Warm Sea Surface Research Articles

Elevated temperature and nutrients lead to increased N2O emissions from salt marsh soils from cold and warm climates

Salt marshes can attenuate nutrient pollution and store large amounts of ‘blue carbon’ in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N2O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types, Sporobolus alterniflorus (= Spartina alterniflora) and Sporobolus pumilus (= Spartina patens), using 24-h laboratory incubation experiments. Potential N2O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N2O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N2O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N2O:N2 indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in Louisiana S. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios.

Why was Pakistan extreme precipitation stronger in 2022 than in 2010?

In Pakistan, five continuous extreme precipitation events in the summer (July‒August) of 2022 caused disastrous floods, depriving thousands of people's lives and ruining millions of hometowns. This tremendous disaster in Pakistan also happened in 2010 but with three concentrated extreme precipitation events in the middle of summer. The amount of Pakistan heavy daily precipitation in 2022 surpasses that in 2010 record, making it the strongest precipitation event ever recorded. To comprehensively understand the causes of extreme precipitation in Pakistan, this study investigated the anomalies in atmospheric circulation and moisture contribution of 2010/2022 extreme precipitation and compared their differences. The results show that an atmospheric blocking over northern Europe in both 2022 and 2010 enhanced convection in Pakistan by transporting cold‒dry air from the high-latitude region and benefiting warm‒wet monsoonal air marching to Pakistan. By employing a moisture track model, the main moisture sources for summer precipitation in Pakistan were identified. It is found that moisture contributions except from Eurasia were enhanced, causing extreme precipitation. In particular, enhanced moisture contribution from the southern Indian Ocean and the northern Indian continent in 2022 are more prominent than that in 2010. The increased moisture contribution in 2010 was due to increased evaporation induced by warming sea surface in the Indian Ocean, while much richer moisture transport in 2022 was attributed to the enhanced cross‒equatorial flow induced by the anomalous subtropical high in the Southern Hemisphere. Therefore, attention should be paid to the role of subtropical high in the Southern Hemisphere in addition to those in Northern Hemisphere in understanding disastrous extreme precipitation events in Pakistan.

Decadal Enhancement in the Effect of El Niño in the Decaying Stage on the Pre–Flood Season Precipitation over Southern China

Abstract The abundant precipitation over Southern China during the pre–flood season (PFS) [i.e., April–June (AMJ)] has important socioeconomic impacts on this densely populated region. Using observational and reanalyzed datasets, this study explores how El Niño affected the subsequent PFS precipitation over Southern China during 1961–2020. The results show that the El Niño–related anomalies in sea surface temperature forced a northwestern Pacific anomalous anticyclone (NPAAC) in the decaying AMJ. This NPAAC featured southwesterly wind anomalies in its northwestern flank, which could transport moisture from the South China Sea, and accompanying the NPAAC there was abnormal descending motion over the tropical western Pacific, resulting in weakened regional Hadley circulation with abnormal ascending motion over subtropical East Asia. Before the 1990s, this abnormal ascending motion was located mainly to the east of Southern China with insignificant impacts on the PFS precipitation there. In contrast, after the early 1990s, El Niño–related warm sea surface temperature anomalies were stronger and longer-lasting with westward extension. This enhanced the NPAAC with a decadal westward extension, and consequently, the anomalous regional Hadley circulation was more evident over Southern China after the early 1990s during the El Niño decaying AMJ, causing strong abnormal upward motion and excessive precipitation there. The present results emphasize an enhancing influence of El Niño on the subsequent PFS precipitation over Southern China since the early 1990s, offering better understanding of the interannual precipitation variability over Southern China and with important implications for regional seasonal climate prediction. Significance Statement The precipitation over Southern China during the pre–flood season (PFS) contributes between 40% and 50% of the local annual precipitation, with severe floods occurring frequently and tremendous socioeconomic impacts on the region, including on its agriculture, water resources, food security, ecosystems, disaster mitigation, infrastructure construction, and human health. This study reveals a decadal enhancement in the effect of El Niño on the subsequent PFS precipitation over Southern China since the early 1990s. This is ascribed to a decadal enhancement of El Niño–related warm sea surface temperature anomalies over the equatorial central Pacific, as a result of El Niño warming occurring more frequently over the central Pacific against the backdrop of global warming. To the extent that the central Pacific El Niño events would occur even more frequently for the projected future warming scenarios, a stronger effect of El Niño on the PFS precipitation over Southern China would be expected, implying potentially enhanced seasonal predictability of the regional climate in the future.

Sensitivity of Arctic Surface Temperature to Including a Comprehensive Ocean Interior Reflectance to the Ocean Surface Albedo Within the Fully Coupled CESM2

AbstractAlmost all current climate models simplify the ocean surface albedo (OSA) by assuming the reflected solar energy without the ocean interior contribution. In this study, an improved ocean surface albedo scheme is incorporated into the Community Earth System Model version 2 (CESM2) to assess the sensitivity of Arctic surface temperature to including ocean interior reflectance to the OSA. Fully coupled CESM2 simulations with and without ocean interior reflectance are subsequently performed, we focus on the analysis of Arctic surface temperature responses. Incorporating ocean interior reflectance increases absorbed solar radiation and warms the ocean, enhancing seasonal heat storage and release across the Arctic Ocean, and increasing sea ice reduction and positive climate feedbacks that elevates Arctic surface temperature. Seasonal variations in air‐surface temperature differences induce changes in turbulent heat flux patterns, concurrently modifying dynamic advection and moisture processes that affect boundary layer humidity and low clouds, especially in winter. Based on partitioning physical processes in the thermodynamic energy equation, surface air warming is induced primarily through positive heating anomalies of vertical advection, latent heat release, and longwave radiative forcing. Through an examination of the surface energy budget, skin temperature warming is driven predominantly by increased downward longwave radiation, positive surface albedo feedback in summer, and increased conductive heat transport from the ocean particularly in winter. Significant effects of ocean interior reflectance on the Arctic Ocean, including sea surface warming and sea ice reduction, justify the importance of ocean interior reflectance in climate models for better understanding of ongoing Arctic climate changes.

Projections of central Arctic summer sea surface temperatures in CMIP6

One of the most dramatic climate responses to future global warming is the near-disappearance of the perennial sea ice cover in the central Arctic Ocean, a phenomenon known as the ice-free summer Arctic. The immediate consequence of an ice-free Arctic would be the surface warming of the central Arctic Ocean, where sea surface temperatures (SSTs) used to be at freezing levels. Through an analysis of climate models participating in the climate model intercomparison project phase-6, this study demonstrates a wide range of responses in the central Arctic SSTs in August–September as a result of the projected ice-free summer Arctic. These responses vary from 0.7 to 8 °C in the shared socioeconomic pathway 2 (SSP2-4.5), referred to as the ‘middle of the road’ scenario, in which socioeconomic and technological trends do not significantly deviate from historical patterns. The extent of the central Arctic sea surface warming in August–September is found to have a loose correlation with the September sea ice extent (SIE), but a stronger connection to the SIE during spring to early summer (May–July), when incoming shortwave radiation is most intense. In certain climate models, the perennial sea ice cover disappears completely in September, causing central Arctic SSTs to rise by 5 °C–8 °C by the end of the 21st century. This leads to a bimodal distribution of annual SSTs. Further analysis reveals a close relationship between mid-summer SSTs in the central Arctic Ocean and preceding winter sea ice thickness (SIT). This underscores the significance of winter SIT in predicting future Arctic surface warming and marine heatwaves.

Changes in ENSO Modulation of the Distribution of Rapidly Intensifying Tropical Cyclones over the Western North Pacific in Boreal Autumn

Abstract This work explores the modulation of the Pacific decadal oscillation (PDO) on the relationship between the occurrence position of rapid intensification (RI) events of tropical cyclones (TCs) over the western North Pacific (WNP) in boreal autumn and El Niño–Southern Oscillation (ENSO). From the warm to cold phase of the PDO, the occurrence position of WNP RI events experiences a significant westward shift of 5.5° in El Niño years and a significant northward shift of 4.5° in La Niña years. The strengthening of thermodynamic conditions west of 160°N plays a dominant role in the westward shift of RI events in El Niño years, and the northward shift in La Niña years is associated with the expansion of areas with warm sea surface temperature, high tropical cyclone heat potential and midlevel relative humidity, strengthening of relative vorticity north of 20°N, and weakening of dynamic conditions within 10°–20°N. During the PDO cold phase, the descending branch of the Walker circulation over the western Pacific is weak and shifts west of 140°E in El Niño years, whereas it is much stronger in La Niña years. In addition, the Hadley circulation over the WNP shows little change during El Niño, but the ascending branch around 10°N expands to 20°N during La Niña. These trends reflect the changing responses of the WNP environment to ENSO variation and are consistent with the changing distribution of WNP RI events. Moreover, during the PDO cold phases, SST over the north Indian Ocean is much warmer, and anomalous anticyclonic circulation occurs in the WNP in boreal spring (summer and autumn) during the developing phase of El Niño (La Niña) years, which may also contribute to strengthening the thermodynamic conditions over the WNP.

Eelgrass (Zostera marina L.) populations are threatened by high sea-surface temperatures and impaired waters on Nantucket Island, USA

Eelgrass (Zostera marina L.) is a key foundation species that provides multiple ecosystem services to shallow coastal and estuarine systems in the Northern Hemisphere. It is estimated that, over the last century, up to 50 % of all Z. marina habitat has been lost along the east coast of the USA due to factors including light reduction, eutrophication, and physical disturbance. Warming sea surface temperatures are also believed to be exacerbating losses and the future of this ecosystem is unclear. Here, we assess Z. marina meadows on Nantucket, an island system located 50 km off-shore of Massachusetts, by using common indicators of seagrass plant health and environmental quality. Our results show that Z. marina meadows on Nantucket Island are thermally stressed and light-limited during parts of their peak growing season. This suggests that sea-surface temperatures are a pivotal factor, along with cultural eutrophication, in observed large-scale losses of Z. marina and that further degradation could be expected in the future as the climate continues to warm. Methods from this study may be used by managers as a guide to assess seagrass ecosystem status in degrading systems.

Emergence of the Central Atlantic Niño.

The Atlantic Niño is characterized by sea surface warming in the equatorial Atlantic, which can trigger La Niña, the cold phase of El Niño-Southern Oscillation (ENSO). Although observations show that the Atlantic Niño has weakened by approximately 30% since the 1970s, its remote influence on ENSO remains strong. Here, we show that this apparent discrepancy is due to the existence of two types of Atlantic Niño with distinct patterns and climatic impacts, which we refer to as the central and eastern Atlantic Niño. Our results show that with equal strength, the central Atlantic Niño has a stronger influence on tropical climate than its eastern counterpart. Meanwhile, the eastern Atlantic Niño has weakened by approximately 50% in recent decades, allowing the central Atlantic Niño to emerge and dominate the remote impact on ENSO. Given the distinct climatic impacts of the two types, it is necessary to distinguish between them and investigate their behaviors and influences on climate in future studies.

Paleocene–Eocene boundary in the Gabal El-Gir, east Qena, Egypt: Paleoenvironment and sequence stratigraphy interpretations

Comprehensive field, litho-, biostratigraphy, and paleoenvironmental studies of the upper Paleocene–lower Eocene (P–E) boundary interval in the Gabal El-Gir area were conducted. Two selected sections El-Gir-1 and El-Gir-2 were investigated. The prevailed paleoenvironmental conditions were discussed in the frame of the nannofossils, planktonic, and benthic foraminiferal assemblages. Benthic foraminifera assemblages indicate a remarkable turnover across the PETM. The extinction of Angulogavelinella avnimelechi at the base of the PETM represents a significant marker for the base of the benthic foraminifera extinction event at this site and other neritic sites in the Tethyan region. The dominance of calcareous planktonic excursion taxa at the base of the PETM indicates adaptation to extremely warm sea surface temperatures and low nutrient levels. Also, our data suggests that the biotic responses to environmental change in the early Eocene resembled those observed in the early Eocene hyperthermals at the nearbay Dababiya and El-Ballas sections. This could indicate that similar processes characterized the Egyptian sedimentary basin during the early Eocene. Stratigraphically, despite the limited distance between the studied sections, the Dababiya Quarry Member (DQM) in El-Gir-1 resemble those of the Global Stratotype Section and Point (GSSP) but reduced in thickness. While in El-Gir-2, the lower part of the DQM was missing. These findings could indicate that the DQM was deposited in a submarine channel. Three third-order depositional sequences were recognized covering the P–E interval. The reconstructed sea level curve shows the presence of eustatic similarities with the global sea level.

Processes Controlling the Seasonally Varying Emergence of Forced Arctic Longwave Radiation Changes

Abstract Most observed patterns of recent Arctic surface warming and sea ice loss lie outside of unforced internal climate variability. In contrast, human influence on related changes in outgoing longwave radiation has not been assessed. Outgoing longwave radiation captures the flow of thermal energy from the surface through the atmosphere to space, making it an essential indicator of Arctic change. Furthermore, satellites have measured pan-Arctic radiation for two decades while surface temperature observations remain spatially and temporally sparse. Here, two climate model initial-condition large ensembles and satellite observations are used to investigate when and why twenty-first-century Arctic outgoing longwave radiation changes emerge from unforced internal climate variability. Observationally, outgoing longwave radiation changes from 2001 to 2021 are within the range of unforced internal variability for all months except October. The model-predicted timing of Arctic longwave radiation emergence varies throughout the year. Specifically, fall emergence occurs a decade earlier than spring emergence. These large emergence timing differences result from seasonally dependent sea ice loss and surface warming. The atmosphere and clouds then widen these seasonal differences by delaying emergence more in the spring and winter than in the fall. Finally, comparison of the two ensembles shows that more sea ice and a more transparent atmosphere during the melt season led to an earlier emergence of forced longwave radiation changes. Overall, these findings demonstrate that attributing changes in Arctic outgoing longwave radiation to human influence requires understanding the seasonality of both forced change and internal climate variability.

Extremely long-lived marine heatwave in South China Sea during summer 2020: Combined effects of the seasonal and intraseasonal variations

South China Sea (SCS) experienced an exceptionally long-lived marine heatwave (MHW) event in summer 2020, which broke the historical records in terms of its time duration since 1982. This long-lasting MHW was primarily attributed to the combined effects of the seasonal cycle (SC) and 30–60-day intraseasonal oscillation (ISO) components of anomalous atmospheric circulations. During the SC developing phase, the superimposed anticyclonic circulation regimes of the SC and 30–60-day ISO jointly contributed to the first warming sea surface temperature (SST) peak over the central SCS in July, via the combined effects of net downward shortwave radiation (SWR) and latent heat flux (LHF) anomalies. In contrast, during the SC decaying phase, the SCS MHW-related warming SST tendency was mostly due to 30–60-day ISO signals. During the growth phase of ISO, the joint warming effect of the SWR and LHF anomalies was the dominant factor maintaining the SCS MHW. In the decaying phase of 30–60-day ISO, however, the second MHW-related warming peak around Beibu Gulf in September was mainly contributed by the sharply reduced LHF loss due to strong southwesterly anomalies over the northern SCS on the 30–60-day timescale. The varying SC component plays a crucial role in the occurrence and maintenance of MHWs, while the 30–60-day ISO can effectively regulate the development and pattern of SCS MHWs. These results suggest that, besides the vital role of the SC component, the 30–60-day ISO should also be considered for fully understanding the extreme MHW events.

Warm Clouds Biases in CMIP6 Models Linked to Indirect Effects of Falling Ice‐Radiation Interactions Over the Tropical and Subtropical Pacific

AbstractWe examine the spatial distributions of CMIP6‐simulated cloud liquid water path (CLWP) and content (CLWC) against MODIS and CloudSat synthesized data over the tropical and subtropical Pacific. Three subsets of models are categorized based on their treatments of frozen ice‐radiative interactions. CLWP/CLWC are generally well simulated in subset with separately‐calculated radiative effects of cloud ice and falling ice (SON2). Too much warm clouds above 750 hPa are produced in either subset with total frozen ice radiative effects (SON1) or subset without radiative effects of falling ice (NOS) and thus CLWP/CLWC are overestimated over the open ocean including the trade‐wind regions. Stratocumulus clouds off the coasts of North and South America are severely underestimated in NOS models. We attribute the overestimates of clouds above the trade‐wind boundary layers to anomalous ascending motion associated with warmer sea surface temperature and weaker surface wind stress linked to indirect effects of falling ice‐radiation interactions.

Diagnosing Hurricane Barry Track Errors and Evaluating Physics Scalability in the UFS Short-Range Weather Application

To assess the performance and scalability of the Unified Forecast System (UFS) Short-Range Weather (SRW) application, case studies are chosen to cover a wide variety of forecast applications. Here, model forecasts of Hurricane Barry (July 2019) are examined and analyzed. Several versions of the Global Forecast System (GFS) and Rapid Refresh Forecast System (RRFS) physics suites are run in the UFS-SRW at grid spacings of 25 km, 13 km, and 3 km. All model configurations produce significant track errors of up to 350 km at landfall. The track errors are investigated, and several commonalities are seen between model configurations. A westerly bias in the environmental steering flow surrounding the tropical cyclone (TC) is seen across forecasts, and this bias is coincident with a warm sea surface temperature (SST) bias and overactive convection on the eastern side of the forecasted TC. Positive feedback between the surface winds, latent heating, moisture, convection, and TC intensification is initiated by this SST bias. The asymmetric divergent flow induced by the excess convection results in all model TC tracks being diverted to the east as compared to the track derived from reanalysis. The large differences between runs using the same physics packages at different grid spacing suggest a deficiency in the scalability of these packages with respect to hurricane forecasting in vertical wind shear.

Dietary Plasticity Mitigates Impacts to Reproduction for the Gull-Billed Tern Gelochelidon nilotica During Abnormally Warm Sea Surface Temperature Events in California, U.S.A.

Dietary Plasticity Mitigates Impacts to Reproduction for the Gull-Billed Tern Gelochelidon nilotica During Abnormally Warm Sea Surface Temperature Events in California, U.S.A.

Dynamic controls of the interannual and interdecadal variations of the freezing level height over the Tibetan Plateau

The variation of freezing level height (FLH) is significantly indicated the cryosphere change under global warming. In this study, the FLH during boreal summer in the TP regions is investigated, with the leading mode of a mono-sign pattern both on interannual and interdecadal timescales. The interannual variation of the summer FLH is dominantly coupled with the summer Arctic Oscillation (AO). When the AO is in positive phase, positive geopotential height anomalies are prolonged in the mid-latitude and negative anomalies are present over the Arctic regions. Meanwhile, the westerly shifts northward and strengthened, preventing the cold air propagating southward. These conditions are beneficial for the surface and free-air temperature increasing and thus enhance the FLH over the TP. On the interdecadal timescale, the Pacific Decadal Oscillation (PDO) has mainly contributed to the variation of summer FLH. In the negative PDO phase, the warming sea surface temperature anomalies appear in the North Pacific, which could drive wave trains propagating eastward and induce high pressure around the northern TP. With the anomalous anticyclone from TP to western North Pacific, the southeasterly wind is prevalent in the TP. Consequently, the northern TP becomes warming remarkably and leads to the rising of FLH. This study provides an insight perspective for understanding the changes of FLH over the TP, which also suggests the influence on the cryosphere processes at multi-timescale variations.

The Role of Tropical Easterly Jet on the Bay of Bengal’s Tropical Cyclones: Observed Climatology and Future Projection

Abstract Tropical cyclones (TCs) in the Bay of Bengal (BoB) are among the most devastating events in nature, significantly influencing human life. They affect the region between April and December, being separated into two distinct phases: the premonsoon (April–June, AMJ) and postmonsoon (October–December, OND), with little activity during the monsoon peak. In this article, the influence of the tropical easterly jet (TEJ) on the observed BoB TC activity, and the expected changes under global warming conditions, are investigated using reanalyses and Coupled Model Intercomparison Project phase 6 (CMIP6) models. The general assumption is that TC activity should increase in response to warmer sea surface temperature (SST). However, several dynamical processes are needed to create organized convection, which is a prerequisite for TC development. The results show that the frequency and accumulated energy should decrease in the future (2020–49), compared to the historical climate (1950–79), during both phases. However, the northern BoB TCs’ intensity increases during OND. The change of the mean TEJ and intermittent shear relaxations in the future climate are found to play a major role in the change of TC activity rather than the SST. During AMJ, a southward shift of the mean TEJ and its intermittent shear relaxations are associated with reduced TC activity. During OND, the decrease of the frequency of TEJ and subtropical jet (STJ) intermittent shear relaxations are associated with the reduction of TC activity. This study clarifies the importance of both the mean and intermittency of the TEJ in determining the future of TC activity in the BoB.

Contribution of sea surface temperature anomalies in the Barents and Yellow-Japan Seas to summer high temperature anomalies over the eastern Tibetan Plateau

In the summer of 2022, extremely high surface air temperature (SAT) anomalies occurred over the Tibetan Plateau (TP), especially the eastern TP (ETP). Using the 2022 high SAT anomaly over the ETP as an example, the present study investigates the characteristics of the variability of summer SATs over the ETP and associated oceanic forcing factors. The results reveal that the warmer sea surface temperature anomalies (SSTAs) in the Barents Sea (BS) and Yellow-Japan Sea (YJS) are jointly responsible for the higher summer SATs over the ETP for the period 1961–2022 and can also explain the higher ETP SAT anomaly in the summer of 2022. The warmer BS SSTA can lead to an upper-tropospheric high-pressure anomaly around the ETP (i.e., a strengthened and northeastward-extended South Asian high) and the associated higher ETP SAT anomaly by exciting local upper-tropospheric geopotential height anomaly over the BS and accordingly modulating the downstream Rossby wave from the BS to the ETP. The warmer YJS SSTA can directly cause an upper-tropospheric high-pressure anomaly around the ETP and YJS and eventually enhance the SATs over the ETP through stimulating local upward motion and guiding the Rossby wave from the upstream region to the ETP and YJS. This mechanism can also explain the extremely high SAT anomaly over the ETP in the summer of 2022. Moreover, the BS and YJS SSTAs can effectively modulate the summer SATs over the ETP even though the absence of the effect of El Niño/La Niña. These findings may provide valuable insights into the reason for the variability of summer SATs over the TP.

Understanding changes in heat waves, droughts, and compound events in Yangtze River Valley and the corresponding atmospheric circulation patterns

Heat waves, droughts, and compound drought and heat waves (CDHWs) have received extensive attention because of their disastrous impacts on agriculture, ecosystems, human health, and society. Here, we computed the heat wave magnitude index (HWMI), drought magnitude index (DMI), and compound drought and heat wave magnitude index (CDHMI) for Yangtze River Valley (YRV) from July to August during 1961–2022. We compared the large-scale atmospheric circulation characteristics of different extreme events based on these indexes. The results show that the positive center with sink motion in East Asia provides a favorable circulation background for heat wave events. Drought events are mainly affected by the zonal wave train dominated by a significant negative anomaly in Siberia and a high-pressure anomaly upstream, and a anticyclonic water vapor with strong divergence over the Yangtze River basin. During CDHW events, both anomalous systems that affect heat waves and droughts appear and strengthen simultaneously. Specifically, in the middle and upper troposphere, the positive height anomaly center in YRV expands abnormally, and the “+–+” wave train over the northern 50° N region of East Asia becomes more obvious. Therefore, the positive anomaly and water vapor anomaly brought by the two circulation patterns at different latitudes are superimposed over the YRV, leading to severe CDHWs. At the same time, the warm positive eddy center and cold negative eddy center in high latitudes exhibit more stable positive pressure features, which are conducive to the persistent development and strengthening of CDHWs. In addition, the anomalous warm sea surface temperature in western Pacific moderating the favorable circulation patterns may also promote the occurrence of CDHWs in the YRV during the same period.

Origins of Southern Ocean warm sea surface temperature bias in CMIP6 models

The warm sea surface temperature (SST) bias in the Southern Ocean (SO) has persisted in several generations of Coupled Model Inter-comparison Project (CMIP) models, yet the origins of such a bias remain controversial. Using the latest CMIP6 models, here we find that the warm SST bias in the SO features a zonally oriented non-uniform pattern mainly located between the northern and southern fronts of the Antarctic Circumpolar Current. This common bias is not likely to be caused by the biases in the surface heat flux or the strength of the Atlantic meridional overturning circulation (AMOC) — the two previously suggested sources of the SO bias based on CMIP5 models. Instead, it is linked to the robust common warm bias in the Northern Atlantic deep ocean through the AMOC transport as an adiabatic process. Our findings indicate that remote oceanic biases that are dynamically connected to the SO should be taken into account to reduce the SO SST bias in climate models.

The Influence of Large-Scale Radiation Anomalies on Tropical Cyclone Frequency

Abstract The response of tropical cyclone (TC) frequency to sea surface warming is uncertain in climate models. We hypothesize that one source of uncertainty is the anomalies of large-scale atmospheric radiation in response to climate change, and whose influence on TC frequency is investigated. Given two atmospheric models with opposite TC frequency responses to uniform sea surface warming, we interchange their atmospheric radiation anomalies in experiments with prescribed radiative heating rates. The largest model discrepancy occurs in the western North Pacific, where the TC frequency tends to increase with anomalous large-scale ascent caused by prescribed positive radiation anomalies, while the TC frequency tends to decrease with anomalous large-scale descent caused by prescribed negative radiation anomalies. The model spread in TC frequency response is approximated by the model spread in the frequency response of pre-TC vortices (seeds), which is explained by changes in the large-scale circulation using a downscaling formula known as the seed propensity index. We further generalize the index to predict the influence of large-scale radiation anomalies on TC seed frequency. The results show that model spread in TC and seed frequency response can be reduced when constraining the large-scale radiation anomalies. Significance Statement It is difficult to predict whether tropical cyclones will occur more or less frequently in the future and by how much. We show that tropical cyclone frequency is strongly influenced by the global pattern of heating and cooling due to radiation, a process that has been neglected in existing theories. Our theory improves understanding of how tropical cyclones respond to climate change, explaining why one model may predict a frequency increase while a different but equally realistic model may predict a frequency decrease. One reason for the difficulty in predicting tropical cyclone frequency is found to be the difficulty in predicting how global cloud distribution will change in the future.