Improving Sea Level Reconstructions Using Non-Sea Level Measurements

We present a new method for reconstructing sea level involving cyclostationary empirical orthogonal functions (CSEOFs). While we show results from a CSEOF reconstruction using basis functions computed from satellite altimetry and subsequently fit to tide gauge data, our focus is on how other ocean observations such as sea surface temperature can be leveraged to create an improved reconstructed sea level data set spanning the time period from 1900 to present. Basis functions are computed using satellite measurements of sea surface temperature, and using a simple regression technique, these basis functions are transformed to represent a similar temporal evolution to corresponding satellite altimeter-derived sea level basis functions. The resulting sea level and sea surface temperature basis functions are fit to tide gauge data and historical sea surface temperature data, respectively, to produce a reconstructed sea level data set spanning the period from 1900 to present. We demonstrate the use of this reconstructed data set for climate monitoring, focusing primarily on climate signals in the Pacific Ocean. The CSEOF reconstruction technique can be used to create indices computed solely from sea level measurements for monitoring signals such as the eastern Pacific (EP) El Nio-Southern Oscillation (ENSO), Central Pacific (CP) ENSO, and Pacific Decadal Oscillation (PDO). The EP ENSO, CP ENSO, and PDO signals are all well represented in the CSEOF reconstruction relying solely on sea level measurements from 1950 to present; however, significant improvement can be made in reconstructing these signals during the first half of the twentieth century by including sea surface temperature measurements in the sea level reconstruction procedure

Cyclostationary Empirical Orthogonal Function Sea-Level Reconstruction

Since 1993, satellite altimetry has provided accurate measurements of sea surface height with near-global coverage. These measurements led to the first definitive estimates of global mean sea-level (GMSL) rise and have improved understanding of how sea levels are changing regionally at decadal time scales. These relatively short records, however, provide no information about the state of the ocean prior to 1993, and with the modern altimetry record spanning only 20 years, the lower frequency signals that are known to be present in the ocean are difficult or impossible to resolve. Tide gauges, on the other hand, have measured sea level over the last 200 years, with some records extending back to 1807. While providing longer records, the spatial resolution of tide gauge sampling is poor, making studies of the large-scale patterns of ocean variability and estimates of GMSL difficult. Combining the satellite altimetry with the tide gauges using a technique known as sea-level reconstruction results in a data set with the record length of the tide gauges and the near-global coverage of satellite altimetry. Cyclostationary empirical orthogonal functions (CSEOFs), derived from satellite altimetry, are combined with historical sea-level measurements from tide gauges to create the Reconstructed Sea Level data set spanning from 1950 to 2009. Previous sea-level reconstructions have utilized empirical orthogonal functions (EOFs) as basis functions, but by using CSEOFs and by addressing other aspects of the reconstruction procedure, an alternative sea-level reconstruction can be computed. The resulting reconstructed sea-level data set has weekly temporal resolution and half-degree spatial resolution

Separating Decadal Global Water Cycle Variability from Sea Level Rise

Under a warming climate, amplification of the water cycle and changes in precipitation patterns over land are expected to occur, subsequently impacting the terrestrial water balance. On global scales, such changes in terrestrial water storage (TWS) will be reflected in the water contained in the ocean and can manifest as global sea level variations. Naturally occurring climate-driven TWS variability can temporarily obscure the long-term trend in sea level rise, in addition to modulating the impacts of sea level rise through natural periodic undulation in regional and global sea level. The internal variability of the global water cycle, therefore, confounds both the detection and attribution of sea level rise. Here, we use a suite of observations to quantify and map the contribution of TWS variability to sea level variability on decadal timescales. In particular, we find that decadal sea level variability centered in the Pacific Ocean is closely tied to low frequency variability of TWS in key areas across the globe. The unambiguous identification and clean separation of this component of variability is the missing step in uncovering the anthropogenic trend in sea level and understanding the potential for low-frequency modulation of future TWS impacts including flooding and drought

Reconstructing Sea Level Using Cyclostationary Empirical Orthogonal Functions

Cyclostationary empirical orthogonal functions, derived from satellite altimetry, are combined with historical sea level measurements from tide gauges to reconstruct sea level fields from 1950 through 2009. Previous sea level reconstructions have utilized empirical orthogonal functions as basis functions, but by using cyclostationary empirical orthogonal functions and by addressing other aspects of the reconstruction procedure, an alternative sea level reconstruction can be computed. The procedure introduced here is capable of capturing the annual cycle and El Nio-Southern Oscillation (ENSO) signals back to 1950, with correlations between the reconstructed ENSO signal and common ENSO indices found to be over 0.9. The regional trends computed from the new reconstruction show good agreement with the trends obtained from the satellite altimetry, but some discrepancies are seen when comparing with previous sea level reconstructions over longer time periods. The computed rate of global mean sea level rise from the reconstructed time series is 1.97 mm/yr from 1950 to 2009 and 3.22 mm/yr from 1993 to 2009

Variations in sea surface roughness induced by the 2004 Sumatra-Andaman tsunami

Observations of tsunamis away from shore are critically important for improving early warning systems and understanding of tsunami generation and propagation. Tsunamis are difficult to detect and measure in the open ocean because the wave amplitude there is much smaller than it is close to shore. Currently, tsunami observations in deep water rely on measurements of variations in the sea surface height or bottom pressure. Here we demonstrate that there exists a different observable, specifically, ocean surface roughness, which can be used to reveal tsunamis away from shore. The first detailed measurements of the tsunami effect on sea surface height and radar backscattering strength in the open ocean were obtained from satellite altimeters during passage of the 2004 Sumatra-Andaman tsunami. Through statistical analyses of satellite altimeter observations, we show that the Sumatra-Andaman tsunami effected distinct, detectable changes in sea surface roughness. The magnitude and spatial structure of the observed variations in radar backscattering strength are consistent with hydrodynamic models predicting variations in the near-surface wind across the tsunami wave front. Tsunami-induced changes in sea surface roughness can be potentially used for early tsunami detection by orbiting microwave radars and radiometers, which have broad surface coverage across the satellite ground track

Contribution of the Pacific Decadal Oscillation to Global Mean Sea Level Trends

Understanding and explaining the trend in global mean sea level (GMSL) have important implications for future projections of sea level rise. While measurements from satellite altimetry have provided accurate estimates of GMSL, the modern altimetry record has only now reached 20 years in length, making it difficult to assess the contribution of decadal to multidecadal climate signals to the global trend. Here, we use a sea level reconstruction to study the 20 year trends in sea level since 1950. In particular, we show that the Pacific Decadal Oscillation (PDO) contributes significantly to the 20 year trends in GMSL. We estimate the PDO contribution to the GMSL trend over the past 20 years to be approximately 0.49 ± 0.25 mm/year and find that removing the PDO contribution reduces the acceleration in GMSL estimated over the past 60 years. Key Points The PDO has contributed 0.49 mm/yr to the current altimetry GMSL trend The PDO has a large impact on regional and global sea level trends Reconstructions allow for the study of decadal-scale climate variability

Detection of the 2010 Chilean Tsunami Using Satellite Altimetry

Tsunamis are difficult to detect and measure in the open ocean because the wave amplitude is much smaller than it is closer to shore. An effective early warning system, however, must be able to observe an impending tsunami threat far away from the shore in order to provide the necessary lead-time for coastal inhabitants to find safety. Given the expansiveness of the ocean, sensors capable of detecting the tsunami must also have very broad areal coverage. The 2004 Sumatra-Andaman tsunami was definitively detected in the open ocean from both sea surface height and sea surface roughness measurements provided by satellite altimeters. This tsunami, however, was exceptionally strong and questions remain about the ability to use such measurements for the detection of weaker tsunamis. Here we study the 2010 Chilean tsunami and demonstrate the ability to detect the tsunami in the open ocean. Specifically, we analyze the utility of filtering in extracting the tsunami signal from sea surface height measurements, and, through the use of statistical analyses of satellite altimeter observations, we demonstrate that the 2010 Chilean tsunami induced distinct and detectable changes in sea surface roughness. While satellite altimeters do not provide the temporal and spatial coverage necessary to form the basis of an effective early warning system, tsunami-induced changes in sea surface roughness can be detected using orbiting microwave radars and radiometers, which have a broad surface coverage across the satellite ground track

Could Satellite Altimetry Have Improved Early Detection and Warning of the 2011 Tohoku Tsunami?

The 2011 Tohoku tsunami devastated Japan and affected coastal populations all around the Pacific Ocean. Accurate early warning of an impending tsunami requires the detection of the tsunami in the open ocean. While the lead-time was not sufficient for use in warning coastal populations in Japan, satellite altimetry observations of the tsunami could have been used to improve predictions and warnings for other affected areas. By comparing to both model results and historical satellite altimeter data, we use near-real-time satellite altimeter measurements to demonstrate the potential for detecting the 2011 Tohoku tsunami within a few hours of the tsunami being generated. We show how satellite altimeter data could be used to both directly detect tsunamis in the open ocean and also improve predictions made by models

The Effect of the El Nino-Southern Oscillation on U.S. Regional and Coastal Sea Level

Although much of the focus on future sea level rise concerns the long-term trend associated with anthropogenic warming, on shorter time scales, internal climate variability can contribute significantly to regional sea level. Such sea level variability should be taken into consideration when planning efforts to mitigate the effects of future sea level change. In this study, we quantify the contribution to regional sea level of the El Niño-Southern Oscillation (ENSO). Through cyclostationary empirical orthogonal function analysis (CSEOF) of the long reconstructed sea level data set and of a set of U.S. tide gauges, two global modes dominated by Pacific Ocean variability are identified and related to ENSO and, by extension, the Pacific Decadal Oscillation. By estimating the combined contribution of these two modes to regional sea level, we find that ENSO can contribute significantly on short time scales, with contributions of up to 20 cm along the west coast of the U.S. The CSEOF decomposition of the long tide gauge records around the U.S. highlights the influence of ENSO on the U.S. east coast. Tandem analyses of both the reconstructed and tide gauge records also examine the utility of the sea level reconstructions for near-coast studies