The Role of Air-Sea Coupling on November-April Intraseasonal Rainfall Variability Over the South Pacific

Abstract We investigate the impact of resolving air-sea interaction on the simulation of the intraseasonal rainfall variability over the South Pacific using the ECHAM5 atmospheric general circulation model coupled with the Snow-Ice-Thermocline (SIT) ocean model. We compare the fully coupled simulation with two uncoupled ECHAM5 simulations, one forced with sea surface temperature (SST) climatology and one forced with daily SST from the coupled model. The intraseasonal rainfall variability over the South Pacific is reduced by 17% in the uncoupled model forced with SST climatology and increased by 8% in the uncoupled simulation forced with daily SST, suggesting the role of air-sea coupling and SST variability. The coupled model best simulates the key characteristics of two intraseasonal rainfall modes over the South Pacific with reasonable propagation and correct periodicity. The spatial structure of the two rainfall modes in all three simulations is very similar, suggesting these modes are primarily generated by the dynamics of the atmosphere. The southeastward propagation of rainfall anomalies associated with two leading rainfall modes in the South Pacific depends upon the eastward propagating MJO signals over the Indian Ocean and western Pacific. Air-sea interaction seems crucial for such propagation as both eastward and southeastward propagations are substantially reduced in the uncoupled model forced with SST climatology. The simulation of both eastward and southeastward propagations improved considerably in the uncoupled model forced with daily SST; however, the periodicity differs from the coupled model. Such discrepancy in the periodicity is attributed to the changes in the SST-rainfall relationship with weaker correlations and the nearly in-phase relationship.

Download Full-text

The role of air-sea coupling on the simulation of intraseasonal rainfall variability over the South Pacific in ECHAM5-SIT

10.5194/egusphere-egu21-12770 ◽

2021 ◽

Author(s):

Sunil Kumar Pariyar ◽

Noel Keenlyside ◽

Wan-Ling Tseng

Keyword(s):

General Circulation ◽

Rainfall Variability ◽

Intraseasonal Variability ◽

Phase Relationship ◽

Coupled Model ◽

South Pacific ◽

South Pacific Convergence Zone ◽

The South ◽

The Impact

We investigate the impact of air-sea coupling on the simulation of the intraseasonal variability of rainfall over the South Pacific using the ECHAM5 atmospheric general circulation model coupled with Snow-Ice-Thermocline (SIT) ocean model. We compare the fully coupled simulation with two uncoupled simulations forced with sea surface temperature (SST) climatology and daily SST from the coupled model. The intraseasonal rainfall variability over the South Pacific Convergence Zone (SPCZ) is reduced by 17% in the uncoupled model forced with SST climatology and increased by 8% in the uncoupled simulation forced with daily SST. The coupled model best simulates the key characteristics of the two intraseasonal rainfall modes of variability in the South Pacific, as identified by an Empirical Orthogonal Function (EOF) analysis. The spatial structure of the two EOF modes in all three simulations is very similar, suggesting these modes are independent of air-sea coupling and primarily generated by the dynamics of the atmosphere. The southeastward propagation of rainfall anomalies associated with two leading rainfall modes in the South Pacific depends upon the eastward propagating Madden-Julian Oscillation (MJO) signals over the Indian Ocean and western Pacific. Air-sea interaction seems crucial for such propagation as both eastward and southeastward propagations substantially reduced in the uncoupled model forced with SST climatology. Prescribing daily SST from the coupled model improves the simulation of both eastward and southeastward propagations in the uncoupled model forced with daily SST, showing the role of SST variability on the propagation of the intraseasonal variability, but the periodicity differs from the coupled model. The change in the periodicity is attributed to a weaker SST-rainfall relationship that shifts from SST leading rainfall to a nearly in-phase relationship in the uncoupled model forced with daily SST.

Download Full-text

Decadal Sea Level Variability in the South Pacific in a Global Eddy-Resolving Ocean Model Hindcast

Journal of Physical Oceanography ◽

10.1175/2007jpo3915.1 ◽

2008 ◽

Vol 38 (8) ◽

pp. 1731-1747 ◽

Cited By ~ 46

Author(s):

Yoshi N. Sasaki ◽

Shoshiro Minobe ◽

Niklas Schneider ◽

Takashi Kagimoto ◽

Masami Nonaka ◽

...

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

General Circulation ◽

Decadal Variability ◽

Southern Oscillation ◽

Circulation Model ◽

South Pacific ◽

The South ◽

Sea Level Variability ◽

The Impact

Abstract Sea level variability and related oceanic changes in the South Pacific from 1970 to 2003 are investigated using a hindcast simulation of an eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES), along with sea level data from tide gauges since 1970 and a satellite altimeter since 1992. The first empirical orthogonal function mode of sea level anomalies (SLAs) of OFES exhibits broad positive SLAs over the central and western South Pacific. The corresponding principal component indicates roughly stable high, low, and high SLAs, separated by a rapid sea level fall in the late 1970s and sea level rise in the late 1990s, consistent with tide gauge and satellite observations. These decadal changes are accompanied by circulation changes of the subtropical gyre at 1000-m depth, and changes of upper-ocean zonal current and eddy activity around the Tasman Front. In general agreement with previous related studies, it is found that sea level variations in the Tasman Sea can be explained by propagation of long baroclinic Rossby waves forced by wind stress curl anomalies, if the impact of New Zealand is taken into account. The corresponding atmospheric variations are associated with decadal variability of El Niño–Southern Oscillation (ENSO). Thus, decadal sea level variability in the western and central South Pacific in the past three and half decades and decadal ENSO variability are likely to be connected. The sea level rise in the 1990s, which attracted much attention in relation to the global warming, is likely associated with the decadal cooling in the tropical Pacific.

Download Full-text

The Effect of a Well-Resolved Stratosphere on Surface Climate: Differences between CMIP5 Simulations with High and Low Top Versions of the Met Office Climate Model

Journal of Climate ◽

10.1175/jcli-d-11-00579.1 ◽

2012 ◽

Vol 25 (20) ◽

pp. 7083-7099 ◽

Cited By ~ 44

Author(s):

S. C. Hardiman ◽

N. Butchart ◽

T. J. Hinton ◽

S. M. Osprey ◽

L. J. Gray

Keyword(s):

General Circulation ◽

Climate Model ◽

Southern Oscillation ◽

Coupled Model ◽

Circulation Model ◽

Climate Simulation ◽

Reanalysis Dataset ◽

Quasi Biennial Oscillation ◽

Surface Climate ◽

The Impact

Abstract The importance of using a general circulation model that includes a well-resolved stratosphere for climate simulations, and particularly the influence this has on surface climate, is investigated. High top model simulations are run with the Met Office Unified Model for the Coupled Model Intercomparison Project Phase 5 (CMIP5). These simulations are compared to equivalent simulations run using a low top model differing only in vertical extent and vertical resolution above 15 km. The period 1960–2002 is analyzed and compared to observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. Long-term climatology, variability, and trends in surface temperature and sea ice, along with the variability of the annular mode index, are found to be insensitive to the addition of a well-resolved stratosphere. The inclusion of a well-resolved stratosphere, however, does improve the impact of atmospheric teleconnections on surface climate, in particular the response to El Niño–Southern Oscillation, the quasi-biennial oscillation, and midwinter stratospheric sudden warmings (i.e., zonal mean wind reversals in the middle stratosphere). Thus, including a well-represented stratosphere could improve climate simulation on intraseasonal to interannual time scales.

Download Full-text

Origin and Pathway of Equatorial 13°C Water in the Pacific Identified by a Simulated Passive Tracer and Its Adjoint*

Journal of Physical Oceanography ◽

10.1175/2009jpo4045.1 ◽

2009 ◽

Vol 39 (8) ◽

pp. 1836-1853 ◽

Cited By ~ 32

Author(s):

Tangdong Qu ◽

Shan Gao ◽

Ichiro Fukumori ◽

Rana A. Fine ◽

Eric J. Lindstrom

Keyword(s):

General Circulation ◽

Circulation Model ◽

South Pacific ◽

Equatorial Region ◽

Equatorial Pacific ◽

Western Boundary ◽

Passive Tracer ◽

The South ◽

Mode Water ◽

Eastern Equatorial Pacific

Abstract The origin and pathway of the thermostad water in the eastern equatorial Pacific Ocean, often referred to as the equatorial 13°C Water, are investigated using a simulated passive tracer and its adjoint, based on circulation estimates of a global general circulation model. Results demonstrate that the source region of the 13°C Water lies well outside the tropics. In the South Pacific, some 13°C Water is formed northeast of New Zealand, confirming an earlier hypothesis on the water’s origin. The South Pacific origin of the 13°C Water is also related to the formation of the Eastern Subtropical Mode Water (ESTMW) and the Sub-Antarctic Mode Water (SAMW). The portion of the ESTMW and SAMW that eventually enters the density range of the 13°C Water (25.8 < σθ < 26.6 kg m−3) does so largely by mixing. Water formed in the subtropics enters the equatorial region predominantly through the western boundary, while its interior transport is relatively small. The fresher North Pacific ESTMW and Central Mode Water (CMW) are also important sources of the 13°C Water. The ratio of the southern versus the northern origins of the water mass is about 2 to 1 and tends to increase with time elapsed from its origin. Of the total volume of initially tracer-tagged water in the eastern equatorial Pacific, approximately 47.5% originates from depths above σθ = 25.8 kg m−3 and 34.6% from depths below σθ = 26.6 kg m−3, indicative of a dramatic impact of mixing on the route of subtropical water to becoming the 13°C Water. Still only a small portion of the water formed in the subtropics reaches the equatorial region, because most of the water is trapped and recirculates in the subtropical gyre.

Download Full-text

Orographic Influences on Subtropical Stratocumulus

Journal of the Atmospheric Sciences ◽

10.1175/jas3756.1 ◽

2006 ◽

Vol 63 (10) ◽

pp. 2585-2601 ◽

Cited By ~ 34

Author(s):

I. Richter ◽

C. R. Mechoso

Keyword(s):

General Circulation ◽

Atmospheric General Circulation Model ◽

Circulation Model ◽

South American ◽

Instability Mechanism ◽

Peruvian Coast ◽

Westerly Winds ◽

Stratocumulus Clouds ◽

The Impact

Abstract The impact of South American orography on subtropical stratocumulus clouds off the Peruvian coast is investigated in the context of an atmospheric general circulation model. It is found that stratocumulus incidence is significantly reduced when South American orography is removed. Key to this behavior is a decrease in lower tropospheric stability (LTS) that allows for more frequent stratocumulus destruction through the model’s cloud-top entrainment instability mechanism. The role of orography in enhancing Peruvian stratocumulus is as follows. Within the PBL, orography deflects the midlatitude westerly winds equatorward in association with cold air advection and blocking of the low-level flow from the continent. Above the PBL, the steep and high South American orography deflects a significant portion of the midlatitude westerlies equatorward. This flow sinks along the equatorward sloping isentropes, thus promoting subsidence. Both processes increase LTS over the stratocumulus region. In further AGCM experiments, the sensitivity of Peruvian stratocumulus to the use of unsmoothed orographic boundary conditions is assessed. The results show no significant differences to the control simulation, which uses smoothed orography. This suggests that, in the context of GCMs, a representation of South American orography more detailed than is generally used has little potential for improving the performance of coupled ocean–atmosphere models in the eastern tropical Pacific.

Download Full-text

The Role of Radiative Interactions in Tropical Cyclone Development under Realistic Boundary Conditions

Journal of Climate ◽

10.1175/jcli-d-20-0574.1 ◽

2020 ◽

pp. 1-38

Author(s):

Bosong Zhang ◽

Brian J. Soden ◽

Gabriel A. Vecchi ◽

Wenchang Yang

Keyword(s):

Boundary Conditions ◽

General Circulation ◽

Circulation Model ◽

Sea Surface ◽

Latent Heat Release ◽

Synoptic Scale ◽

Spatially Varying ◽

The Impact ◽

Realistic Boundary Conditions

AbstractThe impact of radiative interactions on tropical cyclones (TC) climatology is investigated using a global, TC-permitting general circulation model (GCM) with realistic boundary conditions. In this model, synoptic-scale radiative interactions are suppressed by overwriting the model-generated atmospheric radiative cooling rates with its monthly-varying climatological values. When radiative interactions are suppressed, the global TC frequency is significantly reduced, indicating that radiative interactions are a critical component of TC development even in the presence of spatially varying boundary conditions. The reduced TC activity is primarily due to a decrease in the frequency of pre-TC synoptic disturbances (“seeds”), whereas the likelihood that the seeds undergo cyclogenesis is less affected. When radiative interactions are suppressed, TC genesis shifts toward coastal regions, whereas TC lysis locations stay almost unchanged; together the distance between genesis and lysis is shortened, reducing TC duration. In a warmer climate, the magnitude of TC reduction from suppressing radiative interactions is diminished due to the larger contribution from latent heat release with increased sea surface temperatures. These results highlight the importance of radiative interactions in modulating the frequency and duration of TCs.

Download Full-text

Influence of Surface Topography on the Critical Carbon Dioxide Level Required for the Formation of a Modern Snowball Earth

Journal of Climate ◽

10.1175/jcli-d-17-0821.1 ◽

2018 ◽

Vol 31 (20) ◽

pp. 8463-8479 ◽

Cited By ~ 2

Author(s):

Yonggang Liu ◽

W. Richard Peltier ◽

Jun Yang ◽

Yongyun Hu

Keyword(s):

Sea Ice ◽

General Circulation Model ◽

Land Surface ◽

General Circulation ◽

Coupled Model ◽

Circulation Model ◽

Ocean General Circulation Model ◽

Ocean Model ◽

Snowball Earth ◽

Ice Dynamics

The influence of continental topography on the initiation of a global glaciation (i.e., snowball Earth) is studied with both a fully coupled atmosphere–ocean general circulation model (AOGCM), CCSM3, and an atmospheric general circulation model (AGCM), CAM3 coupled to a slab ocean model. It is found that when the climate is very cold, snow cover over the central region of the Eurasian continent decreases when the atmospheric CO2 concentration ( pCO2) is reduced. In the coupled model, this constitutes a negative feedback due to the reduction of land surface albedo that counteracts the positive feedback due to sea ice expansion toward the equator. When the solar insolation is 94% of the present-day value, Earth enters a snowball state when pCO2 is ~35 ppmv. On the other hand, if the continents are assumed to be flat topographically (with the global mean elevation as in the more realistic present-day case), Earth enters a snowball state more easily at pCO2 = ~60 ppmv. Therefore, the presence of topography may increase the stability of Earth against descent into a snowball state. On the contrary, a snowball Earth is found to form much more easily when complex topography is present than when it is not in CAM3. This happens despite the fact that the mid- to high-latitude climate is much warmer (by ~10°C) when topography is present than when it is not. Analyses show that neglecting sea ice dynamics in this model prevents the warming anomaly in the mid- to high latitudes from being efficiently transmitted into the tropics.

Download Full-text

A nested Atlantic-Mediterranean Sea general circulation model for operational forecasting

Ocean Science ◽

10.5194/os-5-461-2009 ◽

2009 ◽

Vol 5 (4) ◽

pp. 461-473 ◽

Cited By ~ 124

Author(s):

P. Oddo ◽

M. Adani ◽

N. Pinardi ◽

C. Fratianni ◽

M. Tonani ◽

...

Keyword(s):

Mediterranean Sea ◽

General Circulation Model ◽

Ocean Circulation ◽

General Circulation ◽

Circulation Model ◽

Ocean Model ◽

Global Ocean ◽

Lateral Boundary ◽

The Mediterranean ◽

The Impact

Abstract. A new numerical general circulation ocean model for the Mediterranean Sea has been implemented nested within an Atlantic general circulation model within the framework of the Marine Environment and Security for the European Area project (MERSEA, Desaubies, 2006). A 4-year twin experiment was carried out from January 2004 to December 2007 with two different models to evaluate the impact on the Mediterranean Sea circulation of open lateral boundary conditions in the Atlantic Ocean. One model considers a closed lateral boundary in a large Atlantic box and the other is nested in the same box in a global ocean circulation model. Impact was observed comparing the two simulations with independent observations: ARGO for temperature and salinity profiles and tide gauges and along-track satellite observations for the sea surface height. The improvement in the nested Atlantic-Mediterranean model with respect to the closed one is particularly evident in the salinity characteristics of the Modified Atlantic Water and in the Mediterranean sea level seasonal variability.

Download Full-text

Role of Diurnal Cycle in the Maritime Continent Barrier Effect on MJO Propagation in an AGCM

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0112.1 ◽

2021 ◽

Vol 78 (5) ◽

pp. 1545-1565

Author(s):

R. S. Ajayamohan ◽

Boualem Khouider ◽

V. Praveen ◽

Andrew J. Majda

Keyword(s):

Diurnal Cycle ◽

General Circulation ◽

Dominant Role ◽

Circulation Model ◽

Striking Difference ◽

Barrier Effect ◽

Maritime Continent ◽

Stochastic Case ◽

The Impact

AbstractThe barrier effect of the Maritime Continent (MC) in stalling or modifying the propagation characteristics of the MJO is widely accepted. The strong diurnal cycle of convection over the MC is believed to play a dominant role in this regard. This hypothesis is studied here, with the help of a coarse-resolution atmospheric general circulation model (AGCM). The dry dynamical core of the AGCM is coupled to the multicloud parameterization piggybacked with a dynamical bulk boundary layer model. A set of sensitivity experiments is carried out by systematically varying the strength of the MC diurnal flux to assess the impact of the diurnal convective variability on the MJO propagation. The effects of deterministic and stochastic diurnal forcings on MJO characteristics are compared. It is found that the precipitation and zonal wind variance, on the intraseasonal time scales, over the western Pacific region decreases with the increase in diurnal forcing, indicating the blocking of MC precipitation. An increase in precipitation variance over the MC associated with the weakening of precipitation variance over the west Pacific is evident in all experiments. The striking difference between deterministic and stochastic diurnal forcing experiments is that the strength needed for the deterministic case to achieve the same degree of blocking is almost double that of stochastic case. The stochastic diurnal flux over the MC seems to be more detrimental in blocking the MJO propagation. This hints at the notion that the models with inadequate representation of organized convection tend to suffer from the MC-barrier effect.

Download Full-text

Relationships between South Atlantic SST Variability and Atmospheric Circulation over the South African Region during Austral Winter

Journal of Climate ◽

10.1175/jcli3474.1 ◽

2005 ◽

Vol 18 (16) ◽

pp. 3339-3355 ◽

Cited By ~ 42

Author(s):

C. J. C. Reason ◽

D. Jagadheesha

Keyword(s):

Interannual Variability ◽

General Circulation ◽

Large Scale ◽

Rainfall Variability ◽

Circulation Model ◽

South Atlantic ◽

The South ◽

Winter Rainfall ◽

Southwest Atlantic ◽

Sst Anomalies

Abstract The Southwestern Cape (SWC) region of South Africa is characterized by winter rainfall brought mainly via cold fronts and by substantial interannual variability. Previous work has found evidence that the interannual variability in SWC winter rainfall may be related to sea surface temperature (SST) in the South Atlantic Ocean and to large-scale ocean–atmosphere interaction in this region. During wet winters, SST tends to be anomalously warm (cool) in the southwest Atlantic and southeast Atlantic (central South Atlantic). Atmospheric general circulation model experiments with various idealized SST anomalies in the South Atlantic are used to explore mechanisms potentially associated with the rainfall variability. The model results suggest that the atmosphere is sensitive to subtropical–midlatitude SST anomalies in the South Atlantic during winter. Locally, there are changes to the jet position and strength, low-level relative vorticity, and convergence of moisture and latent heat flux that lead to changes in rainfall over the SWC. The model response to the SST forcing also shows large-scale anomalies in the midlatitude Southern Hemisphere circulation, namely, an Antarctic Oscillation–type mode and wavenumber-3 changes, similar to those observed during anomalous winters in the region.

Download Full-text