Baroclinic Rossby Waves Research Articles

Interannual variations of the North Equatorial Current/Undercurrent from mooring array observations

Interannual variability of the North Equatorial Current (NEC)/Undercurrent (NEUC) in the northwestern Pacific was investigated with the mooring array measurements at 130°E during 2014-2021, in combination with the satellite altimetry. Mooring observations indicate that the velocity of the NEC/NEUC in the upper 900 m exhibits significant variations on the interannual time scale. The westward-flowing NEC strengthens when the underlying eastward-flowing NEUC weakens, and the NEUC branch at 8.5°N is intensified during the mature phase of El Niño and reaches the maximum velocity during the decay phase of El Niño. The phase of the interannual variation of the currents delays with the increasing latitude, with the signal at 15°N lagging that at 8.5°N by about one year. Based on a 1.5 layer reduced gravity model, the interannual variation is suggested to be controlled mainly by the westward propagating baroclinic Rossby wave induced by the wind stress curl forcing in the central Pacific. Different propagating speed of the baroclinic Rossby wave at different latitudes explains the meridional phase lag of the interannual signal. Empirical Orthogonal Function and vertical mode decomposition analysis suggest that the interannual variation of the NEC/NEUC velocity in the northern part is dominated by surface-intensified signals with a vertical structure of the first baroclinic mode, while that in the southern part is dominated by subsurface-intensified signals which is associated with the combination of the first two baroclinic modes. The low-order mode baroclinic response of the ocean to the wind forcing accounts for the interannual fluctuation of the NEC/NEUC velocity observed by the mooring array.

Second Baroclinic Mode Rossby Waves in the South Indian Ocean

Abstract Many previous studies of midlatitude Rossby waves have examined satellite altimetry data, which reflect variability near the surface above the pycnocline. Argo float observations provide hydrographic data in the upper 2000 m, which likely monitor subsurface variability below the pycnocline. This study examines the variability in meridional velocity at midlatitudes and investigates Rossby waves in the southern Indian Ocean using an ocean reanalysis generated by a 4DVAR method. The results show two modes of variability. One is trapped near the surface and propagates to the west at a phase speed close to that of first baroclinic mode Rossby waves. This mode is representative of variability detected by satellite altimetry. The other mode has a local peak in amplitude at ∼600-m depth and propagates to the west at a phase speed 3 times slower than the first baroclinic mode. Such slowly propagating signals are observed globally, but they are largest in amplitude in the southern Indian Ocean and consistent in phase speed with the second baroclinic mode. Results from numerical experiments using an OGCM show that zonal winds in the tropical Pacific Ocean related to ENSO are the primary driver of slowly propagating signals in the southern Indian Ocean. Wind forcing in the tropical Pacific Ocean drives a surface trapped jet that propagates via the Indonesian Archipelago and excites subsurface variability in meridional velocity in the southern Indian Ocean. In addition, surface heat flux and meridional winds near the west coast of Australia can drive subsurface variability. Significance Statement Many previous studies of midlatitude Rossby waves have used satellite altimetry measurements, which reflect variability in the upper few hundred meters of the ocean. Argo float observations have provided in situ hydrographic observations in the upper 2000 m, and these enable us to examine subsurface variability with high reliability. In this study, we used output from an ocean reanalysis, which assimilates in situ observations, and found that the meridional velocity below the surface (∼600-m depth) of the southern Indian Ocean propagates at a phase speed 3 times slower than that of surface variability. These slowly propagating signals can be of climatic importance because of their possible impact on meridional heat transport. We also discuss the driving force of these slowly propagating signals.

Wind-Forced Variability of the Zonal Overturning Circulation

AbstractThe mechanisms of wind-forced variability of the zonal overturning circulation (ZOC) are explored using an idealized shallow water numerical model, quasigeostrophic theory, and simple analytic conceptual models. Two wind-forcing scenarios are considered: midlatitude variability in the subtropical/subpolar gyres and large-scale variability spanning the equator. It is shown that the midlatitude ZOC exchanges water with the western boundary current and attains maximum amplitude on the same order of magnitude as the Ekman transport at a forcing period close to the basin-crossing time scale for baroclinic Rossby waves. Near the equator, large-scale wind variations force a ZOC that increases in amplitude with decreasing forcing period such that wind stress variability on annual time scales forces a ZOC ofO(50) Sv (1 Sv ≡ 106m3s−1). For both midlatitude and low-latitude variability the ZOC and its related heat transport are comparable to those of the meridional overturning circulation. The underlying physics of the ZOC relies on the influences of the variation of the Coriolis parameter with latitude on both the geostrophic flow and the baroclinic Rossby wave phase speed as the fluid adjusts to time-varying winds.Significance StatementThe purpose of this study is to better understand how large-scale winds at mid- and low latitudes move water eastward or westward, even in the deep ocean that is not in direct contact with the atmosphere. This is important because these currents can shift where heat is stored in the ocean and if it might be released into the atmosphere. It is shown that large-scale winds can drive rapid cross-basin transports of water masses, especially so at low latitudes. The present results provide a guide on what controls this motion and highlight the importance of large-scale ocean waves on the water movement and heat storage.

Interannual Modulation of Subthermocline Eddy Kinetic Energy East of the Philippines

AbstractInterannual variation of subthermocline eddy kinetic energy (EKE) east of the Philippines is investigated based on mooring measurements during 2015–2019 and ocean‐state estimates during 1995–2017 from the Oceanic General Circulation Model for the Earth Simulator (OFES). Prominent interannual variation of EKE is detected below the thermocline east of the Philippine coast, which is closely related to the El Niño and Southern Oscillation (ENSO) events and generally lags the Nino3.4 index by 14 months. Further energy diagnostic analysis indicates that the interannual variation of subthermocline EKE is controlled by both baroclinic and barotropic instability of the background flows and dominated by the barotropic instability especially. Barotropic instability in the southern part of the Philippine coast is associated with the subsurface component of the quasi‐permanent anticyclonic Halmahera Eddy (HE), while that in the northern part is closely related to the Mindanao Undercurrent (MUC). Both HE and MUC are modulated by the ENSO events. When El Niño occurs, negative sea surface height anomalies appear near the dateline and propagate westward in the form of the first‐mode baroclinic Rossby wave, exerting delayed impacts upon the western boundary currents east of the Philippine coast and further modulating the interannual variation of subthermocline EKE. Moreover, the barotropic energy conversion rate and its corresponding subthermocline EKE at lower latitudes respond relatively faster to ENSO due to the higher Rossby wave group velocity there.

Dynamics of the Baroclinic Rossby Waves Regulating the Abyssal South China Sea

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.

Synchronized decadal variabilities in the Kuroshio and Kuroshio Extension system

Using satellite altimetry and wind stress data, the spatially coherent decadal variabilities in current intensity and current path during 1993–2018 were investigated for a western boundary current system, namely, the Kuroshio from the east of Luzon Island to the south of Japan and the Kuroshio Extension (KE). Analyses based on Hovmöller diagrams showing decadal variabilities along the entire current demonstrated three types of synchronic relationship between different regions: 1) an out-of-phase relationship in current intensity between the Kuroshio south of Japan and the KE during 1993–2018; 2) an out-of-phase relationship in current intensity between the Kuroshio from the east of Taiwan (ETW) to the East China Sea (ECS) and the KE during 2006–2014; 3) an out-of-phase relationship in current position between the Kuroshio paths in the ETW to ECS and the Luzon Strait during 1993–2018. These patterns were dynamically regulated by the Pacific Decadal Oscillation (PDO). The synchronized current-intensity variability in 2006–2014, which governed almost the entire current system, was caused by coherent phenomena comprising two kinds of baroclinic Rossby wave propagations along the KE and the subtropical countercurrent (STCC), and a regional sea surface height (SSH) anomaly advection from the South China Sea (SCS) to the ECS via the Taiwan Strait. However, the synchronized current-path variability between 1993 and 2018 was caused by an SSH anomaly migration advected by the Kuroshio from the western North Equatorial Current zone to the ECS and an SSH variability localized in the SCS.In this study, we paid particular attention to that the current intensity of the Kuroshio in the ETW to ECS had a positive no-lag correlation with the SSH-based PDO index in 1996–2005 while a positive 3–4 years lag correlation with that index during 2006–2014. It was emphasized that this transition was due to the following mechanism: the current-intensity variability in 1996–2005 was mainly caused by the variability of the eddy activity in the western STCC zone, while that variability in 2006–2014 was mainly caused by the amplification of baroclinic Rossby waves propagating along the STCC zone; the shift from the former to the latter was generated by a rapid phase shift with a relatively large amplitude for the wind stress curl anomaly and a timescale change of the eddy kinetic energy over the STCC zone around 2006.

Divergence and Dispersion of Global Eddy Propagation from Satellite Altimetry

Abstract It is well understood that isolated eddies are presumed to propagate westward intrinsically at the speed of the annual baroclinic Rossby wave. This classic description, however, is known to be frequently violated in both propagation speed and its direction in the real ocean. Here, we present a systematic analysis on the divergence of eddy propagation direction (i.e., global pattern of departure from due west) and dispersion of eddy propagation speed (i.e., zonal pattern of departure from Rossby wave phase speed). Our main findings include the following: 1) A global climatological phase map (the first of its kind to our knowledge) indicating localized direction of most likely eddy propagation has been derived from 28 years (1993–2020) of satellite altimetry, leading to a leaf-like full-angle pattern in its overall divergence. 2) A meridional deflection map of eddy motion is created with prominent equatorward/poleward deflecting zones identified, revealing that it is more geographically correlated rather than polarity determined as previously thought (i.e., poleward for cyclonic eddies and equatorward for anticyclonic ones). 3) The eddy–Rossby wave relationship has a duality nature (waves riding by eddies) in five subtropical bands centered around 27°N and 26°S in the two hemispheres, outside which their relationship has a dispersive nature with dominant waves (eddies) propagating faster in the tropical (extratropical) oceans. Current, wind, and topographic effects are major external forcings responsible for the observed divergence and dispersion of eddy propagations. These results are expected to make a significant contribution to eddy trajectory prediction using physically based and/or data-driven models.

Opposite responses of sea level variations to ENSO in the Northwestern Pacific: A transition latitude at 20°N

The interannual sea level variations in the Northwestern Pacific are significantly tied to the El Niño-Southern Oscillation (ENSO), with opposite responses and asymmetric strength to the south and north of 20°N. Using a 1.5-layer reduced gravity model, we show that the spatial pattern of sea level interannual variations is not sensitive to the existence of the Luzon Strait located at around 20°N, but is dominated by the basin-scale wind stress curl associated with ENSO. Dynamically, sea level variations tied to ENSO are attributed to long baroclinic Rossby wave adjustment driven by wind stress curl in the western/central Pacific, as previous studies have indicated. However, the responses of wind stress curl to ENSO are dynamically different to the south and north of 20°N. In the tropics, the wind stress curl anomaly results from the ENSO event directly, through the Matsuno-Gill response with the transition of zonal wind anomaly at approximately 20°N, while in the subtropics, the wind stress curl anomaly is mainly set up by atmospheric vertical motions through the Hadley Cell.

On the contribution of Rossby waves driven by surface buoyancy fluxes to low-frequency North Atlantic steric sea surface height variations

Previous studies have shown that wind-forced baroclinic Rossby waves can capture a large portion of low-frequency steric sea surface height (SSH) variations in the North Atlantic. In this paper, the classical wind-driven Rossby wave model derived in a 1.5-layer ocean is extended to include surface buoyancy forcing, and the new model is then used to assess the contribution from buoyancy-forced Rossby waves to low-frequency North Atlantic steric SSH variations. Buoyancy forcing is determined from surface heating as freshwater fluxes are negligible. It is found that buoyancy-forced Rossby waves are important in only a few regions belonging to the subtropical-to-midlatitude and eastern subpolar North Atlantic. In these regions, the new Rossby wave model accounts for 25%–70% of low-frequency steric SSH variations. Furthermore, as part of the analysis it is also shown that a simple static model driven by local surface heat fluxes captures 60%–75% of low-frequency steric SSH variations in the Labrador Sea, which is a region where Rossby waves are found to have no influence on the steric SSH.

Seasonal Behaviour of Mesoscale Eddy Trajectories in the North Indian Ocean Based on Satellite Altimetry

In the north Indian Ocean (NIO), maps of sea level anomaly from satellite altimetry were analysed from January-1995 to December-2000. The study attempted to trace the trajectories of the individual mesoscale anomalies manually and to understand seasonal changes in terms of phase speed. Mesoscale anomalies are detected as concentric circular shapes and diameters of ~90 km to 600 km and the minimum 30 days life cycle. Relatively higher eddy kinetic energy was noticed in the northwestern region of the NIO. Individual mesoscale anomalies, namely positive (warm, anticyclonic eddies) and negative (cold, cyclonic eddies) showing travelling direction westward in the NIO basins. In autumn, the number of negative anomalies detected is more than positive anomalies and vice versa during summer. The westward propagating positive (negative) anomalies in the Arabian Sea start appearing in winter (spring) along (away from) the west coast of India and west of 65°E; individual anomalies move to the west in spring/summer/autumn and collide along Somalia’s & Arabian coast. A group of positive (negative) anomalies trajectories appears as a tail at the southern tip of India are located west of the Laccadive ridge in winter (summer to autumn) associated with LH (LL). The Bay of Bengal (BB) trajectories show southwestward in northern BB, westward in central BB and northwestward in southern BB; individual anomalies are appearing along the west coast of Andaman & Nicobar ridge. The zonal phase speed decreases away from the equator, and the magnitude varies longitudinally in each season in the form of a wave-like pattern propagating westward from autumn to summer; the life cycle of the wave is almost 365 days (a year). The theoretical phase speed of the first mode of the baroclinic Rossby waves is quite similar to that of averaged zonal speed. Therefore mesoscale anomalies (eddies) are embedded into the large waves like phenomenon (Rossby waves), responsible for creating high variability and EKE in the region of NIO along the western boundaries.

Across-Shore Propagation of Subthermocline Eddies in the California Current System

AbstractThough subthermocline eddies (STEs) have often been observed in the world oceans, characteristics of STEs such as their patterns of generation and propagation are less understood. Here, the across-shore propagation of STEs in the California Current System (CCS) is observed and described using 13 years of sustained coastal glider measurements on three glider transect lines off central and southern California as part of the California Underwater Glider Network (CUGN). The across-shore propagation speed of anticyclonic STEs is estimated as 1.35–1.49 ± 0.33 cm s−1over the three transects, line 66.7, line 80.0, and line 90.0, close to the westward long first baroclinic Rossby wave speed in the region. Anticyclonic STEs are found with high salinity, high temperature, and low dissolved oxygen anomalies in their cores, consistent with transporting California Undercurrent water from the coast to offshore. Comparisons to satellite sea level anomaly indicate that STEs are only weakly correlated to a sea surface height expression. The observations suggest that STEs are important for the salt balance and mixing of water masses across-shore in the CCS.

High‐Frequency Fluctuations in Antarctic Bottom Water Transport Driven by Southern Ocean Winds

AbstractNorthward flow of Antarctic Bottom Water (AABW) across the Southern Ocean comprises a key component of the global overturning circulation. Yet AABW transport remains poorly constrained by observations and state estimates, and there is presently no means of directly monitoring any component of the Southern Ocean overturning. However, AABW flow is dynamically linked to Southern Ocean surface circulation via the zonal momentum balance, offering potential routes to indirect monitoring of the transport. Exploiting this dynamical link, this study shows that wind stress (WS) fluctuations drive large AABW transport fluctuations on time scales shorter than 2 years, which comprise almost all of the transport variance. This connection occurs due to differing time scales on which topographic and interfacial form stresses respond to wind variability, likely associated with differences in barotropic versus baroclinic Rossby wave propagation. These findings imply that AABW transport variability can largely be reconstructed from the surface WS alone.

Reciprocity in the Indian Ocean: Intraseasonal Oscillation and Ocean Planetary Waves

AbstractDuring the intraseasonal oscillation (ISO) in the tropical Indian Ocean, a recent study observed an interesting relationship between atmospheric convection and ocean heat content anomalies. Ocean heat content anomalies maximize during periods of enhanced convection. Using that study as motivation, the processes responsible for this behavior are further investigated using satellite observations and a state‐of‐the‐art ocean reanalysis. These data show that ocean dynamics linked to first baroclinic mode equatorial Rossby (ER) waves are responsible for the routine synchronization of anomalous ocean heat content and atmospheric convection during the ISO. Using a novel index to identify oceanic ER waves in the Indian Ocean, we observe the following sequence of events indicative of two‐way feedbacks between large‐scale intraseasonal modes in the atmosphere and ocean: (a) Westerly wind stress associated with the enhanced convective phase of the ISO piles mass in the eastern Indian Ocean forcing a westward oriented pressure gradient along the equator. (b) The easterly wind stress that immediately follows combines with the pressure gradient forcing to generate downwelling ER waves that slowly propagate westward. (c) The waves maximize in the central Indian Ocean where they depress the isotherms and increase the ocean heat content as the next enhanced convective phase of the ISO passes and intensifies. The entire feedback loop transpires over 90 days.

An idealized modeling study of the mid-latitude variability of the wind-driven meridional overturning circulation

AbstractThe frequency and latitudinal dependence of the mid-latitude wind-driven meridional overturning circulation (MOC) is studied using theory and linear and nonlinear applications of a quasi-geostrophic numerical model. Wind-forcing is varied by either changing the strength of the wind or by shifting the meridional location of the wind stress curl pattern. At forcing periods less than the first mode baroclinic Rossby wave basin crossing time scale the linear response in the mid-depth and deep ocean is in phase and opposite to the Ekman transport. For forcing periods close to the Rossby wave basin crossing time scale, the upper and deep MOC are enhanced, and the mid-depth MOC becomes phase shifted, relative to the Ekman transport. At longer forcing periods the deep MOC weakens and the mid-depth MOC increases, but eventually for long enough forcing periods (decadal) the entire wind-driven MOC spins down. Nonlinearities and mesoscale eddies are found to be important in two ways. First, baroclinic instability causes the mid-depth MOC to weaken, lose correlation with the Ekman transport, and lose correlation with the MOC in the opposite gyre. Second, eddy thickness fluxes extend the MOC beyond the latitudes of direct wind forcing. These results are consistent with several recent studies describing the four-dimensional structure of the MOC in the North Atlantic.

Mechanisms of Decadal North Atlantic Climate Variability and Implications for the Recent Cold Anomaly

AbstractDecadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations, and preindustrial control simulations, we identify a decadal mode of atmosphere–ocean variability in the North Atlantic with a dominant time scale of 13–18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air–sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability.

A Strong Sub‐Thermocline Intrusion of the North Equatorial Subsurface Current Into the Makassar Strait in 2016–2017

AbstractThe poorly resolved pathway of the sub‐thermocline waters (>300 m) within the Makassar Strait, the primary inflow path of the Indonesian Throughflow, is investigated using in situ mooring measurements and Argo profiles at the entrance to the Indonesian Seas. We focus on the strong sub‐thermocline intrusion in the summer of 2016, when significant changes of sub‐thermocline transport thrice as large as the interannual standard deviation occurred. Analysis suggests that the intrusion was drawn from the North Equatorial Subsurface Current (NESC) flowing westward below the North Equatorial Countercurrent, which was composed of a mixture of intermediate‐depth waters from both the North and South Pacific. The anomalously strong NESC to the Makassar sub‐thermocline in the summer 2016 is suggested to be in response to the 2015/2016 extreme El Niño event, forced by the trade wind anomalies over the western‐central Pacific Ocean through westward and downward propagating baroclinic Rossby waves.

The Sloshing and Diapycnal Meridional Overturning Circulations in the Indian Ocean

AbstractThe meridional overturning circulation (MOC) seasonality in the Indian Ocean is investigated with the ocean state estimate product ECCO v4r3. The vertical movements of water parcels are predominantly due to the heaving of the isopycnals all over the basin except off the western coast. Aided by the linear propagation equation of long baroclinic Rossby waves, the driving factor determining the strength of the seasonal MOC in the Indian Ocean is identified as the zonally integrated Ekman pumping anomaly, rather than the Ekman transport concluded in earlier studies. A new concept of sloshing MOC is proposed, and its difference with the classic Eulerian MOC leads to the so-called diapycnal MOC. The striking resemblance of the Eulerian and sloshing MOCs implies the seasonal variation of the Eulerian MOC in the Indian Ocean is a sloshing mode. The shallow overturning cells manifest themselves in the diapycnal MOC as the most remarkable structure. New perspectives on the upwelling branch of the shallow overturn in the Indian Ocean are offered based on diapycnal vertical velocity. The discrepancy among the observation-based estimates on the bottom inflow across 32°S of the basin is interpreted with the seasonal sloshing mode. Consequently, the “missing mixing” in the deep Indian Ocean is attributed to the overestimated diapycnal volume fluxes. Decomposition of meridional heat transport (MHT) into sloshing and diapycnal components clearly shows the dominant mechanism of MHT in the Indian Ocean in various seasons.

Dynamics of zonally elongated transient flows

Abstract

Interdecadal variability of the Western Subarctic Gyre in the North Pacific Ocean

Interdecadal variations of the Western Subarctic Gyre (WSAG) in the North Pacific were examined mainly by analysis of absolute dynamic topography (ADT) during 1993–2017. The ADT was based on altimetry-derived sea level anomalies superimposed on state-of-the-art mean dynamic topography. We specified geostrophic surface streamlines of the WSAG associated with closed isolines of the ADT. The WSAG intensity was the strongest in the late 1990s, when the WSAG was zonally contracted, and it decreased to the mid-2010s in a linear manner with time. The WSAG expanded abruptly toward the east around 2001 in a step function–like manner. After this abrupt shift, the WSAG was connected most strongly to the Alaskan Gyre. This condition persisted even into the late 2010s. The shift to the very elongated pattern was also accompanied by a drastic lowering of the ADT around 170°W, 50°N, where the WSAG and Alaskan Gyre had been distinctly separated by a peak of the ADT during the previous period when the shape of the gyre was less elongated. A reduced-gravity model of wind-induced, long, baroclinic Rossby waves with weak dissipation could account for both the interdecadal weakening of the WSAG and the step function–like lowering of the ADT peak at the time of the shift of the WSAG to the very elongated pattern. In addition to the baroclinic response of the subarctic ocean to basin-scale wind stress, mesoscale clockwise eddies contributed to sea level rise around the center of the WSAG. In particular, there was an interdecadal increase in the frequency of appearance of strong, mesoscale, clockwise eddies that were generated south of the Aleutian Islands at longitudes of 170°E−180°. Those eddies propagated approximately to the west-southwest, and they disappeared around the center of the WSAG.

Observed Triple Mode of Salinity Variability in the Thermocline of Tropical Pacific Ocean

AbstractA well‐defined Triple MOde of Salinity (TMOS) variability in the tropical Pacific thermocline layer (24–25.5 σθ isopycnal surfaces) is revealed from the Argo observations during 2004–2018. Thermocline salinity in the tropical northern and southwestern Pacific Ocean varies out of phase on interannual to near‐decadal time scales with that in the tropical southeastern Pacific Ocean. The TMOS is attributed to anomalous advection of mean salinity as well as advection of anomalous salinity by the mean oceanic current on isopycnal surfaces. While the TMOS pattern is quite different from the thermal patterns associated with the El Niño‐Southern Oscillation (ENSO) or the Pacific Decadal Oscillation (PDO), it is significantly associated with the ENSO and PDO indices with a time lag of 10 months. Isopycnal propagation of salinity anomalies and anomalous freshwater input in the outcropping region, also both associated with ENSO and PDO, are important in the formation of TMOS. The propagation speed of isopycnal salinity anomalies is close to documented phase speeds of baroclinic Rossby waves and velocities of zonal currents. The TMOS acts to shape the basin‐scale Pacific water masses and potentially plays an important role in climate.