Meridional Mode Rossby Waves Research Articles

Atlantic equatorial deep jets in Argo float data

Abstract Equatorial deep jets (EDJ) are zonal currents along the equator in all three ocean basins that alternate in direction with depth and time. In the Atlantic below the thermocline, they are the dominant variability on interannual timescales. Observations of equatorial deep jets are available but scarce, given the EDJs’ location at depth, their small vertical scale and their long periodicity of several years. In the last few years, Argo floats have added a significant amount of measurements at intermediate depth. In this study we therefore revise estimates of the EDJ scales based on Argo float data. Mostly, we use velocity data at 1000 m depth calculated from float displacement, which yield robust estimates of the Atlantic EDJ period (4.6 years), amplitude distribution, phase distribution, zonal wavelength (146.7°), and meridional structure. We also show that the equatorial amplitude of the EDJs’ first meridional mode Rossby wave component (9.8 cm s−1) is larger than that of their Kelvin wave component (2.8 cm s−1). Additionally, we present a new estimation of the EDJs’ vertical structure throughout the Atlantic basin, based on an equatorial geostrophic velocity reconstruction from hydrographic Argo float measurements from depths between 400 and 2000 m. Our new estimates from Argo float data provide the first basin-wide assessment of the Atlantic EDJ scales, as well as having smaller uncertainties than estimates from earlier studies.

Intermediate Intraseasonal Variability in the Western Tropical Pacific Ocean: Meridional Distribution of Equatorial Rossby Waves Influenced by a Tilted Boundary

AbstractIntermediate-depth intraseasonal variability (ISV) at a 20–90-day period, as detected in velocity measurements from seven subsurface moorings in the tropical western Pacific, is interpreted in terms of equatorial Rossby waves. The moorings were deployed between 0° and 7.5°N along 142°E from September 2014 to October 2015. The strongest ISV energy at 1200 m occurs at 4.5°N. Peak energy at 4.5°N is also seen in an eddy-resolving global circulation model. An analysis of the model output identifies the source of the ISV as short equatorial Rossby waves with westward phase speed but southeastward and downward group velocity. Additionally, it is shown that a superposition of first three baroclinic modes is required to represent the ISV energy propagation. Further analysis using a 1.5-layer shallow water model suggests that the first meridional mode Rossby wave accounts for the specific meridional distribution of ISV in the western Pacific. The same model suggests that the tilted coastlines of Irian Jaya and Papua New Guinea, which lie to the south of the moorings, shift the location of the northern peak of meridional velocity oscillation from 3°N to near 4.5°N. The tilt of this boundary with respect to a purely zonal alignment therefore needs to be taken into account to explain this meridional shift of the peak. Calculation of the barotropic conversion rate indicates that the intraseasonal kinetic energy below 1000 m can be transferred into the mean flows, suggesting a possible forcing mechanism for intermediate-depth zonal jets.

Impacts of Indonesian Throughflow on seasonal circulation in the equatorial Indian Ocean

Impacts of the Indonesian Throughflow (ITF) on seasonal circulation in the equatorial eastern Indian Ocean are investigated using the ocean-only model LICOM by opening and closing ITF passages. LICOM had daily forcing from NCEP reanalysis data during 2000-2011. It can reproduce vertical profiles of mean density and buoyancy frequency of World Ocean Atlas 2013 data. The model also simulates well annual oscillation in the central Indian Ocean and semiannual oscillation in the eastern Indian Ocean of sea level anomalies (SLA) using satellite altimeter data, as well as the semiannual oscillation of surface zonal equatorial currents of Ocean Surface Current Analyses Real Time current data in the equatorial Indian Ocean. The wave decomposition method is used to analyze the propagation and reflection of equatorial long waves based on LICOM output. Wave analysis suggests that ITF blockage mainly influences waves generated from the Indian Ocean but not the Pacific Ocean, and eastern boundary reflections play an important role in semiannual oscillations of SLA and zonal current differences in the equatorial Indian Ocean associated with ITF. Reconstructed ITF-caused SLA using wave decomposition coefficient differences between closed and open ITF-passage experiments suggest both Kelvin and Rossby waves from the first baroclinic mode have comparable contributions to the semiannual oscillations of SLA difference. However, reconstructed ITF-caused surface zonal currents at the equator suggest that the first meridional-mode Rossby wave has much greater contribution than the first baroclinic mode Kelvin wave. Both reconstructed sea level and zonal currents demonstrate that the first baroclinic mode has a greater contribution than other baroclinic modes.

Basin-Wavelength Equatorial Deep Jet Signals across Three Oceans

AbstractEquatorial deep jets (EDJs) are equatorially trapped, stacked, zonal currents that reverse direction every few hundred meters in depth throughout much of the water column. This study evaluates their structure observationally in all three oceans using new high-vertical-resolution Argo float conductivity–temperature–depth (CTD) instrument profiles from 2010 to 2014 augmented with historical shipboard CTD data from 1972 to 2014 and lower-vertical-resolution Argo float profiles from 2007 to 2014. The vertical strain of density is calculated from the profiles and analyzed in a stretched vertical coordinate system determined from the mean vertical density structure. The power spectra of vertical strain in each basin are analyzed using wavelet decomposition. In the Indian and Pacific Oceans, there are two distinct peaks in the power spectra, one Kelvin wave–like and the other entirely consistent with the dispersion relation of a linear, first meridional mode, equatorial Rossby wave. In the Atlantic Ocean, the first meridional mode Rossby wave signature is very strong and dominates. In all three ocean basins, Rossby wave–like signatures are coherent across the basin width and appear to have wavelengths the scale of the basin width, with periods of about 5 yr in the Indian and Atlantic Oceans and about 12 yr in the Pacific Ocean. Their observed meridional scales are about 1.5 times the linear theoretical values. Their phase propagation is downward with time, implying upward energy propagation if linear wave dynamics hold.

The dynamics of wind‐driven intraseasonal variability in the equatorial Indian Ocean

Variability in the equatorial Indian Ocean on intraseasonal time scales (defined as periods of 30–110 days) is investigated using satellite and in situ observations and a simple analytical linear long‐wave equatorial β‐plane model. Despite the extreme simplicity of the model, which includes just the two gravest baroclinic mode Kelvin waves and first meridional mode Rossby waves, simulated surface zonal velocity and sea surface height compare very well with observations. Both observations and model are characterized by a red shift in the velocity spectrum relative to the wind forcing spectrum, which is attributable to a combination of factors, including (1) the near resonant excitation of Kelvin waves by eastward propagating winds, (2) constructive interference between wind‐forced waves and Rossby waves reflected from the eastern boundary, and (3) the favored excitation of low‐frequency waves whose zonal wavelengths are long compared to the zonal fetch of the wind. We decomposed variability in two broad period bands, namely, 30–70 days and 70–110 days, for detailed analysis. At periods of 30–70 days, zonal velocity tends to be stationary in the directly forced region along the equator owing to the competing contributions of Kelvin and Rossby waves. In contrast, at 70–110 day periods, zonal velocity propagates westward despite eastward propagation of zonal wind stress because of the combined influence of eastern boundary generated and wind‐forced Rossby waves. Kelvin waves reflected from the western boundary are negligibly small, indicating that basin mode resonances are not prominent as has been previously suggested.

Seasonal cycle in the upper equatorial Atlantic Ocean

The dynamics of the seasonal cycle in the upper equatorial Atlantic ocean are studied using observations and a hierarchy of ocean models. Distinctive features of the seasonal cycle are strong annual and semiannual components: eastward (westward) propagating sea surface height (SSH) and thermocline depth at the equator (off the equator) and westward propagating surface zonal currents at the equator. Modelling results show that linear theory can explain the seasonal cycle in thermocline depth and SSH. While first‐order linear theory can also explain the structure of the seasonal cycle of surface zonal currents at the equator, nonlinear terms are required; they weaken the variability and improve its phase and zonal extent. The important terms are meridional and vertical advection and vertical diffusion of zonal momentum. The linear solution is essentially determined by the four gravest baroclinic modes and Kelvin and first meridional mode Rossby waves. The eastward propagation in thermocline depth at the equator results from the Kelvin wave contribution, while the westward propagation in thermocline depth off the equator and surface zonal currents at the equator result from the first meridional mode Rossby wave. The contribution of Kelvin and Rossby waves generated by boundary reflections equals that of the directly forced waves. The semiannual cycle in zonal winds although much weaker than the annual component forces a strong semiannual component in SSH and surface zonal currents, because it excites the basin mode of the second baroclinc mode. This explains the observed feature in the seasonal cycle from March to August.

Observations of the Dispersion Characteristics and Meridional Sea Level Structure of Equatorial Waves in the Pacific Ocean

Abstract Spectral techniques applied to altimetry data are used to examine the dispersion relation and meridional sea level structure of wavelike variability with periods of about 20 to 200 days in the equatorial Pacific Ocean. Zonal wavenumber–frequency power spectra of sea surface height, when averaged over about 7°S–7°N, exhibit spectral peaks near the theoretical dispersion curves of first baroclinic-mode equatorial Kelvin and Rossby waves. There are distinct, statistically significant ridges of power near the first and second meridional-mode Rossby wave dispersion curves. Sea level variability near the theoretical Kelvin wave and first meridional-mode Rossby wave dispersion curves is dominantly (but not perfectly) symmetric about the equator, while variability near the theoretical second meridional-mode Rossby wave dispersion curve is dominantly antisymmetric. These results are qualitatively consistent with expectations from classical or shear-modified theories of equatorial waves. The meridional structures of these modes resemble the meridional modes of equatorial wave theory, but there are some robust features of the meridional profiles that were not anticipated. The meridional sea level structure in the intraseasonal Kelvin wave band closely resembles the theoretically expected Gaussian profile, but sea level variability coherent with that at the equator is detected as far away as 11.75°S, possibly as a result of the forced nature of these Kelvin waves. Both first and second meridional-mode Rossby waves have larger amplitude in the Northern Hemisphere. The meridional sea level structure of tropical instability waves closely resembles that predicted by Lyman et al. using a model linearized about a realistic equatorial zonal current system.

Distinct 17- and 33-Day Tropical Instability Waves in Subsurface Observations*

Abstract Tropical instability waves (TIWs) within a half-degree of the equator in the Pacific Ocean have been consistently observed in meridional velocity with periods of around 20 days. On the other hand, near 5°N, TIWs have been observed in sea surface height (SSH), thermocline depth, and velocity to have periods near 30 days. Tropical Atmosphere–Ocean (TAO) Project moored equatorial velocity and temperature time series are used to investigate the spatial and temporal structure of TIWs during 3 years of La Niña conditions from 1998 through 2001. Along 140°W, where the TIW temperature and velocity variabilities are at their maxima, these variabilities include two distinct TIWs with periods of 17 and 33 days, rather than one broadbanded process. As predicted by modeling studies, the 17-day TIW variability is shown to occur not only in meridional velocity at the equator, but also in subsurface temperature at 2°N and 2°S, while the 33-day TIW variability is observed primarily in subsurface temperature at 5°N. These two TIWs, respectively, are shown to have characteristics similar to a Yanai wave/surface-trapped instability and an unstable first meridional mode Rossby wave. One implication of such a description is that the velocity variability on the equator is not directly associated with the dominant 33-day variability along 5°N.

Annual cycle and interannual variability of the mid-depth tropical Atlantic Ocean

Several shipboard sections acquired in the western equatorial Atlantic along 35 ∘ W allow for the first time to analyze the annual cycle of the density and velocity field in the upper 2000 m within this region. The amplitude of the annual harmonic of the velocity field shows several distinct maxima at the equator. Strong amplitudes up to 15 cm s - 1 are found in the depth range of the equatorial intermediate current (EIC) between 400 and 1000 m depth that are slightly shifted northward with respect to the equator. The meridional structure of the annual harmonics as well as upward phase propagation is consistent with downward propagating odd meridional mode Rossby waves. The observations are compared with a regional numerical model with very high vertical resolution. Good agreement is found between the simulated and observed structure and the amplitude of the annual harmonics. The model results suggest the presence of equatorial beams composed of wind-generated Kelvin and Rossby beams (causing seasonality in the near surface layer) as well as Rossby beams generated by the reflection of Kelvin beams at the eastern boundary (causing seasonality in the depth range of the EIC at the 35 ∘ W section). The annual cycle of the sea surface height (SSH) observed by TOPEX/Poseidon altimetry indicates an east–west seesaw pattern that corresponds to the fast response (about 30 days lag) of the zonal SSH gradient to the annual cycle of the zonal wind field at the equator due to the propagation of lowest baroclinic mode equatorial waves. The increase of the easterly winds in the western equatorial Atlantic associated with El Nino in the beginning of 1997 led almost instantaneously to an adjustment of the zonal SSH gradient with elevated (depressed) SSH in the western (eastern) basin. In contrast, the velocity field at intermediate depths lags the zonal wind forcing by several months up to a year as the model simulation reveals.

Annual cycle of equatorial zonal currents in the Pacific

Observational (Tropical Atmosphere‐Ocean array) data on the annual cycle of upper ocean zonal currents on the equator are analyzed using a simple dynamical ocean model in order to investigate underlying dynamics. The model, by treating linear and nonlinear terms semi‐independently, allows a separation of various linear and nonlinear effects. The model focuses on linear dynamics of low‐order baroclinic modes. By realistically simulating the vertical structure of annual cycle, the model shows that linear dynamics determines the vertical and meridional structure of the annual cycle. Nonlinearity is weak and only important in the undercurrent, where it provides a simple mechanism for the annual cycle: mean meridional advection of the annual cycle north of the equator onto the equator, with the boreal springtime surge in the undercurrent being a direct result of a surge centered at 2°N. Model results show that annual variations in zonal currents are out of phase across the equator, surging in the corresponding spring. This behavior is a response to trade wind variations, which are also equatorially antisymmetric, and is generated by the second meridional mode Rossby wave.

Equatorial waves and warm pool displacements during the 1992–1998 El Niño Southern Oscillation events: Observation and modeling

In the equatorial Pacific, zonal displacements of the eastern edge of the warm pool represent an intrinsic manifestation of El Nino Southern Oscillation (ENSO) events, with numerous dynamical and biogeochemical consequences. Following a previous work dedicated to the 1986–1989 Geosat period, we focus on the 1992–1998 zonal displacements of the warm pool using mainly TOPEX/Poseidon data. We also used a simple linear model forced by monthly ERS winds to help in the interpretation of the results. We found that the 1992–1998 zonal displacements of the warm pool resulted mainly from horizontal advection by zonal current anomalies, through a combination of interannual equatorial Kelvin and first meridional mode Rossby waves. The interannual equatorial Kelvin waves were essentially wind forced in the western and central equatorial Pacific, with some minor contribution from reflected Rossby waves on the western Pacific boundary. In particular, westerly wind anomalies and the resulting downwelling Kelvin waves (entailing eastward surface current anomalies and thermocline deepening) contributed strongly to the onset of the 1993, 1994–1995 and 1997–1998 El Niño events. In contrast, easterly wind anomalies and the resulting upwelling Kelvin waves (with westward surface current anomalies and thermocline shoaling) played a role in stopping the 1993 El Niño and in shifting the 1994–1995 and 1997–1998 El Niño into La Niña events. Consistently with the 1987–1988 El Niño‐La Niña scenario, two main downwelling Rossby wave packets, originating from eastern boundary reflections and wind forcing, crossed the entire basin in 1993 and 1994–1995. These waves favored the decay of the corresponding El Niño events, in the sense that their associated current anomalies contributed to shifting the displacements of the eastern edge of the warm pool from eastward to westward. Unlike what happened for the termination of the 1993 and 1994–1995 El Niño events, downwelling Rossby wave packets, mostly reflected from impinging Kelvin waves, did not propagate all the way to the western Pacific during the 1997–1998 El Niño. They stopped propagating in the central basin where they met unfavorable eastward migrating westerly wind anomalies which generated upwelling Rossby waves. Hence reflected downwelling and wind‐forced upwelling Rossby waves opposed each other for shifting the eastern edge of the warm pool. The rapid demise of the 1997–1998 El Niño and its shift into La Niña in mid‐1998 are interpreted as resulting mainly from the effect of upwelling Kelvin waves forced by easterly wind anomalies occurring in the west from the end of 1997. The associated thermocline shoaling was further enhanced by the wind‐forced upwelling Rossby waves in the central basin in mid‐1998, strongly influencing the fast sea surface temperature (SST) cooling at times when the thermocline was very close to the surface at the end of the mature phase of the 1997–1998 El Niño.

Vertically Propagating Annual and Interannual Variability in an OGCM Simulation of the Tropical Pacific Ocean in 1985–94

Abstract The annual and interannual variability in the subthermocline equatorial Pacific is studied in a simulation of the tropical Pacific for the 1985–94 decade using a primitive equation high-resolution model. The study focuses on temperature variability and vertical energy fluxes. Similar to the observations made by W. S. Kessler and J. P. McCreary, the annual harmonic of vertical isotherm displacements presents phase lines sloping downward from east to west with off-equatorial maxima. Estimates of zonal and vertical phase speeds, the location of the maxima of isotherm displacements, and an analysis of ernergy fluxes suggest the presence of l = 1 Rossby waves. The simulated field is somewhat more trapped toward the equator than the observations. A linear simulation is carried out for the vertical standing modes with characteristics derived from the OGCM simulation. In the linear simulation, high-order meridional mode Rossby waves are more prominent than in the OGCM simulation. However, the (l = 1) Ros...

Zonal displacement of the western equatorial Pacific “fresh pool”

The distribution of salt in the tropical Pacific Ocean and its variability are potentially important in better understanding the El Niño Southern Oscillation (ENSO) ocean‐atmosphere coupled system. Complementary data sets (including sea surface salinity (SSS) derived from a ship‐of‐opportunity network, Geosat sea level and derived zonal surface currents, and outgoing longwave radiation‐derived precipitation, evaporation, and cruise measurements along the 165°E longitude) are used to describe and understand SSS changes in the western equatorial Pacific over several ENSO cycles. It is first shown that the 1974–1989 zonal displacements of the eastern edge of the western equatorial Pacific “fresh pool” (SSS < 35), marked by a salinity front and closely related to the eastern edge of the warm pool, were dominated by interannual variations that are highly correlated with the Southern Oscillation Index (SOI). Owing to the availability of basin‐wide Geosat surface current data, it is then shown that the eastward extension of the eastern edge of the fresh pool during the 1986–1987 El Niño, as well as the westward retreat during the 1988–1989 La Niña, were chiefly the result of zonal salt advection in the equatorial band. Although the equatorial upwelling extended toward the western Pacific during La Niña, its effectiveness in changing the near‐surface salinity (and temperature) is found questionable. Bearing in mind the uncertainty in determining open‐ocean precipitation, evaporation, and mixed‐layer depth, it appears that the net freshwater flux probably played a minor role in the zonal displacements of the eastern edge of the fresh pool, as compared to zonal salt advection. The roles of first baroclinic Kelvin and first meridional mode Rossby waves are discussed considering that they account in a large part for zonal advection.

Effect of Equatorial Long Waves on the North Equatorial Countercurrent

An analytical, reduced-gravity, equatorial long wave model is employed to study the effect of equatorial long waves on the North Equatorial Countercurrent(NECC).This model is first forced by an abruptly switched-on-off zonal wind stress which is distributed uniformly in the zonal direction. As the wind is turned on, an eastward current with a maximum speed at 5°N and a trough of thermocline at 3°N are generated in the western basin. These features coincide with the observed distribution of NECC. The initial intensification of this eastward current is due to the forced first meridional mode Rossby wave. The duration of intensification is related to the wind fetch and Rossby wave speed. After the forced Rossby wave has passed, the eastward current is decelerated or accelerated by a series of reflected Rossby waves generated at the eastern boundary. As the steady state is reached, the easterly wind is then turned off. The forced first meridional mode Rossby wave, now generated by wind relaxation, decelerates the eastward current and even turns the current westward, due to overshooting. Replacing the uniform wind with a linear wind, we find that the general features remain unchanged but the NECC now has larger amplitude and smaller zonal domain. The present linear wind has similar zonal distribution to the trade wind on the tropical Atlantic Ocean. Finally, the linear wind stress distribution, with a representative of time variations observed along the equator, is applied to force the model. The resultant ocean response is compared with the velocity measurement at 6°N, 28°W. General agreement is found. The equatorial long waves, especially the forced Rossby waves, have significant impact upon the NECC.

Fitting Dynamic Models to the Geosat Sea Level Observations in the Tropical Pacific Ocean. Part I: A Free Wave Model

Free, equatorially trapped sinusoidal wave solutions to a linear model on an equatorial beta plane are used to fit the Geosat altimetric sea level observations in the Tropical Pacific Ocean. The Kalman filter technique is used to estimate the wave amplitude and phase from the data. The estimation is performed at each time step by combining the model forecast with the observation in an optimal fashion utilizing the respective error covariances. The model error covariance is determined such that the performance of the model forecast is optimized. It is found that the dominant observed features can be described qualitatively by basin-scale Kelvin waves and the first meridional mode Rossby waves. Quantitatively, however, only 23% of the signal variance can be accounted for by this simple model.

On the reflection and transmission of low‐frequency energy at the irregular western Pacific Ocean boundary

The western boundary of the tropical Pacific is not continuous, and leakage of low‐frequency energy from the Pacific to the Indian Ocean is possible. At low frequencies, equatorial Kelvin and Rossby waves have very large east‐west scales compared with the east‐west scale of the land masses in the region. Consequently, these land masses may be treated as islands that are infinitesimally thin in the east‐west direction. By generalizing previous theory for a single island, the leakage and multiple reflection of low‐frequency energy through the seven major “islands” forming the boundary of the western Pacific can be studied. The results obtained depend on continuity of mass and large‐scale balances and not on the details of nonlinear and/or frictional flow near island eastern boundaries. The major results are as follows: (1) When a mode 1 low‐frequency Rossby wave is reflected at the discontinuous western Pacific boundary, the eastward reflected Kelvin wave energy flux is about one third of the incoming energy flux, or about two thirds ofthat expected for a solid meridional wall. In other words, the reflected Kelvin wave amplitude is 83% of that which would be reflected from a solid meridional wall. The reflection mainly occurs from the Indonesia/Borneo/Asia land mass and very little energy gets into the Indian Ocean. (2) Sea levels in the western equatorial Pacific and on the western boundaries of the major western Pacific land masses should be in phase and of a similar amplitude. In particular, in‐phase interannual sea levels should occur along Australia's western coast and be highly correlated with sea levels in the western equatorial Pacific. Sea level data at Truk Island and on Australia's western coastline confirm this prediction. (3) The interannual exchange of water between the Pacific and Indian oceans due to interannual oscillations in the Pacific is about 6 Sv (1 Sv = 106 m3 s−1), is highly correlated with El Niño/Southern Oscillation (ENSO) events, and bears definite phase relationships to sea level stations at locations in the western Pacific. For example, for a high sea level at Darwin, the transport is westward from the Pacific Ocean into the Indian Ocean. It seems that interannual transport may be approximately monitored using coastal sea level. (4) Strong narrow low‐frequency currents are predicted to occur westward of some island tips. (5) Interannual Indian Ocean Kelvin wave signals are largely blocked at the Indonesia/Borneo/Asia land mass. (6) In terms of net transport, reflection, and transmission properties, the seven island Pacific western boundary can be reduced to a simpler two‐island problem, in which one island is the Indonesia/Borneo/Asia land mass and the other New Guinea/Australia. (7) The comparatively small Halmahera Sea between Halmahera and New Guinea is dynamically important. Closing this sea for the incident first meridional mode Rossby wave case reduces the interannual transport between the Pacific and Indian Oceans by 40%.

An estimate of equatorial wave energy flux at 9‐ to 90‐day periods in the central Pacific

Deep fluctuations in current along the equator in the Central Pacific are dominated by coherent structures which correspond closely to narrow‐band (in wave number) propagating equatorial waves. Currents were measured roughly at 1500 and 3000 m depths at 5 moorings between 144 and 148°W from January 1981 to March 1983 as part of the Pacific Equatorial Ocean Dynamics (PEQUOD) program. In each frequency band resolved, a single complex empirical orthogonal function accounts for half to three quarters of the observed variance in either zonal or meridional current. Dispersion for equatorial first meridional Rossby and Rossby gravity waves is consistent with the observed vertical‐zonal coherence structure. The observations indicate that energy flux is westward and downward in long first meridional mode Rossby waves at periods 45 days and longer and eastward and downward in short first meridional mode Rossby waves and Rossby‐gravity waves at periods 30 days and shorter. A local minimum in energy flux occurs at periods corresponding to a maximum in upper ocean meridional current energy contributed by tropical instability waves. Total vertical flux (energy transport per unit zonal distance) across the 9‐ to 90‐day period range is 2.5 kW/m.