Browse

You are looking at 71 - 80 of 6,932 items for :

  • Journal of Physical Oceanography x
  • Refine by Access: All Content x
Clear All
Yu Hong, Ying Zhang, and Yan Du

Abstract

The Southern Ocean (SO) is one of the key regions in absorbing and storing anthropogenic heat. An analysis of the CMIP6 models finds a distinct warming minimum/cooling and freshening in the subtropical ocean thermocline of the south Indian Ocean (SIO) under a medium-emission scenario (SSP245). The warming minimum/cooling has also been found in other warming scenarios in previous studies. However, the freshening here has received less attention. On account of increased precipitation in the models, the SO high latitudes get fresher in a warmer world. We show that this freshening anomaly is advected to the north of the deep mixed layer by the horizontal current and then subducts into the ocean interior in the SIO. As a result, the isopycnal surfaces become fresher, deeper, and cooler. This freshening and cooling signal then propagates to the north along isopycnals through the subtropical gyre and leads to freshening and cooling on the depth coordinates where the vertical movement of isopycnals (heaving) is insignificant. Lacking deep-enough mixed layers, the other two basins show smaller freshening and cooling signs in the models. Here the importance of freshening in temperature redistribution in the ocean interior in the SIO under extensive global warming is emphasized. The result helps interpret the future heat storage in the SO in a warmer world.

Significance Statement

The Southern Ocean (SO) is a key region in absorbing and storing anthropogenic heat. The observed past and simulated future warming in the SO show maximum values between 40° and 45°S and minimum values downward and northward of the deep mixed layers. CMIP6 models show the same pattern, and it is found that this pattern is most distinguished in the south Indian Ocean. The freshening anomaly advected to the deep mixed layers subducts into the ocean interior, leading to freshening and cooling signals on isopycnals. These signals spread with the subtropical gyre and induce warming minimum/cooling there. The result would help interpret the warming pattern in the SO in a warmer world.

Restricted access
Andrew J. Lucas and Robert Pinkel

Abstract

Space- and time-continuous seafloor temperature observations captured the three-dimensional structure of shoaling nonlinear internal waves (NLIWs) off of La Jolla, California. NLIWs were tracked for hundreds of meters in the cross- and along-shelf directions using a fiber optic distributed temperature sensing (DTS) seafloor array, complemented by an ocean-wave-powered vertical profiling mooring. Trains of propagating cold-water pulses were observed on the DTS array inshore of the location of polarity transition predicted by weakly nonlinear internal wave theory. The subsequent evolution of the temperature signatures during shoaling was consistent with that of strongly nonlinear internal waves with a large Froude number, highlighting their potential to impact property exchange. Unexpectedly, individual NLIWs were trailed by a coherent, small-scale pattern of seabed temperature variability as they moved across the mid- and inner shelf. A kinematic model was used to demonstrate that the observed patterns were consistent with a transverse instability with an along-crest wavelength of ∼10 m—a distance comparable to the cross-crest width of the wave core—and with an inferred amplitude of several meters. The signature of this instability is consistent with the span-wise vortical circulations generated in three-dimensional direct numerical simulations of shoaling and breaking nonlinear internal waves. The coupling between the small-scale transverse wave wake and turbulent wave core may have an important impact on mass, momentum, and tracer redistribution in the coastal ocean.

Significance Statement

Internal waves permeate the ocean and atmosphere. Their transport of energy and momentum plays a central role in the ocean as a physical system and mediates critical biogeochemical property exchange. In the coastal ocean, internal waves fuel the local ecosystem by redistributing nutrients and shape the local geomorphology by resuspending and transporting sediment. Despite these important impacts, a detailed understanding of nonlinear internal wave evolution in shallow water remains an elusive goal, limited by the difficulty of observing the process in action. Here we describe a transformative observational approach to track internal waves through shoaling to dissipation, combining fiber optic distributed temperature sensing and ocean-wave-powered vertical profiling to track individual waves continuously in the cross- and along-shelf direction. The waves arise from the locally energetic internal tide and undergo rapid nonlinear transformation in the shallow waters of the inner shelf. Our measurements provide the first observational evidence that after evolving into highly nonlinear waves of elevation, the waves develop a trailing, wake-like, three-dimensional instability. This instability resembles the vortical coherent structures generated in high resolution numerical simulations of internal wave shoaling, previous observations of related phenomena in the atmosphere, and in breaking surface gravity waves. The observed transverse structure has an along-crest wavelength of only ∼10 m, making it nearly invisible to traditional ocean sampling techniques. The generation of coherent vortical structures during internal wave shoaling may have a profound influence on the exchange of energy, nutrients, and sediments in coastal oceans and lakes globally.

Open access
Hengling Leng, Michael A. Spall, and Xuezhi Bai

Abstract

A simplified quasigeostrophic (QG) analytical model together with an idealized numerical model are used to study the effect of uneven ice–ocean stress on the temporal evolution of the geostrophic current under sea ice. The tendency of the geostrophic velocity in the QG model is given as a function of the lateral gradient of vertical velocity and is further related to the ice–ocean stress with consideration of a surface boundary layer. Combining the analytical and numerical solutions, we demonstrate that the uneven stress between the ice and an initially surface-intensified, laterally sheared geostrophic current can drive an overturning circulation to trigger the displacement of isopycnals and modify the vertical structure of the geostrophic velocity. When the near-surface isopycnals become tilted in the opposite direction to the deeper ones, a subsurface velocity core is generated (via geostrophic setup). This mechanism should help understand the formation of subsurface currents in the edge of Chukchi and Beaufort Seas seen in observations. Furthermore, our solutions reveal a reversed flow extending from the bottom to the middepth, suggesting that the ice-induced overturning circulation potentially influences the currents in the deep layers of the Arctic Ocean, such as the Atlantic Water boundary current.

Restricted access
Gengxin Chen, Rui Xin Huang, Qihua Peng, and Xiaoqing Chu

Abstract

The Sverdrup relation is the backbone of wind-driven circulation theory; it is a simple relation between the meridional transport of the wind-driven circulation in the upper ocean and the wind stress curl. However, the relation is valid for steady circulation only. In this study, a time-dependent Sverdrup relation is postulated, in which the meridional transport in a time-dependent circulation is the sum of the local wind stress curl term and a time-delayed term representing the effect of the eastern boundary condition. As an example, this time-dependent Sverdrup relation is evaluated through its application to the equatorial circulation in the Indian Ocean, using reanalysis data and a reduced gravity model. Close examination reveals that the southward Somali Current occurring during boreal winter is due to the combination of the local wind stress curl in the Arabian Sea and delayed signals representing the time change of layer thickness at the eastern boundary.

Significance Statement

Sverdrup balance dictates the law of meridional transport of steady circulation in the upper ocean, and has been one of the foundations upon which our understanding of ocean circulation is built. However, for circulation forced by time-varying wind stress, with annual, interannual and decadal frequency, the governing law remains elusive. In this study, we introduce a time-dependent Sverdrup relation applicable to time-dependent wind-driven circulation. As an example, this relation is used to diagnose the monsoon-driven circulation in the Indian Ocean.

Restricted access
Chengyan Liu, Zhaomin Wang, Xi Liang, Xiang Li, Xichen Li, Chen Cheng, and Di Qi

Abstract

Warm deep water intrusion over the Antarctic continental shelves threatens the Antarctic ice sheet stability by enhancing the basal melting of ice shelves. In East Antarctica, the Antarctic Slope Current (ASC), along with the Antarctic Slope Front (ASF), acts as a potential vorticity barrier to prevent the warm modified Circumpolar Deep Water (mCDW) from ventilating the cold and fresh shelf. However, mCDW onshore transport is still observed within certain shelf regions, such as submarine troughs running perpendicular to the continental shelf. This study focuses on the dynamic mechanisms governing mCDW intrusion within a submarine trough over the fresh shelf regions, East Antarctica. Based on an idealized eddy-resolving coupled ocean–ice shelf model, two high-resolution process-oriented numerical experiments are conducted to reveal the mechanisms responsible for the mCDW onshore transport. Three dynamic mechanisms governing cross-slope mCDW intrusion are identified: 1) the bottom pressure torque, 2) the topography beta spiral, and 3) the topography Rossby waves. These three mechanisms simultaneously govern the mCDW intrusion together. The bottom pressure torque plays a leading role in driving the time-mean onshore flow whose vertical structure is determined by the topography beta spiral, while the topography Rossby waves contribute to the high-frequency oscillations in the onshore volume and heat transport. The simulated spatial distribution and seasonality of mCDW intrusion qualitatively coincide with the observed mCDW intrusion over fresh shelf regions, East Antarctica. Both the topography beta spiral and the ASC play an important role in governing the seasonality of mCDW intrusion.

Open access
Ran Liu, Guihua Wang, Christopher Chapman, and Changlin Chen

Abstract

The interaction between the Antarctic Circumpolar Current (ACC) jets and mesoscale eddies plays a prominent role in the dynamics of the ACC. However, few studies have investigated the influence of the jets on the statistics of the mesoscale turbulence field. This study combines a mesoscale eddy detection algorithm with the jet-detection technique to extract their interaction in the Southern Ocean from 1993 to 2016. It is found that stronger jet filaments can effectively shorten eddy lifetime, thus introducing a negative correlation between eddy lifetime and matched jet filament velocities. It can also be seen along the pathway of the ACC whether the eddy is located upstream or downstream of topography. A series of numerical experiments suggest that the stronger zonal jet plays the role of a waveguide to facilitate the stronger and faster eastward linear Rossby wave field induced by the mesoscale eddy. A stronger eastward Rossby wave phase speed, enabled by the jet-induced Doppler shift, results in a more rapid loss of eddy energy and shortens the eddy lifetime.

Restricted access
Edward D. Zaron, Ruth C. Musgrave, and Gary D. Egbert

Abstract

The energetics of baroclinic tides are analyzed using the High Resolution Empirical Tide (HRET) model. The HRET model consists of maps of the sea surface height (SSH) anomaly associated with that component of the tides’ baroclinic pressure fields, which are phase locked with the gravitational tidal potential. The dynamical assumptions underpinning the transformation of SSH into corresponding baroclinic velocity and energy flux are examined critically through comparisons with independent information and term balances in the equations of motion. It is found that the HRET-derived phase speed of the mode-1 baroclinic tide agrees closely with the phase speed predicted by the theory for long waves propagating through the observed climatological stratification. The HRET SSH is decomposed into contributions from separate vertical modes, and the energy, energy flux, and energy flux divergence of mode-1 (for M2, S2, K1, and O1) and mode-2 (for M2) tides are computed, with an emphasis on the most accurately determined mode-1 M2. The flux divergence of HRET mode-1 M2, computed as the contour integral of the outbound normal flux around strong generation regions, is found to correspond with independent estimates of the area-integrated barotropic-to-baroclinic-mode-1 conversion, although, there is considerable uncertainty in both the flux divergence and the barotropic-to-baroclinic conversion. Further progress on mapping the baroclinic tidal energetics from altimeter observations will require more dynamically complete descriptions of the baroclinic tides than can be provided by kinematic models of SSH, such as HRET.

Restricted access
Agathe Germe, Joël J.-M. Hirschi, Adam T. Blaker, and Bablu Sinha

Abstract

This study describes the intra- to interannual variability of the Atlantic meridional overturning circulation (AMOC) and the relative dynamical contributions to the total variability in an eddy-resolving 1/12° resolution ocean model. Based on a 53-yr-long hindcast and two 4-yr-long ensembles, we assess the total AMOC variability as well as the variability arising from small differences in the ocean initial state that rapidly imprints on the mesoscale eddy fields and subsequently on large-scale features. This initial-condition-dependent variability will henceforth be referred to as “chaotic” variability. We find that intra-annual AMOC fluctuations are mainly driven by the atmospheric forcing, with the chaotic variability fraction never exceeding 26% of the total variance in the whole meridional Atlantic domain. To understand the nature of the chaotic variability we decompose the AMOC (into its Ekman, geostrophic, barotropic, and residual components). The barotropic and geostrophic AMOC contributions exhibit strong, partly compensating fluctuations, which are linked to chaotic spatial variations of currents over topography. In the North Atlantic, the largest chaotic divergence of ensemble members is found around 24°, 38°, and 64°N. At 26.5°N, where the AMOC is monitored by the RAPID–MOCHA array, the chaotic fraction of the AMOC variability is 10%. This fraction is slightly overestimated with the reconstruction methodology as used in the observations (∼15%). This higher fraction of chaotic variability is due to the barotropic contribution not being completely captured by the monitoring system. We look at the strong AMOC decline observed in 2009/10 and find that the ensemble spread (our measure for chaotic variability) was not particularly large during this event.

Significance Statement

The ocean is characterized by ubiquitous swirls (eddies) with diameters ranging from more than 100 km (low latitudes) to a few tens of kilometers (high latitudes). There is limited predictability of the timing and location of such eddies. They introduce unpredictable (“chaotic”) variability, which affects the ocean circulation on a wide range of spatial and temporal scales. Any observations of ocean currents contain a fraction of chaotic variability. However, it is, in general, not possible to quantify this chaotic variability from observations. Here we use a set of simulations performed with a state-of-the-art ocean computer model to estimate the fraction of chaotic variability in the amount of warm northward flowing near-surface seawater that delivers large amounts of heat to the North Atlantic, known to scientists as the Atlantic meridional overturning circulation (AMOC). We find that about 10%–25% of the AMOC variance is likely to be chaotic.

Open access
Ryan Abernathey, Christopher Bladwell, Gary Froyland, and Konstantinos Sakellariou

Abstract

The connectivity between ocean basins and subbasin regions strongly influences the transport of ocean tracers and thus plays a role in regulating climate and ocean ecosystems. We describe the application of a new technique from nonlinear dynamical systems to infer the Lagrangian connectivity of the deep global ocean. We approximate the dynamic Laplacian using Argo trajectories from January 2011 to January 2017 and extract the eight dominant coherent (or dynamically self-connected) regions at 1500 m depth. Our approach overcomes issues such as sparsity of observed data and floats continually leaving and entering the dataset; only 10% of floats record continuously for the full six years. The identified coherent regions maximally trap water within them over the six-year time frame, providing a distinct analysis of the deep global ocean and relevant information for planning future float deployment. A key result is that the coherent regions are highly stationary, showing minimal displacement over the six-year period. Although our study is concerned with ocean circulation at a multiyear, global scale, the dynamic Laplacian approach may be applied at any temporal or spatial scale to identify coherent structures in ocean flow from positional time series information arising from observations or models.

Restricted access
Qinbo Xu, Chun Zhou, Wei Zhao, Qianwen Hu, Xin Xiao, Dongqing Zhang, Fan Yang, Xiaodong Huang, and Jiwei Tian

Abstract

Intraseasonal fluctuation with periods of ∼90 days in the South China Sea (SCS) basin is investigated based on an array of seven subsurface moorings. In the deep layer, the 90-day fluctuation is revealed to contribute significantly to the variability in the current, accounting for ∼69% of the subinertial variance. This fluctuation propagates westward along the mooring section with a phase speed of ∼4.6 cm s−1. In the upper layer, the fluctuation also propagates westward with a similar phase speed, but with opposite phase to that of the deep layer. These results suggest that the 90-day fluctuation regulating the abyssal SCS should be the first mode baroclinic Rossby wave. A set of experiments based on a two-layer dynamic model reveal that both the local wind stress curl and the flow originating from the North Pacific through the Luzon Strait contribute to drive the 90-day fluctuation in the deep SCS, while the latter plays the dominant role.

Open access