Showing papers in "Journal of Climate in 2000"

Annular Modes in the Extratropical Circulation. Part I: Month-to-Month Variability*

Abstract: The leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep, zonally symmetric or ‘‘annular’’ structures, with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude. The structure and dynamics of the Southern Hemisphere (SH) annular mode have been extensively documented, whereas the existence of a Northern Hemisphere (NH) mode, herein referred to as the Arctic Oscillation (AO), has only recently been recognized. Like the SH mode, the AO can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields. In this paper the structure and seasonality of the NH and SH modes are compared based on data from the National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis and supplementary datasets. The structures of the NH and SH annular modes are shown to be remarkably similar, not only in the zonally averaged geopotential height and zonal wind fields, but in the mean meridional circulations as well. Both exist year-round in the troposphere, but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave‐mean flow interaction: midwinter in the NH and late spring in the SH. During these ‘‘active seasons,’’ the annular modes modulate the strength of the Lagrangian mean circulation in the lower stratosphere, total column ozone and tropopause height over mid- and high latitudes, and the strength of the trade winds of their respective hemispheres. The NH mode also contains an embedded planetary wave signature with expressions in surface air temperature, precipitation, total column ozone, and tropopause height. It is argued that the horizontal temperature advection by the perturbed zonal-mean zonal wind field in the lower troposphere is instrumental in forcing this pattern. A companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades.

Pacific–East Asian Teleconnection: How Does ENSO Affect East Asian Climate?

Abstract: Observational evidence is presented to show a teleconnection between the central Pacific and East Asia during the extreme phases of ENSO cycles. This Pacific‐East Asian teleconnection is confined to the lower troposphere. The key system that bridges the warm (cold) events in the eastern Pacific and the weak (strong) East Asian winter monsoons is an anomalous lower-tropospheric anticyclone (cyclone) located in the western North Pacific. The western North Pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures. The anomalies persist until the following spring or early summer, causing anomalously wet (dry) conditions along the East Asian polar front stretching from southern China northeastward to the east of Japan (Kuroshio extension). Using atmospheric general circulation and intermediate models, the authors show that the anomalous Philippine Sea anticyclone results from a Rossby-wave response to suppressed convective heating, which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central Pacific warming. The development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling. Both the anticyclone and the cooling region propagate slowly eastward. The development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades. The rapid establishment of the Philippine Sea wind and SST anomalies implies the occurrence of extratropical‐tropical interactions through cold surge‐induced exchanges of surface buoyancy flux. The central Pacific warming plays an essential role in the development of the western Pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone‐ SST interaction and midlatitude‐tropical interaction in the western North Pacific.

Representing Twentieth-Century Space-Time Climate Variability. Part II: Development of 1901-96 Monthly Grids of Terrestrial Surface Climate

Abstract: The authors describe the construction of a 0.58 lat‐long gridded dataset of monthly terrestrial surface climate for the period of 1901‐96. The dataset comprises a suite of seven climate elements: precipitation, mean temperature, diurnal temperature range, wet-day frequency, vapor pressure, cloud cover, and ground frost frequency. The spatial coverage extends over all land areas, including oceanic islands but excluding Antarctica. Fields of monthly climate anomalies, relative to the 1961‐90 mean, were interpolated from surface climate data. The anomaly grids were then combined with a 1961‐90 mean monthly climatology (described in Part I) to arrive at grids of monthly climate over the 96-yr period. The primary variables—precipitation, mean temperature, and diurnal temperature range—were interpolated directly from station observations. The resulting time series are compared with other coarser-resolution datasets of similar temporal extent. The remaining climatic elements, termed secondary variables,were interpolated from merged datasets comprising station observations and, in regions where there were no station data, synthetic data estimated using predictive relationships with the primary variables. These predictive relationships are described and evaluated. It is argued that this new dataset represents an advance over other products because (i) it has higher spatial resolution than other datasets of similar temporal extent, (ii) it has longer temporal coverage than other products of similar spatial resolution, (iii) it encompasses a more extensive suite of surface climate variables than available elsewhere, and (iv) the construction method ensures that strict temporal fidelity is maintained. The dataset should be of particular relevance to a number of applications in applied climatology, including large-scale biogeochemical and hydrological modeling, climate change scenario construction, evaluation of regional climate models, and comparison with satellite products. The dataset is available from the Climatic Research Unit and is currently being updated to 1998.

Annular Modes in the Extratropical Circulation. Part II: Trends

Abstract: The authors exploit the remarkable similarity between recent climate trends and the structure of the ‘‘annular modes’’ in the month-to-month variability (as described in a companion paper) to partition the trends into components linearly congruent with and linearly independent of the annular modes. The index of the Northern Hemisphere (NH) annular mode, referred to as the Arctic Oscillation (AO), has exhibited a trend toward the high index polarity over the past few decades. The largest and most significant trends are observed during the ‘‘active season’’ for stratospheric planetary wave‐mean flow interaction, January‐ March (JFM), when fluctuations in the AO amplify with height into the lower stratosphere. For the periods of record considered, virtually all of the JFM geopotential height falls over the polar cap region and the strengthening of the subpolar westerlies from the surface to the lower stratosphere, ;50% of the JFM warming over the Eurasian continent, ;30% of the JFM warming over the NH as a whole, ;40% of the JFM stratospheric cooling over the polar cap region, and ;40% of the March total column ozone losses poleward of 408N are linearly congruent with month-to-month variations in the AO index. Summertime sea level pressure falls over the Arctic basin are suggestive of a year-round drift toward the positive polarity of the AO, but the evidence is less conclusive. Owing to the photochemical memory inherent in the ozone distribution, roughly half the ozone depletion during the NH summer months is linearly dependent on AO-related ozone losses incurred during the previous active season. Lower-tropospheric geopotential height falls over the Antarctic polar cap region are indicative of a drift toward the high index polarity of the Southern Hemisphere (SH) annular mode with no apparent seasonality. In contrast, the trend toward a cooling and strengthening of the SH stratospheric polar vortex peaks sharply during the stratosphere’s relatively short active season centered in November. The most pronounced SH ozone losses have occurred in September‐October, one or two months prior to this active season. In both hemispheres, positive feedbacks involving ozone destruction, cooling, and a weakening of the wave-driven meridional circulation may be contributing to a delayed breakdown of the polar vortex and enhanced ozone losses during spring.

Interannual and Interdecadal Variations of the East Asian Summer Monsoon and Tropical Pacific SSTs. Part I: Roles of the Subtropical Ridge

Abstract: The interannual relationship between the East Asian summer monsoon and the tropical Pacific SSTs is studied using rainfall data in the Yangtze River Valley and the NCEP reanalysis for 1951‐96. The datasets are also partitioned into two periods, 1951‐77 and 1978‐96, to study the interdecadal variations of this relationship. A wet summer monsoon is preceded by a warm equatorial eastern Pacific in the previous winter and followed by a cold equatorial eastern Pacific in the following fall. This relationship involves primarily the rainfall during the pre-Mei-yu/Mei-yu season (May‐June) but not the post-Mei-yu season (July‐August). In a wet monsoon year, the western North Pacific subtropical ridge is stronger as a result of positive feedback that involves the anomalous Hadley and Walker circulations, an atmospheric Rossby wave response to the western Pacific complementary cooling, and the evaporation‐wind feedback. This ridge extends farther to the west from the previous winter to the following fall, resulting in an 850-hPa anomalous anticyclone near the southeast coast of China. This anticyclone 1) blocks the pre-Mei-yu and Mei-yu fronts from moving southward thereby extending the time that the fronts produce stationary rainfall; 2) enhances the pressure gradient to its northwest resulting in a more intense front; and 3) induces anomalous warming of the South China Sea surface through increased downwelling, which leads to a higher moisture supply to the rain area. A positive feedback from the strong monsoon rainfall also appears to occur, leading to an intensified anomalous anticyclone near the monsoon region. This SST‐subtropical ridge‐monsoon rainfall relationship is observed in both the interannual timescale within each interdecadal period and in the interdecadal scale. The SST anomalies (SSTAs) change sign in northern spring and resemble a tropospheric biennial oscillation (TBO) pattern during the first interdecadal period (1951‐77). In the second interdecadal period (1978‐96) the sign change occurs in northern fall and the TBO pattern in the equatorial eastern Pacific SST is replaced by longer timescales. This interdecadal variation of the monsoon‐SST relationship results from the interdecadal change of the background state of the coupled ocean‐atmosphere system. This difference gives rise to the different degrees of importance of the feedback from the anomalous circulations near the monsoon region to the equatorial eastern Pacific. In a wet monsoon year, the anomalous easterly winds south of the monsoon-enhanced anomalous anticyclone start to propagate slowly eastward toward the eastern Pacific in May and June, apparently as a result of an atmosphere‐ocean coupled wave motion. These anomalous easterlies carry with them a cooling effect on the ocean surface. In 1951‐77 this effect is insignificant as the equatorial eastern Pacific SSTAs, already change from warm to cold in northern spring, probably as a result of negative feedback processes discussed in ENSO mechanisms. In 1978‐96 the equatorial eastern Pacific has a warmer mean SST. A stronger positive feedback between SSTA and the Walker circulation during a warm phase tends to keep the SSTA warm until northern fall, when the eastward-propagating anomalous easterly winds reach the eastern Pacific and reverse the SSTA.

Multimodel Ensemble Forecasts for Weather and Seasonal Climate

Abstract: In this paper the performance of a multimodel ensemble forecast analysis that shows superior forecast skills is illustrated and compared to all individual models used. The model comparisons include global weather, hurricane track and intensity forecasts, and seasonal climate simulations. The performance improvements are completely attributed to the collective information of all models used in the statistical algorithm. The proposed concept is first illustrated for a low-order spectral model from which the multimodels and a “nature run” were constructed. Two hundred time units are divided into a training period (70 time units) and a forecast period (130 time units). The multimodel forecasts and the observed fields (the nature run) during the training period are subjected to a simple linear multiple regression to derive the statistical weights for the member models. The multimodel forecasts, generated for the next 130 forecast units, outperform all the individual models. This procedure was deployed...

Climate Variability in Southern South America Associated with El Niño and La Niña Events.

Abstract: A comprehensive view is given of the precipitation and circulation anomalies associated with the various stages of El Nino (EN) and La Nina (LN) events all over southern South America (SSA). This view comprises the delineation of coherent regions with respect to precipitation anomalies, the identification of the seasons of maximum anomalies, the indication of their magnitude, and the assessment of their consistency during those events. In addition, the spatial and temporal variability of these anomalies is detailed by calculating the expected precipitation percentiles and the consistency of wet and dry anomalies for each station and each three-month running season during EN and LN events. Composites of circulation anomalies and an assessment of their consistency are also presented and their connection with the precipitation anomalies is discussed. Southern Brazil presents the strongest average signal in EN events. The general behavior toward opposite signals in the precipitation and circulation anomalies over SSA during almost the same periods of the EN and LN events indicates a large degree of linearity in the response to these events. The timing of the anomalies changes throughout SSA, leading to the identification of eight different coherent regions in the EN case and six in the LN case. This regionalization is mostly caused by different processes leading to precipitation anomalies in SSA during those events. All these regions show a significant response in some part of each event. The magnitude and consistency of this response show a large spatial variability and some areas present very strong and consistent anomalies sometimes not disclosed when large coherent regions are analyzed. In spite of the differences in timing, some features of the precipitation anomalies are rather uniform throughout the region during EN and LN events. In EN episode, there is a tendency to lower than median precipitation in the year before the event, which continues until March of the year of the event. In a vast region, east of the Andes, the strongest positive precipitation anomalies occur in spring of this year, when the circulation anomalies concur to enhance rainfall over several regions. During the summer of the mature stage the positive precipitation anomalies almost disappear and then reappear in some regions in late summer-early autumn and in winter of the year following the starting year of the event. This description holds partially for the LN event, but with opposite signs, although there is a larger spatial variability in the LN-related anomalies in the following year and some shifts in timing. As for precipitation, the symmetry of the geopotential height anomaly fields with opposite signs between LN and EN cases is also remarkable, especially during the year (0).

The Arctic Ocean Response to the North Atlantic Oscillation

Abstract: The climatically sensitive zone of the Arctic Ocean lies squarely within the domain of the North Atlantic oscillation (NAO), one of the most robust recurrent modes of atmospheric behavior. However, the specific response of the Arctic to annual and longer-period changes in the NAO is not well understood. Here that response is investigated using a wide range of datasets, but concentrating on the winter season when the forcing is maximal and on the postwar period, which includes the most comprehensive instrumental record. This period also contains the largest recorded low-frequency change in NAO activity—from its most persistent and extreme low index phase in the 1960s to its most persistent and extreme high index phase in the late 1980s/early 1990s. This longperiod shift between contrasting NAO extrema was accompanied, among other changes, by an intensifying storm track through the Nordic Seas, a radical increase in the atmospheric moisture flux convergence and winter precipitation in this sector, an increase in the amount and temperature of the Atlantic water inflow to the Arctic Ocean via both inflow branches (Barents Sea Throughflow and West Spitsbergen Current), a decrease in the late-winter extent of sea ice throughout the European subarctic, and (temporarily at least) an increase in the annual volume flux of ice from the Fram Strait.

Arctic Sea Ice Variability in the Context of Recent Atmospheric Circulation Trends

Abstract: Forty years (1958–97) of reanalysis products and corresponding sea ice concentration data are used to document Arctic sea ice variability and its association with surface air temperature (SAT) and sea level pressure (SLP) throughout the Northern Hemisphere extratropics. The dominant mode of winter (January–March) sea ice variability exhibits out-of-phase fluctuations between the western and eastern North Atlantic, together with a weaker dipole in the North Pacific. The time series of this mode has a high winter-to-winter autocorrelation (0.69) and is dominated by decadal-scale variations and a longer-term trend of diminishing ice cover east of Greenland and increasing ice cover west of Greenland. Associated with the dominant pattern of winter sea ice variability are large-scale changes in SAT and SLP that closely resemble the North Atlantic oscillation. The associated SAT and surface sensible and latent heat flux anomalies are largest over the portions of the marginal sea ice zone in which the tr...

Observations of Warm Water Volume Changes in the Equatorial Pacific and Their Relationship to El Niño and La Niña

Abstract: This paper describes observed changes in surface winds, sea surface temperature (SST), and the volume of water warmer than 20°C (WWV) in the equatorial Pacific Ocean for the period 1980–99. The purpose is to test recent hypotheses about the relationship between variations in WWV and the El Nino–Southern Oscillation (ENSO) cycle. The results confirm inferences based on theory, models, and previous empirical analyses using proxy data (namely sea level) that ENSO involves a recharge and discharge of WWV along the equator and that the cyclic nature of ENSO results from a disequilibrium between zonal winds and zonal mean thermocline depth. The authors also find that the magnitude of ENSO SST anomalies is directly related to the magnitude of zonal mean WWV anomalies. Furthermore, for a given change in equatorial WWV, the corresponding warm El Nino SST anomalies are larger than the corresponding cold La Nina anomalies. This asymmetry between the warm and cold phases of the ENSO cycle implies differences...

Northern Hemisphere Snow Cover Variability and Change, 1915-97.

Abstract: Historical and reconstructed snow cover data from stations in Canada, the United States, the former Soviet Union, and the People’s Republic of China were used to reconstruct monthly snow cover extent (SCE) fluctuations over midlatitudinal (∼40°–60°N) regions of North America (NA) and Eurasia back to the early 1900s using an areal snow index approach. The station distribution over NA allowed SCE to be reconstructed back to 1915 for 6 months (November–April), along with estimates of monthly mean snow water equivalent (SWE) from gridded daily snow depth data. Over Eurasia, SCE was able to be reconstructed back to 1922, but major gaps in the station network limited the approach to 3 months (October, March, and April). The reconstruction provided evidence of a general twentieth century increase in NA SCE, with significant increases in winter (December–February) SWE averaging 3.9% per decade. The results are consistent with an observed increasing trend in winter snow depth over Russia and provide furth...

Interannual Variability of Caribbean Rainfall, ENSO, and the Atlantic Ocean

Abstract: The large-scale ocean‐atmosphere patterns that influence the interannual variability of Caribbean‐Central American rainfall are examined. The atmospheric circulation over this region is shaped by the competition between the North Atlantic subtropical high sea level pressure system and the eastern Pacific ITCZ, which influence the convergence patterns on seasonal and interannual timescales. The authors find the leading modes of interannual sea level pressure (SLP) and SST variability associated with Caribbean rainfall, as selected by canonical correlation analysis, to be an interbasin mode, linking the eastern Pacific with the tropical Atlantic, and an Atlantic mode. North Atlantic SLP affects Caribbean rainfall directly, by changing the patterns of surface flow over the region, and indirectly, through SST anomalies. Anomalously high SLP in the region of the North Atlantic high translates into stronger trade winds, hence cooler SSTs, and less Caribbean rainfall. The interbasin mode, which manifests itself as a zonal seesaw in SLP between the tropical Atlantic and the eastern equatorial Pacific, is correlated with ENSO. When SLP is low in the eastern equatorial Pacific, it is high in the tropical Atlantic: the surface atmospheric flow over the basin is divergent, to the west, toward the eastern Pacific ITCZ, and to the east, toward the tropical North Atlantic. A weakened meridional SLP gradient in the tropical North Atlantic signifies weaker trade winds and the opportunity for SSTs to warm up, reaching peak intensity 2‐4 months after the mature phase of an ENSO event. This SST anomaly is particularly evident in the Caribbean‐western Atlantic basin. The tendency is for drier-than-average conditions when the divergent atmospheric flow dominates, during the rainy season preceding the mature phase of a warm ENSO event. The dry season that coincides with the mature phase of ENSO is wetter than average over the northwestern section of the basin, that is, Yucatan, the Caribbean coast of Honduras, and Cuba, and drier than average over the rest of the basin, that is, Costa Rica and northern South America. The following spring, as the atmospheric circulation transitions to normal conditions, the positive SST anomaly that has built up in the basin takes over, favoring convection. The positive precipitation anomaly spreads southeastward, from the northwest to the entire basin. At the start of a new rainy season, it is especially strong over the Greater Antilles.

The Global Monsoon as Seen through the Divergent Atmospheric Circulation

Abstract: A comprehensive description is given of the global monsoon as seen through the large-scale overturning in the atmosphere that changes with the seasons, and it provides a basis for delimiting the monsoon regions of the world. The analysis focuses on the mean annual cycle of the divergent winds and associated vertical motions, as given by the monthly mean fields for 1979–93 reanalyses from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) and European Centre for Medium-Range Weather Forecasts (ECMWF), which are able to reproduce the dominant modes. A complex empirical orthogonal function analysis of the divergent circulation brings out two dominant modes with essentially the same vertical structures in all months of the year. The first mode, which depicts the global monsoon, has a simple vertical structure with a maximum in vertical motion at about 400 mb, divergence in the upper troposphere that is strongest at 150 mb and decays to zero amplitud...

Modulation of Eastern North Pacific Hurricanes by the Madden–Julian Oscillation

Abstract: Hurricane and tropical storm statistics verify the modulation of eastern Pacific tropical systems by the Madden–Julian oscillation (MJO) as hypothesized by Maloney and Hartmann. Over twice as many named tropical systems (hurricanes and tropical storms) accompany equatorial 850-mb westerly anomalies than accompany equatorial easterly anomalies, and the systems that do exist are stronger. Hurricanes are over four times more numerous during westerly phases of the MJO than during easterly phases. The current study constructs a composite life cycle of the MJO during May–November 1979–95 using an index based on the 850-mb equatorial zonal wind. Equatorial Kelvin waves propagating eastward from convective regions of the western Pacific Ocean alter dynamical conditions over the eastern Pacific Ocean. Westerly (easterly) equatorial 850-mb wind anomalies are accompanied by enhanced (suppressed) convection over the eastern Pacific hurricane region. Convection locally amplifies the wind anomalies over the ea...

Changes in the Extremes in an Ensemble of Transient Climate Simulations with a Coupled Atmosphere–Ocean GCM

Abstract: The extremes of surface temperature, precipitation, and wind speed and their changes under projected changes in radiative forcing are examined in an ensemble of three transient climate change simulations for the years 1900–2100 conducted with the global coupled model of the Canadian Centre for Climate Modelling and Analysis. The evolution of the greenhouse gases and aerosols in these simulations is consistent with the Intergovernmental Panel on Climate Change 1992 scenario A. The extremes are analyzed in three 21-yr time periods centered at years 1985, 2050, and 2090. The model simulates reasonably well the extremes of the contemporary near-surface climate. Changes in extremes of daily maximum and daily minimum temperature are distinctively different and are related to changes in the mean screen temperature, soil moisture, and snow and sea-ice cover. Extreme precipitation increases almost everywhere on the globe. Relative change in extreme precipitation is larger than change in total precipitatio...

Variations in Surface Air Temperature Observations in the Arctic, 1979-97.

Abstract: The statistics of surface air temperature observations obtained from buoys, manned drifting stations, and meteorological land stations in the Arctic during 1979‐97 are analyzed. Although the basic statistics agree with what has been published in various climatologies, the seasonal correlation length scales between the observations are shorter than the annual correlation length scales, especially during summer when the inhomogeneity between the ice-covered ocean and the land is most apparent. During autumn, winter, and spring, the monthly mean correlation length scales are approximately constant at about 1000 km; during summer, the length scales are much shorter, that is, as low as 300 km. These revised scales are particularly important in the optimal interpolation of data on surface air temperature (SAT) and are used in the analysis of an improved SAT dataset called International Arctic Buoy Programme/Polar Exchange at the Sea Surface (IABP/POLES). Compared to observations from land stations and the Russian North Pole drift stations, the IABP/POLES dataset has higher correlations and lower rms errors than previous SAT fields and provides better temperature estimates, especially during summer in the marginal ice zones. In addition, the revised correlation length scales allow data taken at interior land stations to be included in the optimal interpretation analysis without introducing land biases to grid points over the ocean. The new analysis provides 12-h fields of air temperatures on a 100-km rectangular grid for all land and ocean areas of the Arctic region for the years 1979‐97. The IABP/POLES dataset is then used to study spatial and temporal variations in SAT. This dataset shows that on average melt begins in the marginal seas by the first week of June and advances rapidly over the Arctic Ocean, reaching the pole by 19 June, 2 weeks later. Freeze begins at the pole on 16 August, and the freeze isotherm advances more slowly than the melt isotherm. Freeze returns to the marginal seas a month later than at the pole, on 21 September. Near the North Pole, the melt season length is about 58 days, while near the margin, the melt season is about 100 days. A trend of 118C (decade)21 is found during winter in the eastern Arctic Ocean, but a trend of 218C (decade)21 is found in the western Arctic Ocean. During spring, almost the entire Arctic shows significant warming trends. In the eastern Arctic Ocean this warming is as much as 28C (decade)21. The spring warming is associated with a trend toward a lengthening of the melt season in the eastern Arctic. The western Arctic, however, shows a slight shortening of the melt season. These changes in surface air temperature over the Arctic Ocean are related to the Arctic Oscillation, which accounts for more than half of the surface air temperature trends over Alaska, Eurasia, and the eastern Arctic Ocean but less than half in the western Arctic Ocean.

Impact of ENSO on the Variability of the Asian–Australian Monsoons as Simulated in GCM Experiments

Abstract: The influences of El Nino–Southern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 1950–95 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios. The essential characteristics of the model climatology for the Asian–Australian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding...

Indian Monsoon–ENSO Relationship on Interdecadal Timescale

Abstract: Empirical evidence is presented to support a hypothesis that the interdecadal variation of the Indian summer monsoon and that of the tropical SST are parts of a tropical coupled ocean-atmosphere mode. The interdecadal variation of the Indian monsoon rainfall (IMR) is strongly correlated with the interdecadal variations of various indices of El Nino-Southern Oscillation (ENSO). It is also shown that the interannual variances of both IMR and ENSO indices vary in phase and follow a common interdecadal variation. However, the correlation between IMR and eastern Pacific SST or between IMR and Southern Oscillation index (SOI) on the interannual timescale does not follow the interdecadal oscillation. The spatial patterns of SST and sea level pressure (SLP) associated with the interdecadal variation of IMR are nearly identical to those associated with the interdecadal variations of ENSO indices. As has been shown earlier in the case of ENSO, the global patterns associated with the interdecadal and interannual variability of the Indian monsoon are quite similar. The physical link through which ENSO is related to decreased monsoon rainfall on both interannual and interdecadal timescales has been investigated using National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis products. The decrease in the Indian monsoon rainfall associated with the warm phases of ENSO is due to an anomalous regional Hadley circulation with descending motion over the Indian continent and ascending motion near the equator sustained by the ascending phase of the anomalous Walker circulation in the equatorial Indian Ocean. It is shown that, to a large extent, both the regional Hadley circulation anomalies and Walker circulation anomalies over the monsoon region associated with the strong (weak) phases of the interdecadal oscillation are similar to those associated with the strong (weak) phases of the interannual variability. However, within a particular phase of the interdecadal oscillation, there are several strong and weak phases of the interannual variation. During a warm eastern Pacific phase of the interdecadal variation, the regional Hadley circulation associated with El Nino reinforces the prevailing anomalous interdecadal Hadley circulation while that associated with La Nina opposes the prevailing interdecadal Hadley circulation. During the warm phase of the interdecadal oscillation, El Nino events are expected to be strongly related to monsoon droughts while La Nina events may not have significant relation. On the other hand, during the cold eastern Pacific phase of the interdecadal SST oscillation, La Nina events are more likely to be strongly related to monsoon floods while El Nino events are unlikely to have a significant relation with the Indian monsoon. This picture explains the observation that the correlations between IMR and ENSO indices on the interannual timescale do not follow the interdecadal oscillation as neither phase of the interdecadal oscillation favors a stronger (or weaker) correlation between monsoon and ENSO indices.

Tropical Cyclone Activity over the Western North Pacific Associated with El Niño and La Niña Events

Abstract: This paper presents the results of an investigation on the variations of tropical cyclone (TC) activity over the western North Pacific (WNP) associated with both El Nino (EN) and La Nina (LN) events. The study is based on the monthly number of TCs that occurred during the period 1959–97. Anomalies within each 5° lat × 5° long box from the year before (EN−1 and LN−1) to the year after (EN+1 and LN+1) are examined. During an EN−1 year, more (less) TCs are found in September and October over the South China Sea (southeast of Japan). In an EN year, TC activity is below normal during these two months over the South China Sea (SCS) but above normal especially in the late season in the eastern part of the WNP. After the mature phase of the warm event (i.e., during an EN+1 year), TC activity over the entire ocean basin tends to be below normal. No significant anomalies are found during an LN−1 year. However, in an LN year, the SCS tends to have more TCs in September and October, but for the rest of the W...

Intraseasonal and Interannual Variability of Rainfall over India

Abstract: A gridded daily rainfall dataset prepared from observations at 3700 stations is used to analyze the intraseasonal and interannual variability of the summer monsoon rainfall over India. It is found that the major drought years are characterized by large-scale negative rainfall anomalies covering nearly all of India and persisting for the entire monsoon season. The intraseasonal variability of rainfall during a monsoon season is characterized by the occurrence of active and break phases. During the active phase, the rainfall is above normal over central India and below normal over northern India (foothills of the Himalaya) and southern India. This pattern is reversed during the break phase. It is found that the nature of the intraseasonal variability is not different during the years of major droughts or major floods. This suggests that a simple conceptual model to explain the interannual variability of the Indian monsoon rainfall should consist of a linear combination of a large-scale persistent s...

The Southern Oscillation Revisited: Sea Level Pressures, Surface Temperatures, and Precipitation

Abstract: An update is given of the global correlation and regression patterns of sea level pressure associated with the Southern Oscillation, based upon the reanalyses from the National Centers for Environmental Prediction‐National Center for Atmospheric Research for 1958‐98, a period independent of that of early work. Features over the oceans are better defined than was previously possible and most features prove to be robust, although climate changes such as the 1976 climate shift have evidently altered some important relationships, such as those with Southeast Asia. Associated surface temperature patterns are also shown over the same interval and reveal striking

A Census of Precipitation Features in the Tropics Using TRMM: Radar, Ice Scattering, and Lightning Observations

Abstract: An algorithm has been developed to identify precipitation features ($75 km2 in size) in two land and two ocean regions during August, September, and October 1998. It uses data from two instruments on the Tropical Rainfall Measuring Mission (TRMM) satellite: near-surface precipitation radar (PR) reflectivities, and TRMM Microwave Imager (TMI) 85.5-GHz polarization corrected temperatures (PCTs). These features were classified by size and intensity criteria to identify mesoscale convective systems (MCSs), precipitation with PCTs below 250 K, and other features without PCTs below 250 K. By using this technique, several hypotheses about the convective intensity and rainfall distributions of tropical precipitation systems can be evaluated. It was shown that features over land were much more intense than similar oceanic features as measured by their minimum PCTs, maximum heights of the 30-dBZ contour, and 6-km reflectivities. The diurnal cycle of precipitation features showed a strong afternoon maximum over land and a rather flat distribution over the ocean, quite similar to those found by others using infrared satellite techniques. Precipitation features with MCSs over the ocean contained significantly more rain outside the 250-K PCT isotherm than land systems, and in general, a significant portion (10%‐15%) of rainfall in the Tropics falls in systems containing no PCTs less than 250 K. Volumetric rainfall and lightning characteristics (as observed by the Lightning Imaging Sensor aboard TRMM) from the systems were classified by feature intensity; similar rain amounts but highly differing lightning flash rates were found among the regions. Oceanic storms have a bimodal contribution of rainfall from two types of systems: very weak systems with little ice scattering and moderately strong systems that do not produce high lightning flash rates. Continental systems that produce the bulk of the rainfall (as sampled) are likely to have higher lightning flash rates, which are shown to be linked to stronger radar and ice-scattering intensities.

Radiative Effects of Cloud-Type Variations

Abstract: Radiative flux changes induced by the occurrence of different cloud types are investigated using International Satellite Cloud Climatology Project cloud data and a refined radiative transfer model from National Aeronautics and Space Administration/Goddard Institute for Space Studies general circulation model. Cloud types are defined by their top height and optical thickness. Cloud-type variations are shown to be as important as cloud cover in modifying the radiation field of the earth–atmosphere system. Other variables, such as the solar insolation and atmospheric and surface properties, also play significant roles in determining regional cloud radiative effects. The largest “annual” mean (approximated by averaging the results of four particular days, one from each season) changes of the global top-of-atmosphere and surface shortwave radiative fluxes are produced by stratocumulus, altostratus, and cirrostratus clouds (i.e., clouds with moderate optical thicknesses). Cirrus, cirrostratus, and deep...

Interdecadal Change of the Structure of the ENSO Mode and Its Impact on the ENSO Frequency

Abstract: In the late 1970s, the ENSO cycle exhibited frequency change. The oscillation period increased from 2–4 yr (high frequency) during 1962–75 to 4–6 yr (low frequency) during 1980–93. Observations suggest that this frequency change was accompanied by a significant change in the structure of the coupled ENSO mode. In comparison with the high-frequency regime, the structure of the coupled mode in the low-frequency regime shows three distinctive features during the warm phase of ENSO: the eastward shift of the westerly anomalies, the meridional expansion of the westerly anomalies, and the weaker intensity of the easterly anomalies in the eastern Pacific. To test the robustness of the relationship between the oscillation period and the structure of the coupled mode, the authors designed empirical atmospheric models based on observations and coupled them with the ocean model of Zebiak and Cane. Numerical experiments demonstrate that the ENSO period is sensitive to changes in the wind anomaly pattern in a...

The Timescale, Power Spectra, and Climate Noise Properties of Teleconnection Patterns

Abstract: This study uses NCEP–NCAR reanalysis data covering the boreal winters of 1958–97 to examine the power spectral, timescale, and climate noise properties of the dominant atmospheric teleconnection patterns. The patterns examined include the North Atlantic oscillation (NAO), the Pacific–North American (PNA), and west Pacific (WP) teleconnections, and a spatial pattern associated with ENSO. The teleconnection patterns are identified by applying a rotated principal component analysis to the daily unfiltered 300-mb geopotential height field. The NAO and PNA were found to be the two dominant patterns on all timescales. The main finding is that the temporal evolution of the NAO, PNA, and WP teleconnections can be interpreted as being a stochastic (Markov) process with an e-folding timescale between 7.4 and 9.5 days. The time series corresponding to the ENSO spatial pattern did not match that of a Markov process, and thus a well-defined timescale could not be specified. The shortness of the above timescal...

Dynamical and Boundary Forcing Characteristics of Regional Components of the Asian Summer Monsoon

Abstract: In this paper, the authors present a description of the internal dynamics and boundary forcing characteristics of two major subcomponents of the Asian summer monsoon (ASM), that is, the South Asian monsoon (SAM) and the East–Southeast Asian monsoon (EAM). The description is based on a new monsoon-climate paradigm in which the variability of ASM is considered as the outcome of the interplay of a “fast” and an “intermediate” monsoon subsystem, under the influence of “slow” external forcings. Two sets of regional monsoon indices derived from dynamically consistent rainfall and wind data are used in this study. Results show that the internal dynamics of SAM are representative of a “classical” monsoon system in which the anomalous circulation is governed by Rossby wave dynamics, where anomalous vorticity induced by an off-equatorial heat source is balanced by the advection of planetary vorticity. On the other hand, the internal dynamics of EAM are characterized by a “hybrid” monsoon system featuring m...

Combined Effects of Deforestation and Doubled Atmospheric CO2 Concentrations on the Climate of Amazonia

Abstract: It is generally expected that the Amazon basin will experience at least two major environmental changes during the next few decades and centuries: 1) increasing areas of forest will be converted to pasture and cropland, and 2) concentrations of atmospheric CO2 will continue to rise. In this study, the authors use the National Center for Atmospheric Research GENESIS atmospheric general circulation model, coupled to the Integrated Biosphere Simulator, to determine the combined effects of large-scale deforestation and increased CO2 concentrations (including both physiological and radiative effects) on Amazonian climate. In these simulations, deforestation decreases basin-average precipitation by 0.73 mm day−1 over the basin, as a consequence of the general reduction in vertical motion above the deforested area (although there are some small regions with increased vertical motion). The overall effect of doubled CO2 concentrations in Amazonia is an increase in basin-average precipitation of 0.28 mm da...

An Analysis of Recent Rainfall Conditions in West Africa, Including the Rainy Seasons of the 1997 El Nino and the 1998 La Nina Years

Abstract: This article examines recent rainfall conditions throughout the Sahel and in other parts of West Africa in detail and presents an overview of changes in rainfall on timescales of decades for Africa as a whole. In West Africa, there has been a pattern of continued aridity since the late 1960s that is most persistent in the more western regions. Some recovery occurred in the easternmost sectors during the 1990s, with rainfall in some years being near or just above the long-term mean. Dry conditions continued during 1997, but that year was not unusually dry compared to others of the last two decades. Hence, it appears that the 1997 El Nino did not have a large impact in the region. A preliminary analysis suggests that in 1998 rainfall was still below the long-term mean in most of the Sahel, but the central Sahel of Niger experienced localized flooding due to high rainfall in September. Throughout the region, the wettest years of the last decades were 1978, 1988, 1994, and possibly 1998, but conditio...

Wave-Maintained Annular Modes of Climate Variability*

Abstract: The leading modes of month-to-month variability in the Northern and Southern Hemispheres are examined by comparing a 100-yr run of the Geophysical Fluid Dynamics Laboratory GCM with the NCEP–NCAR reanalyses of observations. The model simulation is a control experiment in which the SSTs are fixed to the climatological annual cycle without any interannual variability. The leading modes contain a strong zonally symmetric or annular component that describes an expansion and contraction of the polar vortex as the midlatitude jet shifts equatorward and poleward. This fluctuation is strongest during the winter months. The structure and amplitude of the simulated modes are very similar to those derived from observations, indicating that these modes arise from the internal dynamics of the atmosphere. Dynamical diagnosis of both observations and model simulation indicates that variations in the zonally symmetric flow associated with the annular modes are forced by eddy fluxes in the free troposphere, while...

Precipitation and Damaging Floods: Trends in the United States, 1932-97

Abstract: The poor relationship between what climatologists, hydrologists, and other physical scientists call floods, and those floods that actually cause damage to life or property, has limited what can be reliably said about the causes of observed trends in damaging floods. It further limits what can be said about future impacts of floods on society based on predicted changes in the global hydrological cycle. This paper presents a conceptual framework for the systematic assessment of the factors that condition observed trends in flood damage. Using the framework, it assesses the role that variability in precipitation has in damaging flooding in the United States at national and regional levels. Three different measures of flood damage—absolute, per capita, and per unit wealth—each lead to different conclusions about the nature of the flood problem. At a national level, of the 10 precipitation measures examined in this study, the ones most closely related to flood damage are the number of 2-day heavy rainfall events and the number of wet days. Heavy rainfall events are defined relative to a measure of average rainfall in each area, not as absolute thresholds. The study indicates that the growth in recent decades in total damage is related to both climate factors and societal factors: increased damage is associated with increased precipitation and with increasing population and wealth. At the regional level, this study reports a stronger relationship between precipitation measures and flood damage, and indicates that different measures of precipitation are most closely related to damage in different regions. This study suggests that climate plays an important, but by no means determining, role in the growth in damaging floods in the United States in recent decades.