Sea Level Difference Research Articles

Relationship between the flow past Miyakejima and sea-level differences at two island locations

Relationship between the flow past Miyakejima and sea-level differences at two island locations

Evidence for Reduced Black Sea Water Outflow to the North Aegean

AbstractThe Black Sea Waters (BSW) exported into the Aegean Sea largely control its overturning circulation as well as the regional biogeochemical characteristics. Observational evidence from ARGO floats and satellite‐derived sea surface temperature (SST) and chlorophyll indicate a recent drastic reduction of Black Sea Water presence in the surface layer of the North Aegean Sea. This evidence is also supported by the long‐term negative sea level difference trend between the southwestern Black Sea and the northeastern Aegean Sea. The role of the Black Sea in hindering deep water formation processes in the North Aegean seems to be diminishing. The future evolution of the Aegean overturning cell will depend on whether this trend will continue, in addition to the variability of local atmospheric forcing.

Non-linear long-term trend in volume transport through the Korea/Tsushima Strait

Based on the relation between volume transport and sea level difference across the Korea/Tsushima Strait (KTS), in this study, we estimated the volume transport of the Tsushima Warm Current over the past four decades and examined its long-term trend using an ensemble empirical model decomposition (EEMD) method. The estimated geostrophic transports showed very good agreement with those measured by acoustic Doppler current profiler and those estimated from satellite altimetry-based sea level data. This corroborates the reliability of the long-term estimation. EEMD analysis revealed an acceleration in volume transport in the KTS, with a persistent increase in the trend rate occurring after the mid-1980s. Further analyses indicate that the trend shift in the KTS closely coincided with an upper-ocean warming trend in the East/Japan Sea (EJS) after the mid-1980s. Analyzing large-scale wind patterns suggests that long-term trends of wind stress curls over the North Pacific are likely a key driver of the trend shifts in the inflow through the KTS and the upper-ocean warming in the EJS over the past four decades.

Long-term vertical-land-motion investigation with space and terrestrial geodetic techniques near San Leon, Texas, USA

Monitoring vertical land motion (VLM) along coastlines, which influences the dynamics of sea level changes in relation to the land, is a challenging task due to its inherent high spatiotemporal variability and limited availability of observations. This study aimed to investigate the rates, patterns, and drivers of land subsidence near the coastal town of San Leon, TX, United States, since the 1990s, utilizing a range of space and terrestrial geodetic techniques. These techniques included interferometric synthetic aperture radar (InSAR), global navigation satellite systems (GNSS), tide gauge (TG), and satellite radar altimetry (SRA). The small baseline subset (SBAS) InSAR method was adopted to process 254 images from three synthetic aperture radar (SAR) sensors, i.e., ERS-2 between 1995 and 1999, ALOS-1 PALSAR between 2006 and 2011, and Sentinel-1 between 2016 and 2020. The results from InSAR subsidence maps were verified by comparing with high-accuracy vertical positioning observations at ten continuously operating GNSS (cGNSS) stations. Within the study area, a special attention was given to the Eagle Point TG station where sea level has been significantly rising relative to the sinking land. Long-term time series of land subsidence at Eagle Point obtained from sea-level difference between TG and SRA observations were confirmed and compared against InSAR and the cGNSS station observations recorded in close proximity. Gaussian Process regression (GPR) was then employed to model the VLM processes at Eagle Point using: (1) the combined results of InSAR and sea-level difference (i.e., GPR 1), and (2) the InSAR results alone (i.e., GPR 2). A 0.9 mm/yr divergence was found between GPR 1 and GPR 2 models, indicating the potential to accurately estimate long-term VLM with InSAR standalone measurements even if multi-year observation gaps intermittently occur, especially for inland areas where measurement data from other geodetic techniques, such as GNSS, TG, and SRA, are not available. Further investigations suggest that land subsidence around Eagle Point since 1998 was related to anthropogenic activities such as hydrocarbon pumping from oil and gas wells that were situated in close proximity to the TG station.

Numerical study of tidal effect on the water flux across the Korea/Tsushima Strait

Tremendous amounts of materials and energy are transported from the East China Sea (ECS) to the East/Japan Sea (EJS) through the Korea/Tsushima Strait (KTS). Tides undoubtedly play an important role in regulating ocean circulation on the broad continental shelf of the ECS, while the effects of tides on the water exchange between the ECS and EJS remain unclear. Using a three-dimensional Regional Oceanic Modeling System (ROMS) circulation model, we conducted numerical experiments with tides, without tides, and only barotropic tides. The results showed that the water flux across the KTS can increase by up to 13% (in summer) when excluding tides from the numerical simulation. To understand how tidal forcing regulates the KTS water flux, we performed a dynamic diagnostic analysis and revealed that the variation in sea surface height under tidal effect is the main reason for the water flux variation across the KTS. The tidal effect can adjust the sea surface height, weaken the pressure gradient and reduce the water flux across the KTS, which affect the intensity of water exchange between the ECS and EJS. The tidal effect can alter sea level difference between the Taiwan Strait and the KTS, which influences the KTS water flux. Tides can also influence the KTS water flux by altering the sea surface height through interaction with topography and stratification. We also found that tidal effect weakens the northward intrusion of the Yellow Sea Warm Current in winter and in turn enhances the water flux across the KTS according to volume conservation. These modeling results imply that tides must be considered when simulating the ocean environment of the northwestern Pacific Ocean.

50-year volume transport of the Soya Warm Current estimated from the sea-level difference and its relationship with the Tsushima and Tsugaru Warm Currents

This study provides formulae for monthly estimates of the Soya Warm Current (SYC) transport from the sea-level difference (SLD) across the Soya Strait and along the current, and creates a 50-yr data set of the SYC transport. The formulae are based on the dynamical balance both across the strait and along the current by considering the barotropic and baroclinic components. It is suggested that the barotropic and baroclinic components in transport variability are comparable in summer, whereas the barotropic component is dominant in winter. We compared the 50-year time series of the SYC transport with those of the Tsushima (TSC) and Tsugaru Warm Currents estimated from the SLDs from the viewpoint of the Japan Sea Throughflow (JSTF) system. The variabilities of the SYC and TSC transports are mostly shared for any season, showing relatively high coherence at periods of 2–5 years as well as 1 year. The winter variability of the JSTF transport originates partly from that of the SYC transport caused by the winter monsoon variability in the Sea of Okhotsk. The SYC transport is significantly correlated with the transport through the eastern channel of the TSC, but not that through the western channel. We found difficulty in extracting variability and relationships at periods longer than 5 years among the three transports estimated from the SLDs, probably due to incomplete correction of the ground movements for the sea-level data.

ОЦЕНКА ПЕРЕПАДОВ УРОВЕННОЙ ПОВЕРХНОСТИ МОРЯ: СКОРОСТИ ПЕРЕМЕЩЕНИЙ МОРСКИХ МАРКЕРОВ ПО ИК-ИЗОБРАЖЕНИЯМ В СРАВНЕНИИ С ДАННЫМИ СПУТНИКОВОЙ АЛЬТИМЕТРИИ

A quantitative comparison of sea level differences calculated from sea surface current velocities and satellite altimetry measurements was carried out. The velocities were calculated by the automatic method for tracing thermal inhomogeneities based оn a sequence of infrared images with a priori estimation of the calculation accuracy. Comparisons were carried out along the tracks of an altimetry satellite and at the points of the regular grid, to which the values from the tracks are interpolated. Ageostrophic components were estimated for the calculated velocities. At the distances of about 100 km, their contribution did not exceed the accuracy of altimetry measurements (2-4 cm). Along the tracks, the estimates of sea level differences calculated from the velocities were close to those computed from satellite altimetry. At the regular grid points, the estimates of sea level differences by velocities were significantly higher than the estimates of sea level differences by satellite altimetry, which is explained by the drawbacks of the optimal interpolation scheme used. It was concluded that the joint use of two types of measurements to retrieve the World Ocean circulation is reasonable.

Estimation of Sea Level Difference: Velocities of Sea Surface Markers Derived from IR Images in Comparison with Satellite Altimetry Data

Estimation of Sea Level Difference: Velocities of Sea Surface Markers Derived from IR Images in Comparison with Satellite Altimetry Data

Long-term variation in volume transport of the Tsushima warm current estimated from ADCP current measurement and sea level differences in the Korea/Tsushima Strait

ADCP current data and sea level data from tide gauges and satellite altimetry were used in order to analyze long-term variation of the Tsushima Warm Current (TWC) volume transport passing through the Korea/Tsushima Strait. The annual mean transport of the TWC estimated from ADCP current measurement for about 17 years (1997–2013) is 2.64 ± 0.41 Sv (Sv = 106 m3s−1) in the Korea/Tsushima Strait, 1.53 ± 0.36 Sv (about 58% of the total transport) in the western channel, and 1.11 ± 0.14 Sv (about 42%) in the eastern channel. The TWC shows the minimum transport (1.74 Sv) in January and the maximum transport (3.10 Sv) in October, with seasonal variation of up to 1.36 Sv (51.5% of the annual mean transport). The seasonal variation of the total TWC transport in the Korea/Tsushima Strait is mainly similar to that in the western channel. The annual mean transport of the TWC and its seasonal variation in the Korea/Tsushima Strait, the western and eastern channels are all similar in three datasets (ADCP, tide gauge, satellite altimetry). The annual mean volume transport of the TWC estimated from long-term tide gauge data was 2.57 ± 0.37 Sv (total volume transport) for 44 years (1975–2018) in the Korea/Tsushima Strait, 1.51 ± 0.32 Sv in the western channel and 1.06 ± 0.14 Sv in the eastern channel. Long-term variation of the TWC transport shows a decreasing trend for 1975 to 1988, and an increasing trend from 1989 to 2018. For the latter period (1989 to 2018), monthly transport tends to increase in all months throughout the years, showing a greater increase, especially in spring and summer months. The increase of the transport in the eastern channel was about 2.8 times larger than that in the western channel. It is suggested that the increasing trend of the TWC transport since 1989 is related to the Pacific Decadal Oscillation (PDO), which has been changed to negative direction since mid-1980s. During the period when the negative PDO index is strong, the negative wind stress curl weakens in the mid-latitudes of the North Pacific, and the Kuroshio in the East China Sea also weakens. As a result, the flow is strengthened toward the Korea/Tsushima Strait, increasing the TWC transport.

Managed Nitrogen Load Decrease Reduces Chlorophyll and Hypoxia in Warming Temperate Urban Estuary

Many urban estuaries worldwide suffer from excess phytoplankton and hypoxia (low oxygen) due to high nutrient loads. A common water quality management strategy is to require wastewater treatment facility upgrades. This case study examines Narragansett Bay, a warming temperate mid-latitude urban estuary with seasonal periodic hypoxia, during June through September from 2005 to 2019. Within this period, numerous facilities were upgraded to nitrogen removal over several years. The response of the bay is more consistent with “textbook” expectations for reduced chlorophyll and hypoxia than what was seen in many other systems—despite its complex coastline geometry, numerous river inputs, and widely-distributed treatment facilities. River flow drives inter-annual variability with increased load, density stratification, chlorophyll, and hypoxia in wet years. Mean 2013-2019 bay-wide total nitrogen load was 34% less than the 2005-2012 mean, a reduction of about 106 kg yr-1, comparable to the range of flow-driven inter-annual variations. Chlorophyll Index and Hypoxia Index event-based metrics applied to high-frequency time series observations at eight sites quantify exceedances of severe and moderate thresholds. Relatively steady 33% and 16% Chlorophyll Index declines, for severe and moderate thresholds, occurred from about 2007 to 2019. The Hypoxia Index declined markedly by 2009 and 2014 for severe and moderate thresholds, respectively, and remained at or near zero from 2014 to 2019. The load reduction explains chlorophyll and hypoxia declines better than physical processes including river flow, stratification, tidal variations, winds, sea level differences, and temperatures. River flow about 55% higher than the 2005-2019 mean would increase non-treatment facility loads by an amount comparable to the managed load decrease, so future wet summers could partially reverse the improvements. Long-term trends include warming of about 0.5°C decade-1, which reduces oxygen saturation by 0.1 mg l-1 decade-1. This rate is likely a lower bound for temperature-driven oxygen decreases, because warming can also accelerate phytoplankton growth and bacterial consumption. Without warming, the managed load decrease would have curtailed hypoxia more effectively. Climate trends should be at least as important to future eutrophication as the managed load decline because, in addition to warming influences, long-term increases in river flow would increase load and stratification.

Characteristics of Ocean Waves in the Southern Ocean and Seasonal Prediction of Difficulty for the Vessels Crossing the Westerlies

In this study, the European Centre for Medium-Range Weather Forecasts reanalysis v5 (ERA5) wave height reanalysis data and sea-level pressure data from two seasonal prediction systems were used to study the wave characteristics of the Southern Ocean and the seasonal prediction of difficulty for the vessels crossing the westerlies. The results show that significant wave height, extreme wave height, and extreme wave processes were all much stronger in austral winter than in austral summer, making it more difficult for vessels to cross the westerlies in winter. Furthermore, the difficulty of crossing the westerlies has increased over the last 40 years, except in areas east of South America. We found that the monthly frequency of crossing the westerlies between 100° E and 75° W could be accurately estimated point by point using the monthly mean sea-level pressure difference between 30° S and 65° S. With a 1-month lead, the multi-model forecasts gave a fairly accurate forecast of such sea-level pressure difference signal between 150° E to 60° W for target months from May to December. Based on these assessments, we could estimate the difficulty of crossing the westerlies in some specific months in advance using a seasonal prediction system. We also found that the seasonal prediction system produced inaccurate predictions of sea-level pressure at high latitudes, which is the main barrier to further accurately estimating the difficulty of crossing the westerlies. These findings may provide support for predicting difficulty for the vessels crossing the westerlies with a 1-month lead and can provide planning for the route and the allocation of material reserves.

A tidally driven fjord-like strait close to an amphidromic region

Abstract. The strait studied in this paper, “Sundalagið Norður”, is the northern part of a narrow body of seawater separating the two largest islands in the Faroe Islands (Faroes). It has shallow sills in both ends and considerably deeper waters in between. South of the southern end of the strait there is an amphidromic region for the semidiurnal tides so that the tidal range is much lower south of the strait than north of it. The resulting tidal forcing generates periodically varying inflow of seawater across the northern sill, but only a part of that manages to cross the narrow and shallow southern sill. Combined with a large input of freshwater, this gives the strait a fjord-like character. To investigate how this fjord-like character affects the circulation within the strait and its exchanges with outside waters, a pilot project was initiated to simulate the dynamics of the strait with a high-resolution ocean model for a month. The model simulations show clearly the dominance of tidal forcing over freshwater (estuarine) and wind on timescales up to a day. On longer timescales, the simulations indicate systematic variations in the net flows (averaged over a diurnal tidal period) through both the upper and deeper layers. These long-period variations of net flow in the model simulations are forced by sea level differences between both ends of the strait generated by the dominant fortnightly and monthly tidal constituents (Mf, MSf, Mm, MSm). Harmonic analysis of sea level records from two tide gauges located off each end of the strait demonstrates that this behaviour is not a model artefact and it has pronounced effects on the strait. Not only does it induce long-period (mainly fortnightly) variations in the net flow through the strait, but it also generates variations in the estuarine characteristics. According to the model simulations, periods with net southward flow, typically lasting a week, have a strait-like character with net southward flow almost everywhere. Periods with net northward flow, in contrast, have a more fjord-like character with stronger salinity stratification and a southward counter-flow in the deep layer. This also induces a large difference in renewal rate of the deep water between the two periods, which is important to consider for human utilization of the strait, especially the local aquaculture plant. The combination of topographic, freshwater, and tidal characteristics creating these long-period variations is rather unusual, and it is not known whether similar systems exist elsewhere, but the long-period variations tend to be masked by the stronger semidiurnal and diurnal variations and may easily be overlooked.

Exchange of water and contaminants between the Strait of Istanbul and the Golden Horn

The Golden Horn (Estuary) is a hotspot for marine pollution because of its close proximity to the Strait of Istanbul (Strait) with dense maritime traffic and high pollution risk of oil spill. Transport and fate of water and contaminants in Strait and Estuary are analyzed by three-dimensional numerical models. Eddies forming at the Golden Horn junction (Junction), either on a single layer or distributed over the entire water column, block the water exchange partly or entirely across the Junction. In case of an oil spill in adjacent waters, the size and distribution of eddies at Junction control the transport and fate of contaminants inside Estuary. Using twenty test cases, the exchange flows at Junction are classified in four types based on sea level difference between north and south boundaries of Strait and Reynolds number of surface flow. Eddies may prevent the natural water exchange between Strait and Estuary but also block the contaminants penetrating to Estuary. Under predominant conditions of mild and moderate southward net flow in Strait, surface waters enter Estuary. During very strong southward flow, Junction is blocked to Black Sea water and surface contaminants. Strait hydrodynamics control water exchange with Estuary through eddy formation. When horizontal eddies are formed at Junction, their size and distribution control the transport and fate of contaminants in branching inlets and estuaries by blocking the water exchange either entirely or partly in specific layers.

Estimating Vertical Land Motion from Remote Sensing and In-Situ Observations in the Dubrovnik Area (Croatia): A Multi-Method Case Study

Different space-borne geodetic observation methods combined with in-situ measurements enable resolving the single-point vertical land motion (VLM) and/or the VLM of an area. Continuous Global Navigation Satellite System (GNSS) measurements can solely provide very precise VLM trends at specific sites. VLM area monitoring can be performed by Interferometric Synthetic Aperture Radar (InSAR) technology in combination with the GNSS in-situ data. In coastal zones, an effective VLM estimation at tide gauge sites can additionally be derived by comparing the relative sea-level trends computed from tide gauge measurements that are related to the land to which the tide gauges are attached, and absolute trends derived from the radar satellite altimeter data that are independent of the VLM. This study presents the conjoint analysis of VLM of the Dubrovnik area (Croatia) derived from the European Space Agency’s Sentinel-1 InSAR data available from 2014 onwards, continuous GNSS observations at Dubrovnik site obtained from 2000, and differences of the sea-level change obtained from all available satellite altimeter missions for the Dubrovnik area and tide gauge measurements in Dubrovnik from 1992 onwards. The computed VLM estimates for the overlapping period of three observation methods, i.e., from GNSS observations, sea-level differences, and Sentinel-1 InSAR data, are −1.93±0.38 mm/yr, −2.04±0.22 mm/yr, and −2.24±0.46 mm/yr, respectively.

Response of sea level to tide, atmospheric pressure, wind forcing and river discharge in the Kitimat Fjord System

The response of sea level to tide, atmospheric pressure, wind forcing, and river discharge in the Kitimat Fjord System (KFS) is investigated by using observations from tide gauges and results from a high-resolution ocean circulation model. Tidal harmonic analysis of the observed hourly sea level at the head and mouth of Douglas Channel shows that the largest semidiurnal tide (M2) alone accounts for 79% of the total sea level variance, and the top four major tidal constituents (M2, S2, K1, and O1) account for 94% of the total variance. In addition, the amplitude of M2 increases slightly from 1.586 to 1.647 m between the mouth and the head. For the observed subtidal sea level, the local inverse barometer response due to the variation of atmospheric pressure accounts for 59% of the variance. The variation of subtidal sea level difference between the mouth and the head of Douglas Channel is observable and can be largely attributed to the along-channel wind (54%) with an additional 13% of the variation due to the daily-averaged and seasonal-cumulative river discharges.A fjord-shelf three-dimensional ocean circulation model for KFS is developed to simulate the sea level variability. The model performance in simulating the sea level is assessed by comparing model results with observed harmonic constituents of ~70 tidal stations inside the model domain and time series recorded at the two tide gauges of KFS. The modelled tides are in good agreement with observations with a domain-averaged root-mean-square error estimate of ~0.04 m for M2. For the subtidal sea level, the model is able to capture the observed synoptic variations. Further analysis suggests that the underestimates of subtidal sea level in the ocean circulation model are mainly due to the weaker-than-observed wind forcing taken from the atmospheric model especially during the peak of strong down-fjord katabatic wind events.

Global sea level reconstruction for 1900–2015 reveals regional variability in ocean dynamics and an unprecedented long weakening in the Gulf Stream flow since the 1990s

Abstract. A new monthly global sea level reconstruction for 1900–2015 was analyzed and compared with various observations to examine regional variability and trends in the ocean dynamics of the western North Atlantic Ocean and the US East Coast. Proxies of the Gulf Stream (GS) strength in the Mid-Atlantic Bight (GS-MAB) and in the South Atlantic Bight (GS-SAB) were derived from sea level differences across the GS. While decadal oscillations dominate the 116-year record, the analysis showed an unprecedented long period of weakening in the GS flow since the late 1990s. The only other period of long weakening in the record was during the 1960s–1970s, and red noise experiments showed that is very unlikely that those just occurred by chance. Ensemble empirical mode decomposition (EEMD) was used to separate oscillations at different timescales, showing that the low-frequency variability of the GS is connected to the Atlantic Multi-decadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC). The recent weakening of the reconstructed GS-MAB was mostly influenced by weakening of the upper mid-ocean transport component of AMOC as observed by the RAPID measurements for 2005–2015. Comparison between the reconstructed sea level near the coast and tide gauge data for 1927–2015 showed that the reconstruction underestimated observed coastal sea level variability for timescales less than ∼5 years, but lower-frequency variability of coastal sea level was captured very well in both amplitude and phase by the reconstruction. Comparison between the GS-SAB proxy and the observed Florida Current transport for 1982–2015 also showed significant correlations for oscillations with periods longer than ∼5 years. The study demonstrated that despite the coarse horizontal resolution of the global reconstruction (1∘ × 1∘), long-term variations in regional dynamics can be captured quite well, thus making the data useful for studies of long-term variability in other regions as well.

Persistence of coastal upwelling after a plunge in upwelling-favourable wind

Unprecedented coastal upwelling off the southern coast of the Korean Peninsula was reported during the summer of 2013. The upwelling continued for more than a month after a plunge in upwelling-favourable winds and had serious impacts on fisheries. This is a rare phenomenon, as most coastal upwelling events relax a few days after the wind weakens. In this study, observational data and numerical modelling results were analysed to investigate the cause of the upwelling and the reason behind it being sustained for such an extended period. Coastal upwelling was induced by an upwelling-favourable wind in July, resulting in the dynamic uplift of deep, cold water. The dynamic uplift decreased the steric sea level in the coastal region. The sea level difference between the coastal and offshore regions produced an intensified cross-shore pressure gradient that enhanced the surface geostrophic current along the coast. The strong surface current maintained the dynamic uplift due to geostrophic equilibrium. This positive feedback between the dynamic uplift and geostrophic adjustment sustained the coastal upwelling for a month following a plunge in the upwelling-favourable wind.

Sea Level Variability in the Red Sea: A Persistent East–West Pattern

Based on 26 years of satellite altimetry, this study reveals the presence of a persistent east–west pattern in the sea level of the Red Sea, which is visible throughout the years when considering the east–west difference in sea level. This eastern–western (EW) difference is positive during winter when a higher sea level is observed at the eastern coast of the Red Sea and the opposite occurs during summer. May and October are transition months that show a mixed pattern in the sea level difference. The EW difference in the southern Red Sea has a slightly higher range compared to that of the northern region during summer, by an average of 0.2 cm. Wavelet analysis shows a significant annual cycle along with other signals of lower magnitude for both the northern and southern Red Sea. Removing the annual cycle reveals two energy peaks with periodicities of <12 months and 3–7 years, representing the intraseasonal and El Nino—Southern Oscillation (ENSO) signals, respectively. Empirical Orthogonal Function (EOF) analysis shows that EOF1 corresponds to 98% of total variability, EOF2 to 1.3%, and EOF3 to 0.4%. The remote response of ENSO is evident in the variability in the atmospheric bridge, while that of the Indian Ocean Dipole (IOD) and North Atlantic Oscillation (NAO) is weak. Three physical mechanisms are responsible for the occurrence of this EW difference phenomenon, namely wind, buoyancy, and the polarity of eddies.

Control of Bering Strait Transport by the Meridional Overturning Circulation

AbstractIt is well established that the mean transport through Bering Strait is balanced by a sea level difference between the North Pacific and the Arctic Ocean, but no mechanism has been proposed to explain this sea level difference. It is argued that the sea level difference across Bering Strait, which geostrophically balances the northward throughflow, is associated with the sea level difference between the North Pacific and the North Atlantic/Arctic. In turn, the latter difference is caused by deeper middepth isopycnals in the Indo-Pacific than in the Atlantic, especially in the northern high latitudes because there is deep water formation in the Atlantic, but not in the Pacific. Because the depth of the middepth isopycnals is associated with the dynamics of the upper branch of the meridional overturning circulation (MOC), a model is formulated that quantitatively relates the sea level difference between the North Pacific and the Arctic/North Atlantic with the wind stress in the Antarctic Circumpolar region, since this forcing powers the MOC, and with the outcropping isopycnals shared between the Northern Hemisphere and the Antarctic circumpolar region, since this controls the location of deep water formation. This implies that if the sinking associated with the MOC were to occur in the North Pacific, rather than the North Atlantic, then the Bering Strait flow would reverse. These predictions, formalized in a theoretical box model, are confirmed by a series of numerical experiments in a simplified geometry of the World Ocean, forced by steady surface wind stress, temperature, and freshwater flux.

A comparison of ocean model data and satellite observations of features affecting the growth of the North Equatorial Counter Current during the strong 1997–1998 El Niño

Abstract. Descriptions of the ocean's role in the El Niño usually focus on equatorial Kelvin waves and the ability of such waves to change the mean thermocline depth and the sea surface temperature (SST) in the central and eastern Pacific. In contrast, starting from a study of the transport of water with temperatures greater than 28 ∘C, sufficient to trigger deep atmospheric convection, Webb (2018) found that, during the strong El Niños of 1983–1984 and 1997–1998, advection by the North Equatorial Counter Current (NECC) had a much greater impact on sea surface temperatures than processes occurring near the Equator. Webb's analysis, which supports the scheme proposed by Wyrtki (1973, 1974), made use of archived data from a high-resolution ocean model. Previously the model had been checked in a preliminary comparison against SST observations in the equatorial Pacific, but, given the contentious nature of the new analysis, the model's behaviour in key areas needs to be checked further against observations. In this paper this is done for the 1987–1988 El Niño, making use of satellite observations of SST and sea level. SST is used to check the movement of warm water near the Equator and at the latitudes of the NECC. Sea level is used to check the model results at the Equator and at 6∘ N in the North Equatorial Trough. Sea level differences between these latitudes affect the transport of the NECC, the increased transport at the start of each strong El Niño being associated with a drop in sea level at 6∘ N in the western Pacific. Later rises in sea level at the Equator increase the transport of the NECC in mid-ocean. The variability of sea level at 6∘ N is also used to compare the strength of tropical instability waves in the model and in the observations. The model showed that in a normal year these act to dilute the temperature in the core of the NECC. However their strength declined during the development of the strong El Niños, allowing the NECC to carry warm water much further than normal across the Pacific. The results of this paper should not be taken as providing proof of the hypotheses of Wyrtki (1973, 1974) or Webb (2018) but instead as a failure of a targeted study, using satellite observations, to disprove the hypotheses.