During December-January-February Research Articles

A drier Orinoco basin during the twenty-first century: the role of the Orinoco low-level jet

AbstractThis study focuses on the analysis of the simulation of the main climatological features of the Orinoco low-level jet (OLLJ) by a set of models included in the Sixth Phase of the Coupled Model Intercomparison Project (CMIP6) and their projected changes under three Shared Socioeconomic Pathways (SSPs): SSP2-4.5, SSP3-7.0, and SSP5-8.5. We consider the 1979–2014 period to evaluate the historical simulations using the ERA5 reanalysis as the reference dataset. In general, CMIP6 models are able to capture the activation of the OLLJ during December-January–February (DJF) in the Orinoco basin, as well as the main links between this circulation and low-level moisture transport patterns in northern South America. Regarding the analysis of projections, CMIP6 models suggest a weakening and shrinking of the OLLJ, especially in its exit region, by the end of the twenty-first century, which in turn induces changes in atmospheric moisture transport patterns in the region during DJF. The projected changes of the OLLJ are associated with variations in the regional gradients of mean sea level pressure, near-surface air temperature, and surface sensible heat flux in association with drier conditions in the Orinoco basin. These projections are consistent with previous studies suggesting a drier Orinoco river basin throughout the twenty-first century. Assessing the projected changes of this low-level jet in northern South America improves our understanding of the different phenomena that modulate atmospheric moisture transport in the region, which is particularly important given its high vulnerability to climate change.

The Importance of Cumulus Parameterization and Resolution in Simulating Rainfall over Peninsular Malaysia

In this study, five simulations were conducted using the weather research and forecasting (WRF) model with different cumulus parameterizations schemes (CPSs) for the period from 2013 until 2018. A one-year simulation of 2013 with three different horizontal resolutions of 25, 5, and 1.6 km was also performed. The CPSs used were Kain–Fritsch (KF), Grell–Devenyi (GR), Betts–Miller–Janjic (BM), and a non-parameterized scheme (NC). In assessments of model resolutions, both the 25 and 5 km resolutions depicted a strong negative bias in the northeastern part of Peninsular Malaysia during December–January–February (DJF), with marginal differences between the two simulations. Among all 5 km experiments, the best performing scheme was the BM scheme for almost all seasons. Furthermore, the 5 km simulation did not exhibit significant differences relative to the 25 km of the diurnal cycle. The 1.6 km simulation showed significant added value as it was the only simulation that was able to simulate the high precipitation intensity in the morning and a precipitation peak during the evening. The 1.6 km resolution was also the only resolution capable of picking up the precipitation signals in the R4 region (South Peninsular Malaysia) compared to the other two resolutions. While both CPSs and resolutions are important for accurate predictions, the role of CPSs became less significant in a higher resolution simulation.

Genesis of Upper-Tropospheric Anticyclones over the Asian–Western Pacific Sector from Tropical–Extratropical Interaction Perspective

Abstract Upper-tropospheric anticyclones (UTACs) emerge throughout the seasons with changing location and intensity. Here, the formation mechanisms of these UTACs, especially in the Asian–Australian–western Pacific sector, were investigated based on the diagnosis of the vorticity equation as well as the contribution of the planetary waves. During June–July–August (JJA), a vigorous UTAC corresponding to the South Asian high (SAH) forms over South Asia, to the south of the Tibetan Plateau, where intense heating associated with the Asian summer monsoon rainfall and the resultant baroclinic Rossby response are the important physical processes. Meanwhile, the produced anticyclonic vorticity is farther transported by the interhemispheric divergent wind toward the Southern Hemisphere (SH), creating the SH UTAC centered over the Maritime Continent. During December–January–February (DJF), two zonally elongated UTACs reside on each side of the equator (∼10° poleward), mainly over the Maritime Continent–western Pacific sector. Upon a closer look at the NH winter, we observed that the northern parts of UTAC cannot be explained by this vorticity balance alone. Diagnosis of the wave activity flux indicated that planetary waves emanating from the cold Eurasian continent converges around the northern parts of the UTAC with its peak in the NH winter, which weakens the subtropical jet, thus generating UTAC. Configuration of the SH summer (DJF) UTAC bears resemblance with that of SAH. These results suggest that the creation of anticyclonic vorticity and its interhemispheric transportation as well as the propagation of planetary wave are the selectively important agents for the genesis of seasonally varying UTACs. Significance Statement Recent studies have provided evidence that the South Asian high (formerly Tibetan high) is not a purely thermally driven system only maintained over the elevated Tibetan Plateau. This study aims to understand the physical processes responsible for the genesis of the upper-tropospheric anticyclone, especially in the Asian–Australian–western Pacific sector, throughout the season. During summer in the Northern Hemisphere, deep heating caused by South Asian monsoon rainfall plays a crucial role in the genesis of the South Asian high. The wintertime anticyclone emerging over the subtropical western North Pacific is caused via remote influences anchored with tropical convection and the cold Eurasian continent in which atmospheric teleconnections are important. These findings provide new avenues for research on tropical–extratropical interactions with respect to the formation and variability of important climate phenomena.

Intensified modulation of winter aerosol pollution in China by El Niño with short duration

Abstract. El Niño–Southern Oscillation (ENSO), a phenomenon of periodic changes in sea surface temperature in the equatorial central-eastern Pacific Ocean, is the strongest signal of interannual variability in the climate system with a quasi-period of 2–7 years. El Niño events have been shown to have important influences on meteorological conditions in China. In this study, the impacts of El Niño with different durations on aerosol concentrations and haze days during December–January–February (DJF) in China are quantitatively examined using the state-of-the-art Energy Exascale Earth System Model version 1 (E3SMv1). We find that PM2.5 concentrations are increased by 1–2 µg m−3 in northeastern and southern China and decreased by up to 2.4 µg m−3 in central-eastern China during El Niño events relative to the climatological means. Compared to long-duration (LD) El Niño events, El Niño with short duration (SD) but strong intensity causes northerly wind anomalies over central-eastern China, which is favorable for aerosol dispersion over this region. Moreover, the anomalous southeasterly winds weaken the wintertime prevailing northwesterly in northeastern China and facilitate aerosol transport from southern and southeast Asia, enhancing aerosol increase in northeastern China during SD El Niño events relative to LD El Niño events. In addition, the modulation effect on haze days by SD El Niño events is 2–3 times more than that by LD El Niño events in China. The aerosol variations during El Niño events are mainly controlled by anomalous aerosol accumulation/dispersion and transport due to changes in atmospheric circulation, while El Niño-induced precipitation change has little effect. The occurrence frequency of SD El Niño events has been increasing significantly in recent decades, especially after the 1940s, suggesting that El Niño with short duration has exerted an increasingly intense modulation on aerosol pollution in China over the past few decades.

Impacts of Onset Time of El Niño Events on Summer Rainfall over Southeastern Australia during 1980–2017

El Niño–Southern Oscillation (ENSO) has large impacts on Australia’s rainfall. A composite analysis technique was utilized to distinguish the impact of onset time of El Niño on summer rainfall over southeastern Australia. Summer rainfall tended to be lower than normal in austral autumn El Niño events during December–January–February (DJF) and higher than normal in austral winter El Niño events, in 1980–2017. During autumn El Niño events, the Walker circulation and meridional cells served as a bridge, linking the warmer sea surface temperature (SST) in the eastern equatorial Pacific (EEP) and lower summer rainfall over southeastern Australia. This physical process can be described as follows: During DJF, a positive SST anomaly in the EEP was concurrent with anomalous downdraft over southeastern Australia via zonal anomalous Walker circulation, meridional anomalous cells along 170° E–170° W, and a Pacific South American (PSA) teleconnection wave train at 500 hPa. In addition, an anomalous convergence at 200 hPa depressed the convection. Meanwhile, an 850 hPa abnormal westerly was not conducive to transport marine water vapor into this area. These factors resulted in below-normal rainfall. During winter El Niño events, a positive SST anomaly in the central equatorial Pacific (CEP) and the changes in Walker circulation and meridional cells were weaker. The PSA teleconnection wave train shifted westward and northward, and there was a low-level anomalous ascent over southeastern Australia. At the western flank of the anomalous anticyclone, northerly transported water vapor from the ocean to southeastern Australia resulted in a sink of water vapor over this area. The development of low-level convective activity and the plentiful water vapor supply favored more rainfall over southeastern Australia. Onset time of El Niño may be a useful metric for improving the low predictive skill of southeastern Australian summer rainfall.

Rainfall Trends and Variability over Onitsha, Nigeria

Analysis of trend and variability in rainfall can provide the necessary information required for water resources planning and management in any geographical setting. Therefore, the present study applied standard tests to investigate rainfall trends and variability using monthly, seasonal and annual series over Onitsha, Nigeria between 1971 and 2008. Rainfall variability index revealed the year 1983 as the driest (-2.38) and 1997, the wettest (+2.0), with more dry years observed between 2000 and 2008. Variability was relatively low annually as compared to seasonal and monthly series. September (15.8%) has the highest contributions to total annual rainfall, while January contributed the least (0.6%). Seasonally, about 40% of the annual rainfall was received in June-July-August (JJA), while the lowest rainfall was during December-January-February (DJF) (3.75%). Trends were mostly insignificant on monthly basis with 5 of the 12 months exhibiting negative trends, while only January depicted positive significant trend (p< 0.05). Similarly, only JJA exhibited insignificant upward trend while other seasons showed downward trends that are also not significant. On the annual basis, an insignificant negative trend was observed for the period under study. Hence, farmers and water resources managers may need to develop appropriate management strategies which include construction of more water storage and diversion structures such as reservoirs and dams to combat recurrent flooding during summer seasons and potential future water scarcity in the area. Keywords. Onitsha, rainfall, trend analysis, variability index

Importance of the El Niño Teleconnection to the 21st Century California Wintertime Extreme Precipitation Increase

AbstractUnder continued climate warming, California (CA) hydrological projections, particularly precipitation, exhibit significant uncertainty. Recent analyses, however, have indicated a tendency for increased CA precipitation through the 21st century, particularly during December‐January‐February (DJF). Here we show that this increase is due entirely to an increase in extreme (&gt;90th percentile) daily precipitation. This response is consistent with enhanced horizontal vapor transport off the CA coast, most of which is caused by thermodynamical effects due to increases in atmospheric moisture. Furthermore, observations over the late twentieth century show that CA DJF extreme precipitation is associated with an El Niño‐like sea surface temperature anomaly pattern. Models that better simulate the observed El Niño teleconnection with CA DJF precipitation also better reproduce the El Niño‐like sea surface temperature anomaly pattern associated with extreme precipitation, including the associated thermodynamical and dynamical atmospheric responses. In turn, these models simulate a significantly larger increase in extreme precipitation under warming.

ENSO modulation of seasonal rainfall and extremes in Indonesia

This paper provides a detailed description of how ENSO events affect seasonal and extreme precipitation over Indonesia. Daily precipitation data from 97 stations across Indonesia covering the period from 1981 to 2012 were used to investigate the effects of El Nino and La Nina on extreme precipitation characteristics including intensity, frequency and duration, as defined based on a subset of the Expert Team on Climate Change Detection and Indices (ETCCDI). Although anomalous signals in these three indices were consistent with those of total rainfall, anomalies in the duration of extremes [i.e., consecutive dry days (CDD) and consecutive wet days (CWD)] were much more robust. El Nino impacts were particularly prominent during June–July–August (JJA) and September–October–November (SON), when anomalously dry conditions were experienced throughout the country. However, from SON, a wet anomaly appeared over northern Sumatra, later expanding eastward during December–January–February (DJF) and March–April–May (MAM), creating contrasting conditions of wet in the west and dry in the east. We attribute this apparent eastward expansion of a wet anomaly during El Nino progression to the equatorial convergence of two anti-cyclonic circulations, one residing north of the equator and the other south of the equator. These anti-cyclonic circulations strengthen and weaken according to seasonal changes and their coupling with regional seas, hence shaping moisture transport and convergence. During La Nina events, the eastward expansion of an opposite (i.e., dry) anomaly was also present but less prominent than that of El Nino. We attribute this to differences in regional ocean—atmosphere coupling, which result in the contrasting seasonal evolution of the two corresponding anomalous cyclonic circulations and in turn suggests the strong nonlinearity of El Nino and La Nina responses over the Maritime Continent. Based on the seasonal behaviour of anomalous CDD and CWD, we propose five sub-divisions of the Indonesian region for both El Nino and La Nina.

Characteristics of precipitation extremes in Malaysia associated with El Niño and La Niña events

ABSTRACTThis study investigates the El Niño‐Southern Oscillation (ENSO)‐related seasonal variations in precipitation extremes based on the observed daily precipitation dataset of 23 meteorological stations in Malaysia, spanning a period of 46 years from 1966 to 2011. The extreme indices were a subset of the Expert Team for Climate Change Detection and Indices (ETCCDI) that covers the duration, frequency and intensity aspects of precipitation extremes. Seasonal composites of ‘El Niño minus neutral’ and ‘La Niña minus neutral’ years of these indices were computed based on El Niño and La Niña occurrences during the period. The results showed that the ENSO‐related variations in precipitation extremes were generally coherent with the variations in total precipitation. Generally, dry (wet) precipitation extremes tended to enhance during El Niño (La Niña); however, this was dependent on season and location. The El Niño and La Niña influences on precipitation extremes were not entirely linear. While the impacts of El Niño and La Niña were generally opposite for most locations and seasons, there were cases where both exerted in‐phase influences. The impacts were also dependent on the intensity of the event itself. While the El Niño impacts were generally coherent across different intensities, La Niña can have entirely different impacts among different categories. During December–January–February (DJF), strong (moderate) La Niña caused a significant decrease (increase) in wet precipitation extremes over the Peninsular Malaysia. This was related to the broadening (narrowing and westward displacement) of the anomalous cyclonic circulation over the western north Pacific during strong (moderate) La Niña. Hence, the likelihood for widespread flooding over the east coast of the Peninsular Malaysia during DJF increases during moderate but not during strong La Niña events.

Global analysis of carbon disulfide (CS&lt;sub&gt;2&lt;/sub&gt;) using the 3-D chemistry transport model STOCHEM

Carbon disulfide (CS2), a precursor to the long-lived carbonyl sulphide (OCS) is one of the main contributors to the atmospheric sulfate layer. The annual fluxes from its sources and sinks are investigated using a 3-D chemistry transport model, STOCHEM-CRI. In terms of the flux analysis, the oxidation of CS2 by OH is found to be the main removal process (76–88% of the total loss) and the dry deposition loss contributes 11–24% to the total loss of CS2. The global burden of CS2 was calculated, varying between 6.1 to 19.2 Tg and the lifetime of CS2 was determined to be within the range of 2.8–3.4 days. The global distribution of CS2 found the Northern Hemisphere (NH) continental landmasses to be the areas of concentration maxima with peak concentrations reaching up to 20 ppt during June-July-August (J-J-A) season and 40 ppt during December-January-February (D-J-F) season in anthropogenic source regions. Oceanic regions returned low CS2 levels of less than 2 ppt. The vertical profile of CS2 shows higher levels up to 3 ppt at 30°N–45°N during J-J-A and up to 4 ppt at 30°N–55°N during D-J-F. The oxidation of CS2 by OH can produce a substantial amount (0.58 Tg/yr) of atmospheric OCS and the annual average surface distribution of this flux shows up to 6 Gg/yr OCS formed in the regions with highest anthropogenic pollution (e.g., South east Asia). In general, the model-measurement comparison reveals an underprediction of model CS2 compared with measured CS2 for most of the regions. It is likely that the emissions of CS2 are being underestimated and there are likely much larger emission sources of atmospheric CS2 than previously suggested.

Performance of Decadal Prediction in Coupled Model Intercomparisson Project Phase 5 (CMIP5) on Projecting Climate in Tropical Area

Skill of BCC-CSM1.1, MPI-ESM-LR and IPSL-CM5A-LR (climate models which are collected together in Coupled Model Intercomparison Project Phase 5) on projecting precipitation, air temperature and sea surface temperature (SST) in tropical region were analyzed during December-January-February (DJF) and June-July-August (JJA) using Empirical Orthogonal Functions (EOF), climatology, bias and Root Mean Square Error (RMSE) parameters. These models were compared with the reanalysis/observation data. The results showed that dominant modes (EOF1) of all models for air temperature were similar with the mode of reanalysis data. In general, EOF1 of all models for precipitation are similar with mode of the observation which is dominated by positive (negative) anomaly in northern (southern) equator. Climatology of precipitation, air temperature and sea surface temperature showed that high values were located in southern equator during DJF and shifted to the north during JJA. The highest air temperature bias was located in Africa (around 6°C during DJF and JJA) for MPI-ESM-LR. In addition all models captured the ITCZ signal. The highest RMSE and bias of precipitation (> 9 and 5mm/day) were located in western Pacific in BCC-CSM1.1 during DJF and IPSL-CM5A-LR during JJA. BCC-CSM1.1 had higher RMSE (> 9 mm/day) in projecting precipitation in Papua and Borneo, MPI-ESM-LR detected higher bias in Sumatra, Java, Nusa Tenggara, while IPSL-CM5A-LR in Sumatra. The highest RMSE for SST (more than 2°C) was located in the eastern Pacific.

Unusual past dry and wet rainy seasons over Southern Africa and South America from a climate perspective

Southern Africa and Southern South America have experienced recent extremes in dry and wet rainy seasons which have caused severe socio-economic damages. Selected past extreme events are here studied, to estimate how human activity has changed the risk of the occurrence of such events, by applying an event attribution approach (Stott et al., 2004)comprising global climate models of Coupled Model Intercomparison Project 5 (CMIP5). Our assessment shows that models' representation of mean precipitation variability over Southern South America is not adequate to make a robust attribution statement about seasonal rainfall extremes in this region. Over Southern Africa, we show that unusually dry austral summers as occurred during 2002/2003 have become more likely, whereas unusually wet austral summers like that of 1999/2000 have become less likely due to anthropogenic climate change. There is some tentative evidence that the risk of extreme high 5-day precipitation totals (as observed in 1999/2000) have increased in the region. These results are consistent with CMIP5 models projecting a general drying trend over SAF during December–January–February (DJF) but also an increase in atmospheric moisture availability to feed heavy rainfall events when they do occur. Bootstrapping the confidence intervals of the fraction of attributable risk has demonstrated estimates of attributable risk are very uncertain, if the events are very rare. The study highlights some of the challenges in making an event attribution study for precipitation using seasonal precipitation and extreme 5-day precipitation totals and considering natural drivers such as ENSO in coupled ocean–atmosphere models.

A quantitative assessment of changes in seasonal potential predictability for the twentieth century

Changes over the twentieth century in seasonal mean potential predictability (PP) of global precipitation, 200 hPa height and land surface temperature are examined by using 100-member ensemble. The ensemble simulations have been conducted by using an intermediate complexity atmospheric general circulation model of the International Center for Theoretical Physics, Italy. Using the Hadley Centre sea surface temperature (SST) dataset on a 1 grid, two 31 year periods of 1920-1950 and 1970-2000 are separated to distinguish the periods of low and high SST variability, respectively. The standard deviation values averaged for the (''Nino-3.4''; 5S-5N, 170W-120W) region are 0.71 and 1.15 C, for the periods of low and high SST variability, respectively, with a percentage change of 62 % during December-January-February (DJF). The leading eigenvector and the associated principal compo- nent time series, also indicate that the amplitude of SST variations have positive trend since 1920s to recent years, particularly over the El Nino Southern Oscillation (ENSO) region. Our hypothesis states that the increase in SST variability has increased the PP for precipitation, 200 hPa height and land surface temperature during the DJF. The analysis of signal and noise shows that the signal-to-noise (S/N) ratio is much increased over most of the globe, particularly over the tropics and subtropics for DJF pre- cipitation. This occurs because of a larger increase in the signal and at the same time a reduction in the noise, over most of the tropical areas. For 200 hPa height, the S/N ratio over the Pacific North American (PNA) region is increas- ing more than that for the other extratropical regions, because of a larger percentage increase in the signal and only a small increase in noise. It is also found that the increase in seasonal mean transient signal over the PNA region is 50 %, while increase in the noise is only 12 %, during the high SST variability period, which indicates that the increase in signal is more than the noise. For DJF land surface temperature, the perfect model notion is utilized to confirm the changes in PP during the low and high SST variability periods. The correlation between the perfect model and the other members clearly reveal that the sea- sonal mean PP changed. In particular, the PP for the 31 years period of 1970-2000 is higher than that for the 31 years period of 1920-1950. The land surface tempera- ture PP is increased in northern and southern Africa, central Europe, southern South America, eastern United States and over Canada. The increase of the signal and hence the seasonal mean PP is coincides with an increase in tropical Pacific SST variability, particularly in the ENSO region.

Anomalies of the South American summer monsoon associated with the 1997–99 El Niño–southern oscillation

AbstractWe describe the rainfall and circulation anomalies of the South American summer monsoon (SASM) during December–January–February (DJF) of 1997–98 (El Niño) and 1998–99 (La Niña). The most pronounced rainfall signals in DJF 1997–98 include (a) excessive rainfall over northern Peru and Ecuador, (b) deficient rainfall over northern and central Brazil, and (c) above‐normal rainfall over southeastern subtropical South America. The rainfall anomalies in (a) and (b) are associated with the excitation of an anomalous east–west overturning cell with rising motion and low‐level westerlies over the equatorial eastern Pacific, coupled to sinking motion and low‐level easterlies over northern Brazil. The easterlies turn sharply southeastward on encountering the steep topography of the Andes, enhancing the summertime low‐level jet (LLJ) along the eastern foothills of the Andes near 15–20° S, possibly contributing to the increased rainfall in (c).During DJF 1997–98, the sea‐surface temperature‐induced warming spreads and expands over the entire tropical troposphere. The eastward expansion of a warm upper tropospheric geopotential and temperature ridge from the Niño‐3 region, across subtropical South America to the southeast Atlantic, enhances warming over the Altiplano Plateau, hydrostatically strengthening the Bolivia high. Similar to previous warming events, the South Pacific high is weakened, and the South Atlantic high is strengthened. During DJF 1998–99, as cold water develops over the equatorial central Pacific, the SASM anomalies in the tropics are weaker and less organized and appear to be in transition to the opposite phase to those found in DJF 97–98. In the subtropics, notable features include a weakening of the LLJ, a rainfall pattern associated with a poleward shift of the South Atlantic convergence zone, and development of the Pacific–South America teleconnection pattern. Published in 2003 by John Wiley &amp; Sons, Ltd.

A technique for extracting potentially predictable patterns from climate data

We propose a computational technique which makes it possible to extract long-range potentially predictable patterns of interannual variability of meteorological seasonal mean fields. These patterns arise from slowly varying external forcing, such as sea surface temperatures, and slowly varying internal dynamics. The method provides a means of decomposing the covariance matrix of a seasonal mean field into covariance matrices for the potentially predictable and the chaotic, or weather-noise, components, separately. We illustrate the technique using Australian surface maximum temperatures during December-January-February (DJF) for the period 1958--1991. The dominant patterns, arising from the potentially predictable covariance matrix, are shown to be more closely related to slowly varying external forcing and slowly varying internal dynamics than those from a conventional analysis. The importance of tropical sea surface temperatures in forcing the potentially predictable patterns is also discussed.

The effect of vegetation in a climate model simulation on the Younger Dryas

The effect of vegetation on the Younger Dryas (YD) climate is studied by comparing the results of four experiments performed with the ECHAM-4 atmospheric general circulation model (AGCM): (1) modern control climate, (2) simulation with YD boundary conditions, but with modern vegetation, (3 and 4) identical to (2), but with paleo-vegetation. Prescribing paleo-vegetation instead of modern vegetation resulted in temperature anomalies (both positive and negative) of up to 4°C in the Northern Hemisphere mid-latitudes, mainly as an effect of changes in forest cover (change in albedo). Moreover, changes in precipitation and evaporation were found, most notably during December–January–February (DJF) in the tropics and were caused by the replacement of forests by grasslands. These results are consistent with other model studies on the role of vegetation changes on climate and they suggest that it is important in paleoclimate simulation studies to prescribe realistic vegetation types, belonging to the period of interest. However, in our case the addition of YD vegetation did not improve the agreement with proxy data in Europe, as the temperatures were increasing during winter compared to the YD simulation with modern vegetation. It must be noted that this increase was not statistically significant. The model-data mismatch suggests that other factors probably played an important role, such as permafrost and atmospheric dust. We infer that during the last glacial-interglacial transition, the time lag between the first temperature increase and the northward migration of trees, estimated at 500–1000 years, could have delayed the warming of the Eurasian continent. The relatively open vegetation that existed during the early stages of the last glacial-interglacial transition had a relatively high albedo, thus tempering warming up of the Eurasian land surfaces.

Sensitivity of the CCM1 hydrologic cycle to CO2

To better understand the global hydrologic cycle, climatological simulations with the National Center for Atmospheric Research global atmospheric general circulation model (GCM) or community climate model (CCM1), coupled to a mixed layer model, are compared to available global hydrologic observations. We find that CCM1 simulates, reasonably well, the large‐scale precipitable water, moisture flux convergence, precipitation, and evaporation potentials. Regional discrepancies are noticeable, though, especially in the eastern tropical Pacific. Smaller‐scale moisture flux convergence, precipitation, and evaporation also compare less well since effects of resolution can be discerned between low‐resolution simulations and higher‐resolution global observations. Sensitivity experiments to changed CO2 show that the CCM1 hydrologic response is similar to other GCMs. Precipitable water, evaporation, and precipitation increase while snow decreases with increasing CO2. Soil moisture changes are seasonally and regionally dependent. In middle latitudes, soil moisture increases during the winter and decreases during the summer with increasing CO2. Changes in moisture transport between land and ocean are also seasonally dependent. Increased CO2 induces increased moisture divergence over the central equatorial Pacific during December, January, February (DJF). This water is transported to the rest of the northern hemisphere land and ocean where precipitation is much greater than evaporation over the climatologically cold land surface. During June, July, August (JJA), water is preferentially converged over the western Pacific Ocean.