scholarly journals Interannual Agulhas Leakage Variability and Its Regional Climate Imprints

2018 ◽  
Vol 31 (24) ◽  
pp. 10105-10121 ◽  
Author(s):  
Yu Cheng ◽  
Lisa M. Beal ◽  
Ben P. Kirtman ◽  
Dian Putrasahan
Keyword(s):  
Time Scales ◽  
Large Scale ◽  
Regional Climate ◽  
Austral Summer ◽  
Heat Fluxes ◽  
Convective Rainfall ◽  
Surface Heat Fluxes ◽  
Model Studies ◽  
Large Scale Field ◽  
The Indian Ocean

We investigate the interannual variability of Agulhas leakage in an ocean-eddy-resolving coupled simulation and characterize its influence on regional climate. Many observational leakage estimates are based on the study of Agulhas rings, whereas recent model studies suggest that rings and eddies carry less than half of leakage transport. While leakage variability is dominated by eddies at seasonal time scales, the noneddy leakage transport is likely to be constrained by large-scale forcing at longer time scales. To investigate this, leakage transport is quantified using an offline Lagrangian particle tracking approach. We decompose the velocity field into eddying and large-scale fields and then recreate a number of total velocity fields by modifying the eddying component to assess the dependence of leakage variability on the eddies. We find that the resulting leakage time series show strong coherence at periods longer than 1000 days and that 50% of the variance at interannual time scales is linked to the smoothed, large-scale field. As shown previously in ocean models, we find Agulhas leakage variability to be related to a meridional shift and/or strengthening of the westerlies. High leakage periods are associated with east–west contrasting patterns of sea surface temperature, surface heat fluxes, and convective rainfall, with positive anomalies over the retroflection region and negative anomalies within the Indian Ocean to the east. High leakage periods are also related to reduced inland convective rainfall over southeastern Africa in austral summer.

scholarly journals The role of large-scale moistening by adiabatic lifting in the Madden-Julian Oscillation convective onset

2021 ◽  
pp. 1-49
Author(s):  
Chelsea E. Snide ◽  
Ángel F. Adames ◽  
Scott W. Powell ◽  
Víctor C. Mayta
Keyword(s):  
Large Scale ◽  
Heat Fluxes ◽  
Data Sets ◽  
Madden Julian Oscillation ◽  
Onset Of Convection ◽  
Surface Heat Fluxes ◽  
Planetary Scale ◽  
Moisture Advection ◽  
The Indian Ocean

AbstractThe initiation of the Madden-Julian Oscillation over the Indian Ocean is examined through the use of a moisture budget that applies a version of the weak temperature gradient (WTG) approximation that does not neglect dry adiabatic vertical motions. Examination of this budget in ERA-Interim reveals that horizontal moisture advection and vertical advection by adiabatic lifting govern the moistening of the troposphere for both primary and successive MJO initiation events. For both types of initiation events, horizontal moisture advection peaks prior to the maximum moisture tendency, while adiabatic lifting peaks after the maximum moisture tendency. Once convection initiates, moisture is maintained by anomalous radiative and adiabatic lifting. Adiabatic lifting during successive MJO initiation is attributed to the return of the circumnavigating circulation from a previous MJO event, while in primary events the planetary-scale circulation appears to originate over South America. Examination of the same budget with data from the DYNAMO northern sounding array shows that adiabatic lifting contributes significantly to MJO maintenance, with a contribution that is comparable to that of surface heat fluxes. However, results from the DYNAMO data disagree with those from ERA-Interim over the importance of adiabatic lifting to the moistening of the troposphere prior to the onset of convection. In spite of these differences, the results from the two data sets show that small departures from WTG balance in the form of dry adiabatic motions cannot be neglected when considering MJO convective onset.

scholarly journals From Synoptic to Interdecadal Variability in Southern African Rainfall: Toward a Unified View across Time Scales

2018 ◽  
Vol 31 (15) ◽  
pp. 5845-5872 ◽  
Author(s):  
Benjamin Pohl ◽  
Bastien Dieppois ◽  
Julien Crétat ◽  
Damian Lawler ◽  
Mathieu Rouault
Keyword(s):  
Time Scales ◽  
Large Scale ◽  
Southern Oscillation ◽  
Rainfall Variability ◽  
Regional Climate ◽  
Low Frequency ◽  
Austral Summer ◽  
Longwave Radiation ◽  
African Rainfall ◽  
The Pacific

During the austral summer season (November–February), southern African rainfall, south of 20°S, has been shown to vary over a range of time scales, from synoptic variability (3–7 days, mostly tropical temperate troughs) to interannual variability (2–8 years, reflecting the regional effects of El Niño–Southern Oscillation). There is also evidence for variability at quasi-decadal (8–13 years) and interdecadal (15–28 years) time scales, linked to the interdecadal Pacific oscillation and the Pacific decadal oscillation, respectively. This study aims to provide an overview of these ranges of variability and their influence on regional climate and large-scale atmospheric convection and quantify uncertainties associated with each time scale. We do this by applying k-means clustering onto long-term (1901–2011) daily outgoing longwave radiation anomalies derived from the 56 individual members of the Twentieth Century Reanalysis. Eight large-scale convective regimes are identified. Results show that 1) the seasonal occurrence of the regimes significantly varies at the low-frequency time scales mentioned above; 2) these modulations account for a significant fraction of seasonal rainfall variability over the region; 3) significant associations are found between some of the regimes and the aforementioned modes of climate variability; and 4) associated uncertainties in the regime occurrence and convection anomalies strongly decrease with time, especially the phasing of transient variability. The short-lived synoptic anomalies and the low-frequency anomalies are shown to be approximately additive, but even if they combine their respective influence at both scales, the magnitude of short-lived perturbations remains much larger.

scholarly journals Fifty-Seven-Year California Reanalysis Downscaling at 10 km (CaRD10). Part I: System Detail and Validation with Observations

2007 ◽  
Vol 20 (22) ◽  
pp. 5553-5571 ◽  
Author(s):  
Masao Kanamitsu ◽  
Hideki Kanamaru
Keyword(s):  
Time Scales ◽  
Large Scale ◽  
Global Analysis ◽  
Regional Climate ◽  
Regional Scale ◽  
Surface Wind ◽  
Lateral Boundary ◽  
Climate Change Research ◽  
Near Surface ◽  
Precipitation Analysis

Abstract For the purpose of producing datasets for regional-scale climate change research and application, the NCEP–NCAR reanalysis for the period 1948–2005 was dynamically downscaled to hourly, 10-km resolution over California using the Regional Spectral Model. This is Part I of a two-part paper, describing the details of the downscaling system and comparing the downscaled analysis [California Reanalysis Downscaling at 10 km (CaRD10)] against observation and global analysis. An extensive validation of the downscaled analysis was performed using station observations, Higgins gridded precipitation analysis, and Precipitation-Elevation Regression on Independent Slopes Model (PRISM) precipitation analysis. In general, the CaRD10 near-surface wind and temperature fit better to regional-scale station observations than the NCEP–NCAR reanalysis used to force the regional model, supporting the premise that the regional downscaling is a viable method to attain regional detail from large-scale analysis. This advantage of CaRD10 was found on all time scales, ranging from hourly to decadal scales (i.e., from diurnal variation to multidecadal trend). Dynamically downscaled analysis provides ways to study various regional climate phenomena of different time scales because all produced variables are dynamically, physically, and hydrologically consistent. However, the CaRD10 is not free from problems. It suffers from positive bias in precipitation for heavy precipitation events. The CaRD10 is inaccurate near the lateral boundary where regional detail is damped by the lateral boundary relaxation. It is important to understand these limitations before the downscaled analysis is used for research.

scholarly journals Oceanic Response to Surface Loading Effects Neglected in Volume-Conserving Models

2006 ◽  
Vol 36 (3) ◽  
pp. 426-434 ◽  
Author(s):  
Rui M. Ponte
Keyword(s):  
Time Scales ◽  
Deep Ocean ◽  
Heat Fluxes ◽  
Freshwater Flux ◽  
Surface Heat Fluxes ◽  
Sea Level Fluctuations ◽  
Freshwater Fluxes ◽  
Variable Surface ◽  
Loading Effects ◽  
Surface Loads

Abstract Forcing by freshwater fluxes implies variable surface loads that are not treated in volume-conserving ocean models. A similar problem exists with the representation of volume changes implied by surface heat fluxes. Under the assumption of an equilibrium response, such surface loads merely lead to spatially uniform sea level fluctuations, which carry no dynamical significance. A barotropic model forced by realistic freshwater fluxes is used to test the validity of the equilibrium assumption on seasonal to daily time scales. The simulated nonequilibrium signals have amplitudes much weaker than those of the forcing, with standard deviations well below 1 mm over most of the deep ocean. Larger values (up to ∼1 cm) can be found in shallow and semienclosed coastal areas, where the equilibrium assumption can lead to substantial errors even at monthly and longer time scales. Forcing by mean seasonal river runoff yields similar results, and heat flux effects lead to weaker nonequilibrium signals. In contrast, nonequilibrium signals driven by atmospheric pressure loading are at least an order of magnitude larger than those forced by freshwater fluxes. The exceptions occur for some shallow, coastal regions in the Tropics and at the longest time scales, in general, where forcing by freshwater flux is much stronger than by pressure.

Evaporation measurements with an Optical-Microwave Scintillometer system over a Saline lake in the Atacama Desert

2020 ◽  
Author(s):  
Felipe Lobos Roco ◽  
Oscar Hartogensis ◽  
Jordi Vila ◽  
Alberto de la Fuente ◽  
Francisco Suarez
Keyword(s):  
Large Scale ◽  
Saline Lake ◽  
Regional Scale ◽  
Atacama Desert ◽  
Open Water ◽  
Heat Fluxes ◽  
Sudden Increase ◽  
Surface Heat Fluxes ◽  
Advantages And Disadvantages ◽  
Water Outflow

<p>Evaporation is the main water outflow and a key component of the water and surface energy balance in the endorheic basins of the Atacama Desert. This is very localized to confined environments such as saline lakes, wetlands and crop fields. In these environments, the understanding of evaporation is challenging due to the interaction between the large-scale forcing and local scale turbulence over heterogeneous surfaces. Here, the advection of momentum, heat or moisture plays an important role in the enhancement of evaporation. To understand the evaporation dynamics over such environments, we performed a comprehensive 10-days experiment: the E-DATA (<strong>E</strong>vaporation caused by <strong>D</strong>ry <strong>A</strong>ir <strong>T</strong>ransport over the <strong>A</strong>tacama Desert), localized under extreme conditions in the Salar del Huasco saline lake (22,3°S - 68,8°W - 3790 m a.s.l.), Chile. The measurement strategy was based on spatially distributed high-resolution surface and airborne observations in combination with WRF (Weather Research Forecasting) modeling. The main findings of the experiment show that evaporation is mainly controlled by the lack of turbulence in the morning and by regional-scale forcing in the afternoon, which leads to a sudden increase in mechanical turbulence, therefore in the evaporation flux.  </p><p>This work compares two in-situ independent measurements of surface heat fluxes over the saline lake, by using an Eddy Covariance (EC) system and an Optical-Microwave Scintillometer (OMS). Our results show in general a good agreement between EC and OMS measurements of latent (L<sub>v</sub>E) and sensible (H) heat fluxes over the water surface (R<sup>2</sup>: 0,90-0,96). During the morning, slight differences are observed between the EC and OMS measurements. However, differences up to 200 W m<sup>-2</sup> are observed in the afternoon for L<sub>v</sub>E and up to 20 Wm<sup>-2</sup> for H. The first analysis shows that these differences given during the afternoon are likely attributed to Monin-Obukhov stability (MOST) functions, which need to be developed yet for open water surfaces. Moreover, differences in the footprint of both measurement systems together with dramatic wind changes between the morning and afternoon may play a role. Finally, inaccurate bandpass filtering of the raw scintillometer signal may be a factor in the differences between EC and OMS, where we are currently working to refine our results. Our findings highlight the advantages and disadvantages of each measurement method over an open water body and provide a discussion about its performance.    </p>

scholarly journals The sensitivity of the Seychelles–Chagos thermocline ridge to large-scale wind anomalies

2009 ◽  
Vol 66 (7) ◽  
pp. 1455-1466 ◽  
Author(s):  
Juliet C. Hermes ◽  
Chris J. C. Reason
Keyword(s):  
Indian Ocean ◽  
Large Scale ◽  
Regional Climate ◽  
Austral Summer ◽  
Tropical Indian Ocean ◽  
Ocean Model ◽  
The South ◽  
South Indian Ocean ◽  
Southwest Indian Ocean ◽  
South Indian

Abstract Hermes, J. C., and Reason, C. J. C. 2009. The sensitivity of the Seychelles–Chagos thermocline ridge to large-scale wind anomalies. – ICES Journal of Marine Science, 66: 1455–1466. The Seychelles–Chagos thermocline ridge (SCTR) in the southwest tropical Indian Ocean is important for regional climate, the Madden–Julian Oscillation, as well as upper-ocean nutrients and related phytoplankton and zooplankton densities. Subsurface variability in this region has been proved to influence the overlying sea surface temperatures, which in turn can influence eastern African rainfall. There is evidence that austral summers with a deeper (shallower) SCTR tend to have more (less) tropical cyclone (TC) days in the Southwest Indian Ocean. The importance of this relationship was underlined during the 2006/2007 austral summer, when areas of Madagascar and central Mozambique experienced devastating floods, because of ten named tropical storms, including several intense TCs, effecting on these areas. At the same time, the SCTR during this season was anomalously deep, partly because of a downwelling Rossby wave that propagated across the South Indian Ocean during the previous austral winter/spring. In this paper, a regional ocean model is used to investigate the effect of remote forcing on this region and to study the sensitivity of the SCTR to changes in the large-scale winds over the South Indian Ocean, with a particular focus on the events of the 2006/2007 austral summer.

scholarly journals Atmospherically Forced Exchange through the Bab el Mandeb Strait

2012 ◽  
Vol 42 (7) ◽  
pp. 1143-1157 ◽  
Author(s):  
William E. Johns ◽  
Sarantis S. Sofianos
Keyword(s):  
Time Scales ◽  
Red Sea ◽  
Large Scale ◽  
Barometric Pressure ◽  
Forced Response ◽  
Low Frequencies ◽  
The Indian Ocean ◽  
Annual Evaporation ◽  
Forcing Mechanisms ◽  
Stress Variability

Abstract The exchange between the Red Sea and the Indian Ocean on synoptic time scales (days to weeks) is investigated using moored current meter data collected in the strait of Bab el Mandeb from June 1995 to November 1996. Transport variations through the strait on these time scales can reach amplitudes of up to 0.6 Sv (1 Sv ≡ 106 m3 s−1), or nearly twice as large as the mean rate of exchange through the strait driven by annual evaporation over the Red Sea. The synoptic transport variability appears to be driven by two primary forcing mechanisms: 1) local wind stress variability over the strait and 2) variation in the large-scale barometric pressure over the Red Sea. Simple models of the forced response are developed and are shown to reproduce the essential features of the observations. The response to barometric pressure forcing over the Red Sea is fundamentally barotropic, whereas the response to along-strait winds is barotropic at high frequencies and tends toward a two-layer exchange at low frequencies. The responses to both types of forcing show enhanced amplitude at the Helmholtz resonance frequency for the Red Sea, which occurs at a period of about 5 days. A linear two-layer model, incorporating both types of forcing and a reasonable frictional parameterization, is shown to account for about 70% of the observed transport variance within the strait.

Genesis of Indian Ocean Mixed Layer Temperature Anomalies: A Heat Budget Analysis

2010 ◽  
Vol 23 (20) ◽  
pp. 5375-5403 ◽  
Author(s):  
Agus Santoso ◽  
Alexander Sen Gupta ◽  
Matthew H. England
Keyword(s):  
Indian Ocean ◽  
Time Scales ◽  
Mixed Layer ◽  
Heat Budget ◽  
Heat Fluxes ◽  
Temperature Anomalies ◽  
Mixed Layer Temperature ◽  
The Indian Ocean ◽  
The Tropics

Abstract The genesis of mixed layer temperature anomalies across the Indian Ocean are analyzed in terms of the underlying heat budget components. Observational data, for which a seasonal budget can be computed, and a climate model output, which provides improved spatial and temporal coverage for longer time scales, are examined. The seasonal climatology of the model heat budget is broadly consistent with the observational reconstruction, thus providing certain confidence in extending the model analysis to interannual time scales. To identify the dominant heat budget components, covariance analysis is applied based on the heat budget equation. In addition, the role of the heat budget terms on the generation and decay of temperature anomalies is revealed via a novel temperature variance budget approach. The seasonal evolution of the mixed layer temperature is found to be largely controlled by air–sea heat fluxes, except in the tropics where advection and entrainment are important. A distinct shift in the importance and role of certain heat budget components is shown to be apparent in moving from seasonal to interannual time scales. On these longer time scales, advection gains importance in generating and sustaining anomalies over extensive regions, including the trade wind and midlatitude wind regimes. On the other hand, air–sea heat fluxes tend to drive the evolution of thermal anomalies over subtropical regions including off northwestern Australia. In the tropics, however, they limit the growth of anomalies. Entrainment plays a role in the generation and maintenance of interannual anomalies over localized regions, particularly off Sumatra and over the Seychelles–Chagos Thermocline Ridge. It is further shown that the spatial distribution of the role and importance of these terms is related to oceanographic features of the Indian Ocean. Mixed layer depth effects and the influence of model biases are discussed.

scholarly journals Enhanced Persistence of Equatorial Waves via Convergence Coupling in the Stochastic Multicloud Model

2015 ◽  
Vol 72 (12) ◽  
pp. 4701-4720 ◽  
Author(s):  
Noah D. Brenowitz ◽  
Yevgeniy Frenkel ◽  
Andrew J. Majda
Keyword(s):  
Large Scale ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Theoretical Studies ◽  
Equatorial Waves ◽  
Moist Convection ◽  
Kelvin Wave ◽  
Surface Heat Fluxes ◽  
Walker Cell ◽  
Conditional Instability

Abstract Recent observational and theoretical studies show a systematic relationship between tropical moist convection and measures related to large-scale convergence. It has been suggested that cloud fields in the column stochastic multicloud model compare better with observations when using predictors related to convergence rather than moist energetics (e.g., CAPE) as per Peters et al. Here, this work is extended to a fully prognostic multicloud model. A nonlocal convergence-coupled formulation of the stochastic multicloud model is implemented without wind-dependent surface heat fluxes. In a series of idealized Walker cell simulations, this convergence coupling enhances the persistence of Kelvin wave analogs in dry regions of the domain while leaving the dynamics in moist regions largely unaltered. This effect is robust for changes in the amplitude of the imposed sea surface temperature (SST) gradient. In essence, this method provides a soft convergence coupling that allows for increased interaction between cumulus convection and the large-scale circulation but does not suffer from the deleterious wave–conditional instability of the second kind (CISK) behavior of the Kuo-type moisture-convergence closures.

scholarly journals A 20-Yr Average of the Indonesian Throughflow: Regional Currents and the Interbasin Exchange

2008 ◽  
Vol 38 (9) ◽  
pp. 1965-1978 ◽  
Author(s):  
Susan E. Wijffels ◽  
Gary Meyers ◽  
J. Stuart Godfrey
Keyword(s):  
Heat Fluxes ◽  
Ekman Transport ◽  
Indonesian Throughflow ◽  
South Equatorial Current ◽  
Near Surface ◽  
Surface Heat Fluxes ◽  
Interbasin Exchange ◽  
The Indian Ocean ◽  
Equatorial Current ◽  
Pass Through

Abstract Twenty years of monthly or more frequent repeat expendable bathythermograph data are used to estimate the mean geostrophic velocity and transport relative to 750 m of the Indonesian Throughflow (ITF) and its partitioning through the major outflow straits into the Indian Ocean. Ekman transports are estimated from satellite and atmospheric reanalysis wind climatologies. A subsurface maximum near 100 m characterizes the geostrophic ITF, but Ekman flows drive a warm near-surface component as well. A subsurface intensified fresh Makassar Jet feeds the Lombok Strait Throughflow (∼2 Sv; 1Sv ≡ 106 m3 s−1) and an eastward flow along the Nusa Tenggara island chain [the Nusa Tenggara Current (6 Sv)]. This flow feeds a relatively cold 3.0-Sv flow through the Ombai Strait and Savu Sea. About 4–5 Sv pass through Timor Passage, fed by both the Nusa Tenggara Current and likely warmer and saltier flow from the eastern Banda Sea. The Ombai and Timor Throughflow feature distinctly different shear profiles; Ombai has deep-reaching shear with a subsurface velocity maximum near 150 m and so is cold (∼15.5°–17.1°C), while Timor Passage has a surface intensified flow and is warm (∼21.6°–23°C). At the western end of Timor Passage the nascent South Equatorial Current is augmented by recirculation from a strong eastward shallow flow south of the passage. South of the western tip of Java are two mean eastward flows—the very shallow, warm, and fresh South Java Current and a cold salty South Java Undercurrent. These, along with the inflow of the Eastern Gyral Current, recirculate to augment the South Equatorial Current, and greatly increase its salinity compared to that at the outflow passages. The best estimate of the 20-yr-average geostrophic plus Ekman transport is 8.9 ± 1.7 Sv with a transport-weighted temperature of 21.2°C and transport-weighted salinity of 34.73 near 110°E. The warm temperatures of the flow can be reconciled with the much cooler estimates based on mooring data in Makassar Strait by accounting for an unmeasured barotropic and deep component, and local surface heat fluxes that warm the ITF by 2°–4°C during its passage through the region.

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