Negative Sea Surface Height Anomalies Research Articles

On Barotropic Response of Arctic Seas to Polar Lows: A Case Study in the Barents Sea

In the present paper, we investigate the sea surface height (SSH) anomalies caused by polar lows (PLs) crossing the central part of the Barents Sea and verify if the barotropic response is detectable in the shallow Arctic seas. Analysis of the SSH anomalies in response to the passage of two PLs is performed using satellite altimeter measurements and model simulations. The observed SSH anomalies contained an inverse barometer correction; therefore, they were presumably caused only by the action of surface wind stress in the PLs. The SSH anomalies along the satellite altimeter tracks had the shape of a trough, with the lowest surface height near the center of the PL. The observed anomalies were well distinguished within about one day after the PL passage, with the largest negative value of 0.6 m. The SSH anomalies are analyzed using a simplified model of the ocean barotropic response to the surface wind stress, derived from the hourly wind fields provided in the ERA5 reanalysis dataset. The model quantitatively reproduced the SSH anomalies along most satellite altimeter tracks crossing the PL trajectories. The model simulations revealed that the largest negative SSH anomalies were observed in areas where the PL translation velocity was low and its moving direction changed with the trajectory curvature radius, which was much smaller than the barotropic radius of deformation. The estimated quasi-geostrophic current velocities corresponding to the SSH anomalies in the wakes of the PLs reached 0.15 m/s, which were comparable to the current velocities observed in the Barents Sea.

A Mesoscale Ocean–Atmosphere Coupled Pathway for Decadal Variability of the Kuroshio Extension System

Abstract The Kuroshio Extension (KE) system has been observed to experience a decadal cycle between dynamically stable and unstable states. However, divergent conclusions on its interaction with the atmosphere obfuscate the understanding of its oscillatory nature at the preferred decadal time scale. Here, using satellite observations and the ERA-Interim reanalysis in 2002–16, physical process-oriented diagnoses suggest that the wintertime finer-scale thermodynamic response to mesoscale oceanic surface conditions and slow oceanic Rossby wave adjustment frame a coupled ocean–atmosphere delayed oscillator for the decadal KE variability. During the stable state of the KE system, the downstream KE transition region is rich in mesoscale oceanic warming associated with warm eddies, which induces surface wind convergence and upward motion, probably via the enhanced turbulent mixing. Meanwhile, increased finer-scale diabatic heating in the lower troposphere with abundant moisture supply from warmer water likely facilitates the deep-reaching updraft that adiabatically cools the middle troposphere. The background northwesterly wind helps to spread out the cooling, leading to southward deflection of local atmospheric eddy available potential energy (EAPE) production by baroclinic conversion. Consequently, the synoptic eddy activity displaces southward across the basin with additional energy supply from the increased diabatic production of EAPE downstream. Anomalous synoptic eddy thermal and vorticity forcing eventually fosters the basin-scale equivalent-barotropic cyclonic circulation anomaly, which is further maintained by energy conversion from the background state. The resultant wind-driven negative sea surface height anomalies propagate westward into the upstream KE region with a delay of ∼4 years and can trigger the unstable state of the KE system.

The generation mechanism of cold eddies and the related heat flux exchanges in the upper ocean during two sequential tropical cyclones

The impacts of two sequential tropical cyclones (TCs), Kyarr and Maha, [from October 24 to November 06, 2019, over the Arabian Sea (AS)] on upper ocean environments were investigated using multiple satellite observations, Argo float profiles and numerical model outputs. To obtain a realistic TC strength, the Weather Research and Forecasting (WRF) model was used to reproduce Kyarr and Maha. During Kyarr and Maha, three distinct cold patches were observed at the sea surface with a maximum sea surface cooling of approximately 5°C. The comparison between WRF model simulation results and ERA5 wind field showed that the WRF model simulation indicated high simulation accuracy with respect to the SST decrease in the AS under the influence of Kyarr and Maha’s wind stress curls. Meanwhile, concentration of chlorophyll a (chl-a) and positive relative vorticity of sea surface also appeared in the three cold patch areas. Through the use of eddy detection algorithms, three mesoscale cold cyclonic eddies were identified along the track of TC Kyarr, and the locations of these cold eddies were highly correlated with three obvious negative sea surface height anomalies (SSHAs). The radii of the three cold eddies were 69 km, 50 km, and 41 km. With a focus on the thermodynamic responses of the three cold eddy fields to Kyarr and Maha, the central regions of the three cold eddies were explored. The central regions of the three cold eddies exhibited relatively shallow mixed-layer depths (MLDs) and low mixed-layer temperatures (MLTs). The depth integrated heat (DIH) content was also calculated to explore the heat flux exchanges occurring in different layers in the upper 200 m of the centre of each eddy. The results showed that DIH in each eddy centre varied by one order of magnitude, accounting for between 127.3 MJ m-2 and 1220.0 MJ m-2 of heat loss. This study suggests that the effect of long forcing time on intense positive wind stress curls can produce upwelling caused by Ekman response, which is the main influencing factor of the three cold eddies generation mechanism. At the same time, the positive relative vorticity injected into the sea surface also has some contribution. TC-induced vertical mixing and upwelling (strengthened by unstable structures inside the cold eddies) cause substantial redistribution of the DIH, and related heat flux exchanges at different layers occur in the eddy fields.

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.

Winter–Spring Phytoplankton Phenology Associated with the Kuroshio Extension Instability

We used ocean color data of chlorophyll-a (CHL) from the period 1998 to 2017 to investigate phytoplankton phenology during winter–spring in association with the Kuroshio Extension (KE) instability. In the areas south of the KE, CHLs tended to be higher in winter during periods of unstable KEs (compared to stable KEs) which were attributed to the increases in nutrient and light availability. Nutrients were supplied from the deep layer due to physical processes indicated by negative sea surface height anomalies (SSHAs) and shallow mixed layer depths (MLDs). The increase in light availability could be attributed to greater exposure of phytoplankton to light in the shallower MLD. The same physical processes also explained higher CHLs in spring during unstable KEs. We also found that CHLs could possibly be lower during unstable KEs in spring which might be related to warmer SSTs in winter–spring. On average, the onset of the phytoplankton spring bloom south of the KE tended to be 1–3 weeks earlier during the period of unstable KEs than during the period of stable KEs. Whether this difference of 1–3 weeks impacts high-trophic-level organisms should be investigated in future studies.

Occurrence and Evolution of Mesoscale Thermodynamic Phenomena in the Northern Part of the East Sea (Japan Sea) Derived from Satellite Altimeter Data

Based on satellite measurements and oceanic reanalysis data, it has been possible to investigate the spatiotemporal variability of the mesoscale phenomena in the northern part of the East Sea (NES) where direct observations of currents and hydrographical conditions are scarce. For the first time, this study identifies the detailed spatiotemporal structure of the mesoscale features in the NES and the mechanism of its occurrence and evolution, which have important consequences on the distribution of the intermediate water masses in the East Sea. Here, we show that mesoscale thermodynamic phenomena in the northwestern region of the East Sea are characterized by a dipole structure associated with positive and negative sea surface height anomalies. These result in a strong thermal gradient between the seasonally non-persistent anomalies, which emerge and strengthen during late fall and early winter. In contrast to the previous finding of the relationship between winter monsoon winds and mesoscale features in the NES, we found that this relationship is crucial only to the emergence of the mesoscale phenomena. Consequently, we present a new perspective on the evolution mechanism of the mesoscale features in the NES. Of direct significance to the present study, thermohaline transport into the northwestern region of the East Sea regulates the strengthening and weakening of mesoscale features in the NES. Wind forcing may contribute to the emergence of the mesoscale features in the NES and then the intensification of the mesoscale activities is attributed to the intrusion of warm and fresh surface water advected from the southern part of the East Sea.

Facing extremes: Cory’s shearwaters adjust their foraging behaviour differently in response to contrasting phases of North Atlantic Oscillation

Climate projections predict increases in the frequency and severity of extreme climate events over the next decades. Hence, phases of extreme climatic indices are emerging as one of the most dangerous effects of climate change, though their impacts on wildlife populations are still poorly understood. Here, we studied the foraging behaviour, body condition and breeding performance of a neritic (Berlenga Island) and oceanic (Corvo Island) population of Cory’s shearwaters (Calonectris borealis) in the mid-North Atlantic, during the two most positive and negative phases of North Atlantic Oscillation (NAO) reported in recent decades. We showed that during an extreme negative NAO phase, birds from Berlenga spent less time foraging and provided less food to their chicks, which subsequently grew more slowly and were in poorer body condition. In contrast, the opposite pattern was found during the strong positive NAO phase in this population. Interestingly, during the same extreme negative NAO phase, birds from Corvo were more successful in terms of their foraging and breeding performance, taking advantage of the enhanced productivity associated with the cyclonic eddies (negative sea surface height anomalies) which occurred close to the colony. However, when anticyclonic eddies (positive sea surface height anomalies) were prevalent near the colony during the strong positive NAO phase, birds travelled longer distances, which negatively impacted their own body condition and that of their chicks. Our study shows that populations breeding in neritic and oceanic areas of the eastern North Atlantic Ocean make contrasting foraging behavioural decisions in response to climate extremes and highlights the importance of mesoscale eddies for oceanic populations of pelagic seabirds.

Variations of the North Equatorial Current Bifurcation and the SSH in the Western Pacific Associated With El Niño Flavors

AbstractThis study presents different variations of the North Equatorial Current Bifurcation (NECB) in the development of the canonical El Niño, the Central Pacific El Niño I (CP‐I El Niño), and the Central Pacific El Niño II (CP‐II El Niño) in autumn and investigates the dynamic mechanism by analyzing Simple Ocean Data Assimilation data sets. It is suggested that the meridional shifts of the NECB are negatively related to the sea surface height (SSH) near the coast of the Philippines. The NECB shifts northward in autumn of the development phase of both canonical and CP‐II El Niño events, whereas it moves insignificantly for CP‐I El Niño events. During the development phase of canonical and CP‐II El Niños, the remote positive wind stress curl anomalies in the central and western tropical Pacific between 12°N and 15°N are responsible for the variations of the NECB, which can induce the westward Rossby waves of negative SSH anomalies and thus result in the northward shift of the NECB several months later. However, the local wind stress curl anomalies near the Philippine coast are significantly negative and result in locally positive SSH anomalies for CP‐I El Niños. Such anomalous positive SSHs are offset by the negative counterparts that result from the remote wind forcing in the western tropical Pacific. Therefore, the meridional shifts of the NECB are not significant in autumn for CP‐I El Niños. A one and a half layer‐reduced gravity model is used to conduct experiments to further confirm the above results.

Seasonal Variation in the Correlation Between Anomalies of Sea Level and Chlorophyll in the Antarctic Circumpolar Current

AbstractThe Antarctic Circumpolar Current has highly energetic mesoscale phenomena, but their impacts on phytoplankton biomass, productivity, and biogeochemical cycling are not understood well. We analyze satellite observations and an eddy‐rich ocean model to show that they drive chlorophyll anomalies of opposite sign in winter versus summer. In winter, deeper mixed layers in positive sea surface height (SSH) anomalies reduce light availability, leading to anomalously low chlorophyll concentrations. In summer with abundant light, however, positive SSH anomalies show elevated chlorophyll concentration due to higher iron level, and an iron budget analysis reveals that anomalously strong vertical mixing enhances iron supply to the mixed layer. Features with negative SSH anomalies exhibit the opposite tendencies: higher chlorophyll concentration in winter and lower in summer. Our results suggest that mesoscale modulation of iron supply, light availability, and vertical mixing plays an important role in causing systematic variations in primary productivity over the seasonal cycle.

Optimal precursors triggering the Kuroshio Extension state transition obtained by the Conditional Nonlinear Optimal Perturbation approach

In this study, the initial perturbations that are the easiest to trigger the Kuroshio Extension (KE) transition connecting a basic weak jet state and a strong, fairly stable meandering state, are investigated using a reduced-gravity shallow water ocean model and the CNOP (Conditional Nonlinear Optimal Perturbation) approach. This kind of initial perturbation is called an optimal precursor (OPR). The spatial structures and evolutionary processes of the OPRs are analyzed in detail. The results show that most of the OPRs are in the form of negative sea surface height (SSH) anomalies mainly located in a narrow band region south of the KE jet, in basic agreement with altimetric observations. These negative SSH anomalies reduce the meridional SSH gradient within the KE, thus weakening the strength of the jet. The KE jet then becomes more convoluted, with a high-frequency and large-amplitude variability corresponding to a high eddy kinetic energy level; this gradually strengthens the KE jet through an inverse energy cascade. Eventually, the KE reaches a high-energy state characterized by two well defined and fairly stable anticyclonic meanders. Moreover, sensitivity experiments indicate that the spatial structures of the OPRs are not sensitive to the model parameters and to the optimization times used in the analysis.

Asymmetric evolution of El Niño and La Niña: the recharge/discharge processes and role of the off-equatorial sea surface height anomaly

Observed oceanic and atmospheric anomalies in the tropical Pacific are analyzed to understand the symmetric and asymmetric characteristics between El Nino and La Nina evolutions. It is noted that the evolutions are largely symmetric for El Nino and La Nina prior to and in their mature phase, but become asymmetric afterwards. It is further demonstrated that this asymmetry is due to the fact that, on average, the discharge process associated with El Nino is stronger than the recharge counterpart associated with La Nina. The symmetric and asymmetric evolution for different phases of El Nino-Southern Oscillation (ENSO) is consistent with the associated recharge and discharge processes that are linked with the evolution of anomalous ocean surface current and gradient of sea surface height anomalies (SSHAs). It is suggested that the evolution of the meridional gradient of SSHAs in the central and eastern equatorial Pacific is different between El Nino and La Nina years due to relatively different rates of the SSHA changes on and off the equator. During the decay of a warm event, the equatorial SSHA quickly switches from positive to negative, meanwhile the off-equator areas are occupied by positive SSHAs. Such changes on the equator and in the off-equatorial regions make the concavity of the meridional SSHA favorable for growth and persistence of westward surface zonal current anomaly in the equatorial Pacific, then for the ENSO phase transition. During the decay of a cold event, the slow decrease of the negative SSHA on the equator combined with the disappearance of the negative SSHA in the off-equatorial regions makes the meridional concavity reverse easily and is unfavorable for the steady growth of the eastward surface zonal current anomaly and for the phase transition. Thus, the asymmetric evolution of ENSO is associated with both the atmosphere and ocean anomalies on the equator and off the equator.

Influence of positive and negative Indian Ocean Dipoles on ENSO via the Indonesian Throughflow: Results from sensitivity experiments

The role of the Indonesian Throughflow (ITF) in the influence of the Indian Ocean Dipole (IOD) on ENSO is investigated using version 2 of the Parallel Ocean Program (POP2) ocean general circulation model. We demonstrate the results through sensitivity experiments on both positive and negative IOD events from observations and coupled general circulation model simulations. By shutting down the atmospheric bridge while maintaining the tropical oceanic channel, the IOD forcing is shown to influence the ENSO event in the following year, and the role of the ITF is emphasized. During positive IOD events, negative sea surface height anomalies (SSHAs) occur in the eastern Indian Ocean, indicating the existence of upwelling. These upwelling anomalies pass through the Indonesian seas and enter the western tropical Pacific, resulting in cold anomalies there. These cold temperature anomalies further propagate to the eastern equatorial Pacific, and ultimately induce a La Nina-like mode in the following year. In contrast, during negative IOD events, positive SSHAs are established in the eastern Indian Ocean, leading to downwelling anomalies that can also propagate into the subsurface of the western Pacific Ocean and travel further eastward. These downwelling anomalies induce negative ITF transport anomalies, and an El Nino-like mode in the tropical eastern Pacific Ocean that persists into the following year. The effects of negative and positive IOD events on ENSO via the ITF are symmetric. Finally, we also estimate the contribution of IOD forcing in explaining the Pacific variability associated with ENSO via ITF.

The Bay of Bengal upper‐ocean response to tropical cyclone forcing during 1999

The regional upper‐ocean dynamical and thermodynamical responses to two consecutive, strong tropical cyclones (TCs)—04B (15–19 October) and 05B (25 October to 3 November) in 1999 (hereafter, TC1 and TC2) in the Bay of Bengal (BoB) and the associated oceanic processes are investigated using an eddy‐permitting Hybrid Coordinate Ocean Model. The TC winds induce positive sea surface height anomalies (SSHA) along the northern BoB coastline and Andaman Sea due to onshore convergence, and negative SSHA along the TC tracks due to Ekman divergence, which in turn induce geostrophic flow. The TC‐associated radiation and precipitation have negligible effects on the northwestern BoB top 30 m averaged temperature (T0–30 m), while the strong TC winds significantly enhance turbulent heat flux causing T0–30 m decrease. Due to the existence of the barrier layer and subsurface warm advection in the northwestern BoB, vertical mixing may induce near‐surface warming by entraining warm water from below. As a result, the proportion of the T0–30 m cooling caused by turbulent heat flux is likely elevated in the 6° × 6° footprints of the TCs. Both TC wind‐induced vertical mixing and upwelling significantly cool T0–30 m in TC1's wake, while upwelling dominates the maximum cooling region in TC2's wake, likely due to the preceding deepening of the mixed layer by TC1. The near‐surface cooling for both TCs has rightward bias because of the higher winds and the resonant response on the right, and Ekman divergence extends the cooling areas outward. TC1 and TC2's sizes and prestorm oceanic conditions are found to be the most influential factors for near‐surface cooling.

Variability in the Tropical Southwest Indian Ocean and Influence on Southern African Climate

This study considers satellite era reanalysis products to characterize inter-annual variability in the SW Indian Ocean and influence on the climate of southern Africa. Decorrelation of temperature from a buoy area (8°S, 55°E) is found to vary by season; widespread in the first half of the year and almost zero in the second. This leads to an analysis of the annual cycle and inter-annual variability via wavelet filtered principal components. Contrasts between SODA and GODAS reanalysis products reveal the former exhibits higher amplitude annual to inter-annual variability. Good agreement is found for sub-surface temperature in the SW Indian Ocean thermocline ridge, with rhythmic fluctuations of 2.5-5 yr period. When an ocean Rossby wave crest arrives in the southwestern part of the basin, increased SST and local convective rainfall alters the large-scale circulation. The NW monsoon becomes active and moisture that would normally be exported to southern Africa is retained in the SW Indian Ocean. Daily data from a mooring at 8°S, 55°E reveals intra-seasonal pulses at 15-50 days in Nov.-Mar. season. Intercomparisons with satellite and reanalysis data reveal a cool bias in NCEP during the SE monsoon. Hovmoller analysis highlights how surges in NW monsoon winds relate to heat fluxes and rossby wave patterns. A case study is made to contrast Dec.-Mar. 2011 vs 2012, the former having negative sea surface height anomalies, the latter being positive. Regional wind responses to SW Indian Ocean heat anomalies contribute to a 15% change in seasonal rainfall and maize production in South Africa.

Equatorial Waves and the Evolution of 2008 Indian Ocean Dipole

Observations and a linear wave model were used to evaluate the role of equatorial oceanic wave processes in affecting the evolution of the 2006 positive Indian Ocean Dipole (IOD), focusing on the activation phase of the event. The observations indicate the presence of upwelling equatorial waves and westward near-surface zonal currents along the equator during the activation phase of the event in August 2006. These upwelling equatorial waves (negative sea surface height anomalies) and westward zonal current anomalies contributed to significant sea surface cooling in the eastern equatorial Indian Ocean. The model results reveal that the upwelling equatorial waves and westward nearsurface zonal currents were mainly generated by wind-forced Kelvin waves associated with the easterly wind anomalies. On the other hand, anomalous easterly winds along the equator during June caused downwelling waves (positive sea surface height anomalies) in the off-equatorial region which propagated westward and elevated sea levels in the western region. The model further shows that a complex interplay of wind-forced and boundary-generated Rossby waves elevated sea levels in the western equatorial Indian Ocean during the activations phase of the 2006 IOD event.

Independence of SST skewness from thermocline feedback in the eastern equatorial Indian Ocean

The observed sea surface height (SSH)‐sea surface temperature (SST) in east equatorial Indian Ocean (EEIO) shows a significant asymmetric relation between the positive and negative SSH anomalies during boreal autumn. Whether or not the thermocline‐ SST feedback is responsible for the negative SST skewness in the EEIO is explored by diagnosing various air‐sea feedback processes and a mixed layer heat budget. Our analysis based on observations suggests that the SSH‐subsurface temperature relation is approximately symmetric between the positive and negative episode, which is significantly different from the asymmetric SSH‐SST relation. This implies that the observed SSH‐SST asymmetry is not attributed to the ocean thermocline feedback. A further analysis of the SST‐precipitation, precipitation‐wind stress and wind stress‐SSH relations demonstrates that the asymmetry arises from asymmetric atmospheric heating/wind responses to the SST anomaly. A mixed layer heat budget analysis reveals that the ratio of the mixed‐layer temperature tendency between the positive and negative events keeps approximately unchanged with and without vertical temperature advection, suggesting that the thermocline‐feedback is not crucial for generating the negative SST skewness in the EEIO.

Comparative Influence of Ocean Conditions on Yellowfin and Atlantic Bluefin Tuna Catch from Longlines in the Gulf of Mexico

Directed fishing effort for Atlantic bluefin tuna in the Gulf of Mexico (GOM), their primary spawning grounds in the western Atlantic, has been prohibited since the 1980s due to a precipitous decline of the spawning stock biomass. However, pelagic longlines targeted at other species, primarily yellowfin tuna and swordfish, continue to catch Atlantic bluefin tuna in the GOM as bycatch. Spatial and temporal management measures minimizing bluefin tuna bycatch in the GOM will likely become important in rebuilding the western Atlantic bluefin stock. In order to help inform management policy and understand the relative distribution of target and bycatch species in the GOM, we compared the spatiotemporal variability and environmental influences on the catch per unit effort (CPUE) of yellowfin (target) and bluefin tuna (bycatch). Catch and effort data from pelagic longline fisheries observers (1993–2005) and scientific tagging cruises (1998–2002) were coupled with environmental and biological data. Negative binomial models were used to fit the data for both species and Akaike's Information Criterion (corrected for small sample size) was used to determine the best model. Our results indicate that bluefin CPUE had higher spatiotemporal variability as compared to yellowfin CPUE. Bluefin CPUE increased substantially during the breeding months (March-June) and peaked in April and May, while yellowfin CPUE remained relatively high throughout the year. In addition, bluefin CPUE was significantly higher in areas with negative sea surface height anomalies and cooler sea surface temperatures, which are characteristic of mesoscale cyclonic eddies. In contrast, yellowfin CPUE was less sensitive to environmental variability. These differences in seasonal variability and sensitivity to environmental influences suggest that bluefin tuna bycatch in the GOM can be reduced substantially by managing the spatial and temporal distribution of the pelagic longline effort without substantially impacting yellowfin tuna catches.

ANOMALOUS OCEANIC CONDITIONS IN THE TROPICAL INDIAN OCEAN DURING 2006 AS REVEALED BY MULTI-SATELLITE SENSORS

A positive Indian Ocean Dipole (IOD) took place in the tropical Indian Ocean during 2006. The evolution of this event started in July 2006 and intensified during August 2006. It was indicated by negative sea surface temperature anomalies, lower than normal sea level and supressed convection in the southeastern equatorial Indian Ocean in contrast to western counterpart. Peak negative SST anomalies exceeding 1°C were observed in the eastern basin during September-November coinciding with anomaous easterly winds along the equator and strong southeasterly winds along the coast of Sumatra and Java. The expression of this positive IOD was also seen in other physical variables: negative sea surface height anomalies about 30 cm and negative rainfall anomalies exceeding 8 mm/day were observed in the eastern basin. The events was terminated in December 2006 mainly due to warming of the eastern pole. The magnitude of this event determined by the east-west temperature gradient across the tropical Indian Ocean was the third largest in the last 30 years, after 1997 and 1994 events.

Bathymetry and frontal system interactions influence seasonal foraging movements of lactating subantarctic fur seals from Marion Island

Sixteen lactating subantarctic fur seals Arctocephalus tropicalis were satellite-tracked during the winter of 2006 (n = 6), summer of 2006/07 (n = 6) and autumn/winter (n = 4) of 2007, from Marion Island, Southern Ocean. Despite varied individual movement patterns, a favoured foraging area lay to the northeast of the island. In contrast to findings for populations at similar latitudes, seals from Marion Island did not undertake short overnight foraging trips, but trips consistently went beyond 300 km from the island. This aligns with the at-sea duration of lactating seals’ foraging trips from temperate Amsterdam Island, but differs from subantarctic Crozet and Macquarie islands. Time spent at sea, maximum distances travelled and movement variation of tracks from the island varied seasonally. Faecal analysis suggests the diet comprised primarily myctophid fish with limited seasonal variation. Well-defined areas of restricted movement coincided with significant bathymetric features to the west/northwest of the Crozet Plateau, with the Del Cano Rise clearly being important. Positive and negative sea-surface height anomalies (compared to the mean) appeared to be preferred by most seals across seasons. Higher summer sea-surface temperatures correlated with the movements of some seals. Higher chlorophyll a concentrations dictated transit and foraging areas during summer. Bathymetrically influenced oceanographic variables likely explain these preferred long-distance eastward movements. The Iles Crozet and Marion island subantarctic fur seals differ in their foraging ecology despite being neighbours. Conversely, the subantarctic fur seal populations from the distant Amsterdam and Marion islands appear to be similarly influenced by such environmental factors.

Comment on “Importance of pre‐existing oceanic conditions to upper ocean response induced by Super Typhoon Hai‐Tang” by Z.‐W. Zheng, C.‐R. Ho, and N.‐J. Kuo

[1] Understanding the impact of pre-existing oceanic conditions on the upper ocean response to tropical cyclones (TCs) is one of the important issues for understanding the short-term occurrence of TC-ocean interaction and its impact on TC intensity. Zheng et al. [2008] showed that preexisting cyclonic flow represented by negative sea-surface height anomalies (SSHA) played a crucial role in enhancing sea-surface cooling (SSC) caused by Typhoon Hai-Tang in 2005. On the other hand, the impact of initial oceanic mixedlayer depth and the vertical sea-temperature gradient in the thermocline on the amplitude of SSC has been considered to be small compared with that of the Ekman pumping and entrainment/vertical turbulent mixing [e.g., Wada, 2002]. Therefore, we hardly accept the discussion of Zheng et al. [2008] that the impact of Ekman pumping on SSC is less significant for local SSC occurred in certain areas than that of pre-existing oceanic condition. [2] Zheng et al. [2008] provide no quantitative evidence that SSC caused by the passage of Hai-Tang is irrelevant to the Ekman pumping or that the amplitude of SSC caused by the Ekman pumping is negligibly smaller than that due to relatively low sea surface height (SSH). In this paper, we revisit the relationship between SSC and SSH using a daily oceanic reanalysis dataset [Wada and Usui, 2007] produced by the North Pacific version of the Japan Meteorological Agency/Meteorological Research Institute Multivariate Ocean Variational Estimation system (MOVE) [Usui et al., 2006] in section 2. In section 3, we revisit the impact of the pre-existing oceanic condition on the ocean response to HaiTang using the Argo profiling float data. Section 4 presents our concluding remarks.