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Yunxia Zheng
,
Zhanhong Ma
,
Jie Tang
, and
Zheliang Zhang

Abstract

The characteristics of in-storm cooling occurring ahead of the eye center are investigated based on a combination of observations and numerical simulations, as well as its sensitivity to tropical cyclone (TC) characteristics and oceanic climatological conditions. A composite of drifter and remote sensing observations from 1979 to 2020 in the Northern Hemisphere statistically evidences that the percentage of TC-induced ahead-of-eye-center cooling is enhanced remarkably over the coastal ocean compared with that over the open sea, no matter what the TC intensity, translation speed, and prestorm SST conditions are. Results are statistically similar when the actual ahead-of-eye SST cooling is used. Idealized numerical simulation results show that as the TC center approaches the coastline, the percentage of ahead-of-eye-center cooling increases steadily with the water depth shallowing below 100 m. This phenomenon may not be caused by strong stratification of the coastal ocean, as previous studies suggested. An ocean heat balance analysis reveals a new mechanism responsible for the enhanced percentage of ahead-of-eye-center cooling near the coast: although the vertical mixing dominates in the surface cooling process over the open sea, broad and intense advection is largely responsible for the rapid increase of the percentage of ahead-of-eye-center cooling over the coastal ocean, owing to less cold-water entrainment from below. A series of sensitivity experiments are conducted by varying TC characteristics in terms of intensity, translation speed, radius of maximum wind speed, and ocean characteristics in terms of temperature profiles and slope rates of the shelf. The percentage of ahead-of-eye-center cooling is dependent on the intensity and translation speed of TCs but shows little sensitivity to other parameters.

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Motoki Nagura
and
Michael J. McPhaden

Abstract

Frequency spectra of in situ meridional velocity measurements in the central equatorial Indian Ocean show two distinct peaks at “quasi biweekly” periods of 10–30 days. One is near the surface at frequencies of 0.06–0.1 cpd (periods of 10–17 days) and the other is in the pycnocline (∼100-m depth) at lower frequencies of 0.04–0.06 cpd (periods 17–25 days). Analysis of a wind-forced ocean general circulation model shows that variability in the two frequency bands represents wind-driven mixed Rossby–gravity waves. The waves share a similar horizontal structure, but the meridional scale of lower-frequency variability is about one-half of that of higher-frequency variations. Higher-frequency variability has its largest amplitude in the eastern basin while the lower-frequency variability has its largest amplitude in the central basin. The vertical wavelength of lower-frequency variability is smaller by a factor of 3–4 than that of higher-frequency variability. These results are consistent with expectations from linear mixed Rossby–gravity wave theory. Numerical simulations show that the primary driver of these waves is surface wind forcing in the central and eastern Indian Ocean and that dynamical instability does not play a major role in their generation.

Significance Statement

Spectra of meridional velocity in the central equatorial Indian Ocean from in situ measurements show two distinct peaks in the biweekly period band with different spatial structures. This study uses an ocean general circulation model to show that variability in these two bands is driven by surface winds that are themselves spatially structured. The variability in both period bands is consistent with linear mixed Rossby–gravity wave theory, but the spatial structures, including meridional trapping scale, vertical wavelength, and zonal distribution of energy, are very different.

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Kristin Svingen
,
Ailin Brakstad
,
Kjetil Våge
,
Wilken-Jon von Appen
, and
Lukas Papritz

Abstract

The Greenland Sea produces a significant portion of the dense water from the Nordic seas that supplies the lower limb of the Atlantic meridional overturning circulation. Here, we use a continuous 10-yr hydrographic record from moored profilers to examine dense-water formation in the central Greenland Sea between 1999 and 2009. Of primary importance for dense-water formation is air–sea heat exchange, and 60%–80% of the heat lost to the atmosphere during winter occurs during intense, short-lived events called cold-air outbreaks (CAOs). The long duration and high temporal resolution of the moored record has for the first time facilitated a statistical quantification of the direct impact of CAOs on the wintertime mixed layer in the Greenland Sea. The mixed layer development can be divided into two phases: a cooling phase and a deepening phase. During the cooling phase (typically between November and January), CAOs cooled the mixed layer by up to 0.08 K day−1, depending on the intensity of the events, while the mixed layer depth remained nearly constant. Later in winter (February–April), heat fluxes during CAOs primarily led to mixed layer deepening of up to 38 m day−1. Considerable variability was observed in the mixed layer response, indicating that lateral fluxes of heat and salt were also important. The magnitude and vertical distributions of these fluxes were quantified, and idealized mixed layer simulations suggest that their combined effect is a reduction in the mixed layer depth at the end of winter of up to several hundred meters.

Open access
Qinbiao Ni
,
Xiaoming Zhai
,
Zhibin Yang
, and
Dake Chen

Abstract

Mesoscale eddies are ubiquitous features of the global ocean circulation. Traditionally, anticyclonic eddies are thought to be associated with positive temperature anomalies while cyclonic eddies are associated with negative temperature anomalies. However, our recent study found that about one-fifth of the eddies identified from global satellite observations are cold-core anticyclonic eddies (CAEs) and warm-core cyclonic eddies (WCEs). Here we show that in the tropical oceans where the probabilities of CAEs and WCEs are high, there are significantly more CAEs and WCEs in summer than in winter. We conduct a suite of idealized numerical model experiments initialized with composite eddy structures obtained from Argo profiles as well as a heat budget analysis. The results highlight the key role of relative wind-stress-induced Ekman pumping, surface mixed layer depth, and vertical entrainment in the formation and seasonal cycle of these unconventional eddies. The relative wind stress is found to be particularly effective in converting conventional eddies into CAEs or WCEs when the surface mixed layer is shallow. The abundance of CAEs and WCEs in the global ocean calls for further research on this topic.

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Takuro Matsuta
and
Yukio Masumoto

Abstract

Recent studies suggest that the eddy kinetic energy is localized in the lee of significant topographic features in the Antarctic Circumpolar Current (ACC). Here we explore the importance of the local dynamics quantitatively using the outputs from the realistic ocean general circulation model hindcast with the aid of the modified Lorentz energy cycle. Results confirm the importance of energy transfer among reservoirs in the downstream region of standing meanders, showing that the major five standing meanders are responsible for more than 70% of the kinetic energy transfer to eddies and dissipation over the Antarctic Circumpolar Current region. The eddy kinetic energy is generated in the upper 3000-m depth downstream of the standing meanders and transported due to the vertical energy redistribution governed by the vertical pressure flux toward the deeper layer where the eddy energy is dissipated. Moreover, we also calculate the work done by the Ekman transport to confirm that the wind energy input works as the dominant energy source for the baroclinic energy pathway. The advantage of this quantity against the vertical mean density flux is that it is independent of the reference states defined arbitrarily. It is shown that the westerlies can supply sufficient energy locally to initiate baroclinic instability in the Indian and Pacific sectors of the ACC, whereas the nonlocal process is important in the Atlantic sector. Our results suggest that the five narrow regions associated with significant topography play key roles in the energy balance of the ACC region.

Significance Statement

The purpose of this study is to understand the eddy–mean flow interactions in the Antarctic Circumpolar Current from the energetic viewpoint. Our results show that the five narrow regions called “hotspots” in our study are responsible for the energy transfer from the mean flow to eddies. It is also found that the hotspots are important for the energy sink in the Southern Ocean. These findings suggest that the five hotspots are likely to play key roles in the responses of the Antarctic Circumpolar Current to the changes in westerlies in these decades.

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Gunnar Voet
,
Matthew H. Alford
,
Jesse M. Cusack
,
Larry J. Pratt
,
James B. Girton
,
Glenn S. Carter
,
Jody M. Klymak
,
Shuwen Tan
, and
Andreas M. Thurnherr

Abstract

The energy and momentum balance of an abyssal overflow across a major sill in the Samoan Passage is estimated from two highly resolved towed sections, set 16 months apart, and results from a two-dimensional numerical simulation. Driven by the density anomaly across the sill, the flow is relatively steady. The system gains energy from divergence of horizontal pressure work O ( 5 ) kW m 1 and flux of available potential energy O ( 2 ) kW m 1 . Approximately half of these gains are transferred into kinetic energy while the other half is lost to turbulent dissipation, bottom drag, and divergence in vertical pressure work. Small-scale internal waves emanating downstream of the sill within the overflow layer radiate O ( 1 ) kW m 1 upward but dissipate most of their energy within the dense overflow layer and at its upper interface. The strongly sheared and highly stratified upper interface acts as a critical layer inhibiting any appreciable upward radiation of energy via topographically generated lee waves. Form drag of O ( 2 ) N m 2 , estimated from the pressure drop across the sill, is consistent with energy lost to dissipation and internal wave fluxes. The topographic drag removes momentum from the mean flow, slowing it down and feeding a countercurrent aloft. The processes discussed in this study combine to convert about one-third of the energy released from the cross-sill density difference into turbulent mixing within the overflow and at its upper interface. The observed and modeled vertical momentum flux divergence sustains gradients in shear and stratification, thereby maintaining an efficient route for abyssal water mass transformation downstream of this Samoan Passage sill.

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Xiaojiang Zhang
,
Xiaodong Huang
,
Yunchao Yang
,
Wei Zhao
,
Huizan Wang
,
Chunxin Yuan
, and
Jiwei Tian

Abstract

The high-resolution mooring observations reported here reveal a cascade process from internal solitary waves (ISWs) to turbulent mixing via high-frequency internal waves near the maximum local buoyancy frequency (near-N waves) in the deep water of the northern South China Sea (SCS). Riding on the parent ISW, near-N waves with a peak frequency of 20 cph emerged at the trough of the ISW and extended to the rear face of the ISW. Most of the near-N waves occurred around the thermocline, where the isothermal displacements induced by the near-N waves were largest with an amplitude of 12 m. The energy of near-N waves was 5% of that of the parent ISW, and instability investigations showed that due to the strong shear, Ri in the region of strong near-N waves was less than 1/4, suggesting that the near-N waves were unstable and might dissipate rapidly. Simulations based on the Korteweg–de Vries (KdV)–Burgers equation reproduced the formation of observed near-N waves due to the energy cascade from ISWs. Our observational results demonstrate a new energy cascade route from ISWs to turbulence in the deep water, deepening the understanding of the energy dissipation process of ISWs and their roles in the enhanced mixing in the northern SCS.

Open access
Yandong Lang
,
Geoffrey J. Stanley
, and
Trevor J. McDougall

Abstract

An existing approximately neutral surface, the ω surface, minimizes the neutrality error and hence also exhibits very small fictitious dianeutral diffusivity Df that arises when lateral diffusion is applied along the surface, in nonneutral directions. However, there is also a spurious dianeutral advection that arises when lateral advection is applied nonneutrally along the surface; equivalently, lateral advection applied along the neutral tangent planes creates a vertical velocity e sp through the ω surface. Mathematically, e sp = us, where u is the lateral velocity and s is the slope error of the surface. We find that e sp produces a leading-order term in the evolution equations of temperature and salinity, being similar in magnitude to the influence of cabbeling and thermobaricity. We introduce a new method to form an approximately neutral surface, called an ω u · s surface, that minimizes e sp by adjusting its depth so that the slope error is nearly perpendicular to the lateral velocity. The e sp on a surface cannot be reduced to zero when closed streamlines contain nonzero neutral helicity. While e sp on the ω u · s surface is over 100 times smaller than that on the ω surface, the fictitious dianeutral diffusivity on the ω u · s surface is larger, nearly equal to the canonical 10−5 m2 s−1 background diffusivity. Thus, we also develop a method to minimize a combination of e sp and Df , yielding the ω u · s + s 2 surface, which is recommended for inverse models since it has low Df and it significantly decreases e sp through the surface, which otherwise would be a leading term that cannot be ignored in the conservation equations.

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Free access
Carl Wunsch

Abstract

Simplified descriptions of the ocean are useful both for formulating explanatory theories and for conveying meaningful global attributes. Here, using a 26-yr average of a global state estimate from ECCO, the basis for Munk’s “abyssal recipes” is evaluated on a global scale between 1000- and 3000-m depth. The two specific hydrographic stations he used prove untypical, with potential temperature and salinity more generally displaying different vertical scale heights, and thus differing in one-dimensional (in the vertical) values of mixing coefficients and/or vertical velocities. The simplest explanation is that the circulation is fully three-dimensional with temperature and salinity fields not describable with a one-dimensional steady balance. In contrast, the potential density and buoyancy are quantitatively describable through a one-dimensional exponential balance, and which calls for an explanation in terms of turbulent mixing processes.

Open access