the intrinsic predictability of its global teleconnections is of great interest. As the tropical diabatic heating associated with the MJO ultimately drives these teleconnections, the variability in heating among ensemble forecasts initialized from the same episode of the MJO will limit this predictability. In order to assess this limitation, a suite of 60 d ensemble reforecasts has been carried out with the ECMWF forecast model, spanning 13 starting dates from 1 November and 1 January for different years. The initial dates were chosen so that phases 2 and 3 of the MJO (with anomalous tropical heating in the Indian Ocean sector) were present in the observed initial conditions. The 51 members of an individual ensemble use identical initial conditions for the atmosphere and ocean. Stochastic perturbations to the tendencies produced by the atmospheric physics parameterizations are applied only over the Indian Ocean region (50–120∘ E). This guarantees that the spread between reforecasts within an ensemble is due to perturbations in heat sources only in the Indian Ocean sector. The point-wise spread in the intra-ensemble (or error) variance of vertically integrated tropical heating Q is larger than the average ensemble mean signal even at early forecast times; however the planetary wave (PW) component of Q (zonal waves 1–3) is predictable for 25 to 45 d, the time taken for the error variance to reach 50 % to 70 % of saturation. These scales never reach 90 % of saturation during the forecasts. The upper-level tropical PW divergence is even more predictable than Q (40 to 50 d). In contrast, the PW component of the 200 hPa Rossby wave source, which is responsible for propagating the influence of tropical heating to the extratropics, is only predictable for 20 to 30 d. A substantial ensemble spread of 300 hPa meridional wind propagates from the tropics to the Northern Hemisphere storm-track regions by days 15–16. Following the growth of upper-tropospheric spread in planetary wave heat flux, the stratosphere provides a feedback in enhancing the error via downward propagation towards the end of the reforecasts.

Plausible Drying and Wetting Scenarios for Summer in Southeastern South America

Abstract Summer rainfall trends in southeastern South America (SE-SA) have received attention in recent decades because of their importance for climate impacts. More than one driving mechanism has been identified for the trends, some of which have opposing effects. It is still not clear how much each mechanism has contributed to the observed trends or how their combined influence will affect future changes. Here, we address the second question and study how the CMIP6 summer SE-SA rainfall response to greenhouse warming can be explained by mechanisms related to large-scale extratropical circulation responses in the Southern Hemisphere to remote drivers (RDs) of regional climate change. We find that the regional uncertainty is well represented by combining the influence of four RDs: tropical upper-tropospheric amplification of surface warming, the delay in the stratospheric polar vortex breakdown date, and two RDs characterizing recognized tropical Pacific SST warming patterns. Applying a storyline framework, we identify the combination of RD responses that lead to the most extreme drying and wetting scenarios. Although most scenarios involve wetting, SE-SA drying can result if high upper-tropospheric tropical warming and early stratospheric polar vortex breakdown conditions are combined with low central and eastern Pacific warming. We also show how the definition of the SE-SA regional box can impact the results since the spatial patterns characterizing the dynamical influences are complex and the rainfall changes can be averaged out if these are not considered when aggregating. This article’s perspective and the associated methodology are applicable to other regions of the globe. Significance Statement Summer rainfall in southeastern South America (SE-SA) affects an area where around 200 million people live. The observed trends suggest long-term wetting, and most climate models predict a wetting response to greenhouse warming. However, in this work, we find that there is a physically plausible combination of large-scale circulation changes that can promote drying, which means SE-SA drying is a possibility that cannot be ignored. We also show that the definition of the SE-SA regional box can impact regional rainfall analysis since the spatial patterns characterizing the dynamical influences are complex and the changes can be averaged out if these are not considered when aggregating. This perspective and the associated methodology are applicable to other regions of the globe.

The Influence of Equatorial Cooling on Axially Symmetric Atmospheric Circulation Forced by Annually Averaged Heating

Abstract Axisymmetric theory of atmospheric circulation is extended for the case of concentrated equatorial cooling and annually averaged heating. The solutions are derived in a 1.5-layer shallow water model on the spherical Earth, which includes vertical mixing with diagnostic surface momentum. The axisymmetric solutions capture the sensitivity of the large-scale circulation to equatorial cooling seen in observations and in eddy-permitting models, namely, (i) weakening and widening of the meridional overturning circulation (MOC) and (ii) weakening and poleward shift of the subtropical jet. For sufficiently large equatorial cooling, a tropical anti-Hadley cell emerges that transports energy equatorward, balancing the equatorial energetic sink. The analytic solutions predict the critical cooling required for the emergence of the anti-Hadley cell and provide a simple mechanism for the response of the MOC to equatorial cooling. Specifically, equatorial cooling reduces net tropical heating, which weakens the circulation and shifts the edge of the rising branch poleward. This in turn reduces upper-level momentum which is set by surface momentum in the rising branch. The subtropical meridional temperature gradient decreases with upper-level momentum, requiring a widening of the circulation to close the energy budget. The subtropical jet therefore shifts poleward with the edge of the MOC and weakens due to the reduced upper-level angular momentum. The strong sensitivity to equatorial cooling seen in the axisymmetric system suggests that the above mechanism may have an important role in the sensitivity of the MOC to equatorial temperature anomalies on seasonal or longer time scales. Significance Statement Axisymmetric solutions for the response of the atmospheric circulation to concentrated equatorial cooling are derived, motivated by the equatorial cooling by the cold tongues of the Pacific and Atlantic. The solutions capture the response of the large-scale realistic atmosphere to equatorial cooling, namely a weakening and widening of the meridional overturning circulation and a weakening and poleward shift of the subtropical jet. In addition, for sufficiently strong cooling, a tropical anti-Hadley cell emerges. The solutions provide insight into the influence of biases in the representation of the cold tongues in climate models, the relation of interannual equatorial variability to the large-scale circulation, and changes in the large-scale atmospheric circulation on geological time scales.

Strengthening gradients in the tropical west Pacific connect to European summer temperatures on sub-seasonal timescales

Abstract. Recent work has shown that (sub-)seasonal variability in tropical Pacific convection, closely linked to the El Niño–Southern Oscillation (ENSO), relates to summertime circulation over the Euro-Atlantic. The teleconnection is non-stationary, probably due to long-term changes in both the tropical Pacific and extra-tropical Atlantic. It also appears imperfectly captured by numerical models. A dipole in west Pacific sea surface temperatures (SSTs) was found to be the best predictor of errors in numerical sub-seasonal forecasts of European temperature. In this diagnostic study we use reanalysis data to further investigate the teleconnection pathway and the processes behind its non-stationarity. We show that SST gradients associated with the dipole represent a combination of ENSO variability and west Pacific warming, and have become stronger since 1980. Associated patterns of suppressed and enhanced tropical heating are followed by quasi-stationary waves that linger for multiple weeks. Situations with La Niña-like gradients are followed by high-pressure centres over eastern Europe and Russia, three to six weeks later. Inverted situations are followed by high pressure over western Europe, three to six weeks later. The latter situation is conditional on a strong meridional tripole in north Atlantic SSTs and a co-located jet stream. Overall, the sub-seasonal pathway diagnosed in this study connects to patterns detected on seasonal scales, and confirms earlier findings that the summertime connectivity between the Pacific and Europe has shifted in recent decades.

Atmospheric bias teleconnections in boreal winter associated with systematic sea surface temperature errors in the tropical Indian Ocean

Abstract. Coupled climate models suffer from significant sea surface temperature (SST) biases in the tropical Indian Ocean (TIO), leading to errors in global climate predictions. In this study, we investigate the local and remote effects of the TIO SST bias on the simulated atmospheric circulation and spatio-temporal variability – bias teleconnections. A set of century-long simulations forced by idealized SST perturbations, which resemble various (monopolar or dipolar, positive or negative) TIO SST biases in coupled climate models, are conducted with an intermediate-complexity atmospheric model. Bias teleconnections with a focus on boreal wintertime are analysed using the normal-mode function (NMF) decomposition, which can differentiate between balanced and unbalanced flows across spatial scales. The results show that the atmospheric circulation biases caused by the TIO SST bias have the Gill–Matsuno-type pattern in the tropics and Rossby-wave-train structure in the extratropics, similar to the steady-state response to tropical heating perturbations. The teleconnections between the tropical and extratropical biases are set up by Rossby wave activity flux emanating from the subtropics. Over 90 % of the bias variance (i.e. the square of the bias amplitude) is contained in zonal wavenumbers k≤5. The northward shift of the SST bias away from the Equator weakens the amplitude but does not change the spatial structure of the atmospheric response. Besides, the positive SST bias produces stronger bias teleconnections than the negative one of the same size and magnitude. In the NMF framework, the change in the spatial variance of the time-mean state (i.e. energy) is equal to the sum of the bias variance and the covariance between the circulation bias and the reference state (i.e. bias covariance). Due to the TIO SST biases, the global unbalanced zonal-mean (k=0) flow energy decreases, whereas its balanced counterpart increases. These changes primarily arise from the strong bias covariance. For k>0, both the global unbalanced and the tropical balanced energies increase in the case of a monopolar SST bias and decrease in the case of a dipolar SST bias. The increase appears mainly as the bias variance, whereas the decrease is associated with a strong negative bias covariance at k=1 and 2. In contrast, the extratropical balanced wave energy decreases (increases) when the TIO SST bias is positive (negative), which is mainly associated with the bias covariance at k=1. The change in the interannual variance (IAV) is contingent upon the sign of the TIO SST bias. A positive bias reduces, whereas a negative one increases, the IAV in both balanced and unbalanced flows. Geographically, large IAV changes are observed in the tropical Indo-West Pacific region, Australia, South and Northeast Asia, the Pacific-North America region, and Europe, where the background IAVs are strong.

Refined fine-scale mapping of tree cover using time series of Planet-NICFI and Sentinel-1 imagery for Southeast Asia (2016–2021)

Abstract. High-resolution mapping of tree cover is indispensable for effectively addressing tropical forest carbon loss, climate warming, biodiversity conservation and sustainable development. However, the availability of precise, high-resolution tree cover map products remains inadequate due to the inherent limitations of mapping techniques utilizing medium- to coarse-resolution satellite imagery, such as Landsat and Sentinel-2 imagery. In this study, we have generated an annual tree cover map product at a resolution of 4.77 m for Southeast Asia (SEA) for the years 2016–2021 by integrating Norway's International Climate and Forests Initiative imagery from Planet (hereafter Planet-NICFI) and Sentinel-1 synthetic aperture radar data. We have also collected annual tree cover/non-tree cover samples to assess the accuracy of our Planet-NICFI tree cover map product. The results show that our Planet-NICFI tree cover map product for 2016–2021 achieves high accuracy, with an overall accuracy of ≥0.867 ± 0.017 and a mean F1 score of 0.921. Furthermore, our tree cover map product exhibits high temporal consistency from 2016 to 2021. Compared with existing map products (e.g., Finer Resolution Observation and Monitoring of Global Land Cover 10 m, FROM-GLC10, and European Space Agency WorldCover 2020 and 2021), our tree cover map product exhibits better performance, both statistically and visually. However, the imagery obtained from Planet-NICFI does not perform as well with respect to mapping tree cover in areas with diverse vegetation or complex landscapes due to insufficient spectral information. Nevertheless, we highlight the capability of Planet-NICFI imagery to provide quick and fine-scale tree cover mapping to a large extent. The consistent characterization of tree cover dynamics in SEA's tropical forests can be further applied in various disciplines. Our data from 2016 to 2021 at a 4.77 m resolution are publicly available at https://doi.org/10.57760/sciencedb.07173 (Yang and Zeng, 2023).

Impact of tropical Indian Ocean warming on the surface phytoplankton biomass at two significant coastal upwelling zones in the Arabian Sea

The Sea Surface Temperature (SST) of the Arabian Sea undergoes large seasonal variations owing to the monsoonal forcing and upwelling. Warming of the ocean adversely affects its biological productivity. The present study examined the role of the rapid warming of the tropical Indian Ocean and associated changes in physical forcing parameters on phytoplankton biomass in the Arabian Sea. SST during the summer monsoon period (June-September) of 1971–2020 showed an increasing trend of 0.6 °C in the southeastern Arabian Sea (SEAS) and 1.6 °C in the northwestern Arabian Sea. During the recent summer monsoon period of 1998–2020, surface chlorophyll-a (chl-a) concentration showed a decreasing trend of – 0.32 mg m−3 in the SEAS and an increasing trend of 0.56 mg m−3 in the northwestern Arabian Sea. High-resolution satellite and reanalysis data of physical forcing parameters such as surface winds, SST, Sea Level Anomaly (SLA) that influence the surface chl-a concentration at two distinct upwelling locations in the Arabian Sea, were analysed for the recent two decades (1998–2020). Multiple linear regression (MLR) analysis showed that SLA was the most important parameter that determines the surface chl-a variability in the SEAS, whereas alongshore wind stress was dominant in the northwestern Arabian Sea. An epochal analysis showed that in the most recent decade, SLA in the SEAS became less favourable for upwelling, whereas summer monsoon winds became increasingly favourable for upwelling in the northwestern Arabian Sea. These differences corroborate the contrasting trends in surface chl-a in the two locations in the Arabian Sea. The present study has shown that the inconsistent response of surface chl-a at distinct locations within the Arabian Sea depends on the relative strength of the influencing physical forcing mechanisms.

Seasonal Tropical–Extratropical Teleconnections Originating from Tropical Rainfall Modes Independent of the Niño-3.4 Index in Northern Winters

Abstract This study, based on an analysis with observational and reanalysis data, highlights seasonal tropical–extratropical atmospheric teleconnections originating from tropical rainfall modes unrelated to the Niño-3.4 index for northern winters. The mode decomposition for tropical rainfall is done by first removing the Niño-3.4 index–related variability from the tropical rainfall and then applying rotated empirical orthogonal function (REOF) analysis to the residual. The corresponding teleconnection patterns are obtained by regressing global atmospheric fields against the time series of the rainfall modes. Analyses of the tropical heating–atmospheric circulation relationship indicate that the circulation anomalies corresponding to the rainfall modes are forced responses to the corresponding rainfall mode. The teleconnection patterns reveal some new features and show that some intrinsic midlatitude patterns can be triggered by tropical forcing with different rainfall patterns. Results from this study are relevant to seasonal climate attribution and prediction analyses and climate model evaluation. As an illustration, the teleconnections from the rainfall modes, together with that related to the Niño-3.4 index and linear trend, are applied to the attribution analyses for the global circulation anomalies of 2019/20 winter and the California dry condition during the strong El Niño winter of 2015/16. The overall impact of these modes in the period of 1980–2021 is also discussed. Significance Statement This study highlights the seasonal tropical–extratropical atmospheric teleconnections independent of the Niño-3.4 index using tropical rainfall modes for northern winters. The reason for using rainfall rather than SST in the mode decomposition is that rainfall represents vertically integrated latent heat, which is the direct forcing of the tropical atmosphere, while SST may have no definite relationship with rainfall in the Indo-Pacific warm pool region. The results of this study are applicable to the analysis of climate attribution and prediction and climate model evaluation, and further, may also have the potential to help improve seasonal climate forecasts.

Effects of simulated tropical heat waves during development on the morphological and reproductive traits of Africanized honey bee

Effects of simulated tropical heat waves during development on the morphological and reproductive traits of Africanized honey bee

Cold Springs Over Mid‐Latitude North America Induced by Tropical Atlantic Warming

AbstractIn recent decades, severe cold winters and springs have frequently occurred over mid‐latitude North America, despite the anthropogenic global warming trend. In this study, we reveal a possible mechanism by investigating the teleconnection between tropical oceans and North America. Through observational analysis and numerical experiments, we reveal that an anomalous tropical Atlantic warming can trigger a cold spring over central‐western mid‐latitude North America. The tropical Atlantic warming intensifies regional deep atmospheric convection and generates a stationary Rossby wave train propagating poleward, forming an anomalous low pressure center over the mid‐latitude North Atlantic. This low‐level circulation adjustment further intensifies the cold advection and increases the cloud cover over central‐western North America, cooling the surface through cloud radiative feedback. The mechanisms revealed in this study may contribute to the improvement of predictability of cold springs over North America, and have broad implications for agriculture production, power supply, and public health.

Tropical tropospheric warming pattern explained by shifts in convective heating in the Matsuno–Gill model

AbstractHorizontal temperature gradients in the tropical free troposphere are fairly weak, and tropical tropospheric warming is usually treated as uniform. However, we show here that projected tropospheric warming is spatially inhomogeneous in Coupled Model Intercomparison Project Phase 6 models, as well as in a storm‐resolving climate model. We relate the upper tropospheric warming pattern to sea‐surface temperature changes that reorganise convection and thereby cause spatial shifts in convective heating. Using the classical Gill model for tropical circulation and forcing it with precipitation changes that arise due to greenhouse gas warming, we can understand and reproduce the different warming patterns simulated by a range of global climate models. Forcing the Gill model with precipitation changes from a certain region demonstrates how local tropospheric temperature changes depend on local changes in convective heating. Close to the Equator, anomalous geopotential gradients are balanced by the dissipation term in the Gill model. The optimal dissipation time‐scale to reproduce the warming pattern varies depending on the Coupled Model Intercomparison Project Phase 6 model, and is between 1 and 10 days. We demonstrate that horizontal advection and eddy momentum fluxes have large enough equivalent dissipation time‐scales to balance the gradients in geopotential and thereby shape the warming pattern. Though climate models show a large spread in projections of tropical sea‐surface temperature and precipitation changes, our results imply that, once these predictions improve, our confidence in the predicted upper tropospheric warming pattern should also increase.