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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.

Kilpatrick, TJ, Xie SP.  2016.  Circumventing rain-related errors in scatterometer wind observations. Journal of Geophysical Research-Atmospheres. 121:9422-9440.   10.1002/2016jd025105   AbstractWebsite

Satellite scatterometer observations of surface winds over the global oceans are critical for climate research and applications like weather forecasting. However, rain-related errors remain an important limitation, largely precluding satellite study of winds in rainy areas. Here we utilize a novel technique to compute divergence and curl from satellite observations of surface winds and surface wind stress in rainy areas. This technique circumvents rain-related errors by computing line integrals around rainy patches, using valid wind vector observations that border the rainy patches. The area-averaged divergence and wind stress curl inside each rainy patch are recovered via the divergence and curl theorems. We process the 10 year Quick Scatterometer (QuikSCAT) data set and show that the line-integral method brings the QuikSCAT winds into better agreement with an atmospheric reanalysis, largely removing both the "divergence bias" and "anticyclonic curl bias" in rainy areas noted in previous studies. The corrected QuikSCAT wind stress curl reduces the North Pacific midlatitude Sverdrup transport by 20-30%. We test several methods of computing divergence and curl on winds from an atmospheric model simulation and show that the line-integral method has the smallest errors. We anticipate that scatterometer winds processed with the line-integral method will improve ocean model simulations and help illuminate the coupling between atmospheric convection and circulation.

Chow, CH, Liu QY, Xie SP.  2015.  Effects of Kuroshio Intrusions on the atmosphere northeast of Taiwan Island. Geophysical Research Letters. 42:1465-1470.   10.1002/2014gl062796   AbstractWebsite

The Kuroshio loses bathymetric support off northeast Taiwan Island, causing large variability in its path. The resultant covariability of sea surface temperature (SST) and the lower atmosphere is investigated using satellite observations. In winter and spring off northeast Taiwan Island, the intrusions of warm Kuroshio water onto the continental shelf cause a large increase in local SST, intensify the northeasterly monsoonal winds, and lead to the increases in water vapor and rainfall. Key to this air-sea interaction is the existence of anomalous heat advection by the Kuroshio intrusions. The Kuroshio intrusions are partly due to westward propagating ocean eddies east of Taiwan Island with a lead time of 3weeks, hinting at the possibility of improved weather prediction near northeast Taiwan Island by considering ocean variability east of Taiwan Island.

Tomita, H, Xie SP, Tokinaga H, Kawai Y.  2013.  Cloud response to the meandering Kuroshio extension front. Journal of Climate. 26:9393-9398.   10.1175/jcli-d-13-00133.1   AbstractWebsite

A unique set of observations on board research vessel (R/V) Mirai in April 2010 captured a striking cloud hole over a cold meander of the Kuroshio Extension (KE) east of Japan as corroborated by atmospheric soundings, ceilometer, shipboard radiation data, and satellite cloud images. Distinct differences were also observed between the warm meander farther to the north and warm water south of the KE. The atmosphere is highly unstable over the warm meander, promoting a well-mixed marine atmospheric boundary layer (MABL) and a layer of solid stratocumulus clouds capped by a strong inversion. Over the warm water south of the KE, MABL deepens and is decoupled from the ocean surface. Scattered cumulus clouds develop as captured by rapid variations in ceilometer-derived cloud base. The results show that the meandering KE front affects the entire MABL and the clouds. Such atmospheric response can potentially intensify the baroclinicity in the lower atmosphere.

Xie, SP.  2013.  Advancing climate dynamics toward reliable regional climate projections. Journal of Ocean University of China. 12:191-200.   10.1007/s11802-013-2277-7   AbstractWebsite

With a scientific consensus reached regarding the anthropogenic effect on global mean temperature, developing reliable regional climate projections has emerged as a new challenge for climate science. A national project was launched in China in 2012 to study ocean's role in regional climate change. This paper starts with a review of recent advances in the study of regional climate response to global warming, followed by a description of the Chinese project including the rationale, objectives, and plan for field observations. The 15 research articles that follow in the special issue are highlighted, representing some of the initial results from the project.