Strong Surface Heating Research Articles

The Potential Role of Seasonal Surface Heating on the Chaotic Origins of the El Niño/Southern Oscillation Spring Predictability Barrier

AbstractThe Spring Predictability Barrier (SPB) phenomenon is characterized by the reduced accuracy of El Niño/Southern Oscillation (ENSO) forecasts during the spring, which substantially limits our ability to predict ENSO events. By investigating the nonlinear dynamic characteristics of ENSO systems simulated by a box model, we found that the strong surface heating process in spring may contribute to the SPB by regulating the different coupling processes between the ocean and atmosphere. Specifically, the intensified springtime surface heating increases the Sea Surface Temperature (SST), further amplifying the thermal damping effect of SST anomalies and reducing the dynamic connection between zonal SST gradient and upwelling process, and finally increasing the chaotic degree of ENSO systems simulated by the box model. The enhanced chaotic degree of ENSO systems leads to a more rapid growth of initial errors in the forecast model in spring, potentially leading to the SPB phenomenon.

Intrinsic Low-Frequency Variability in the Upper-Layer Circulation of the East Sea (Sea of Japan)

Abstract The effects of external forcing variation on the intrinsic variability in the upper-layer circulation occurring within the East Sea (Sea of Japan) and its physical mechanism are analyzed using numerical experiments. In this study, the experiments were conducted with climatological annual/monthly mean forcings (constant/seasonal forcings). The intrinsic variability is mainly distributed in the meandering regions around the main current path with the comparatively large variability limited to the southern region. The reason of greater intrinsic variability mainly in the southern part of the East Sea than in the northern part is that more energy is required from external forcings to change the thicker upper layer formed in the northern part due to seasonal forcings (strong wind stress and surface heat flux). Although the experiments show slight differences, westward propagation of the Rossby wave appears in areas where the variability is large. The transport of the eddy momentum flux associated with the Rossby wave modulates the strength of the eastward jet and the north–south shift of its axis. Among the external forcings, the volume transport through the Korea/Tsushima Strait is the most important driver of intrinsic variability, and wind stress plays an important role in expanding and strengthening intrinsic variability. Significance Statement Intrinsic variability is an important factor in understanding the total variability in the upper-layer circulation in the East Sea (Sea of Japan). However, the physical mechanisms of intrinsic variability remain unclear. This study investigates the physical mechanisms of intrinsic variability occurring within the East Sea. The intrinsic variability occurs mainly around the main current path, especially the meandering current. Based on numerical experiments, the mechanism of the intrinsic variability is represented by the fluctuation in the strength of the eastward current or the north–south movement of its axis caused by the Rossby wave.

Vertical Structure of Clouds and Precipitation During Arctic Cold‐Air Outbreaks and Warm‐Air Intrusions: Observations From COMBLE

AbstractThe Arctic is marked by deep intrusions of warm, moist air, alternating with outbreaks of cold air down to lower latitudes. The typical vertical structure of clouds and precipitation during these two synoptic weather extremes is examined at a coastal site at 69°N in Norway. The Norwegian Sea is a corridor for warm‐air intrusions (WAIs) and frequently witnesses cold‐air outbreaks (CAOs). This study uses data from profiling radar, lidar, and microwave radiometer, radiosondes and other probes that were collected during the CAOs in the Marine Boundary Layer Experiment (COMBLE) between 1 December 2019 and 31 May 2020. Marine CAOs are defined in terms of thermal instability relative to the sea surface temperature, and WAIs in terms of equivalent potential temperature stratification between the surface and 850 hPa. Cloud structures in CAOs are convective, driven by strong surface heat fluxes over a long fetch of open water, with cloud tops rarely exceeding 6 km above sea level. The mostly open‐cellular convection produces intermittent moderately‐heavy precipitation at the observational site, notwithstanding the low precipitable water vapor (PWV). In contrast, WAIs are marked by high values of PWV and integrated vapor transport. WAI clouds are synoptically driven, stratiform, with cloud tops often exceeding 5 km, sometimes layered, and generally producing persistent precipitation that can be heavier than in CAOs.

The disparities in thickness between the thermal and seismic lithosphere beneath the global Precambrian cratons and its geodynamic implications

Cratons play an important role in the evolution of Earth and usually exhibit extremely thick and strong lithosphere roots and low surface heat flow. Most cratons remain in a stable state throughout their long evolutionary process. However, the stability of a few cratons was lost, the thickness of the lithosphere was greatly reduced, and surface heat flow greatly increased. Significant thinning of the lithosphere is one of the most obvious features of craton destruction. However, even within the same area, the lithospheric thickness values estimated from different methods can vary a lot. The thickness disparities between the thermal and seismic lithosphere remain unclear. On the basis of the latest global heat flow data and crustal stratification information, we calculated the thickness of the thermal lithosphere of cratons globally and conducted a detailed comparison to that of the seismic lithosphere. We found that 1) the thickness of the thermal lithosphere of most cratons is more than 150 km, while that of the Bundelkhand, Singhbhum, and upper Yangtze craton ranges from approximately 120–150 km. The thickness of the eastern North China craton, lower Yangtze craton and Wyoming craton ranges from only approximately 80–100 km. 2) For most cratons, the thickness of the seismic lithosphere is tens of kilometers (30–141 km) larger than that of the thermal lithosphere, except for the Wyoming craton, eastern North China craton, upper Yangtze craton, and Dahawar craton. 3) The part between the bottom boundaries of the seismic and thermal lithosphere can be considered as the rheological boundary layer (RBL), which is related to the craton stability. The thicker the RBL is, the higher the craton stability. 4) Due to plate subduction, the water content in the upper mantle is high, the viscosity is low, and the thickness of the RBL of cratons is small.

Wintertime Synoptic Patterns of Midlatitude Boundary Layer Clouds Over the Western North Atlantic: Climatology and Insights From In Situ ACTIVATE Observations

AbstractThe winter synoptic evolution of the western North Atlantic and its influence on the atmospheric boundary layer is described by means of a regime classification based on Self‐Organizing Maps applied to 12 years of data (2009–2020). The regimes are classified into categories according to daily 600‐hPa geopotential height: dominant ridge, trough to ridge eastward transition (trough‐ridge), dominant trough, and ridge to trough eastward transition (ridge–trough). A fifth synoptic regime resembles the winter climatological mean. Coherent changes in sea‐level pressure and large‐scale winds are in concert with the synoptic regimes: (a) the ridge regime is associated with a well‐developed anticyclone; (b) the trough‐ridge gives rise to a low‐pressure center over the ocean, ascents, and northerly winds over the coastal zone; (c) trough is associated with the eastward displacement of a cyclone, coastal subsidence, and northerly winds, all representative characteristics of cold‐air outbreaks; and (d) the ridge–trough regime features the development of an anticyclone and weak coastal winds. Low clouds are characteristic of the trough regime, with both trough and trough–ridge featuring synoptic maxima in cloud droplet number concentration (Nd). The Nd increase is primarily observed near the coast, concomitant with strong surface heat fluxes exceeding by more than 400 W m−2 compared to fluxes further east. Five consecutive days of aircraft observations collected during the ACTIVATE campaign corroborates the climatological characterization, confirming the occurrence of high Nd for days identified as trough. This study emphasizes the role of boundary‐layer dynamics and aerosol activation and their roles in modulating cloud microphysics.

Urban effects on local cloud patterns

Urbanization extensively modifies surface roughness and properties, impacting regional climate and hydrological cycles. Urban effects on temperature and precipitation have drawn considerable attention. These associated physical processes are also closely linked to clouds' formation and dynamics. Cloud is one of the critical components in regulating urban hydrometeorological cycles but remains less understood in urban-atmospheric systems. We analyzed satellite-derived cloud patterns spanning two decades over 447 US cities and quantified the urban-influenced cloud patterns diurnally and seasonally. The systematic assessment suggests that most cities experience enhanced daytime cloud cover in both summer and winter; nocturnal cloud enhancement prevails in summer by 5.8%, while there is modest cloud suppression in winter nights. Statistically linking the cloud patterns with city properties, geographic locations, and climate backgrounds, we found that larger city size and stronger surface heating are primarily responsible for summer local cloud enhancement diurnally. Moisture and energy background control the urban cloud cover anomalies seasonally. Under strong mesoscale circulations induced by terrains and land-water contrasts, urban clouds exhibit considerable nighttime enhancement during warm seasons, which is relevant to strong urban surface heating interacting with these circulations, but other local and climate impacts remain complicated and inconclusive. Our research unveils extensive urban influences on local cloud patterns, but the effects are diverse depending on time, location, and city properties. The comprehensive observational study on urban-cloud interactions calls for more in-depth research on urban cloud life cycles and their radiative and hydrologic implications under the urban warming context.

Effects of Surface Heating on Coastal Upwelling Intensity

AbstractNumerical experiments were conducted to evaluate the effects of surface heating on coastal upwelling intensity. Offshore transport, isopycnal slope, and the sea surface temperature (SST) difference between coastal and offshore regions, which represent the upwelling intensity, were estimated. Surface heating decreases Ekman transport but increases Ekman pumping by changing air‐sea stability conditions. However, offshore transport does not change significantly with surface heating. Our experimental results revealed that the increase in surface heating decreases the isopycnal slope but increases the SST difference. Both the isopycnal slope and SST difference are closely related to the change in the surface boundary layer (SBL). Strong surface heating thins the surface‐mixed layer, which decreases the vertical eddy viscosity. The decreased vertical eddy viscosity thins the SBL and enhances its offshore velocity. The isopycnal slope weakens because of the thin SBL, while the SST difference becomes stronger because of the enhanced offshore velocity despite the same offshore transport. The present study may be relevant to the change in upwelling systems as the increase in surface heating under future global warming.

Turbulent Thermal-Wind-Driven Coastal Upwelling: Current Observations and Dynamics

Abstract A remarkably consistent Lagrangian upwelling circulation at monthly and longer time scales is observed in a 17-yr time series of current profiles in 12 m of water on the southern New England inner shelf. The upwelling circulation is strongest in summer, with a current magnitude of ∼1 cm s−1, which flushes the inner shelf in ∼2.5 days. The average winter upwelling circulation is about one-half of the average summer upwelling circulation, but with larger month-to-month variations driven, in part, by cross-shelf wind stresses. The persistent upwelling circulation is not wind-driven; it is driven by a cross-shelf buoyancy force associated with less-dense water near the coast. The cross-shelf density gradient is primarily due to temperature in summer, when strong surface heating warms shallower nearshore water more than deeper offshore water, and to salinity in winter, caused by fresher water near the coast. In the absence of turbulent stresses, the cross-shelf density gradient would be in a geostrophic, thermal-wind balance with the vertical shear in the along-shelf current. However, turbulent stresses over the inner shelf attributable to strong tidal currents and wind stress cause a partial breakdown of the thermal-wind balance that releases the buoyancy force, which drives the observed upwelling circulation. The presence of a cross-shelf density gradient has a profound impact on exchange across this inner shelf. Many inner shelves are characterized by turbulent stresses and cross-shelf density gradients with lighter water near the coast, suggesting turbulent thermal-wind-driven coastal upwelling may be a broadly important cross-shelf exchange mechanism. Significance Statement A remarkably consistent upwelling circulation at monthly time scales is observed in a 17-yr time series of current profiles in shallow water off southern New England. This is not the traditional wind-driven coastal upwelling; instead, it is forced by cross-shelf buoyancy (density) gradients, released by turbulent stresses in shallow water. The persistent upwelling circulation is strongest in summer, when wind and wave forcing are weak, and flushes the inner portion of the continental shelf in a few days. Consequently, this buoyancy-driven coastal upwelling is important for cooling the inner shelf and provides a reliable mechanism for cross-shelf exchange. Many inner shelves are characterized by cross-shelf density gradients and turbulent stresses, suggesting this may be a broadly important cross-shelf exchange mechanism.

Submesoscale Frontal Instabilities Modulate Large‐Scale Distribution of the Winter Deep Mixed Layer in the Kuroshio‐Oyashio Extension

AbstractIn the mid‐latitude North Pacific, the wintertime ocean heat loss reaches the maximum in the Kuroshio‐Oyashio Extension (KOE) region. However, the mixed layer depth (MLD) there is shallow (<50 m), flanked by a zonal band of deep MLD (>100 m) on either side. Such an observed mixed layer pattern is not reproduced by coarse‐resolution or mesoscale eddy‐resolving (1/10°) models. What causes the observed MLD shoaling along the KOE frontal region? Here we show that the mixed layer shoaling in the front is well reproduced by a 1/30°, submesoscale‐permitting ocean general circulation model, indicating that it is closely related to the submesoscale frontal dynamics. The geostrophic strain rate is strong along the sharp KOE front, favoring submesoscale frontal instabilities. The surface buoyancy loss during winter reduces stratification and enhances the fronts (i.e., with steeper, northward rising isopycnals). The available potential energy stored in the narrow fronts can be released in the form of eddy kinetic energy by the mixed layer baroclinic instabilities (MLI). The associated vigorous ageostrophic motions associated with MLI efficiently slump the vertically oriented isopycnals and restratify the mixed layer. Thus, the MLD is shallow in the KOE frontal region despite the strong surface heat loss and wind stirring during winter. The submesoscale processes revealed here have important implications for improving climate models.

From a Shapiro–Keyser extratropical cyclone to the subtropical cyclone Raoni: An unusual winter synoptic situation over the South Atlantic Ocean

AbstractThe eastern coast of South America is a cyclogenetic region in terms of extratropical cyclones and, in lower number, of subtropical cyclones that are more frequent in austral summer and autumn. However, in June 2021, an unusual cyclone developed near the boundary of Uruguay and southern Brazil, initially having extratropical features and later undergoing a subtropical transition. At 1200 UTC 29 June, the Brazilian Navy named it as subtropical cyclone Raoni. This study aims to describe the synoptic evolution of the cyclone and address physical drivers for the subtropical transition based on the ECMWF‐ERA5 reanalysis and numerical experiments with the WRF model. The cyclone precursor of Raoni had its genesis at 1800 UTC 26 June 2021 forced by a trough at mid–upper levels that crossed the Andes Mountains and caused a rapid surface pressure drop. Less than 24 hr later, the cyclone presented a frontal T‐bone pattern and warm seclusion, following the Shapiro–Keyser development model. Strong surface heat fluxes, a deep moist troposphere, and the vertical alignment of the warm seclusion with an upper‐level cut‐off pattern provided the adequate environment for organising convection and, consequently, for subtropical transition at 0600 UTC 28 June. The fundamental role of the surface turbulent heat fluxes for the transition is confirmed through numerical experiments. This study is unprecedented in the sense that no subtropical cyclone originating from a warm seclusion has been documented over the South Atlantic before. These findings emphasise the need of monitoring cold‐season extratropical Shapiro–Keyser cyclones in the region since they can evolve to a subtropical or tropical cyclone and can cause damage to the maritime activities and coastal region due to the strong winds.

Recent nutrient enrichment and high biological productivity in the Labrador Sea is tied to enhanced winter convection

The Labrador Sea is known for strong surface heat losses and deep ocean ventilation in the wintertime as well as high biological productivity and carbon export associated with extensive spring blooms. Using satellite-derived estimates of chlorophyll-a and net primary productivity, this study documents that the Labrador Sea has recently become more productive, evident in the form of more intense and spatially extensive phytoplankton blooms. The spatial and temporal variability in chlorophyll-a was compared against trends in nutrient concentrations across the Labrador Sea. Nutrient concentrations were obtained on a repeat hydrography line running from the Labrador to Greenland shelves. The 25-year long time series of nitrate, phosphate and silicate concentrations along the AR7 W line reveal decadal trends in the upper Labrador Sea. A comparison of these nutrients with basin-averaged annual mean surface chlorophyll-a concentrations reveals positive correlations in the upper 1000 m of the Labrador Sea. Furthermore, nutrient concentrations in the upper 1000 m were strongly correlated with wintertime convection depth. During years of strong winter convection, more nutrients are entrained from the deeper Labrador Sea, which then can be distributed through the Western Greenland and Irminger Currents and eddy activity, allowing sustained phytoplankton growth further north in the Labrador Sea.

An investigation into the vertical structures of low‐altitude atmosphere over the Central Taklimakan Desert in summer

AbstractIn this study, 1‐month continuous radiosonde observational data were applied to present the low‐altitude vertical structures and their evolutions over the Central Taklimakan Desert (CTD). The primary focus is to highlight the vertical structures near the ground with the high‐resolution (10 m in height and 6 hr in time) radiosonde data. One of the unique features evident in our results is an obvious diurnal transition in lower layers near the ground due to strong surface heating or cooling. Unlike a traditional vertical structure in the boundary layer observed over a nondesert surface, both superadiabatic and inversion layers are distinct during the day. More specifically, the superadiabatic layer is obvious in the daytime because of strong solar radiation over the desert, and the superadiabatic can reach up to 0.2 km. In contrast, an apparent inversion layer forms in the nighttime due to the surface cooling. It is found that the surface forcing mainly dominates the structures in the boundary layer. At last, the vertical structures from the observations are compared with those from the ERA‐Interim and MERRA2 reanalysis data sets. The results indicate that both reanalysis products can provide similar vertical profile patterns and diurnal variations. However, the diurnal transitions of temperature and wind profiles over the CTD are underestimated severely by both reanalysis data. Besides, the reanalysis data sets completely miss the superadiabatic near the ground in the daytime.

The Ocean Heat Content Changes in the South China Sea due to Tropical Cyclones with Different Tracks

The influences of the tropical cyclones (TCs) on the ocean heat content (OHC) of the South China Sea (SCS) was investigated by observations and numerical simulations. Ensemble analyses based on observations showed notable differences in OHC changes caused by TCs with different tracks: (1) TCs which generated over the western North Pacific (WNP) and did not enter the SCS (TC-P) decreased the OHC mainly in the central SCS while increasing the OHC in southern SCS and coastal areas along South China; (2) TCs which generated over the WNP and entered the SCS (TC-PS) decreased the OHC in most of the SCS beyond the coastal areas with a higher amplitude than that caused by TC-P; and (3) TCs which generated over the SCS (TC-S) induced the strongest OHC decrease and a distinct OHC increase in the southeastern SCS and Sulu Sea (SS). The distinct response of the SCS OHC was mainly attributed to the different surface heat fluxes and oceanic transports caused by TCs. Strong surface heat flux proximal to TC tracks led to ocean heat release to the atmosphere and decreased the OHC in these areas, while the oceanic current caused by the TC cyclonic wind field transported the OHC to areas distant from the TC center. Eastward transport of the OHC south of 15° N increased the OHC in the southeastern SCS and SS. These results expressed the detailed response of the OHC to TCs with different tracks over the WNP and SCS, and clarified the influences of TCs on the ocean environments of the SCS and for predicting the changes in the SCS.

Sea surface temperature (SST) anomalies of the Yellow and East China Seas in July of 2020

With the global warming, the long term variations of sea surface temperature and its anomalies in the Yellow and East China Seas, especially for the July of 2020 due to the abnormally torrential rain along Changjiang/Yangtze River Valley, have been investigated based on the Merged Satellite and In-situ Data Global Daily sea surface temperature (MGDSST) provided by the Japan Meteorological Agency (JMA) using the methods of composite and correlation analyses. The results suggest, contrary to warming anomalies in the North-western Pacifica Ocean, the sea surface temperature in the East China Sea is cooler around 0.5°C and that in the East China Sea is cooler around 1.3°C than the normal values. The sea surface temperatures approach the extreme low value in the Yellow Sea and East China Sea in July, and warm up to the normal year in August. In addition, the south-westerly summer monsoon over this region, is proposed to contribute the transport of Kuroshio and its pathway. The obvious westerly wind anomalies, correspond to the lower sea surface temperature over the Yellow and East China Seas in July of 2020, leads to a clear less heat advection from Kuroshio to this region. Further, the low sea surface temperature, leading a downward air motion with a convergence at near sea surface level, is helpful for the enhance of the westerly wind anomalies until the strong surface heat flux in August. This study suggests that the local horizontal circulation advection and net heat flux are also dominated on the heat content of the East China waters. Further quantitative studies are worth conducting.

A Case Study on the Variability of Summer Water Properties in the Southeastern Yellow Sea Based on the Hydrological Data from 1995 to 2019

The long-term variability of the summer water properties in the southeastern Yellow Sea is described using the hydrological data. The results indicate warming trend in the upper layer and cooling trend in the deeper layer resulting in a strengthen thermocline. While, the mean and the core temperatures in the southeastern Yellow Sea Cold Water Mass both tend to rise slowly, which are previously thought to be fallen. At the deep layer (below 30 m depth), the temperature cooling trend in summer is even stronger than that in winter, which contradict to the stronger surface heat flux in summer. This study proposes that the strengthen thermocline during warming seasons (spring and summer) prevents the heat transferring from surface to the deep layer across isopycnal lines. Furthermore, combined with the metrological data, the strengthen thermocline induces stronger southward tidal residual current at the deep layer, which facilitates the enhanced complementary wind-driven current induced by the southerly monsoon. Thus, the southward tidal induced residual current effectively contributes to the southward motion of the colder water from the northern area. The declined trending of salinity due to the increase of the Changjiang River discharge can be conducive to the strengthen summer thermocline.

Environmental sensitivities of shallow-cumulus dilution – Part 1: Selected thermodynamic conditions

Abstract. Cumulus entrainment, and its consequent dilution of buoyant cloud cores, strongly regulates the life cycle of shallow cumuli yet remains poorly understood. Herein, new insights into this problem are obtained through large-eddy simulations that systematically investigate the sensitivity of shallow-cumulus dilution to cloud-layer relative humidity (RH), cloud- and subcloud-layer depths, and continentality (i.e., the land–ocean contrast). The simulated cloud-core dilution is found to be strongly sensitive to continentality, with fractional dilution rates twice as large over the ocean as over land. Using a similarity theory based on the turbulent-kinetic-energy (TKE) budget, the reduced cloud-core dilution over land is attributed to larger cloud-base mass flux (mb), driven by stronger surface heating and subcloud turbulence. As mb increases, the fractional dilution rate must decrease to maintain energetic equilibrium. A positive sensitivity is also found to cloud-layer RH, with the core dilution increasing by 25 %–50 % for a 10 % enhancement in RH. This sensitivity is interpreted using the buoyancy-sorting hypothesis, in that mixtures of cloud and environmental air are more likely to become negatively buoyant and detrain (rather than diluting the cloud core) in drier cloud layers. By contrast, the sensitivities of (marine) shallow-cumulus dilution to cloud- and subcloud-layer depths are weak, with a 3 % decrease for a doubling for the former and a 4 % reduction in dilution for a 50 % deeper subcloud layer. These surprisingly weak sensitivities are readily explained by offsetting effects in the TKE similarity theory. Altogether, these experimental findings provide useful, though still incomplete, guidance for flow-dependent shallow-cumulus entrainment formulations in large-scale models.

Machine learning methods for the detection of polar lows in satellite mosaics: major issues and their solutions

Polar mesocyclones (PMCs) and their intense subclass polar lows (PLs) are relatively small atmospheric vortices that form mostly over the ocean in high latitudes. PLs can strongly influence deep ocean water formation since they are associated with strong surface winds and heat fluxes. Detection and tracking of PLs are crucial for understanding the climatological dynamics of PLs and for the analysis of their impacts on other components of the climatic system. At the same time, visual tracking of PLs is a highly time-consuming procedure that requires expert knowledge and extensive examination of source data.There are known procedures involving deep convolutional neural networks (DCNNs) for the detection of large-scale atmospheric phenomena in reanalysis data that demonstrate a high quality of detection. However, one cannot apply these procedures to satellite data directly since, unlike reanalyses, satellite products register all the scales of atmospheric vortices. It is also known that DCNNs were originally designed to be scale-invariant. This leads to the problem of filtering of the scale of detected phenomena. There are other problems to be solved, such as low signal-to-noise ratio of satellite data, and an unbalanced number of negative (without PLs) and positive (where a PL is presented) classes in a satellite dataset.In our study, we propose a deep learning approach for the detection of PLs and PMCs in remote sensing data, which addresses class imbalance and scale filtering problems. We also outline potential solutions for other problems, along with promising improvements to the presented approach.

Seasonal stratification of a deep, high-altitude, dimictic lake: Nam Co, Tibetan Plateau

We investigate the seasonal evolution of stratification in a deep (100 m), high-altitude (4,730 m a.s.l.) dimictic lake on the Tibetan Plateau using three years of observation of Nam Co. The lake is situated at relatively low latitude (30 °N) where it receives high solar radiative forcing (observed maximum daily average 400 W m−2), yet the annual average air temperature is close to 0 °C at this high altitude. These features make Nam Co distinct from most well-documented dimictic lakes, which are usually located at higher latitude and lower altitude. We classify seasonal stratification into six phases based on strength of stratification, surface temperature relative to the temperature of maximum density, ice-cover, and heat and mixing dynamics, which we use to compare Nam Co with better-documented, higher-latitude dimictic lakes. While the three warm stratification phases (i.e. when water is warmer than the temperature of maximum density) in Nam Co are relatively cold, they are otherwise similar to that observed in high-latitude dimictic lakes. Conversely, two of Nam Co’s cold stratification phases are distinct from that reported in high-latitude lakes. These two phases are characterized by the interplay between the relatively strong radiative forcing and surface heat flux, and include the following: 1) during fall turnover, persistent winds aided by radiatively driven convection prolong vertical mixing (i.e. turnover) and surface heat loss such that the entire water column cools well below the temperature of maximum density (as low as 1 °C); and 2) in contrast, under ice-cover with relatively little snow, the entire water column of the lake warms continuously due to through-ice solar radiative flux. The intense cooling and heating during these two phases counteract each other such that hypolimnetic temperature at spring turnover is similar to that observed in high latitude lakes. Our observations highlight the relative importance of radiatively driven convection on the seasonal stratification dynamics of Nam Co, and underscore that these dynamics must be considered when attempting to predict climate change impacts on high-altitude, low-latitude lakes, including the >1100, largely unstudied, lakes on the Tibetan Plateau.

Strong North Pacific Subtropical Mode Water Volume and Density Decrease in Year 1999

AbstractA strong decrease of volume and density of North Pacific Subtropical Mode Water (NPSTMW) in 1999 was analyzed in a regional high‐resolution (0.1°) numerical ocean circulation model simulation. Both shoaling of the bottom and deepening of the top of the NPSTMW layer contributed equally to volume decrease. They were locally governed by different physical processes, but both seem to be associated with basin‐wide changes in wind. A westward propagating negative thermocline depth anomaly that developed in the Central Pacific when the Pacific Decadal Oscillation index changed from a positive to a negative phase in 1998 caused shoaling of the bottom of the NPSTMW layer. Deepening of the top of the NPSTMW layer was due to an increase in the near surface stratification, caused by an increase in wind‐driven lateral heat transport convergence by the Kuroshio Extension jet starting in 1997. Both processes increased the potential vorticity in the NPSTMW region, decreasing the volume of water in the NPSTMW density range that satisfied the low potential vorticity constraint that is part of the definition of “mode water.” The strong near‐surface density decrease provided preconditioning for preferential surface formation of a lighter variety of NPSTMW, further decreasing its density. It also resulted in decrease of the outcrop window in the NPSTMW density range, strongly reducing its formation rate in 1998 and 1999 despite strong surface heat loss.

Thermal Effects of the Surface Heat Flux on Cloud Systems over the Tibetan Plateau in Boreal Summer

ABSTRACT The influence of surface heat fluxes on the generation and development of cloud and precipitation and its relative importance to the large-scale circulation patterns are investigated via cloud-resolving model (CRM) simulations over the Tibetan Plateau (TP) during boreal summer. Over the lowland (e.g., along the middle and lower reaches of the Yangtze River), the dynamical and thermal properties of the atmosphere take more responsibility than the surface heat fluxes for the triggering of heavy rainfall events. However, the surface thermal driving force is a necessary criterion for the triggering of heavy rainfall in the eastern and western TP (ETP and WTP). Strong surface heat fluxes can trigger shallow convections in the TP. Furthermore, moisture that is mainly transported from the southern tropical ocean has a greater influence on the heavy rainfall events of the WTP than those of the ETP. Cloud microphysical processes are substantially less active and heavy rainfall cannot be produced when surface heat fluxes are weakened by half in magnitude over the TP. In addition, surface heating effects are largely responsible for the high occurrence frequency of convection during the afternoon, and the cloud tops of convective systems show a positive relationship with the intensity of surface heat fluxes.