Mean Seasonal Cycle Research Articles

The Nondeterministic Nature of Kuroshio Penetration and Eddy Shedding in the South China Sea*

A ⅛°, 6-layer Pacific version of the Naval Research Laboratory Layered Ocean Model is used to investigate the nondeterministic nature of Kuroshio intrusion and eddy shedding into the South China Sea (SCS) on annual and interannual timescales. Four simulations, which only differ in the initial state, are forced with 1979–93 European Centre for Medium-Range Weather Forecasts reanalysis 1000 hectopascal (hPa) winds and then continued in 1994–97 with ECMWF operational 1000-hPa winds. The model shows differing amounts of Kuroshio penetration across all four simulations for the yearly means, indicating a large degree of nondeterminism at this timescale. This nondeterminism is quantified by a technique that separates the variability of a model variable into deterministic (caused by direct atmospheric forcing) and nondeterministic (caused by mesoscale flow instabilities) components. Analysis indicates substantial nondeterministic sea surface height and upper-layer velocity variability in the vicinity of Luzon Strait. A quantitative measure of Kuroshio intrusion into the SCS is presented that allows interexperiment comparisons and investigation of interannual variability, and attempts are made to positively correlate it with the oceanic and atmospheric environment. Yearly mean Kuroshio intrusion is not strongly linked to Luzon Strait transports or to changes in the North Equatorial Current bifurcation latitude (which is related to the northward Kuroshio transport east of Luzon). Likewise, no relationship could be found that linked interannual variability of yearly mean Kuroshio intrusion or monsoon season mean Luzon Strait transport with the corresponding zonal or meridional wind components, wind stress magnitude, or wind stress curl. However, there was a close relationship between the mean seasonal cycles of the Luzon Strait transport and the northeast–southwest monsoon. Eddy shedding and deep Kuroshio intrusion are rare events during the period of ECMWF reanalysis forcing, but are persistent features during the ECMWF operational time frame. While the wind stress is consistent across the reanalysis/operational time boundary, large differences exist in the wind stress curl pattern over the Luzon Strait and interior of the SCS basin. For contemporaneous years, the ECMWF operational winds produce higher curl extrema (by a factor of 2) and a much sharper north–south gradient in Luzon Strait. The net effect is to produce more Ekman pumping, a deepening of the thermocline, and a more deeply penetrating Kuroshio during the 1994–97 ECMWF operational forcing time frame. Thus, while normal interannual variations of the wind curl did not produce a deterministic response of simulated Kuroshio intrusion, the marked differences in curl between the two atmospheric products did have a substantial impact.

A Model Study of Oceanic Mechanisms Affecting Equatorial Pacific Sea Surface Temperature during the 1997–98 El Niño

In this study, the processes affecting sea surface temperature variability over the 1992-98 period, encompassing the very strong 1997-98 El Nino event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1-2and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The model reproduces the main features of the tropical Pacific mean state, despite a weaker than observed thermal stratification, a 0.1 m s21 too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a slight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 108N-108S band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during the 1992-98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s 21 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing. This good agreement with observations provides confidence in the comprehensive three-dimensional circulation and thermal structure of the model. A close examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993-96 period and the 1997-98 El Nino. In the eastern Pacific, cooling by exchanges with the subsurface (vertical advection, mixing, and entrainment), the atmospheric forcing, and the eddies (mainly the tropical instability waves) are the three main contributors to the heat budget. In the central-western Pacific, the zonal advection by low-frequency currents becomes the main contributor. Westerly wind bursts (in December 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997-98 El Nino. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward advection of the warm pool that they generated temperature anomalies in the central Pacific. The end of El Nino can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from the end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical processes. In the central Pacific, easterlies pushed the warm pool back to the west. In the east, they led to a shallower thermocline, which ultimately allowed subsurface cooling to resume and to quickly cool the surface layer.

Role of ocean dynamics in determining the mean seasonal cycle of the South China Sea surface temperature

The mean seasonal cycle of surface heat budget is examined using historical temperature data combined with climatological wind stress and surface heat flux in the South China Sea. In most parts of the basin, we see a negative correlation between sea surface temperature (SST) and mixed layer depth (MLD); that is, SST tends to be higher (lower) when MLD is shallower (deeper). Given the characteristics of the MLD distribution, we further partition the South China Sea into four smaller areas: one along the continental slope south of China, one off west Luzon, one in the central part of the basin, and one near the coast of Vietnam. Heat budget assessment in these smaller areas indicates that although surface heat flux is fundamental to the mean seasonal cycle of SST, the effect of ocean dynamics is not negligible. Adding the contribution of ocean dynamics better explains the SST tendency. Ekman advection forced by the northeast monsoon is the primary heating process in winter but becomes less important as the horizontal temperature gradient decreases in the following seasons. Vertical entrainment is effective at cooling when the southwest monsoon prevails, which works against the surface heating and leads to a decrease of SST by up to 5 months earlier than surface heat flux starts to cool the ocean.

Impact of ozone layer depletion I: ozone depletion climatology

A monthly ozone profile depletion climatology is constructed for the time period 11/1978–4/1993 for 10° latitude bands using the TOMS total ozone data and SAGE trends. Monthly mean Nimbus-7/TOMS total ozone data are analysed by multiple linear regression which takes into account the seasonal variation, quasi-biennial oscillations (QBO), solar cycle and the month-to-month correlation of ozone values. Total ozone depletion trends and the mean seasonal cycle are determined. To construct the monthly ozone profile depletion climatology the SAGE trends are used down to an altitude of 20 km. The trends between 20 km and the tropopause are estimated from both the TOMS total ozone fields for 1978/79 (the mean seasonal cycle) and 1992/93 (the mean seasonal cycle minus depletion trends) and the SAGE trends; these estimates take into account general information about the seasonal and altitude dependence of trends that is derived from an ozonesonde analysis.

What is the mean seasonal cycle of surface heat flux in the equatorial Pacific?

The mean seasonal cycles of six state‐of‐the‐art surface heat flux products (three based on widely available data and three based on numerical model reanalysis fields) are compared in the equatorial Pacific with heat fluxes computed from Tropical Atmosphere‐Ocean (TAO) buoy data. Net surface heat flux and individual flux components derived from these products exhibit large deviations from TAO. We find that a significant contribution to these differences, which are often 50 W m−2 or more for net flux, is the use of systematically biased bulk variables in the computation of turbulent surface heat fluxes. We also find that for some products, compensating errors in bulk variables lead to fortuitous agreement with turbulent heat fluxes estimated from TAO. Finally, comparisons of TAO‐derived fluxes with tuned and untuned heat flux estimates from the Comprehensive Ocean Atmosphere Data Set indicate better agreement with untuned fluxes, suggesting that commonly used ad hoc strategies to close the global ocean heat budget are not strictly valid.

Biogeochemical modelling of the tropical Pacific Ocean. I: Seasonal and interannual variability

Abstract A coupled physical–biogeochemical model has been developed in order to study physical–biological interactions in the tropical Pacific Ocean on seasonal-to-interannual timescales. The model incorporates both iron- and nitrogen-limited phytoplankton growth, and succession of phytoplankton size classes in accordance with the “ecumenical” iron hypothesis. The model shows a strong El Nino–Southern Oscillation component to phytoplankton variability in the central equatorial Pacific over the period 1980–1998. It is possible that this mode is more dominant in the model than in nature, although the correlation of modelled and observed chlorophyll in this region is strong. The model results show that interannual variability dominates over the mean seasonal cycle for both physical and biogeochemical fields, with the exception of undercurrent transport west of the dateline. Physical and biogeochemical fields show consistent seasonal phasing among the four El Nino events simulated, although there is variability in the magnitudes and exact timing. Nutrient concentration anomalies at constant temperature in the thermocline of the central equatorial Pacific appear to be largely advected from the west, and the strong seasonal cycle of the equatorial undercurrent in the west introduces a significant annual component to the variability of nutrient concentrations at longitudes where the mean seasonal cycle of the local physical forcing is negligible. The biogeochemical model maintains realistic nutrient pools over the time scales required for interannual simulation, and responds in a realistic fashion to changing upper ocean hydrography and circulation. The model is quite sensitive to the temporal resolution of the wind forcing, which introduces additional uncertainty into the validation and prediction of biogeochemical fields.

Surface measurements of global warming causing atmospheric constituents in Korea.

The expansion of the industrial economy and the increase of population in Northeast Asian countries have caused much interest in climate monitoring related to global warming. However, new techniques and better platforms for the measurement of global warming and regional databases are still old-fashioned and are not being developed sufficiently. With respect to this agenda, since 1993, at the request of the World Meteorological Organization (WMO), to monitor functions of global warming, the Korea Meteorological Administration (KMA) has set up a Global Atmospheric Watch (GAW) Station on the western coast of Korea (Anmyun-do) and has been actively monitoring global warming over Northeast Asia. In addition, atmospheric carbon dioxide (CO2) has been measured for a similar KMA global warming program at Kosan, Cheju Island since 1990. Aerosol and radiation have also been measured at both sites as well as in Seoul. The observations have been analyzed using diagnostics of climate change in Northeast Asia and also have been internationally compared. Results indicate that greenhouse gases are in good statistic agreement with the NOAA/Climate Monitoring and Diagnostics Laboratory (CMDL) long-term trends of monthly mean concentrations and seasonal cycles. Atmospheric particulate matter has also been analyzed for particular Asian types in terms of optical depth, number concentration and size distribution.

On the Variability of the Tropical Pacific Thermal Structure during the 1979–96 Period, as Deduced from XBT Sections

The thermal structure variability of the tropical Pacific is investigated using an objective analysis of about 250 000 temperature profiles (mainly XBT) collected during the 1979–96 time period. Mean conditions and seasonal variability are briefly described to set the context, and temperature anomalies are constructed relative to a mean seasonal cycle to focus on the ENSO (El Niño–Southern Oscillation) timescale. Heat content anomalies (0–450 m) built up in the western equatorial basin prior to the 1982–83, 1986–87, and 1997–98 El Niño events but not clearly prior to the 1991–92, 1993, and 1994–95 events, which are thus found “atypical.” Low-frequency migration of temperature anomalies located at the mean thermocline depth is evidenced eastward in the equatorial band, as well as westward along a narrow zonal band located slightly north of the mean position of the intertropical convergence zone (say, 10°–20°N). This indicates that ENSO-related temperature anomalies in the subsurface ocean are present generally in the western equatorial Pacific about 1–2 years before the appearance of temperature anomalies in the eastern equatorial Pacific. Similarly, this indicates that subsurface temperature anomalies tend to be present in the eastern Pacific basin around 14°N about 1–2 years before the appearance of temperature anomalies in the western Pacific basin at the same latitude. The likely mechanisms responsible for these migrations and the possible link between the eastward and westward migration are discussed.

Comparison of rainfall structures between NCEP/NCAR reanalyses and observed data over tropical Africa

A comparison is made between modelled (NCEP/NCAR reanalysis) and observed (CRC and CRU dataset) annual and monthly precipitation over tropical Africa during the period 1958–1997. The split moving-windows dissimilarity analysis (SMWDA) is used to locate abrupt changes in rainfall time series. In the NCEP reanalysis data, we identify a main abrupt shift, which occurs in 1967 and concerns more than 50% of grid points. In the observation, this shift is only found over parts of tropical North Africa. Three other NCEP abrupt shifts (1975, 1983 and 1987) in the reanalysis shown by the SMWDA, each concerning about 20% of tropical Africa, are not identified in the observation. One hypothesis concerning the 1967 marked abrupt shift is a problem of data assimilation in the NCEP/NCAR model which generated artificial shifts in the time series. In view of this result, further comparisons have been restricted to the period 1968–1997. On a continental scale, while the CRC and NCEP mean seasonal rainfall patterns are almost the same, however, some regional features are not well reproduced. Using five regional rainfall indexes, the mean seasonal rainfall cycle is correctly reproduced, but the NCEP reanalysis generally underestimates the amounts during the rainy season. The NCEP reanalysis rainfall is closer to the observation when the region shows a single rainy season. The correlation values between NCEP and CRC interannual rainfall variations over the period 1968–1997 are very low and seldom significant. The NCEP four main structures of rainfall variability as deducted from rotated principal component analysis are not realistic at all and the associated time series are systematically dominated by a marked low-frequency variability not present in the observation. However, the main teleconnections between ENSO and African rainfall variations are fairly well reproduced, with a correct location of the main structures, but with lower correlation values than those found in the observation.

Pour mieux comprendre la radiothérapie

The water-balance components of 16 Soil–Vegetation Atmospheric Transfer (SVAT) schemes were evaluated by comparing predicted and observed streamflow, predicted evapotranspiration and evapotranspiration inferred from an atmospheric moisture budget analysis, and soil moisture storage changes for a seven-year period (1980–1986) using data from the Red–Arkansas River basins of the Southern Great Plains of the USA. The evaluations support the following suggestions: (a) The mean annual runoff of all models follows, at least generally, the strong climatic East–West gradient of precipitation, although most models predict too much runoff in the dry part of the basin. (b) The mean monthly storage change tends to be underestimated, even though all models capture reasonably well the seasonality of the evapotranspiration. (c) The wide range of conceptualizations used for generation of surface and subsurface runoff strongly affect runoff generation on seasonal, and shorter, time scales. Model responses to summer precipitation ranged from almost no summer runoff (one model) to the (more common) situation of persistent overprediction of summer runoff, especially in the driest part of the basin. (d) All models tended to underpredict evapotranspiration in summer and overpredict in winter. (e) Model-derived mean seasonal cycles of changes in soil moisture storage are qualitatively similar to those inferred from observations, but most models do not predict the decrease in April soil moisture storage and the increase in October that is inferred from observations.

Measurements of the carbon and hydrogen isotopes of atmospheric methane at Izaña, Tenerife: Seasonal cycles and synoptic‐scale variations

Atmospheric CH4 and its stable isotope ratios 13C/12C and D/H have been investigated at the Global Atmospheric Watch station Izaña, Tenerife (28°N, 16°W, 2370 m above sea level), since late 1996. Every fortnight both spot samples and integral samples were taken, the latter collected continuously over 2‐week periods. While spot samples show considerable synoptic‐scale variability, the continuous samples clearly define seasonal cycles of CH4, δ13C, and δD, with peak‐to‐peak amplitudes of 30 ppb, 0.2‰ and 3.5‰, respectively. The measurement of the δD seasonality is the first ever reported for atmospheric background CH4 and has been made possible by the development of a tunable diode laser based optical Methane Isotopomer Spectrometer (MISOS). The δD is well in phase with CH4 mixing ratios, and the compact correlation between them allows to derive the average kinetic isotope effect (KIE) of the tropospheric sinks to be 1.23±0.04 (1σ), consistent with recent laboratory measurements of the kinetic isotope effect in the reaction of CH4±OH. In contrast to δD, δ13C is out of phase to CH4 mixing ratios, clearly indicating that δ13C is not only effected by the KIE of the CH4±OH reaction, but also by seasonally varying source mixtures. Considerable short‐term variations are observed in the isotopic composition and mixing ratios of the spot samples. This can be attributed to different origin of air masses arriving at Izaña. Significant CH4 enhancement is observed for air masses originating from the North American continent or Europe, of 12.7±11.7 and 13.4±0.3 ppb, respectively, while air from the African continent or the North Atlantic is depleted (−11.2±2.8 and −7.0 ± 8.2 ppb). Deviations from the mean seasonal CH4 cycle are correlated with significant deviations of δ13C and δD, allowing to estimate the δ13C and δD signatures of major source regions. Furthermore, deviations in CH4 mixing ratio are also clearly correlated with deviations of CO and SF6 mixing ratios. A three‐dimensional inverse model is employed in order to assist with the interpretation of observational data. In general, the model shows excellent agreement with the observed mean seasonal cycles of CH4, δ13C, and δD, including the observed phase behavior.

A model of plankton dynamics for the coastal upwelling system of Monterey Bay, California

A seven-box plankton model for the mixed layer of the open ocean (Fasham et al., 1990, Journal of Marine Research 48, 591–639) was modified and applied to the coastal upwelling ecosystem of Monterey Bay, California. Upwelling velocities, photosynthetically active radiation and changes in mixed-layer depth drove the model. The observed long-term mean (1989–1995) seasonal cycles of nitrate concentration, primary production and phytoplankton concentration within the mixed layer were reproduced. Bacteria and zooplankton biomass, and ammonium concentration were within the same order of magnitude as observations. Modeled dissolved organic nitrogen (labile) values were an order of magnitude lower than observations (total). New production estimates were high, leading to an annual f-ratio of 0.84, typical of productive coastal upwelling systems. Export production was much lower, and horizontal export by upwelling-induced advection was the dominant loss term for phytoplankton primary production. It exceeded sinking and grazing and the flux of zooplankton-derived detritus throughout most of the year. The model suggests that a large proportion of the daily phytoplankton growth is ungrazed in the mixed layer, and is advected downstream and offshore of the production site.

Col du Dôme (Mt Blanc Massif, French Alps) suitability for ice-core studies in relation with past atmospheric chemistry over Europe

The site of Col du Dôme glacier, located at 4250 m a.s.l. nearby the Mont Blanc summit (French Alps), was investigated for its suitability for reconstructing the anthropogenic perturbation of the atmosphere chemistry over Europe via glacio-chemical ice core studies. For this purpose, a 126 m long ice core drilled close to bedrock has been dedicated for glacio-chemical studies. Major ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl-,>NO3-, and SO42-) and D/H isotope ratios have been measured with high seasonal resolution along the upper 60 m of this core (covering 13 years). For dating by annual layer counting, a highly resolved ammonium profile was obtained for the entire core. To assess the spatial representativity of the chemical signals obtained from this core, additional chemical profiles were obtained from two shallow firn cores (13 and 20 m long) drilled at about 100 m from the deep ice core. All ice core parameters show regular seasonal variations with low winter and high summer values. Mean summer to winter ratios (averaged over the period 1981 to 1994) are close to 4 for NO3-, SO42- and Ca2+, but reach 14 for NH4+. While the shape of the mean seasonal cycles of NH4+, NO3-, and SO42- show a flat winter minimum followed by persistently high summer concentrations, more flickered variations are observed in the case of Ca2+. In contrast to the chemical species, δD shows a smoothed seasonal cycle. The chemical impurity levels and the δD content in snow deposits from Col du Dôme have been compared with those from Colle Gnifetti (4450 m a.s.l., located in the Swiss Alps, 80 km east of Col du Dôme) over a time period of 10 years. This comparison suggests that the two sites may experience similar atmospheric pollution conditions throughout the whole year, at least for NH4+, NO3-, and SO42-. Precise dating of the ice core drilled in 1994 was achieved by annual layer counting using the NH4+ stratigraphy. The latter reveals that the glacier of Col du Dôme records well preserved snow deposits, arising from summer as well as from winter precipitation, over, at least, the last 75 years. However, the seasonal signal of the δD content appears to be disturbed at increasing depth, in particular below of 115 m. A systematic decrease in the ratio of the winter to summer net snow accumulation found with increasing depth is shown to exert an important influence on mean temporal ice core trends for parameters underlying a strong seasonal variation. The comparison of chemical impurities between Col du Dôme and Colle Gnifetti indicates that glacio-chemical ice core records from Col du Dôme will provide seasonally resolved records over the 20th century which are at least representative on a regional scale.

Estimates of vertical heat flux and stratification from Southern Georges Bank, interannual variability, 1985–1995

Interannual variability (IAV) of net vertical heat flux ( Q), potential energy anomaly or “stratification index” ( V) and date of onset for water column stratification were studied using five-day averaged model output over an 11 yr (1985–1995) period for three depth zones which span water depths from less than 60 to 100 m on southern Georges Bank. Comparison between five-day averaged model Q estimates from this study and Q measurements from US-GLOBEC mooring ST1 during winter–summer 1995 shows that nearly all of the difference between uncalibrated model estimates and calibrated ST1 measurements is attributable to the difference between the sum of the latent, sensible and long-wave components ( Q e+ Q s+ Q lw) from each data set. Furthermore, there is virtually no IAV in residual five-day averaged Q e+ Q s+ Q lw values relative to the long-term (1985–1995) mean seasonal cycle, suggesting strongly that the calibration of model Q estimates using measurements from mooring ST1 during 1995 may be applied to other years (1985–1994). Calibrated long-term (1985–1995) monthly mean Q values from this study show a similar seasonal cycle to climatological values determined from ship-of-opportunity data. However, values from this study are higher relative to the ship-of-opportunity long-term monthly mean climatology, with largest differences occurring during winter. IAV of monthly mean Q for zones 1–3 on southern Georges Bank is significant with the magnitude of residuals increasing from zones 1 to 3. Long-term (1985–1995) monthly mean model-derived V computed for zones 1–3 shows zone 1 (<60 m depth) remaining well-mixed throughout the year with no stratification development and therefore no detectable IAV for V. However, within zone 2 (60–80 m depth) and zone 3 (80–100 m depth) significant permanent stratification begins to develop during May and April, respectively, increasing almost linearly from May–July and reaching a maximum during July and August, respectively. IAV of V is large for zones 2 and 3 with typical standard deviations of 1000–5000 J m −2 occurring from late-spring-summer with the largest coefficient of variation during spring and fall. Results from linear regression for zones 2–3 show that IAV of V is controlled largely by IAV of Q, explaining up to ∼80% of the variance, with IAV of wind mixing being of secondary importance. Results from model-derived five-day averaged V show that temporary or “transient” stratification nearly always occurs in early-May and mid-April for zones 2 and 3, respectively, over the 11 yr study period and is associated with reduced tidal stirring during periods of neap tide. The mean date of first transient stratification for zone 3 coincides with the historical maximum abundance of early-stage (<6 mm) cod larvae and copepod nauplii for Georges Bank. In addition, the historical maximum abundance of haddock larvae occurs on Georges Bank at the same time that permanent or “seasonal” stratification is established in zone 3.

Microphysics of Clouds with the Relaxed Arakawa–Schubert Scheme (McRAS). Part II: Implementation and Performance in GEOS II GCM

Abstract A prognostic cloud scheme named the Microphysics of Clouds with the Relaxed Arakawa–Schubert Scheme (McRAS) and the Simple Biosphere Model have been implemented in a version of the Goddard Earth Observing System (GEOS) II GCM at a 4° latitude × 5° longitude × 20 sigma-layer resolution. The McRAS GCM was integrated for 50 months. The integration was initialized with the European Centre for Medium-Range Weather Forecasts analysis of observations for 1 January 1987 and was forced with the observed sea surface temperatures and sea-ice distribution; on land, the permanent ice and vegetation properties (biomes and soils) were climatological, while the soil moisture and snow cover were prognostic. The simulation shows that the McRAS GCM yields realistic structures of in-cloud water and ice, and cloud-radiative forcing (CRF) even though the cloudiness has some discernible systematic errors. The simulated intertropical convergence zone (ITCZ) has a realistic time mean structure and seasonal cycle. The sim...

The Surface-Layer Heat Balance in the Equatorial Pacific Ocean. Part I: Mean Seasonal Cycle*

Abstract The surface-layer heat balance in the equatorial Pacific is examined in order to determine the processes responsible for the mean seasonal cycle of sea surface temperature (SST). Principal datasets include multiyear time series of surface winds, upper-ocean temperature, and velocity obtained from Tropical Atmosphere Ocean (TAO) buoy array at four locations along the equator in the western (165°E), central (170°W), and eastern (140° and 110°W) Pacific. A blended satellite–in situ SST product and climatological surface heat fluxes based on the Comprehensive Ocean–Atmosphere Data Set are also used. Changes in heat storage, horizontal heat advection, and heat fluxes at the surface are estimated directly from data; vertical fluxes of heat out of the base of the mixed layer are calculated as a residual. Results indicate that, of the terms that can be directly estimated, the net surface heat flux is generally the largest term in heat balance. Zonal heat advection is important at all locations and is gen...

On the predictability of the interannual behaviour of the Madden‐Julian oscillation and its relationship with el Nin̄o

AbstractThe Madden‐Julian Oscillation (MJO) is the dominant mode of tropical variability at intraseasonal time‐scales. It displays substantial interannual variability in intensity which may have important implications for the predictability of the coupled system. the reasons for this interannual variability are not understood. the aim of this paper is to investigate whether the interannual behaviour of the MJO is related to tropical sea surface temperature (SST) anomalies, particularly El Niño, and hence whether it is predictable.The interannual behaviour of the MJO has been diagnosed initially in the 40‐year National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis. the results suggest that prior to the mid‐1970s the activity of the MJO was consistently lower than during the latter part of the record. This may be related either to inadequacies in the data coverage, particularly over the tropical Indian Ocean prior to the introduction of satellite observations. or to the real effects of a decadal time‐scale warming in the tropical SSTs. the teleconnection patterns between interannual variations in MJO activity and SST show only a weak, barely significant, influence of El Niño in which the MJO is more active during the cold phase.As well as the NCEP/NCAR Reanalysis, a 4‐member ensemble of 45‐year integrations with the Hadley Centre climate model (HadAM2a), forced by observed SSTs for 1949‐93, has been used to investigate the relationship between MJO activity and SST. HadAM2a is known to give a reasonable simulation of the MJO, and the extended record provided by this ensemble of integrations allows a more robust investigation of the predictability of MJO activity than was possible with the 40‐year NCEP/NCAR Reanalysis. the results have shown that, for the uncoupled system, with the atmosphere being driven by imposed SSTs, there is no reproducibility of the activity of the MJO from year‐to‐year. the interannual behaviour of the MJO is not controlled by the phase of El Niño and would appear to be mainly chaotic in character. However, the model results have confirmed the low‐frequency, decadal time‐scale variability of MJO activity seen in the NCEP/NCAR Reanalysis. the activity of the MJO is consistently lower in all realizations prior to the mid‐1970s, suggesting that the MJO may become more active as tropical SSTs increase. This result may have implications for the effects of global warming on the coupled tropical atmosphere‐ocean system.Since the observed and simulated MJOs display clear seasonality in their occurrence, the relationship with interannual changes in the mean seasonal cycle of the tropical circulation has also been investigated. In contrast to the MJO, the interannual variability in the mean seasonal cycle is reproducible and influenced by the phase of El Niño. the implications of these results for the predictability of the tropical atmosphere‐ocean system are discussed, particularly with reference to the strong El Niño event of 1997 which developed in association with a period of intense MJO activity.

The Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) phase 2(c) Red–Arkansas River basin experiment:: 3. Spatial and temporal analysis of water fluxes

The water-balance components of 16 Soil–Vegetation Atmospheric Transfer (SVAT) schemes were evaluated by comparing predicted and observed streamflow, predicted evapotranspiration and evapotranspiration inferred from an atmospheric moisture budget analysis, and soil moisture storage changes for a seven-year period (1980–1986) using data from the Red–Arkansas River basins of the Southern Great Plains of the USA. The evaluations support the following suggestions: (a) The mean annual runoff of all models follows, at least generally, the strong climatic East–West gradient of precipitation, although most models predict too much runoff in the dry part of the basin. (b) The mean monthly storage change tends to be underestimated, even though all models capture reasonably well the seasonality of the evapotranspiration. (c) The wide range of conceptualizations used for generation of surface and subsurface runoff strongly affect runoff generation on seasonal, and shorter, time scales. Model responses to summer precipitation ranged from almost no summer runoff (one model) to the (more common) situation of persistent overprediction of summer runoff, especially in the driest part of the basin. (d) All models tended to underpredict evapotranspiration in summer and overpredict in winter. (e) Model-derived mean seasonal cycles of changes in soil moisture storage are qualitatively similar to those inferred from observations, but most models do not predict the decrease in April soil moisture storage and the increase in October that is inferred from observations.

Surface Air Temperature Simulations by AMIP General Circulation Models: Volcanic and ENSO Signals and Systematic Errors

Thirty surface air temperature simulations for 1979–88 by 29 atmospheric general circulation models are analyzed and compared with the observations over land. These models were run as part of the Atmospheric Model Intercomparison Project (AMIP). Several simulations showed serious systematic errors, up to 4°–5°C, in globally averaged land air temperature. The 16 best simulations gave rather realistic reproductions of the mean climate and seasonal cycle of global land air temperature, with an average error of −0.9°C for the 10-yr period. The general coldness of the model simulations is consistent with previous intercomparison studies. The regional systematic errors showed very large cold biases in areas with topography and permanent ice, which implies a common deficiency in the representation of snow-ice albedo in the diverse models. The SST and sea ice specification of climatology rather than observations at high latitudes for the first three years (1979–81) caused a noticeable drift in the neighboring land air temperature simulations, compared to the rest of the years (1982–88). Unsuccessful simulation of the extreme warm (1981) and cold (1984–85) periods implies that some variations are chaotic or unpredictable, produced by internal atmospheric dynamics and not forced by global SST patterns. Among the 16 best simulations, 8 reproduced the dominant El Niño–Southern Oscillation (ENSO) mode in the 10-yr period, which includes the 1982–83 and 1986–87 warm episodes and the 1988 cold episode. On the average, the ENSO mode explains about 30% of the total variance in surface air temperature fluctuation and has a 2-month lag from the Southern Oscillation index. In this mode, North America displays a Pacific–North American–like anomaly pattern, but Eurasia gave little response to warm SSTs in the eastern equatorial Pacific, in good agreement with results based on historical data. The special design of the AMIP experiment provides a unique opportunity to estimate the effects of the El Chichón volcanic eruption in spring 1982, which was not included in the model forcing. Comparison of the simulations with data delineated a visible global cooling in the first months following the El Chichón eruption, in addition to the cooling from the volcanic eruption of Nyamuragira in December 1981, due to the reduction of incoming solar radiation by volcanic aerosols. However, the mean climate shift in the AMIP experiment due to the forcing data discontinuity at the end of 1981 made the quantitative estimate of El Chichón global cooling influence impossible. The contrast between the simulated ENSO signal and observations shows that the major warming over the northern continents during the 1982/83 winter (DJF) is not an ENSO-like signal. Instead it is most likely a pattern resulting from the enhanced polar vortex produced by a larger pole-to-equator temperature gradient. This gradient was due to the larger absorption of radiation in low latitudes by the El Chichón volcanic sulfate aerosols in the stratosphere. These results suggest that during the Northern Hemisphere wintertime, the stratospheric polar vortex has substantial influence on surface air temperature fluctuations through its effects on vertically propagating planetary waves of the troposphere, and imply that current GCMs are deficient in simulation of stratospheric processes and their coupling with the troposphere.

Atmospheric CO2 simulated by the National Center for Atmospheric Research Community Climate Model: 1. Mean fields and seasonal cycles

Atmospheric CO2 has been simulated on‐line in a coupled model of the atmosphere and terrestrial land surface. The atmospheric model is a precursor to version 3 of the Community Climate Model developed at the National Center for Atmospheric Research. It interacts with a mechanistic parameterization of the land surface at each time step. Photosynthesis and respiration fluxes are calculated at each time step and are linked to the tracer transport scheme that advects CO2 through the model atmosphere. For the first time in atmospheric CO2 modeling, these two fluxes are not forced to balance annually at each grid point. The model was run for 10 years with climatological sea surface temperatures, and the results were saved as monthly means. In this paper, an average year is calculated from the output of the 10 year run and then evaluated in terms of mean distributions and seasonal cycles. The model reproduces many of the observed features of the atmospheric CO2 distribution although there are discrepancies in some areas. The seasonality of tropical carbon exchange is exaggerated which leads to unrealistic CO2 fields near the equator. Carbon uptake is too strong at high northern latitudes in the boreal summer which results in a spurious annual carbon sink and exaggerated seasonal CO2 cycle amplitudes at these latitudes. The annual sink strongly affects the simulated interhemispheric CO2 gradient. Errors in the temperature and precipitation simulation cause anomalies in the CO2 field in specific regions such as the United States and India. Possible model improvements are discussed.