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Jian Ma and Jin-Yi Yu

Abstract

This study analyzes representative concentration pathway 4.5 projections by 18 models from phase 5 of the Coupled Model Intercomparison Project to show that surface warming patterns in the equatorial Pacific during the twenty-first century (centennial warming) are influenced by the relative strengths of the Walker and Hadley circulations. The stronger the Hadley (Walker) circulation is, the greater the surface warming in the central Pacific (CP) [eastern Pacific (EP)]. The EP warming is associated with the Bjerknes feedback, while the CP warming is associated with the wind–evaporation–sea surface temperature feedback. This atmospheric circulation influence on the centennial warming is similar to that found for the EP and CP El Niño. This suggests a methodology to constrain the estimate of the projected surface warming patterns in the equatorial Pacific using recent El Niño activity. The constraint indicates that the “most likely” centennial warming patterns have a maximum in the EP and are 39% weaker than the warming projected by the 18-model mean. The most-likely projection also shows alternating stronger and weaker warming in the subtropical North Pacific, which is not predicted by the 18-model mean projection. Nevertheless, the two projections agree on the minimum warming in the southeastern subtropical Pacific.

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Jian Ma and Shang-Ping Xie

Abstract

Precipitation change in response to global warming has profound impacts on environment for life but is highly uncertain. Effects of sea surface temperature (SST) warming on the response of rainfall and atmospheric overturning circulation are investigated using Coupled Model Intercomparison Project simulations. The SST warming is decomposed into a spatially uniform SST increase (SUSI) and deviations from it. The SST pattern effect is found to be important in explaining both the multimodel ensemble mean distribution and intermodel variability of rainfall change over tropical oceans. In the ensemble mean, the annual rainfall change follows a “warmer-get-wetter” pattern, increasing where the SST warming exceeds the tropical mean, and vice versa. Two SST patterns stand out both in the ensemble mean and intermodel variability: an equatorial peak anchoring a local precipitation increase and a meridional dipole mode with increased rainfall and weakened trade winds over the warmer hemisphere. These two modes of intermodel variability in SST account for one-third of intermodel spread in rainfall projection.

The SST patterns can explain up to four-fifths of the intermodel variability in intensity changes of overturning circulations. SUSI causes both the Hadley and Walker circulations to slow down, as articulated by previous studies. The weakening of the Walker circulation is robust across models as the SST pattern effect is weak. The Hadley circulation change, by contrast, is significantly affected by SST warming patterns. As a result, near and south of the equator, the Hadley circulation strength change is weak in the multimodel ensemble mean and subject to large intermodel variability due to the differences in SST warming patterns.

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Jian Cao, Bin Wang, and Libin Ma

Abstract

Investigation of global monsoon (GM) responses to external forcings is instrumental for understanding its formation mechanism and projected future changes. Coupled climate model experiments are performed to assess how the individual and full Last Glacial Maximum (LGM) forcings change GM precipitation. Under the full LGM forcing, the annual and local summer-mean GM precipitation are reduced by 8.5% and 10.8%, respectively, compared to the results in the preindustrial control run; and the reduction of Northern Hemisphere (NH) summer monsoon (NHSM) precipitation is twice as large as its Southern Hemisphere (SH) counterpart (SHSM). The NH–SH asymmetric response is mainly caused by the monsoon circulation change–induced moisture convergence rather than the reduction of moisture content, but the root cause is the continental ice sheet forcing. The NHSM precipitation changes dramatically differ among various single-forcing experiments, while this is not the case for their SH counterparts. The moisture budget analysis indicates the NHSM is dynamically oriented, but SHSM is thermodynamically oriented. The markedly different NHSM circulation changes are caused by different forcing-induced sea surface temperature (SST) patterns, including the North Atlantic cooling pattern forced by the continental ice sheet, the mega–La Niña–like pattern resulting from the greenhouse gas forcing, and the Indian Ocean dipole–like SST pattern caused by the land–sea configuration forcing. Moreover, the distinctive change of “monsoonality” in the Australian–Indonesian monsoon is predominantly forced by the exposure of the land shelf, which enhances precipitation during early summer (November–December) but weakens it in the rest of the year.

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Jian Ma, Shang-Ping Xie, and Yu Kosaka

Abstract

The annual-mean tropospheric circulation change in global warming is studied by comparing the response of an atmospheric general circulation model (GCM) to a spatial-uniform sea surface temperature (SST) increase (SUSI) with the response of a coupled ocean–atmosphere GCM to increased greenhouse gas concentrations following the A1B scenario. In both simulations, tropospheric warming follows the moist adiabat in the tropics, and static stability increases globally in response to SST warming. A diagnostic framework is developed based on a linear baroclinic model (LBM) of the atmosphere. The mean advection of stratification change (MASC) by climatological vertical motion, often neglected in interannual variability, is an important thermodynamic term for global warming. Once MASC effect is included, LBM shows skills in reproducing GCM results by prescribing latent heating diagnosed from the GCMs.

MASC acts to slow down the tropical circulation. This is most clear in the SUSI run where the Walker circulation slows down over the Pacific without any change in SST gradient. MASC is used to decelerate the Hadley circulation, but spatial patterns of SST warming play an important role. Specifically, the SST warming is greater in the Northern than Southern Hemisphere, an interhemispheric asymmetry that decelerates the Hadley cell north, but accelerates it south of the equator. The MASC and SST-pattern effects are on the same order of magnitude in our LBM simulations. The former is presumably comparable across GCMs, while SST warming patterns show variations among models in both shape and magnitude. Uncertainties in SST patterns account for intermodel variability in Hadley circulation response to global warming (especially on and south of the equator).

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Yimin Ma, Noel E. Davidson, Yi Xiao, and Jian-Wen Bao

Abstract

In high-wind conditions, sea spray, in conjunction with a generally decreasing drag coefficient for increasing winds, greatly modulates surface heat and momentum fluxes. It has been suggested that the process can be particularly important for the prediction of tropical cyclones (TCs), yet its robust application in operational forecast systems has remained elusive. A sea spray inclusion scheme and a modified algorithm for momentum exchange have been implemented in the Australian Bureau of Meteorology’s current operational TC model. Forecasts for a limited sample of TCs demonstrate that the revised parameterizations improve initialized and forecast intensities, while mostly maintaining track prediction skill. TC Yasi (2011) has been studied for impacts of the revised parameterization on rapid intensification (RI). Compared with the conventional bulk air–sea exchange parameterization, the revised version simulates a cooler and moister region near the surface in the eyewall/eye region, adjusts the RI evolution by an earlier and stronger subsidence in the eye, and simulates a stronger radial pulsating of the eye and eyewall convection on relatively short time scales. The inclusion of the new scheme enhances RI features characterized by eyewall ascent, radial convergence, and inertial stability inside the radius of azimuthal-mean maximum wind over low- to midlevels, and by a ringlike radial distribution of relative vorticity above the boundary layer. In addition, it allows a higher maximum intensity wind speed based on Emanuel’s maximum potential intensity theory. It is shown that, as expected, this is mainly because of a larger ratio of enthalpy and momentum exchange coefficients.

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Jie Ma, Ming Feng, Bernadette M. Sloyan, and Jian Lan

Abstract

In this study, low-frequency variability of the meridional temperature transport in the Indian Ocean is examined using a mesoscale-eddy-resolving global ocean circulation model for the period 1979–2014. The dominant empirical orthogonal function (EOF) of the meridional temperature transport is found to be highly influenced by Pacific El Niño–Southern Oscillation (ENSO) through both oceanic and atmospheric waveguides, with the southward temperature transport being stronger during La Niña and weaker during El Niño. A dynamical decomposition of the meridional streamfunction and temperature transport shows that the relative importance of different dynamic modes varies with latitude; these modes act together to contribute to the coherent ENSO response. The Ekman mode explains a larger part of low-frequency variability in overturning and temperature transport north of the equator. Between 25° and 3°S, variations associated with vertical shear mode are of greater importance. The external mode has an important contribution between 30° and 25°S where the western boundary currents impinge on topography. South of 25°S, the variability of the external mode contribution has significant negative correlations with the vertical shear mode, suggesting that the large variability of external mode depends on the joint effects of baroclinicity and topography, such that hydrographic sections alone may not be suitable for deducing changes in the meridional temperature transport at these latitudes.

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Jiapei Ma, Hongyi Li, Jian Wang, Xiaohua Hao, Donghang Shao, and Huajin Lei

Abstract

Gridded precipitation data are very important for hydrological and meteorological studies. However, gridded precipitation can exhibit significant statistical bias that needs to be corrected before application, especially in regions where high wind speeds, frequent snowfall, and sparse observation networks can induce significant uncertainties in the final gridded datasets. In this paper, we present a method for the production of gridded precipitation on the Tibetan Plateau (TP). This method reduces the statistical distribution error by correcting for wind-induced undercatch and optimizing the interpolation method. A gridded precipitation product constructed by this method was compared with previous products on the TP. The results show that undercatch correction is necessary for station data, which can reduce the distributional error by 30% at most. A thin-plate splines interpolation algorithm considering altitude as a covariate is helpful to reduce the statistical distributional error in general. Our method effectively inhibits the smoothing effect in gridded precipitation, and compared to previous products, results in a higher mean value, larger 98th percentile, and greater temporal variance. This study can help to improve the quality of gridded precipitation over the TP.

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Evan Weller, Ming Feng, Harry Hendon, Jian Ma, Shang-Ping Xie, and Nick Caputi

Abstract

Off the Western Australia coast, interannual variations of wind regime during the austral winter and spring are significantly correlated with the Indian Ocean dipole (IOD) and the southern annular mode (SAM) variability. Atmospheric general circulation model experiments forced by an idealized IOD sea surface temperature anomaly field suggest that the IOD-generated deep atmospheric convection anomalies trigger a Rossby wave train in the upper troposphere that propagates into the southern extratropics and induces positive geopotential height anomalies over southern Australia, independent of the SAM. The positive geopotential height anomalies extended from the upper troposphere to the surface, south of the Australian continent, resulting in easterly wind anomalies off the Western Australia coast and a reduction of the high-frequency synoptic storm events that deliver the majority of southwest Australia rainfall during austral winter and spring. In the marine environment, the wind anomalies and reduction of storm events may hamper the western rock lobster recruitment process.

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Shang-Ping Xie, Clara Deser, Gabriel A. Vecchi, Jian Ma, Haiyan Teng, and Andrew T. Wittenberg

Abstract

Spatial variations in sea surface temperature (SST) and rainfall changes over the tropics are investigated based on ensemble simulations for the first half of the twenty-first century under the greenhouse gas (GHG) emission scenario A1B with coupled ocean–atmosphere general circulation models of the Geophysical Fluid Dynamics Laboratory (GFDL) and National Center for Atmospheric Research (NCAR). Despite a GHG increase that is nearly uniform in space, pronounced patterns emerge in both SST and precipitation. Regional differences in SST warming can be as large as the tropical-mean warming. Specifically, the tropical Pacific warming features a conspicuous maximum along the equator and a minimum in the southeast subtropics. The former is associated with westerly wind anomalies whereas the latter is linked to intensified southeast trade winds, suggestive of wind–evaporation–SST feedback. There is a tendency for a greater warming in the northern subtropics than in the southern subtropics in accordance with asymmetries in trade wind changes. Over the equatorial Indian Ocean, surface wind anomalies are easterly, the thermocline shoals, and the warming is reduced in the east, indicative of Bjerknes feedback. In the midlatitudes, ocean circulation changes generate narrow banded structures in SST warming. The warming is negatively correlated with wind speed change over the tropics and positively correlated with ocean heat transport change in the northern extratropics. A diagnostic method based on the ocean mixed layer heat budget is developed to investigate mechanisms for SST pattern formation.

Tropical precipitation changes are positively correlated with spatial deviations of SST warming from the tropical mean. In particular, the equatorial maximum in SST warming over the Pacific anchors a band of pronounced rainfall increase. The gross moist instability follows closely relative SST change as equatorial wave adjustments flatten upper-tropospheric warming. The comparison with atmospheric simulations in response to a spatially uniform SST warming illustrates the importance of SST patterns for rainfall change, an effect overlooked in current discussion of precipitation response to global warming. Implications for the global and regional response of tropical cyclones are discussed.

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Jian Ma, Sara C. da Silva, Aaron Levine, Yang Yang, Paul Fuentes, Li Zhou, Chuan-Chi Tu, Jia Hu, I. M. Shiromani Jayawardena, Antti Pessi, and DaNa Carlis

A four-day educational cruise navigated around the leeward side of Oahu and Kauai to observe the thermodynamic and dynamic features of the trade-wind wakes of these small islands by using weather balloons and other onboard atmospheric and oceanographic sensors. This cruise was proposed, designed, and implemented completely by graduate students from the School of Ocean and Earth Science and Technology at the University of Hawaii. The data collected during the cruise show, for the first time, strong sea/land breezes during day/night and their thermal effects on the island wake. This cruise provided the students with a significant, valuable, and meaningful opportunity to experience the complete process of proposing and undertaking field observations, as well as analyzing data and writing a scientific article.

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