Evaporation Minus Precipitation Research Articles

Exogenous moisture deficit fuels drought risks across China

Intensifying droughts under climatic warming are of widespread concern owing to their devastating impacts on water resources, societies and ecosystems. However, the effects of exogeneous drivers on regional droughts remain poorly understood. Using the Lagrangian method, atmospheric reanalysis data and climate projections from the Coupled Model Inter-comparison Project phase 6 (CMIP6), we show how exogenous precipitation minus evaporation (PME) deficit drives droughts across China. More specifically, we demonstrate that four distinct trajectories of such exogenous PME deficit fuel regional droughts. Three of these trajectories relate to oceanic PME deficit originating from the North Atlantic, eastern Bering Sea and Indian Ocean, and one trajectory characterizes exogenous terrestrial PME deficit from the Siberian Plateau. We show that during 1980–2020, droughts induced by exogenous PME deficit account for 45% of all droughts that occurred in China’s coastal region, and for 7% of all droughts in the northwestern regions. Under climate scenario SSP245 (SSP585), limiting warming to 1.5 °C compared to 2 °C above pre-industrial levels could avoid 60% (84%) of exogenous drought exposure. This would in turn reduce population exposure by 40% (49%), and economic exposure by 73% (66%). Our study unravels how exogenous PME deficit drives droughts in China, underscoring the role that external drivers have on regional droughts and associated future prediction.

Last interglacial seasonal hydroclimate in the North Sea–Baltic Sea region

The Last Interglacial (LIG) experienced substantial changes in seasonal insolation compared with the present day, which may have affected the hydrography and water-mass exchange in the North Sea and Baltic Sea region. Here we investigate the effects of solar radiation and greenhouse gas (GHG) forcing on the regional climate by analyzing model simulations of the LIG (127 ka BP), pre-industrial (PI, 1850 CE), and present-day (PD, 1990 CE) climates. We also interpret the reconstructed seasonal bottom water conditions using benthic foraminifera and geochemistry data.Our simulations reveal that during the LIG, the Baltic Sea region (including the Kattegat and the Danish Straits) experienced more saline and colder bottom waters than those in the PD, in agreement with the reconstruction data. This can be attributed to lower GHG levels and enhanced water exchange of cooler, saline North Sea water into the Baltic Sea during the LIG. The thermocline was stronger during the summer months in the LIG, mainly due to the higher sea surface temperature (SST) compared to that of the PD resulting from increased summer insolation.Further, the temperature anomalies (LIG–PD) show significant inverse correlations with the precipitation–minus–evaporation (P–E) at the Baltic Sea entrance. However, the P–E balance appears to have had minimal impact on salinity changes in the North Sea, the Baltic Proper, and the open sea area. Our findings indicate that monthly surface and bottom water salinity anomalies of LIG-PI exhibit strong positive correlations with the North Atlantic Oscillation (NAO) anomalies in the Baltic entrance region. During the LIG, a more positive phase of the NAO index in autumn played a crucial role in wind-driven major inflows and led to more intensive water exchange in the North Sea–Baltic Sea region compared to the late Holocene.

Oceanic climate changes threaten the sustainability of Asia’s water tower

Water resources sustainability in High Mountain Asia (HMA) surrounding the Tibetan Plateau (TP)—known as Asia’s water tower—has triggered widespread concerns because HMA protects millions of people against water stress1,2. However, the mechanisms behind the heterogeneous trends observed in terrestrial water storage (TWS) over the TP remain poorly understood. Here we use a Lagrangian particle dispersion model and satellite observations to attribute about 1 Gt of monthly TWS decline in the southern TP during 2003–2016 to westerlies-carried deficit in precipitation minus evaporation (PME) from the southeast North Atlantic. We further show that HMA blocks the propagation of PME deficit into the central TP, causing a monthly TWS increase by about 0.5 Gt. Furthermore, warming-induced snow and glacial melt as well as drying-induced TWS depletion in HMA weaken the blocking of HMA’s mountains, causing persistent northward expansion of the TP’s TWS deficit since 2009. Future projections under two emissions scenarios verified by satellite observations during 2020–2021 indicate that, by the end of the twenty-first century, up to 84% (for scenario SSP245) and 97% (for scenario SSP585) of the TP could be afflicted by TWS deficits. Our findings indicate a trajectory towards unsustainable water systems in HMA that could exacerbate downstream water stress.

Modeling Land‐Atmosphere Coupling at Cloud‐Resolving Scale Within the Multiple Atmosphere Multiple Land (MAML) Framework in SP‐E3SM

AbstractRepresenting subgrid variabilities of land surface processes and their upscaled effects is crucial for global climate modeling. Here, we implement a multiple atmosphere multiple land (MAML) framework in the superparamaterized version of E3SM (SP‐E3SM) to explicitly simulate the subgrid variabilities of land states and fluxes at cloud‐resolving scale and their interactions with atmosphere. Comparing to the standard SP‐E3SM in which all the atmospheric columns of the cloud resolving model embedded within the global atmospheric model grid interact with the same land surface (i.e., multiple atmosphere single land (MASL)), the impact of MAML on the strength of land‐atmosphere coupling is limited, partly because the current implementation mainly facilitates one‐way coupling between the cloud‐resolving model and the land surface model. Despite such limitation, MAML increases the surface latent heat flux at the expense of sensible heat flux, and increases precipitation in India, Amazon, and Central Africa, reducing the model dry bias compared to the standard SP‐E3SM. By employing a normalized gross moist stability (NGMS) diagnostic framework, we find that the increase in precipitation minus evaporation (P‐E) is primarily driven by the change in large‐scale moisture convergence, particularly by the increase of water vapor in the lower atmosphere, while the local effect of total surface energy flux plays a minor role in the P‐E change. More specifically, MAML changes the surface energy partitioning (evaporative fraction), increases the atmosphere water vapor, and further increases P‐E by decreasing the NGMS. Finally, future development in the MAML framework is discussed.

Low Sea Surface Salinity Event of the Japan Sea During the Last Glacial Maximum

AbstractAbnormal lightening of the oxygen isotope ratio (δ18O) of planktonic foraminifera during the Last Glacial Maximum (LGM, ∼21 kyr BP) suggests that the Japan Sea had experienced a low sea surface salinity event at that time. However, the exact value and timing of minimum salinity have been controversial so far. To address this issue, we adopt a simple box model and reconstruct the sea surface salinity in the Japan Sea (SJP) over the past 35 kyr with a focus on the LGM period. In particular, as input data for the box model, the inflow transport through the Tsushima Strait (Q) is converted from sea level evolution using a newly defined relationship, in which Q reduces non‐linearly with the sea level reduction through a dynamically‐constrained realistic ocean model. Meanwhile, another input data of the box model, sea surface freshwater flux (precipitation minus evaporation (P‐E) evolution), is obtained by averaging multi‐paleoclimate models (PMIP3 and MIROC4m models) results. The reconstructed SJP using the box model reached its minimum value (20.2) at 20 kyr BP with a high coefficient of determination (R2) for δ18O (0.81, p << 0.01). Further analysis demonstrates that the above non‐linear relationship, determined by h3/2 (h is the strait depth), promises a more reasonable reconstruction of the SJP evolution. It is also concluded that both the value and timing of the minimum SJP depend on the Q evolution, and the P‐E evolution can modify the former. Therefore, the combination of Q and P‐E determines the exact value and timing of minimum salinity.

Role of local and remote forcing on the decadal variability of Mixed Layer Depth in the Bay of Bengal

The Mixed Layer Depth (MLD) of an ocean acts as a storage of the incoming heat and releases via air-sea exchange processes. The present study tries to understand the decadal variability of MLD over the Bay of Bengal (BoB) and its association with local as well as external forcing. The time series and wavelet analysis confirm the significant decadal signals for the period from 1980 to 2015. The BoB MLD is found to have opposing trend prior to and post of 1998. In the decadal timescale, the BoB MLD is found to be comparably deep for regime-1 (1980–1998) and shallow for regime-2 (1999–2014) with stronger variation in the western bay. The local forcing like wind, wind-related processes, and Evaporation minus Precipitation (E-P) are found to have a prominent role to modulate the decadal variability of BoB MLD. The leading mode of the decadal Empirical Orthogonal Function (EOF-1) of MLD over the BoB shows a strong spatial pattern with the strongest in the western part of the Bay. The corresponding principal component shows a similar variability to Interdecadal Pacific Oscillation (IPO) signal. The enhancement of descending (ascending) branch of the Indian branch of Walker circulation (IWC) in the positive (negative) phase of IPO influences the changes over the equatorial Indian Ocean remotely modulate MLD of the coastal region of the BoB via the propagating coastal Kelvin waves. The decadal variability of MLD in the coastal region of the BoB is caused mainly by remote effect but the interior BoB is by the local factors.

Uncertainty in Projected Changes in Precipitation Minus Evaporation: Dominant Role of Dynamic Circulation Changes and Weak Role for Thermodynamic Changes

AbstractEnd of century projections from Coupled Model Intercomparison Project (CMIP) models show a decrease in precipitation over subtropical oceans that often extends into surrounding land areas, but with substantial intermodel spread. Changes in precipitation are controlled by both thermodynamical and dynamical processes, though the importance of these processes for regional scales and for intermodel spread is not well understood. The contribution of dynamic and thermodynamic processes to the model spread in regional precipitation minus evaporation (P − E) is computed for 48 CMIP models. The intermodel spread is dominated essentially everywhere by the change of the dynamic term, including in most regions where thermodynamic changes drive the multi‐model mean response. The dominant role of dynamic changes is insensitive to zonal averaging which removes any influence of stationary wave changes, and is also evident in subtropical oceanic regions. Relatedly, intermodel spread in P − E is generally unrelated to climate sensitivity.

Representation of Spatial Variability of the Water Fluxes over the Congo Basin Region.

The Congo Basin, being one of the major basins in the tropics, is important to the global climate, yet its hydrology is perhaps the least understood. Although various reanalysis/analysis datasets have been used to improve our understanding of the basin’s hydroclimate, they have been historically difficult to validate due to sparse in situ measurements. This study analyzes the impact of model resolution on the spatial variability of the Basin’s hydroclimate using the Decorrelation Length Scale (DLCS) technique, as it is not subject to uniform model bias. The spatial variability within the precipitation (P), evaporation/evapotranspiration (E), and precipitation-minus-evaporation (P-E) fields were investigated across four spatial resolutions using reanalysis/analysis datasets from the ECMWF ranging from 9–75 km. Results show that the representation of P and P-E fields over the Basin and the equatorial Atlantic Ocean are sensitive to model resolution, as the spatial patterns of their DCLS results are resolution-dependent. However, the resolution-independent features are predominantly found in the E field. Furthermore, the P field is the dominant source of spatial variability of P-E, occurring over the land and the equatorial Atlantic Ocean, while over the Southern Atlantic, P-E is mainly governed by the E field, with both showing weak spatial variability.

Evaluating the large-scale hydrological cycle response within the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) ensemble

Abstract. The mid-Pliocene (∼3 Ma) is one of the most recent warm periods with high CO2 concentrations in the atmosphere and resulting high temperatures, and it is often cited as an analog for near-term future climate change. Here, we apply a moisture budget analysis to investigate the response of the large-scale hydrological cycle at low latitudes within a 13-model ensemble from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The results show that increased atmospheric moisture content within the mid-Pliocene ensemble (due to the thermodynamic effect) results in wetter conditions over the deep tropics, i.e., the Pacific intertropical convergence zone (ITCZ) and the Maritime Continent, and drier conditions over the subtropics. Note that the dynamic effect plays a more important role than the thermodynamic effect in regional precipitation minus evaporation (PmE) changes (i.e., northward ITCZ shift and wetter northern Indian Ocean). The thermodynamic effect is offset to some extent by a dynamic effect involving a northward shift of the Hadley circulation that dries the deep tropics and moistens the subtropics in the Northern Hemisphere (i.e., the subtropical Pacific). From the perspective of Earth's energy budget, the enhanced southward cross-equatorial atmospheric transport (0.22 PW), induced by the hemispheric asymmetries of the atmospheric energy, favors an approximately 1∘ northward shift of the ITCZ. The shift of the ITCZ reorganizes atmospheric circulation, favoring a northward shift of the Hadley circulation. In addition, the Walker circulation consistently shifts westward within PlioMIP2 models, leading to wetter conditions over the northern Indian Ocean. The PlioMIP2 ensemble highlights that an imbalance of interhemispheric atmospheric energy during the mid-Pliocene could have led to changes in the dynamic effect, offsetting the thermodynamic effect and, hence, altering mid-Pliocene hydroclimate.

Redistribution of riverine and rainfall freshwater by the Bay of Bengal circulation

We use satellite-derived currents and a Lagrangian approach to investigate the redistribution of the precipitation minus evaporation (P-E) and river freshwater inputs into Bay of Bengal (BoB) by the oceanic circulation. We find a key role of Ekman transport in shaping the BoB freshwater distribution. Until September, the summer monsoon winds induce eastward Ekman transport, which maintains freshwater near its major rivers and rain sources in the northeastern BoB. The winter monsoon Ekman transport strongly contributes to the surface flow in many areas of the interior BoB. This ~ 0.15 m s−1 westward transport overcomes the weaker offshore transport by mesoscale motions and pushes a ~ 40/45% mixture of P-E and Ganges–Brahmaputra freshwater into the East Indian Coastal Current (EICC). In agreement with previous studies, we find that the EICC then transports Ganges–Brahmaputra freshwater southward, allowing the formation of a narrow freshwater tongue or “river in the sea” along the coast east of India in November. Ekman transport thus operates jointly with the EICC to allow the “river in the sea” formation. The EICC is nonetheless a “leaky pipe” as only ~ 22% of the Ganges–Brahmaputra, and ~ 9% of the P-E monsoonal freshwater inputs exit the BoB near Sri Lanka. The winter monsoon anticyclonic circulation in fact brings more rain freshwater from the equatorial Indian Ocean into the southeastern BoB than it exports freshwater through the EICC. As a result, the BoB circulation contributes to a net freshwater gain that amounts to 11% of the local rain and freshwater inputs.

Enhanced hydrological cycle increases ocean heat uptake and moderates transient climate change.

The large-scale moistening of the atmosphere in response to increasing greenhouse gases amplifies the existing patterns of precipitation minus evaporation (P-E) which, in turn, amplifies the spatial contrast in sea surface salinity (SSS). Through a series of transient CO2 doubling experiments, we demonstrate that surface salinification driven by the amplified dry conditions (P-E < 0), primarily in the subtropical ocean, accelerates ocean heat uptake. The salinification also drives the sequestration of upper-level heat into the deeper ocean, reducing the thermal stratification and increasing the heat uptake through a positive feedback. The change in Atlantic Meridional Overturning Circulation due to salinification plays a secondary role in heat uptake. Consistent with the heat uptake changes, the transient climate response would increase by approximately 0.4 K without this process. Observed multi-decadal changes in subsurface temperature and salinity resembles those simulated, indicating that anthropogenically-forced changes in salinity are likely enhancing the ocean heat uptake.

Future Summer Drying in the U.S. Corn Belt and the Role of Midlatitude Storm Tracks

AbstractDuring the summer, the Midwest United States, which covers the main US corn belt, has a net loss of surface water as evapotranspiration exceeds precipitation. The net moisture gain into the atmosphere is transported out of the region to northern high latitudes through transient eddy moisture fluxes. How this process may change in the future is not entirely clear despite the fact that the corn belt region is responsible for a large portion of the global supply of corn and soybeans. We find that increased CO2 and the associated warming increases evapotranspiration. while precipitation reduces in the region leading to further reduction in precipitation minus evaporation (P-E) in the future. At the same time, the poleward transient moisture flux increases leading to enhanced atmospheric moistures export from the corn belt region. However, storm track intensity is generally weakened in the summer due to reduced north-south temperature gradient associated with amplified warming in the midlatitudes. The intensified transient eddy moisture transport as storm track weakens can be reconciled by the stronger mean moisture gradient in the future. This is found to be caused by the climatological low-level jet transporting more moisture into the Great Plains region due to the thermodynamic mechanism under warmer conditions. Our results, for the first time, show that in the future, the US Midwest corn belt will experience more hydrological stress due to intensified transient eddy moisture export leading to drier soils in the region.

Observation, Preconditioning and Recurrence of Exceptionally High Salinities in the Adriatic Sea

The paper aims to describe the preconditioning and observations of exceptionally high salinity values that were observed in summer and autumn of 2017 in the Adriatic. The observations encompassed CTD measurements carried out along the well-surveyed climatological transect in the Middle Adriatic (the Palagruža Sill, 1961–2020), Argo profiling floats and several glider missions, accompanied with satellite altimetry and operational ocean numerical model (Mediterranean Forecasting System) products. Typically, subsurface salinity maximum, with values lower than 39.0, is observed in the Southern Adriatic (usually between 200 and 400 m), related to ingressions of saltier and warmer waters originating in the eastern Mediterranean (Levantine Intermediate Water—LIW). However, seasonally strong inflow of warm and high salinity waters (S &amp;gt; 38.8) has been observed much closer to the surface since spring 2015. The main LIW core deepened at the same time (to 400–700 m). Such double-maxima vertical pattern was eventually disturbed by winter convection at the beginning of 2017, increasing salinities throughout the water column. A new episode of very strong inflow of high salinity waters from the Northern Ionian was observed in late winter and spring of 2017, this time restricted almost to the surface. As most of 2017 was characterized by extremely dry conditions, low riverine inputs and warmer than usual summer over the Adriatic and Northern Ionian, salinity values above the sharp and shallow (15–40 m) thermocline significantly increased. The maximum recorded salinity was 39.26, as measured by the Argo float in the Southern Adriatic. Surface salinity maximum events, but with much lower intensity, have been documented in the past. Both past events and the 2017 event were characterized by (i) concurrence with overall high salinity conditions and cyclonic or transitional phase of the Adriatic-Ionian Bimodal Oscillating System, (ii) very low river discharges preconditioning the events for a year or more, (iii) higher-than-average heat fluxes during most of the summer and early autumn periods, forming a stable warm layer above the thermocline, and (iv) higher-than-average E-P (evaporation minus precipitation) acting on this warm surface layer. Importantly, the 2017 event was also preceded by strong near-surface inflow of very saline waters from the Northern Ionian in early 2017.

Monsoon‐Induced Zonal Asymmetries in Moisture Transport Cause Anomalous Pacific Precipitation Minus Evaporation

AbstractBasin‐integrated precipitation minus evaporation (P − E) in the Pacific is near neutral while the Atlantic shows net evaporation. We link this P − E asymmetry to atmospheric moisture fluxes across the boundaries of the ocean drainage basins. Adopting an objective approach based on a comparison between actual fluxes and a zonally averaged circulation, we show that the asymmetry is dominated by moisture fluxes associated with the monthly mean flow at low latitudes rather than by differences in moisture fluxes into the Southern Ocean and Arctic catchments. In boreal summer, the eastward moisture flux, due to the Asian Summer Monsoon flow, opposes the zonal mean westward flux in the Trade Winds and results in more positive P − E over the Pacific than both the Atlantic and Indian Oceans, even in the annual mean. Our analysis reveals that moisture flux across Southeast Asia, rather than across Central America, is the dominant factor in the P − E asymmetry.

Evaluating simulated climate patterns from the CMIP archives using satellite and reanalysis datasets using the Climate Model Assessment Tool (CMATv1)

Abstract. An objective approach is presented for scoring coupled climate simulations through an evaluation against satellite and reanalysis datasets during the satellite era (i.e., since 1979). The approach is motivated, described, and applied to available Coupled Model Intercomparison Project (CMIP) archives and the Community Earth System Model (CESM) Version 1 Large Ensemble archives with the goal of robustly benchmarking model performance and its evolution across CMIP generations. A scoring system is employed that minimizes sensitivity to internal variability, external forcings, and model tuning. Scores are based on pattern correlations of the simulated mean state, seasonal contrasts, and ENSO teleconnections. A broad range of feedback-relevant fields is considered and summarized on discrete timescales (climatology, seasonal, interannual) and physical realms (energy budget, water cycle, dynamics). Fields are also generally chosen for which observational uncertainty is small compared to model structural differences. Highest mean variable scores across models are reported for well-observed fields such as sea level pressure, precipitable water, and outgoing longwave radiation, while the lowest scores are reported for 500 hPa vertical velocity, net surface energy flux, and precipitation minus evaporation. The fidelity of models is found to vary widely both within and across CMIP generations. Systematic increases in model fidelity in more recent CMIP generations are identified, with the greatest improvements occurring in dynamic and energetic fields. Such examples include shortwave cloud forcing and 500 hPa eddy geopotential height and relative humidity. Improvements in ENSO scores with time are substantially greater than for climatology or seasonal timescales. Analysis output data generated by this approach are made freely available online from a broad range of model ensembles, including the CMIP archives and various single-model large ensembles. These multimodel archives allow for an expeditious analysis of performance across a range of simulations, while the CESM large ensemble archive allows for estimation of the influence of internal variability on computed scores. The entire output archive, updated and expanded regularly, can be accessed at http://webext.cgd.ucar.edu/Multi-Case/CMAT/index.html (last access: 18 August 2020).

Freshwater budget in the Persian (Arabian) Gulf and exchanges at the Strait of Hormuz.

Excess evaporation within the Persian (also referred as the Arabian) Gulf induces an inverse-estuary circulation. Surface waters are imported, via the Strait of Hormuz, while saltier waters are exported in the deeper layers. Using output of a 1/12-Degree horizontal resolution ocean general circulation model, the spatial structure and time variability of the circulation and the exchanges of volume and salt through the Strait of Hormuz are investigated in detail. The model's circulation pattern in the Gulf is found to be in good agreement with observations and other studies based on numerical models. The mean export of salty waters in the bottom layer is of 0.26±0.05Sv (Sverdrup = 1.0 × 106 m3 s-1). The net freshwater import, the equivalent of the salt export divided by a reference salinity, done by the baroclinic circulation across that vertical section is decomposed in an overturning and a horizontal components, with mean values of 7.2±2.1 × 10-3 Sv and 5.0±1.7 × 10-3 Sv respectively. An important, novel finding of this work is that the horizontal component is confined to the deeper layers, mainly in the winter. It is also described for the first time that both components are correlated at the same level with the basin averaged evaporation minus precipitation (E-P) over the Persian Gulf. The highest correlation (r2 = 0.59) of the total freshwater transport across 26°N with E-P over the Gulf is found with a one-month time lag, with E-P leading. The time series of freshwater import does not show any significant trend in the period from 1980 to 2015. Power spectra analysis shows that most of the energy is concentrated in the seasonal cycle. Some intraseasonal variability, likely related to the Shamal wind phenomenon, and possible impacts of El-Nino are also detected. These results suggest that the overturning and the horizontal components of freshwater exchange across the Strait of Hormuz are both driven by dynamic and thermodynamic processes inside the Persian Gulf.

Monsoon variability and upper ocean stratification during the last ∼66 ka over the Andaman Sea: Inferences from the δ18O records of planktonic foraminifera

Using new oxygen isotope (δ18O) data of three planktonic foraminiferal (Globigerinoides ruber, Globigerinoides sacculifer and Globorotalia menardii) species from a sediment core SK-234-60, collected from the Andaman Sea, we infer that summer monsoon was weaker during Marine Isotopic Stage-3 (MIS-3) than the early Holocene but was stronger than that during MIS-2. Further, an abrupt decrease in a monsoon during the mid-Holocene between ∼7 and 4.5 ka is observed. The difference in δ18O between species with different habitats depth also enables the reconstruction of past ocean stratification over the Andaman Sea. Distinct weak and strong freshwater-induced upper ocean stratifications are observed during the last ∼66 ka. A weaker upper ocean stratification during the Last Glacial Maximum (LGM) and a stronger stratification during the early Holocene is observed. This is due to the prevailing positive and negative Evaporation minus Precipitation (E-P) conditions, respectively. Weaker stratifications during ∼32 ka and early to mid-MIS-3 are observed, whereas the stratification was strong during ∼42-39 ka. Stronger monsoon associated with higher upper ocean stratification was seen during ∼62 ka. Likewise, weaker monsoon during the Heinrich events was associated with weaker stratification.

Detectability of Decadal Anthropogenic Hydroclimate Changes over North America

Regional hydroclimate changes on decadal time scales contain substantial natural variability. This presents a challenge for the detection of anthropogenically forced hydroclimate changes on these spatiotemporal scales because the signal of anthropogenic changes is modest, compared to the noise of natural variability. However, previous studies have shown that this signal-to-noise ratio can be greatly improved in a large model ensemble where each member contains the same signal but different noise. Here, using multiple state-of-the-art large ensembles from two climate models, the authors quantitatively assess the detectability of anthropogenically caused decadal shifts in precipitation-minus-evaporation (PmE) mean state against natural variability, focusing on North America during 2000–50. Anthropogenic forcing is projected to cause detectable (signal larger than noise) shifts in PmE mean state relative to the 1950–99 climatology over 50%–70% of North America by 2050. The earliest detectable signals include, during November–April, a moistening over northeastern North America and a drying over southwestern North America and, during May–October, a drying over central North America. Different processes are responsible for these signals. Changes in submonthly transient eddy moisture fluxes account for the northeastern moistening and central drying, while monthly atmospheric circulation changes explain the southwestern drying. These model findings suggest that despite the dominant role of natural internal variability on decadal time scales, anthropogenic shifts in PmE mean state can be detected over most of North America before the middle of the current century.

Assessing recent air-sea freshwater flux changes using a surface temperature-salinity space framework

A novel assessment of recent changes in air-sea freshwater fluxes has been conducted using a surface temperature-salinity framework applied to four atmospheric reanalyses. Viewed in the T-S space of the ocean surface, the complex pattern of the longitude-latitude space mean global Precipitation minus Evaporation (PME) reduces to three distinct regions. The analysis is conducted for the period 1979-2007 for which there is most evidence for a broadening of the (atmospheric) tropical belt. All four of the reanalyses display an increase in strength of the water cycle. The range of increase is between 2%-30% over the period analysed, with an average of 14%. Considering the average across the reanalyses, the water cycle changes are dominated by changes in tropical as opposed to mid-high latitude precipitation. The increases in the water cycle strength, are consistent in sign, but larger than in a 1% greenhouse gas run of the HadGEM3 climate model. In the model a shift of the precipitation/evaporation cells to higher temperatures is more evident, due to the much stronger global warming signal. The observed changes in freshwater fluxes appear to be reflected in changes in the T-S distribution of the Global Ocean. Specifically, across the diverse range of atmospheric reanalyses considered here, there was an acceleration of the hydrological cycle during 1979-2007 which led to a broadening of the ocean's salinity distribution. Finally, although the reanalyses indicate that the warm temperature tropical precipitation dominated water cycle change, ocean observations suggest that ocean processes redistributed the freshening to lower ocean temperatures.

Centennial Changes of the Global Water Cycle in CMIP5 Models

Abstract The global water cycle is predicted to intensify under various greenhouse gas emissions scenarios. Here the nature and strength of the expected changes for the ocean in the coming century are assessed by examining the output of several CMIP5 model runs for the periods 1990–2000 and 2090–2100 and comparing them to a dataset built from modern observations. Key elements of the water cycle, such as the atmospheric vapor transport, the evaporation minus precipitation over the ocean, and the surface salinity, show significant changes over the coming century. The intensification of the water cycle leads to increased salinity contrasts in the ocean, both within and between basins. Regional projections for several areas important to large-scale ocean circulation are presented, including the export of atmospheric moisture across the tropical Americas from Atlantic to Pacific Ocean, the freshwater gain of high-latitude deep water formation sites, and the basin averaged evaporation minus precipitation with implications for interbasin mass transports.