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Simon P. de Szoeke and Shang-Ping Xie

Abstract

Warmer SST and more rain in the Northern Hemisphere are observed year-round in the tropical eastern Pacific with southerly wind crossing the equator toward the atmospheric heating. The southerlies are minimal during boreal spring, when two precipitation maxima straddle the equator. Fourteen atmosphere–ocean coupled GCMs from the Coupled Model Intercomparison Project (CMIP3) and one coupled regional model are evaluated against observations with simple metrics that diagnose the seasonal cycle and meridional migration of warm SST and rain. Intermodel correlations of the metrics elucidate common coupled physics. These models variously simulate the climatology of SST and ITCZ rain.

In 8 out of 15 models the ITCZ alternates symmetrically between the hemispheres with the seasons. This seasonally alternating ITCZ error generates two wind speed maxima per year—one northerly and one southerly—resulting in spurious cooling in March and a cool SST error of the equatorial ocean. Most models have too much rain in the Southern Hemisphere so that SST and rain are too symmetric about the equator in the annual mean. Weak meridional wind on the equator near the South American coast (2°S–2°N, 80°–90°W) explains the warm SST error there.

Northeasterly wind jets blow over the Central American isthmus in winter and cool the SST in the eastern Pacific warm pool. In some models the strength of these winds contributes to the early demise of their northern ITCZ relative to observations. The February–April northerly wind bias on the equator is correlated to the antecedent December–February Central American Pacific wind speed at −0.88. The representation of southern-tropical stratus clouds affects the underlying SST through solar radiation, but its effect on the meridional atmospheric circulation is difficult to discern from the multimodel ensemble, indicating that errors other than the simulation of stratus clouds are also important for accurate simulation of the meridional asymmetry.

This study identifies several features to be improved in atmospheric and coupled GCMs, including the northeasterly cross–Central American wind in winter and meridional wind on the equator. Improved simulation of the seasonal cycle of meridional wind could alleviate biases in equatorial SST and improve simulation of ENSO and its teleconnections.

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H. Annamalai, S. P. Xie, J. P. McCreary, and R. Murtugudde

Abstract

Prior to the 1976–77 climate shift (1950–76), sea surface temperature (SST) anomalies in the tropical Indian Ocean consisted of a basinwide warming during boreal fall of the developing phase of most El Niños, whereas after the shift (1977–99) they had an east–west asymmetry—a consequence of El Niño being associated with the Indian Ocean Dipole/Zonal mode. In this study, the possible impact of these contrasting SST patterns on the ongoing El Niño is investigated, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), with the latter used to identify basic processes. Specifically, analyses of reanalysis products during the El Niño onset indicate that after the climate shift a low-level anticyclone over the South China Sea was shifted into the Bay of Bengal and that equatorial westerly anomalies in the Pacific Ocean were considerably stronger. The present study focuses on determining influence of Indian Ocean SST on these changes.

A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipitation anomalies over the equatorial Indian Ocean. Solutions indicate that both local and remote SST anomalies are necessary for realistic simulations, with convection in the tropical west Pacific and the subsequent development of the South China Sea anticyclone being particularly sensitive to Indian Ocean SST anomalies. Prior to the climate shift, the basinwide Indian Ocean SST anomalies generate an atmospheric Kelvin wave associated with easterly flow over the equatorial west-central Pacific, thereby weakening the westerly anomalies associated with the developing El Niño. In contrast, after the shift, the east–west contrast in Indian Ocean SST anomalies does not generate a significant Kelvin wave response, and there is little effect on the El Niño–induced westerlies. The Linear Baroclinic Model (LBM) solutions confirm the AGCM’s results.

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Masami Nonaka, Julian P. McCreary Jr., and Shang-Ping Xie

Abstract

The stratification of the equatorial thermocline is a key variable for tropical climate dynamics, through its influence on the temperature of the water that upwells in the eastern equatorial ocean. In this study, two types of ocean models are used, an ocean general circulation model (GCM) and a 1½-layer model, to investigate processes by which changes in the midlatitude winds affect the equatorial stratification. Specifically, the influences of anomalous mode-water formation, Ekman pumping, and entrainment in the subpolar ocean are examined. The effects of a “sponge layer” adjacent to the northern boundary of the basin are also assessed. Solutions are forced by idealized zonal winds with strong or weak midlatitude westerlies, and they are found in rectangular basins that extend from the equator to 36°N (small basin) or to 60°N (large basin). In the GCM solutions, a prominent response to reduced winds is the thinning of the mixed layer in the northwestern region of the subtropical gyre, leading to less subduction of low-potential-vorticity mode water and hence thinning of the upper thermocline in the central-to-eastern subtropics. Almost all of this thinning signal, however, recirculates within the subtropics, and does not extend to the equator. Another midlatitude response is shallowing (deepening) of the thermocline in the subtropical (subpolar) ocean in response to Ekman pumping. This, primarily, first-baroclinic-mode (n = 1) response has the most influence on the equatorial thermocline. First-baroclinic-mode Rossby waves propagate to the western boundary of the basin where they reflect as packets of coastal Kelvin and short-wavelength Rossby waves that carry the midlatitude signal to the equator. Subsequently, equatorial Kelvin waves spread it along the equator, leading to a shoaling and thinning of the equatorial thermocline. The layer-thickness field h in the 1½-layer model corresponds to thermocline depth in the GCM. Both the sponge layer and subpolar Ekman suction are important factors for the 1½-layer model solutions, requiring water upwelled in the interior ocean to be transported into the sponge layer via the western boundary layer. In the small basin, equatorial h thins in response to weakened westerlies when there is a sponge layer, but it thickens when there is not. In the large basin, equatorial h is unaffected by weakened westerlies when there is a sponge layer, but it thins when water is allowed to entrain into the layer in the subpolar gyre. It is concluded that the thinning of the equatorial thermocline in the GCM solutions is caused by the sponge layer in the small basin and by entrainment in the subpolar ocean in the large one.

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W. Timothy Liu, Xiaosu Xie, and Pearn P. Niiler

Abstract

Many years of high-resolution measurements by a number of space-based sensors and from Lagrangian drifters became available recently and are used to examine the persistent atmospheric imprints of the semipermanent meanders of the Agulhas Extension Current (AEC), where strong surface current and temperature gradients are found. The sea surface temperature (SST) measured by the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and the chlorophyll concentration measured by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) support the identification of the meanders and related ocean circulation by the drifters. The collocation of high and low magnitudes of equivalent neutral wind (ENW) measured by Quick Scatterometer (QuikSCAT), which is uniquely related to surface stress by definition, illustrates not only the stability dependence of turbulent mixing but also the unique stress measuring capability of the scatterometer. The observed rotation of ENW in opposition to the rotation of the surface current clearly demonstrates that the scatterometer measures stress rather than winds. The clear differences between the distributions of wind and stress and the possible inadequacy of turbulent parameterization affirm the need of surface stress vector measurements, which were not available before the scatterometers. The opposite sign of the stress vorticity to current vorticity implies that the atmosphere spins down the current rotation through momentum transport. Coincident high SST and ENW over the southern extension of the meander enhance evaporation and latent heat flux, which cools the ocean. The atmosphere is found to provide negative feedback to ocean current and temperature gradients. Distribution of ENW convergence implies ascending motion on the downwind side of local SST maxima and descending air on the upwind side and acceleration of surface wind stress over warm water (deceleration over cool water); the convection may escalate the contrast of ENW over warm and cool water set up by the dependence of turbulent mixing on stability; this relation exerts a positive feedback to the ENW–SST relation. The temperature sounding measured by the Atmospheric Infrared Sounder (AIRS) is consistent with the spatial coherence between the cloud-top temperature provided by the International Satellite Cloud Climatology Project (ISCCP) and SST. Thus ocean mesoscale SST anomalies associated with the persistent meanders may have a long-term effect well above the midlatitude atmospheric boundary layer, an observation not addressed in the past.

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R. W. Higgins, V. E. Kousky, and P. Xie

Abstract

An analysis of extreme daily precipitation events that occurred in the south-central United States during May and June 2010 is carried out using gridded station data and reanalysis products in use at the National Centers for Environmental Prediction (NCEP). Various aspects of the daily extremes are examined from a climate perspective using a 62-yr (1948–2010) period of record, including their historical ranking, common circulation features, moisture plumes, and the possible influence of ENSO. The analysis also considers how the frequency and intensity of daily extremes is changing in the United States. Each of the 2010 flash flood events examined here was associated with historic daily rainfall totals. Several of the events had meteorological conditions in common at upper and lower levels of the atmosphere, and all of the events fit well into an existing classification scheme for heavy precipitation events associated with flash flooding. Each case exhibited characteristics of the “Maya Express” flood events that link tropical moisture plumes from the Caribbean and Gulf of Mexico to midlatitude flooding over the central United States. Consistent with recent assessment reports, it is shown that extreme daily precipitation events in the United States have increased in frequency during the most recent 30-yr period (1980–2009) when compared to the previous 30-yr period (1950–79), though the increases are relatively small during May and June.

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N. H. Saji, S-P. Xie, and T. Yamagata

Abstract

The twentieth-century simulations using by 17 coupled ocean–atmosphere general circulation models (CGCMs) submitted to the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) are evaluated for their skill in reproducing the observed modes of Indian Ocean (IO) climate variability. Most models successfully capture the IO’s delayed, basinwide warming response a few months after El Niño–Southern Oscillation (ENSO) peaks in the Pacific. ENSO’s oceanic teleconnection into the IO, by coastal waves through the Indonesian archipelago, is poorly simulated in these models, with significant shifts in the turning latitude of radiating Rossby waves. In observations, ENSO forces, by the atmospheric bridge mechanism, strong ocean Rossby waves that induce anomalies of SST, atmospheric convection, and tropical cyclones in a thermocline dome over the southwestern tropical IO. While the southwestern IO thermocline dome is simulated in nearly all of the models, this ocean Rossby wave response to ENSO is present only in a few of the models examined, suggesting difficulties in simulating ENSO’s teleconnection in surface wind.

A majority of the models display an equatorial zonal mode of the Bjerknes feedback with spatial structures and seasonality similar to the Indian Ocean dipole (IOD) in observations. This success appears to be due to their skills in simulating the mean state of the equatorial IO. Corroborating the role of the Bjerknes feedback in the IOD, the thermocline depth, SST, precipitation, and zonal wind are mutually positively correlated in these models, as in observations. The IOD–ENSO correlation during boreal fall ranges from −0.43 to 0.74 in the different models, suggesting that ENSO is one, but not the only, trigger for the IOD.

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M. R. P. Sapiano, J. E. Janowiak, P. A. Arkin, H. Lee, T. M. Smith, and P. Xie

Abstract

The longest record of precipitation estimated from satellites is the outgoing longwave radiation (OLR) precipitation index (OPI), which is based on polar-orbiting infrared observations from the Advanced Very High Resolution Radiometer (AVHRR) instrument that has flown onboard successive NOAA satellites. A significant barrier to the use of these data in studies of the climate of tropical precipitation (among other things) is the large bias caused by orbital drift that is present in the OLR data. Because the AVHRR instruments are deployed on the polar-orbiting spacecraft, OLR observations are recorded at specific times for each earth location for each day. Discontinuities are caused by the use of multiple satellites with different observing times as well as the orbital drift that occurs throughout the lifetime of each satellite. A regression-based correction is proposed based solely on the equator crossing time (ECT). The correction allows for separate means for each satellite as well as separate coefficients for each satellite ECT. The correction is calculated separately for each grid box but is applied only at locations where the correction is correlated with the OLR estimate. Thus, the correction is applied only where deemed necessary.

The OPI is used to estimate precipitation from the OLR estimates based on the new corrected version of the OLR, the uncorrected OLR, and two earlier published corrected versions. One of the earlier corrections is derived by removing variations from AVHRR based on EOFs that are identified as containing spurious variations related to the ECT bias, whereas the other is based on OLR estimates from the High Resolution Infrared Radiation Sounder (HIRS) that have been corrected using diurnal models for each grid box. The new corrected version is shown to be free of nearly all of the ECT bias and has the lowest root mean square difference when compared to gauges and passive microwave estimates of precipitation. The EOF-based correction fails to remove all of the variations related to the ECT bias, whereas the correction based on HIRS removes much of the bias but appears to introduce erroneous trends caused by the water vapor signal to which these data are sensitive. The new correction for AVHRR OLR works well in the tropics where the OPI has the most skill, but users should be careful when interpreting trends outside this region.

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Simon P. de Szoeke, Shang-Ping Xie, Toru Miyama, Kelvin J. Richards, and R. Justin O. Small

Abstract

A coupled ocean–atmosphere regional model suggests a mechanism for formation of a sharp sea surface temperature (SST) front north of the equator in the eastern Pacific Ocean in boreal summer and fall. Meridional convergence of Ekman transport at 5°N is forced by eastward turning of the southeasterly cross-equatorial wind, but the SST front forms considerably south of the maximum Ekman convergence. Geostrophic equatorward flow at 3°N in the lower half of the isothermally mixed layer enhances mixed layer convergence.

Cold water is upwelled on or south of the equator and is advected poleward by mean mixed layer flow and by eddies. The mixed layer current convergence in the north confines the cold advection, so the SST front stays close to the equator. Warm advection from the north and cold advection from the south strengthen the front. In the Southern Hemisphere, a continuous southwestward current advects cold water far from the upwelling core.

The cold tongue is warmed by the net surface flux, which is dominated by solar radiation. Evaporation and net surface cooling are at a maximum just north of the SST front where relatively cool dry air is advected northward over warm SST. The surface heat flux is decomposed into a response to SST alone, and an atmospheric feedback. The atmospheric feedback enhances cooling on the north side of the front by 178 W m−2, about half of which is due to enhanced evaporation from cold dry advection, while the other half is due to cloud radiative forcing.

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Richard Justin O. Small, Simon P. de Szoeke, and Shang-Ping Xie

Abstract

The midsummer drought (MSD) is a diminution in rainfall experienced during the middle of the rainy season in southern Mexico and Central America, as well as in the adjacent Caribbean, Gulf of Mexico, and eastern Pacific seas. The aim of this paper is to describe the regional characteristics of the MSD and to propose some possible forcing mechanisms. Satellite and in situ data are used to form a composite of the evolution of a typical MSD, which highlights its coincidence with a low-level anticyclone centered over the Gulf of Mexico and associated easterly flow across Central America. The diurnal cycle of precipitation over the region is reduced in amplitude during midsummer. The MSD is also coincident with heavy precipitation over the Sierra Madre Occidental (part of the North American monsoon). Reanalysis data are used to show that the divergence of the anomalous low-level flow during the MSD is the main factor governing the variations in precipitation. A linear baroclinic model is used to show that the seasonal progression of the Pacific intertropical convergence zone (ITCZ), which moves northward following warm sea surface temperature (SST) during the early summer, and of the Atlantic subtropical high, which moves westward, are the most important remote factors that contribute toward the low-level easterly flow and divergence during the MSD. The circulation associated with the MSD precipitation deficit helps to maintain the deficit by reinforcing the low-level anticyclonic flow over the Gulf of Mexico. Surface heating over land also plays a role: a large thermal low over the northern United States in early summer is accompanied by enhanced subsidence over the North Atlantic. This thermal low is seen to decrease considerably in midsummer, allowing the high pressure anomalies in the Atlantic and Pacific Oceans to extend into the Gulf of Mexico. These anomalies are maintained until late summer, when an increase in rainfall from the surge in Atlantic tropical depressions induces anomalous surface cyclonic flow with westerlies fluxing moisture from the Pacific ITCZ toward Central America.

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R. W. Higgins, V. E. Kousky, V. B. S. Silva, E. Becker, and P. Xie

Abstract

A comparison of the statistics of daily precipitation over the conterminous United States is carried out using gridded station data and three generations of reanalysis products in use at the National Centers for Environmental Prediction (NCEP). The reanalysis products are the NCEP–NCAR reanalysis (Kalnay et al.), the NCEP–Department of Energy (DOE) reanalysis (Kanamitsu et al.), and the NCEP Climate Forecast System (CFS) reanalysis (Saha et al.). Several simple measures are used to characterize relationships between the observations and the reanalysis products, including bias, precipitation probability, variance, and correlation. Seasonality is accounted for by examining these measures for four nonoverlapping seasons, using daily data in each case. Relationships between daily precipitation and El Niño–Southern Oscillation (ENSO) phase are also considered.

It is shown that the CFS reanalysis represents a clear improvement over the earlier reanalysis products, though significant biases remain. Comparisons of the error patterns in the reanalysis products provide a suitable basis for confident conversion of the Climate Prediction Center (CPC) operational monitoring and prediction products to the new generation of analyses based on CFS.

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