Synoptic Weather Events Research Articles

Simulated Future Shifts in Wildfire Regimes in Moist Forests of Pacific Northwest, USA

AbstractFire is an integral natural disturbance in the moist temperate forests of the Pacific Northwest of the United States, but future changes remain uncertain. Fire regimes in this climatically and biophysically diverse region are complex, but typically climate limited. One challenge for interpreting potential changes is conveying projection uncertainty. Using projections of Energy Release Component (ERC) derived from 12 global climate models (GCM) that vary in performance relative to the region's contemporary climate, we simulated thousands of plausible fire seasons with the stochastic spatial fire spread model FSim for mid‐21st century (2035–2064) under RCP8.5 emissions scenario for five northwestern pyromes. The magnitude of projected changes to burn probability, fire size, and number of fires varied among pyromes and GCMs. We projected the largest increases in burn probability and fire size in the cooler and wetter northern parts of the region (North Cascades, Olympics & Puget Lowlands) and Oregon West Cascades, with more moderate changes projected for the Washington West Cascades and Oregon Coast Range. We provide new insights into changing fire regimes characterized by the possibility of shifts toward more frequent and large fires (especially >40,000 ha), as well as shifts in seasonality, including more fires burning at the beginning of fall when extreme synoptic weather events have the potential to increase fire spread. Our work highlights the potential geographic variability in climate change effects in some of the most productive moist temperate forests of the world and points to a rapid acceleration of fire in the coming decades.

Drastic Fluctuation in Water Exchange Between the Yellow Sea and Bohai Sea Caused by Typhoon Lekima in August 2019: A Numerical Study

AbstractConnecting the Bohai Sea (BS) and Yellow Sea (YS), the Bohai Strait is vital in controlling physical‐biogeochemical conditions in the East Asian marginal seas. In August 2019, Typhoon Lekima moved northward inland into eastern China and caused drastic water exchange through the Bohai Strait. The specific variation and underlying dynamics involved were investigated with observations and high‐resolution numerical simulations. Volume transport featured an intense inflow followed by an intense outflow, with a fluctuation magnitude of 1.88 Sv, which was much greater than the seasonal‐interannual variations. The corresponding current patterns presented three phases: strait‐wide inflow, strait‐wide outflow, and outflow in the south and inflow in the north. The strait‐wide inflow was driven by strong east‐northeasterly winds through local Ekman dynamics. The strait‐wide outflow resulted from the dynamic adjustment of ocean currents accompanying the rapid weakening of easterly winds. The third current pattern was part of a large‐scale cyclonic circulation in the BS and North YS and was governed by different factors in the southern and northern straits. Further comparisons indicated that transport caused by winter storms and other typhoons in the year had opposite phase variations to that caused by Lekima. The underlying dynamics mainly involved local Ekman dynamics and/or remote wind forcing via the propagation of coastal trapped waves but could differ greatly from events. Finally, we found that to a large extent, geostrophic transport derived from the sea surface height difference across the Bohai Strait could be used as actual transport during synoptic weather events.

Projected 21st Century Wind‐Wave Climate of Bass Strait and South‐East Australia: Comparison of EC‐Earth3 and ACCESS‐CM2 Climate Model Forcing

AbstractThis study analyses the past and future wave climate of Bass Strait and south‐east Australia by comparing two high‐resolution regional wave climate models (RWCMs), respectively forced by the surface wind speed of EC‐Earth3 and ACCESS‐CM2 global circulation models (GCMs) under two emission scenarios (SSP1‐2.6 and SSP5‐8.5). The two RWCMs are verified against long‐term buoy and satellite observations as well as hindcast model data. The comparison with long‐term satellite altimeter observations shows good agreement between the long‐term model climate and the observations. Both RWCMs project a future increase in swell, approaching the coast of Tasmania and mainland Australia, generated from distant storms in the Southern Ocean. However, different projected wave height spatial distributions are found in the coastal regions, affected by local synoptic weather events and by the coarse resolution of the GCMs winds. We also show that the long‐term wave climate in the domain is impacted by the Southern Annular Mode. As low‐pressure systems are projected to move southward, especially in the SSP5‐8.5 scenario, the swells approaching south‐east Australia are projected to be longer and more energetic in the future. The combination of more energetic swell systems and a counterclockwise rotation in peak wave direction, projected by both RWCMs, may influence the coastal equilibrium, impacting critical coastal locations.

The influence of recent and future climate change on spring Arctic cyclones

In recent decades, the Arctic has experienced rapid atmospheric warming and sea ice loss, with an ice-free Arctic projected by the end of this century. Cyclones are synoptic weather events that transport heat and moisture into the Arctic, and have complex impacts on sea ice, and the local and global climate. However, the effect of a changing climate on Arctic cyclone behavior remains poorly understood. This study uses high resolution (4 km), regional modeling techniques and downscaled global climate reconstructions and projections to examine how recent and future climatic changes alter cyclone behavior. Results suggest that recent climate change has not yet had an appreciable effect on Arctic cyclone characteristics. However, future sea ice loss and increasing surface temperatures drive large increases in the near-surface temperature gradient, sensible and latent heat fluxes, and convection during cyclones. The future climate can alter cyclone trajectories and increase and prolong intensity with greatly augmented wind speeds, temperatures, and precipitation. Such changes in cyclone characteristics could exacerbate sea ice loss and Arctic warming through positive feedbacks. The increasing extreme nature of these weather events has implications for local ecosystems, communities, and socio-economic activities.

Variability of surface and subsurface phytoplankton blooms in a seasonal coastal upwelling system

Understanding how phytoplankton respond to their physical environment is key to predicting shifts in bloom dynamics under a changing climate. This research investigates phytoplankton dynamics in a seasonal coastal upwelling system of the eastern Great Australian Bight (GAB), South Australia, using >10 years of in-situ mooring and CTD observations together with >18 years of satellite and wind data. Earlier studies concluded that upwelling in the region is driven by interactions between upwelling-favourable winds associated with synoptic weather events during austral summer months (Jan–March) and the formation of a pool of nutrient-rich shelf-bottom water on the Lincoln Shelf, known as the Kangaroo Island (KI) Pool. Surface phytoplankton blooms in the upwelling centre, which form off the southern tip of the Eyre Peninsula, are characterized by chlorophyll-a levels of ∼1–2 mg/m3. Our analysis of satellite chl-a data indicates the absence of surface phytoplankton blooms in the upwelling centre in more than a third of all upwelling seasons, despite the existence of statistically similar upwelling-favourable winds. The KI Pool is located ∼100 km upstream from the upwelling centre in a marine region that generally displays very low surface phytoplankton levels. Analysis of water-column data indicates strong interannual variability the KI Pool that is correlated with the intensity of surface phytoplankton blooms in the upwelling centre. The analysis also reveals the existence of substantial (∼1 mg/m3 chl-a) subsurface phytoplankton blooms at depths between ∼20 and 70 m in shallower waters of the Lincoln Shelf, where nutrient-rich shelf-bottom water of the KI Pool reaches into the euphotic zone. Our data suggest that subsurface phytoplankton blooms, forming at depths that cannot be detected by satellites, represent a major driver of the upwelling ecosystem of the eastern GAB.

Impact of Anticyclonic Eddies Under Stormy Weather on the Mixed Layer Variability in April South of the Kuroshio Extension

AbstractSubtropical mode water (STMW) plays an important role in the North Pacific climate system by recording climate variability and by taking in anthropogenic carbon dioxide. A thick STMW forms in anticyclonic eddies (AEs) with deeper mixed layers during winter and early spring. However, the mixed layer depth (MLD) variability and mode water formation in AEs under synoptic weather events are not well known. In this study, we tracked an AE located south of the Kuroshio Extension for several months by 17 Argo floats with daily sampling. We found that the MLD inside the target AE deepened sharply to more than 150 m during storms in April, while the MLD outside had little change and was only 50 m on average. We further compared the different mixed layer deepening processes inside and outside other AEs over the same region based on historical Argo data from 2004 to 2019. Since the relatively weak stratification in AEs is easily destroyed by storms with strong wind bursts and significant surface cooling in early spring, the mixed layer deepening time period lasts longer (December–April) inside AEs than outside AEs (January–March). The mode water ventilation and formation period are correspondingly extended in AEs, with important implications for North Pacific climate variability and the biogeochemical cycle.

К вопросу о взаимосвязи экстремумов лунно-солнечных приливов 2021 года с изменениями погоды

The transformations of synoptic processes and severe weather events that sometimes accompany them occur near the extremes (maxima and minima) of tidal fluctuations in the Earth’s angular velocity. A graph of tidal fluctuations in the Earth’s angular velocity for 2021 is presented. The extremes of tidal fluctuations in the Earth’s angular velocity for 2021 are identified and their calendar dates are determined. On these dates, the transformation of synoptic processes and sometimes hydrometeorological hazards may be expected. The case of catastrophic icing in the Vladivostok area during the extreme on November 18, 2020 is described. Keywords: transformation of synoptic processes, weather forecasts, severe weather events, tidal fluctuations in the Earth’s angular velocity

Estimated Heat Budget During Summer Melt of Arctic First‐Year Sea Ice

AbstractMelt ponds on summer Arctic sea ice control surface albedo, governing energy and mass balance of the ice. The date ponds first form has been connected to interannual variations in ice retreat. Here we evaluate the surface energy balance that governs this critical pond formation date. A three‐dimensional sea ice model with resolved melt ponds is used to diagnose pond onset date at a coastal site across years with observed surface fluxes but incomplete pond observations. Results show that the combined sensible and latent heat flux is the best predictor of pond formation date. This finding supports the hypothesis that synoptic weather events transporting warm, moist air into the Arctic are key to initiating pond formation, triggering albedo feedbacks, and, by extension, ice retreat. Changes in timing and frequency of spring warm air incursions may have significant implications on the ice cover and provide predictive power over seasonal ice retreat.

Identification of synoptic weather types over Taiwan area with multiple classifiers

In this study, a novel machine learning approach was used to classify three types of synoptic weather events in Taiwan area from 2001 to 2010. We used reanalysis data with three machine learning algorithms to recognize weather systems and evaluated their performance. Overall, the classifiers successfully identified 52–83% of weather events (hit rate), which is higher than the performance of traditional objective methods.The results showed that the machine learning approach gave low false alarm rate in general, while the support vector machine (SVM) with more principal components of reanalysis data had higher hit rate on all tested weather events. The sensitivity tests of grid data resolution indicated that the differences between the high‐ and low‐resolution datasets are limited, which implied that the proposed method can achieve reasonable performance in weather forecasting with minimal resources.By identifying daily weather systems in historical reanalysis data, this method can be used to study long‐term weather changes, to monitor climatological‐scale variations, and to provide better estimate of climate projections. Furthermore, this method can also serve as an alternative of model output statistics and potentially be used for synoptic weather forecasting.

Large-Eddy Simulation of Shallow Cumulus over Land: A Composite Case Based on ARM Long-Term Observations at Its Southern Great Plains Site

Abstract Based on long-term observations by the Atmospheric Radiation Measurement program at its Southern Great Plains site, a new composite case of continental shallow cumulus (ShCu) convection is constructed for large-eddy simulations (LES) and single-column models. The case represents a typical daytime nonprecipitating ShCu whose formation and dissipation are driven by the local atmospheric conditions and land surface forcing and are not influenced by synoptic weather events. The case includes early morning initial profiles of temperature and moisture with a residual layer; diurnally varying sensible and latent heat fluxes, which represent a domain average over different land surface types; simplified large-scale horizontal advective tendencies and subsidence; and horizontal winds with prevailing direction and average speed. Observed composite cloud statistics are provided for model evaluation. The observed diurnal cycle is well reproduced by LES; however, the cloud amount, liquid water path, and shortwave radiative effect are generally underestimated. LES are compared between simulations with an all-or-nothing bulk microphysics and a spectral bin microphysics. The latter shows improved agreement with observations in the total cloud cover and the amount of clouds with depths greater than 300 m. When compared with radar retrievals of in-cloud air motion, LES produce comparable downdraft vertical velocities, but a larger updraft area, velocity, and updraft mass flux. Both observations and LES show a significantly larger in-cloud downdraft fraction and downdraft mass flux than marine ShCu.

Wave Events: Climatology, Trends, and Relationship to Northern Hemisphere Winter Blocking and Weather Extremes

AbstractDiagnostics of finite-amplitude local wave activity (LWA) are applied to the 500-hPa geopotential height field to diagnose persistent synoptic weather events of anomalously large wave activity in the Northern Hemisphere. By considering the cyclonic and anticyclonic components of LWA separately, persistent weather systems associated with large-amplitude troughs and ridges are detected. While anticyclonic wave events are predominantly found over Europe and Alaska, cyclonic wave events usually occur over East Asia and northeastern Canada. Those preferred regions correspond to the location of planetary-scale ridges and troughs, which contribute, together with transient anomalies, to the formation of wave events. Although wave events are not blocking events per definition, they are typically associated with increased blocking in their vicinity. Their spatial relationship to blocking, however, varies depending on their cyclonic or anticyclonic nature and the type of wave-breaking signatures. Wave events are also shown to be accompanied by warm or cold temperature extremes, whose spatial pattern depends on the type of events, cyclonic or anticyclonic, and the sector affected. Trends in the frequency of wave events indicate that cyclonic wave events and the associated cold extremes affecting East Asia have become more frequent in recent decades and could be linked to recent trends toward La Niña–like conditions in the Pacific and trends toward the negative phase of Arctic Oscillation.

Spectral structure of 5 year time series of horizontal wind speed at the Boulder Atmospheric Observatory

AbstractWe investigate the spectral structures of 5 year, 1 min time series of horizontal wind speeds at 100 and 10 m heights at the Boulder Atmospheric Observatory tower located in the eastern slope of the Rocky Mountains, USA. In the full‐scale spectra, the diurnal spectral peak, which is usually insignificant at a coastal or offshore site, is the most significant at both heights. The spectrum is enhanced on the low‐frequency side of the diurnal peak during winter, but on the high‐frequency side during summer, which indicates frequent synoptic weather events during winter supplanted by mesoscale events during summer. In terms of the spectral density in the spectral gap of Van der Hoven (1957), separating boundary layer turbulence from the synoptic‐scale fluctuations, at a frequency between 10−4 and 10−3 Hz, we rank the daily time series at 100 m height and sample the summer top and winter bottom 10 percentile cases. The winter cases of the reduced spectral density in the gap region present the f−3 spectrum (f is frequency) and negatively skewed velocity increment distributions, which are the signatures of enstrophy (the integral of squared vorticity) cascade of turbulent two‐dimensional (2‐D) flows. In contrast, the summer cases of the enhanced spectral density present the f−5/3 spectrum and positively skewed velocity increment distributions, which are the signatures of upscale energy cascade of 2‐D flows. In these mesoscale events that fill up the gap, the turbulence intensity‐wind speed relationship is very sensitive to the choice of the averaging period.

Large CO2 effluxes at night and during synoptic weather events significantly contribute to CO2 emissions from a reservoir

CO2 emissions from inland waters are commonly determined by indirect methods that are based on the product of a gas transfer coefficient and the concentration gradient at the air water interface (e.g., wind-based gas transfer models). The measurements of concentration gradient are typically collected during the day in fair weather throughout the course of a year. Direct measurements of eddy covariance CO2 fluxes from a large inland water body (Ross Barnett reservoir, Mississippi, USA) show that CO2 effluxes at night are approximately 70% greater than those during the day. At longer time scales, frequent synoptic weather events associated with extratropical cyclones induce CO2 flux pulses, resulting in further increase in annual CO2 effluxes by 16%. Therefore, CO2 emission rates from this reservoir, if these diel and synoptic processes are under-sampled, are likely to be underestimated by approximately 40%. Our results also indicate that the CO2 emission rates from global inland waters reported in the literature, when based on indirect methods, are likely underestimated. Field samplings and indirect modeling frameworks that estimate CO2 emissions should account for both daytime–nighttime efflux difference and enhanced emissions during synoptic weather events. The analysis here can guide carbon emission sampling to improve regional carbon estimates.

Influence of synoptic weather events on the isotopic composition of atmospheric moisture in a coastal city of the western United States

Synoptic weather events are known to strongly influence the isotope composition ofprecipitation in continental locations. In this study, we present hourly values of water vaporisotopologues (HDO and H218O) measured over a 30 day period in locally extreme weatherconditions, including Santa Ana winds and winter rainstorms, in San Diego, California,USA. We investigate how atmospheric and hydrological processes influence HDO andH218O using an isotope-enabled GCM model (IsoGSM). Combining measurements andIsoGSM simulation, we demonstrate that convective mixing of marine and continental airmasses are responsible for the isotopic variation of near-surface water vapor in this coastallocation. The isotopic variability is most pronounced during Santa Ana winds. The SantaAna winds represent a unique boundary layer condition in which atmospheric mixingbecomes the process that dominantly controls the changes in the isotopic compositionrelative to air humidity. We demonstrate that a two-source mixing approach (Keeling plot)can reliably be used to estimate the isotopic composition of the source moisture, and fromthat, to infer the location of the moisture origin that contributes to the atmospheric moisturecontent in southern California. The present study is unique because it combines large-scaleisotope GCM modeling with a robust and high-resolution isotope data set to disentangle thecontrol of atmospheric and hydrologic processes on the atmospheric humidity in anextratropical climate. Our results demonstrate the utility of using single-point, ground-basedisotope observations as a complementary resource to existing satellite isotopemeasurements for constraining isotope-enabled GCMs in future investigation ofatmospheric water cycle.

Photometric observations of 630.0‐nm OI and 427.8‐nm N2+ emission from South Pole and McMurdo Stations during winter: Analysis of temporal variations spanning minutes to hourly timescales

Time series analysis of zenith‐viewing, wide field‐of‐view photometer measurements of thermospheric OI 630.0‐nm and N2+ 427.8‐nm emissions taken simultaneously from South Pole station (90°S geographic) and from McMurdo station (78°S geographic) during the austral winter months of 2003, 2004, and 2005 are presented. The high sampling rate and continuous, extended duration of measurements at both locations allow for the analysis of phenomena with periods spanning the order of minutes to hours. It is shown that high frequency variations (i.e., with periods of order 1 and 10 min) are often observed at both sites which may be, as supported by colocated magnetometer instrumentation, related to periodic injection of plasma associated with magnetospheric pulsations or their generating processes. We also show that oscillations with periods on the order of 1 h are evident in the power spectrum from both South Pole and McMurdo at both altitudes and emissions and are highly variable. The observations of 12‐, 8‐, 6‐, 4.8‐, and 4‐h oscillations are believed to be due to daily sampling of the auroral zone emissions along with potential moonlight contamination. Other observed periods with order 1 h require further study but are likely due to auroral poleward boundary intensifications. The approach presented herein serves as a means to quickly identify time periods of geophysical activity which will be useful in a future analysis of a much greater, extended climatology of the various spectral features, e.g., a 2 decade survey of poleward moving auroral form events currently in progress. The results herein also assist in providing context to the analysis of synoptic space weather events with photometers and provide support for recently reported auroral intensity variations in the Pc5 band.

Eddy covariance measurements of surface energy budget and evaporation in a cool season over southern open water in Mississippi

Eddy covariance measurements of sensible (H) and latent (LE) heat fluxes were made over a large southern open water surface of Ross Barnett Reservoir (the Reservoir hereafter) in Mississippi during the cool season with frequent incursions of cold fronts from 1 September 2007 to 31 January 2008. The eddy covariance tower was located in the middle of the main body of the Reservoir with the tower fetches exceeding 2.0 km in all directions. The Reservoir was ice‐free in winter and the water temperatures always decreased with depth. Over the entire cool season, the averaged water surface temperatures were 1.8°C higher than the overlying air (i.e., positive temperature gradients that led to thermally convective conditions) and the averaged vapor pressure near the water surface was 0.8 kPa greater than the overlying air (i.e., positive vapor pressure gradients), though occasionally negative gradients for temperature and vapor pressure were also observed for short periods. On average, the wind speeds were considerably large (3.9 m s−1) to maintain adequate turbulent mixing mechanically. As a consequence of the combined effect of thermally and mechanically generated turbulent mixing, consistently positive H (with a mean H of 20.0 W m−2) and LE (with a mean LE of 80.0 W m−2) occurred during the entire season. These continuous energy losses via H and LE resulted in release of a large amount of energy stored in the water to the atmosphere. The mean Bowen ratio was low for this open water surface (i.e., 0.3), suggesting that most of the energy released from the water fueled evaporation rather than sensible heating of the atmosphere. Nighttime evaporative water losses were substantial, contributing to 45% of the total evaporative water loss in this cool season. Frequent incursions of cold fronts with windy, cold, and dry air masses significantly promoted turbulent exchanges of sensible and latent heat through enhanced turbulent mixing thermally and mechanically, leading to large H and LE events. Daily H and LE (i.e., evaporation) during the passages of cold fronts were on average 2.7 and 7.3 times those in nonevent days, respectively. Given the fact that large H and LE events occurred 26% of the time for our site, these cold front events caused an increase in the seasonal H by 42% and LE by 157%. Therefore changes in frequency, intensity, and duration of synoptic weather events, particularly the incursions of cold fronts, have significant impacts on the surface energy budget and evaporation over water at this site.

Seasonal climate forecasts for agriculture: Towards better understanding and value

Two approaches to obtaining more valuable seasonal climate forecasts for Australian agriculture are described. The first involves obtaining a better understanding of the climate system by exploring the individual synoptic weather events that make up seasonal climate. An examination of rainfall events in north-west Victoria indicates that there is one dominant synoptic system that is responsible for the majority of useful rainfall: the cutoff low. An exploration of the physics of these systems reveals that the moisture sources are most likely to be from the oceans north of Australia, while the frequency and intensity might be controlled by ocean temperatures to the south. The second approach involves an examination of the potential value of different forecast systems using a century-long simulation of the growth of a wheat crop in north-west Victoria. It is concluded that a perfect forecast of the growing season rainfall total is likely to provide less than half the potential value of an ideal forecast system.

Surface albedo observations of Hudson Bay (Canada) landfast sea ice during the spring melt

AbstractThe shortwave albedo is a major component in determining the surface energy balance and thus the evolution of the spring melt cycle. As the melt commences, the ice is partitioned into multiple surface types ranging from highly reflective white ice to absorptive blue ice. The reflectance from these surfaces shows significant spatial and temporal variability. Spectral albedo measurements were made at six different sites encompassing these two surface types, from 19 March to 3 May 2005, on 1.5 m thick landfast sea ice in southwestern Hudson Bay, Canada (58˚ N). Furthermore, the broadband albedo and the surface energy balance were continuously recorded at a nearby site during the 1 month period. Rapid changes in the albedo were found to relate to typical subarctic climate conditions, i.e. frequent incursions of southerly air, resulting snow and rain events and the generally high maximum solar insolation levels. Subsequently, diurnal variations in snow surface temperature were evident, often causing daytime melting and night-time refreezing resulting in the formation of ice lenses and superimposed ice. After rain events and extensive melting, the snowpack was transformed throughout into melt/freeze metamorphosed snow and superimposed ice. The integrated (350–1050 nm) albedo varied between 0.52 and 0.95 at the blue-ice sites, while it varied between 0.73 and 0.91 at white-ice sites. Variability on the order of ±10% in the white-ice broadband albedo resulted from the diurnal freeze–thaw cycle, but also synoptic weather events, such as snowfall and rain events, could rapidly change the surface conditions.

Seasonal evolution of the albedo of multiyear Arctic sea ice

As part of ice albedo feedback studies during the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment, we measured spectral and wavelength‐integrated albedo on multiyear sea ice. Measurements were made every 2.5 m along a 200‐m survey line from April through October. Initially, this line was completely snow covered, but as the melt season progressed, it became a mixture of bare ice and melt ponds. Observed changes in albedo were a combination of a gradual evolution due to seasonal transitions and abrupt shifts resulting from synoptic weather events. There were five distinct phases in the evolution of albedo: dry snow, melting snow, pond formation, pond evolution, and fall freeze‐up. In April the surface albedo was high (0.8–0.9) and spatially uniform. By the end of July the average albedo along the line was 0.4, and there was significant spatial variability, with values ranging from 0.1 for deep, dark ponds to 0.65 for bare, white ice. There was good agreement between surface‐based albedos and measurements made from the University of Washington's Convair‐580 research aircraft. A comparison between net solar irradiance computed using observed albedos and a simplified model of seasonal evolution shows good agreement as long as the timing of the transitions is accurately determined.

Variations of Sensible and Latent Heat Fluxes from a Great Lakes Buoy and Associated Synoptic Weather Patterns

An investigation of sensible and latent heat fluxes and their relation to synoptic weather events was performed using hourly meteorological measurements from National Data Buoy Center buoy 45003, located in northern Lake Huron, during April‐November of 1984. Two temporal heat flux regimes were found to exist over Lake Huron. The first period extended from April through July and was characterized by periods of modest negative (downward) heat fluxes. The second regime was marked by periods of large positive (upward) heat fluxes and occurred from August through November. This later period accounted for 95%‐100% of both the total positive sensible and latent heat fluxes. In addition, a comparison of the seasonal evolution of sensible and latent heat fluxes showed the transition from the negative to the positive flux regime occurred 10‐20 days earlier for latent heat flux than for sensible heat flux. A notable, statistically significant increase of the surface heat flux variability from the negative to positive flux regimes with a general decrease in the near-surface atmospheric stability during the positive flux regime was found. During both flux regimes, the magnitude of surface sensible and latent heat fluxes remained coupled to transient synoptic-scale weather events. On average, the occurrences of minimum (maximum) heat fluxes preceded time periods of low (high) sea level pressure by 0‐3 h during the negative flux regime. For the positive flux regime, maximum (minimum) surface heat fluxes followed the passage of low (high) pressure by approximately 24 h. In addition, maximum (minimum) sensible and latent heat fluxes preceded synoptic high (low) pressure by approximately 16 h. Typical synoptic surface weather patterns were identified for both significant positive and negative heat flux events, time periods when the atmosphere‐lake heat exchange was maximized.