Global mean sea surface temperature (T) is a variable of primary interest in studies of climate v... more Global mean sea surface temperature (T) is a variable of primary interest in studies of climate variability and change. The temporal evolution of T can be influenced by surface heat fluxes () and by diffusion () and advection () processes internal to the ocean, but quantifying the contribution of these different factors from data alone is prone to substantial uncertainties. Here we derive a closed T budget for the period 1993-2015 based on a global ocean state estimate, which is an exact solution of a general circulation model constrained to most extant ocean observations through advanced optimization methods. The estimated average temperature of the top (10-m thick) level in the model, taken to represent T, shows relatively small variability at most time scales compared to , , or , reflecting the tendency for largely balancing effects from all the latter terms. The seasonal cycle in T is mostly determined by small imbalances between and , with negligible contributions from . While seems to simply damp at the annual period, a different dynamical role for at semiannual period is suggested by it being larger than . At periods longer than annual, contributes importantly to T variability, pointing to the direct influence of the variable ocean circulation on T and mean surface climate. Plain Language Summary Global mean sea surface temperature T is a key metric when defining the Earth's climate. Determining what controls the evolution of T is thus vital for understanding past climate variability and predicting its future evolution. Processes that control T involve forcing surface heat fluxes, as well as advection and diffusion of heat internal to the ocean, but their relative contributions are poorly known and difficult to assess from observations alone. Here we use advanced methods to combine models and data and derive a closed budget for T variability in terms of the forcing, advection, and diffusion processes. The estimated T shows relatively small variability compared to surface forcing, advection, or diffusion, reflecting the tendency for largely balancing effects from all the latter terms. The seasonal cycle in T is mostly determined by small imbalances between forcing and diffusion, with negligible contributions from advection. Diffusion does not always act as a simple damping of forcing surface fluxes, however. In addition, at periods longer than annual, advection contributes importantly to T variability. The results point to the direct influence of the variable ocean circulation on T and the Earth's surface climate.
A major challenge for managing impacts and implementing effective mitigation measures and adaptat... more A major challenge for managing impacts and implementing effective mitigation measures and adaptation strategies for coastal zones affected by future sea level (SL) rise is our limited capacity to predict SL change at the coast on relevant spatial and temporal scales. Predicting coastal SL requires the ability to monitor and simulate a multitude of physical processes affecting SL, from local effects of wind waves and river runoff to
The solar diurnal and semidiurnal tidal oscillations in surface pressure are extracted from the o... more The solar diurnal and semidiurnal tidal oscillations in surface pressure are extracted from the operational analysis product of the European Centre for Medium Range Weather Forecasting (ECMWF). For the semidiurnal tide this involves a special temporal interpolation, following Van den Dool et al. (1997). The resulting tides are compared with a ground truth tide data set, a compilation of well-determined tide estimates deduced from many long time series of station barometer measurements. These comparisons show that the ECMWF (analysis) tides are significantly more accurate than the tides deduced from two other widely available reanalysis products. Spectral analysis of ECMWF pressure series shows that the tides consist of sharp central peaks with modulating sidelines at integer multiples of 1 cycle/year, superimposed on a broad cusp of stochastic energy. The integrated energy in the cusp dominates that of the side-lines. This complicates the development of a simple empirical model that can characterize the full temporal variability of the tides.
The possibility of El Niño being a result of the random superposition of stochastically forced eq... more The possibility of El Niño being a result of the random superposition of stochastically forced equatorial Kelvin waves is investigated, with the help of the theory of statistics of extremes, which provides us with the tools to evaluate the threshold crossing statistics of the sea level (SL) and sea surface temperature (SST) anomaly fields. Knowledge of the probability density function and the power spectrum of a random process allows one to compute the threshold crossing statistics of the process (e.g;, the mean rate of crossings of some level, the mean time or distance between successive crossings, or the mean time or distance it stays above some level after each crossing). A linear, stratified, equatorial β plane Kelvin wave ocean model is forced by stochastic zonal winds. The zonal velocity field is used to advect mean zonal temperature gradients to produce SST anomalies. Solutions are obtained in terms of the zonal wave number-frequency spectra of SL and SST. These spectra are used to compute the threshold crossing statistics of the respective fields. The results are sensitive to some of the assumptions (e.g., spectral shape of forcing and values of spectral cutoffs). The time and space scales for the SL and SST excursions above 2 standard deviations are found to be small, when compared with observed El Niño scales. The stochastic assumption is reinterpreted as a possible triggering mechanism, rather than as a full explanation, for El Niño occurrences. Better knowledge of the wind stress forcing spectrum (zonal wave number and frequency) is needed in order to test the plausibility of the stochastic argument in a more conclusive way.
Http Dx Doi Org 10 1175 Jcli D 13 00316 1, Jun 24, 2014
ABSTRACT A recent state estimate covering the period 1992-2010 from the Estimating the Circulatio... more ABSTRACT A recent state estimate covering the period 1992-2010 from the Estimating the Circulation and Climate of the Ocean (ECCO) project is utilized to quantify the upper-ocean heat budget in the North Atlantic on monthly to interannual time scales (seasonal cycle removed). Three novel techniques are introduced: 1) the heat budget is integrated over the maximum climatological mixed layer depth (integral denoted as H), which gives results that are relevant for explaining SST while avoiding strong contributions from vertical diffusion and entrainment; 2) advective convergences are separated into Ekman and geostrophic parts, a technique that is successful away from ocean boundaries; and 3) air sea heat fluxes and Ekman advection are combined into one local forcing term. The central results of our analysis are as follows: 1) In the interior of subtropical gyre, local forcing explains the majority of H variance on all time scales resolved by the ECCO estimate. 2) In the Gulf Stream region, low-frequency H anomalies are forced by geostrophic convergences and damped by air sea heat fluxes. 3) In the interior of the subpolar gyre, diffusion and bolus transports play a leading order role in H variability, and these transports are correlated with low-frequency variability in wintertime mixed layer depths.
The possibility of El Nifio being a result of the random superposition of stochastically forced e... more The possibility of El Nifio being a result of the random superposition of stochastically forced equatorial Kelvin waves is investigated, with the help of the theory of statistics of extremes, which provides us with the tools to evaluate the threshold crossing statistics of the sea level (SL) and sea surface temperature (SST) anomaly fields. Knowledge of the probability density function and the power spectrum of a random process allows one to compute the threshold crossing statistics of the process (e.g:, the mean rate of crossings of some level, the mean time or distance between successive crossings, or the mean time or distance it stays above some level after each crossing). A linear, stratified, equatorial/• plane Kelvin wave ocean model is forced by stochastic zonal winds. The zonal velocity field is used to advect mean zonal temperature gradients to produce SST anomalies. Solutions are obtained in terms of the zonal wave number-frequency spectra of SL and SST. These spectra are used to compute the threshold crossing statistics of the respective fields. The results are sensitive to some of the assumptions (e.g., spectral shape of forcing and values of spectral cutoffs). The time and space scales for the SL and SST excursions above 2 standard deviations are found to be small, when compared with observed El Nifio scales. The stochastic assumption is reinterpreted as a possible triggering mechanism, rather than as a full explanation, for El Nifio occurrences. Better knowledge of the wind stress forcing spectrum (zonal wave number and frequency) is needed in order to test the plausibility of the stochastic argument in a more conclusive way.
Http Dx Doi Org 10 1175 Jpo D 14 0150 1, Mar 1, 2015
ABSTRACT The oceanic response to surface loading, such as that related to atmospheric pressure, f... more ABSTRACT The oceanic response to surface loading, such as that related to atmospheric pressure, freshwater exchange, and changes in the gravity field, is essential to our understanding of sea level variability. In particular, so-called self-attraction and loading (SAL) effects caused by the redistribution of mass within the land-atmosphere-ocean system can have a measurable impact on sea level. In this study, the nature of SAL-induced variability in sea level is examined in terms of its equilibrium (static) and nonequilibrium (dynamic) components, using a general circulationmodel that implicitly includes the physics ofSAL. The additional SAL forcing is derived by decomposing ocean mass anomalies into spherical harmonics and then applying Love numbers to infer associated crustal displacements and gravitational shifts. This implementation of SALphysics incurs only a relatively small computational cost. Effects of SAL on sea level amount to about 10% of the applied surface loading on average but depend strongly on location. The dynamic component exhibits large-scale basinwide patterns, with considerable contributions from subweekly time scales. Departures from equilibrium decrease toward longer time scales but are not totally negligible in many places. Ocean modeling studies should benefit from using a dynamical implementation of SAL as used here.
Atmospheric angular momentum (AAM) reached extremely high values during the large 1982-83 El Niño... more Atmospheric angular momentum (AAM) reached extremely high values during the large 1982-83 El Niño event. The mechanisms responsible for the anomalously high AAM are examined using mountain torque (m) and friction torque (f) time series computed from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalyses. AAM anomalies, defined with respect to a 29-yr climatology (1968-96), are mostly positive from mid-1982 onward, but notably they double in amplitude over a 2-week period in early 1983. Analysis of the torque series reveals that this sharp AAM increase is mostly related to anomalies in m, primarily associated with American and Eurasian orography. After reaching its peak value in January, AAM anomalies decay slowly to near-normal values over the next three months, with anomalies in f, especially over the subtropical North Pacific, playing a dominant role in this downturn. The relevant anomalies in m and f are discussed in relation to rapid synoptic-scale variability and longer-term, large-scale anomalous patterns in surface pressure and winds that characterized this El Niño event.
Anomalies in the angular momentum of the atmosphere (M) during the 1982-83 El Niño event and the ... more Anomalies in the angular momentum of the atmosphere (M) during the 1982-83 El Niño event and the torques responsible for these anomalies are investigated using output from the Canadian Climate Centre general circulation model. Model values of M during the year of the event are generally larger than those for the model climatology, thereby capturing the observed tendency toward higher values of M during El Niñto. Differences exist between the model and observations in the timing and amplitude of the largest anomalies, but these differences may he due to natural variability and not necessarily directly associated with the 1982-83 El Niño conditions.In late September and October 1982, the model atmosphere acquires momentum more rapidly than usual, leading to the development of the largest deviations from mean conditions at the end of this period, mostly associated with strong westerly momentum signals centered at 25°N. Large, sustained positive anomalies in tangential stress torques over the northern tropics are the major mechanism responsible for the modeled increase in M, but mountain torque anomalies centered at 35°N are also important at the end of October. A secondary maximum in the departure from mean M values occurs in January 1983 and is related to a general strengthening of westerly momentum anomalies over the model's tropical and midlatitude regions. Both mountain and tangential stress torques are involved in this episode, but no particular mechanism or region dominates the anomalous exchange of momentum.
ABSTRACT Following on the heels of the World Ocean Circulation Experiment, the Estimating the Cir... more ABSTRACT Following on the heels of the World Ocean Circulation Experiment, the Estimating the Circulation and Climate of the Ocean (ECCO) consortium has been directed at making the best possible estimates of ocean circulation and its role in climate. ECCO is combining state-of-the-art ocean general circulation models with the nearly complete global ocean data sets for 1992 to present. Solutions are now available that adequately fit almost all types of ocean observations and that are, simultaneously, consistent with the model. These solutions are being applied to understanding ocean variability, biological cycles, coastal physics, geodesy, and many other areas.
Global mean sea surface temperature (T) is a variable of primary interest in studies of climate v... more Global mean sea surface temperature (T) is a variable of primary interest in studies of climate variability and change. The temporal evolution of T can be influenced by surface heat fluxes () and by diffusion () and advection () processes internal to the ocean, but quantifying the contribution of these different factors from data alone is prone to substantial uncertainties. Here we derive a closed T budget for the period 1993-2015 based on a global ocean state estimate, which is an exact solution of a general circulation model constrained to most extant ocean observations through advanced optimization methods. The estimated average temperature of the top (10-m thick) level in the model, taken to represent T, shows relatively small variability at most time scales compared to , , or , reflecting the tendency for largely balancing effects from all the latter terms. The seasonal cycle in T is mostly determined by small imbalances between and , with negligible contributions from . While seems to simply damp at the annual period, a different dynamical role for at semiannual period is suggested by it being larger than . At periods longer than annual, contributes importantly to T variability, pointing to the direct influence of the variable ocean circulation on T and mean surface climate. Plain Language Summary Global mean sea surface temperature T is a key metric when defining the Earth's climate. Determining what controls the evolution of T is thus vital for understanding past climate variability and predicting its future evolution. Processes that control T involve forcing surface heat fluxes, as well as advection and diffusion of heat internal to the ocean, but their relative contributions are poorly known and difficult to assess from observations alone. Here we use advanced methods to combine models and data and derive a closed budget for T variability in terms of the forcing, advection, and diffusion processes. The estimated T shows relatively small variability compared to surface forcing, advection, or diffusion, reflecting the tendency for largely balancing effects from all the latter terms. The seasonal cycle in T is mostly determined by small imbalances between forcing and diffusion, with negligible contributions from advection. Diffusion does not always act as a simple damping of forcing surface fluxes, however. In addition, at periods longer than annual, advection contributes importantly to T variability. The results point to the direct influence of the variable ocean circulation on T and the Earth's surface climate.
A major challenge for managing impacts and implementing effective mitigation measures and adaptat... more A major challenge for managing impacts and implementing effective mitigation measures and adaptation strategies for coastal zones affected by future sea level (SL) rise is our limited capacity to predict SL change at the coast on relevant spatial and temporal scales. Predicting coastal SL requires the ability to monitor and simulate a multitude of physical processes affecting SL, from local effects of wind waves and river runoff to
The solar diurnal and semidiurnal tidal oscillations in surface pressure are extracted from the o... more The solar diurnal and semidiurnal tidal oscillations in surface pressure are extracted from the operational analysis product of the European Centre for Medium Range Weather Forecasting (ECMWF). For the semidiurnal tide this involves a special temporal interpolation, following Van den Dool et al. (1997). The resulting tides are compared with a ground truth tide data set, a compilation of well-determined tide estimates deduced from many long time series of station barometer measurements. These comparisons show that the ECMWF (analysis) tides are significantly more accurate than the tides deduced from two other widely available reanalysis products. Spectral analysis of ECMWF pressure series shows that the tides consist of sharp central peaks with modulating sidelines at integer multiples of 1 cycle/year, superimposed on a broad cusp of stochastic energy. The integrated energy in the cusp dominates that of the side-lines. This complicates the development of a simple empirical model that can characterize the full temporal variability of the tides.
The possibility of El Niño being a result of the random superposition of stochastically forced eq... more The possibility of El Niño being a result of the random superposition of stochastically forced equatorial Kelvin waves is investigated, with the help of the theory of statistics of extremes, which provides us with the tools to evaluate the threshold crossing statistics of the sea level (SL) and sea surface temperature (SST) anomaly fields. Knowledge of the probability density function and the power spectrum of a random process allows one to compute the threshold crossing statistics of the process (e.g;, the mean rate of crossings of some level, the mean time or distance between successive crossings, or the mean time or distance it stays above some level after each crossing). A linear, stratified, equatorial β plane Kelvin wave ocean model is forced by stochastic zonal winds. The zonal velocity field is used to advect mean zonal temperature gradients to produce SST anomalies. Solutions are obtained in terms of the zonal wave number-frequency spectra of SL and SST. These spectra are used to compute the threshold crossing statistics of the respective fields. The results are sensitive to some of the assumptions (e.g., spectral shape of forcing and values of spectral cutoffs). The time and space scales for the SL and SST excursions above 2 standard deviations are found to be small, when compared with observed El Niño scales. The stochastic assumption is reinterpreted as a possible triggering mechanism, rather than as a full explanation, for El Niño occurrences. Better knowledge of the wind stress forcing spectrum (zonal wave number and frequency) is needed in order to test the plausibility of the stochastic argument in a more conclusive way.
Http Dx Doi Org 10 1175 Jcli D 13 00316 1, Jun 24, 2014
ABSTRACT A recent state estimate covering the period 1992-2010 from the Estimating the Circulatio... more ABSTRACT A recent state estimate covering the period 1992-2010 from the Estimating the Circulation and Climate of the Ocean (ECCO) project is utilized to quantify the upper-ocean heat budget in the North Atlantic on monthly to interannual time scales (seasonal cycle removed). Three novel techniques are introduced: 1) the heat budget is integrated over the maximum climatological mixed layer depth (integral denoted as H), which gives results that are relevant for explaining SST while avoiding strong contributions from vertical diffusion and entrainment; 2) advective convergences are separated into Ekman and geostrophic parts, a technique that is successful away from ocean boundaries; and 3) air sea heat fluxes and Ekman advection are combined into one local forcing term. The central results of our analysis are as follows: 1) In the interior of subtropical gyre, local forcing explains the majority of H variance on all time scales resolved by the ECCO estimate. 2) In the Gulf Stream region, low-frequency H anomalies are forced by geostrophic convergences and damped by air sea heat fluxes. 3) In the interior of the subpolar gyre, diffusion and bolus transports play a leading order role in H variability, and these transports are correlated with low-frequency variability in wintertime mixed layer depths.
The possibility of El Nifio being a result of the random superposition of stochastically forced e... more The possibility of El Nifio being a result of the random superposition of stochastically forced equatorial Kelvin waves is investigated, with the help of the theory of statistics of extremes, which provides us with the tools to evaluate the threshold crossing statistics of the sea level (SL) and sea surface temperature (SST) anomaly fields. Knowledge of the probability density function and the power spectrum of a random process allows one to compute the threshold crossing statistics of the process (e.g:, the mean rate of crossings of some level, the mean time or distance between successive crossings, or the mean time or distance it stays above some level after each crossing). A linear, stratified, equatorial/• plane Kelvin wave ocean model is forced by stochastic zonal winds. The zonal velocity field is used to advect mean zonal temperature gradients to produce SST anomalies. Solutions are obtained in terms of the zonal wave number-frequency spectra of SL and SST. These spectra are used to compute the threshold crossing statistics of the respective fields. The results are sensitive to some of the assumptions (e.g., spectral shape of forcing and values of spectral cutoffs). The time and space scales for the SL and SST excursions above 2 standard deviations are found to be small, when compared with observed El Nifio scales. The stochastic assumption is reinterpreted as a possible triggering mechanism, rather than as a full explanation, for El Nifio occurrences. Better knowledge of the wind stress forcing spectrum (zonal wave number and frequency) is needed in order to test the plausibility of the stochastic argument in a more conclusive way.
Http Dx Doi Org 10 1175 Jpo D 14 0150 1, Mar 1, 2015
ABSTRACT The oceanic response to surface loading, such as that related to atmospheric pressure, f... more ABSTRACT The oceanic response to surface loading, such as that related to atmospheric pressure, freshwater exchange, and changes in the gravity field, is essential to our understanding of sea level variability. In particular, so-called self-attraction and loading (SAL) effects caused by the redistribution of mass within the land-atmosphere-ocean system can have a measurable impact on sea level. In this study, the nature of SAL-induced variability in sea level is examined in terms of its equilibrium (static) and nonequilibrium (dynamic) components, using a general circulationmodel that implicitly includes the physics ofSAL. The additional SAL forcing is derived by decomposing ocean mass anomalies into spherical harmonics and then applying Love numbers to infer associated crustal displacements and gravitational shifts. This implementation of SALphysics incurs only a relatively small computational cost. Effects of SAL on sea level amount to about 10% of the applied surface loading on average but depend strongly on location. The dynamic component exhibits large-scale basinwide patterns, with considerable contributions from subweekly time scales. Departures from equilibrium decrease toward longer time scales but are not totally negligible in many places. Ocean modeling studies should benefit from using a dynamical implementation of SAL as used here.
Atmospheric angular momentum (AAM) reached extremely high values during the large 1982-83 El Niño... more Atmospheric angular momentum (AAM) reached extremely high values during the large 1982-83 El Niño event. The mechanisms responsible for the anomalously high AAM are examined using mountain torque (m) and friction torque (f) time series computed from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalyses. AAM anomalies, defined with respect to a 29-yr climatology (1968-96), are mostly positive from mid-1982 onward, but notably they double in amplitude over a 2-week period in early 1983. Analysis of the torque series reveals that this sharp AAM increase is mostly related to anomalies in m, primarily associated with American and Eurasian orography. After reaching its peak value in January, AAM anomalies decay slowly to near-normal values over the next three months, with anomalies in f, especially over the subtropical North Pacific, playing a dominant role in this downturn. The relevant anomalies in m and f are discussed in relation to rapid synoptic-scale variability and longer-term, large-scale anomalous patterns in surface pressure and winds that characterized this El Niño event.
Anomalies in the angular momentum of the atmosphere (M) during the 1982-83 El Niño event and the ... more Anomalies in the angular momentum of the atmosphere (M) during the 1982-83 El Niño event and the torques responsible for these anomalies are investigated using output from the Canadian Climate Centre general circulation model. Model values of M during the year of the event are generally larger than those for the model climatology, thereby capturing the observed tendency toward higher values of M during El Niñto. Differences exist between the model and observations in the timing and amplitude of the largest anomalies, but these differences may he due to natural variability and not necessarily directly associated with the 1982-83 El Niño conditions.In late September and October 1982, the model atmosphere acquires momentum more rapidly than usual, leading to the development of the largest deviations from mean conditions at the end of this period, mostly associated with strong westerly momentum signals centered at 25°N. Large, sustained positive anomalies in tangential stress torques over the northern tropics are the major mechanism responsible for the modeled increase in M, but mountain torque anomalies centered at 35°N are also important at the end of October. A secondary maximum in the departure from mean M values occurs in January 1983 and is related to a general strengthening of westerly momentum anomalies over the model's tropical and midlatitude regions. Both mountain and tangential stress torques are involved in this episode, but no particular mechanism or region dominates the anomalous exchange of momentum.
ABSTRACT Following on the heels of the World Ocean Circulation Experiment, the Estimating the Cir... more ABSTRACT Following on the heels of the World Ocean Circulation Experiment, the Estimating the Circulation and Climate of the Ocean (ECCO) consortium has been directed at making the best possible estimates of ocean circulation and its role in climate. ECCO is combining state-of-the-art ocean general circulation models with the nearly complete global ocean data sets for 1992 to present. Solutions are now available that adequately fit almost all types of ocean observations and that are, simultaneously, consistent with the model. These solutions are being applied to understanding ocean variability, biological cycles, coastal physics, geodesy, and many other areas.
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Papers by Rui Ponte