Mooring Data Research Articles

Analysis of a Halocline Mesoscale Eddy in the Northwind Basin, Arctic Ocean

Eddies have been observed at all depths and in all regions of the Arctic Ocean. However, given the complex geographic conditions and dynamic environments of this ocean, the synchronized observations of temperature, salinity, and currents, and the detailed analysis of individual eddies are still lacking in the Northwind Basin. Our study aims to address these research gaps. We observed an eddy from a mooring in the Northwind Basin in late October 2017. It is a large anticyclonic cold eddy within the Arctic halocline, with a maximum azimuthal velocity reaching 52.63 cm/s and a horizontal scale (∼56 km) that significantly exceeds the first local baroclinic Rossby radius of deformation, i.e., it is in mesoscale. Its azimuthal velocity and scale are larger compared with those of nearby eddies, suggesting a relatively young state. This eddy possibly originated from the northern Chukchi Sea shelf, converging near Hanna Shoal with the Chukchi Slope Current before being advected northward into the Northwind Basin. Our study outlines detailed steps for extracting and analyzing eddies from mooring data and contributes to improving the understanding of the characteristics of Arctic Ocean eddies, providing a typical case for the investigation of eddies in the Northwind Basin.

Tidal and residual currents in the inner shelf of East China Sea detected from underway ADCP observations

Characterizing the spatial variability of net flow transport in a tide-dominated area like the East China Sea (ECS) is key to understanding the material and heat budget as well as their impacts on local ecosystem. However, the residual circulation in the ECS was much less confirmed by observations as it can only be derived from very sparse mooring data. Here for the first time we obtain the spatial patterns of tidal and residual currents in the inner ECS shelf from ADCP underway measurements conducted in 12 cruises using a spatial least squares (SLS) fitting technique. The SLS method is validated with the traditional harmonic analysis using numerical simulation results and then implemented with the observational data. The tidal current is dominated by two semidiurnal components M2 and S2 in this area, with the major axis of tidal ellipse along northwest-southeast direction. The ADCP-derived tide parameters are in good agreement with those from the mooring data and barotropic tide model. The annual spatial pattern of residual circulation is characterized by a southward coastal current and a northward shelf current, which are identified as the East China Sea Coastal Current (ECSCC) from Changjiang River runoff and the Taiwan Warm Current (TWC) originated from Taiwan Strait and Kuroshio. Their magnitudes are comparable but of one order smaller than tides. The vertical structure of TWC shows a subsurface-intensified feature in line with previous mooring observations. The north limit of TWC is approximately at 31.5°N around the submerged valley, beyond which the northward TWC becomes almost invisible. By a series of virtual cruise experiments, we show that the observation data length is the most crucial factor controlling the SLS fitting accuracy, while the role of other factors like sampling frequency is secondary. With a fixed-length dataset, the degree of polynomial function used for SLS fitting must be properly determined to minimize the under-fitting and over-fitting error, and there is also a trade-off in the fitting accuracy between tidal and residual currents depending on the fitting strategy.

Advanced Moored Data Buoys for Catching Typhoons in the Western North Pacific

Abstract With the western North Pacific being one of the most active typhoon hotspots, a reliable buoy system is essential for in situ observations during typhoon passages. This study details the development and implementation of two improved buoys designed by National Taiwan University based on the Autonomous Temperature Line Acquisition System. It aims to establish a buoy network for near real-time, high-resolution meteorological and oceanic data collection during typhoon periods in the western North Pacific. The buoy system was distinguished by its reliable design, low power-consumption system, dual satellite and radio communication system, and inductive modem-based data acquisition system. Moreover, the system was proven robust to extreme weather conditions and fishing challenges. From 2015 to 2022, we conducted sea trials in typhoon hotspot regions every summer (normally June to October), demonstrating the buoys' durability and capability to withstand 21 typhoons while collecting valuable scientific data.

Effects of internal waves on bottom thermal structures and turbulent mixing in the Xisha Islands

Using velocity and high-resolution temperature mooring data from the fore-reef slope of Yongxing Island in the northwest South China Sea (water depth of 69 m), we examine the effects of internal waves on the temporal variations in temperature, bottom mixed layer (BML) and turbulent mixing. The diurnal tide is found to be the dominant tidal force and the baroclinic tide is highly active, which would account for the 11 d abnormal spring-neap cycle in the barotropic tidal current. During the ebb period (tidal elevation decreases), the bottom diurnal baroclinic current transports cold water upslope, resulting in a decrease in temperature, and vice versa during the flood period. The BML thickness Hbml widely varies around 1.5m, approximately 2% of the water depth. The bottom turbulent mixing is not so active, indicated by the bulk dissipation Eɛ of 10 mWm−2 and turbulent diffusivity κz of 2×10−4m2s−1. Both Hbml and Eɛ approximate a log-normal distribution, demonstrating strong intermittency. The high-frequency (ω) internal bores can increase Hbml by four times and enhance the turbulent mixing by one order, which should be responsible for the slow cascade of Hbml(∼ω−1.5) and Eɛ(∼ω−1.0). In the downslope phase, Hbml is about 30% thicker, and turbulent mixing is enhanced by 3 times stronger than those in the upslope phase. It is revealed that under the forcing of internal waves, turbulent mixing corresponds to a thick BML with Hbml∼〈κz〉0.25, and stratification (N) has a significantly negative correlation with the development of BML, Hbml∼〈N〉−1.8.

Internal solitary waves in the central Andaman sea observed by combining mooring data and satellite remote sensing

The behavioral characteristics of internal solitary waves (ISWs) in the central Andaman Sea were revealed by analyzing multisource satellite remote sensing images combined with synchronous mooring data. The relationship between the generation time and tides of ISWs in the central Andaman Sea was analyzed and discussed. The ISWs in the central Andaman Sea exhibit a notable semidiurnal tidal cycle, primarily occurring during the spring tide. Additionally, it was observed that larger currents can generate ISWs with larger amplitudes. The propagation velocity of the ISWs initially increases and then decreases from the source, with no significant overall change in the deep areas. Based on the analysis of satellite remote sensing images that cover the source of ISWs in the central Andaman Sea, the generation time of ISWs was estimated. A comparison was made between the estimated generation time and the tidal flow predicted from the TPXO 9 global tidal model at the location of the ISW source. It was found that ISWs are generated when the westward tidal flow reaches its maximum. The ISWs in the central Andaman Sea are generated between the islands of Car Nicobar and Teresa Island. They propagate eastward into the Andaman Sea, grow in amplitude, split into multiple waves, dissipate, and vanish in the waters near the Danao Islands. The entire life cycle of these waves lasts for approximately 3∼4 days.

Sea water temperature and light intensity at high-Arctic subtidal shallows – 16 years perspective

Manifestations of climate change in the Arctic include an increase in water temperatures and massive loss of sea ice enabling more light penetration. Yet to understand tempo and scale of these parameters change over time, constant monitoring is needed. We present 16-yr long-term datasets of sea water temperature and relative light intensity at two depth strata (8 and 14 ± 1 m) of two hard-bottom sites in southern Isfjorden proper (Spitsbergen, 78°N). The high temporal resolution of the datasets (every 30 min, between 2006–2022) makes them suitable for studying changes at a local scale, correlating environmental variability with observed processes in benthic assemblages, and serving as ground-truth for comparison with, for example, remotely sensed or mooring data. These datasets serve as baseline for long-term investigations in the shallows of a high-Arctic fjord undergoing severe environmental changes.

Variability of Inflowing Current Into the Dotson Ice Shelf and Its Cause in the Amundsen Sea

AbstractThe inflow of warm and salty Circumpolar Deep Water affects the melting of the ice shelf on the Amundsen Sea, a significant contributor to global sea level rise. Multi‐year mooring data (2014–2016 and 2018–2020) from the front of the Dotson Ice Shelf show the modified Circumpolar Deep Water layer was thicker during 2018–2020 than during 2014–2016. During 2014–2016, Ocean surface stress curl influenced the barotropic process and strengthened southward velocity, while during 2018–2020, it caused lift and downwelling of thermocline depth, increasing the impact of the baroclinic process in ocean circulation. The heat transport to the ice shelf during 2018–2020 (57.42 MW m−1) was half as much as it was during 2014–2016 (111.06 MW m−1) due to a weaker lower layer current. The difference in ocean circulation between two periods, caused by a difference in warm layer thickness, ultimately impacts the heat transport entering the ice shelf cavity.

Multi-Timescale Characteristics of Southwestern Australia Nearshore Surface Current and Its Response to ENSO Revealed by High-Frequency Radar

The surface currents in coastal areas are closely related to the ecological environment and human activities, and are influenced by both local and remote factors of different timescales, resulting in complex genesis and multi-timescale characteristics. In this research, 9-year-long, hourly high-frequency radar (HFR) surface current observations are utilized together with satellite remote sensing reanalysis products and mooring data, and based on the Empirical Orthogonal Function (EOF) and correlation analysis, we revealed the multi-timescale characteristics of the surface currents in Fremantle Sea (32°S), Southwestern Australia, and explored the corresponding driving factors as well as the impact of El Niño-Southern Oscillation (ENSO) on the nearshore currents. Results show that the currents on the slope are dominated by the southward Leeuwin Current (LC), and the currents within the shelf are dominated by winds, which are subject to obvious diurnal and seasonal variations. The strong bathymetry variation there, from a wide shelf in the north to a narrow shelf in this study region, also plays an important role, resulting in the frequent occurrence of nearshore eddies. In addition, the near-zonal winds south of 30°S in winter contribute to the interannual variability of the Leeuwin Current at Fremantle, especially in 2011, when the onshore shelf circulation is particularly strong because of the climatic factors, together with the wind-driven offshore circulation, which results in significant and long-lasting eddies. The southward Leeuwin Current along Southwestern Australia shows a strong response to interannual climatic variability. During La Niña years, the equatorial thermal anomalies generate the westward anomalies in winds and equatorial currents, which in turn strengthen the Leeuwin Current and trigger the cross-shelf current as well as downwelling within the shelf at Fremantle, whereas during El Niño years, the climate anomalies and the response of coastal currents are opposite. This paper provides insights into the multi-timescale nature of coastal surface currents and the relative importance of different driving mechanisms. It also demonstrates the potential of HFR to reveal the response of nearshore currents to climate anomalies when combined with other multivariate data. Meanwhile, the methodology adopted in this research is applicable to other coastal regions with long-term available HFR observations.

Study of Interannual Variability of the Winter Mesothermal Temperature Maximum Layer in Southern Baikal

This paper is devoted to the study of the mesothermal temperature maximum layer (MTML) in Lake Baikal, which is observed during the period of winter stratification. On the one hand, this is a rather well-known phenomenon; on the other hand, it is not sufficiently studied, although it has a significant impact on the thermal regime in winter and water dynamics during the periods of formation and breakdown of inverse temperature stratification. Our work presents the results of analyzing the spatial and temporal variability of the main MTML parameters and their dependence on hydrometeorological factors. For this purpose, CTD soundings and mooring data obtained in the western part of the southern basin of Lake Baikal in 2000–2022 were analyzed in comparison to ERA5-Land reanalysis. The MTML parameters have noticeable within-season and interannual spatial and temporal variability. This is obviously related to the influence of the processes of vertical turbulent mixing, internal wave action, and current patterns. The analysis of interannual differences revealed four types of behavior of the maximum MTML temperature during the ice season. The influence of wind conditions on the main MTML parameters (maximum MTML temperature, depth of its occurrence, and depth of the upper MTML boundary) was shown not only in the fall, but also in the summer period, when heat accumulation in the Baikal water column takes place. With the increased wind activity in the late fall, the MTML is formed deeper and has lower maximum temperature values. At lower wind activity in the fall, the MTML is closer to the surface and the values of the maximum MTML temperature are higher. A change in wind activity in the summer leads to the opposite effect. In spite of the essential trends over the study period in the dates of the occurrence of hydrological events, no noticeable trends were registered for the maximum MTML temperature, its depth, and the depth of the upper boundary of the MTML.

Reflection and Scattering of Low-Mode Internal Tides on the Continental Slope of the South China Sea

Abstract It is well known that strong low-mode internal tides generated in Luzon Strait propagate westward to impinge continental slopes in the northeastern South China Sea (SCS). The reflection and scattering of these internal tides including diurnal and semidiurnal components on the slopes are quantitatively investigated using two sets of mooring data and a linear internal tide model with realistic topography and stratification. Flux reflections computed from mooring data collected on the continental slopes are consistent with the linear model. Based on the results of the observations and simulations, a map of low-mode internal tide reflection and scattering coefficients along the continental margin in the northeastern SCS is revealed. On average, diurnal internal tides lose 38% of their energy to high modes (≥mode 4) that are assumed to dissipate on the slopes, transmit 28% onto the continental shelf, and reflect 31% back to the deep ocean. On the contrary, most of the semidiurnal energy (89%) transmits onto the continental shelf, and only 11% is scattered to high modes (7%) and reflected back to the deep ocean (4%). For diurnal internal tides, a large fraction of energy that is scattered to high modes and reflected back to the deep sea can be attributed to the critical–supercritical slopes, while the weak reflection for the semidiurnal energy is due to the subcritical slopes. These quantitative descriptions for evolutions of low-mode internal tides incident to the slopes provide an energy budget map on the continental slopes in the northeastern SCS.

Dynamical interactions between Loop Current and Loop Current Frontal Eddies in a HYCOM ensemble of the circulation in the Gulf of Mexico

The dynamics of the Loop Current (LC) system in the Gulf of Mexico (GoM), specifically during the shedding of Eddy Franklin in 2010, is investigated using an ensemble of simulations. The ensemble members differed in their initial conditions of the West Florida Cyclonic Eddy (WFCE), which in turn significantly influences the timing and occurrence of the Loop Current Eddy (LCE) detachment. The results reveal that a stronger and larger WFCE leads to an early LCE detachment, while a weaker and smaller WFCE results in late or even no detachment within the 60-day simulation period. The initial WFCE’s size and strength are also found to impact the evolution of Campeche Bank Cyclonic Eddies (CBCE). The intrusion of a large and strong WFCE into the LC leads to a rapid growth of potential vorticity (PV) over the eastern Campeche Bank (CB), associated with the formation of a CBCE. In addition, ensemble members with stronger and larger WFCE generally agree with mooring data regarding the velocity evolution over the eastern CB, as well as the CBCE’s northeastward offshore displacement. Our results suggest that the size and strength of the WFCE may serve as predictors of the formation of a CBCE and of an LCE detachment occurrence. This finding has implications for future studies and forecasting methodologies for the GoM circulation.

Basin-Scale to Submesoscale Variability of the East Mediterranean Sea Upper Circulation

Abstract The East Mediterranean Sea (EMS) circulation has previously been characterized as dominated by gyres, mesoscale eddies, and disjoint boundary currents. We develop nested high-resolution numerical simulations in the EMS to examine the circulation variability with an emphasis on the yet unexplored regional submesoscale currents. Rather than several disjoint currents, a continuous cyclonic boundary current (BC) encircling the Levantine basin is identified in both model solution and altimetry data. This EMS BC advects eddy chains downstream and is identified as a principal source of regional mesoscale and submesoscale current variability. During the seasonal fall to winter mixed layer deepening, energetic submesoscale [O(10) km] eddies, fronts, and filaments emerge throughout the basin, characterized by O(1) Rossby numbers. A submesoscale time scale range of ≈1–5 days is identified using spatiotemporal analysis of the numerical solutions and confirmed through mooring data. The submesoscale kinetic energy (KE) wavenumber (k) spectral slope is found to be k−2, shallower than the quasigeostrophic-like ∼k−3 slope diagnosed in summer. The shallowness of the winter spectral slope is shown to be due to divergent subinertial motions, consistent with the Boyd theoretical model, rather than with the surface quasigeostrophic model. Using a coarse-graining approach, we diagnose a seasonal inverse (forward) KE cascade above (below) 30-km scales due to rotational (divergent) motions and show that these commence after completion of the fall submesoscale energization. We also show that at scales larger than several hundred kilometers, the spectral density becomes near constant and a weak forward cascade occurs, from gyre scales to mesoscales.

An automatic identification algorithm of internal solitary wave for mooring data based on geometric characteristics of the flow field

An automatic identification algorithm of internal solitary wave for mooring data based on geometric characteristics of the flow field

Balancing Volume, Temperature, and Salinity Budgets During 2014–2018 in the Tropical Pacific Ocean State Estimate

AbstractA state estimate of the tropical Pacific Ocean is used to analyze regional volume, temperature, and salinity budgets during 2014–2018. The simulated ocean state is constrained by both model dynamics and assimilated observations. Comparisons with independent mooring data show that the state estimate is consistent with the observed variability in temperature and velocity. Budgets are analyzed between 5°S and 5°N in the upper 300 m, inside a box defined to represent the central and eastern equatorial Pacific. Transports through the faces of this box are quantified to understand the processes responsible for variability in box‐mean properties. Vertical mixing across 300 m is negligible; temperature and salinity tendencies are balanced by surface fluxes and advective divergence, which is decomposed into geostrophic and ageostrophic components. The onset and recovery of the 2015/2016 El Niño event is found to be dominated by anomalous surface fluxes and horizontal advection. During the onset phase, weaker trade winds cause the shallow meridional overturning circulation to slow down, which reduces the poleward transport of heat and leads to upper ocean warming. Anomalous precipitation and advection of fresh water from the western Pacific drive the net freshening of the region. Relaxation from El Niño conditions is dominated by wind‐driven meridional advection at 5°N. As the meridional advection regains strength, Ekman advection efficiently exports the warm, fresh surface water out of the equatorial region. Quantifying the heat and salt transport changes in response to wind variability strengthens our understanding of global ocean heat transport.

Formation and circulation of newly ventilated winter water in the western Beaufort Sea

Newly ventilated winter water (NVWW) is a cold, salty, nutrient-rich water mass that is critical for supporting the ecosystem of the western Arctic Ocean. In this study, NVWW formation is documented using timeseries from an 8-mooring array deployed from 2008 to 2009 across the shelf and slope in the western Beaufort Sea near 150°W. The saltiest (densest) class of the winter water (salinity > 32.6), which is able to ventilate the cold halocline in the interior Canada Basin, was only observed on the inner shelf from December to May. The reasons for this are three-fold: (1) In mid- to late-fall the water on the outer shelf is largely influenced by fresher water in the vicinity of the shelfbreak, limiting the density of the winter product there; (2) during winter the variance in ice cover is significantly higher on the inner shelf than the outer shelf, and this enhanced presence of leads and polynyas results in more re-freezing and hence formation of saltier winter water; and (3) the cross-shelf velocities during the cold months (December–May) were negligible at the array site. To better understand the ultimate fate of the salty NVWW, the components of the western Beaufort Sea boundary current system were quantified using the array data plus additional mooring data to the west. It is argued that the salty NVWW can be fluxed off the shelf due to the convergence of the westward-flowing current on the shelf and the eastward-flowing outflow from Barrow Canyon.

Validating the spatial variability in the semidiurnal internal tide in a realistic global ocean simulation with Argo and mooring data

Abstract. The autocovariance of the semidiurnal internal tide (IT) is examined in a 32 d segment of a global run of the HYbrid Coordinate Ocean Model (HYCOM). This numerical simulation, with 41 vertical layers and 1/25∘ horizontal resolution, includes tidal and atmospheric forcing, allowing for the generation and propagation of ITs to take place within a realistic eddying general circulation. The HYCOM data are in turn compared with global observations of the IT around 1000 dbar, from Argo float park-phase data and mooring records. HYCOM is found to be globally biased low in terms of the IT variance and decay of the IT autocovariance over timescales shorter than 32 d. Except in the Southern Ocean, where limitations in the model cause the discrepancy with in situ measurements to grow poleward, the spatial correlation between the Argo and HYCOM tidal variance suggests that the generation of low-mode semidiurnal ITs is globally well captured by the model.

Suspended particulate matter dynamics in the inner sector of a Ría under fair-weather conditions: A synthetic model

In this work, we analyse the sediment particulate matter (SPM) dynamics. This subject is poorly understood in the Rías Baixas system (NW Iberian Peninsula), thus the bottom sediment dynamics of the inner sector of a ría is investigated for the first time. The current, turbidity, temperature and salinity were measured from 15 June to 17 July 2009, in the Ría de Muros-Noia, the northernmost embayment of the Rías Baixas. The parameters were measured by two moorings placed in the central axis of the inner sector; one equipped with two current meters, at 0.4 m and 1 m above the bottom (mab), and the second mooring equipped with one current meter at 0.4 mab. The wave data inside the ría during the survey was obtained using a numerical model (SWAN) computed from available wave data in open waters (provided by Organismo Público Puertos del Estado, OPPE). The survey was characterized by upwelling and downwelling events and punctual precipitation periods. The mooring data revealed the alternation of different subtidal processes; deep-water upwelling, relaxation periods and a large downwelling. SPM of fluvial origin was transferred to the bottom from the surface under downwelling conditions, whilst the arriving of oceanic water under upwelling conditions showed lowest turbidity values. The subtidal circulation of the water masses inside the ría controlled mainly the bottom turbidity dynamics in the inner sector. The SW waves were likely to produce punctual resuspension in the shallowest areas, and the tidal currents only transported locally the SPM clouds when the subtidal circulation made it available in the bottom.

The Atlantic Meridional Overturning Circulation at 35°N From Deep Moorings, Floats, and Satellite Altimeter

AbstractFrom 2004 to 2014, the Line W moorings measured a 0.7 Sv yr−1 slowing of the deep western boundary current (DWBC) offshore of Cape Cod. Here, we combine these deep mooring observations with float and satellite altimeter data and find that this DWBC change corresponded to a slowing of the cross‐basin Atlantic Meridional Overturning Circulation (AMOC) of about 0.3 Sv yr−1. Our AMOC transport time series corresponds well with the Estimating the Circulation and Climate of the Ocean state estimate, particularly when the Line W mooring data influences our volume closure. We compare our 35°N time series with a similar time series at 41°N as well as the 26°N RAPID AMOC, and find AMOC declines across datasets from 2004 to 2014. However, when we extend our analysis to 2004–2019, there are no significant trends at any latitude. These observations suggest that AMOC decadal variability is meridionally coherent from 26°N to 41°N and that the DWBC may reflect this variability.

Warming waters of the Gulf of Maine: The role of Shelf, Slope and Gulf Stream Water masses

We present an analysis of a 19-year time series of mooring data (T, S,) collected since 2003 in Jordan Basin in the eastern Gulf of Maine and beginning in 2016, in situ nitrate; we also include hydrographic station data collected by NOAA’s ECOMON Program. Analyses support, in part, conclusions drawn by others that the Gulf of Maine has warmed and continues to warm as a result of long-term and shorter term air-sea heat fluxes. We show that warming is to a large extent a result of changes in advective heat fluxes via lateral exchanges with water masses in neighboring shelf and slope regions (Warm Slope Water and Scotian Shelf Water). Water properties of the source waters and their inflows to the Gulf have been shown to have been changing over time, and now reveal a mix that includes warm, salty, low nutrient Gulf Stream Water. The result is a new baseline in the Gulf of Maine, of warmer temperatures and higher salinities, which appears to have become established after 2010. Annual cycles of spring-to summer water column warming and fall-to-winter cooling and convective mixing are presented and discussed, showing that the maximum depth of convective mixing in the eastern interior Gulf of Maine does not usually exceed 150 m, a depth constrained by the presence a strong vertical density gradient created by layer of high salinity bottom waters. Approximately-six months after the annual late-summer influx to the Gulf of warm and salty water masses at depth and along the bottom, a mass of cold, low salinity Scotian Shelf Water arrives in the upper water layers in winter – these two major water mass fluxes are a half year out of phase with one another, every year. We suggest that these two water mass intrusion events are linked – that the winter-spring inflow of Scotian Shelf Water creates a barotropic pressure gradient in the Gulf that persists into the summer, thereby impeding the inflow of warm and salty Slope, and/or Gulf Stream Water, until that Scotian Shelf Water mass has become sufficiently dispersed in late summer, thereby effectively opening a valve that allows warm and salty deep waters from outside the Gulf to penetrate.

A simple diagnostic based on sea surface height with an application to central Pacific ENSO

Abstract. We use output from a freely running NEMO model simulation for the equatorial Pacific to investigate the utility of linearly removing the local influence of vertical displacements of the thermocline from variations in sea surface height. We show that the resulting time series of residual sea surface height, denoted ηnlti, measures variations in near-surface heat content that are independent of the local vertical displacement of the thermocline and can arise from horizontal advection, surface heat flux, and diapycnal mixing processes. We find that the variance of ηnlti and its correlation with sea surface temperature are focused on the Niño4 region. Furthermore, ηnlti averaged over the Niño4 region is highly correlated with indices of central Pacific El Niño–Southern Oscillation (CP ENSO), and its variance in 21-year running windows shows a strong upward trend over the past 50 years, corresponding to the emergence of CP ENSO following the 1976/77 climate shift. We show that ηnlti can be estimated from observations, using satellite altimeter data and a linear multi-mode model. The time series of ηnlti, especially when estimated using the linear model, show pronounced westward propagation in the western equatorial Pacific, arguing for an important role for zonal advective feedback in the dynamics of CP ENSO, in particular for cold events. We also present evidence that the role of the thermocline displacement in influencing sea surface height increased strongly after 2000 in the eastern part of the Niño4 region, at a time when CP ENSO was particularly active. Finally, the diagnostic is easy to compute and can be easily applied to mooring data or coupled climate models.