Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena

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Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena

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Mcgregor S.,Atmosphere and EnvironmentMonash UniversityClayton | Sen Gupta A.,University of New South Wales | Dommenget D.,Atmosphere and EnvironmentMonash UniversityClayton | Lee T.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena | And 2 more authors.
Journal of Geophysical Research: Oceans | Year: 2017

Given the importance of tropical Pacific winds to global climate, it is interesting to examine differences in the mean and trend among various wind products, and their implications for ocean circulation. Past analysis has revealed that despite the assimilation of observational data, there remain large differences among reanalysis products. Thus, here we examine if satellite-based synthesis products may provide more consistent estimate than reanalysis. Reanalysis product winds are, however, typically used as a background constraint in constructing the synthesis products to fill spatiotemporal gaps and to deal with satellite wind direction ambiguity. Our study identified two important factors that influence both the mean and trends from synthesized wind products. First, the choice of background wind product in synthesized satellite wind products affects the mean and long-term trends, which has implications for simulations of ocean circulation, sea level, and presumably SST. Second, we identify a clear need for developing a better understanding of, and correcting differences between in situ observations of absolute winds with the satellite-derived relative winds prior to synthesizing. This correction requires careful analysis of satellite surface winds with existing colocated in situ measurements of surface winds and currents, and will benefit from near-surface current observations of the proposed Tropical Pacific Observing System. These results also illustrate the difficulty in independently evaluating the synthesis wind products because the in situ data have been utilized at numerous steps during their development. Addressing these identified issues effectively, will require enhanced collaborations among the wind observation (both satellite and in situ), reanalysis, and synthesis communities. © 2016. American Geophysical Union.


Menemenlis D.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena | Schodlok M.,University of California at Los Angeles
Journal of Geophysical Research: Oceans | Year: 2017

Recent studies suggest that the thickness of Winter Water (WW), that is, water with potential temperature below ∼-1°C located below Antarctic Surface Water and above Circumpolar Deep Water (CDW) is critical in determining the ice shelf melt rate, especially for the Pine Island Glacier (PIG). Existing model studies, however, misrepresent WW thickness and properties in the Amundsen Sea (AS). Here, we adjust a small number of model parameters in a regional Amundsen and Bellingshausen Seas configuration of the Massachusetts Institute of Technology general circulation model in order to reproduce properties and thickness of WW and CDW close to observations, with significant improvement for WW compared to previous studies. The cost, which is defined as weighted model-data difference squared, is reduced by 23%. Although a previous modeling study points out that the local surface heat loss upstream from Pine Island Polynya could be the reason for the observed 2012 PIG melt decline and WW thickening, they did not show WW freshening, which was observed at the same time. Model sensitivity experiments for surface heat loss, PIG melt rate, and precipitation fail to replicate WW freshening concurrent with PIG melt decline, implying that these processes cannot fully explain the observed PIG melt decrease. © 2017. American Geophysical Union. All Rights Reserved.


Bingham F.M.,University of North Carolina WilmingtonWilmington | Lee T.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena
Journal of Geophysical Research: Oceans | Year: 2017

Using Aquarius Version-4.0 data, we have investigated the time and space scales of sea surface salinity (SSS) over the global ocean between 60°S and 60°N. Decorrelation time scales of SSS were found to be divided among less than 80 days (covering 1/2 of ocean area), 80-100 days (1/3), and greater than 100 days (remainder). Once the seasonal cycle is removed, shorter time scales (less than 80 days) dominate. Spatial scales are largest in the tropics along the intertropical convergence zones of all oceans and the South Pacific convergence zone in the South Pacific. Time scales were also calculated for time-integrated (cumulative) surface freshwater forcing (CFWF) using precipitation from Tropical Rainfall Measurement Mission and evaporation from OAFlux data. These showed little spatial pattern, but a dominance of the seasonal and longer time scales over the globe. The lack of correspondence between dominant temporal and spatial scales of SSS and CFWF highlights the importance of ocean processes in regulating SSS variability. © 2017. American Geophysical Union. All Rights Reserved.


Ravichandran M.,National Institute of Oceanography of India | Wang W.,National Oceanic and Atmospheric Administration | Shinoda T.,Texas A&M University | Lee T.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena
Journal of Geophysical Research: Oceans | Year: 2017

The huge freshwater flux of the Indian summer monsoon (ISM; May-October) gives rise to strong salinity stratification in the Bay of Bengal (BoB), causing a shallow mixed layer and a thick barrier layer, which potentially affects intraseasonal oscillations of the monsoon (MISOs). In this study, intraseasonal variability of the mixed-layer depth (MLD) and barrier layer thickness (BLT) is investigated using in situ observations from Argo floats and moored buoys and an ocean general circulation model (OGCM). The average MLD in the BoB is typically 20-30 m during the ISM, while the BLT increases from ∼10 m in May-June to 20-40 m in September-October. MISOs induce in-phase variations in MLD and isothermal layer depth (ILD), both of which are deepened by 8-15 m during MISO active phase, while the change of BLT is small and within the error range of Argo data sampling. In the northern (southern) bay, BLT increases by ∼5 m (2 m) during MISOs owing to a larger deepening of ILD than MLD. OGCM experiments are performed to understand the underlying mechanism. In the BoB intraseasonal variations of MLD, ILD and BLT arise largely from ocean internal instability, whereas those induced by MISOs are weaker. The in-phase variations of MLD and ILD during MISOs are induced by different processes. The MLD deepening is primarily caused by wind stress forcing, while the ILD deepening is driven by surface heat fluxes via surface cooling. The limited variability of BLT is due to the offsetting of different forcing processes. © 2017. American Geophysical Union. All Rights Reserved.


Song Y.T.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena
Journal of Geophysical Research: Oceans | Year: 2017

Conventional tsunami theories suggest that earthquakes with significant vertical motions are more likely to generate tsunamis. In tsunami models, the vertical seafloor elevation is directly transferred to the sea-surface as the only initial condition. However, evidence from the 2011 Tohoku earthquake indicates otherwise; the vertical seafloor uplift was only 3-5 m, too small to account for the resultant tsunami. Surprisingly, the horizontal displacement was undeniably larger than anyone's expectation; about 60 m at the frontal wedge of the fault plate, the largest slip ever recorded by in situ instruments. The question is whether the horizontal motion of seafloor slopes had enhanced the tsunami to become as destructive as observed. In this study, we provide proof: (1) combining various measurements from the 2011 Tohoku event, we show that the earthquake transferred a total energy of 3.1e+15 joule to the ocean, in which the potential energy (PE) due to the vertical seafloor elevation (including seafloor uplift/subsidence plus the contribution from the horizontal displacement) was less than a half, while the kinetic energy (KE) due to the horizontal displacement velocity of the continental slope contributed a majority portion; (2) using two modern state-of-the-art wave flumes and a three-dimensional tsunami model, we have reproduced the source energy and tsunamis consistent with observations, including the 2004 Sumatra event. Based on the unified source energy formulation, we offer a competing theory to explain why some earthquakes generate destructive tsunamis, while others do not. © 2017. American Geophysical Union. All Rights Reserved.


Song Y.T.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena
Journal of Geophysical Research C: Oceans | Year: 2015

The Indonesian throughflow (ITF) from the Pacific to the Indian Ocean plays an important role in global ocean circulation and climate. Yet, continuous ITF measurement is difficult and expensive. The longest time series of in situ measurements of the ITF to date were taken in the Makassar Strait, the main pathway of the ITF. Here we have demonstrated a plausible approach to derive the ITF transport proxy using satellite altimetry sea surface height (SSH), gravimetry ocean bottom pressure (OBP) data, in situ measurements from the Makassar Strait from 1996 to 1998 and 2004 to 2011, and a theoretical formulation. We first identified the optimal locations of the correlation between the observed ITF transport through the Makassar Strait and the pressure gradients, represented by the SSH and OBP differences between the Pacific and Indian Oceans at a 1° × 1° horizontal resolution. The optimal locations were found centered at 162°E and 11°N in the Pacific Ocean and 80°E and 0° in the Indian Ocean, then were used in the theoretical formulation to estimate the throughflow. The proxy time series follow the observation time series quite well, with the 1993-2011 mean proxy transport of 11.6±3.2 Sv southward, varying from 5.6 Sv during the strong 1997 El Niño to 16.9 Sv during the 2007 La Nina period, which are consistent with previous estimates. The observed Makassar mean transport is 13.0±3.8 Sv southward over 2004-2011, while the SSH proxy (for the same period) gives an ITF mean transport of 13.9±4.1 Sv and the SSH+OBP proxy (for 2004-2010) is 15.8±3.2 Sv. © 2015. American Geophysical Union. All Rights Reserved.


Landerer F.W.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena | Wiese D.N.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena
Water Resources Research | Year: 2016

Basin-scale groundwater storage trends calculated from long-term streamflow records provide insight into the evolution of watershed behaviors. Our study presents the first spatially relevant validation of recession-based trend approaches by comparing three independent storage trend estimates using GRACE-derived groundwater storage, in situ groundwater elevation observations, and recession-based approaches for the time period of 2003-2015. Results documented consistent agreement between spatially interpolated groundwater observation trends and recession-based storage trends, while GRACE-derived groundwater trends were found to exhibit variable, poor comparisons. A decreasing trend in watershed storage was identified in the southeastern U.S. while increasing trends were identified in the northeast and upper Midwest estimated from recession-based approaches. Our recession-based approach conducted using nested watershed streamflow records identified variable watershed storage trends at scales directly applicable for comparative hydrology studies and for assisting in watershed-based water resources management decisions. © 2016. American Geophysical Union.


Kwok R.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena | Morison J.,Applied Physics LaboratoryUniversity of Washington Seattle 98105
Journal of Geophysical Research: Oceans | Year: 2016

We examine 4 years (2011-2014) of sea surface heights (SSH) from CryoSat-2 (CS-2) over the ice-covered Arctic and Southern Oceans. Results are from a procedure that identifies and determines the heights of sea surface returns. Along 25 km segments of satellite ground tracks, variability in the retrieved SSHs is between ∼2 and 3 cm (standard deviation) in the Arctic and is slightly higher (∼3 cm) in the summer and the Southern Ocean. Average sea surface tilts (along these 25 km segments) are 0.01±3.8 cm/10 km in the Arctic, and slightly lower (0.01±2.0 cm/10 km) in the Southern Ocean. Intra-seasonal variability of CS-2 dynamic ocean topography (DOT) in the ice-covered Arctic is nearly twice as high as that of the Southern Ocean. In the Arctic, we find a correlation of 0.92 between 3 years of DOT and dynamic heights (DH) from hydrographic stations. Further, correlation of 4 years of area-averaged CS-2 DOT near the North Pole with time-variable ocean-bottom pressure from a pressure gauge and from GRACE, yields coefficients of 0.83 and 0.77, with corresponding differences of <3 cm (RMS). These comparisons contrast the length scale of baroclinic and barotropic features and reveal the smaller amplitude barotropic signals in the Arctic Ocean. Broadly, the mean DOT from CS-2 for both poles compares well with those from the ICESat campaigns and the DOT2008A and DTU13MDT fields. Short length scale topographic variations, due to oceanographic signals and geoid residuals, are especially prominent in the Arctic Basin but less so in the Southern Ocean. Key Points:: Time-varying dynamic topography of ice-covered oceans from CryoSat-2 Assessment with dynamic height from hydrographic stations and ocean bottom pressure Relative length scale and amplitude of baroclinic and barotropic signals in Arctic Ocean © 2015. American Geophysical Union. All Rights Reserved. January 2016 10.1002/2015JC011357 Research Article Research Articles © 2015. American Geophysical Union.


Wiese D.N.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena | Killett B.,Pasadena California United States | Watkins M.M.,University of Texas at AustinAustin | Yuan D.-N.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena
Journal of Geophysical Research: Oceans | Year: 2016

The extended length of the GRACE data time series (now 13.5 years) provides the unique opportunity to estimate global mass variations due to ocean tides at large (∼300 km) spatial scales. State-of-the-art global tide models rely heavily on satellite altimetry data, which are sparse for latitudes higher than 66°. Thus, the performance of the models is typically worse at higher latitudes. GRACE data, alternately, extend to polar latitudes and therefore provide information for both model validation and improvement at the higher latitudes. In this work, 11 years of GRACE inter-satellite range-acceleration measurements are inverted to solve for corrections to the amplitudes and phases of the major solar and lunar ocean tidal constituents (M2, K1, S2, and O1) from the GOT4.7 ocean tide model at latitudes south of 50°S. Two independent inversion and regularization methods are employed and compared against one another. Uncertainty estimates are derived by subtracting two independent solutions, each spanning a unique 5.5 years of data. Features above the noise floor in the derived solutions likely represent errors in GOT4.7. We find the GOT4.7 amplitudes to be generally too small for M2 and K1, and too large for S2 and O1, and to spatially correlate with geographic regions where GOT4.7 predicts the largest tidal amplitudes. In particular, we find GOT4.7 errors to be dominant over the Patagonia shelf (M2), the Filchner-Ronne Ice Shelf (M2 and S2), the Ross Ice Shelf (S2), and the Weddell and Ross Seas (K1 and O1). © 2016. American Geophysical Union. All Rights Reserved.


Zeng L.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Liu W.T.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena | Xue H.,University of Chinese Academy of Sciences | Xiu P.,University of Chinese Academy of Sciences | Wang D.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology
Journal of Geophysical Research C: Oceans | Year: 2014

Newly available sea surface salinity (SSS) data from the Aquarius together with in situ hydrographic data are used to explore the spatial and temporal characteristics of SSS in the South China Sea (SCS). Using in situ observations as the reference, an evaluation of daily Aquarius data indicates that there exists a negative bias of 0.45 psu for the version 3.0 data set. The root-mean-square difference for daily Aquarius SSS is about 0.53 psu after correcting the systematic bias, and those for weekly and monthly Aquarius SSSs are 0.45 and 0.29 psu, respectively. Nevertheless, the Aquarius SSS shows a reliable freshening in the SCS in 2012, which is larger than the Aquarius uncertainty. The freshening of up to 0.4 psu in the upper-ocean of the northern SCS was confirmed by in situ observations. This freshening in 2012 was caused by a combined effect of abundant local freshwater flux and limited Kuroshio intrusion. By comparing the Kuroshio intrusion in 2012 with that in 2011, we found the reduction as a relatively important cause for the freshening over the northern SCS. In contrast to the northern SCS, reduced river discharge in 2012 played the leading role to the saltier surface in the region near the Mekong River mouth with respect to 2011. © 2014. American Geophysical Union. All Rights Reserved.

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