Baroclinic Rossby Waves Research Articles

Baroclinic Characteristics and Energetics of Annual Rossby Waves in the Southern Tropical Indian Ocean

AbstractThe first baroclinic mode Rossby wave is known to be of critical importance to the annual sea level variability in the southern tropical Indian Ocean (STIO; 0°–20°S, 50°–115°E). In this study, an analysis of continuously stratified linear ocean model reveals that the second baroclinic mode also has significant contribution to the annual sea level variability (as high as 81% of the first baroclinic mode). The contributions of residual high-order modes (3 ≤ n ≤ 25) are much less. The superposition of low-order (first and second) baroclinic Rossby waves (BRWs) primarily contribute to the high energy center of sea level variability at ~10°S in the STIO and the vertical energy penetration below the seasonal thermocline. We have found that 1) the low-order BRWs, having longer zonal wavelengths and weaker damping, can couple more efficiently to the local large-scale wind forcing than the high-order modes and 2) the zonal coherency of the Ekman pumping results in the latitudinal energy maximum of low-order BRWs. Overall, this study extends the traditional analysis to suggest the characteristics of the second baroclinic mode need to be taken into account in interpreting the annual variability in the STIO.

Seasonal Variability and Dynamics of the Pacific North Equatorial Subsurface Current

AbstractThe North Equatorial Subsurface Current (NESC) is a subthermocline ocean current uncovered recently in the tropical Pacific Ocean, flowing westward below the North Equatorial Countercurrent. In this study, the dynamics of the seasonal cycle of this current are studied using historical shipboard acoustic Doppler current profiler measurements and Argo absolute geostrophic currents. Both data show a westward current at the depths of 200–1000 m between 4° and 6°N, with a typical core speed of about 5 and 2 cm s−1, respectively. The subsurface current originates in the eastern Pacific, with its core descending to deeper isopycnal surfaces and moving to the equator as it flows westward. The zonal velocity of the NESC shows pronounced seasonal variability, with the annual-cycle harmonics of vertical isothermal displacement and zonal velocity presenting characters of vertically propagating baroclinic Rossby waves. A simple analytical Rossby wave model is employed to simulate the propagation of the seasonal variations of the westward zonal currents successfully, which is the basis for exploring the wind forcing dynamics. The results suggest that the wind curl forcing in the central-eastern basin between 170° and 140°W associated with the meridional movement of the intertropical convergence zone dominates the NESC seasonal variability in the western Pacific, with the winds west of 170°W and east of 140°W playing a minor role in the forcing.

Midlatitude unstable air-sea interaction with atmospheric transient eddy dynamical forcing in an analytical coupled model

While recent observational studies have shown the critical role of atmospheric transient eddy (TE) activities in midlatitude unstable air-sea interaction, there is still a lack of a theoretical framework characterizing such an interaction. In this study, an analytical coupled air-sea model with inclusion of the TE dynamical forcing is developed to investigate the role of such a forcing in midlatitude unstable air-sea interaction. In this model, the atmosphere is governed by a barotropic quasi-geostrophic potential vorticity equation forced by surface diabatic heating and TE vorticity forcing. The ocean is governed by a baroclinic Rossby wave equation driven by wind stress. Sea surface temperature (SST) is determined by mixing layer physics. Based on detailed observational analyses, a parameterized linear relationship between TE vorticity forcing and meridional second-order derivative of SST is proposed to close the equations. Analytical solutions of the coupled model show that the midlatitude air-sea interaction with atmospheric TE dynamical forcing can destabilize the oceanic Rossby wave within a wide range of wavelengths. For the most unstable growing mode, characteristic atmospheric streamfunction anomalies are nearly in phase with their oceanic counterparts and both have a northeastward phase shift relative to SST anomalies, as the observed. Although both surface diabatic heating and TE vorticity forcing can lead to unstable air-sea interaction, the latter has a dominant contribution to the unstable growth. Sensitivity analyses further show that the growth rate of the unstable coupled mode is also influenced by the background zonal wind and the air–sea coupling strength. Such an unstable air-sea interaction provides a key positive feedback mechanism for midlatitude coupled climate variabilities.

Random Movement of Mesoscale Eddies in the Global Ocean

AbstractIn this study we track and analyze eddy movement in the global ocean using 20 years of altimeter data and show that, in addition to the well-known westward propagation and slight polarity-based meridional deflections, mesoscale eddies also move randomly in all directions at all latitudes as a result of eddy–eddy interaction. The speed of this random eddy movement decreases with latitude and equals the baroclinic Rossby wave speed at about 25° of latitude. The tracked eddies are on average isotropic at mid- and high latitudes, but become noticeably more elongated in the zonal direction at low latitudes. Our analyses suggest a critical latitude of approximately 25° that separates the global ocean into a low-latitude anisotropic wavelike regime and a high-latitude isotropic turbulence regime. One important consequence of random eddy movement is that it results in lateral diffusion of eddy energy. The associated eddy energy diffusivity, estimated using two different methods, is found to be a function of latitude. The zonal-mean eddy energy diffusivity varies from over 1500 m2 s−1 at low latitudes to around 500 m2 s−1 at high latitudes, but significantly larger values are found in the eddy energy hotspots at all latitudes, in excess of 5000 m2 s−1. Results from this study have important implications for recently developed energetically consistent mesoscale eddy parameterization schemes which require solving the eddy energy budget.

Variabilities of the Sea Surface Height in the Kuril Basin of the Sea of Okhotsk: Coherent Shelf-Trapped Mode and Rossby Normal Modes

AbstractA time series analysis of the sea surface height anomaly (SSHa) was conducted in the Kuril Basin of the Sea of Okhotsk. The mapping of the satellite-derived SSHa data was optimized to mitigate the effects of sea ice on the SSHa field during winter and early spring. Complex empirical orthogonal functions (CEOFs) were then used to analyze the SSHa field, revealing that the first three modes account for 55% of the signal variance. Mode 1 mainly represents the coherent variability trapped over the shelves all along the coastal regions and the Kuril Islands. Both seasonal and interannual variations are strongly correlated with the alongshore wind stress and can be well explained by an arrested topographic wave. Mode 3 is a propagating mode that exhibits trains of southwestward-propagating, high-amplitude anomalies. One possible mechanism for this is first-mode baroclinic Rossby waves, whose energy propagates from the Kuril Straits toward the Kuril Basin. However, mode 3 can be better interpreted as barotropic Rossby normal modes generated in the deep Kuril Basin. Mode 2 is a standing mode that may encompass the baroclinic variability in the basin. The monthly mean of the SSHa in the Kuril Basin is primarily governed by variability in mode 1, with mode 2 contributing to a lesser extent, and mode 3 being insignificant.

Numerical study on interaction between eddies and the Kuroshio Current east of Taiwan, China

We investigated the interaction between mesoscale eddies and the Kuroshio Current east of Taiwan, China, using a fine-resolution regional general circulation model. Mesoscale eddies are injected into a region east of Taiwan, China, according to the quasi-geostrophic theory of stratified fluids. Modeled eddies propagated westward at the velocity of the first baroclinic mode Rossby wave. When eddies collide with the Kuroshio Current east of Taiwan, China, the spatial structure and volume transport of the Kuroshio Current shows a significant variation. The upper 600 m of the anticyclonic eddy cannot cross the Kuroshio Current to reach the region west of the Kuroshio Current; rather, these waters flow northward along the eastern side of the Kuroshio Current. The upper water carried by the anticyclonic eddies cannot reach the shelf of the East China Sea (ECS). In contrast, the waters in the upper layer of the cyclonic eddy reach the western side of the Kuroshio Current and then flow northward. The dynamic mechanism analysis shows that the interaction between the Kuroshio Current and the cyclonic (anticyclonic) eddy decrease (increase) the horizontal potential vorticity (PV) gradient, or PV barrier, whereby the cyclonic (anticyclonic) eddy can (cannot) cross the Kuroshio Current. This study implies that the continental shelf could potentially be influenced by cyclonic eddies in the open ocean, which can transport heat and material from the upper open ocean across the Kuroshio Current to the shelf waters.

The characteristics of the mid-depth striations in the North Indian Ocean

Argo float trajectory data has been used to estimate the velocities of mid-depth (1000 db) currents in the North Indian Ocean (NIO). The structure of the upper ocean absolute geostrophic currents can be derived more accurately from the Argo temperature and salinity profiles by referring to the estimated velocities rather than by assuming a level of “no motion”. The derived flow field reveals that the eastward zonal velocities generate striation-like structures in the Arabian Sea, which is most prominent above the layer of 500 db layer, with a meridional scale of ~300 km; however, such structures are barely observed in the Bay of Bengal. This meridional scale as well as the distribution of the mid-depth striations are unique to the NIO, as compared with the other ocean basins. The nonlinear 112-layer reduced-gravity model combined with the triad baroclinic Rossby wave interaction theory have been used to determine the essential factors controlling the characteristics of the quasi-zonal striation structure. Compared with the North Pacific Ocean, the meridional scale in the NIO is narrower, and is caused by the smaller basin size in the equatorial zone, rather than by the semiannual wind stress forcing period or the eastern boundary angle. Furthermore, the coastal trapped Kelvin waves significantly contribute to the generation of the zonal striations in the Arabian Sea.

Seasonal Variations of the North Equatorial Current Across the Pacific Ocean

AbstractThe seasonal variations of North Equatorial Current (NEC) axis (YCM), intensity (INT), and transport across the Pacific basin are investigated based on altimetry data, Argo data, and ocean reanalysis data. In the western Pacific (WP), the NEC shifts southward/northward when it is intensified/weakened in the first/latter half of the year. However, in the interior Pacific (IP), the INT shows antiphase fluctuations with that in the WP with obvious westward propagations of the meridional shift of theYCM. In the eastern Pacific (EP), the phase relationship between the YCM and INT is opposite to that in the WP with the strengthening/weakening of the NEC being accompanied by a northward/southward shift of its axis. Different locations of the NEC seasonal thermocline departure maxima/minima in the WP, IP, and EP contribute to its distinct seasonal variations in these three basins. One of the new findings is that the seasonal shallowest thermocline generally corresponds to its largest meridional slope in the IP and EP, hence favoring the intensification of the NEC in these two basins, but not the case in the WP. Model experiments by blocking the baroclinic Rossby wave propagations and Ekman pumpings in different basins suggest that the seasonal variations of the NEC in the WP and EP are mainly controlled by the Ekman pumping, while those in the IP are controlled by the joint effect of the local Ekman pumping and the Rossby waves. The Sverdrup theory tends to overestimate the NEC's mean transport.

Remote Forcing of Tasman Sea Marine Heatwaves

AbstractRecent marine heatwave (MHW) events in the Tasman Sea have had dramatic impacts on the ecosystems, fisheries, and aquaculture off Tasmania’s east coast. However, our understanding of the large-scale drivers (forcing) and potential predictability of MHW events in this region off southeast Australia is still in its infancy. Here, we investigate the role of oceanic Rossby waves forced in the interior South Pacific on observed MHW occurrences off southeast Australia from 1994 to 2016, including the extreme 2015/16 MHW event. First, we used an upper-ocean heat budget analysis to show that 51% of these historical Tasman Sea MHWs were primarily due to increased East Australian Current (EAC) Extension poleward transports through the region. Second, we used lagged correlation analysis to empirically connect the EAC Extension intensification to incoming westward-propagating sea surface height (SSH) anomalies from the interior South Pacific. Third, we dynamically analyzed these SSH anomalies using simple process-based baroclinic and barotropic Rossby wave models forced by wind stress curl changes across the South Pacific. Finally, we show that associated monthly SSH changes around New Zealand may be a useful index of western Tasman Sea MHW predictability, with a lead time of 2–3 years. In conclusion, our findings demonstrate that there is potential predictability of advection-dominated MHW event likelihoods in the EAC Extension region up to several years in advance, due to the deterministic contribution from baroclinic and barotropic Rossby waves in modulating the EAC Extension transports.

Structure and Seasonal Variation of the Indian Ocean Tropical Gyre Based on Surface Drifters

AbstractUsing the Gauss‐Markov decomposition method, this study investigates the mean structure and seasonal variation of the tropical gyre in the Indian Ocean based on the observations of surface drifters. In the climatological mean, the clockwise tropical gyre consists of the equatorial Wyrtki Jets (WJs), the South Equatorial Current (SEC), and the eastern and western boundary currents. This gyre system redistributes the water mass over the entire tropical Indian Ocean basin. Its variations are associated with the monsoon transitions, featuring a typical clockwise pattern in the boreal spring and fall seasons. The relative importance of the geostrophic and Ekman components of the surface currents as well as the role of eddy activity were further examined. It was found that the geostrophic component dominates the overall features of the tropical gyre, including the SEC meandering, the broad eastern boundary current, and the axes of the WJs in boreal spring and fall, whereas the Ekman component strengthens the intensity of the WJs and SEC. Eddies are active over the southeastern tropical Indian Ocean and transport a warm and fresh water mass westward, with direct impact on the southern branch of the tropical gyre. In particular, the trajectories of drifters reveal that during strong Indian Ocean Dipole or El Niño‐Southern Oscillation events, long‐lived eddies were able to reach the southwestern Indian Ocean with a moving speed close to that of the first baroclinic Rossby waves.

Wind-Forced Variability of the Remote Meridional Overturning Circulation

AbstractThe mechanisms by which time-dependent wind stress anomalies at midlatitudes can force variability in the meridional overturning circulation at low latitudes are explored. It is shown that winds are effective at forcing remote variability in the overturning circulation when forcing periods are near the midlatitude baroclinic Rossby wave basin-crossing time. Remote overturning is required by an imbalance in the midlatitude mass storage and release resulting from the dependence of the Rossby wave phase speed on latitude. A heuristic theory is developed that predicts the strength and frequency dependence of the remote overturning well when compared to a two-layer numerical model. The theory indicates that the variable overturning strength, relative to the anomalous Ekman transport, depends primarily on the ratio of the meridional spatial scale of the anomalous wind stress curl to its latitude. For strongly forced systems, a mean deep western boundary current can also significantly enhance the overturning variability at all latitudes. For sufficiently large thermocline displacements, the deep western boundary current alternates between interior and near-boundary pathways in response to fluctuations in the wind, leading to large anomalies in the volume of North Atlantic Deep Water stored at midlatitudes and in the downstream deep western boundary current transport.

Revisit of seasonal variability of subsurface temperature in the tropical Pacific with Argo data

Abstract Seasonal variability of temperature in the tropical Pacific is studied with Argo data from 2001 to 2017. It is found that the strongest seasonal variation is in the subsurface layer corresponding to the thermocline, which is located in three zonal bands centering at 12°N, 5°N and 5°S across the entire tropical Pacific. Further analysis indicates that seasonal variation of subsurface temperature is caused by the vertical movement of the thermocline, and the signal is generated in the eastern Pacific and propagates westward to the western Pacific. A linear wind-driven first-mode baroclinic Rossby wave model is then employed to investigate the seasonal fluctuation of thermocline. The seasonal shoaling and deepening of thermocline and associated temperature variations are well captured by this Rossby wave model. Basin-scale seasonal anomalies of wind stress curl in the tropical Pacific produce significant thermocline fluctuations, which propagate westward in the form of annual Rossby wave, and cause the seasonal variation of subsurface temperature. Thermocline fluctuations are revealed to be tightly associated with the meridional shift of equatorial currents such as North Equatorial Current, North Equatorial Countercurrent and South Equatorial Current.

Dynamics of intraseasonal oscillations in the Bay of Bengal during summer monsoons captured by mooring observations

The intraseasonal variabilities of current and temperature in the southern Bay of Bengal (BoB) were examined using in-situ observations collected in the upper 500 m of the water column for a period of 20 months, between December 2013 and August 2015. Data were sampled by six subsurface moorings deployed by the United States Naval Research Laboratory as part of an international effort focused on the dynamics of the northern Indian Ocean. The moored observations captured motions of multiple time-scales, including seasonal circulations, eddies, and planetary waves that underlie intraseasonal variability. This study focuses on 30–60-day intraseasonal oscillations (ISOs). Spectral analysis of currents shows enhanced energy and momentum fluxes for the period 46–52-days. The ISO velocity fluctuations were coherent in the upper 500 m of the water column, strongest near the surface and nearly uniform below 100 m. The ISO kinetic energy appeared at the beginning of the 2014 summer monsoon and lasted throughout the summer, but the associated potential energy dropped rapidly after it peaked at the beginning of the monsoon season.The correlation between the 30–60 day ISOs of local winds and the oceanic variability below 120 m depth was fund to be as small as 25% suggesting thus that the local winds were not the main forcing mechanism for ISOs. The 30–60-day band-passed sea-surface-height anomalies indicate westward propagating anomalies with phase speeds between 0.17 m s−1 at 8°N and 0.43 m s−1 at 4°N, within the phase-speed ranges of mode-1 and mode-2 linear baroclinic Rossby waves, pointing to the role of Rossby waves in 30–60-day ISOs.The background mean flow acceleration resulting from the meridional divergence of wave momentum flux in the thermocline was about 10−8 m s−2. As a result, within a wave period, ISOs can enhance the eastward flow in the thermocline by about 25%. The negative shear production computed for the same period is consistent with this finding, suggesting that the mean flow has gained kinetic energy at the expense of the ISO band at a rate of about 0.5×10−8 m2 s−2. The meridional heat-flux divergence was -10−7 °C s−1, and has a tendency for cooling the thermocline by about 0.5 °C when the ISOs are active. The ISOs appear to play an important role in the ocean dynamics of the southern BoB during the summer monsoon.

Remote influences on the Indian monsoon low-level jet intraseasonal variations

A strong low-level jet (LLJ), also known as the Findlater jet, develops over the Arabian Sea during the Indian summer monsoon. This jet is an essential source of moisture for monsoonal rainfall over the densely-populated Indian subcontinent and is a key contributor to the Indian Ocean oceanic productivity by sustaining the western Arabian Sea upwelling systems. The LLJ intensity fluctuates intraseasonally within the ~ 20- to 90-day band, in relation with the northward-propagating active and break phases of the Indian summer monsoon. Our observational analyses reveal that these large-scale regional convective perturbations only explain about half of the intraseasonal LLJ variance, the other half being unrelated to large-scale convective perturbations over the Indian Ocean. We show that convective fluctuations in two regions outside the Indian Ocean can remotely force a LLJ intensification, four days later. Enhanced atmospheric deep convection over the northwestern tropical Pacific yields westerly wind anomalies that propagate westward to the Arabian Sea as baroclinic atmospheric Rossby Waves. Suppressed convection over the eastern Pacific/North American monsoon region yields westerly wind anomalies that propagate eastward to the Indian Ocean as dry baroclinic equatorial Kelvin waves. Those largely independent remote influences jointly explain ~ 40% of the intraseasonal LLJ variance that is not related to convective perturbations over the Indian Ocean (i.e. ~ 20% of the total), with the northwestern Pacific contributing twice as much as the eastern Pacific. Taking into account these two remote influences should thus enhance the ability to predict the LLJ.

The Structure of Baroclinic Modes in the Presence of Baroclinic Mean Flow

AbstractThis contribution seeks to understand the vertical structure of linearized quasigeostrophic baroclinic modes when they are modified by the presence of a baroclinic mean flow and associated potential vorticity gradients. It is found that even modest, O(0.05 m s−1), mean flows can give rise to very substantial changes in modal structures, often in the sense of increased surface intensification. The extent to which stable modes are modified depends strongly on the direction of Rossby wave propagation. Further, baroclinically unstable solutions can appear, and a meaningful inviscid critical-layer solution can occur at the transition to instability when the horizontal gradient of potential vorticity changes sign at some depth within the water column. In addition, the gravest, n = 0, vertical stable mode is no longer strictly barotropic, but rather it can carry density variability at frequencies much higher than those possible for baroclinic (higher) Rossby wave modes. This finding appears to be consistent with oceanic current-meter observations that suggest temperature variability propagation even when the frequency is too high for traditional baroclinic Rossby waves to exist.

Commonly used methods fail to detect known propagation speeds of simulated signals from time–longitude (Hovmöller) diagrams

Abstract. This work examines the accuracy and validity of two variants of Radon transform and two variants of the two-dimensional fast Fourier transform (2-D FFT) that have been previously used for estimating the propagation speed of oceanic signals such as sea surface height anomalies (SSHAs) derived from satellite-borne altimeters based on time–longitude (Hovmöller) diagrams. The examination employs numerically simulated signals made up of 20 or 50 modes where one, randomly selected, mode has a larger amplitude than the uniform amplitude of the other modes. Since the dominant input mode is known ab initio, we can clearly define “success” in detecting its phase/propagation speed. We show that all previously employed variants fail to detect the phase speed of the dominant input mode when its amplitude is smaller than 5 times the amplitude of the other modes and that they successfully detect the phase speed of the dominant input mode only when its amplitude is at least 10 times the amplitude of the other modes. This requirement is an unrealistic limitation on oceanic observations such as SSHA. In addition, three of the variant methods detect a dominant mode even when all modes have the exact same amplitude. The accuracy with which the four methods identify a dominant input mode decreases with the increase in the number of modes in the signal. Our findings are relevant to the reliability of phase speed estimates of SSHA observations and the reported “too fast” a phase speed of baroclinic Rossby waves in the ocean.

Resonance of baroclinic waves in the tropical oceans: The Indian Ocean and the far western Pacific

The Indian Ocean has a particularity, its width is close to half the wavelength of a Rossby wave of biannual frequency, this coincidence having been capitalized on by several authors to give the observations a physical basis. The purpose of this article is to show that this is not the case since the resonance of tropical baroclinic waves occurs in all three oceans. This is because the westward-propagating Rossby wave is retroflexed at the western boundary to form off-equatorial Rossby waves dragged by countercurrents before receding and turning back as a Kelvin wave. Thus a quasi-stationary baroclinic wave is formed, whose mean period is tuned to the forcing period. Two independent basin modes resonantly forced are highlighted – 1) a nearly symmetric zonal 1/2-yr period Quasi-Stationary Wave (QSW) that is resonantly forced by the biannual monsoon. It is formed from first baroclinic mode equatorial-trapped Rossby and Kelvin waves and off-equatorial Rossby waves at the western antinode. This QSW controls the Equatorial Counter Current at the node. The Indian Ocean Dipole (IOD) results from a subharmonic mode locking resulting from the coupling of this QSW and the 2nd, 3rd and 4th baroclinic modes - 2) a 1-yr period QSW formed from an off-equatorial baroclinic Rossby wave, which is induced from the southernmost current of the Indonesian Throughflow through the Timor passage, propagating in the southern and northern hemispheres: the drivers are south-easterlies in the southern hemisphere and monsoon wind in the northern hemisphere.

Wind-driven decadal sea surface height and main pycnocline depth changes in the western subarctic North Pacific

The northward shrinkage of the North Pacific western subarctic gyre (WSAG) in the early 2000s is associated with a sea surface height (SSH) elevation and is correlated to sea surface wind stress change. By using a Rossby wave model forced by wind stress, which computes the component variations due to the barotropic and first to fourth baroclinic modes, we estimated decadal changes in SSH and main pycnocline depth in the subpolar region. Realistic decadal SSH elevation and deepening of the main pycnocline depth associated with the northward shrinkage of the western subarctic gyre from the late 1990s to the mid-2000s were reproduced by the model. The sea surface elevation was caused primarily by the barotropic Rossby wave response to the relaxation of the Ekman suction due to the attenuation of the Aleutian Low by frequent La Niña occurrences after the late 1990s in addition to the long-term weakening of the westerly wind. The northward shrinkage of the WSAG was found to be associated with the intensification of an anticyclonic circulation centered around 43–44 ∘ N, 170–175 ∘ E. The westerly wind weakening deepened the main pycnocline in the western subarctic region through the baroclinic Rossby wave mode response to the wind stress change, which mostly accounts the equivalent halocline deepening at station K2 (47 ∘ N, 160 ∘ E). While the first baroclinic mode variation of the water density significantly attenuates during propagation, the higher mode variations, particularly the second and third mode variations, are locally excited through a quasi-resonant amplification mechanism and have profound impacts on the depth of the upper main pycnocline.

South Pacific Decadal Climate Variability and Potential Predictability

Abstract The South Pacific decadal oscillation (SPDO) characterizes the Southern Hemisphere contribution to the Pacific-wide interdecadal Pacific oscillation (IPO) and is analogous to the Pacific decadal oscillation (PDO) centered in the North Pacific. In this study, upper ocean variability and potential predictability of the SPDO is examined in HadISST data and an atmosphere-forced ocean general circulation model. The potential predictability of the IPO-related variability is investigated in terms of both the fractional contribution made by the decadal component in the South, tropical and North Pacific Oceans and in terms of a doubly integrated first-order autoregressive (AR1) model. Despite explaining a smaller fraction of the total variance, we find larger potential predictability of the SPDO relative to the PDO. We identify distinct local drivers in the western subtropical South Pacific, where nonlinear baroclinic Rossby wave–topographic interactions act to low-pass filter decadal variability. In particular, we show that the Kermadec Ridge in the southwest Pacific enhances the decadal signature more prominently than anywhere else in the Pacific basin. Applying the doubly integrated AR1 model, we demonstrate that variability associated with the Pacific–South American pattern is a critically important atmospheric driver of the SPDO via a reddening process analogous to the relationship between the Aleutian low and PDO in the North Pacific—albeit that the relationship in the South Pacific appears to be even stronger. Our results point to the largely unrecognized importance of South Pacific processes as a key source of decadal variability and predictability.

Interannual Variability of Antarctic Intermediate Water in the Tropical North Atlantic

AbstractInterannual variability of Antarctic Intermediate Water (AAIW) in the tropical North Atlantic is investigated using the GECCO2 ocean state estimate and Argo data. AAIW salinity variability near the western boundary is highly correlated with the transport along the western boundary on interannual timescales. Northward propagating anomalies are associated with the western boundary transport variability that, to some extent, is related to the large‐scale wind stress curl forcing by means of the Sverdrup balance. AAIW anomalies also propagate westward with the speed of baroclinic Rossby waves, indicating that the displacement of the meridional salinity gradient by westward propagation of baroclinic Rossby waves plays a role in the variability of AAIW characteristics. Slower eastward spreading of AAIW anomalies is identified on decadal timescales likely associated with the advection of salinity anomalies by weak eastward current bands. Understanding the observed interannual and decadal variability of AAIW salinity is important to properly interpret salinity changes reported in response to changes in the hydrological cycle.