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  • 1
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    Global variations in oceanic evaporation (1958–2005) : the role of the changing wind speed (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 20 (2007): 5376–5390, doi:10.1175/2007JCLI1714.1.
    Description: Global estimates of oceanic evaporation (Evp) from 1958 to 2005 have been recently developed by the Objectively Analyzed Air–Sea Fluxes (OAFlux) project at the Woods Hole Oceanographic Institution (WHOI). The nearly 50-yr time series shows that the decadal change of the global oceanic evaporation (Evp) is marked by a distinct transition from a downward trend to an upward trend around 1977–78. Since the transition, the global oceanic Evp has been up about 11 cm yr−1 (10%), from a low at 103 cm yr−1 in 1977 to a peak at 114 cm yr−1 in 2003. The increase in Evp was most dramatic during the 1990s. The uncertainty of the estimates is about ±2.74 cm yr−1. By utilizing the newly developed datasets of Evp and related air–sea variables, the study investigated the cause of the decadal change in oceanic Evp. The decadal differences between the 1990s and the 1970s indicates that the increase of Evp in the 1990s occurred over a global scale and had spatially coherent structures. Larger Evp is most pronounced in two key regions—one is the paths of the global western boundary currents and their extensions, and the other is the tropical Indo-Pacific warm water pools. It is also found that Evp was enlarged primarily during the hemispheric wintertime (defined as the mean of December–February for the northern oceans and June–August for the southern oceans). Despite the dominant upward tendency over the global basins, a slight reduction in Evp appeared in such regions as the subtropical centers of the Evp maxima as well as the eastern equatorial Pacific and Atlantic cold tongues. An empirical orthogonal function (EOF) analysis was performed for the yearly winter-mean time series of Evp and the related air–sea variables [i.e., wind speed (U) and air–sea humidity differences (dq)]. The analysis suggested a dominant role of the wind forcing in the decadal change of both Evp and dq. It is hypothesized that wind impacts Evp in two ways. The first way is direct: the greater wind speed induces more evaporation by carrying water vapor away from the evaporating surface to allow the air–sea humidity gradients to be reestablished at a faster pace. The second way is indirect: the enhanced surface wind strengthens the wind-driven subtropical gyre, which in turn drives a greater heat transport by the western boundary currents, warms up SST along the paths of the currents and extensions, and causes more evaporation by enlarging the air–sea humidity gradients. The EOF analysis performed for the time series of the global annual-mean Evp fields showed that the first three EOF modes account for nearly 50% of the total variance. The mode 1 variability represents the upward trend in Evp after 1978 and is attributable to the increased U, and the mode 2 variability explains much of the downward trend in Evp before 1978 and is correlated to the global dq variability. The EOF mode 3 of Evp captures the interannual variability of Evp on time scales of the El Niño–Southern Oscillation, with the center of action over the eastern equatorial Pacific.
    Description: The author acknowledges the supporting grants from the NOAA Office of Climate Observations (OCO) and Climate Change Data and Detection (CCDD) and from the NASA Ocean Vector Wind Science Team.
    Keywords: Evaporation ; Winds ; Climatology ; Air-sea interaction ; Decadal variability
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Role of net surface heat flux in seasonal variations of sea surface temperature in the tropical Atlantic Ocean (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society 2006. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 19 (2006): 6153–6169, doi:10.1175/JCLI3970.1.
    Description: The present study used a new net surface heat flux (Qnet) product obtained from the Objective Analyzed Air–Sea Fluxes (OAFlux) project and the International Satellite Cloud Climatology Project (ISCCP) to examine two specific issues—one is to which degree Qnet controls seasonal variations of sea surface temperature (SST) in the tropical Atlantic Ocean (20°S–20°N, east of 60°W), and the other is whether the physical relation can serve as a measure to evaluate the physical representation of a heat flux product. To better address the two issues, the study included the analysis of three additional heat flux products: the Southampton Oceanographic Centre (SOC) heat flux analysis based on ship reports, and the model fluxes from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The study also uses the monthly subsurface temperature fields from the World Ocean Atlas to help analyze the seasonal changes of the mixed layer depth (hMLD). The study showed that the tropical Atlantic sector could be divided into two regimes based on the influence level of Qnet. SST variability poleward of 5°S and 10°N is dominated by the annual cycle of Qnet. In these regions the warming (cooling) of the sea surface is highly correlated with the increased (decreased) Qnet confined in a relatively shallow (deep) hMLD. The seasonal evolution of SST variability is well predicted by simply relating the local Qnet with a variable hMLD. On the other hand, the influence of Qnet diminishes in the deep Tropics within 5°S and 10°N and ocean dynamic processes play a dominant role. The dynamics-induced changes in SST are most evident along the two belts, one of which is located on the equator and the other off the equator at about 3°N in the west, which tilts to about 10°N near the northwestern African coast. The study also showed that if the degree of consistency between the correlation relationships of Qnet, hMLD, and SST variability serves as a measure of the quality of the Qnet product, then the Qnet from OAFlux + ISCCP and ERA-40 are most physically representative, followed by SOC. The NCEP–NCAR Qnet is least representative. It should be noted that the Qnet from OAFlux + ISCCP and ERA-40 have a quite different annual mean pattern. OAFlux + ISCCP agrees with SOC in that the tropical Atlantic sector gains heat from the atmosphere on the annual mean basis, where the ERA-40 and the NCEP–NCAR model reanalyses indicate that positive Qnet occurs only in the narrow equatorial band and in the eastern portion of the tropical basin. Nevertheless, seasonal variances of the Qnet from OAFlux + ISCCP and ERA-40 are very similar once the respective mean is removed, which explains why the two agree with each other in accounting for the seasonal variability of SST. In summary, the study suggests that an accurate estimation of surface heat flux is crucially important for understanding and predicting SST fluctuations in the tropical Atlantic Ocean. It also suggests that future emphasis on improving the surface heat flux estimation should be placed more on reducing the mean bias.
    Description: This study is support by the NOAA CLIVAR Atlantic under Grant NA06GP0453 and NOAA Climate observations and Climate Change and Data Detection under Grant NA17RJ1223.
    Keywords: Sea surface temperature ; Surface fluxes ; Seasonal variability
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    Annual, seasonal, and interannual variability of air-sea heat fluxes in the Indian Ocean (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 20 (2007): 3190-3209, doi:10.1175/JCLI4163.1.
    Description: This study investigated the accuracy and physical representation of air–sea surface heat flux estimates for the Indian Ocean on annual, seasonal, and interannual time scales. Six heat flux products were analyzed, including the newly developed latent and sensible heat fluxes from the Objectively Analyzed Air–Sea Heat Fluxes (OAFlux) project and net shortwave and longwave radiation results from the International Satellite Cloud Climatology Project (ISCCP), the heat flux analysis from the Southampton Oceanography Centre (SOC), the National Centers for Environmental Prediction reanalysis 1 (NCEP1) and reanalysis-2 (NCEP2) datasets, and the European Centre for Medium-Range Weather Forecasts operational (ECMWF-OP) and 40-yr Re-Analysis (ERA-40) products. This paper presents the analysis of the six products in depicting the mean, the seasonal cycle, and the interannual variability of the net heat flux into the ocean. Two time series of in situ flux measurements, one taken from a 1-yr Arabian Sea Experiment field program and the other from a 1-month Joint Air–Sea Monsoon Interaction Experiment (JASMINE) field program in the Bay of Bengal were used to evaluate the statistical properties of the flux products over the measurement periods. The consistency between the six products on seasonal and interannual time scales was investigated using a standard deviation analysis and a physically based correlation analysis. The study has three findings. First of all, large differences exist in the mean value of the six heat flux products. Part of the differences may be attributable to the bias in the numerical weather prediction (NWP) models that underestimates the net heat flux into the Indian Ocean. Along the JASMINE ship tracks, the four NWP modeled mean fluxes all have a sign opposite to the observations, with NCEP1 being underestimated by 53 W m−2 (the least biased) and ECMWF-OP by 108 W m−2 (the most biased). At the Arabian Sea buoy site, the NWP mean fluxes also have an underestimation bias, with the smallest bias of 26 W m−2 (ERA-40) and the largest bias of 69 W m−2 (NCEP1). On the other hand, the OAFlux+ISCCP has the best comparison at both measurement sites. Second, the bias effect changes with the time scale. Despite the fact that the mean is biased significantly, there is no major bias in the seasonal cycle of all the products except for ECMWF-OP. The latter does not have a fixed mean due to the frequent updates of the model platform. Finally, among the four products (OAFlux+ISCCP, ERA-40, NCEP1, and NCEP2) that can be used for studying interannual variability, OAFlux+ISCCP and ERA-40 Qnet have good consistency as judged from both statistical and physical measures. NCEP1 shows broad agreement with the two products, with varying details. By comparison, NCEP2 is the least representative of the Qnet variabilities over the basin scale.
    Description: This work is supported by the NOAA Office of Climate Observation and the Office of Climate Change and Data Detection under Grant NA17RJ1223.
    Keywords: Indian Ocean ; Interannual variability ; Seasonal variability ; Fluxes ; Air-sea interaction
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
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    Variations of the global net air–sea heat flux during the “hiatus” period (2001–10) (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 29 (2016): 3647-3660, doi:10.1175/JCLI-D-15-0626.1.
    Description: An assessment is made of the mean and variability of the net air–sea heat flux, Qnet, from four products (ECCO, OAFlux–CERES, ERA-Interim, and NCEP1) over the global ice-free ocean from January 2001 to December 2010. For the 10-yr “hiatus” period, all products agree on an overall net heat gain over the global ice-free ocean, but the magnitude varies from 1.7 to 9.5 W m−2. The differences among products are particularly large in the Southern Ocean, where they cannot even agree on whether the region gains or loses heat on the annual mean basis. Decadal trends of Qnet differ significantly between products. ECCO and OAFlux–CERES show almost no trend, whereas ERA-Interim suggests a downward trend and NCEP1 shows an upward trend. Therefore, numerical simulations utilizing different surface flux forcing products will likely produce diverged trends of the ocean heat content during this period. The downward trend in ERA-Interim started from 2006, driven by a peculiar pattern change in the tropical regions. ECCO, which used ERA-Interim as initial surface forcings and is constrained by ocean dynamics and ocean observations, corrected the pattern. Among the four products, ECCO and OAFlux–CERES show great similarities in the examined spatial and temporal patterns. Given that the two estimates were obtained using different approaches and based on largely independent observations, these similarities are encouraging and instructive. It is more likely that the global net air–sea heat flux does not change much during the so-called hiatus period.
    Description: This paper is funded in part by the NOAA Climate Observation Division, Climate Program Office, under Grant NA09OAR4320129 and by the NOAA MAPP Climate Reanalysis Task Force Team under Grant NA13OAR4310106. The study was initiated when X. Liang was a postdoc at MIT, where he was supported in part by the NSF through Grant OCE-0961713, by NOAA through Grant NA10OAR4310135, and by the NASA Physical Oceanography Program through ECCO.
    Description: 2016-11-15
    Keywords: Physical Meteorology and Climatology ; Heat budgets/fluxes ; Surface fluxes ; Models and modeling ; Reanalysis data ; Variability ; Climate variability ; Interannual variability ; Seasonal variability
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Air-sea interaction regimes in the sub-Antarctic Southern Ocean and Antarctic marginal ice zone revealed by icebreaker measurements (2017)
    John Wiley & Sons
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    Publication Date: 2022-05-25
    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
    Keywords: Air-sea interaction ; Sub-Antarctic Southern Ocean ; Antarctic marginal ice zone ; Icebreaker measurements
    Repository Name: Woods Hole Open Access Server
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  • 6
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    A global relationship between the ocean water cycle and near-surface salinity (2011)
    American Geophysical Union
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    Publication Date: 2022-05-25
    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.
    Keywords: Air-sea interaction ; Ocean salinity ; Water cycle ; Upper ocean and mixed layer processes
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Surface heat budget in the Southern Ocean from 42 degrees S to the Antarctic marginal ice zone: Four atmospheric reanalyses versus icebreaker Aurora Australis measurements (2019)
    Norsk Polarinstitutt
    Details
    Publication Date: 2022-10-26
    Description: © The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Yu, L., Jin, X., & Schulz, E. W. Surface heat budget in the Southern Ocean from 42 degrees S to the Antarctic marginal ice zone: Four atmospheric reanalyses versus icebreaker Aurora Australis measurements. Polar Research, 38, (2019): 3349, doi:10.33265/polar.v38.3349.
    Description: Surface heat fluxes from four atmospheric reanalyses in the Southern Ocean are evaluated using air–sea measurements obtained from the Aurora Australis during off-winter seasons in 2010–12. The icebreaker tracked between Hobart, Tasmania (ca. 42°S), and the Antarctic continent, providing in situ benchmarks for the surface energy budget change in the Subantarctic Southern Ocean (58–42°S) and the eastern Antarctic marginal ice zone (MIZ, 68–58°S). We find that the reanalyses show a high-level agreement among themselves, but this agreement reflects a universal bias, not a “truth.” Downward shortwave radiation (SW↓) is overestimated (warm biased) and downward longwave radiation (LW↓) is underestimated (cold biased), an indication that the cloud amount in all models is too low. The ocean surface in both regimes shows a heat gain from the atmosphere when averaged over the seven months (October–April). However, the ocean heat gain in reanalyses is overestimated by 10–36 W m−2 (80–220%) in the MIZ but underestimated by 6–20 W m−2 (7–25%) in the Subantarctic. The biases in SW↓ and LW↓ cancel out each other in the MIZ, causing the surface heat budget to be dictated by the underestimation bias in sensible heat loss. These reanalyses biases affect the surface energy budget in the Southern Ocean by meaningfully affecting the timing of the seasonal transition from net heat gain to net heat loss at the surface and the relative strength of SW↓ at different regimes in summer, when the length-of-day effect can lead to increased SW↓ at high latitudes.
    Description: The study is supported by the NOAA Climate Observation Division grant NA14OAR4320158 and NOAA Modeling, Analysis, Predictions, and Projections Program’s Climate Reanalysis Task Force through grant no. NA13OAR4310106.
    Keywords: Surface fluxes ; Surface energy budget ; Overestimation bias ; Underestimation bias ; Surface meteorology ; Icebreaker-based meteorological measurements
    Repository Name: Woods Hole Open Access Server
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  • 8
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    Closing the water cycle from observations across scales: where do we stand? (2022)
    American Meteorological Society
    Details
    Publication Date: 2022-05-27
    Description: Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 102(10), (2021): E1897–E1935, https://doi.org/10.1175/BAMS-D-19-0316.1.
    Description: Life on Earth vitally depends on the availability of water. Human pressure on freshwater resources is increasing, as is human exposure to weather-related extremes (droughts, storms, floods) caused by climate change. Understanding these changes is pivotal for developing mitigation and adaptation strategies. The Global Climate Observing System (GCOS) defines a suite of essential climate variables (ECVs), many related to the water cycle, required to systematically monitor Earth’s climate system. Since long-term observations of these ECVs are derived from different observation techniques, platforms, instruments, and retrieval algorithms, they often lack the accuracy, completeness, and resolution, to consistently characterize water cycle variability at multiple spatial and temporal scales. Here, we review the capability of ground-based and remotely sensed observations of water cycle ECVs to consistently observe the hydrological cycle. We evaluate the relevant land, atmosphere, and ocean water storages and the fluxes between them, including anthropogenic water use. Particularly, we assess how well they close on multiple temporal and spatial scales. On this basis, we discuss gaps in observation systems and formulate guidelines for future water cycle observation strategies. We conclude that, while long-term water cycle monitoring has greatly advanced in the past, many observational gaps still need to be overcome to close the water budget and enable a comprehensive and consistent assessment across scales. Trends in water cycle components can only be observed with great uncertainty, mainly due to insufficient length and homogeneity. An advanced closure of the water cycle requires improved model–data synthesis capabilities, particularly at regional to local scales.
    Description: WD acknowledges ESA’s QA4EO (ISMN) and CCI Soil Moisture projects. WD, CRV, AG, and KL acknowledge the G3P project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement 870353. MIH and MS acknowledge ESA’s CCI Water Vapour project. MS and RH acknowledges the support by the EUMETSAT member states through CM SAF. DGM acknowledges support from the European Research Council (ERC) under Grant Agreement 715254 (DRY–2–DRY). Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).
    Description: 2022-04-01
    Keywords: Hydrologic cycle ; Satellite observations ; Surface fluxes ; Surface observations ; Water masses/storage ; Water budget/balance
    Repository Name: Woods Hole Open Access Server
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