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  • AMS (American Meteorological Society)  (8)
  • 2015-2019  (6)
  • 2010-2014  (2)
  • 1
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    Influences of Pacific Climate Variability on Decadal Subsurface Ocean Heat Content Variations in the Indian Ocean (2018)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 31 (10). pp. 4157-4174.
    Details
    Publication Date: 2021-02-08
    Description: Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC; 50–300 m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal time scales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean Basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the interdecadal Pacific oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the Maritime Continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, a linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JCLI-D-17-0654.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Decadal variability of Eddy Kinetic Energy in the South Pacific Subtropical Countercurrent in an Ocean General Circulation Model (2018)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 48 (4). pp. 757-771.
    Details
    Publication Date: 2021-02-08
    Description: The Eddy Kinetic Energy (EKE) associated with the Subtropical Countercurrent (STCC) in the western subtropical South Pacific is known to exhibit substantial seasonal and decadal variability. Using an eddy-permitting ocean general circulation model, which is able to reproduce the observed, salient features of the seasonal cycles of shear, stratification, baroclinic production and the associated EKE, we investigate the decadal changes of EKE. We show that the STCC region exhibits, uniquely among the subtropical gyres of the world’s oceans, significant, atmospherically forced, decadal EKE variability. The decadal variations are driven by changing vertical shear between the STCC in the upper 300 m and the South Equatorial Current below, predominantly caused by variations in STCC strength associated with a changing meridional density gradient. In the 1970s, an increased meridional density gradient results in EKE twice as large as in later decades in the model. Utilizing sensitivity experiments, decadal variations in the wind field are shown to be the essential driver. Local wind stress curl anomalies associated with the Interdecadal Pacific Oscillation (IPO) lead to up- and downwelling of the thermocline, inducing strengthening or weakening of the STCC and the associated EKE. Additionally, remote wind stress curl anomalies in the eastern subtropical South Pacific, which are not related to the IPO, generate density anomalies that propagate westward as Rossby waves and can account for up to 30–40 % of the density anomalies in the investigated region.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-17-0173.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Annual and semiannual cycle of equatorial Atlantic circulation associated with basin mode resonance (2016)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 46 (10). pp. 3011-3029.
    Details
    Publication Date: 2020-08-04
    Description: Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semi-annual variability is also pronounced, despite weak forcing at that period. Here we use multi-year, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semi-annual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the 4th mode and the semi-annual cycle by the 2nd mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semi-annual cycles of the respective dominant baroclinic modes are associated with characteristic basin-wide structures. Using an idealized linear reduced-gravity model to simulate the dynamics of individual baroclinic modes, it is shown that the observed circulation variability can be explained by resonant equatorial basin modes. Corollary simulations of the reduced-gravity model with varying basin geometry (i.e. square basin versus realistic coastlines) or forcing (i.e. spatially uniform versus spatially variable wind) show a structural robustness of the simulated basin modes. A main focus of this study is the seasonal variability of the Equatorial Undercurrent (EUC) as identified in recent observational studies. Main characteristics of the observed EUC including seasonal variability of transport, core depth, and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced-gravity model.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1175/JPO-D-15-0248.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Regional Patterns of Sea Level Change Related to Interannual Variability and Multidecadal Trends in the Atlantic Meridional Overturning Circulation (2010)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 23 (15). pp. 4243-4254.
    Details
    Publication Date: 2020-08-04
    Description: Some studies of ocean climate model experiments suggest that regional changes in dynamic sea level could provide a valuable indicator of trends in the strength of the Atlantic meridional overturning circulation (MOC). This paper describes the use of a sequence of global ocean–ice model experiments to show that the diagnosed patterns of sea surface height (SSH) anomalies associated with changes in the MOC in the North Atlantic (NA) depend critically on the time scales of interest. Model hindcast simulations for 1958–2004 reproduce the observed pattern of SSH variability with extrema occurring along the Gulf Stream (GS) and in the subpolar gyre (SPG), but they also show that the pattern is primarily related to the wind-driven variability of MOC and gyre circulation on interannual time scales; it is reflected also in the leading EOF of SSH variability over the NA Ocean, as described in previous studies. The pattern, however, is not useful as a “fingerprint” of longer-term changes in the MOC: as shown with a companion experiment, a multidecadal, gradual decline in the MOC [of 5 Sv (1 Sv ≡ 106 m3 s−1) over 5 decades] induces a much broader, basin-scale SSH rise over the mid-to-high-latitude NA, with amplitudes of 20 cm. The detectability of such a trend is low along the GS since low-frequency SSH changes are effectively masked here by strong variability on shorter time scales. More favorable signal-to-noise ratios are found in the SPG and the eastern NA, where a MOC trend of 0.1 Sv yr−1 would leave a significant imprint in SSH already after about 20 years.
    Type: Article , PeerReviewed
    Format: text
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    Format: text
    DOI: 10.1175/2010JCLI3341.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Multidecadal Indian Ocean variability linked to the Pacific and implications for pre-conditioning Indian Ocean Dipole events (2017)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 30 (5). pp. 1739-1751.
    Details
    Publication Date: 2020-02-06
    Description: The Indian Ocean has sustained robust surface warming in recent decades, but the role of multi-decadal variability remains unclear. Using ocean model hindcasts, characteristics of low-frequency Indian Ocean temperature variations are explored. Simulated upper-ocean temperature changes across the Indian Ocean in the hindcast are consistent with those recorded in observational products and ocean reanalyses. Indian Ocean temperatures exhibit strong warming trends since the 1950s limited to the surface and south of 30°S, while extensive subsurface cooling occurs over much of the tropical Indian Ocean. Previous work focused on diagnosing causes of these long-term trends in the Indian Ocean over the second half of the 20th Century. Instead, the temporal evolution of Indian Ocean subsurface heat content is shown here to reveal distinct multi-decadal variations associated with the Pacific Decadal Oscillation and the long-term trends are thus interpreted to result from aliasing of the low-frequency variability. Transmission of the multi-decadal signal occurs via an oceanic pathway through the Indonesian Throughflow and is manifest across the Indian Ocean centered along 12°S as westward propagating Rossby waves modulating thermocline and subsurface heat content variations. Resulting low-frequency changes in the eastern Indian Ocean thermocline depth are associated with decadal variations in the frequency of Indian Ocean Dipole (IOD) events, with positive IOD events unusually common in the 1960s and 1990s with a relatively shallow thermocline. In contrast, the deeper thermocline depth in the 1970s and 1980s is associated with frequent negative IOD and rare positive IOD events. Changes in Pacific wind forcing in recent decades and associated rapid increases in Indian Ocean subsurface heat content can thus affect the basin’s leading mode of variability, with implications for regional climate and vulnerable societies in surrounding countries.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1175/JCLI-D-16-0200.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Loop Current Variability as Trigger of Coherent Gulf Stream Transport Anomalies (2019)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 49 (8). pp. 2115-2132.
    Details
    Publication Date: 2022-01-31
    Description: Satellite observations and output from a high-resolution ocean model are used to investigate how the Loop Current in the Gulf of Mexico affects the Gulf Stream transport through the Florida Straits. We find that the expansion (contraction) of the Loop Current leads to lower (higher) transports through the Straits of Florida. The associated surface velocity anomalies are coherent from the southwestern tip of Florida to Cape Hatteras. A simple continuity-based argument can be used to explain the link between the Loop Current and the downstream Gulf Stream transport: as the Loop Current lengthens (shortens) its path in the Gulf of Mexico, the flow out of the Gulf decreases (increases). Anomalies in the surface velocity field are first seen to the southwest of Florida and within 4 weeks propagate through the Florida Straits up to Cape Hatteras and into the Gulf Stream Extension. In both the observations and the model this propagation can be seen as pulses in the surface velocities. We estimate that the Loop Current variability can be linked to a variability of several Sverdrups (1Sv = 10(6) m(3) s(-1)) through the Florida Straits. The exact timing of the Loop Current variability is largely unpredictable beyond a few weeks and its variability is therefore likely a major contributor to the chaotic/intrinsic variability of the Gulf Stream. However, the time lag between the Loop Current and the flow downstream of the Gulf of Mexico means that if a lengthening/shortening of the Loop Current is observed this introduces some predictability in the downstream flow for a few weeks.
    Type: Article , PeerReviewed
    Format: archive
    Format: text
    DOI: 10.1175/JPO-D-18-0236.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    The Nature of Eddy Kinetic Energy in the Labrador Sea: Different Types of Mesoscale Eddies, their Temporal Variability and Impact on Deep Convection (2019)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2022-02-25
    Description: Oceanic eddies are an important component in preconditioning the central Labrador Sea (LS) for deep convection and in restratifying the convected water. This study investigates the different sources and impacts of Eddy Kinetic Energy (EKE) and its temporal variability in the LS with the help of a 52-year long hindcast simulation of a 1/20° ocean model. Irminger Rings (IR) are generated in the West Greenland Current (WGC) between 60 and 62°N, mainly affect preconditioning and limit the northward extent of the convection area. The IR exhibit a seasonal cycle and decadal variations linked to the WGC strength, varying with the circulation of the subpolar gyre. The mean and temporal variations of IR generation can be attributed to changes in deep ocean baroclinic and upper ocean barotropic instabilities at comparable magnitudes. The main source of EKE and restratification in the central LS are Convective Eddies (CE). They are generated by baroclinic instabilities near the bottom of the mixed layer during and after convection. The CE have a mid-depth core and reflect the hydrographic properties of the convected water mass with a distinct minimum in potential vorticity. Their seasonal to decadal variability is tightly connected to the local atmospheric forcing and the associated air-sea heat fluxes. A third class of eddies in the LS are the Boundary Current Eddies shed from the Labrador Current (LC). Since they are mostly confined to the vicinity of the LC, these eddies appear to exert only minor influence on preconditioning and restratification.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Pacific Ocean Contribution to the Asymmetry in Eastern Indian Ocean Variability (2013)
    AMS (American Meteorological Society)
    Details
    Publication Date: 2022-03-10
    Description: Variations in eastern Indian Ocean upper-ocean thermal properties are assessed for the period 1970–2004, with a particular focus on asymmetric features related to opposite phases of Indian Ocean Dipole events, using high-resolution ocean model hindcasts. Sensitivity experiments, where atmospheric forcing variability is restricted to the Indian or Pacific Ocean only, support the interpretation of forcing mechanisms for large-scale asymmetric behavior in eastern Indian Ocean variability. Years are classified according to eastern Indian Ocean subsurface heat content (HC) as proxy of thermocline variations. Years characterized by anomalous low HC feature a zonal gradient in upper-ocean properties near the equator, while high events have a meridional gradient from the tropics into the subtropics. The spatial and temporal characteristics of the seasonal evolution of HC anomalies for the two cases is distinct, as is the relative contribution from Indian Ocean atmospheric forcing versus remote influences from Pacific wind forcing: low events develop rapidly during austral winter/spring in response to Indian Ocean wind forcing associated with an enhanced southeasterly monsoon driving coastal upwelling and a shoaling thermocline in the east; in contrast, formation of anomalous high eastern Indian Ocean HC is more gradual, with anomalies earlier in the year expanding from the Indonesian Throughflow (ITF) region, initiated by remote Pacific wind forcing and transmitted through the ITF via coastal wave dynamics. Implications for seasonal predictions arise with high HC events offering extended lead times for predicting thermocline variations and upper-ocean properties across the eastern Indian Ocean.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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