Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Reviews of Geophysics 50 (2012): RG4003, doi:10.1029/2012RG000389.

Description: The most important sources of atmospheric moisture at the global scale are herein identified, both oceanic and terrestrial, and a characterization is made of how continental regions are influenced by water from different moisture source regions. The methods used to establish source-sink relationships of atmospheric water vapor are reviewed, and the advantages and caveats associated with each technique are discussed. The methods described include analytical and box models, numerical water vapor tracers, and physical water vapor tracers (isotopes). In particular, consideration is given to the wide range of recently developed Lagrangian techniques suitable both for evaluating the origin of water that falls during extreme precipitation events and for establishing climatologies of moisture source-sink relationships. As far as oceanic sources are concerned, the important role of the subtropical northern Atlantic Ocean provides moisture for precipitation to the largest continental area, extending from Mexico to parts of Eurasia, and even to the South American continent during the Northern Hemisphere winter. In contrast, the influence of the southern Indian Ocean and North Pacific Ocean sources extends only over smaller continental areas. The South Pacific and the Indian Ocean represent the principal source of moisture for both Australia and Indonesia. Some landmasses only receive moisture from the evaporation that occurs in the same hemisphere (e.g., northern Europe and eastern North America), while others receive moisture from both hemispheres with large seasonal variations (e.g., northern South America). The monsoonal regimes in India, tropical Africa, and North America are provided with moisture from a large number of regions, highlighting the complexities of the global patterns of precipitation. Some very important contributions are also seen from relatively small areas of ocean, such as the Mediterranean Basin (important for Europe and North Africa) and the Red Sea, which provides water for a large area between the Gulf of Guinea and Indochina (summer) and between the African Great Lakes and Asia (winter). The geographical regions of Eurasia, North and South America, and Africa, and also the internationally important basins of the Mississippi, Amazon, Congo, and Yangtze Rivers, are also considered, as is the importance of terrestrial sources in monsoonal regimes. The role of atmospheric rivers, and particularly their relationship with extreme events, is discussed. Droughts can be caused by the reduced supply of water vapor from oceanic moisture source regions. Some of the implications of climate change for the hydrological cycle are also reviewed, including changes in water vapor concentrations, precipitation, soil moisture, and aridity. It is important to achieve a combined diagnosis of moisture sources using all available information, including stable water isotope measurements. A summary is given of the major research questions that remain unanswered, including (1) the lack of a full understanding of how moisture sources influence precipitation isotopes; (2) the stationarity of moisture sources over long periods; (3) the way in which possible changes in intensity (where evaporation exceeds precipitation to a greater of lesser degree), and the locations of the sources, (could) affect the distribution of continental precipitation in a changing climate; and (4) the role played by the main modes of climate variability, such as the North Atlantic Oscillation or the El Niño–Southern Oscillation, in the variability of the moisture source regions, as well as a full evaluation of the moisture transported by low-level jets and atmospheric rivers.

Description: Luis Gimeno would like to thank the Spanish Ministry of Science and FEDER for their partial funding of this research through the project MSM. A. Stohl was supported by the Norwegian Research Council within the framework of the WATER‐SIP project. The work of Ricardo Trigo was partially supported by the FCT (Portugal) through the ENAC project (PTDC/AAC-CLI/103567/2008).

Description: 2013-05-08

Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 4205–4225, doi:10.1002/2015JC010790.

Description: The Intertropical Convergence Zone (ITCZ) is a major source of the surface freshwater input to the tropical open ocean. Under the ITCZ, sea-surface salinity (SSS) fronts that extend zonally across the basins are observed by the Aquarius/SAC-D mission and Argo floats. This study examined the evolution and forcing mechanisms of the SSS fronts. It is found that, although the SSS fronts are sourced from the ITCZ-freshened surface waters, the formation, structure, and propagation of these fronts are governed by the trade wind driven Ekman processes. Three features characterize the governing role of Ekman forcing. First, the SSS fronts are associated with near-surface salinity-minimum zones (SMZs) of 50–80 m deep. The SMZs are formed during December–March when the near-equatorial Ekman convergence zone concurs with an equatorward displaced ITCZ. Second, after the formation, the SMZs are carried poleward away at a speed of ∼3.5 km d−1 by Ekman transport. The monotonic poleward propagation is a sharp contrast to the seasonal north/south oscillation of the ITCZ. Lastly, each SMZ lasts about 12–15 months until dissipated at latitudes beyond 10°N/S. The persistence of more than 1 calendar year allows two SMZs to coexist during the formation season (December–March), with the newly formed SMZ located near the equator while the SMZ that is formed in the previous year located near the latitudes of 10–15° poleward after 1 year's propagation. The contrast between the ITCZ and SMZ highlights the dominance of Ekman dynamics on the relationship between the SSS and the ocean water cycle.

Description: The study was supported by the NASA Ocean Salinity Science Team (OSST) under grant NNX12AG93G. Support from the NOAA Office of Climate Observation (OCO) under grant NA09OAR4320129 and NASA Ocean Vector Wind Science Team (OVWST) under grant NNA10AO86G in developing OAFlux evaporation and surface wind stress used in the study is gratefully acknowledged.

Description: 2015-12-08

Description: Author Posting. © Sears Foundation for Marine Research, 2010. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 68 (2010): 569-595, doi:10.1357/002224010794657164.

Description: The current capabilities of quantifying the oceanic freshwater cycle are shown based on new observations from satellite data and re-analysis models for evaporation and precipitation over the ocean. For this purpose, we analyze the homogeneity and internal consistency of eight evaporation and seven precipitation products. Discontinuities are found around 1987 for all datasets, attributable to the launch of a microwave imaging satellite. Based on a review of comparisons with independent data and these analyses, the Global Precipitation Climatology Project (GPCP) and the Objectively Analyzed Ocean-Atmosphere Fluxes (OAFlux) evaporation product are combined with a state-of-the-art river discharge dataset to produce a new estimate of the global oceanic freshwater cycle for 1987-2006. The annual mean precipitation into the ocean averaged over 19 years is estimated at 12.2±1.2 Sv, the evaporative loss at 13.0±1.3 Sv, and the total freshwater input from land at 1.25±0.1 Sv. The oceanic budget closes within the errors estimated for each data set with an imbalance of 0.5±1.8 Sv. Based on this quantification, the global patterns of oceanic freshwater fluxes are described and a global mean is integrated to provide estimates of freshwater fluxes between basins. We find the Atlantic to be less evaporative and the Pacific less precipitative than previous in-situ estimates.

Description: The authorswould like to acknowledge support from the National Science Foundation, grant #OCE-0647949.

Description: Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 117 (2012): C11013, doi:10.1029/2012JC008069.

Description: The study used 126 buoy time series as a benchmark to evaluate a satellite-based daily, 0.25-degree gridded global ocean surface vector wind analysis developed by the Objectively Analyzed airs-sea Fluxes (OAFlux) project. The OAFlux winds were produced from synthesizing wind speed and direction retrievals from 12 sensors acquired during the satellite era from July 1987 onward. The 12 sensors included scatterometers (QuikSCAT and ASCAT), passive microwave radiometers (AMSRE, SSMI and SSMIS series), and the passive polarimetric microwave radiometer from WindSat. Accuracy and consistency of the OAFlux time series are the key issues examined here. A total of 168,836 daily buoy measurements were assembled from 126 buoys, including both active and archive sites deployed during 1988–2010. With 106 buoys from the tropical array network, the buoy winds are a good reference for wind speeds in low and mid-range. The buoy comparison shows that OAFlux wind speed has a mean difference of −0.13 ms−1 and an RMS difference of 0.71 ms−1, and wind direction has a mean difference of −0.55 degree and an RMS difference of 17 degrees. Vector correlation of OAFlux and buoy winds is of 0.9 and higher over almost all the sites. Influence of surface currents on the OAFlux/buoy mean difference pattern is displayed in the tropical Pacific, with higher (lower) OAFlux wind speed in regions where wind and current have the opposite (same) sign. Improved representation of daily wind variability by the OAFlux synthesis is suggested, and a decadal signal in global wind speed is evident.

Description: The authors are grateful for the support of the NASA Ocean Vector Wind Science Team (OVWST) under grant NNA10AO86G during the five-year development of the OAFlux wind synthesis products. Support from the NOAA Office of Climate Observation (OCO) under grant NA09OAR4320129 in establishing and maintaining the buoy validation database for surface fluxes is gratefully acknowledged.

Description: 2013-05-14

Description: Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 5353–5375, doi:10.1002/jgrc.20386.

Description: A satellite-based global analysis of high-resolution (0.25°) ocean surface turbulent latent and sensible heat fluxes was developed by the objectively analyzed air-sea fluxes (OAFlux) project. Resolving air-sea flux down to the order to 0.25° is critical for the description of the air-sea interaction on mesoscale scales. In this study, we evaluate the high-resolution product in depicting air-sea exchange in the eddy-rich Gulf Stream region. Two approaches were used for evaluation, one is point-to-point validation based on six moored buoys in the region, and another is basin-scale analysis in terms of wave number spectra and probability density functions. An intercomparison is also carried out between OAFlux-0.25°, OAFlux-1°, and four atmospheric reanalyses. Results indicate that OAFlux-0.25° is able to depict sharp oceanic fronts and has the best performance among the six participating products in comparison with buoy measurements. The mean OAFlux-0.25° differences in latent and sensible heat flux with respect to the buoy are 7.6 Wm−2 (7.7%) with root-mean-square (RMS) difference of 44.9 Wm−2, and 0.0 Wm−2 with RMS difference of 19.4 Wm−2, respectively. Large differences are primarily due to mismatch in SST between gridded data and point measurements when strong spatial gradients are presented. The wave number spectra and decorrelation length scale analysis indicate OAFlux-0.25° depicts eddy variability much better than OAFlux-1° and the four reanalyses; however, its capability in detecting eddies with smaller scale still needs to be improved. Among the four reanalyses, CFSR stands out as the best in comparison with OAFlux-0.25°.

Description: This study was supported by NOAA Ocean Climate Observations program (OCO) under grant NA09OAR4320129 and the NASA Ocean Vector Wind Science Team (OVWST) under grant NNA10AO86G.

Description: 2014-04-15

Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 6547–6564, doi:10.1002/2016JC012281.

Description: This study analyzed shipboard air-sea measurements acquired by the icebreaker Aurora Australis during its off-winter operation in December 2010 to May 2012. Mean conditions over 7 months (October–April) were compiled from a total of 22 ship tracks. The icebreaker traversed the water between Hobart, Tasmania, and the Antarctic continent, providing valuable in situ insight into two dynamically important, yet poorly sampled, regimes: the sub-Antarctic Southern Ocean and the Antarctic marginal ice zone (MIZ) in the Indian Ocean sector. The transition from the open water to the ice-covered surface creates sharp changes in albedo, surface roughness, and air temperature, leading to consequential effects on air-sea variables and fluxes. Major effort was made to estimate the air-sea fluxes in the MIZ using the bulk flux algorithms that are tuned specifically for the sea-ice effects, while computing the fluxes over the sub-Antarctic section using the COARE3.0 algorithm. The study evidenced strong sea-ice modulations on winds, with the southerly airflow showing deceleration (convergence) in the MIZ and acceleration (divergence) when moving away from the MIZ. Marked seasonal variations in heat exchanges between the atmosphere and the ice margin were noted. The monotonic increase in turbulent latent and sensible heat fluxes after summer turned the MIZ quickly into a heat loss regime, while at the same time the sub-Antarctic surface water continued to receive heat from the atmosphere. The drastic increase in turbulent heat loss in the MIZ contrasted sharply to the nonsignificant and seasonally invariant turbulent heat loss over the sub-Antarctic open water.

Description: NOAA Climate Observation Division Grant Number: NA09OAR4320129

Description: 2018-02-23

Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): C10025, doi:10.1029/2010JC006937.

Description: Ocean evaporation (E) and precipitation (P) are the fundamental components of the global water cycle. They are also the freshwater flux forcing (i.e., E-P) for the open ocean salinity. The apparent connection between ocean salinity and the global water cycle leads to the proposition of using the oceans as a rain gauge. However, the exact relationship between E-P and salinity is governed by complex upper ocean dynamics, which may complicate the inference of the water cycle from salinity observations. To gain a better understanding of the ocean rain gauge concept, here we address a fundamental issue as to how E-P and salinity are related on the seasonal timescales. A global map that outlines the dominant process for the mixed-layer salinity (MLS) in different regions is thus derived, using a lower-order MLS dynamics that allows key balance terms (i.e., E-P, the Ekman and geostrophic advection, vertical entrainment, and horizontal diffusion) to be computed from satellite-derived data sets and a salinity climatology. Major E-P control on seasonal MLS variability is found in two regions: the tropical convergence zones featuring heavy rainfall and the western North Pacific and Atlantic under the influence of high evaporation. Within this regime, E-P accounts for 40–70% MLS variance with peak correlations occurring at 2–4 month lead time. Outside of the tropics, the MLS variations are governed predominantly by the Ekman advection, and then vertical entrainment. The study suggests that the E-P regime could serve as a window of opportunity for testing the ocean rain gauge concept once satellite salinity observations are available.

Description: The study was supported by the NASA Remote Sensing Science for Carbon and Climate program under grant NNX07AF97G and by the NSF Physical Oceanography program under grant OCE‐0647949.

Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 7625–7644, doi:10.1002/2014JC010030.

Description: Aquarius observations feature a prominent zonal sea-surface salinity (SSS) front that extends across the tropical Pacific between 2–10°N. By linking to Argo subsurface salinity observations and satellite-derived surface forcing datasets, the study discovered that the SSS front is not a stand-alone feature; it is in fact the surface manifestation of a low-salinity convergence zone (LSCZ) located within 100 m of the upper ocean. The near-surface salinity budget analysis suggested that, although the LSCZ is sourced from the rainfall in the Inter-tropical convergence zone (ITCZ), its generation and maintenance are governed by the wind-driven Ekman dynamics, not the surface evaporation-minus-precipitation flux. Three distinct features highlight the relationship between the oceanic LSCZ and the atmospheric ITCZ. First, the seasonal movement of the LSCZ is characterized by a monotonic northward displacement starting from the near-equatorial latitudes in boreal spring, unlike the ITCZ that is known for its seasonal north-south displacement. Second, the lowest SSS waters in the LSCZ are locked to the northern edge of the Ekman salt convergence throughout the year, but have no fixed relationship with the ITCZ rain band. Collocation between the LSCZ and ITCZ occurs only during August-October, the time that the ITCZ rain band coincides with the Ekman convergence zone. Lastly, the SSS front couples with the Ekman convergence zone but not the ITCZ. The evidence reinforces the findings of the study that the Ekman processes are the leading mechanism of the oceanic LSCZ and the SSS front is the surface manifestation of the LSCZ.

Description: The study was supported by the NASA Ocean Salinity Science Team (OSST) under grant NNX12AG93G. Support from the NOAA Office of Climate Observation (OCO) under grant NA09OAR4320129 and NASA Ocean Vector Wind Science Team (OVWST) under grant NNA10AO86G in developing OAFlux evaporation and surface wind stress used in the study is gratefully acknowledged.

Description: 2015-05-18

Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 5244–5269, doi:10.1002/2013JC009648.

Description: A high-resolution global daily analysis of ocean surface vector winds (1987 onward) was developed by the Objectively Analyzed air-sea Fluxes (OAFlux) project. This study addressed the issues related to the development of the time series through objective synthesis of 12 satellite sensors (two scatterometers and 10 passive microwave radiometers) using a least-variance linear statistical estimation. The issues include the rationale that supports the multisensor synthesis, the methodology and strategy that were developed, the challenges that were encountered, and the comparison of the synthesized daily mean fields with reference to scatterometers and atmospheric reanalyses. The synthesis was established on the bases that the low and moderate winds (〈15 m s−1) constitute 98% of global daily wind fields, and they are the range of winds that are retrieved with best quality and consistency by both scatterometers and radiometers. Yet, challenges are presented in situations of synoptic weather systems due mainly to three factors: (i) the lack of radiometer retrievals in rain conditions, (ii) the inability to fill in the data voids caused by eliminating rain-flagged QuikSCAT wind vector cells, and (iii) the persistent differences between QuikSCAT and ASCAT high winds. The study showed that the daily mean surface winds can be confidently constructed from merging scatterometers with radiometers over the global oceans, except for the regions influenced by synoptic weather storms. The uncertainties in present scatterometer and radiometer observations under high winds and rain conditions lead to uncertainties in the synthesized synoptic structures.

Description: The project is sponsored by the NASA Ocean Vector Wind Science Team (OVWST) activities under grant NNA10AO86G.

Description: 2015-02-19

Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(4), (2021): e2020JC016789, https://doi.org/10.1029/2020JC016789.

Description: Argo profiling floats and L-band passive microwave remote sensing have significantly improved the global sampling of sea surface salinity (SSS) in the past 15 years, allowing the study of the range of SSS seasonal variability using concurrent satellite and in situ platforms. Here, harmonic analysis was applied to four 0.25° satellite products and two 1° in situ products between 2016 and 2018 to determine seasonal harmonic patterns. The 0.25° World Ocean Atlas (WOA) version 2018 was referenced to help assess the harmonic patterns from a long-term perspective based on the 3-year period. The results show that annual harmonic is the most characteristic signal of the seasonal cycle, and semiannual harmonic is important in regions influenced by monsoon and major rivers. The percentage of the observed variance that can be explained by harmonic modes varies with products, with values ranging between 50% and 72% for annual harmonic and between 15% and 19% for semiannual harmonic. The large spread in the explained variance by the annual harmonic reflects the large disparity in nonseasonal variance (or noise) in the different products. Satellite products are capable of capturing sharp SSS features on meso- and frontal scales and the patterns agree well with the WOA 2018. These products are, however, subject to the impacts of radiometric noises and are algorithm dependent. The coarser-resolution in situ products may underrepresent the full range of high-frequency small scale SSS variability when data record is short, which may have enlarged the explained SSS variance by the annual harmonic.

Description: L. Yu was funded by NASA Ocean Salinity Science Team (OSST) activities through Grant 80NSSC18K1335. FMB was funded by the NASA OSST through Grant 80NSSC18K1322. E. P. Dinnat was funded by NASA through Grant 80NSSC18K1443. This research is carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.

Description: 2021-09-17