Upward Latent Heat Flux Research Articles

The Inter‐Model Uncertainty of Projected Precipitation Change in Northern China: The Modulating Role of North Atlantic Sea Surface Temperature

AbstractPrecipitation changes in northern China are projected to increase in the Coupled Model Inter‐comparison Project Phase 6 (CMIP6) multi‐model ensemble. However, these projections are accompanied by notable inter‐model uncertainty, and the sources of this uncertainty remain largely unexplored. By analyzing 30 CMIP6 models, this research explores the source of inter‐model uncertainty in projected precipitation change and reveals the fundamental mechanism driving uncertainty spread. Following the empirical orthogonal function of inter‐model projected precipitation change, the leading mode displays a seesaw spatial pattern between northwest and north China. This phenomenon predominantly stems from the inter‐model divergence of projected sea surface temperature (SST) warming in the North Atlantic. Further scrutinizing the ocean mixed layer heat budget, we discover that the combined effect of surface sensible heat flux, net surface shortwave radiation flux, and ocean heat transport convergence influences heat flux and SST change of North Atlantic. The multi‐model projections indicate that localized increases in solar radiation and heat convergence warm sea surface, raising SST and initiating convective motion. This convective motion subsequently transforms the 200 hPa teleconnection wave train, leading to an anti‐phase pattern over northern China. This wave pattern modulates total cloud cover percentage, influences surface upward latent heat flux, and adjusts the top of atmosphere outgoing longwave radiation, collectively resulting in the seesaw pattern. Our study underscores the pivotal role of inter‐model disparities in North Atlantic SST warming projection, which is a primary driver of precipitation uncertainty in northern China. These insights offer an essential foundation for refining and diminishing inter‐model uncertainty.

An underlying factor of increasing early winter precipitation in the Hokuriku region of Japan in recent decades

AbstractUsing a reanalysis dataset and large‐ensemble simulation results, this study examines a possible factor of increasing trend in early winter precipitation in recent decades in the Hokuriku region of Japan. Monthly precipitation in December has a significant increasing trend after the early 1990s, which is different from those in January and February. The increasing precipitation in December is related to that in the sea surface upward latent heat flux due to intensified winter monsoon circulation and warming sea surface temperatures (SSTs) over the Sea of Japan. December averaged SSTs show a trend pattern in recent decades that is similar to the negative phase of the interdecadal Pacific oscillation (IPO), accompanied by positive trends from the eastern Indian Ocean to the western tropical Pacific. The enhanced trend of convection over the Bay of Bengal is seen; suggesting a combined effect of climatologically high SSTs and IPO‐related warmed SSTs over the region. Trends in recent decades of an upper‐level wavy pattern from South Asia to near Japan along the subtropical jet associated with enhanced convection near the Bay of Bengal and the related pressure drop from Japan to the north are seen, which contribute to intensified winter monsoon circulation.

Long-term warming and interannual variability contributions’ to marine heatwaves in the Mediterranean

In the past 40 years, marine heatwaves (MHWs) have experienced a worldwide increase in duration, intensity, frequency and spatial extent. This trend has been particularly evident in the Mediterranean, where exceptional events were observed during the summers of 2022, 2018 and 2003. This study proposes a twofold analysis of MHWs in the Mediterranean, focusing on their statistical characteristics and physical causes. A satellite dataset is utilized to analyze MHWs via an index, called activity, which aggregates the occurrence, duration, intensity and spatial extent of events. Our results show that the trend toward more active summers for MHWs is strongest in the western Mediterranean basin and long-term warming is the main driver in the whole Mediterranean basin. We also show that in the western and Adriatic Mediterranean region, the increase of SST variability contributes about a third to the MHW activity long-term trend whereas in the central, eastern and Aegean basins, the variability of SST mostly acts to diminish this trend. Through principal component analysis (PCA) of MHW activity, we found that the three most severe summer MHW events in the Mediterranean occur at the same location where the overall trend is highest. Interannual variability increased MHW activity in 2022 around the Balearic Sea, in 2018 in the eastern basins and in 2003 in the central basins. A joint PCA revealed that the long-term trend in MHW activity co-varies with a positive geopotential height anomaly over the Mediterranean, which is consistent with the generation of atmospheric-driven MHWs and which, at the North Atlantic scale, resembles the positive phase of the summer East Atlantic. The additional interannual variability contribution to these three severe summers was associated with western warming and projected onto the positive phase of the summer North Atlantic Oscillation. The increase in MHW over the last 40 years is also associated in the western, central and Adriatic regions with increased downward short-wave radiation and in the eastern Mediterranean with decreased upward long-wave radiation. Increased upward latent heat flux partly compensated for the MHW long-term increase over the whole Mediterranean basin. The interannual variability of MHW activity is related in the western, central and Adriatic basins to increased downward sensible and decreased upward latent heat flux possibly due to warm and humid air intrusion.

Asymmetric modulation of solar activity on tropical cyclone frequency over the western North Pacific and the possible mechanism

The impacts of solar activity on the tropical Pacific climate have been widely reported. However, few studies focus on the effects of solar activity on the tropical cyclone (TC). Based on the observational and reanalysis data for 1979–2020, this study investigated the solar modulation of TC frequency over the western North Pacific in different solar cycle phases. Results suggest that the regressions of TC frequency to solar activity are asymmetric in the high- and low-solar activity years (HS and LS). Specifically, the intensified solar activity could markedly induce more TCs in HS; however, no significant modulation can be found in LS. Further exploration reveals a possible air–sea coupled mechanism for this interesting phenomenon. The relatively cloud-free area in the western North Pacific could receive more incoming solar radiation at the surface in HS than in LS. This increased regional surface net solar radiation in HS could produce a stronger surface upward latent heat flux and, thus, greater evaporation. Along with that, the local upward motion is dramatically enhanced over the TC source. Then, for compensation, the regional sea level pressure is reduced, and the low-level winds become cyclonic over the TC origin. All of these solar-caused regional atmospheric anomalies in HS contribute to more TC generations. The key to this possible mechanism is the increased regional solar forcing at the ocean surface that is amplified by regionally enhanced upward latent heat flux and evaporation.

Estimation of Daily Arctic Winter Sea Ice Thickness from Thermodynamic Parameters Using a Self-Attention Convolutional Neural Network

Thermodynamic parameters play a crucial role in determining polar sea ice thickness (SIT); however, modeling their relationship is difficult due to the complexity of the influencing mechanisms. In this study, we propose a self-attention convolutional neural network (SAC-Net), which aims to model the relationship between thermodynamic parameters and SIT more parsimoniously, allowing us to estimate SIT directly from these parameters. SAC-Net uses a fully convolutional network as a baseline model to detect the spatial information of the thermodynamic parameters. Furthermore, a self-attention block is introduced to enhance the correlation among features. SAC-Net was trained on a dataset of SIT observations and thermodynamic data from the 2012–2019 freeze-up period, including surface upward sensible heat flux, surface upward latent heat flux, 2 m temperature, skin temperature, and surface snow temperature. The results show that our neural network model outperforms two thermodynamic-based SIT products in terms of accuracy and can provide reliable estimates of SIT. This study demonstrates the potential of the neural network to provide accurate and automated predictions of Arctic winter SIT from thermodynamic data, and, thus, the network can be used to support decision-making in certain fields, such as polar shipping, environmental protection, and climate science.

Assessing the impact of the recent warming in the East China Sea on a torrential rain event in northern Kyushu (Japan) in early July 2017

Sea surface temperature (SST) in the East China Sea (ECS) has undergone a rapid rise in recent decades, but the associated impact on extreme weather remains under debate. Here, using a cloud-permitting model, we assess the impact of the ECS warming observed since the 1980s on a torrential rain event that caused devastating floods and landslides in the Kyushu Island, western Japan, in July 2017. Without the increasing trends of SST and air temperature, the model cannot reproduce the observed extremely high amount of precipitation during the event, i.e., >700 mm/12-h. The SST increase is found more influential in determining the precipitation amount. Without the ocean warming, increases in precipitable water and horizontal moisture transport due to the atmospheric warming would not lead to precipitation increase during this event. The change in the amount of precipitation can be largely explained by the change in the updraft intensity of the convective system. Higher SST suppresses downward surface sensible heat flux and enhances upward latent heat flux along the paths of air parcels flowing into the convective system in this case. This increases the equivalent potential temperature in the lower troposphere, which enhances the convective available potential energy in the lower troposphere, leading to intensification of the convective system and thereby the increase of precipitation. The findings of this case study suggest an important role of the warming ECS in the intensification of torrential rain events around Japan and the necessity of further assessment of the role of the ocean warming in the torrential rains.

Atmospheric conditions conducive to marine fog over the northeast Pacific in winters of 1979–2019

Observations show that the northeast Pacific (NEP) is a fog-prone area in winter compared with the northwest and central Pacific where fog rarely occurs in winter. By synthesizing observations and reanalysis results from 1979 to 2019, this study investigates the atmospheric circulation and marine atmospheric boundary layer structure associated with marine fog over the NEP in winter. Composite analysis shows that the eastern flank of the Aleutian low and the northwestern flank of the Pacific subtropical high jointly contribute to a northward air flow over the NEP. Under such conditions, warm and moist air flows through a cooler sea surface and facilitates the formation of advection-cooling fog. The air near the sea surface in foggy areas is cooled by the downward sensible heat flux. The smaller upward latent heat flux (∼10 W m−2) compared to the surrounding area (>60 W m−2) demonstrates that the moisture originates from the advection instead of local evaporation. The lower (at 925 to 875 hPa) and stronger (up to 0.08 K hPa−1) inversion layer, compared with cloudy cases and the turbulence in the lower atmosphere (below 975 hPa), also promotes fog formation and evolution. Approximately 68% of all fog cases (42242) show positive differences between surface air temperature (SAT) and sea surface temperature (SST), while 32% are negative, during southerly winds. Composite analysis of the latter shows lower specific humidity above the inversion bottom compared to the former. Dry air enhances longwave radiative cooling from the fog top, favoring cooling of the fog layer, gradually causing SAT to fall below SST.

Land Use Change Impact on Normalized Difference Vegetation Index, Surface Albedo, and Heat Fluxes in Jambi Province: Implications to Rainfall

Jambi covers various land uses with different characteristics related to biogeophysical cycle. Land use plays an important role in the atmosphere-surface interaction and energy balance partition, which influenced rainfall pattern. Two proxies widely used to differentiate various land uses are albedo and normalized difference vegetation index (NDVI). However, study on albedo and NDVI relationship with rainfall in Jambi is still limited. This study aims to analyze the correlation of NDVI and albedo with rainfall and their distribution in Jambi and Muaro Jambi in 2013 and 2017. The research used Landsat 8 OLI TIRS satellite image data to derived NDVI and albedo, and CHIRPS data for rainfall. A simple linear regression was used to calculate the correlation of NDVI and albedo with rainfall. The results showed that the distribution of albedo for each land use class from the lowest to the highest was forest, plantation, cropland, shrubs, and settlements, respectively. On the contrary, the distribution of NDVI and rainfall is the inverse to albedo. Albedo and NDVI had a strong influence on rainfall through surface energy balance partition. This was indicated by the high R-square between albedo and rainfall (0.99) and between NDVI and rainfall (0.97). Increasing upward latent heat flux from the land surface to atmosphere leads to a rainfall increase. In other words, rainfall may also increase with the decrease in albedo, increase in NDVI, or land use change.

Separating the Influences of Low-Latitude Warming and Sea Ice Loss on Northern Hemisphere Climate Change

AbstractAnalyzing a multimodel ensemble of coupled climate model simulations forced with Arctic sea ice loss using a two-parameter pattern-scaling technique to remove the cross-coupling between low- and high-latitude responses, the sensitivity to high-latitude sea ice loss is isolated and contrasted to the sensitivity to low-latitude warming. Despite some differences in experimental design, the Northern Hemisphere near-surface atmospheric sensitivity to sea ice loss is found to be robust across models in the cold season; however, a larger intermodel spread is found at the surface in boreal summer, and in the free tropospheric circulation. In contrast, the sensitivity to low-latitude warming is most robust in the free troposphere and in the warm season, with more intermodel spread in the surface ocean and surface heat flux over the Northern Hemisphere. The robust signals associated with sea ice loss include upward turbulent and longwave heat fluxes where sea ice is lost, warming and freshening of the Arctic Ocean, warming of the eastern North Pacific Ocean relative to the western North Pacific with upward turbulent heat fluxes in the Kuroshio Extension, and salinification of the shallow shelf seas of the Arctic Ocean alongside freshening in the subpolar North Atlantic Ocean. In contrast, the robust signals associated with low-latitude warming include intensified ocean warming and upward latent heat fluxes near the western boundary currents, freshening of the Pacific Ocean, salinification of the North Atlantic, and downward sensible and longwave fluxes over the ocean.

Different Influencing Mechanisms of Two ENSO Types on the Interannual Variation in Diurnal SST over the Niño-3 and Niño-4 Regions

AbstractIn this paper, the different effects of the eastern equatorial Pacific (EP) and central equatorial Pacific (CP) Ocean El Niño–Southern Oscillation (ENSO) events on interannual variation in the diurnal sea surface temperature (SST) are explored in both the Niño-3 and Niño-4 regions. In the Niño-3 region, the diurnal SST anomaly (DSSTA) is negative during both EP and CP El Niño events and becomes positive during both EP and CP La Niña events. However, the DSSTA in the Niño-4 region is positive in El Niño years and negative in La Niña years, which is opposite to that in the Niño-3 region. Further analysis indicates that the incident shortwave radiation (SWR), wind stress (WS), and upward latent heat flux (LHF) are the main factors causing the different interannual variations in the DSST. In the Niño-3 region, decreased SWR and increased LHF lead to a negative DSSTA in EP El Niño years, and enhanced WS and increased LHF cause a negative DSSTA in CP El Niño years. Conversely, in that same region, increased SWR and decreased LHF lead to a positive DSSTA in EP La Niña years, and reduced WS and decreased LHF cause a positive DSSTA in CP La Niña years. In the Niño-4 region, the reduced trade wind plays a key role in producing the positive DSSTA, whereas the decreased SWR has an opposite effect that reduces the range of the DSSTA during both EP and CP El Niño events, and conversely the enhanced trade wind plays a key role in producing the negative DSSTA, whereas the increased SWR has an opposite effect that increases the range of the DSSTA during both EP and CP La Niña events.

Different Influences of Southeastern Indian Ocean and Western Indian Ocean SST Anomalies on Eastern China Rainfall during the Decaying Summer of the 2015/16 Extreme El Niño

ABSTRACTPrevious studies linked the increase of the middle and low reaches of the Yangtze River (MLRYR) rainfall to tropical Indian Ocean warming during extreme El Niños’ (e.g., 1982/83 and 1997/98 extreme El Niños) decaying summer. This study finds the linkage to be different for the recent 2015/16 extreme El Niño’s decaying summer, during which the above-normal rainfalls over MLRYR and northern China are respectively linked to southeastern Indian Ocean warming and western tropical Indian Ocean cooling in sea surface temperatures (SSTs). The southeastern Indian Ocean warming helps to maintain the El Niño–induced anomalous lower-level anticyclone over the western North Pacific Ocean and southern China, which enhances moisture transport to increase rainfall over MLRYR. The western tropical Indian Ocean cooling first enhances the rainfall over central-northern India through a regional atmospheric circulation, the latent heating of which further excites a midlatitude Asian teleconnection pattern (part of circumglobal teleconnection) that results in an above-normal rainfall over northern China. The western tropical Indian Ocean cooling during the 2015/16 extreme El Niño is contributed by the increased upward latent heat flux anomalies associated with enhanced surface wind speeds, opposite to the earlier two extreme El Niños.

The impact of north tropical Atlantic sea surface temperature anomalies in the ensuing spring of El Niño on the tropical Indian Ocean and Northwest Pacific

AbstractBased on the fifth generation reanalysis of European Centre for Medium–Range Weather Forecasts (ECMWF), Hadley Centre sea ice and sea surface temperature data, and ENSEMLES hindcasts, the impact of north tropical Atlantic (NTA) sea surface temperature (SST) anomalies in the ensuing spring of El Niño on the tropical Indian Ocean (TIO) and Northwest Pacific (NWP) is investigated using composites, singular value decomposition, and correlations. The anomalous SST warming in the NTA region in the decay spring of El Niño leads to a Gill‐type response of the overlying atmosphere in spring–summer. The Kelvin wave over the TIO stimulated by the NTA warming causes the anomalous easterlies in the regions of TIO and NWP, thereby the strengthening of the NWP subtropical anticyclone (NWPSA). The NTA SST warming also forces a local ascending motion, leading to a downward branch over the NWP region. This descent gives rise to the TIO‐NWP low‐level easterly anomalies and an intensification of the NWPSA. The anomalous easterly weakens the background southwesterly monsoon, accompanied by the increased atmospheric boundary layer specific humidity caused by the troposphere warming, suppressing upward latent heat fluxes, finally contributing to the North Indian Ocean (NIO) warming in summer. This indicates that the second warming of the NIO region in the following summer of El Niño may be partially attributed to the NTA warming in the ensuing spring of El Niño. This study improves our understanding of the impact of the NTA SST anomalies in the El Niño decay spring on the TIO and NWP regions, and emphasizes the importance of the NTA region in the pantropical inter‐basin interactions.

Heat budget responses of the eastern China seas to global warming in a coupled atmosphere-ocean model

Impacts of climate change on heat budget in the Eastern China Seas (ECSs) are estimated under the historical, RCP4.5 and RCP8.5 scenarios using an atmosphere-ocean coupled regional climate model system (REMO/MPIOM). The results suggest that the recent and future ocean warming over the ECSs is linked overall to an increased oceanic heat transport by currents, which is partly compensated by the air-sea heat exchange. The Taiwan Strait is the major source of oceanic heat into the ECSs, whereas the shelf break section (SBS) acts as a heat sink. An increased net oceanic heat transport into the ECSs is projected under both considered RCP scenarios, mainly resulting from a reduction of the outward heat transport through the SBS. The mean relative contribution of SBS to the oceanic heat transport thus decreases by 4%–5% under both RCP scenarios, relative to historical run. Regarding the surface air-sea exchange, the heat loss caused by thermal radiation, latent and sensible heat in the ECSs exceeds the heat gain achieved by solar radiation. Under the RCP scenarios, warmer SST and stronger surface wind will enhance the upward latent heat flux, eventually leading to a more pronounced heat loss from the ECSs. The mean relative contributions of the latent heat flux to the air-sea heat exchange notably increases by 2%–3% under both projection scenarios, relative to historical run. Taking into account all components of the ECSs heat balance, we deduce that the increased horizontal heat transport will enhance the surface evaporation over the ECSs under future warming.

Is there an effect of Bay of Bengal salinity on the northern Indian Ocean climatological rainfall?

The northern Bay of Bengal (BoB) receives a large amount of freshwater directly from monsoonal rains over the ocean, and indirectly through river runoffs. It has been proposed that the resulting strong salinity stratification inhibits vertical mixing of heat, thus contributing to maintain warm sea surface temperature and high climatological rainfall over the BoB. In the present paper, we explore this positive feedback loop by performing sensitivity experiments with a 25-km resolution regional coupled climate model, that captures the main BoB features reasonably well. We confirm that salinity stratification tends to stabilize the upper ocean, thereby increasing the mixed layer warming due to vertical mixing by ∼+0.5 °C.month−1 on annual average. Salinity however also induces a compensating cooling by altering the mixed layer heating rate by air-sea heat fluxes, so that the net effect on climatological surface temperature is negligible. During and shortly after the southwest monsoon, this compensation predominantly occurs through increased cooling by upward latent heat fluxes. During boreal winter, it occurs because salinity favours a thinner mixed layer, which is more efficiently cooled by negative air-sea heat fluxes. These compensations result in a negligible climatological surface temperature and rainfall change at all seasons. This weak influence of salinity stratification on climatological surface temperature and rainfall in our model is robust when applying a flux correction to alleviate model biases, when neglecting the solar absorption below the mixed layer and when using different atmospheric radiation and convective parameterizations.

Interdecadal Indian Ocean Basin Mode Driven by Interdecadal Pacific Oscillation: A Season-Dependent Growth Mechanism

The interdecadal variability of basinwide sea surface temperature anomalies (SSTAs) in the tropical Indian Ocean (TIO), referred to as the interdecadal Indian Ocean basin mode (ID-IOBM), is caused by remote forcing of the interdecadal Pacific oscillation (IPO), as demonstrated by the observational datasets and tropical Pacific pacemaker experiments of the Community Earth System Model (CESM). It is noted that the growth of the ID-IOBM shows a season-dependent characteristic, with a maximum tendency of mixed layer heat anomalies occurring in early boreal winter. Three factors contribute to this maximum tendency. In response to the positive IPO forcing, the eastern TIO is covered by the descending branch of the anomalous Walker circulation. Thus, the convection over the southeastern TIO is suppressed, which increases local downward shortwave radiative fluxes. Meanwhile, the equatorial easterly anomalies to the west of the suppressed convection weaken the background mean westerly and thus decrease the upward latent heat fluxes over the equatorial Indian Ocean. Third, anomalous westward Ekman currents driven by the equatorial easterly anomalies advect climatological warm water westward and thus warm the western TIO. In summer, the TIO is out of the control of the positive IPO remote forcing. The ID-IOBM gradually decays due to the Newtonian damping effect.

Study of upper ocean parameters during passage of tropical cyclones over Indian seas

ABSTRACTThe primary energy supply for tropical cyclones is the upward latent heat fluxes that are directly related to Sea Surface Temperatures (SST). The strong winds induce a negative SST anomaly in the tropical cyclone wake. This is usually referred to as the ‘cold wake’. Many studies have suggested that the cold wake results in a significant reduction of upward latent heat fluxes that supply energy to the tropical cyclone, and hence provides a negative feedback on its intensity. The cold-wake feedback on the intensity of tropical cyclones is a strong motivation to understand the oceanic response to tropical cyclones. A recent study of re-examining the mechanisms controlling the cold wake has shown the importance of vertical Ekman pumping. Under the core of tropical cyclones (typically over a disk of 100 km radius), vertical pumping is responsible for a cooling of the entire water column, while surface heat fluxes and vertical mixing both contribute to the surface cooling during the cyclone passage. Farther away from the cyclone core, vertical mixing generally overwhelms the effect of vertical pumping, and also the effect of heat fluxes in the case of strong tropical cyclones. The study examines the Ekman pumping during the passage of few cyclonic storms in the Arabian Sea (AS) and Bay of Bengal (BOB) during last decade using different datasets such as Satellite (QuikScat) and reanalysis (ERA-Interim) data and calls up the value for satellite deliverables. Surface currents and subsurface temperature structure is examined further and for different Ekman pumping velocity (EPV), variable subsurface response is illustrated. The changes in ocean surface temperature, sea level and heat content obtained from the remote sensing data products, in the wake of cyclone are also reported and thus the usefulness of satellite products to study the coupled tropical system is demonstrated.

Short‐Term Impacts of the Megaurbanizations of New Delhi and Los Angeles Between 2000 and 2009

AbstractUrban areas are expanding worldwide due to increasing population, standard of living, and migration from rural areas. This study uses satellite and road data to quantify the urbanization of two megacities, New Delhi and Los Angeles, between 2000 and 2009. It then estimates, with a three‐dimensional nested global‐through‐urban climate, weather, and air pollution model, Gas, Aerosol, Transport, Radiation, General Circulation, Mesoscale, and Ocean Model, the short‐term atmospheric impacts of such urbanization alone. The simulations account for changes in meteorologically driven natural emissions, but not anthropogenic emissions, between 2000 and 2009. New Delhi's urban extent, defined based on the physical existence of its built structures and the transitional gradient from buildings to rural areas rather than on abrupt administrative borders, increased by ~80% and Los Angeles's by ~22.5% between 2000 and 2009. New Delhi experienced a larger increase in its urban extent relative to its population during this period than did Los Angeles. In both megacities, urbanization increased surface roughness, increasing shearing stress and vertical turbulent kinetic energy, decreasing near‐surface and boundary layer wind speed, contributing to higher column pollution levels. Urbanization may also have increased downward solar plus thermal infrared radiation fluxes to the ground and consequently upward latent and sensible heat fluxes from the ground to the air, increasing near‐surface air temperatures. As such, urbanization alone may have had notable impacts on both meteorology and air quality.

Sensitivity of Extreme Temperature Events to Urbanization in the Pearl River Delta Region

The urbanization effect on extreme temperature related to heat wave events in the Pearl River Delta (PRD) of China is investigated using a regional model. Several experiments with different urbanization levels are carried out. Results indicate that the air temperature of heat wave events is sensitive to urban land cover and the urbanized warming intensifies along with urban expansion. The regional model performs better on the simulations of maximum and minimum 2-m air temperature when urban land cover is close to the observed value. Because of the urbanization effect, the surface skin temperature shows a maximum warming (~2.5 °C) at daytime, while the maximum warming of 2-m air temperature (~1.5 °C) occurs at nighttime. Consequently, an increased (a decreased) diurnal range in surface skin temperature (2-m air temperature) is induced. According to the diagnostic equations, the 2-m air temperature is not only related to surface skin temperature, but also regulated by surface sensible heat flux. At daytime, although the increased solar absorption and the decreased upward latent heat flux (due to the deficit of surface evaporation) sharply enhance the surface skin temperature in the higher urbanization experiment, the simultaneously increased sensible heat flux suppresses the 2-m air temperature. At nighttime, however, the difference in surface sensible heat flux is relatively small, thus 2-m air temperature largely depends on surface skin temperature influenced by ground heat release. This study emphasizes the different variations and physical mechanisms of surface skin temperature and 2-m air temperature under the urbanization in the PRD region.

Why SST trend in North Pacific is peculiarly negative against warming trend elsewhere since 1958

A simple theoretical model is constructed to understand the cause of a peculiar cooling trend in North Pacific under the background of the greenhouse gases induced global warming during the past 50 years. It is found that the North Pacific cooling is caused by the increase of surface upward latent heat flux due to the atmosphere and the decrease of surface downward shortwave radiative flux. The former is attributed to enhanced low-level westerlies, while the latter is caused by the increase of stratus cloud over North Pacific. An atmosphere general circulation model is utilized to investigate the cause of the wind and low-level cloud changes. It is found that the strengthened westerly in North Pacific is the result of an atmospheric teleconnection pattern forced by the SSTAs warming in the tropical Pacific. The SSTAs warming in other tropical basins, along with the local cooling in North Pacific, tends to reduce the tropical Pacific SSTAs forcing effect. In addition, the increased local low-level cloud response to the tropical Pacific SSTAs forcing is also responsible for the cooling trend in North Pacific. The increased local stratus cloud may enhance the cooling through a positive feedback among the SST, atmospheric static stability and stratus cloud.

SST biases over the Northwest Pacific and possible causes in CMIP5 models

In this paper, the features and possible causes of sea surface temperature (SST) biases over the Northwest Pacific are investigated based on a mixed-layer heat budget analysis in 21 coupled general circulation models (CGCMs) from phase 5 of the Coupled Model Inter-comparison Project (CMIP5). Most CMIP5 models show cold SST biases throughout the year over the Northwest Pacific. The largest biases appear during summer, and the smallest biases occur during winter. These cold SST biases are seen at the basin scale and are mainly located in the inner region of the low and mid-latitudes. According to the mixed-layer heat budget analysis, overestimation of upward net sea surface heat fluxes associated with atmospheric processes are primarily responsible for the cold SST biases. Among the different components of surface heat fluxes, overestimated upward latent heat fluxes induced by the excessively strong surface winds contribute the most to the cold SST biases during the spring, autumn, and winter seasons. Conversely, during the summer, overestimated upward latent heat fluxes and underestimated downward solar radiations at the sea surface are equally important. Further analysis suggests that the overly strong surface winds over the Northwest Pacific during winter and spring are associated with excessive precipitation over the Maritime Continent region, whereas those occurring during summer and autumn are associated with the excessive northward extension of the intertropical convergence zone (ITCZ). The excessive precipitation over the Maritime Continent region and the biases in the simulated ITCZ induce anomalous northeasterlies, which are in favor of enhancing low-level winds over the North Pacific. The enhanced surface wind increases the sea surface evaporation, which contributes to the excessive upward latent heat fluxes. Thus, the SST over the Northwest Pacific cools.