Meridional Mass Research Articles

Continental cold-air-outbreaks under the varying stratosphere-troposphere coupling regimes during stratospheric Northern Annular Mode events

A Stratospheric Northern Annular Mode (SNAM) phase-based composite analysis reveals that continental Cold Air Outbreaks (CAOs) can occur during both positive and negative SNAM events. CAOs tend to occur over Asia, characterized by a meridional-dipole surface temperature anomaly pattern (cold midlatitudes and warm high-latitudes) when the SNAM index is decreasing or the stratospheric polar vortex is weakening, but over North America and Europe with a meridionally-homogeneous pattern when the SNAM index is increasing or the stratospheric polar vortex is strengthening. While the decreasing SNAM is dominated by a stronger stratospheric poleward warm branch (WB-ST) of the isentropic meridional mass circulation and vice versa, the CAOs always follow a stronger tropospheric poleward warm branch (WB-TR) and an equatorward cold branch (CB) of the isentropic meridional mass circulation. The correspondence between the stronger/weaker WB-ST and stronger/weaker WB-TR&CB during majority of SNAM phases (referred to as stratosphere-troposphere coupling regimes) is responsible for the CAOs in Asia. During the remaining phases (stratosphere-troposphere decoupling regimes), in accompany with a weaker/stronger WB-ST, the WB-TR&CB are stronger/weaker and relates to the CAOs occurred in North America and Europe. The coupling regimes when the stratospheric polar vortex is weakening/strengthening are mainly attributed to the E-P flux convergence/divergence from the middle troposphere to the lower stratosphere, the larger wave amplitude throughout the column, and anomalous tropospheric wave flux mainly in the Asia in subpolar latitudes. The decoupling regimes, however, are mainly related to the anomalous westward-tilting of waves and the wave flux reflection toward the North America or Europe.

Sensitivity of ocean circulation to warming during the Early Eocene greenhouse

Multiple abrupt warming events ("hyperthermals") punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ13C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation's sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ13C and δ18O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ13C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ13C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ13C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ13C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.

A multivariate probabilistic framework for tracking the regional tropical edges: analysis of inter-annual variations and long-term trends

In the present study, a multivariate probabilistic framework is used to identify the meridional positions of regional tropical edges (RTEs), which are based on two variables: sea level pressure and precipitation minus evaporation. This new defined metric effectively captures inter-annual variability and long-term trend of the commonly adopted zonal mean tropical edge based on meridional mass stream function and near-surface winds. Besides, pronounced RTE trends are primarily located over the oceanic regions, and the terrestrial areas exhibit substantial inter-annual variability. These results are consistent among three modern reanalysis datasets. Moreover, the impacts of climate modes on RTE are investigated. The El Niño-Southern Oscillation, the Atlantic multi-decadal oscillation, and the Southern Annular Mode are important both on the inter-annual variations and long-term trends of RTE. The Pacific Decadal Oscillation is more inclined to affect long-term contribution rather than inter-annual relationship, and the Pacific–North American teleconnection, the North Atlantic Oscillation, and the Arctic oscillation highlight the inter-annual relationship with RTE in the specific regions, such as North Pacific, North Atlantic, and North Africa, respectively.

Effects of variable ice–ocean surface properties and air mass transformation on the Arctic radiative energy budget

Abstract. Low-level airborne observations of the Arctic surface radiative energy budget are discussed. We focus on the terrestrial part of the budget, quantified by the thermal-infrared net irradiance (TNI). The data were collected in cloudy and cloud-free conditions over and in the vicinity of the marginal sea ice zone (MIZ) close to Svalbard during two aircraft campaigns conducted in the spring of 2019 and in the early summer of 2017. The measurements, complemented by ground-based observations available from the literature and radiative transfer simulations, are used to evaluate the influence of surface type (sea ice, open ocean, MIZ), seasonal characteristics, and synoptically driven meridional air mass transports into and out of the Arctic on the near-surface TNI. The analysis reveals a typical four-mode structure of the frequency distribution of the TNI as a function of surface albedo, the sea ice fraction, and surface brightness temperature. Two modes prevail over sea ice and another two over open ocean, each representing cloud-free and cloudy radiative states. Characteristic shifts and modifications of the TNI modes during the transition from winter to spring and early summer conditions are discussed. Furthermore, the influence of warm air intrusions (WAIs) and marine cold-air outbreaks (MCAOs) on the near-surface downward thermal-infrared irradiances and the TNI is highlighted for several case studies. It is concluded that during WAIs the surface warming depends on cloud properties and evolution. Lifted clouds embedded in warmer air masses over a colder sea ice surface, decoupled from the ground by a surface-based temperature inversion, have the potential to warm the surface more strongly than near-surface fog or thin low-level boundary layer clouds because of a higher cloud base temperature. For MCAOs it is found that the thermodynamic profile of the southward-moving air mass adapts only slowly to the warmer ocean surface.

When and How Can the Stratosphere Modify the Midlatitude Cold Air Outbreaks in Northern Winter: An Isentropic Meridional Mass Circulation View

AbstractCold air outbreaks (CAOs) in northern winter can be attributed to the variations in the lower‐tropospheric equatorward cold branch (CB) of the isentropic meridional mass circulation (IMMC) across the subpolar latitudes. Understanding when and how the variations of CB are related to the stratosphere is crucial for improving the extended‐range forecast skills of CAOs in winter. Investigation on the vertical coupling of the stratospheric and tropospheric branches of IMMC suggests that the CB (or CAOs) exhibits two major timescales: sub‐monthly (10–30 days) and monthly‐to‐seasonal (>30 days). The CB can be modified by the stratospheric variability that is represented by the stratospheric poleward warm branch (WB‐ST) of the IMMC at monthly‐to‐seasonal timescales via the two‐way stratosphere‐troposphere coupling. At sub‐monthly timescales, however, the upward one‐way coupling from the troposphere to the stratosphere dominates. While changes of the CB always precede those of the WB‐ST at both timescales, indicating the upward tropospheric wave forcing into the stratosphere, the downward impacts of the WB‐ST on the CB (particularly CAOs in North America) are observed only at monthly‐to‐seasonal timescales. Specifically, the accumulated effect of the monthly‐to‐seasonal mass transport by the poleward WB‐ST can be strong enough to dominate the Arctic total column airmass changes, producing not only a barotropic structure of the Northern Annular Mode but also direct changes to the low‐level ageostrophic zonal‐mean meridional flow. The anomalous low‐level ageostrophic flow, in turn, plays a greater role in intensifying (weakening) the equatorward CB following a stronger (weaker) WB‐ST than tropospheric wave activities.

On the coupled interdecadal variations of the extra‐tropical surface air temperature and the isentropic meridional airmass transport in Northern winter

AbstractBased on the ERA5 reanalysis since the 1950s, this study investigates the coherent inter‐decadal variations of the winter continental surface air temperature (SAT) between the Eurasian (EUA) and the North American continents in the past 70 years. The SAT variation over the two continents can be characterized with five typical periods successively from a ‘both‐warmer’ (1950s–1960s), to a ‘both‐colder’ (1960s–1980s), and then a ‘EUA‐warmer’ (1980s–1990s) followed by another ‘both‐warmer’ (1990s–2000s) and a ‘EUA‐colder’ (2000s–2010s) period. Such periodic variation of the SAT is closely coupled with changes of the low isentropic‐level meridional mass transport (IMMT), as manifested by the coupling relationship between the midlatitude SAT and the net IMMT out of the polar circle, in terms of their zonal patterns and geographical patterns. The SAT variation can be well reconstructed with the first two leading maximum covariance analysis (MCA) modes between the SAT and IMMT. The first leading MCA mode (MCA1) mainly dominates a ‘EUA‐warmer/colder’ pattern, while the second (MCA2) dominates the ‘both‐warmer/colder’ pattern. There exists a significant lead–lag correlation between the MCA1 and MCA2, which determines the changing SAT pattern in the five successive periods. Furthermore, the MCA1 and MCA2, especially their lead–lag coupling after the 1975 is significantly associated with the Pacific decadal oscillation (PDO) which dominates the much‐intensified MCA1 and the related ‘EUA‐warmer/colder’ SAT pattern since then. The Atlantic multi‐decadal oscillation mainly dominates the third leading MCA mode with it's effects on the SAT over Europe coincidently to amplify the PDO‐related SAT changes.

Strong subsurface meridional current forced by monsoon intraseasonal oscillation in the southern Bay of Bengal during summer 2020

The meridional current in the southern Bay of Bengal (BOB) exhibits prominent intraseasonal variability (ISV), which exerts a critical influence on meridional mass and energy exchange. However, its relationship with the northward propagating monsoon intraseasonal oscillation (MISO), which is the predominant variability in the tropical Indian Ocean during summer, is not well understood. Using a one-year mooring deployed at 5.5 °N, 90 °E, a strong ISV of the meridional current is observed between 150 and 250 m during summer, exhibiting amplitudes exceeding 0.4 ms−1 and periods of 30–70 d. Further analysis shows that the summer ISV is forced by a strong MISO event with the following dynamic processes. The MISO first drives the zonal wind stress at the equator, leading to the equatorial Kelvin wave and the reflected equatorial Rossby wave at 5 °N. Then, the MISO propagates northward and generates local wind stress curl-induced Ekman pumping near the mooring site, leading to the subsequent off-equatorial Rossby wave at 8 °N. As a result, the synergy of the remotely- and locally-forced Rossby wave causes strong subsurface ISV in southern BOB. This study provides a new insight into the dynamic relationship between the MISO and the meridional current in the southern BOB, which has important implications for regional climate research and prediction.

Precursory Signals in the Stratospheric Meridional Mass Circulation for Mid‐Latitude Cold Air Outbreak Events of High and Low Sub‐Seasonal Predictability

AbstractUsing the NCEP reforecast data (1999–2010) and NCEP's CFSv2 real‐time forecast data (2012–2019) from the S2S (Sub‐seasonal to Seasonal) project, we evaluated the sub‐seasonal skill by the CFSv2 model with respect to mid‐latitude cold air outbreak events (CAOs) in winter (November–February) and investigated the distinct features of the anomalies of surface temperature, stratospheric mass circulation, and wave activities between the top 11 “high‐skill” CAO events and the top 10 “low‐skill” events. Results show that the predictability limit tends to be longer for wide‐impact and long‐lasting CAOs occurred in Eurasia or both continents, but such tendency is not very robust. The model possesses a better sub‐seasonal skill when there is generally stronger and longer‐lasting equatorward cold air branch of isentropic meridional mass circulation, which is coupled with a stronger stratospheric poleward warm branch in the 3 weeks prior to the peak time of “high‐skill” CAOs. The largest contrast between “high‐skill” and “low‐skill” CAOs lies in the wavenumber‐1 component of stratospheric poleward warm branch. Both the amplitude in the middle stratosphere and vertically westward‐tilting of wavenumber‐1 waves in the lower stratosphere tend to be stronger (weaker) prior to “high‐skill” (“low‐skill”) CAOs, collaboratively contributing to the stronger (weaker) stratospheric poleward warm branch. The temporal evolution of intensity of stratospheric poleward warm branch, wave amplitude, and westward‐tilting are better captured by CFSv2 model at sub‐seasonal scale in the month before “high‐skill” CAOs. These results reveal the important role of stratosphere–troposphere connection in determining the windows of opportunity for skillful sub‐seasonal forecasts of CAOs.

A dissection of the topographic effects from Eurasia and North America on the isentropic meridional mass circulation in Northern Winter

The topographic dynamical effect from Eurasia (EA_Topo) and North America (NA_Topo) on the winter isentropic meridional mass circulation (IMMC) is investigated using the WACCM. The independent effect of EA_Topo and that of NA_Topo, with the former much stronger, are both to strengthen the IMMC that is composed of the lower equatorward cold air branch (CB) and the upper poleward warm air branch in the extratropical tropopshere (WB_TR) and stratosphere (WB_ST). Further investigation of the individual contributions from changes in stationary vs. transient and zonal-mean flow vs. waves reveals that, due to the topography-forced mass redistribution, changes in the low-level meridional pressure gradient force a zonal-mean counter-clockwise/clockwise meridional cell in the southern/northern side of topography. This weakens/strengthens the IMMC south/north of 30° N from the troposphere to lower stratosphere, acting as a dominant contributor to the IMMC changes south of 50° N. Meanwhile, the EA/NA_Topo-forced amplification of stationary waves constructively interacts with those determined by land-sea contrast, making the dominant/minor contributions to the strengthening of CB and WB_TR north of 50° N. The related increase in the upward wave propagation further dominates the WB_ST strengthening in the subpolar region. Meanwhile, transient eddy activities are depressed by EA/NA_Topo along with the weakened background westerly, which partly-offset/dominate-over the contribution from stationary flow in midlatitudes and subpolar region. The coexistence of the other topography (NA/EA_Topo) yields destructive mutual interferrence, which can weaken/offset the independent-EA/NA_Topo-forced meridional mass transport mainly via changing the zonal-mean as well as the downstream wave pattern of mass and meridional wind.

Complex Networks Reveal Heatwave Patterns and Propagations Over the USA

AbstractQuantifying the dynamical linkage, co‐evolution, and propagation of regional heatwaves is essential to minimize socio‐economic losses. Here, we investigate such network structure and propagation characteristics for warm period (May–September) heatwaves over Conterminous United States using a complex network approach based on daily maximum temperature. The concept of Event Synchronization (ES) is applied to identify the source and sink regions primarily responsible for heatwave propagations and the strength of association between these regions. The network coefficients are derived to evaluate the extremal dependence, co‐evolution, and spatial propagation of large scale heatwavc events. The topology and propagation of heatwaves are influenced by the spatial distribution of the zonal and meridional air mass transport. Furthermore, we demonstrated the application of ES metrics and the network coefficients for heatwave days prediction between source and sink regions with true positive rate of 63% at a lead time of 2 days.

An application of a solar-type dynamo model for ε Eridani

ABSTRACT During the last decade, the relation between activity cycle periods and stellar parameters has received special attention. The construction of reliable registries of activity reveals that solar-type stars exhibit activity cycles with periods from few years to decades and, in some cases, long and short activity cycles coexist suggesting that two dynamos could operate in these stars. In particular, ε Eridani is an active young K2V star (0.8 Gyr), which exhibits short and long-term chromospheric cycles of ∼3 and ∼13-yr periods. Additionally, between 1985 and 1992, the star went through a broad activity minimum, similar to the solar Maunder Minimum state. Motivated by these results, we found in ε Eridani a great opportunity to test the dynamo theory. Based on the model developed in Sraibman & Minotti, in this work we built a non-linear axisymmetric dynamo for ε Eridani. The time series of the simulated magnetic field components near the surface integrated in all the stellar disc exhibits both the long and short activity cycles with periods similar to the ones detected from observations and also time intervals of low activity that could be associated with the broad Minimum. The short activity cycle associated with the magnetic reversal could be explained by the differential rotation, while the long cycle is associated with the meridional mass flows induced by the Lorentz force. In this way, we show that a single non-linear dynamo model derived from first principles with accurate stellar parameters could reproduce coexisting activity cycles.

Oceanic Density/Pressure Gradients and Slope Currents

AbstractEastern boundary currents are some of the most energetic features of the global ocean, contributing significantly to meridional mass, heat, and salt transports. We take a new look at the form of an oceanic slope current in equilibrium with oceanic density gradients. We depth integrate the linearized x and y momentum and continuity equations and assume an equilibrium force balance in the along-slope direction (no along-slope variation in the along-slope flow) and zero cross-slope flow at a coastal boundary. We relate the bottom stress to a bottom velocity via a simple boundary friction law (the precise details are easily modified) and then derive an expression for the slope current velocity by integrating upward including thermal wind shear. This provides an expression for the slope current as a function of depth and of cross-slope coordinate, dependent on the oceanic density field and surface and bottom stresses. This new expression for the slope current allows for more general forms of oceanic density fields than have been treated previously. Wind stress is also now considered. The emphasis here is on understanding the simplified equilibrium force balance rather than the evolution toward that balance. There is a direct relationship between the slope current strength, friction, and along-slope forcing (e.g., wind), and also between the total along-slope forcing and bottom Ekman transport, illustrating that “slippery” bottom boundaries in literature are a direct consequence of unrealistically assuming zero along-slope pressure gradient. We demonstrate the utility of the new expression by comparison with a high-resolution hydrodynamic numerical model.

The Atlantic Subtropical Cells Inferred from Observations

AbstractThe Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres, they are characterized by equatorward transport at thermocline level, upwelling at the equator, and poleward Ekman transport in the surface layer. This study uses recent data from Argo floats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward flow and the thermocline equatorward flow. The latter is bounded by the 26.0 kg m−3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ± 1.1 Sv from the southern hemisphere compared to 2.9 ± 1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind‐driven Ekman divergence in the surface layer is more symmetric, being 20.4 ± 3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ± 0.8 Sv at 10°S and 6.4 ± 1.4 Sv at 10°N). A diapycnal transport of about 3 Sv across the 26.0 kg m−3 isopycnal is required in order to maintain the mass balance of the STC circulation.

Meridional mass transport of bottom water in the South Atlantic

Estimates of the meridional mass transport of Antarctic Bottom Water, calculated using the coupled ocean- atmosphere model called “Earth System Model” in conjunction with the original data assimilation method are presented. We used the data of the latitudinal CTD sections of temperature and salinity of the WOCE international experiment in 1991-1995 for assimilation. Estimates of the current velocities of Antarctic Bottom Water with the assimilation of observational data are given. We used the author’s data assimilation method, which was previously referred as the generalized Kalman Filter (GKF) method. In the particular case it coincides with the classical Kalman method (EnKF). We also analyze the estimates of the mass transport by the standard dynamic scheme. It is shown that model calculations with data assimilation are qualitatively the same and are quantitatively close to the estimates of geostrophic flow transport based on the dynamic method.

Meridional Mass Transport of Bottom Water in the South Atlantic

Estimates of the meridional mass transport of Antarctic Bottom Water, calculated using the coupled ocean-atmosphere Earth System Model on the basis of the original data assimilation method are presented. For assimilation, we use data of the latitudinal CTD sections of temperature and salinity of the WOCE international experiment in 1991–1995. Estimates of the current velocities of Antarctic Bottom Water with the assimilation of observational data are given. We used the author’s data-assimilation method, which was previously referred to as the generalized Kalman Filter (GKF) method. In this particular case, it coincides with the classical Kalman method (EnKF). We also present the estimates of mass transport based on a standard geostrophic dynamic scheme. It is shown that model calculations with data assimilation are qualitatively the same and are quantitatively close to the estimates of the geostrophic flow transport based on the dynamic method.

Two-step closure of the Miocene Indian Ocean Gateway to the Mediterranean

The Tethys Ocean was compartmentalized into the Mediterranean Sea and Indian Ocean during the early Miocene, yet the exact nature and timing of this disconnection are not well understood. Here we present two new neodymium isotope records from isolated carbonate platforms on both sides of the closing seaway, Malta (outcrop sampling) and the Maldives (IODP Site U1468), to constrain the evolution of past water mass exchange between the present day Mediterranean Sea and Indian Ocean via the Mesopotamian Seaway. Combining these data with box modeling results indicates that water mass exchange was reduced by ~90% in a first step at ca. 20 Ma. The terminal closure of the seaway then coincided with the sea level drop caused by the onset of permanent glaciation of Antarctica at ca. 13.8 Ma. The termination of meridional water mass exchange through the Tethyan Seaway resulted in a global reorganization of currents, paved the way to the development of upwelling in the Arabian Sea and possibly led to a strengthening of South Asian Monsoon.

Sub-seasonal prediction skill for the stratospheric meridional mass circulation variability in CFSv2

This study evaluates the prediction skill for the stratospheric mass circulation variability in winter (November–March) in the Climate Forecast System, version 2 (CFSv2), from 2011 to 2018. Three stratospheric mass circulation indices measuring meridional mass transport into the polar stratosphere by the total flow (ST60N), wavenumber-1 (ST60N_W1), and wavenumber-2 waves (ST60N_W2) are considered. The variability of ST60N is mainly contributed by ST60N_W1 and ST60N_W2, and these indices are good indicators of the timing and locations of the continental-scale cold-air outbreaks (CAOs) at mid-latitudes. Thus, their potentially useful prediction skill can be utilized to make sub-seasonal forecasts of CAOs in a dynamical and statistical hybrid paradigm. Systematic forecast bias is found in both the 7-year averaged winter mean and seasonal cycle, which is tied to the overestimation of damping in amplitude and westward tilting variations of total waves and difficulties in forecasting the exact contributions from different spatial scales of waves. The intraseasonal variations of stratospheric mass circulation indices, with the systematic forecast bias corrected, can be modestly predicted at a forecast lead time of about 20 days, in terms of both the anomaly value and timing of negative and positive peak events. The sub-seasonal predictability of the ST60N value mainly comes from the ST60N_W1, while the predictability of the timing of positive and negative peaks comes from the ST60N_W2. The 20-day prediction limit of the stratospheric mass circulation indices is mainly due to the 2-week limit of the CFSv2 model in predicting the variability of anomalous wave tilt angle, whereas the prediction limit of the wave amplitude anomaly can exceed 50 days.

Understanding the variation of stratosphere–troposphere coupling during stratospheric northern annular mode events from a mass circulation perspective

We revisit the various stratosphere–troposphere coupling relation from the perspective of the meridional mass circulation. We constructed 10-hPa northern annular mode (NAM) phase composites to show the typical spatiotemporal evolution of circulation anomalies during the NAM’s life cycle. Our results indicate that there is large case-to-case difference in the temporal evolution and vertical profile of polar temperature anomalies during NAM events, which shows no strong dependence on the intensity and duration of NAM events, but agrees well with the variations of the three branches of mass circulation at 60°N: the stratospheric poleward warm air branch (ST), the poleward warm air branch in the upper troposphere (WB), and the equatorward cold air branch in the lower troposphere (CB). Such correspondence is due to the dynamic heating and cooling anomalies associated with the redistribution of air masses by the anomalous meridional mass circulation in different isentropic layers. The various relationship among the three mass circulation branches is attributed to anomalous wave activities. The amplitude and westward tilt of waves are always stronger (weaker) throughout the stratosphere before (after) the peak time of negative NAM events, leading to a stronger (weaker) ST before (after) the peak time. Variations in WB and CB are mostly dependent on wave variabilities in the mid- to lower troposphere, leading to variations in the timing of in- or out-of-phase coupling of the ST with the WB and CB, and thus various thermostructure during NAM events. At a later stage of the negative NAM events when the polar temperature becomes colder and the polar jet recovers, the weakened baroclinic instability in the lower stratosphere provides favorable conditions for the strengthening of the WB and CB if wave activities strengthen in the troposphere during that period.

The Effect of Wind Stress Anomalies and Location in Driving Pacific Subtropical Cells and Tropical Climate

Abstract The importance of subtropical and extratropical zonal wind stress anomalies on Pacific subtropical cell (STC) strength is assessed through several idealized and realistic numerical experiments with a global ocean model. Different zonal wind stress anomalies are employed, and their intensity is strengthened or weakened with respect to the climatological value throughout a suite of simulations. Subtropical strengthened (weakened) zonal wind stress anomalies result in increased (decreased) STC meridional mass and energy transport. When upwelling of subsurface water into the tropics is intensified (reduced), a distinct cold (warm) anomaly appears in the equatorial thermocline and up to the surface, resulting in significant tropical sea surface temperature (SST) anomalies. The use of realistic wind stress anomalies also suggests a potential impact of midlatitude atmospheric modes of variability on tropical climate through STC dynamics. The remotely driven response is compared with a set of simulations where an equatorial zonal wind stress anomaly is imposed. A dynamically distinct response is achieved, whereby the equatorial thermocline adjusts to the wind stress anomaly, resulting in significant equatorial SST anomalies as in the remotely forced simulations but with no role for STCs. Significant anomalies in Indonesian Throughflow transport are generated only when equatorial wind stress anomalies are applied, leading to remarkable heat content anomalies in the Indian Ocean. Equatorial wind stress anomalies do not involve modifications of STC transport but could set up the appropriate initial conditions for a tropical–extratropical teleconnection involving Hadley cells, exciting an STC anomalous transport, which ultimately feeds back on the tropics.

Characterization of the long-term changes in moisture, clouds and precipitation in the ascending and descending branches of the Hadley Circulation

Climate model simulations and observations show that there is a poleward expansion of the Hadley Circulation (HC) as well as a strengthening of the hydrological cycle in a warming climate. However, to establish a relation between the two phenomena in present day climate, simultaneous investigations of the HC and hydrological cycle changes using observational/reanalysis data are necessary, which is limited as compared to model simulations. In this regard, the present study employs relative humidity (RH), cloud fraction (CF) and precipitation (RF) parameters of the hydrological cycle and analyse their long term changes within the HC ascending and descending regions, simultaneously. Long term RH and CF data (1979–2016) are obtained from ERA-I reanalysis, and RF from GPCP precipitation dataset. The boundaries of the HC are identified using the meridional Mass Stream Function (MSF) metric, a metric that can track the mass motion in the atmosphere in the meridional direction. The analysis brings out the spatial pattern of the distribution of trends in hydrological parameters within the HC boundaries. The trends are observed to be significantly positive at the edges of the HC ascending region and significantly negative in the regions near the HC edges, insignificant (and in some cases, negative) in the deep tropics. This pattern is more or less consistent in RH, CF, as well as RF parameters. Thus the study shows that there are regions of positive as well as negative trends within the both ascending and descending regions. The results are found to be in accordance with the poleward expansion of HC and strengthening of the hydrological cycle. Besides, a northward shift in the HC ascending regions are also indicated from the analysis of the annual cycle of trend in precipitation. The current investigation is thus envisaged to contribute to further exploration on the relation between the HC changes and the intensification of the hydrological cycle in a warmer climate.