Meridional Circulation Cells Research Articles

Meridional flow profile measurements with SOHO/MDI

AbstractWe present meridional flow measurements of the Sun using a novel helioseismic approach for analyzing SOHO/MDI data in order to push the current limits in radial depth. Analyzing three consecutive months of data during solar minimum, we find that the meridional flow is as expected poleward in the upper convection zone, turns equatorward at a depth of around 40 Mm (∼ 0.95 R⊙), and possibly changes direction again in the lower convection zone. This may indicate two meridional circulation cells in each hemisphere, one beneath the other. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Global Ocean Meridional Overturning

Abstract A decade-mean global ocean circulation is estimated using inverse techniques, incorporating air–sea fluxes of heat and freshwater, recent hydrographic sections, and direct current measurements. This information is used to determine mass, heat, freshwater, and other chemical transports, and to constrain boundary currents and dense overflows. The 18 boxes defined by these sections are divided into 45 isopycnal (neutral density) layers. Diapycnal transfers within the boxes are allowed, representing advective fluxes and mixing processes. Air–sea fluxes at the surface produce transfers between outcropping layers. The model obtains a global overturning circulation consistent with the various observations, revealing two global-scale meridional circulation cells: an upper cell, with sinking in the Arctic and subarctic regions and upwelling in the Southern Ocean, and a lower cell, with sinking around the Antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges.

Origin and transport of tropical cirrus clouds observed over Paramaribo, Suriname (5.8°N, 55.2°W)

The Intertropical Convergence Zone (ITCZ) passes twice a year over tropical Suriname, bringing two wet and two dry seasons. During a pilot study campaign in Suriname, cirrus clouds were observed with a mobile aerosol Raman lidar (MARL) and with balloon sondes containing a frost point hygrometer called Snow White, over the period October–November 2004. These observations are used to study the origin of cirrus clouds and the dynamical processes that determine their transport, using European Centre for Medium‐Range Weather Forecasts (ECMWF) operational analyses. The height of cirrus occurrence is in phase with the height of the cold point tropopause, with maximum heights occurring during Northern Hemisphere winter that are about 2 km above the minimum values in summertime. The occurrence of cirrus often corresponds with a northerly meridional wind flow (in a layer underneath the tropopause), also when the ITCZ lies to the south in the period January–May. ECMWF analyses point out that inertial instability flow, in the form of vertically stacked meridional circulation cells in the upper troposphere (UT), can explain the transport of these cirrus events. Also evident is that radiative cooling of a moist layer transported in the UT leads to a thermal wind in the form of an easterly/westerly jet associated with the southward/northward transport of moist air. An interactive play between the inertial instability and thermal wind mechanisms explains many of the observed features of cirrus occurrence over Suriname. The observed cirrus mostly originates from the ITCZ or from deep convective centers to the south that form during the early summer monsoon.

Vertical profiles of HCN, HC3N, and C2H2 in Titan's atmosphere derived from Cassini/CIRS data

Mid-infrared limb spectra in the range 600–1400 cm−1 taken with the Composite InfraRed Spectrometer (CIRS) on-board the Cassini spacecraft were used to determine vertical profiles of HCN, HC3N, C2H2, and temperature in Titan's atmosphere. Both high (0.5 cm−1) and low (13.5 cm−1) spectral resolution data were used. The 0.5 cm−1 data gave profiles at four latitudes and the 13.5 cm−1 data gave almost complete latitudinal coverage of the atmosphere. Both datasets were found to be consistent with each other. High temperatures in the upper stratosphere and mesosphere were observed at Titan's northern winter pole and were attributed to adiabatic heating in the subsiding branch of a meridional circulation cell. On the other hand, the lower stratosphere was much colder in the north than at the equator, which can be explained by the lack of solar radiation and increased IR emission from volatile enriched air. HC3N had a vertical profile consistent with previous ground based observations at southern and equatorial latitudes, but was massively enriched near the north pole. This can also be explained in terms of subsidence at the winter pole. A boundary observed at 60° N between enriched and un-enriched air is consistent with a confining polar vortex at 60° N and HC3N's short lifetime. In the far north, layers were observed in the HC3N profile that were reminiscent of haze layers observed by Cassini's imaging cameras. HCN was also enriched over the north pole, which gives further evidence for subsidence. However, the atmospheric cross section obtained from 13.5 cm−1 data indicated a HCN enriched layer at 200–250 km, extending into the southern hemisphere. This could be interpreted as advection of polar enriched air towards the south by a meridional circulation cell. This is observed for HCN but not for HC3N due to HCN's longer photochemical lifetime. C2H2 appears to have a uniform abundance with altitude and is not significantly enriched in the north. This is consistent with observations from previous CIRS analysis that show increased abundances of nitriles and hydrocarbons but not C2H2 towards the north pole.

Atmospheric forcing of the eastern tropical Pacific: A review

The increase in marine, land surface, atmospheric and satellite data during recent decades has led to an improved understanding of the air–sea interaction processes in the eastern tropical Pacific. This is also thanks to extensive diagnoses from conceptual and coupled ocean–atmosphere numerical models. In this paper, mean fields of atmospheric variables, such as incoming solar radiation, sea level pressure, winds, wind stress curl, precipitation, evaporation, and surface energy fluxes, are derived from global atmospheric data sets in order to examine the dominant features of the low level atmospheric circulations of the region. The seasonal march of the atmospheric circulations is presented to depict the role of radiative forcing on atmospheric perturbations, especially those dominating the atmosphere at low levels. In the tropics, the trade winds constitute an important north–south energy and moisture exchange mechanism (as part of the low level branch of the Hadley circulation), that determines to a large extent the precipitation distribution in the region, i.e., that associated with the Inter-Tropical Convergence Zone (ITCZ). Monsoonal circulations also play an important role in determining the warm season precipitation distribution over the eastern tropical Pacific through a large variety of air–sea–land interaction mechanisms. Westward traveling waves, tropical cyclones, low latitude cold air intrusions, and other synoptic and mesoscale perturbations associated with the ITCZ are also important elements that modulate the annual rainfall cycle. The low-level jets of the Gulf of California, the Intra-Americas Sea (Gulf of Mexico and Caribbean Sea) and Chocó, Colombia are prominent features of the eastern tropical Pacific low-level circulations related to sub-regional and regional scale precipitation patterns. Observations show that the Intra-Americas Low-Level Jet intensity varies with El Niño/Southern Oscillation (ENSO) phases, however its origin and role in the westward propagation and development of disturbances that may hit the eastern tropical Pacific, such as easterly waves and tropical cyclones, are still unclear. Changes in the intensity of the trade winds in the Caribbean Sea and the Gulf of Mexico (associated with eastern tropical Pacific wind jets) exert an important control on precipitation by means of wind–topography interactions. Gaps in the mountains of southern Mexico and Central America allow strong wind jets to pass over the continent imprinting a unique signal in sea surface temperatures and ocean dynamics of the eastern tropical Pacific. The warm pools of the Americas constitute an important source of moisture for the North American Monsoon System. The northeastern tropical Pacific is a region of intense cyclogenetic activity, just west of the coast of Mesoamerica. Over the oceanic regions, large-scale properties of key variables such as precipitation, moisture, surface energy fluxes and wind stress curl are still uncertain, which inhibits a more comprehensive view of the region and stresses the importance of regional field experiments. Progress has been substantial in the understanding of the ocean and atmospheric dynamics of the eastern tropical Pacific, however, recent observational evidence such as that of a shallow meridional circulation cell in that region, in contrast to the classic concept of the Hadley-type deep meridional circulation, suggests that more in situ observations to validate theories are still necessary. This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific Ocean.

Southern meridional atmospheric circulation associated with IOD

Using the monthly wind and sea surface temperature (SST) data, southern meridional atmospheric circulation cells associated with the Indian Ocean Dipole Mode (IOD) events in the Indian Ocean are for the first time described and examined. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. During the four different phases of the positive IOD events, the anomalous meridional Hadley circulation over the western Indian Ocean shows that the air rises in the tropics, flows poleward in the upper troposphere, sinks in the subtropics, and returns back to the tropics in the lower troposphere. The anomalous Hadley circulation over the eastern Indian Ocean is opposite to that over the western Indian Ocean. During positive IOD events, the meridional Hadley circulation over the eastern Indian Ocean is weakened while it is strengthened over the western Indian Ocean. Correlation analysis between the IOD index and the indices of the Hadley cells also proves that, the atmospheric circulation patterns are evident in every IOD event over the period of record.

A flux‐transport dynamo with a multi‐cell meridional circulation

AbstractWe discuss the effect of a non‐trivial meridional circulation pattern on a flux‐transport type of solar dynamo. The critical value of the turbulent helicity and the periods are calculated as a function of the meridional flow strength. We found that the dynamo mechanism is mainly determined by the global topology of the meridional flow. In particular the equatorwards migration in the butterfly diagram can be easily obtained by the combined action of two cells of meridional circulation. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Torsional Oscillation, Meridional Flows, and Vorticity Inferred in the Upper Convection Zone of the Sun by Time‐Distance Helioseismology

By applying time-distance helioseismology measurements and inversions to Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) dynamics campaign data, we obtain synoptic maps of subsurface plasma-flow fields at a depth of 0-12 Mm for seven solar Carrington rotations, covering the years 1996-2002, from solar-activity minimum to maximum. Vorticity distribution and both zonal and meridional flows are derived from such synoptic flow maps, which contain an enormous amount of information about solar dynamics. The results for the zonal flows agree well with previous results. The meridional flows of an order of 20 m s-1 are found to remain poleward during the whole period of observations. In addition to the poleward meridional flows observed at the solar minimum, extra meridional circulation cells of flow converging toward the activity belts are found in both hemispheres, which may imply plasma downdrafts in the activity belts. These converging flow cells migrate toward the solar equator together with the activity belts as the solar cycle evolves. The vorticity distributions are largely linear with latitude, and the deviations from the vorticity caused by the mean differential rotation are presented. Patterns of large-scale flows are investigated for a large active region at different depths. Converging flows toward the center of the active region are found near the solar surface, and divergent flows in this large active region are found to be rooted much deeper than similar flows observed in individual sunspots. We conclude that the extremely rich and complicated dynamics of the upper convection zone reveal remarkable organization on the large scale, which can be correlated with the magnetic activity zones.

Shallow Meridional Circulation in the Tropical Eastern Pacific*

Abstract Based on four independent datasets, this study presents observational evidence of a shallow meridional circulation cell in the eastern tropical Pacific. In this shallow meridional circulation cell the northerly cross-equatorial return flow from the ITCZ into the Southern Hemisphere is found immediately above the atmospheric boundary layer, in contrast to the classic concept of the Hadley-type deep meridional circulation whose northerly return flow resides in the upper troposphere. The strength and depth of this shallow meridional circulation undergo a distinct annual cycle. Possible causes and climatic implications of this shallow meridional circulation are discussed.

On the Coupling between Helium Settling and Rotation‐induced Mixing in Stellar Radiative Zones. III. Applications to Light Elements in Population I Main‐Sequence Stars

In the two previous papers of this series, we have discussed the importance of the μ-gradients due to helium settling in rotation-induced mixing, first in an approximate analytical way, second in a two-dimensional numerical simulation. We have found that, for slowly rotating low-mass stars, a process of paralysis, in which the circulation and the diffusion are nearly frozen, may take place below the convective zone. Here we apply this theory to the case of and beryllium in Galactic clusters, especially the Hyades. We take into account the rotational braking with rotation velocities adjusted to the present observations. We find that two different cells of meridional circulation appear on the hot side of the lithium dip and that the creeping paralysis process occurs, not directly below the convective zone, but deeper inside the radiative zone, at the top of the second cell. As a consequence, the two cells are disconnected, which may be the basic reason for the increase with effective temperature on this side of the dip. On the cool side, there is just one cell of circulation, and the paralysis has not yet set in at the age of the Hyades; the same modeling accounts nicely for the beryllium observations as well as for the ones.

Spatial/temporal patterns in Weddell gyre characteristics and their relationship to global climate

We examine the spatiotemporal variability of the upper ocean sea ice system of the Atlantic sector of the Southern Ocean subpolar seas (Weddell gyre) and the nature of its covariability with extrapolar climate. To systematically evaluate the sporadic and sparse distribution of subpolar data, we employed an optimal analysis involving empirical orthogonal functions (EOFs). The EOFs reveal that the pattern of spatial covariability of Weddell gyre characteristics is dominated by high interannual variability near the northern (circumpolar) rim of the gyre and lesser variability (10–20% of the variance) in the gyre's central core region. We find considerable, statistically significant teleconnections between subpolar and extrapolar climate. The dominant link is with El Niño–Southern Oscillation. Our analysis of the gyre characteristics suggest that during El Niño, the Atlantic subpolar (Weddell) gyre spins up while the eastern Pacific subpolar gyre spins down; the opposite occurs for La Niña. This regional subpolar response is consistent with recent GCM modeling analyses. These show that equatorial warm anomalies drive changes in the meridional and zonal atmospheric circulation cells. This results in opposite shifts of the subpolar jets in the Pacific and Atlantic oceans that lead to a change in the interaction (especially in cyclones) between the subtropical and polar front jets in those basins, and ultimately in the cyclonic forcing of the subpolar gyres. We also find that the Weddell gyre interior upper ocean characteristics covary positively with sea ice extent in the Atlantic region and negatively with the sea ice extent in the Amundsen/Bellingshausen regions, reflecting a strong Antarctic dipole.

Seasonal Variations of Tropospheric Wind over Indonesia: Comparison between Collected Operational Rawinsonde Data and NCEP Reanalysis for 1992-99

Seasonal variations of wind in the troposphere are analyzed based on operational rawinsondes at 11 stations over Indonesia for 1992-1999 and the results are compared with NCEP reanalysis. For meridional wind, an annual oscillation caused by north-south shift of a meridional (Hadley-type) circulation is clearly found. Winter-poleward flow of the upper-tropospheric meridional circulation is stronger than summer-poleward flow. The winter (southern) hemispheric cell in northern summer has a larger invasion across the equator (as suggested in the zonal-mean Hadley cell), and weaker meridional flows than the winter (northern) hemispheric cell in northern winter. Along the boundary of twin meridional circulation cells, a zonal (Walker-type) circulation seems to exist, and has easterly (westerly) wind in the upper (lower) troposphere in the Indonesian region. The zonal circulation is also shifted from the equator associated with the meridional circulation cell boundary. However, a semiannual oscillation, which has easterly maxima in January-February and July-August is more clearly observed in the upper-tropospheric zonal wind, which is consistent with a consideration on conservation of absolute angular momentum for the upper-tropospheric winter-poleward (equatorward) air mass transport associated with the winter-hemispheric meridional circulation cell, invading the summer hemisphere across the equator. In comparison with the seasonal-vertical variations based on the operational rawinsonde data, the NCEP reanalysis seems to overestimate the upper-tropospheric northerly and easterly maxima during northern summer. In other words, the NCEP reanalysis produces a winter hemispheric side meridional circulation cell with similar intensities between both solsticial seasons, in spite that in the operational rawinsonde data the winter hemispheric side meridional circulation cell in northern summer is weaker than that in northern winter. The overestimation of easterly may also be related with an insufficient reproduction of the winter-hemispheric meridional circulation cell-such as too large northward invasion.

Evolving Submerged Meridional Circulation Cells within the Upper Convection Zone Revealed by Ring‐Diagram Analysis

Using the local helioseismic technique of ring-diagram analysis applied to Michelson Doppler Imager (MDI) Dynamics Program data from the Solar and Heliospheric Observatory, we have discovered that the meridional flow within the upper convection zone can develop additional circulation cells whose boundaries wander in latitude and depth as the solar cycle progresses. We report on the large-scale meridional and zonal flows that we observe from 1996 to 2001. In particular, we discuss the appearance and evolution of a submerged meridional cell during the years 1998-2001, which arose in the northern hemisphere and disrupted the orderly poleward flow and symmetry about the equator that is typically observed. The meridional flows in the southern and northern hemispheres exhibit striking asymmetry during the past four years of the advancing solar cycle. Such asymmetry and additional circulation cells should have profound impact on the transport of angular momentum and magnetic field within the surface layers. These flows may have a significant role in the establishment and maintenance of the near-surface rotational shear layer.

Atmospheric Circulation Cells Associated with the El Niño–Southern Oscillation

Abstract Atmospheric circulation cells associated with the El Nino–Southern Oscillation (ENSO) are described and examined using the NCEP–NCAR reanalysis field and the NCEP sea surface temperature (SST) from January 1950 to December 1999. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. The warm phase of ENSO shows positive SST anomalies in the equatorial eastern Pacific and along the east coast of Asia and the west coast of North America, and negative SST anomalies in the off-equatorial western Pacific and in the central North Pacific. Associated with this SST anomaly distribution are variations of atmospheric zonal and meridional circulation cells over the Pacific. The equatorial zonal Walker circulation cell is weakened, consistent with previous schematic diagrams. The anomalous meridional Hadley circulation cell in the eastern Pacific shows the air rising in the Tropics, flowing poleward in the upper troposphere, sinking in the subt...

Impact of the Indian Ocean dipole on the relationship between the Indian monsoon rainfall and ENSO

The influence of the recently discovered Indian Ocean Dipole (IOD) on the interannual variability of the Indian summer monsoon rainfall (ISMR) has been investigated for the period 1958‐1997. The IOD and the El Niño/Southern Oscillation (ENSO) have complementarily affected the ISMR during the last four decades. Whenever the ENSO‐ISMR correlation is low (high), the IOD‐ISMR correlation is high (low). The IOD plays an important role as a modulator of the Indian monsoon rainfall, and influences the correlation between the ISMR and ENSO. We have discovered that the ENSO‐induced anomalous circulation over the Indian region is either countered or supported by the IOD‐induced anomalous meridional circulation cell, depending upon the phase and amplitude of the two major tropical phenomena in the Indo‐Pacific sector.

The Signature of the Annular Modes in the Moisture Budget

Abstract The mid- and high-latitude variations of mass and momentum, variously termed the Arctic and Antarctic oscillations and/or the Northern and Southern Annular Modes also have a signature in the moisture budget. This is investigated, using associated regression pattern analysis, in the control simulation of the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled climate model and in NCEP reanalysis data. The analyzed behavior is similar in the two hemispheres and is characterized, when pressure is low in polar regions, by anomalous westerlies and a thermodynamically indirect meridional cell poleward of midlatitudes, and by anomalous easterlies and a thermodynamically direct meridional cell equatorward of midlatitudes. Moisture transports are associated primarily with advection by the lower branches of these anomalous meridional circulation cells and secondarily with anomalous eddy transports giving, in this phase of the mode, moisture divergence and decreased precipitation at midlatitu...

Preliminary result of Central Equatorial Pacific Ocean Tomography Experiment

JAMSTEC (Japan Marine Science and Technology Center) and APL (Applied Physics Laboratory, University of Washington) started a joint program to directly measure part of the circulation cell of the Central Equatorial Pacific Ocean using acoustic tomography. The tropical and subtropical Pacific Ocean are suggested to connect via a shallow meridional circulation cell by Gu and Philander (1997). This subtropical cell (STC) has been hypothesized to the El Nino-Southern Oscillation phenomena in the tropics, which then, in turn, can affect the subtropical ocean via rapid atmospheric forcing. But no direct measurements exist and indirect measurements and modeling support the hypothesis. The observational centerpiece is tomography systems located in the Central Equatorial Pacific Region. JAMSTEC began the field work by deploying five acoustic transceivers in the observational region and this work will continue until January 2001 (two-year plan). Measurement data were sent to JAMSTEC in realtime using satellite telemetry system. These data were analyzed and compared with CTD observations. The time series fluctuation of the temperature was solved by this analysis.

On the crossing of the equator by intermediate water masses in the western Atlantic ocean: Identification and pathways of Antarctic Intermediate Water and Upper Circumpolar Water

The flow of intermediate water masses across the equator in the Atlantic Ocean is of fundamental interest in the context of the global meridional circulation cell associated with the formation of the North Atlantic Deep Water. This paper describes the flow and pathways of the Antarctic Intermediate Water (AAIW) and the Upper Circumpolar Water (UCPW) at between 500‐ and 1200‐m depths in the western equatorial Atlantic (5°S–7°30′N). These have been deduced from hydrological and geochemical tracer (nutrients and chlorofluorocarbons) data sets from CITHER 1 (R/V L'Atalante, January‐March 1993), ETAMBOT 1 (R/V Le Noroit, September‐October 1995), and ETAMBOT 2 (R/V Edwin Link, April‐May 1996) cruises. Both the AAIW and UCPW enter, on the isopycnals σθ = 27.25 (676±36 dbar) and σθ = 27.40 (919±35 dbar), respectively, the equatorial belt as narrow, northwestward flows around the northeast tip of Brazil near 5°S. During transit within this zone the core properties of UCPW erode more than those of AAIW. Flow patterns of both the water masses show westward spreading and eastward recirculations on either side of the equator. Temporal variations in spreading and recirculation occur at both levels, but they are more pronounced at the AAIW level, in agreement with earlier observations in the upper layers. At the northern boundary of the equatorial belt (7°30'N) the AAIW flows along the western boundary while the UCPW, instead, recirculates into the interior of the ocean.

熱帯大西洋における大気海洋相互作用の10年スケール変動とブラジル北東部の降雨変動との関係

The moisture budget over the Atlantic Ocean and South America is evaluated for two contrasting 5-year periods, 1985-1989 (wet) and 1990-1994 (dry), of rainfall over Northeast Brazil (NEB) using NCEP/NCAR reanalysis data. The linear correlation coefficients between the 5-year averages of rainfall over NEB and sea surface temperature (SST) anomalies over the Atlantic Ocean show a dipole configuration over the Tropical Atlantic. It is found that evaporation is higher over the Tropical North Atlantic, and lower over the Tropical South Atlantic during the 5-year period of higher rainfall over NEB. Higher evaporation causes lower SST and vice versa. This seems to be the main mechanism which generates dipole configuration in the Atlantic SST. During the rainy season (February through May) in NEB moisture is transported towards NEB and converges, providing the necessary moisture source for the rainfall. This low level convergence is associated with the rising branch of a local meridional circulation cell, which in turn is generated by the SST dipole. This series of interlinked mechanisms seems to fit in a physical picture responsible for the rainfall decadal variation over NEB.

Meridional Circulation Cells and the Source Waters of the Pacific Equatorial Undercurrent

Abstract A 3½-layer model is used to study the meridional circulation cells that provide the source waters of the Pacific Equatorial Undercurrent (EUC). Its three active layers represent tropical, thermocline, and upper-intermediate waters, respectively, and across-interface flow between the layers parameterizes the processes of upwelling, subduction, and diapycnal mixing. Solutions are driven by climatological winds in a domain resembling the Pacific basin from 35°S to 55°N. An additional forcing mechanism is a specified inflow into layer 3 across the open southern boundary and a compensating outflow from layers 1 and 2 along the western boundary just north of the equator; the resulting circulation simulates the Pacific interocean circulation (IOC), in which intermediate water enters the South Pacific and the same amount of thermocline and tropical waters exit via the Indonesian Throughflow. Five meridional cells contribute to the EUC in the main-run solution: north and south Subtropical Cells (STCs), no...