Eastward Currents Research Articles

Reconstruction of Extreme Geomagnetic Storms: Breaking the Data Paucity Curse

AbstractReconstruction of the magnetic field, electric current, and plasma pressure is provided using a new data mining (DM) method with weighted nearest neighbors (NN) for strong storms with the storm activity index Sym‐H < −300 nT, the Bastille Day event (July 2000), and the 20 November 2003 superstorm. It is shown that the new method significantly reduces the statistical bias of the original NN algorithm toward weaker storms. In the DM approach the magnetic field is reconstructed using a small NN subset of the large historical database, with the subset number KNN ≫ 1 being still much larger than any simultaneous multiprobe observation number. This allows one to fit with observations a very flexible magnetic field model using basis function expansions for equatorial and field‐aligned currents, and at the same time, to keep the model sensitive to storm variability. This also allows one to calculate the plasma pressure by integrating the quasi‐static force balance equation with the isotropic plasma approximation. For strong storms of particular importance becomes the resolution of the eastward current, which prevents the divergence of the pressure integral. It is shown that in spite of the strong reduction of the dominant NN number in the new weighted NN algorithm to capture strong storm features, it is still possible to resolve the eastward current and to retrieve plasma pressure distributions. It is found that the pressure peak for strong storms may be as close as ≈2.1RE to Earth and its value may exceed 300 nPa.

From Mixing to the Large Scale Circulation: How the Inverse Cascade Is Involved in the Formation of the Subsurface Currents in the Gulf of Guinea

In this paper, we analyse the results from a numerical model at high resolution. We focus on the formation and maintenance of subsurface equatorial currents in the Gulf of Guinea and we base our analysis on the evolution of potential vorticity (PV). We highlight the link between submesoscale processes (involving mixing, friction and filamentation), mesoscale vortices and the mean currents in the area. In the simulation, eastward currents, the South and North Equatorial Undercurrents (SEUC and NEUC respectively) and the Guinea Undercurrent (GUC), are shown to be linked to the westward currents located equatorward. We show that east of 20° W, both westward and eastward currents are associated with the spreading of PV tongues by mesoscale vortices. The Equatorial Undercurrent (EUC) brings salty waters into the Gulf of Guinea. Mixing diffuses the salty anomaly downward. Meridional advection, mixing and friction are involved in the formation of fluid parcels with PV anomalies in the lower part and below the pycnocline, north and south of the EUC, in the Gulf of Guinea. These parcels gradually merge and vertically align, forming nonlinear anticyclonic vortices that propagate westward, spreading and horizontally mixing their PV content by stirring filamentation and diffusion, up to 20° W. When averaged over time, this creates regions of nearly homogeneous PV within zonal bands between 1.5° and 5° S or N. This mean PV field is associated with westward and eastward zonal jets flanking the EUC with the homogeneous PV tongues corresponding to the westward currents, and the strong PV gradient regions at their edges corresponding to the eastward currents. Mesoscale vortices strongly modulate the mean fields explaining the high spatial and temporal variability of the jets.

Rare earth element input and transport in the near-surface zonal current system of the Tropical Western Pacific

Continental sources and current transport play a major role in rare earth element (REE, and other trace element) input and distribution in the Tropical Western Pacific. Here, we present spatially highly resolved distributions of dissolved REE concentrations ([REE]) along three transects in the zonal (extra-)equatorial current system and the Solomon Strait of the Tropical Western Pacific. We use seawater [REE] in combination with direct physical oceanographic observations (e.g., current velocity data) to characterize the geochemical composition, origin and pathways of the complex surface and upper layer currents of the Tropical Western Pacific and to quantify the input fluxes of REEs. We identify Papua New Guinea (PNG) volcanic rocks, sediments, and/or river particles as the key source adding trace elements to the equatorial eastward zonal currents of the Tropical Western Pacific. Our and published data indicate temporal and spatial variability of this input and transport in the PNG source area and the equatorial eastward currents. The westward currents, on the other hand, lack this REE input signal suggesting lateral transport of preformed seawater [REE]. At the transition between these zonal eastward and westward currents, our data indicate lateral mixing of Eastern and Western Pacific source waters.

Average Ionospheric Middle and Low Latitudes Nighttime Zonal Currents Deduced From CHAMP

AbstractBy utilizing magnetic field measurements collected by CHAMP during the years 2001–2009, average characteristics of nighttime ionospheric zonal currents at middle and low latitudes are investigated. We provide for the first time the detailed dependencies of these currents on location, local time, season, and the level of solar activity. The main results obtained are (1) the nighttime zonal ionospheric currents at F‐region altitude, derived from the vertical magnetic component, exhibit on average height‐integrated current densities up to 10 mA/m. The amplitude varies quasi‐linearly with F10.7 flux level. (2) During equinoxes symmetric current systems at F‐region altitude are found in the two hemispheres. After sunset, eastward currents dominate at low latitudes, fading away toward midnight. After midnight regions of eastward currents are observed at midlatitude. During solstice seasons, eastward currents are enhanced in the summer hemisphere; westward currents are stronger in local winter. (3) Longitudinal variations of F‐region currents show clear wave‐1 patterns. This is related to the effect of the stationary planetary wave‐1 and the diurnal nonmigrating tide, D0. This dominant tidal signal, enhancing eastward currents over the Pacific region, appears at middle and low latitudes and during all seasons. (4) At the magnetic equator, signs of the equatorial electrojet (EEJ) are evident after 02:30 MLT. Intensity of westward currents in the narrow band increases toward sunrise. Longitudinal variations of the EEJ show a similar wave‐1 pattern as F‐region currents. Here the westward current density is enhanced over Africa/India. The EEJ wave‐1 modulation is strongest around June solstice.

Deep Eddy Kinetic Energy in the Tropical Pacific From Lagrangian Floats

AbstractAt the ocean surface, satellite observations have shown evidence of a large spectrum of waves at low latitudes. However, very little is known about the existence and properties of the deep variability. Most of the subsurface observations rely on localized measurements, which do not allow for a global estimation of this variability. In this study, we use velocity estimates, provided by Argo float drifts at 1,000 m, to analyze the spatial and temporal distribution of the deep eddy kinetic energy (EKE) and its spectral signature with an unprecedented time and space coverage. In the tropical Pacific, high EKE is found along the equator, at the western boundary and poleward of 7°N. EKE meridional distribution is also found to vary at the scale of the meridionally alternating mean zonal jets: it is higher inside eastward currents. We develop an original statistical scale analysis to determine the temporal and spatial scale dependence of this deep EKE footprint. We show the presence of periodic features whose characteristics are compatible with theoretical equatorial waves dispersion relations. Annual and semiannual Rossby waves are observed at the equator, as well as ∼30‐day Yanai waves, consistent with surface tropical instability waves. The location and intensification of these waves match the downward energy propagation predicted by ray tracing linear theory. Short‐scale variability (with ∼70‐day periods and 500‐km wavelength) has also been detected poleward of 7°N. The generation mechanisms of this variability are discussed, as well as its potential importance for the mean circulation.

On the Generation and Salinity Impacts of Intraseasonal Westward Jets in the Equatorial Indian Ocean

AbstractWhile westerly winds dominate the equatorial Indian Ocean and generate the well‐known eastward flowing Wyrtki Jets during boreal spring and fall, there is evidence of a strong westward surface jet during winter that is swifter than eastward currents during that season. A weaker westward jet is found in summer. In this study, we report the occurrence, characteristics, and intraseasonal variability of this westward jet and its impact on mixed layer salinity in the equatorial Indian Ocean using the HYbrid Coordinate Ocean Model (HYCOM) reanalysis with the Navy Coupled Ocean Data Assimilation (NCODA). The westward jet typically occurs in the upper 50 m, above an eastward flowing equatorial undercurrent, with peak westward volume transport of approximately −8 Sv. The westward jet builds up gradually, decays rapidly, and is primarily forced by local intraseasonal wind stress anomalies generated by atmospheric intraseasonal convection. Westward acceleration of the jet occurs when the dominant intraseasonal westward wind anomaly is not balanced by the zonal pressure gradient (ZPG) force. The intraseasonal westward jet generates strong horizontal advection and is the leading cause of mixed layer freshening in the western equatorial Indian Ocean. Without it, a saltier mixed layer would persist and weaken any barrier layers. Existing barrier layers are strengthened following the passage of freshwater‐laden westward jets. Deceleration of the westward jet occurs when the eastward ZPG becomes increasingly important and the westward intraseasonal wind anomalies weaken. A rapid reversal of atmospheric intraseasonal convection‐driven surface winds eventually terminates the westward jet.

Warming Trends in the Central Equatorial Indian Ocean and the Associated Coupled Feedback Processes

Mohapatra, S. and Gnanaseelan, C., 2020. Warming trends in the central equatorial Indian Ocean and the associated coupled feedback processes. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 39-45. Coconut Creek (Florida), ISSN 0749-0208.The recent increasing trends in the tropical Indian Ocean (TIO) temperature have strong impact on the regional as well as global climate variability. In this work we studied the changes in the boreal summer monsoon winds and the associated monsoon currents on modulating the surface and subsurface temperature of the equatorial Indian Ocean. The reduction in the meridional sea surface temperature (SST) gradient and strengthening of zonal SST gradient over the Indian Ocean reduced the northward propagation of south-westerly wind, thereby enhancing the surface westerlies over the equator. The strengthened surface westerlies increased the equatorial downwelling Kelvin waves and deepened the thermocline. The present study unravels the causes for the observed confinement of strong SST trend over the central equatorial Indian Ocean (CEIO) even though surface westerlies and the associated downwelling extend upto the eastern equatorial Indian Ocean (EEIO). In contrast to the CEIO surface warming, EEIO experienced strong subsurface warming at thermocline. Our analysis suggests that the upper ocean stratification plays an important role in the observed warming trend of both CEIO surface and EEIO subsurface. The eastward equatorial currents bring warm salty water from Arabian Sea and WEIO towards east and warms CEIO, however the high saline surface water is found to sink in the EEIO, thereby warming the subsurface instead of surface. The composite of temperature anomaly for the period 1978-2002, the period of warm Pacific Decadal Oscillation, shows patterns similar to temperature trends, which clearly suggests the dominance of decadal variability on the TIO temperature trends.

Dynamical Ocean Response Controlling the Eastward Movement of a Heat Content Anomaly Caused by the 18.6‐Year Modulation of Localized Tidally Induced Mixing

AbstractNumerical experiments based on a long‐term ocean state estimate have been used in previous work to demonstrate that the 18.6‐year modulation of localized tidally induced vertical mixing can influence wintertime sea surface temperature (SST) over a broad area of the northern North Pacific, and may play a role in determining the spatial structure of the Pacific Decadal Oscillation (PDO). This study investigates the mechanism driving the model annual mean surface layer heat content anomaly (SHCa), which is correlated with the SST anomaly, at a location near the center of the PDO in the eastern Pacific. The modulation of mixing first induces an 18.6‐year oscillatory SHCa in the western boundary region, which moves eastward along the boundary between the subtropical and subarctic gyres, taking 10 years to reach the eastern region. Adjoint sensitivity analysis is applied to confirm the importance of the dynamical response of the ocean to the eastward motion of the SHCa. It is shown that the generation of a temperature anomaly by wave‐related current anomalies works to delay the eastward motion of the SHCa. Results also suggest that the generation of a spiciness anomaly in the open ocean may play a role. Although further studies are required to clarify whether the mixing modulation has an impact on the actual climate variability, the mechanism can be applied to any location along an eastward current, independent of the periodicity of the phenomena, and thus might have wide applicability in efforts to understand the role of the ocean in the climate system.

GOCI-Observed Chlorophyll Belts Associated With Sea-Surface Fronts in the East China Sea

Using chlorophyll-a observational data from the Geostationary Ocean Color Imager (GOCI) satellite operated by the Korean Satellite Center, thousands of high chlorophyll belts are found along the coast of East China Sea (ECS). These belts extend tens to hundreds of kilometers offshore with an average length of roughly 90 km and 18.3% occurrence probability. Through statistical analysis, appearance of these belts is characterized with a band oriented along the coastline direction around 100-km offshore with a maximum likely occurrence near 28.5°N. The instability of the coastal front plays an important role in the formation of the high-appearance band, and the eastward current associated with the estuarine front contributes to the maximum occurrence region around 28.5°N. These chlorophyll belts not only provide additional nutrients for phytoplankton growth but also are an important factor for the healthy coastal ecosystems.

Continental slope-confined canyons in the Pearl River Mouth Basin in the South China Sea dominated by erosion, 2004–2018

Repeat multibeam bathymetric surveys conducted in 2004, 2005 and 2018 show that seven slope-confined canyons on the continental slope of the Pearl River Mouth Basin, South China Sea, were dominated by large volume, widespread erosion. Erosion volumes were up to 3.4 times greater than deposition volumes. Erosion-dominated areas of the canyons are up to 2.3 times greater than areas dominated by deposition. Average rates of erosion (ranging from 0.7 to 0.8 m/yr) were greater than average rates of deposition (ranging from 0.5 to 0.8 m/yr). In plan view, the erosion-dominated zones exhibit two characteristic shapes: (1) linear, found mainly in upper canyon reaches, distributed predominantly along canyon axes and at the base of eastern canyon walls, and; (2) blocky, found mainly in lower canyon reaches, widely distributed along the steep canyon walls and on lower-canyon interfluves. The deposition-dominated zones are scattered along canyon floors and walls. Seismic reflection data show lateral shifts of canyon fill deposits through time, indicative of longer-term eastward canyon migration. The linear erosion-dominated zones may be attributed to erosive turbidity currents triggered by energetic internal solitary waves shoaling on the shelf. The eastward canyon shifts were likely induced by rapid near-bed eastward currents generated mainly by westward propagating powerful internal solitary waves of depression. The widespread erosion in the deeper canyon areas and interfluves is likely a consequence of slope instabilities associated with the presence of gas hydrates. This study indicates that active sedimentary processes can occur in slope-confined canyons even during sea level highstands. Local, site-specific oceanographic and geological features (e.g., internal solitary waves, gas hydrates) can significantly increase sedimentary activity in and around submarine canyons.

Direct Measurements of Upper Ocean Horizontal Velocity and Vertical Shear in the Tropical North Atlantic at 4°Ν, 23°W

AbstractThe Tropical Atlantic Current Observations Study measured upper ocean horizontal velocity from a mooring at 4°N, 23°W, at discrete depths between 7 and 87 m, in order to observe the temporal and vertical structure of the currents. Between March 2017 and March 2018, mean zonal velocity and vertical shear were strongest between 32 and 37 m. Near‐surface mean eastward currents during this period were weaker than the long‐term mean but within the range of previously observed values given the high interannual variability. Interannual variability of the zonal velocity exceeded that of the meridional velocity, was primarily geostrophically driven, and was strongly influenced by the large‐scale currents. Energetic tropical instability waves (TIWs) were observed in early summer and late fall of 2017. Meridional velocity fluctuations associated with the TIWs were far larger than those of zonal velocity and extended down to 87 m. These fluctuations propagated upward to the surface with vertical phase speeds between 12 and 15 m/day. Coherent velocity and vertical shear variations emerged in a TIW composite. However, the vertical shears observed during the TIW season were modest compared to the large shear measured in spring 2017 and winter 2018. As a result, TIWs may be less crucial for vertical turbulent cooling at 4°N, 23°W than they are near the equator. To observe the intense winter/spring vertical shear events, fine vertical (less than 10 m) and diurnal temporal sampling is needed.

The Equatorial Undercurrent and Its Origin in the Region Between Mindanao and New Guinea

AbstractHerein, the spatial characteristics of the Equatorial Undercurrent (EUC) in the region between Mindanao and New Guinea and the origins of the EUC and the North Equatorial Countercurrent (NECC) were investigated using a combination of high‐resolution Argo hydrographic data and numerical modeling. The Mindanao Current (MC) results in the formation of a strong EUC‐depth eastward current that splits into two branches near the Halmahera Eddy west of 132°E, with one branch directly flowing eastward without experiencing the Halmahera Eddy rotation effect, and the other branch flowing southwestward and then turning northeastward. The EUC and NECC were difficult to distinguish between 132°E and 136°E, since the former combined with the latter to form a continuous equator‐tilted current. At ~137°E, the EUC axis started turning toward the equator, and an isolated EUC core was observed at 143°E and ~200‐m depth. The Pacific EUC was shown to originate at ~130°E, where it featured an isolated current core with a maximum velocity of 5–10 cm/s at ~1°N and 200‐ to 400‐m depth. The EUC and NECC in the western tropical Pacific were found to have the same water sources, namely, the MC and the New Guinea Coastal Undercurrent, with the shallow parts (centered at ~100‐m depth) of MC and New Guinea Coastal Undercurrent finally reaching the NECC and their deeper parts (centered at ~200‐m depth) finally reaching the EUC.

Nearshore-offshore exchanges in multi-basin coastal waters: Observations and three-dimensional modeling in Lake Erie

Abstract Nearshore-offshore exchanges through upwelling events and subsequent formation of internal Kelvin waves and coastal-jets are expected in the multi-basin coastal waters of Lake Erie; however, these phenomena have yet to be confirmed in the field or numerically modeled. Here, we demonstrate these physical processes in Lake Erie through extensive field data analysis and high-resolution three-dimensional hydrodynamic modeling. The validated model successfully reproduces dominant physical processes in the offshore and nearshore waters including surface seiches (~14 h), near-inertial waves (~17 h) and upwelling events (5–10 days). We show that upwelling events are the predominant nearshore physical processes, and are energized when winds accelerate the epilimnetic waters to the south, causing the thermocline front to move up into the north shore. After the wind subsides, the elevated thermoclines simultaneously form two separate cyclonically propagating internal Kelvin waves in the central and the eastern basins following by two coastal-jets at phase speeds of 0.22 ms−1 and 0.37 ms−1 within 5–15 km off the shoreline, respectively. The predominant south-west winds limit the presence of Kelvin waves only to the northern parts of each basin, where the bathymetry allows, and disappear when shoreline morphology no longer maintain these waves due to nonzero cross-shore transports at the coastal boundary layer. Intrusions of hypolimnetic waters through upwelling events contribute 10–30% to the net cross-shore transport, and are most pronounced in May and June when the offshore thermocline is shallow. The intermittent strong westward nearshore currents by Kelvin waves are in the opposite direction of seasonal eastward currents.

Impact of tropical cyclones on the evolution of the monsoon-driven upwelling system in the coastal waters of the northern South China Sea

An upwelling system exists in the coastal waters of the northern South China Sea (NSCS), a region that is frequently affected by tropical cyclones in summer. This study investigates the evolution of the NSCS monsoon-driven upwelling system and the effects of the Talim and Doksuri tropical cyclones on the system using in situ observational data obtained at three mooring stations, one land-based meteorological station, and concurrent satellite remote sensing data for the NSCS coastal waters from May to July 2012. The results show that the occurrence and evolution of the upwelling system were mainly controlled by the Asian southwest monsoon, while the eastward current also made important contributions to the upwelling intensity. A decrease in the bottom water temperature and shifts in the along-shore and cross-shore currents were direct evidence of the establishment, existence, and recovery of this upwelling. Tropical cyclones have significant impacts on hydrodynamics and can thus influence the evolution of the NSCS upwelling system by changing the local wind and current fields. Variations in water level and local current systems impeded the development of upwelling during tropical cyclones Talim and Doksuri in the study area, which have low-frequency fluctuations of approximately 2–10 days. These variations were the results of the coupled interactions between local wind fields, coastal trapped waves, and other factors. The hydrodynamic environment of the marine water (including coastal upwelling system) rapidly recovered to normal sea conditions after each cyclone passed due to the relatively short duration of the impact of a tropical cyclone on the dynamic environment of the waters.

Zonal migration and transport variations of the Kuroshio east of Taiwan induced by eddy impingements

Variability of the Kuroshio east of Taiwan was observed at a cross-stream transect ~ 50km south of the PCM-1 line with an array of three moored ADCPs measuring for ~ 23 months, supplemented with eleven repeated shipboard surveys. Observations of the Kuroshio's velocity structure reveal the absence of an obvious regular seasonal signal, but significant variability at 70–200 day period for both maximum velocity axis migration and transport due to interactions with mesoscale eddies. Empirical orthogonal function (EOF) analysis shows the migration and transport modes explain 46% and 29% of the total variance, respectively, which is in contrast to the findings at the PCM-1 line where the transport mode explained more variance than did the migration mode. The Kuroshio transport in the upper 500m across a 150km section is 17.2Sv with a standard deviation of 5Sv. The estimated Kuroshio transport is 4.3Sv lower than that reported for the PCM-1 line, likely due to the interannual variations related to abundance of mesoscale eddies in the Subtropical Counter Current (STCC) region. Transport variability east of Taiwan is mostly caused by Kuroshio-eddy interactions. When single anticyclonic (cyclonic) eddies encounter the Kuroshio, they enhance (reduce) poleward transport, presumably by increasing (decreasing) the sea level anomaly (SLA) along the eastern flank of the Kuroshio (correlation = 0.82). When a pair of eddies impinges on the Kuroshio, the upstream confluence and diffluence caused by the dipole eddies increases and decreases the Kuroshio transport, respectively. Furthermore, the eastward (westward) currents that result from either the single eddy or the dipole eddy produce flow divergence (convergence) adjacent to the Kuroshio's eastern edge, favoring the offshore (onshore) migration of the Kuroshio axis.

Nearshore circulation on a sea breeze dominated beach during intense wind events

A field experiment was conducted on the northern Yucatan coast from April 1 to April 12, 2014 to investigate the role of intense wind events on coastal circulation from the inner shelf to the swash zone. The study area is characterized by a micro-tidal environment, low-energy wave conditions, and a wide and shallow continental shelf. Furthermore, easterly trade winds, local breezes, and synoptic-scale events, associated with the passage of cold-fronts known as Nortes, are ubiquitous in this region. Currents were measured concurrently at different cross-shore locations during both local and synoptic-scale intense wind events to investigate the influence of different forcing mechanisms (i.e., large-scale currents, winds, tides, and waves) on the nearshore circulation. Field observations revealed that nearshore circulation across the shelf is predominantly alongshore-directed (westward) during intense winds. However, the mechanisms responsible for driving instantaneous spatial and temporal current variability depend on the weather conditions and the across-shelf location. During local strong sea breeze events (W > 10ms-1 from the NE) occurring during spring tide, westward circulation is controlled by the tides, wind, and waves at the inner-shelf, shallow waters, and inside the surf/swash zone, respectively. The nearshore circulation is relaxed during intense land breeze events (W ≈ 9ms-1 from the SE) associated with the low atmospheric pressure system that preceded a Norte event. During the Norte event (Wmax≈ 15ms-1 from the NNW), westward circulation dominated outside the surf zone and was correlated to the Yucatan Current, whereas wave breaking forces eastward currents inside the surf/swash zone. The latter finding implies the existence of large alongshore velocity shear at the offshore edge of the surf zone during the Norte event, which enhances mixing between the surf zone and the inner shelf. These findings suggest that both sea breezes and Nortes play an important role in sediment and pollutant transport along/across the nearshore of the Yucatan shelf.

SPATIAL AND TEMPORAL ANALYSIS OF SEA SURFACE SALINITY USING SATELLITE IMAGERY IN GULF OF MEXICO

Abstract. The recent development of satellite sea surface salinity (SSS) observations has enabled us to analyse SSS variations with high spatiotemporal resolution. In this regards, The Level3-version4 data observed by Aquarius are used to examine the variability of SSS in Gulf of Mexico for the 2012-2014 time periods. The highest SSS value occurred in April 2013 with the value of 36.72 psu while the lowest value (35.91 psu) was observed in July 2014. Based on the monthly distribution maps which will be demonstrated in the literature, it was observed that east part of the region has lower salinity values than the west part for all months mainly because of the currents which originate from low saline waters of the Caribbean Sea and furthermore the eastward currents like loop current. Also the minimum amounts of salinity occur in coastal waters where the river runoffs make fresh the high saline waters. Our next goal here is to study the patterns of sea surface temperature (SST), chlorophyll-a (CHLa) and fresh water flux (FWF) and examine the contributions of them to SSS variations. So by computing correlation coefficients, the values obtained for SST, FWF and CHLa are 0.7, 0.22 and 0.01 respectively which indicated high correlation of SST on SSS variations. Also by considering the spatial distribution based on the annual means, it found that there is a relationship between the SSS, SST, CHLa and the latitude in the study region which can be interpreted by developing a mathematical model.

Decadal Variability of Upper-Ocean Heat Content Associated with Meridional Shifts of Western Boundary Current Extensions in the North Pacific

Generation and propagation processes of upper-ocean heat content (OHC) in the North Pacific are investigated using oceanic subsurface observations and an eddy-resolving ocean general circulation model hindcast simulation. OHC anomalies are decomposed into physically distinct dynamical components (OHC ρ) due to temperature anomalies that are associated with density anomalies and spiciness components (OHC χ) due to temperature anomalies that are density compensating with salinity. Analysis of the observational and model data consistently shows that both dynamical and spiciness components contribute to interannual–decadal OHC variability, with the former (latter) component dominating in the subtropical (subpolar) North Pacific. OHC ρ variability represents heaving of thermocline, propagates westward, and intensifies along the Kuroshio Extension, consistent with jet-trapped Rossby waves, while OHC χ variability propagates eastward along the subarctic frontal zone, suggesting advection by mean eastward currents. OHC χ variability tightly corresponds in space to horizontal mean spiciness gradients. Meanwhile, area-averaged OHC χ anomalies in the western subarctic frontal zone closely correspond in time to meridional shifts of the subarctic frontal zone. Regression coefficient of the OHC χ time series on the frontal displacement anomalies quantitatively agree with the area-averaged mean spiciness gradient in the region, and suggest that OHC χ is generated via frontal variability in the subarctic frontal zone.

Association between mean and interannual equatorial Indian Ocean subsurface temperature bias in a coupled model

In the present study the association between mean and interannual subsurface temperature bias over the equatorial Indian Ocean (EIO) is investigated during boreal summer (June through September; JJAS) in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFSv2) hindcast. Anomalously high subsurface warm bias (greater than 3 °C) over the eastern EIO (EEIO) region is noted in CFSv2 during summer, which is higher compared to other parts of the tropical Indian Ocean. Prominent eastward current bias in the upper 100 m over the EIO region induced by anomalous westerly winds is primarily responsible for subsurface temperature bias. The eastward currents transport warm water to the EEIO and is pushed down to subsurface due to downwelling. Thus biases in both horizontal and vertical currents over the EIO region support subsurface warm bias. The evolution of systematic subsurface warm bias in the model shows strong interannual variability. These maximum subsurface warming episodes over the EEIO are mainly associated with La Nina like forcing. Strong convergence of low level winds over the EEIO and Maritime continent enhanced the westerly wind bias over the EIO during maximum warming years. This low level convergence of wind is induced by the bias in the gradient in the mean sea level pressure with positive bias over western EIO and negative bias over EEIO and parts of western Pacific. Consequently, changes in the atmospheric circulation associated with La Nina like conditions affected the ocean dynamics by modulating the current bias thereby enhancing the subsurface warm bias over the EEIO. It is identified that EEIO subsurface warming is stronger when La Nina co-occurred with negative Indian Ocean Dipole events as compared to La Nina only years in the model. Ocean general circulation model (OGCM) experiments forced with CFSv2 winds clearly support our hypothesis that ocean dynamics influenced by westerly winds bias is primarily responsible for the strong subsurface warm bias over the EEIO. This study advocates the importance of understanding the ability of the models in representing the large scale air–sea interactions over the tropics and their impact on ocean biases for better monsoon forecast.

Dynamics of Andaman Sea circulation and its role in connecting the equatorial Indian Ocean to the Bay of Bengal

AbstractCirculation in the Bay of Bengal (BoB) is driven not only by local winds, but are also strongly forced by the reflection of equatorial Kelvin waves (EKWs) from the eastern boundary of the Indian Ocean. The equatorial influence attains its peak during the monsoon‐transition period when strong eastward currents force the strong EKWs along the equator. The Andaman Sea, lying between the Andaman and Nicobar island chains to its west and Indonesia, Thailand, and Myanmar to the south, east, and north, is connected to the equatorial ocean and the BoB by three primary passages, the southern (6°N), middle (10°N), and northern (15°N) channels. We use ocean circulation models, together with satellite altimeter data, to study the pathways by which equatorial signals pass through the Andaman Sea to the BoB and associated dynamical interactions in the process. The mean coastal circulation within the Andaman Sea and around the islands is primarily driven by equatorial forcing, with the local winds forcing a weak sea‐level signal. On the other hand, the current forced by local winds is comparable to that forced remotely from the equator. Our results suggest that the Andaman and Nicobar Islands not only influence the circulation within the Andaman Sea, but also significantly alter the circulation in the interior bay and along the east coast of India, implying that they need to be represented accurately in numerical models of the Indian Ocean.