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Mark A. Merrifield

Abstract

Pacific Ocean sea surface height trends from satellite altimeter observations for 1993–2009 are examined in the context of longer tide gauge records and wind stress patterns. The dominant regional trends are high rates in the western tropical Pacific and minimal to negative rates in the eastern Pacific, particularly off North America. Interannual sea level variations associated with El Niño–Southern Oscillation events do not account for these trends. In the western tropical Pacific, tide gauge records indicate that the recent high rates represent a significant trend increase in the early 1990s relative to the preceding 40 years. This sea level trend shift in the western Pacific corresponds to an intensification of the easterly trade winds across the tropical Pacific. The wind change appears to be distinct from climate variations centered in the North Pacific, such as the Pacific decadal oscillation. In the eastern Pacific, tide gauge records exhibit higher-amplitude decadal fluctuations than in the western tropical Pacific, and the recent negative sea level trends are indistinguishable from these fluctuations. The shifts in trade wind strength and western Pacific sea level rate resemble changes in dominant global modes of outgoing longwave radiation and sea surface temperature. It is speculated that the western Pacific sea level response indicates a general strengthening of the atmospheric circulation over the tropical Pacific since the early 1990s that has developed in concert with recent warming trends.

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John F. Middleton and Mark A. Merrifield

Abstract

The scattering of coastal-trapped waves (CTWs) by a region of irregular shelf bathymetry is determined from a circulation integral of the depth-integrated momentum equations. For relatively weak stratification the conservation of geostrophic mass flux along isobaths is used to show that bottom pressure of the transmitted waves is equal to bottom pressure p b of the incident waves, when mapped along constant depth contours, plus corrections for the effects of frictional spindown and the rate of change of relative vorticity. These corrections result from changes in the incident wave alongisobath velocity, which can be amplified by the convergence of isobaths between the incident and transmitted regions. For the case of the Labrador shelf, the convergence of isobaths south of the (incident) Hudson Strait region leads to a tenfold increase in the production of relative vorticity and in the correction for pressure for a mode 1 incident wave. This leading order increase in vorticity production violates the assumption of constant geostrophic mass flux and implies that the frictional correction, while small, is invalid. However, the transmitted mode 1 and 2 amplitudes determined are insensitive to these corrections and, in agreement with observations, are of similar magnitude and about 180° out of phase.

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Alejandro Ludert, Bin Wang, and Mark A. Merrifield

Abstract

The U.S.-Affiliated Pacific Islands (USAPIs), located in the tropical western Pacific, are very susceptible to severe drought. Dry season (December–May) rainfall anomalies have different relationships to ENSO for USAPIs north and south of 7°N. South of 7°N, rainfall exhibits a canonical negative correlation with the Oceanic Niño Index (ONI) (i.e., dry conditions during warm periods). To the north, the dry season falls into either “canonical” or “noncanonical” (positively correlated with ONI) regimes. Noncanonical years pose an important forecasting challenge as severe droughts have occurred during cool ONI conditions (referred to here as “cool dry” cases). Composite analysis of the two regimes shows that for noncanonical cool dry years, anticyclonic circulation anomalies over the tropical western North Pacific (TWNP), with a band of anomalous dry conditions extending from the central Pacific toward Micronesia, result in unexpected droughts. In contrast, canonical “cool wet” events show cyclonic TWNP circulation and increased rainfall over the northern USAPIs. Maximum SST anomalies are located near the date line during noncanonical years, and farther east during canonical years. While both regimes show negative rainfall and TWNP anticyclonic circulation anomalies before the onset of the December–May dry season, during the dry season these anomalies persist during noncanonical events but rapidly reverse sign during canonical events. SST anomalies in the noncanonical regime extend eastward from the central Pacific rather than intensify in place over the eastern Pacific in the canonical regime. Differences in the evolution of circulation, precipitation, and SST anomalies suggest distinct physical mechanisms governing the two ENSO regimes, with possible ramifications for seasonal forecasts.

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T. M. Shaun Johnston and Mark A. Merrifield

Abstract

A network of island tide gauges is used to estimate interannual geostrophic current anomalies (GCAs) in the western Pacific from 1975 to 1997. The focus of this study is the zonal component of the current averaged between 160°E and 180° and 2° to 7° north and south of the equator in the mean flow regions associated with the North Equatorial Countercurrent (NECC) and the South Equatorial Current (SEC), respectively. The tide gauge GCA estimates agree closely with similarly derived currents from TOPEX/Poseidon sea level anomalies. The GCAs in the western Pacific relate to a basin-scale adjustment associated with the El Niño–Southern Oscillation, characterized here using empirical orthogonal functions of tide gauge and supporting sea surface temperature and heat storage data. The dominant EOF mode describes the mature phase of ENSO events and correlates (0.8) with the GCA south of the equator. The second mode describes transitions to and from ENSO events and correlates (0.9) with the GCA north of the equator. The typical scenario then is for the NECC to intensify about 6 months prior to the peak of an El Niño, to remain near mean conditions during the peak stage of El Niño, and to later weaken about 6 months following the peak. In contrast, the SEC generally weakens throughout an El Niño displaying eastward anomalies. This equatorial asymmetry in the GCAs is consistent with a similar asymmetry in the wind field over the western Pacific. The phase differences between the NECC and SEC are less apparent during La Niña events. The GCA results provide further evidence that transitional phases of ENSO are more active north than south of the equator in the warm pool region.

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Glenn S. Carter, Michael C. Gregg, and Mark A. Merrifield

Abstract

Microstructure observations over a small seamount on the Kaena Ridge, Hawaii, showed asymmetry in the along- and across-ridge directions. The ∼400-m-high seamount is on the southern edge of the ridge (centered at 21°43′49″N, 158°38′48″W), 42 km northwest of Oahu. A 1-km-resolution numerical simulation shows that the flow within the depth range of the seamount tends to be accelerated around the seamount rather than going up and over it. The flow patterns, however, are more complicated than for an isolated seamount because of the influence of the ∼3000-m-high Kaena Ridge. Comparison with the numerical simulations indicates that the across-ridge asymmetry, in which dissipation on the north-northeastern side of the seamount was higher and more concentrated toward the bed than on the south-southwestern side, is consistent with an M 2 tidal beam generated at the northern edge of the ridge. The along-ridge asymmetry, with higher dissipation on the east-southeastern flank than on the west-northwestern flank, is in qualitative agreement with M 2 shear variance from the model simulation. The average observed dissipation rate over the seamount was ε = 6.2 × 10−8 W kg−1, and diapycnal diffusivity was Kρ = 1.3 × 10−3 m2 s−1. Dissipation measurements following the 1000-m isobath south-southwest of the seamount suggest along-ridge internal tide generation caused by topographic steering that creates an along-ridge current over critical topography northwest of the seamount.

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Jerome Aucan, Mark A. Merrifield, Douglas S. Luther, and Pierre Flament

Abstract

A 3-month mooring deployment (August–November 2002) was made in 2425-m depth, on the south flank of Kaena Ridge, Hawaii, to examine tidal variations within 200 m of the steeply sloping bottom. Horizontal currents and vertical displacements, inferred from temperature fluctuations, are dominated by the semidiurnal internal tide with amplitudes of ≥ 0.1 m s−1 and ∼100 m, respectively. A series of temperature sensors detected tidally driven overturns with vertical scales of ∼100 m. A Thorpe scale analysis of the overturns yields a time-averaged dissipation near the bottom of 1.2 × 10−8 W kg−1, 10–100 times that at similar depths in the ocean interior 50 km from the ridge. Dissipation events much larger than the overall mean (up to 10−6 W kg−1) occur predominantly during two phases of the semidiurnal tide: 1) at peak downslope flows when the tidal stratification is minimum (N = 5 × 10−4 s−1) and 2) at the flow reversal from downslope to upslope flow when the tidal stratification is ordinarily increasing (N = 10−3 s−1). Dissipation associated with flow reversal mixing is 2 times that of downslope flow mixing. Although the overturn events occur at these tidal phases and they exhibit a general spring–neap modulation, they are not as regular as the tidal currents. Shear instabilities, particularly due to tidal strain enhancements, appear to trigger downslope flow mixing. Convective instabilities are proposed as the cause for flow reversal mixing, owing to the oblique propagation of the internal tide down the slope. The generation of similar tidally driven mixing features on continental slopes has been attributed to oblique wave propagation in previous studies. Because the mechanical energy source for mixing is primarily due to the internal tide rather than the surface tide, the observed intermittency of overturn events is attributed to the broadbanded nature of the internal tide.

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Matthew H. Alford, Michael C. Gregg, and Mark A. Merrifield

Abstract

Large semidiurnal vertical displacements (≈100 m) and strong baroclinic currents (≈0.5 m s−1; several times as large as barotropic currents) dominate motions in Mamala Bay, outside the mouth of Pearl Harbor, Hawaii. During September 2002, the authors sought to characterize them with a 2-month McLane moored profiler deployment and a 4-day intensive survey with a towed CTD/ADCP and the Research Vessel (R/V) Revelle hydrographic sonar. Spatial maps and time series of turbulent dissipation rate ϵ, diapycnal diffusivity Kρ, isopycnal displacement η, velocity u, energy E, and energy flux F are presented. Dissipation rate peaks in the lower 150 m during rising isopycnals and high strain and shows a factor-of-50 spring–neap modulation. The largest Kρ values, in the western bay near a submarine ridge, exceed 10−3 m2 s−1. The M 2 phases of η and u increase toward the west, implying a westward phase velocity cp ≈ 1 m s−1 and horizontal wavelength ≈60 km, consistent with theoretical mode-1 values. These phases vary strongly (≈±45°) in time relative to astronomical forcing, implying remotely generated signals. Energy and energy flux peak 1–3 days after spring tide, supporting this interpretation. The group velocity, computed as the ratio F/E, is near ≈1 m s−1, also in agreement with theoretical mode-1 values. Spatial maps of energy flux agree well with results from the Princeton Ocean Model, indicating converging fluxes in the western bay from waves generated to the east and west. The observations indicate a time-varying interference pattern between these waves that is modulated by background stratification between their sources and Mamala Bay.

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Yvonne L. Firing, Mark A. Merrifield, Thomas A. Schroeder, and Bo Qiu

Abstract

Over the past century, tide gauges in Hawaii have recorded interdecadal sea level variations that are coherent along the island chain. The generation of this signal and its relationship to other interdecadal variability are investigated, with a focus on the last decade. Hawaii sea level is correlated with sea surface height (SSH) over a significant portion of the North Pacific Ocean, and with the Pacific–North America (PNA) index, which represents teleconnections between tropical and midlatitude atmospheric variations. Similar variations extend well below the thermocline in World Ocean Atlas temperature. Comparison with NCEP reanalysis wind and pressure shows that high (low) sea level phases around Hawaii are associated with an increase (decrease) in the strength of the Aleutian low. The associated wind stress curl pattern is dynamically consistent with observed sea level anomalies, suggesting that sea level at Hawaii represents large-scale changes that are directly wind-forced in concert with the PNA. Atmospheric modulation, as opposed to Rossby wave propagation, may explain the linkage of Hawaii sea level to North American sea level and ENSO events. A wind-forced, baroclinic Rossby wave model replicates some aspects of the interdecadal SSH variations and their spatial structure but fails to predict them in detail near Hawaii. The accuracy of wind products in this region and over this time period may be a limiting factor. Variations in mixed layer temperature due to surface heat flux anomalies may also contribute to the interdecadal sea level signal at Hawaii.

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Philip R. Thompson, Mark A. Merrifield, Judith R. Wells, and Chantel M. Chang

Abstract

The rate of coastal sea level change in the northeast Pacific (NEP) has decreased in recent decades. The relative contributions to the decreased rate from remote equatorial wind stress, local longshore wind stress, and local windstress curl are examined. Regressions of sea level onto wind stress time series and comparisons between NEP and Fremantle sea levels suggest that the decreased rate in the NEP is primarily due to oceanic adjustment to strengthened trade winds along the equatorial and coastal waveguides. When taking care to account for correlations between the various wind stress time series, the roles of longshore wind stress and local windstress curl are found to be of minor importance in comparison to equatorial forcing. The predictability of decadal sea level change rates along the NEP coastline is therefore largely determined by tropical variability. In addition, the importance of accounting for regional, wind-driven sea level variations when attempting to calculate accelerations in the long-term rate of sea level rise is demonstrated.

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Øyvind Lundesgaard, Brian Powell, Mark Merrifield, Lisa Hahn-Woernle, and Peter Winsor

Abstract

Fjords along the western Antarctic Peninsula are episodically exposed to strong winds flowing down marine-terminating glaciers and out over the ocean. These wind events could potentially be an important mechanism for the ventilation of fjord waters. A strong wind event was observed in Andvord Bay in December 2015, and was associated with significant increases in upper-ocean salinity. We examine the dynamical impacts of such wind events during the ice-free summer season using a numerical model. Passive tracers are used to identify water mass pathways and quantify exchange with the outer ocean. Upwelling and outflow in the model fjord generate an average salinity increase of 0.3 in the upper ocean during the event, similar to observations from Andvord Bay. Down-fjord wind events are a highly efficient mechanism for flushing out the upper fjord waters, but have little net impact on deep waters in the inner fjord. As such, summer episodic wind events likely have a large effect on fjord phytoplankton dynamics and export of glacially modified upper waters, but are an unlikely mechanism for the replenishment of deep basin waters and oceanic heat transport toward inner-fjord glaciers.

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