Mid-summer Rainfall Research Articles

Extreme characteristics and causes of the drought event in the whole Yangtze River Basin in the midsummer of 2022

Due to their huge socio-economic impacts and complex formation causes, extreme and continuous drought events have become the focus and nodus of research in recent years. In the midsummer (July–August) of 2022, a severe drought event occurred in the whole Yangtze River Basin (YRB), China. During that period, the precipitation in the upper, middle and lower reaches of the YRB dropped over 40% less than the 1961–2021 climatic mean, which had never happened previously. Furthermore, the temperature was the highest during 1961–2022. The record-breaking magnitude of less rainfall and high temperature directly led to the continuous development of this extreme drought event. An atmospheric moisture budget analysis revealed that the YRB midsummer rainfall anomaly was dominated by the anomalous powerful vertical moisture advection, which was derived from the strongest descending motion over the whole YRB in the 2022 midsummer during 1981–2022. The western Pacific subtropical high (WPSH) during the midsummer remained stronger, more westward and lasted longer than the climatic mean. As a result, the whole YRB was controlled by a positive geopotential height centre. Further evidence revealed that the anomalous subtropical zonal flow played a crucial role in inducing the extreme descent over the YRB. Moreover, the anomalous upper-tropospheric easterly flow over the YRB in 2022 is the strongest during 1981–2022, modulating the generation of the unprecedented descent anomaly over the YRB. The likelihood that an integrated connection of severe drought in East Asia and flood in West Asia and northwestern South Asia would increase when the extremely strong easterly anomalies in the upper troposphere emerged and induced descending adiabatic flow on the eastern sides of the Tibetan Plateau. The results of this study can provide scientific insights into the predictability of extreme drought events and provide ways to improve predictions.

Multi-year trends and interannual variation in ecosystem resource use efficiencies in a young mixedwood plantation in northern China

Understanding the long-term trends and interannual variability in ecosystem resource use efficiencies (RUEs) and their controlling factors is central to the prediction of ecosystem response to climate change. However, knowledge in this area, particularly as it pertains to young urban plantations, remains largely absent. In this study, we used continuous eddy covariance and site-specific climatic data collected from a 12-year-old mixedwood plantation in Beijing, China, to assess (i) long-term trends, (ii) interannual variability, and (iii) key controlling factors of RUEs with respect to plantation use of carbon (i.e., CUE), water (WUE) and light (LUE). On average, annual CUE and WUE over the 2012–2020 growing seasons increased by about 5.2 and 7.6%. Trends in RUEs corresponded with increases in normalized difference vegetation index (NDVI) when the plantation transitioned from a carbon source to a sink. Analysis of controlling factors indicated that RUEs were regulated by NDVI and other environmental factors at varying degrees dependent on season. Interannual variability in (i) CUE was largely controlled by spring soil temperature, (ii) WUE by summer water vapor pressure deficit and spring soil water content (SWC), and (iii) LUE by photosynthetically active radiation, summer SWC and spring air temperature. Relative seasonal changes in paired RUEs were positively correlated, with soil water availability being the most limiting. Long-term summer drought was shown to evoke severe and persistent decline in same-year RUEs. Ample mid-summer rainfall lessened the impact of early spring drought on plantation performance. Our results emphasized the need to account for the combined effects of climatic controls and canopy-physiological characteristics in the management of young plantations.

Influence of the North Kazakhstan Plains Weather on the Productivity of the Spring Soft Wheat

Spring soft wheat is a primary agricultural crop of North Kazakhstan, the growing process of which is done in harsh weather conditions due to the fact, that early spring drought and the biggest rainfalls at the end of June and beginning of July, are normal in the region. Due to this fact, scientists and producers have mistakenly believed that in the Northern region, spring wheat should be sown later, adjusting the main phase of plant development “tillering – stem elongation” under the maximum summer rainfalls. The research aims to establish the dependence of spring wheat yield on the amount of spring-summer rainfall at different sowing periods. The following methods were used in the research: field method, methods of clustering, variation, correlation and dispersion analysis. Analysis of observations from 2012-2021 showed that years with an early-spring drought and mid-summer maximum rainfalls were less than one-third of ten and about 60% were years with no spring drought, although the maximum rainfall period was shifted to the June month. However, only one year (10%) was characterized by a severe spring drought and a shift of summer rainfalls to August. The highest yield (26.9 c/ha) of the studied wheat species was observed in years with an atmospheric precipitation shift at the beginning of the summer period and early sowing date. During years of early spring drought with the biggest rainfall in mid-summer, relatively high yields (20.5 cwt/ha) are observed in the later sowing dates. The practical value of the research is determined by the fact, that in Northern Kazakhstan the sowing dates of spring wheat are not closely tied to the calendar dates and depend significantly on the climatic conditions of the region and the weather conditions that prevailed in the spring and summer period of a specific year

Strengthened relationship between sea ice in East Siberian Sea and midsummer rainfall in Northeast China

Arctic sea ice displays the high rates of decay during summer-autumn. Previous studies have revealed the impact of autumn Arctic sea ice on local and remote atmospheric circulation. Few attentions have been paid on the relationship between summer Arctic sea ice and climate variation at Eurasia. This study identifies a strengthened relationship between midsummer rainfall at Northeast China (NEC) and simultaneous sea ice area (SIA) in East Siberian Sea after the 1990s. The NEC’s rainfall shows a significant positive correlation with the SIA during 1994–2016, whereas the relationship is insignificant during 1961–1983. The strengthening of the relationship is attributed to the western elongation of background circulation at the North Pacific and the increased interannual variability of the SIA after the 1990s. The former facilitates the western extension of the SIAI-associated circulation over the Central North Pacific. The enlarged amplitudes of sea ice induce surface heat fluxes in situ, and then it leads to stronger meridional temperature gradient anomalies and intensified interaction between synoptic-scale eddies and mean flow over the North Pacific. These conditions jointly contribute to stronger and western elongation of circulation over the Central North Pacific during 1994–2016. Accordingly, the SIA has an intimate connection with NEC’s rainfall though the modulation of moisture transport and vertical movement.

Extremely wet summer events enhance permafrost thaw for multiple years in Siberian tundra

Permafrost thaw can accelerate climate warming by releasing carbon from previously frozen soil in the form of greenhouse gases. Rainfall extremes have been proposed to increase permafrost thaw, but the magnitude and duration of this effect are poorly understood. Here we present empirical evidence showing that one extremely wet summer (+100 mm; 120% increase relative to average June–August rainfall) enhanced thaw depth by up to 35% in a controlled irrigation experiment in an ice-rich Siberian tundra site. The effect persisted over two subsequent summers, demonstrating a carry-over effect of extremely wet summers. Using soil thermal hydrological modelling, we show that rainfall extremes delayed autumn freeze-up and rainfall-induced increases in thaw were most pronounced for warm summers with mid-summer precipitation rainfall extremes. Our results suggest that, with rainfall and temperature both increasing in the Arctic, permafrost will likely degrade and disappear faster than is currently anticipated based on rising air temperatures alone.

Pinus resinosa Tree-Ring Latewood Response to Daily-Scale Precipitation Variability at Lake Itasca, Minnesota

Analysis of daily scale climate observations alongside sub-annual tree-ring data offers new potential for contextualizing climate change in the Great Lakes region. This pilot study combined daily observations from a high-quality station record with a co-located chronology of Pinus resinosa latewood width at Lake Itasca, Minnesota. We evaluate trends in observational data and use multiple methods to compare day-wise aggregated climate observations with tree rings over the eleven-decade common period. The Itasca record exhibited strong increases in warm-season precipitation, minimum temperature in all seasons, and lengthening of the freeze-free season. Correlation analyses verified a strong, multi-month warm-season precipitation response in Pinus resinosa latewood width. Distinct from previous work, daily data analyses were used to fingerprint an ~2-week period starting in late July when rainfall variability was historically a major control on interannual tree growth. Climatologically, the timing of this subseasonal critical climate period corresponds with a relative minimum in mean midsummer precipitation. Since the 1980s, the latewood correlation with midsummer rainfall has vanished, and the seasonal-scale rainfall response diminished considerably. This result, new for Pinus resinosa in Minnesota, is consistent with studies showing a declining relationship between tree growth and drought in the Midwest United States. Further attribution analyses emphasizing daily-scale phenomena are needed to elucidate mechanisms responsible for the tree-growth response to variability, change, and extremes in climate throughout the Great Lakes region, where the biophysical and socioeconomic impacts of climate change are multifaceted issues of increasing urgency.

Reduction of mid-summer rainfall in northern India after the late-1990s induced by the decadal change of the Silk Road pattern

Variation of summer rainfall can pose substantial challenges to water resources and agriculture in northern India, which includes the most fertile land in India, i.e. the Ganges Plain. This study identifies a significant rainfall reduction in northern India during its peak rainy season (July and August) after the late-1990s, based on observational and reanalysis data since 1958. This rainfall reduction is about 0.86 mm d−1, equivalent to 8.3% of the climatological mean. We suggest that the decadal reduction of northern Indian rainfall is induced by the decadal change of the Silk Road pattern (SRP), which is an atmospheric teleconnection pattern along the upper-tropospheric Asian westerly jet. After the late-1990s, corresponding with the decadal phase shift of the SRP, there is a cyclonic anomaly over West Asia, which can reduce rainfall in northern India. Furthermore, this rainfall reduction is unprecedented since the beginning of the 20th century, when observational data are available. Possible roles of the lower-tropospheric circulation changes and the Atlantic Multidecadal Oscillation are discussed.

Impact of early spring sea ice in Barents Sea on midsummer rainfall distribution at Northeast China

The influence of early spring sea ice at Barents Sea on midsummer rainfall in Northeast China (NEC) is identified based on observational analyses and atmospheric modeling experiments in this study. Increased sea ice area (SIA) in the Barents Sea is ensued by positive rainfall anomalies at north of NEC and by negative anomalies at south, and vice verse. Specifically, due to a good seasonal persistence from spring to summer, the preceding sea ice anomalies exert an impact on midsummer surface air temperature anomalies and vertical stability over Barents Sea via the modulation on turbulent heat flux. The anomalous circulation is further triggered over Europe and the Mediterranean Sea through meridional vertical cells, with a barotropic structure. Accordingly, an effective Rossby wave source is excited over the eastern Mediterranean by the advection of vorticity by divergence wind, and causes an eastward propagation of Silk Road Pattern to East Asia. In addition, another SIA-related wave-like train can diffuse directly southeastward from Arctic to NEC in a polar path. Observations and numerical simulations indicate that, in response to increased sea ice at Barents Sea, an anomalous cyclone emerges over NEC, along with easterly or southeasterly over north of NEC and with northwesterly over south, leading to moisture convergence anomalies at north and divergence anomalies at south. Jointly, ascending (descending) motion anomalies favors a wet (dry) summer over north (south) of NEC.

Asymmetric impact of East Asian jet’s variation on midsummer rainfall in North China and Yangtze River Valley

Variations of East Asian subtropical westerly jet (EAWJ) have a close relationship with the midsummer rainfall anomalies in Yangtze River Valley (YRV) and North China (NC). The present study shows that the relationship is asymmetric. When EAWJ is located north (south) of its median position for 1979–2013, EAWJ’s southward displacement is associated with the increased (decreased) rainfall anomalies in NC and no obvious (increased) rainfall anomalies in YRV; and vice versa. The mechanism is as follows. EAWJ’s southward displacement is associated with northward flow anomalies over NC and YRV, when EAWJ is located north of the median position. The associated moisture convergence and warm advection anomalies over NC cause increased rainfall anomalies in NC, while rainfall anomalies in YRV are weak due to the weak thermal advection and water vapor convergence/divergence anomalies. When EAWJ is located south of the median position, anomalous southwestward and eastward flow are observed over NC and YRV, respectively, associated with EAWJ’s southward displacement. The resultant cold advection anomalies over NC cause decreased rainfall anomalies in NC, while the resultant anomalous warm advection and moisture convergence over YRV cause positive rainfall anomalies in YRV. The asymmetric circulation changes responsible for the asymmetric rainfall changes in YRV&NC are quasi-barotropic, implying that EAWJ is among the large-scale environmental forcings for the asymmetric rainfall variability in NC&YRV.

Seasonal forecasts for the Limpopo Province in estimating deviations from grazing capacity

Application of seasonal forecasts in agriculture has significant potential and realized utility. Other sectors that may also benefit from using seasonal forecasts include (but are not limited to) health, hydrology, water, and energy. This paper shows that seasonal forecast model data, satellite Pour l’Observation de la Terre (SPOT), dry matter productivity (DMP) data (proxy of grass biomass) along with other sets of data are effectively used to estimate grazing capacity (GC) over a 12-year test period (1998/1999–2009/2010) in Limpopo Province. GC comprises a vital consideration in agricultural activities, particularly for a province in South Africa like Limpopo, due to its varying climate. The Limpopo Province capitalizes on subsistence farming, including livestock and crop production. Grazing should thus be regulated in order to conserve grass, shrubs, and trees, thereby ensuring sustainability of rangelands. In a statistical downscaling model, the predictor is the 850 geopotential height fields of a coupled ocean–atmosphere general circulation (CGCM) over Southern Africa to predict seasonal DMP values. This model shows that the mid-summer rainfall totals are important predictors for the November through April (NDJFMA) DMP (as well as grazing capacity) growing season. Forecast verification is conducted using the relative operating characteristics (ROC) and reliability diagrams. The CGCM model shows skill in discriminating high and low DMP (GC) seasons in the Limpopo Province, as well as reliability in the probabilistic forecasts. This paper demonstrates the development of a tailored forecast, an avenue that should be explored in enhancing relevance of forecasts in agricultural production.

Upside-down fluxes Down Under: CO<sub>2</sub> net sink in winter and net source in summer in a temperate evergreen broadleaf forest

Abstract. Predicting the seasonal dynamics of ecosystem carbon fluxes is challenging in broadleaved evergreen forests because of their moderate climates and subtle changes in canopy phenology. We assessed the climatic and biotic drivers of the seasonality of net ecosystem–atmosphere CO2 exchange (NEE) of a eucalyptus-dominated forest near Sydney, Australia, using the eddy covariance method. The climate is characterised by a mean annual precipitation of 800 mm and a mean annual temperature of 18 ∘C, hot summers and mild winters, with highly variable precipitation. In the 4-year study, the ecosystem was a sink each year (−225 g C m−2 yr−1 on average, with a standard deviation of 108 g C m−2 yr−1); inter-annual variations were not related to meteorological conditions. Daily net C uptake was always detected during the cooler, drier winter months (June through August), while net C loss occurred during the warmer, wetter summer months (December through February). Gross primary productivity (GPP) seasonality was low, despite longer days with higher light intensity in summer, because vapour pressure deficit (D) and air temperature (Ta) restricted surface conductance during summer while winter temperatures were still high enough to support photosynthesis. Maximum GPP during ideal environmental conditions was significantly correlated with remotely sensed enhanced vegetation index (EVI; r2 = 0.46) and with canopy leaf area index (LAI; r2 = 0.29), which increased rapidly after mid-summer rainfall events. Ecosystem respiration (ER) was highest during summer in wet soils and lowest during winter months. ER had larger seasonal amplitude compared to GPP, and therefore drove the seasonal variation of NEE. Because summer carbon uptake may become increasingly limited by atmospheric demand and high temperature, and because ecosystem respiration could be enhanced by rising temperatures, our results suggest the potential for large-scale seasonal shifts in NEE in sclerophyll vegetation under climate change.

Climatic change on the Gulf of Fonseca (Central America) using two-step statistical downscaling of CMIP5 model outputs

A two-step statistical downscaling method has been reviewed and adapted to simulate twenty-first-century climate projections for the Gulf of Fonseca (Central America, Pacific Coast) using Coupled Model Intercomparison Project (CMIP5) climate models. The downscaling methodology is adjusted after looking for good predictor fields for this area (where the geostrophic approximation fails and the real wind fields are the most applicable). The method’s performance for daily precipitation and maximum and minimum temperature is analysed and revealed suitable results for all variables. For instance, the method is able to simulate the characteristic cycle of the wet season for this area, which includes a mid-summer drought between two peaks. Future projections show a gradual temperature increase throughout the twenty-first century and a change in the features of the wet season (the first peak and mid-summer rainfall being reduced relative to the second peak, earlier onset of the wet season and a broader second peak).

Recent Climate Warming-Related Growth Decline Impairs European Beech in the Center of Its Distribution Range

Increasing summer droughts represent a major threat for the vitality and productivity of forests in the temperate zone. European beech, the most important tree species of Central Europe’s natural forest vegetation, is known to suffer from increased drought intensity at its southern distribution limits, but it is not well known how this species is affected in the center of its distribution range in a sub-oceanic climate. We compared tree-ring chronologies and the climate sensitivity of growth (MS) in 11 mature beech stands along a precipitation gradient (855–576 mm y−1) on two soil types with contrasting water storage capacity (WSC) in northwest Germany to test the hypotheses that recent warming is impairing beech growth also in the center of its distribution below a certain precipitation limit, and stands with low soil WSC are more susceptible. We found a threshold of about 350 mm of mean growing season precipitation below which basal area increment (BAI) showed a consistent decline since the 1970s. The frequency of negative pointer years and MS were highest in low-precipitation stands on sandy soil, but both parameters have increased during the last decades also in the moister stands. The factor with largest impact on BAI was precipitation in June, in combination with high mid-summer temperatures. Contrary to our hypothesis, the edaphic effect on growth dynamics was surprisingly small. We conclude that global warming-related growth decline is affecting European beech even in the center of its distribution below a hydrological threshold that is mainly determined by mid-summer rainfall.

Relationships between North Indian Summer Monsoon and Midsummer Rainfall in Hetao and Its Vicinity Regions of China

Relationships between North Indian Summer Monsoon and Midsummer Rainfall in Hetao and Its Vicinity Regions of China

The coherent large-scale circulation change between dry/wet years over central eastern China simulated by NCAR CAM5

This study evaluates the simulation of the coherent circulation structure correspond to the changes of mid-summer (July–August) rainfall over eastern China (30°–40° N, 110°–120° E) in high-resolution NCAR CAM5. Forced by historical sea surface temperatures (SSTs), the NCAR CAM5 reasonably reproduces coherent changes of temperature and large-scale circulations, corresponding to the changes in rainfall. Results show that when the rainfall decreases over eastern China, the model reproduces a remarkable warm center in the upper troposphere with an anomalous anticyclone appears above and an increase in anomalous westerlies to its north. An anomalous anticyclone also occurs in the lower troposphere, along with anomalous southerlies to its east which indicates strengthening of the East Asian summer monsoon. Both the circulation changes in the upper and lower troposphere favor a decrease in precipitation over central eastern China. There were also good correlations between the simulated upper-tropospheric temperature and other large-scale circulation changes. There are some deficiencies in the NCAR CAM5 simulations in terms of the changes in magnitude and location of the rainfall centers. However, in general, the model reasonably reproduced the coherent configuration of the large-scale circulation patterns and surface rainfall. This study further confirms that the climate variations across East Asia most likely arise from a regional response to global climate change. The well-simulated configuration by NCAR CAM5 also indicates the reliability of the model and its potential to reveal the mechanisms driving the coherent changes of the East Asian summer monsoon system.

Intraspecific variation in stomatal traits, leaf traits and physiology reflects adaptation along aridity gradients in a South African shrub.

Trait-environment relationships are commonly interpreted as evidence for local adaptation in plants. However, even when selection analyses support this interpretation, the mechanisms underlying differential benefits are often unknown. This study addresses this gap in knowledge using the broadly distributed South African shrub Protea repens. Specifically, the study examines whether broad-scale patterns of trait variation are consistent with spatial differences in selection and ecophysiology in the wild. In a common garden study of plants sourced from 19 populations, associations were measured between five morphological traits and three axes describing source climates. Trait-trait and trait-environment associations were analysed in a multi-response model. Within two focal populations in the wild, selection and path analyses were used to test associations between traits, fecundity and physiological performance. Across 19 populations in a common garden, stomatal density increased with the source population's mean annual temperature and decreased with its average amount of rainfall in midsummer. Concordantly, selection analysis in two natural populations revealed positive selection on stomatal density at the hotter, drier site, while failing to detect selection at the cooler, moister site. Dry-site plants with high stomatal density also had higher stomatal conductances, cooler leaf temperatures and higher light-saturated photosynthetic rates than those with low stomatal density, but no such relationships were present among wet-site plants. Leaf area, stomatal pore index and specific leaf area in the garden also co-varied with climate, but within-population differences were not associated with fitness in either wild population. The parallel patterns of broad-scale variation, differences in selection and differences in trait-ecophysiology relationships suggest a mechanism for adaptive differentiation in stomatal density. Densely packed stomata may improve performance by increasing transpiration and cooling, but predominately in drier, hotter climates. This study uniquely shows context-dependent benefits of stomatal density--a trait rarely linked to local adaptation in plants.

SST prediction methodologies and verification considerations for dynamical mid-summer rainfall forecasts for South Africa

Seasonal-to-interannual hindcasts (re-forecasts) for December-January-February (DJF) produced at a 1-month lead-time by the ECHAM4.5 atmospheric general circulation model (AGCM) are verified after calibrating model output to DJF rainfall at 94 districts across South Africa. The AGCM is forced with SST forecasts produced by (i) statistically predicted SSTs, and (ii) predicted SSTs from a dynamically coupled ocean-atmosphere model. The latter SST forecasts in turn consist of an ensemble mean of SST forecasts, and also by considering the individual ensemble members of the SST forecasts. Probabilistic hindcasts produced for two separate category thresholds are verified over a 24-year test period from 1978/79 to 2001/02 by investigating the various AGCM configurations’ attributes of discrimination (whether the forecasts are discernibly different given different outcomes) and reliability (whether the confidence communicated in the forecasts is appropriate). Deterministic hindcast skill is additionally calculated through a range of correlation estimates between hindcast and observed DJF rainfall. For both probabilistic and deterministic verification the hindcasts produced by forcing the AGCM with dynamically predicted SSTs attain higher skill levels than the AGCM forced with statistical SSTs. Moreover, ensemble mean SST forecasts lead to improved skill over forecasts that considered an ensemble distribution of SST forecasts.Keywords: AGCM, SST predictions, seasonal forecasting, South Africa

Seasonal rainfall predictability over the Lake Kariba catchment area

The Lake Kariba catchment area in southern Africa has one of the most variable climates of any major river basin, with an extreme range of conditions across the catchment and through time. Marked seasonal and interannual fluctuations in rainfall are a significant aspect of the catchment. To determine the predictability of seasonal rainfall totals over the Lake Kariba catchment area, this study used the low-level atmospheric circulation (850 hPa geopotential height fields) of a coupled ocean-atmosphere general circulation model (CGCM) over southern Africa, statistically downscaled to gridded seasonal rainfall totals over the catchment. This downscaling configuration was used to retroactively forecast the 3-month rainfall seasons of September-October-November through February-March-April, over a 14-year independent test period extending from 1994. Retroactive forecasts are produced for lead times of up to 5 months and probabilistic forecast performances evaluated for extreme rainfall thresholds of the 25th and 75th percentile values of the climatological record. The verification of the retroactive forecasts shows that rainfall over the catchment is predictable at extended lead-times, but that predictability is primarily found for austral mid-summer rainfall. This season is also associated with the highest potential economic value that can be derived from seasonal forecasts. A forecast case study of a recent extreme rainfall season (2010/11) that lies outside of the verification period is presented as evidence of the statistical downscaling system’s operational capability.Keywords: Lake Kariba catchment, coupled ocean-atmosphere model, statistical downscaling, seasonal forecasting, economic value

Anomalous midsummer rainfall in Yangtze River-Huaihe River valleys and its association with the East Asia westerly jet

In this study, the interannual and interdecadal relationship between midsummer Yangtze River-Huaihe River valley (YHRV) rainfall and the position of the East Asia westerly jet (EAWJ) were investigated. The midsummer YHRV rainfall was found to significantly increase after the 1980s. Moreover, the location of the EAWJ was found abnormally south of the climatic mean during 1980–2008 (ID2) compared to 1951–1979 (ID1). During ID2, associated with the southward movement of the EAWJ, an anomalous upper-level convergence occurred over middle-high latitudes (35°-55°N) and divergence occurred over lower latitudes (∼30°N) of East Asia. Correspondingly, anomalous descending and ascending motion was observed in middle-high and lower latitudes along 90°–130°E, respectively, favoring more precipitation over YHRV. On an interannual time scale, the EAWJ and YHRV rainfall exhibited similar relationships during the two periods. When the EAWJ was centered abnormally southward, rainfall over YHRV tended to increase. However, EAWJrelated circulations were significantly different during the two periods. During ID1, the circulation of the southward-moving EAWJ exhibited alternating positive-negative-positive distributions from low to middle-high latitudes along the East Asian coast; the most significant anomaly appeared west of the Okhotsk Sea. However, during ID2 the EAWJ was more closely correlated with the tropical and subtropical circulations. Significant differences between ID1 and ID2 were also recorded sea surface temperatures (SSTs). During ID1, the EAWJ was influenced by the extratropical SST over the northern Pacific; however, the EAWJ was more significantly affected by the SST of the tropical western Pacific during ID2.

Runoff Temperature Model for Paved Surfaces

Interest in thermal pollution due to storm-water runoff has risen significantly since it was recognized that fish habitat in coldwater streams may deteriorate or even disappear following urban development or logging. The need to project changes in both runoff temperature and volume in response to land use changes has been recognized. Surface runoff hydrographs can be predicted or simulated using a variety of existing models. Few tools exist to predict or simulate the thermograph of that runoff, i.e., the water flow rate and temperature as a function of time. To simulate runoff temperature for small parcels of land of uniform cover such as parking lots, a new hydrothermal runoff model was developed. The runoff portion of the model is semianalytical and spatially integrated. The runoff model is discrete in time, so that it may be used to analyze events with observed rainfall intensity variations at a resolution of 15 min or less. The runoff model closely approximates the simulation results of a one-dimensional kinematic wave model. For runoff temperature simulations, a heat transfer model was linked to the runoff model. The heat transfer model includes heat conduction to the ground and heat exchange with the atmosphere. Weather data at short time resolution (e.g., 15 min) are a model input requirement. To test and illustrate the usefulness of the hydrothermal runoff model, the response of a 24-acre parking lot to three midsummer rainfall events was simulated. The model was found to predict the average runoff temperatures within 0.3–0.5°C of observed values, and total runoff volumes within 1–15%. The root-mean-square error of simulated 2 min runoff temperatures was 1.1°C. The proposed model is to be included in a simulation tool to assess the hydrothermal impact of proposed land development on coldwater streams. Additional features are being added to the hydrothermal model for applications to pervious, vegetated land. Areas targeted for development are divided into small subwatersheds which are simulated individually. The change in total runoff temperature and volume due to development will be obtained by routing of subwatershed outputs.