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Yang, JC, Lin XP, Xie SP.  2017.  A Transbasin Mode of Interannual Variability of the Central American Gap Winds: Seasonality and Large-Scale Forcing. Journal of Climate. 30:8223-8235.   10.1175/jcli-d-17-0021.1   AbstractWebsite

A transbasin mode (TBM) is identified as the leading mode of interannual surface wind variability over the Intra-Americas Seas across Central America based on empirical orthogonal function analysis. The TBM is associated with variability in Central American gap winds, most closely with the Papagayo jet but with considerable signals over the Gulfs of Tehuantepec and Panama. Although El Nino-Southern Oscillation (ENSO) is the main large-scale forcing, the TBM features a distinct seasonality due to sea level pressure (SLP) adjustments across the Pacific and Atlantic. During July-September, ENSO causes meridional SLP gradient anomalies across Central America, intensifying anomalous geostrophic winds funneling through Papagayo to form the TBM. During wintertime, ENSO peaks but imparts little anomalous SLP gradient across Central America with a weak projection on the TBM because of the competing effects of the Pacific-North American teleconnection and tropospheric Kelvin waves. Besides ENSO, tropical Atlantic sea surface temperature anomalies make a weak contribution to the TBM in boreal summer by strengthening the cross-basin gradient. ENSO and the Atlantic forcing constitute a cross-basin seesaw pattern in SLP, manifested as an anomalous Walker circulation across the tropical Americas. The TBM appears to be part of the low-level branch of the anomalous Walker circulation, which modulates Central American wind jets by orographic effect. This study highlights the seasonality of gap wind variability, and calls for further research into its influence on regional climate.

Tokinaga, H, Xie SP, Mukougawa H.  2017.  Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability. Proceedings of the National Academy of Sciences of the United States of America. 114:6227-6232.   10.1073/pnas.1615880114   AbstractWebsite

With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20th-century Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean-atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-to-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region.

Richter, I, Xie SP, Morioka Y, Doi T, Taguchi B, Behera S.  2017.  Phase locking of equatorial Atlantic variability through the seasonal migration of the ITCZ. Climate Dynamics. 48:3615-3629.   10.1007/s00382-016-3289-y   AbstractWebsite

The equatorial Atlantic is marked by significant interannual variability in sea-surface temperature (SST) that is phase-locked to late boreal spring and early summer. The role of the atmosphere in this phase locking is examined using observations, reanalysis data, and model output. The results show that equatorial zonal surface wind anomalies, which are a main driver of warm and cold events, typically start decreasing in June, despite SST and sea-level pressure gradient anomalies being at their peak during this month. This behavior is explained by the seasonal northward migration of the intertropical convergence zone (ITCZ) in early summer. The north-equatorial position of the Atlantic ITCZ contributes to the decay of wind anomalies in three ways: (1) horizontal advection associated with the cross-equatorial winds transports air masses of comparatively low zonal momentum anomalies from the southeast toward the equator. (2) The absence of deep convection leads to changes in vertical momentum transport that reduce the equatorial wind anomalies at the surface, while anomalies aloft remain relatively strong. (3) The cross-equatorial flow is associated with increased total wind speed, which increases surface drag and deposit of momentum into the ocean. Previous studies have shown that convection enhances the surface wind response to SST anomalies. The present study indicates that convection also amplifies the surface zonal wind response to sea-level pressure gradients in the western equatorial Atlantic, where SST anomalies are small. This introduces a new element into coupled air-sea interaction of the tropical Atlantic.

Ma, J, Xie SP, Xu HM.  2017.  Intermember variability of the summer northwest Pacific subtropical anticyclone in the ensemble forecast. Journal of Climate. 30:3927-3941.   10.1175/jcli-d-16-0638.1   AbstractWebsite

The accurate prediction of the East Asian summer monsoon (EASM) remains a major challenge for the climate research community. The northwest Pacific (NWP) subtropical anticyclone (NWPSA) is the dominant feature of the EASM low-level circulation variability. This study identifies two coupled modes between intermember anomalies of the NWPSA and sea surface temperature (SST). The first mode features SST anomalies over the tropical Pacific. This tropical Pacific mode has little impact on East Asian climate. The second mode features a strong coupling between SST in the north Indian Ocean (NIO)-NWP and NWPSA, with large impacts on East Asia. This resembles the Indo-western Pacific Ocean capacitor (IPOC) mode of interannual variability. Major differences exist in temporal evolution of the intermember SST spread between the equatorial Pacific and NIO. In the equatorial Pacific, the intermember SST spread grows gradually with lead time, while the spread of SST and low-level zonal wind grow rapidly from May to June in the NIO. The rapid growth over the NIO is due to positive feedback arising from the coupling between intermember anomalies of SST and winds. In post-El Nino summer, the intermember spread in equatorial Pacific SST forecast represents the variations in the timing of the El Nino phase transition. The late decay of El Nino relates to SST cooling and an anomalous cyclonic circulation over the South China Sea (SCS) but with little impact on East Asian climate. Thus, a better representation of the IPOC mode of regional ocean-atmosphere interaction over the NIO-NWP holds the key to improving the reliability of seasonal forecast of East Asian climate.

Yan, XH, Boyer T, Trenberth K, Karl TR, Xie SP, Nieves V, Tung KK, Roemmich D.  2016.  The global warming hiatus: Slowdown or redistribution? Earths Future. 4:472-482.   10.1002/2016ef000417   AbstractWebsite

Global mean surface temperatures (GMST) exhibited a smaller rate of warming during 1998-2013, compared to the warming in the latter half of the 20th Century. Although, not a "true" hiatus in the strict definition of the word, this has been termed the "global warming hiatus" by IPCC (2013). There have been other periods that have also been defined as the "hiatus" depending on the analysis. There are a number of uncertainties and knowledge gaps regarding the "hiatus." This report reviews these issues and also posits insights from a collective set of diverse information that helps us understand what we do and do not know. One salient insight is that the GMST phenomenon is a surface characteristic that does not represent a slowdown in warming of the climate system but rather is an energy redistribution within the oceans. Improved understanding of the ocean distribution and redistribution of heat will help better monitor Earth's energy budget and its consequences. A review of recent scientific publications on the "hiatus" shows the difficulty and complexities in pinpointing the oceanic sink of the "missing heat" from the atmosphere and the upper layer of the oceans, which defines the "hiatus." Advances in "hiatus" research and outlooks (recommendations) are given in this report.

Zhang, RS, Xie SP, Xu LX, Liu QY.  2016.  Changes in mixed layer depth and spring bloom in the Kuroshio extension under global warming. Advances in Atmospheric Sciences. 33:452-461.   10.1007/s00376-015-5113-8   AbstractWebsite

The mixed layer is deep in January-April in the Kuroshio Extension region. This paper investigates the response in this region of mixed layer depth (MLD) and the spring bloom initiation to global warming using the output of 15 models from CMIP5. The models indicate that in the late 21st century the mixed layer will shoal, and the MLD reduction will be most pronounced in spring at about 33A degrees N on the southern edge of the present deep-MLD region. The advection of temperature change in the upper 100 m by the mean eastward flow explains the spatial pattern of MLD shoaling in the models. Associated with the shoaling mixed layer, the onset of spring bloom inception is projected to advance due to the strengthened stratification in the warming climate.

Gao, WD, Li PL, Xie SP, Xu LX, Liu C.  2016.  Multicore structure of the North Pacific subtropical mode water from enhanced Argo observations. Geophysical Research Letters. 43:1249-1255.   10.1002/2015gl067495   AbstractWebsite

Seventeen Argo profiling floats with enhanced vertical and temporal sampling were deployed in the Kuroshio recirculation gyre in the western North Pacific in late March 2014. The Subtropical Mode Water (STMW) observed in many profiles displays a multicore structure with more than one minima in potential vorticity (PV), corroborated by vertical covariations in apparent oxygen utilization (AOU). These cores are classified into four submodes according to density and AOU. The submode waters are typically 100m thick, in which PV varies by 1x10(-10)m(-1)s(-1) and AOU by 10 mu mole/kg. The STMW multicore structure is most frequently observed in spring, gradually taken over by single-core profiles into summer. The seasonal evolution is suggestive of vertical mixing, especially in STMW of lower density.

Chikamoto, Y, Timmermann A, Luo JJ, Mochizuki T, Kimoto M, Watanabe M, Ishii M, Xie SP, Jin FF.  2015.  Skilful multi-year predictions of tropical trans-basin climate variability. Nature Communications. 6   10.1038/ncomms7869   AbstractWebsite

Tropical Pacific sea surface temperature anomalies influence the atmospheric circulation, impacting climate far beyond the tropics. The predictability of the corresponding atmospheric signals is typically limited to less than 1 year lead time. Here we present observational and modelling evidence for multi-year predictability of coherent trans-basin climate variations that are characterized by a zonal seesaw in tropical sea surface temperature and sea-level pressure between the Pacific and the other two ocean basins. State-of-the-art climate model forecasts initialized from a realistic ocean state show that the low-frequency trans-basin climate variability, which explains part of the El Nino Southern Oscillation flavours, can be predicted up to 3 years ahead, thus exceeding the predictive skill of current tropical climate forecasts for natural variability. This low-frequency variability emerges from the synchronization of ocean anomalies in all basins via global reorganizations of the atmospheric Walker Circulation.

Richter, I, Behera SK, Doi T, Taguchi B, Masumoto Y, Xie SP.  2014.  What controls equatorial Atlantic winds in boreal spring? Climate Dynamics. 43:3091-3104.   10.1007/s00382-014-2170-0   AbstractWebsite

The factors controlling equatorial Atlantic winds in boreal spring are examined using both observations and general circulation model (GCM) simulations from the coupled model intercomparison phase 5. The results show that the prevailing surface easterlies flow against the attendant pressure gradient and must therefore be maintained by other terms in the momentum budget. An important contribution comes from meridional advection of zonal momentum but the dominant contribution is the vertical transport of zonal momentum from the free troposphere to the surface. This implies that surface winds are strongly influenced by conditions in the free troposphere, chiefly pressure gradients and, to a lesser extent, meridional advection. Both factors are linked to the patterns of deep convection. Applying these findings to GCM errors indicates, that, consistent with the results of previous studies, the persistent westerly surface wind bias found in most GCMs is due mostly to precipitation errors, in particular excessive precipitation south of the equator over the ocean and deficient precipitation over equatorial South America. Free tropospheric influences also dominate the interannual variability of surface winds in boreal spring. GCM experiments with prescribed climatological sea-surface temperatures (SSTs) indicate that the free tropospheric influences are mostly associated with internal atmospheric variability. Since the surface wind anomalies in boreal spring are crucial to the development of warm SST events (Atlantic Ninos), the results imply that interannual variability in the region may rely far less on coupled air-sea feedbacks than is the case in the tropical Pacific.

Fuckar, NS, Xie SP, Farneti R, Maroon EA, Frierson DMW.  2013.  Influence of the extratropical ocean circulation on the intertropical convergence zone in an idealized coupled general circulation model. Journal of Climate. 26:4612-4629.   10.1175/jcli-d-12-00294.1   AbstractWebsite

The authors present coupled model simulations in which the ocean's meridional overturning circulation (MOC) sets the zonal mean location of the intertropical convergence zone (ITCZ) in the hemisphere with deep-water production. They use a coarse-resolution single-basin sector coupled general circulation model (CGCM) with simplified atmospheric physics and two idealized land-sea distributions.In an equatorially symmetric closed-basin setting, unforced climate asymmetry develops because of the advective circulation-salinity feedback that amplifies the asymmetry of the deep-MOC cell and the upper-ocean meridional salinity transport. It confines the deep-water production and the dominant extratropical ocean heat release to a randomly selected hemisphere. The resultant ocean heat transport (OHT) toward the hemisphere with the deep-water source is partially compensated by the atmospheric heat transport (AHT) across the equator via an asymmetric Hadley circulation, setting the ITCZ in the hemisphere warmed by the ocean.When a circumpolar channel is open at subpolar latitudes, the circumpolar current disrupts the poleward transport of the upper-ocean saline water and suppresses deep-water formation poleward of the channel. The MOC adjusts by lowering the main pycnocline and shifting the deep-water production into the opposite hemisphere from the channel, and the ITCZ location follows the deep-water source again because of the Hadley circulation adjustment to cross-equatorial OHT. The climate response is sensitive to the sill depth of the channel but becomes saturated when the sill is deeper than the main pycnocline depth in subtropics. In simulations with a circumpolar channel, the ITCZ is in the Northern Hemisphere (NH) because of the Southern Hemisphere (SH) circumpolar flow that forces northward OHT.