Cloud responses to climate variability over the extratropical oceans as observed by MISR and MODIS

Linear temporal trends in cloud fraction over the extratropical oceans, observed by NASA’s Multiangle Imaging Spectro-Radiometer (MISR) during the period 2000-2013, are examined in the context of coincident ECMWF reanalysis data using a maximum covariance analysis. Changes in specific cloud types defined with 10 respect to cloud top height and cloud optical depth are related to trends in reanalysis variables. A pattern of reduced high altitude optically thick cloud and increased low altitude cloud of moderate optical depth is found to be associated with increased temperatures, geopotential heights, and anticyclonicity over the extratropical oceans. These and other trends in cloud occurrence are shown to be correlated with changes in the El Niño Southern Oscillation, the Pacific Decadal Oscillation, the North Pacific Index, and the Southern Annular Mode. 15


Introduction
Clouds play a fundamental role in Earth's climate due to their effect on the planet's radiative budget.Cloud responses to climate change are poorly understood however, and cloud-climate interaction is presently one of the largest sources of uncertainty in climate models (Caldwell et al., 2016;IPCC, 2013;Bony et al., 2006).Several changes in midlatitude cloud are expected under global warming with varying degrees of certainty: including 20 poleward shifts in the storm tracks, rising melting level, rising high cloud tops, and reduced low cloud (IPCC, 2013).
Understanding changes in midlatitude and Southern Ocean cloud in-particular is important, because these clouds have a large radiative impact, influence atmospheric dynamics (Kay et al., 2016;Hwang and Frierson, 2013), and are not adequately captured by climate models (Trenberth and Fasullo, 2010;Bodas-Salcedo et al., 2014).Several studies have observed midlatitude cloud responses to extratropical synoptic variability, for instance, changes in 25 cloud cover associated with the North Atlantic Oscillation or Southern Annular Mode (e.g.Li et al., 2014(a); 2014(b); 2016; Ceppi and Hartmann, 2015;Gordon and Norris, 2010;Gordon et al., 2005;Tselioudis et al., 2000).
Many studies of cloud variability (including several of those cited above) are based on analysis of International Satellite Cloud Climatology Project (ISCCP) datasets.ISCCP is a multi-instrument and multi-satellite product that combines observations from polar orbiting and geostationary weather satellites to determine cloud amount and 30 categorizes clouds by their cloud top pressure and optical depth (Rossow et al., 1999).For example, Bender et al. (2011) use meridional maxima in ISCCP total cloud fraction as a proxy for midlatitude storm track latitude, and identify a 25-year poleward trend in storm track location.Building on early studies, Norris et al. (2016) examine trends in the ISCCP and Extended Pathfinder Atmospheres datasets after applying several empirical corrections (Norris and Evan, 2015).They also show a poleward trend in storm track location between the 1980's and 2000's 100 particularly in the tropics, which is on the order of 0.1-0.3g kg -1 decade -1 , and occurs primarily below 750hPa.We performed a brief comparison to specific humidity data from the Modern Era Retrospective analysis for Research and Applications (MERRA) and found that while it did indicate an increase in low level specific humidity of similar magnitude in some of the Northern Hemisphere, it did not corroborate the pervasive trend in the ECMWF data.Dessler and Davis (2010) provide a more comprehensive inter-comparison of specific humidity trends in different 105 reanalysis datasets, albeit not for the time-period analyzed here, and conclude that while most show recent increases Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-520Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License. in specific humidity (which is expected under global warming), there is large disagreement over the magnitude and spatial distribution.In any case, we show the ECMWF specific humidity data, but caution the reader that these data may not be as robust or reliable as other fields.

Climate Indices 110
Several of the patterns found via the MCA (Sect.4-6) resemble well-documented modes of climate variability.In Section 4, we compare the monthly time series of the MCA modes to monthly indices for various modes of climate variability and northern hemisphere teleconnection patterns maintained by the NOAA Climate Prediction Center (CPC).The indices used are: • The Niño Region 1+2, 3, 3.4, and 4 indices (hereafter referred to as "Niño 3.4," for instance).• The Pacific Decadal Oscillation (PDO) index, which is the first mode of an Empirical Orthogonal Function (EOF) decomposition of SST north of 20°N in the Pacific Ocean (Mantua et al., 1997).
• The Antarctic Oscillation index or "Southern Annular Mode" (SAM), which is defined as the first mode of an EOF analysis of 700hPa geopotential height south of 20°S 1979-2000 (Thompson and 125 Wallace, 2000).
• The Pacific-North American (PNA) mode index.The index is defined by projection of the PNA loading pattern on to the daily 500hPa height anomalies over the entire Northern Hemisphere.The PNA loading pattern is derived by a rotated principal component analysis of 500hPa heights north of 0° between 1950 and 2000 as described in (Barnston and Livezey, 1987).

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• The North Pacific Index (NPI).The NPI is a standardized mean of sea level pressure between 30°-65°N and 160°-220°E (Trenberth & Hurrell, 1994) show trends in ERA interim data and results of the MCA that is discussed in Section 4).The cloud fraction data were spatially averaged over each ocean basin prior to computing trends, and the composited seasonal cycle was removed.The middle row of panels (e)-(h) in Fig. 1 show the MISR cloud fraction trends associated with each optical depth category, here cloud fraction is summed with respect to cloud top height prior to computing trends.

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Finally, the bottom row of panels (i)-(l) show cloud fraction trends for each MODIS optical depth category for both MODIS Aqua and Terra.Note that the MODIS cloud occurrence histograms do not include a "No Retrieval" (NR) category.The MODIS data also include separate bins for optical depths between 60-100 and 100-150 (the high optical depth bin is new for MODIS collection 6), however in Fig. 1, these two optical-depth bins have been summed to create a single bin representing all optical depths greater than 60.This step is taken to make comparison 155 of the two datasets easier.Bold bordered bins in the joint histograms in the top row (a)-(d) indicate that the cloud fraction trend in that bin exceeds a 95% confidence test, while the bars in the lower panels indicate the 95% confidence interval.The confidence intervals were computed using a windowed boot-strapping technique described in Wilks (2006), which was also used to assign confidence to the cloud fraction trends computed in Marchand 2013.This technique involves randomly resampling, with replacement, each bin's cloud-fraction time-series in 12-month 160 chunks 1000 times and computing trends for each of the resampled time-series.The trend associated with the original time series is said to be significant at the 95% level if it exceeds the 25 th most positive or 25 th most negative resampled trend.Bins that account for less than 0.1% of the total cloud fraction are not considered.Figure 1 shows the same dominant pattern of changing extratropical cloud fraction identified in Marchand (2013), but here based on three additional years of MISR data.The results in three of the basins (the North Pacific, South Atlantic, and South but there is a slight reduction in the number of failed cloud-top-height retrievals (that is the NR row) in the North Pacific and North Atlantic.Failed cloud-top-height retrievals most often occur in multilayer cloud conditions (where a low cloud that is visible in the MISR nadir view cannot be located in an off-nadir views due to a visibly opaque or semitransparent higher altitude cloud), which suggest that a small portion of the observed increase in lowlevel clouds is due to a reduction in higher (semi-transparent) cloud.Loeb, 2015;Limbacher and Kahn, 2016).This calibration drift can be expected to reduce the retrieved optical depth, reducing the occurrence of clouds with large-optical depths in the CTH-OD product, and increasing the occurrences of clouds with moderate optical depths.Such a calibration drift will not change the cloud top altitude 180 but will cause clouds at a given altitude to shift toward lower optical depths at that same altitude.Evidence for such a calibration-driven change can be seen to some degree in Fig. 1, where in the South Pacific (panel (d)) and South Atlantic (panel (c)) between 5 and 7 km there is a strong increase of cloud with OD between 9.4 and 23, and strong decrease at this same altitude for optical depths greater than 60.However, Fig. 1 suggests that much of the reduction in optically thick cloud is occurring at high altitudes while the increase in clouds with moderate optical 185 depths is occurring for low level clouds.Limited testing, where the CTH-OD dataset was reprocessed for onemonth with the observed radiances reduced artificially by 2%, suggests a reduction in the occurrence of cloud with large optical depths may well explain 50 to 75% of the MISR trend depending on the region.Plans are underway to reprocess the entire MISR mission, starting from the (level 1) calibrated imagery and eventually including all higher-level datasets (level 2 swath and level 3 global datasets), and this includes the CTH-OD product.This As another note, we add that there is a known issue with the MODIS Terra cloud mask over ocean.MODIS Terra's 8.6 μm channel has undergone warming since around 2010 that has not been corrected though on-board calibration, and this warming has caused a number of clear pixels to be flagged as cloudy in the MODIS cloud product.This On monthly time-scales cloud occurrence is heavily influenced by synoptic conditions, and it seems likely that a large portion of the observed cloud fraction trends are related to trends in synoptic variables.In Fig. 2, we show the 215 spatial distribution of trends in several of the reanalysis variables discussed in Section 2 (500hPa geopotential height, temperature, and absolute vorticity), hashing denotes trends that are significant with 95% confidence (using 5x5-degree bins).These trends have been computed after first de-seasonalizing the data using compositing, and confidence intervals are determined using the windowed bootstrapping technique discussed above.There is a notable increase in both the 500hPa temperature and geopotential height in the center of the North Pacific region 220 (Fig. 2 (d)-(e)).This is accompanied by increased anticyclonicity (f) (a negative trend in absolute vorticity in the northern hemisphere), which is expected given the temperature and pressure changes, though the trends in anticyclonicity often do not pass the significance test, which is perhaps due to the large variability in this field.Both the 500hPa temperature and absolute vorticity fields show increasing trends in both Southern Ocean storm track regions (around 40-50S), with only small areas that pass the confidence test which coincide with the largest positive 225 trends (panels (h), (i), (k), (l)).These are accompanied by an increase in geopotential heights that does not pass the 95% significance test, but is meteorologically consistent with the corresponding changes in temperature and vorticity (panels (g) and (j)).In the South Pacific this primarily occurs to the south and the east of New Zealand.In the South Atlantic there is a meridional dipole with the strongest positive changes in these fields in the poleward part of the region.Notably, the regions in the Southern Ocean that show increased geopotential heights, temperature, and 230 anticyclonicity correspond well with the loading pattern of the Southern Annular Mode which has undergone a positive trend during the study period and will be discussed in more detail later (Sections 5.3 and 5.4).In the North Atlantic, while there is a weak increase in anticyclonicity (c), as well as 500 hPA geopotential height (a) and temperature (b), along the storm track (which is similar in sign to the North Pacific) the trends are generally smaller than in the other three basins, with essentially no significant trends.

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In summary, in the ocean basins studied, ERA-Interim reanalysis shows increased temperature, geopotential heights, and anticyclonicity in the storm track latitudes, though more poleward in the South Atlantic.The largest and most robust changes are in the central North Pacific Ocean and weakest in the North Atlantic.Except perhaps in the North Pacific, most of the trend (linear fit to the change) in the reanalysis variables is not statistically significant, however we stress that this doesn't mean that there has been no change in meteorology or that these changes have no impact 240 on the clouds.To the degree the reanalysis data is accurate there has been an increase in temperature, geopotential heights, and anticyclonicity in some portion of each of these basins over the period examined.Rather, the lack of statistical significance means that relative to the annual variability, the change is small and could be a result of annual variability rather than a true trend in the mean temperature, geopotential heights, and anticyclonicity with time.In the following section we use a maximum covariance analysis to identify linkages between trends in the We wish to identify relationships between two high dimensional datasets.The MISR dataset is a function of time, latitude, longitude, cloud top height and cloud optical depth, while the ERA interim variables described in Section 2 vary in time, height, latitude, and longitude.In particular, we wish to identify which trends in cloud occurrence observed by MISR are related to trends in meteorology.To do this, we identify patterns of trending cloud occurrence, in the context of MISR cloud occurrence histograms, that tend to be co-located with patterns of trending 255 meteorological variables using Maximum Covariance Analysis (MCA), the result of which are shown in Fig. 3.
MCA falls into the family of other linear decomposition techniques such as Singular Value Decomposition (SVD), principal component analysis, empirical orthogonal function analysis, canonical correlation analysis and others, (Bretherton et al., 1992;Hannachi et al., 2007;Von Storch and Zwiers, 2001).Typically, in climate science, SVD (often interchangeably referred to as empirical orthogonal function analysis and principal component analysis) is 260 applied to a single time-varying field to identify important spatial patterns that co-vary in time.This involves decomposition of the field's time-covariance matrix.SVD was used in the atmospheric sciences as early as Lorenz (1956) and Kutzbach (1967), though it became significantly more popular in the 1980-90's when it was used to identify a number of well-known modes of climate variability, such as the North Atlantic Oscillation (Barnston and Livezey, 1987;and Wallace and Gutzler, 1981), the Pacific Decadal Oscillation (Mantua et al., 1997), and the 265 Antarctic Oscillation (Thompson and Wallace, 2000).MCA involves applying SVD to a cross-covariance matrix computed between two datasets.It is similar to canonical correlation analysis which involves decomposition of a cross-correlation matrix.For instance, Prohaska (1976) applies SVD to the temporal correlation matrix computed between monthly mean sea level pressure and temperature fields.MCA has also been used historically to understand climate data, in particular, since about 2000, it has been heavily used to identify interactions between sea surface 270 temperature and atmospheric variables (Czaja and Frankignoul, 1999;Liu et al., 2006;Frankignoul et al., 2011, and references therein).It has been used in other areas of the earth sciences as well, for instance in sea ice modeling (Dirkson et al., 2015) and study of the structure of the Madden Julian Oscillation (Adames and Wallace, 2014).
A complete mathematical description of the formulation used here, as well as a discussion of the significance of MCA modes, is given in Appendix I, and results are shown in Fig. 3.In short, a covariance matrix is computed that 275 represents spatial covariance between trends in the MISR and trends in the ERA variables.A Singular Value Decomposition (SVD) is applied to the covariance matrix.One set of singular vectors identified by the SVD then represent patterns (or modes) of cloud trends in CTH-OD space (shown in the middle column of Fig. 3), and the other set represent corresponding patterns in trends of the vertical profiles of the reanalysis variables (shown in the right column of Figure 3).These patterns (or modes as we will call them in later sections) can be projected on to the 280 original trend data to see corresponding spatial patterns (the left column of Fig. 3).In the left column of Fig. 3, bright red colors represent regions where the cloud occurrence and reanalysis trends shown in the panels to the right have occurred.The same is true for the blue colors except that in these regions the trends are of the opposite sign.
For example, in North Atlantic mode 1 (the first row in Fig. 3 in high thick cloud, and at the same location there has been increased geopotential heights, temperature, and anticyclonicity at most levels.In Section 5, we discuss the modes and their geographic patterns in detail. Unlike an EOF analysis, the original monthly datasets cannot be fully recovered from the MCA modes alone (information is lost when trends are computed and because the SVD is performed on the spatial covariance matrix between the two datasets).However, monthly time series associated with each mode can be derived by linearly 290 projecting the MCA mode patterns shown in Fig. 3 on to the monthly cloud fraction anomaly data.As noted earlier, and discussed further in the next section, many, but not all, of the MCA modes resemble well documented modes of climate variability.In many respects this is not surprising since midlatitude cloud is intimately linked to the synoptic meteorology and one might therefore expect that changes in synoptic conditions captured by the CPC indices will be correlated with changes in cloud cover.We investigate the relationships between the MCA modes 295 and various climate indices (introduced in Section 2) using a time-correlation analysis.In this analysis, both MCAderived time-series and the climate indices are first detrended before Pearson correlation coefficients are computed.
The correlation values (which will be discussed in the next section) are tabulated in Fig. 4, along with information about any linear temporal trends in the associated CPC index time-series.

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We have identified several trends in MISR cloud amount over the extratropical oceans in Fig. 1.Below, cloud fraction trends in each of the ocean basins shown in Fig. 1 are discussed in terms of the MCA decomposition for that basin.We proceed through each of the four basins and examine each MCA mode using shorthand NAX, NPX, SAX, SPX to refer to MCA mode number X in the North Atlantic, North Pacific, South Atlantic, and South Pacific respectively.Under each of the leftmost panels in Fig. 3, the percentage of covariance between the datasets and 305 variance within each dataset that can be explained by that mode are given as: "(% covariance, % ERA-Interim variance, % MISR variance)."These values provide some additional information about the relative importance of each MCA mode.

North Atlantic
NA1 captures an extratropical trend of increased low-level cloud of moderate optical depth and reduced cloud of 310 high optical depth, primarily at high altitudes.This mode is maximized along the storm track (roughly 40 o to 55 o N) and is associated with increased temperature, pressure, and anti-cyclonic flow, as well as divergence at the surface, convergence aloft and downward motion.Figure 2, panels (a)-(c) shows that trends in temperature, geopotential heights, and absolute vorticity in this region do not pass a 95% confidence test, however, in the following discussion we see that similar MCA modes (modes with similar cloud, pressure, temperature, and vorticity patterns) exist in 315 each of the other ocean basins studied where trends in the reanalysis variables are more robust.Figure 4(a) shows there is no particularly strong connection between this mode and any climate index analyzed, but rather weak correlations with the East Atlantic teleconnection patterns and with the North Atlantic Oscillation.Li et al. 2014 identifies relationships between cloud occurrence and the NAO in Cloudsat data, however the correlations shown in  NA2 explains about 15% of the covariance and 12% of the variability in the ERA trends but only 4% of the variance in the MISR trends.As such, this mode appears to be only a minor contributor to the overall (basin average) change in cloud occurrence (shown in Fig. 1).The ERA interim profiles for NA2 show the largest trends in near surface q, T, and divergence, mid-troposphere vertical velocity, as well as the largest SST trend of any of the North Atlantic 325 modes.While the changes in the cloud fraction joint histogram are somewhat noisy, the strongest response is in the low-level cloud.Interestingly, the spatial distribution to the left resembles recent trends in North Atlantic SST, with warming SSTs off the east coast of North America and cooling in the central North Atlantic (Sup.Fig. 1).It has been documented that SSTs in the Gulf Stream influence cloud fraction (Minobe et al. 2008(Minobe et al. , 2010)).Given the apparent connection to SST (and the fact that this mode does not have a strong correlation with any CPC index), we 330 hypothesize that NA2 is a possible link between North Atlantic SST changes and low-level cloud fraction.
NA3 is primarily a subtropical mode (notice location of red colors in left panel of Fig. 3) and is characterized by an increase in low level cloud of moderate optical depth in the southern portion of the study region.This increase in low clouds appears to be due to an increase in specific humidity and increased convergence at low levels in the ERA Interim data set, though as noted in Section 2, we caution this increase in specific humidity may be a spurious 335 feature in ERA interim data or may be over-estimated and is not entirely corroborated by MERRA.A somewhat similar pattern of ERA and cloud fraction trends to NA3 is seen in the North Pacific in mode NP3, but NA3 features a small reduction in mid-level cloud around 4 km and larger reduction in high-clouds above 7 km than NP3.

North Pacific
NP1 is an east-west dipole, showing increased low-level cloud and decreased high-level cloud of median optical 340 depth in the East Pacific, and the opposite to the West.The spatial structure of this mode resembles the spatial structure of the PDO, and Fig. 4 indicates there is a noteworthy time-correlation between the two (r = -0.42).It has been argued that the longer time-scale PDO is a response to ENSO and other factors including variability in the Aleutian low (Schneider andCornuelle 2005, Newman et al. 2016).Newman et al. (2016) show that variability in the Aleutian low as captured by the North Pacific Index (NPI) typically leads the PDO (see their Fig.3b) by several 345 months.Not surprisingly, we find an even stronger correlation between NP1 and the NPI (r=0.56) and to a lesser degree the PNA (r=-0.48)(which is also known to be strongly correlated to the NPI).The panel on the right side of with less low cloud.We note that the sign of the MCA modes is arbitrary and, as shown, is opposite that defined by the PDO (PDO is positive with warmer SST in the eastern pacific and hence the negative correlation).NP1 also shows a weak increase in mid tropospheric down-welling and low-level divergence with convergence aloft, which may explain the reduction in high cloud, and would further enhance stability in the lower troposphere.Finally, NP3 shows a similar subtropical pattern to NA3, with an increase in low level cloud of medium optical depth and an increase in low level specific humidity mostly south of 40°N.This mode explains only a very small 370 amount of the variance in the cloud fraction data (2%), and so it likely does not have a particularly meaningful relation to the cloud fraction trends, but is included because it is consistent with NA 3, and explains a large amount of covariance (12%).

South Atlantic
The MCA results in the South Atlantic are the least tractable of any of the regions studied.SA2 appears to be a

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'correction' to SA1, with nearly identical large scale spatial patterns, (but with SA2 having more fine scale features), and nearly opposite changes in cloud fraction (except in the region between 1.5 and 2.5 km where SA1 shows little change and SA2 a decrease in cloud amount).SA1 also explains much more of the variance in the MISR cloud fraction data than SA2 (21% vs 4%).SA1 is associated with increased high pressure, temperature, and anticyclonicity in the southeast part of the region (consistent with overall ERA-Interim trends shown in Fig. 2 panels 380 (g)-(i).The region of enhanced high pressure in SA1 corresponds with a region of high pressure in the SAM loading pattern, and indeed SA1 shows moderate correlation with the SAM (r = .25)(Fig. 4a).SA3 explains only 7% of the covariance and 1% of the variance in the MISR data and is therefore not included in the analysis.In the NP and NA regions, we chose to include the third MCA modes because there was good agreement between the two basins and a coherent spatial pattern.There is no such agreement in the third MCA modes in the southern hemisphere regions and 385 the spatial patterns associated with the third modes appear noisy.

South Pacific
The changes captured by SP1 are related to the South Pacific Convergence Zone (SPCZ).The period studied was characterized by relatively neutral ENSO conditions, with more La-Niña like conditions later in the time series.The cause this trend to continue even as Antarctic stratospheric ozone begins to recover (Thompson et al. 2011, Zheng et al. 2013).Hartmann and Ceppi 2014 relate changes in South Pacific reflected shortwave radiation observed by Clouds and Earths Radiant Energy System (CERES) Terra to a trend towards La Nina like conditions and to trends in zonal mean winds and the SAM (which are very strongly correlated).They note however, that it is difficult to identify a robust relationship between the SAM and Southern Ocean cloud shortwave radiative effect trends due to 410 the dominant influence of sea ice changes and of ENSO on such a short time period.Ceppi and Hartmann 2015 note that while cloud amount, particularly mid and high-level cloud, responds to changes in the annular modes, associated dynamically forced changes in cloud shortwave radiative effect may be of secondary importance to thermodynamically forced changes in the cloud phase.Regardless, the cloud amount shown in SP2 and SA1 are likely driven by changes in the SAM, and continued data collection should help isolate cloud change responses to 415 the SAM.

Conclusions
In closing, cloud datasets from EOS are now of sufficient quality and length to begin studying the response of cloud to synoptic variability on multi-year time scales.We have found via maximum covariance analysis a number of linkages between trends in synoptic meteorology and trends in cloud fraction.Notably, increased low cloud of 420 moderate optical depth and reduced high cloud of higher optical depth is associated with increased temperature, anticyclonicity, geopotential height, and subsidence in the extratropical storm track regions.We speculate that this could be linked to a strengthening of extratropical warm-core highs during the time-period studied, though this would require additional analysis of daily data to verify.The maximum covariance analysis also revealed linkages between observed extratropical cloud fraction changes and known modes of climatic variability.Cloud changes (or In ascertaining the significance of each MCA mode, it is important to examine the covariance between the two datasets and the variance within each dataset explained by the MCA mode.These values are printed in Fig. 3 underneath each spatial loading pattern in the format: "(% covariance, % ERA-Interim variance, % MISR variance)"

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explained.These measures give an idea of a mode's relative importance in the context of the original data.The percent covariance explained is a measure the importance of a mode relative to other modes, while the percent ERA or MISR variance explained measures the importance of a mode for explaining the trends in each particular dataset.
For instance, the first and second modes in the North Pacific are quite robust because they explain a large amount of the total covariance between the two datasets and explain a substantial amount of the variance in each dataset.On 490 the other hand, the third North Pacific mode is less important, because while it explains a non-negligible fraction of the total covariance, it explains only 2% of the variance in the MISR data and thus does not project strongly on to the MISR trends.While there is no universally agreed upon method for testing the significance of MCA results, a useful metric is the "normalized Root Mean Squared Covariance" (RMSC): Here, || ||  is the Frobenius norm and tr( ) is the trace operator.In general, larger values of this metric imply a robust result while smaller values imply that the MCA modes do not capture a significant portion of the variance in the two datasets.For artificially generated data with similar dimensions to the data used here, poorly correlated fields yield RMSC ≈ 0.09 while well correlated fields yield RMSC ≈ 0.26, (this range will vary depending on the size of the dataset and number of independent samples).The RMSCs computed for each of the study regions are:

500
North Atlantic: 0.17, North Pacific: 0.17, South Atlantic: 0.16, South Pacific: 0.17, which are reassuringly high considering the large variability in the monthly MISR data.Finally, each of the MCA modes, which are shown in

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These indices are based on spatial averages of SST anomaly in various regions in the tropical , and was obtained from the University Corporation for Atmospheric Research (UCAR) website (link provided at bottom of the reference list).Several other similarly defined teleconnection patterns were analyzed, but are less relevant to our results: the East 135 Pacific/North Pacific, Scandinavian, Tropical / Northern Hemisphere, East Atlantic, Pacific Transition, Polar / Eurasia, and West Pacific indices.All indices but the NPI were obtained from the NOAA CPC website (see data availability section).These indices are dimensionless except for the "Niño" indices, which represent temperature anomalies, though we have standardized each index prior to computing any statistics by subtracting the mean, dividing by the standard deviation, and detrending.3 Trends in Cloud Fraction Linear temporal trends were computed on the MISR cloud fraction data.Figure 1 shows the trends computed in each of four extratropical ocean basins for each bin in MISR's CTH-OD joint histograms in panels (a)-(d).The four regions studied are the North Atlantic (25-65N, 280-360E), North Pacific (25-65N, 120-240E), South Atlantic (25-145 65S, 280-360E), and South Pacific (25-65S, 120-240E), (these geographic regions are shown in Figs. 2 and 3, which

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reprocessing will include a correction for this calibration drift among other issues.For the present, an important caveat is that the strength of the optical depth trend in the MISR dataset (and associated statistical confidence) is likely being overestimated, and this may explain why the trends in the MISR dataset are larger than MODIS.The significant changes in cloud top height observed in three of the study regions and the reduced mid-level cloud (between about 2 or 2.5 and 4 km) observed in the South Pacific could not have been caused by this calibration drift 195 however.In the bottom panels (i)-(l) of Fig. 1, trends in MODIS cloud fraction bins only partially corroborate those in the MISR dataset.MODIS Aqua identifies a reduction in optically thick cloud in all the regions studied but the South Pacific, while MODIS Terra shows a reduction in only the North and South Atlantic.MODIS Aqua shows an increase in cloud of moderate optical depth in all regions but the South Atlantic (in agreement with MISR) while 200 MODIS Terra shows little or no change in these bins.As with MISR, there is evidence for drifts in the MODIS calibration.Corbett et al. (2015) compares both MISR and MODIS Terra level 1 radiances to collocated CERES outgoing shortwave radiation observations and finds that while MISR red green and near IR bands have darkened relative to CERES, MODIS Terra red and near IR bands have brightened, which likely explains much of the discrepancy between MODIS Terra and MISR cloud optical thickness trends.Taken in combination, the MISR and 205 MODIS data suggest there has been a reduction in cloud optical thickness during the period studied, at in least in the North and South Atlantic Ocean and likely the North Pacific as well.
210 problem primarily affects the low cloud retrieval fraction in tropical and subtropical regions with low average total cloud fraction and does not appear to have a substantial impact on the MODIS Terra extratropical cloud trends shown in Fig 1.This trend is being corrected in MODIS collection 6.1.Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-520Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License.
245 reanalysis variables and the MISR cloud fraction data.Finally, we show that the changes discussed above are consistent with recent trends in the North Pacific index and the Southern Annular mode and explore correlations with other climate indices.Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-520Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License. 4 Maximum Covariance Analysis 250 ) the red region around 45 N in the left-most panel indicates that in the North Atlantic there has been an increase in low cloud of moderate optical depth and a reduction 285 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-520Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License.

Fig. 4
Fig. 4 and the trends in the NAO and other East Atlantic teleconnection patterns are relatively weak for the time-

Fig. 3
Fig. 3 indicates that NP1 cloud changes are associated with large changes in the thermodynamic variables near the surface, and a particularly large change in SST, with cooler near-surface temperatures and SST in the eastern third of the North Pacific associated with increased low cloud amount, and conversely warmer SSTs being associated Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-520Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License.NP2 is a very similar mode to NA1 (note NA1 not NP1).It shows a reduction in high optically thick cloud and 355 enhanced low cloud of moderate optical depth.The profiles of the ERA interim variables to the right in NP2 show increased temperature, pressure, and anti-cyclonic motion throughout the middle of the domain (along 45 o N), and are nearly identical to the NA1 pattern.Figure 2 shows that both 500hPa temperature and pressure in this area have undergone robust positive trends.The time series derived from NP2 has high correlation with the NPI (r = .42),as does NP1, but NP2 has much weaker correlation with the PDO than NP1.We examine the time-series associated 360 with these two MCA modes and the PNA, PDO, and NPI in panels (a)-(b) of Fig. 5.These time-series illustrate the close connection between both of the North Pacific MCA modes and the NPI, as well as the first mode's relation to the PDO and the second mode's relation to the PNA.However, given the difference in spatial patterns between NP1 and NP2, and the poor correlation of NP2 with the PDO, we interpret the pattern in NP1 as a response to longer time scale forcing related the PDO, while NP2 is associated with shorter time-scale synoptic variability captured by the 365 PNA mode and the NPI.In effect, while NP1 and NP2 both feature increased low clouds and decreased high clouds (albeit with some relatively subtle difference in regards to optical depth), they have very different spatial patterns and do so for different reasons.
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-520Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License.covariance matrix computed between the two sets of normalized trends.  and   contain left and right eigenvectors produced by the singular value decomposition, where the columns of   and   represent modes of spatial covariance in MISR CTH-OD space and in the vertical profiles of the ECMWF reanalysis variables respectively.The matrix  is diagonal, and includes the singular values associated with each mode.The spatial patterns in each trend dataset associated with each column of   and   were retrieved by simply projecting 465   and   on to their respective datasets:   =     and   =    .(2) Here, the columns of   and   are the spatial patterns in their respective trend datasets associated with each of the modes of covariance identified with the MCA.The columns of   and   were then standardized (in this case they are divided by their standard deviation, but the mean is not removed) and again projected on to the original 470 dimensional versions of  and  (which we will call  * and  * ).This yields dimensionalized versions of the MCA modes (  * and   * ) and associated normalized spatial patterns (  and   ) e.g.: Each of the first two or three MCA modes in each ocean basin is displayed in Fig. 3.The left-most panels show the spatial distribution associated with each mode derived using the ECMWF trends (  ).These are normalized.The 475 spatial distributions derived using the MISR data are omitted for space, but are necessarily quite similar, though not identical, to those shown.The middle panels show trends in cloud fraction joint histograms which are dimensionalized (  * ), and the right panels show the associated dimensionalized trends in the profiles of the various ECMWF reanalysis variables (  * ).Recall, that the true trend in the original dataset associated with each mode can be recovered by projection of the contents of the middle or right panel (  * or   * ) on to the spatial distribution in the left panel (  ).Changing the sign of all three panels would then yield the same result, and we have chosen the sign of each mode such that they most resemble the patterns shown in Fig. 1.

Fig. 3 ,Figure 1 :
Fig.3, indicate the percent covariance explained, and the percent variance explained in each of the two datasets by that mode.We discuss these modes in Section 5.The combination of the MISR cloud fraction joint-histogram trends and associated spatial distribution (  and   )

Figure 4 :
Figure 4: Correlation matrix between time series computed for each MCA mode shown in Fig. 3 and each of the CPC climate indices (a).The numerical values in the left panel (a) are correlation coefficients with the decimal point omitted,

Figure 5 :
Figure 5: Time series from the top two North and South Pacific MCA modes, shown along with the most correlated CPC indices identified in Fig. 3.The North Pacific Index is strongly connected to both North Pacific MCA modes (panels a and b), with correlations of .56 and .42 for modes 1 and 2 respectively.In the South Pacific, mode 1 (c) is strongly correlated with the Nino 4 index (-0.50)while mode 2 (d) is correlated with the Southern Annular Mode (0.32).In the plots above,